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Crop yields and phosphorus fertilizer transformations after 25 years of applications to a subtropical soil under groundnut-based cropping systems
Field Crops Research 83 (2003) 283–296
Crop yields and phosphorus fertilizer transformations after 25 years of applications to a subtropical soil under groundnut-based cropping systems Milkha S. Aulakh*, Bachitter S. Kabba, H.S. Baddesha, Gulshan S. Bahl, M.P.S. Gill Department of Soils, Punjab Agricultural University, Ludhiana 141004, Punjab, India Received 20 October 2002; received in revised form 16 February 2003; accepted 4 April 2003
Abstract In subtropical regions where two crops are grown each year, effects of fertilizer P applications to summer, winter or both crops each year were evaluated on the basis of crop yields and transformations of fertilizer P into different soil P pools. Summer-grown groundnut showed small and inconsistent response to fertilizer P. Winter-grown crops responded significantly and consistently suggesting 40 and 60 kg P2O5 ha1 as adequate for rapeseed–mustard and wheat, respectively. Application of P to winter crops produced yields comparable to those when applied to both summer and winter crops. After 25 years of cropping, while Olsen-P decreased slightly in no-P control plots, organic P declined by 39% of the 8-year value. In fertilizer-treated plots, organic P increased by 18–22% of the 8-year value, whereas Olsen-P increased from initial 11.6 to 14–34 kg P ha1 after 3 years, 16–58 kg P ha1 after 8 years and 18–59 kg P ha1 after 25 years. Negligible increase in Olsen-P was observed in between 8 and 25 years. This was probably due to reduction in the applied P rates and raising five crops in between rotational changeovers without applying fertilizer P. Of the total fertilizer P added in 25 years, crops removed 21–54% whereas 33–64% accumulated in plough layer soil leaving 12–32% unaccounted for. This was probably leached to lower soil layers. The residual fertilizer P was transformed into labile (14–18%), moderately labile (28–35%) and non-labile (47–57%) pools of soil P. Results of this study with groundnut-based cropping systems in a subtropical region reveal that (a) long-term applications of fertilizer P to both crops each year significantly increased residual fertilizer P accumulation in soil and accelerated its conversion to non-labile P forms. (b) While application of fertilizer P to winter crops is essential, P requirement of summer-grown groundnut can be met from soilderived and residual fertilizer P; this practice helps enhance fertilizer use efficiency (42–54% as compared to 27–40% in cumulative P treatment), saves costly fertilizer (50%), and reduces P accumulation by 44–76%. (c) The Olsen-P test alone is not adequate to reflect the depletion/accumulation of bio-available P. Therefore, the status of different soil P pools in long-term fertilized soils need to be determined to assess and make use of accumulated fertilizer P for sustainable crop production and environmental safety. # 2003 Elsevier B.V. All rights reserved. Keywords: Inorganic P; Mineral P fertilization; Organic P; Residual P; Groundnut–wheat system; Groundnut–mustard system; Groundnut– rapeseed system; Irrigated semiarid soil; Subtropical region; Fertilizer P-use efficiency; P recovery; Crop yield
* Corresponding author. Tel.: þ91-161-458408; fax: þ91-161-400945 E-mail address: [email protected] (M.S. Aulakh).
0378-4290/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0378-4290(03)00078-9
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1. Introduction India is the second largest oilseed-growing country with an area of 26.2 m ha, out of which groundnut and rapeseed–mustard crops constitute more than 50% (FAI, 2001). Moreover, groundnut and rapeseed–mustard oils constitute 85% of total edible oils consumed in the country and therefore, are the most important oilseed crops. Development of improved cultivars and production technology during last few decades has led to several fold increase in the production of oilseeds. However, India is still not self-reliant and imports 3.5 m t of edible oil to meet the requirement of its ever-increasing population. Clearly, the production level of oilseed crops should be enhanced. Since the farmers of the region are largely cash-limited which restricts their capacity to buy fertilizers, enhancing the nutrient use efficiency in the cropping system is indispensable. While groundnut can meet a major portion of its N requirement through biological N2 fixation, rapeseed–mustard needs 100 kg N ha1 for optimum seed and oil production (Aulakh and Pasricha, 1997). However, phosphorus (P) deficiency has been identified as one of the major constraints in crop production in soils under intensive irrigated cropping systems in subtropical regions of south Asia (Tandon, 1987; Dev, 1994; Santhy et al., 2001). It has been established that crops often utilize only one-quarter to one-third of inorganic fertilizer P while large amounts of residual P can accumulate in soil. Most of such studies have focused primarily on the Olsen-P (plant available inorganic P only as the organic P components are removed during extraction by the addition of activated charcoal) reporting P accumulation with increasing amounts of P applications (Kang and Yususa, 1977; Pasricha et al., 1980; McCollum, 1991; Selles et al., 1999; Benbi and Biswas, 1999; Whalen and Chang, 2001). Only a few attempts have been made, however, to identify the changes in the amount of individual inorganic P (Pi) and organic P (Po) compounds in soil as induced by the long-term applications, despite the fact that plants are able to utilize substantial amounts of P from the less labile Pi and Po pools (Sharpley and Smith, 1985; Beck and Sanchez, 1994, 1996; Selles et al., 1999). Aulakh et al. (1991) reported that P fertilization increased Olsen-P up to 58 kg P ha1 and the total P increased up to 410 kg P ha1 in a soil receiving P applications for 8
years. After 16 years of P applications, Olsen-P accounted for 55–66% of the total P accumulated in soils (Whalen and Chang, 2001). The large difference between the Olsen-P pool and total P pool could present a quantifiable pool of P with remarkable value for plant use (Zhang and MacKenzie, 1997). Thus, besides recovery of fertilizer P by crops, the evaluation of residual fertilizer P in different soil pools illustrating the P budget could lead to a better understanding of P dynamics and hence to develop crop management systems for efficient use of fertilizer P and minimizing accumulation of unutilized fertilizer P in soil to avoid leaching and contamination of groundwater. For precise monitoring of changes in soil that impact on soil fertility and productivity, long-term field experiments provide the best avenue. Several long-term field experiments have been in operation in different temperate regions. For instance, based on the results of longterm experiments at Rothamsted, UK, Jenkinson (1991) reported that the yields could be sustained in monocultures by using adequate and right inorganic fertilizers. However, Hedley et al. (1982) reported that 65 years of wheat–wheat–fallow rotation on black chernozemic soil of Canada reduced total P content of soil by 29% when compared to permanent pasture. Few long-term studies have also been reported from the tropics depicting transformations of residual fertilizer P in different P pools of soil (Pushparajah and Bachik, 1987; Beck and Sanchez, 1994, 1996; Selles et al., 1997). Here we summarize crop responses in the groundnut-based cropping systems in the subtropics where irrigated groundnut–wheat, groundnut–mustard and groundnut–rapeseed rotations were followed in succession. We studied the transformations of fertilizer P into different Pi and Po pools and constructed a P budget after 25 years of fertilizer P applications. Using the data collected after 3, 8 and 25 years, we have attempted to establish the trends of conversion of residual fertilizer P from one pool to another so that efficient fertilizer P management technology could be developed for semiarid subtropical regions. 2. Materials and methods 2.1. Experimental site and soil characteristics A field study was conducted for 25 consecutive years (1975/1976–1999/2000) on a semiarid, irrigated
M.S. Aulakh et al. / Field Crops Research 83 (2003) 283–296
Tolewal sandy loam soil (Typic Ustochrepts) at Punjab Agricultural University Research Farm, Ludhiana, India. Ludhiana is in the subtropical region, situated at 308540 N and 758480 E and is 247 m above mean sea level. Subtropical regions have summer and winter crop-growing seasons where summer is characterized by high temperature and rainfall (i.e. monsoons) and winter is often dry with low temperature, which is suitable for growing field crops such as wheat and rapeseed–mustard under irrigated conditions. During the experimental period, the mean monthly minimum and maximum temperatures fluctuated from 6 and 20 8C in January to 27 and 41 8C in June, respectively. The annual rainfall ranged from 600 to 1200 mm of which 74–85% occurred during the July–September period. The soil (0–15 cm) characteristics at the beginning of study were pH 8.7, electrical conductivity (1:2 soil:water ratio) 0.21 dS m1, organic carbon 3.8 g kg1 and Olsen-P 11.6 kg ha1.
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2.2. Cropping sequences and treatments Table 1 reports the cropping sequences, crop cultivars, and rates of fertilizer P used. For all crop sequences, treatments consisted of four rates of fertilizer P applied each year in three frequencies as follows: (a) applied to summer-grown groundnut (Arachis hypogaea) only: direct for groundnut and residual for succeeding winter-grown crop; (b) applied to winter crop only: direct for winter crop and residual for succeeding summer-grown groundnut; (c) applied both to groundnut and winter crop: two times a year (cumulative). The experimental design was a completely randomized block (CRBD) where 12 treatments (four P rates three P frequencies) were randomized within a block and there were three blocks. Treatments were applied each year on the permanent plots of 3 m 8 m size. Non-experimental plants of same crop on all sides surrounded the experimental area.
Table 1 Groundnut-based cropping sequences, crop cultivars (cv.), and rates and frequencies of fertilizer P followed during 25 years (1975–2000) Year(s)
Summer-grown crop
Groundnut–wheat rotation for 7 years (1975/1976–1981/1982) 1975/1976 Groundnut (cv. M 13)
1976/1977–1981/1982
Groundnut (cv. M 13)
Winter-grown crop
Fertilizer P rates and frequencies
Wheat (cv. WL 357)
Groundnut: general crop without P Wheat: with P (0, 30, 60 and 90 kg P2O5 ha1) to create residual and cumulative P plots for succeeding groundnut crop.
Wheat (cv. WL 357 in 1976/1977) (cv. WL 711 in 1977/1978–1981/1982)
0, 30, 60 and 90 kg P2O5 ha1 applied to groundnut, wheat or to both crops each year
Groundnut–mustard rotation for 10 years (1982/1983–1991/1992) 1982/1983 and 1983/1984 Groundnut (cv. M 13) Mustard (cv. RLM 619) 1984/1985–1991/1992
Groundnut (cv. M 13 in 1984–1987) (cv. M 335 in 1988–1991)
Mustard (cv. RLM 619)
Groundnut–rapeseed rotation for 8 years (1992/1993–1999/2000) 1992/1993 Groundnut (cv. SG 84) Rapeseed (cv. GSL 1)
Without P. 0, 20, 40 and 60 kg P2O5 ha1 applied to groundnut, mustard or to both crops each year Groundnut: without P. Rapeseed: with P (0, 20, 30 and 40 kg P2O5 ha1).
1993/1994–1999/2000
Groundnut (cv. SG 84)
Rapeseed (cv. GSL 1)
0, 20, 30 and 40 kg P2O5 ha1 applied to groundnut, rapeseed or to both crops each year
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For groundnut–wheat sequence, the initial crop of groundnut was seeded as a general crop without fertilizer P in June 1975. At maturity, groundnut was harvested and wheat was seeded in November 1975. To create both residual and cumulative treatments for the succeeding groundnut crop, P was applied to the wheat crop. Thereafter, six cycles of the groundnut–wheat sequence were completed during 1976/1977–1981/1982 with four P rates (0, 30, 60 and 90 kg P2O5 ha1) and three frequencies as described above. Groundnut was seeded in June each year with a presowing irrigation in rows 30 cm apart, with a plant-to-plant distance of 22.5 cm (two kernels per hill). Fertilizer P as triple superphosphate (TSP) was drilled in appropriate treatment plots. A uniform dose of 20 kg S ha1 as gypsum was drilled whereas basal 15 kg N ha1 as urea was broadcast. The crop was irrigated when required. The groundnut crop was harvested in October each year, and pod and total biomass yields were recorded. For wheat (Triticum aestivum L.), fertilizer P as TSP was drilled in the appropriate plots before seeding wheat in November each year in rows 22.5 cm apart. A uniform dose of 120 kg N ha1 in the first year and 150 kg N ha1 in subsequent years as urea was applied in three dressings—at seeding, and with first and second irrigation. All of the aboveground biomass on each plot was harvested in April each year and the yields of grain and total biomass were measured. After discontinuing the groundnut–wheat rotation, the groundnut–mustard sequence was followed for a subsequent 10 years (Table 1). For the initial 2 years (1982/1983–1983/1984), both groundnut and mustard crops were grown without P applications. The rates of fertilizer P were reduced to 0, 20, 40 and 60 kg P2O5 ha1 as P requirement of mustard was lower than wheat. After groundnut, mustard (Brassica juncea L.; local name ‘raya’) was seeded in the first week of November each year in rows 30 cm apart with a plant-to-plant distance of 10 cm. Fertilizer P was applied as diammonium phosphate (DAP). Basal applications of 50 kg N ha1 (26.5–42.2 kg N as urea
after compensating 7.8–23.5 kg N ha1 added in DAP in different treatments) and 25 kg zinc sulfate ha1 were broadcast, and 20 kg S ha1 as gypsum was drilled in all plots. The crop was irrigated 4 weeks after sowing and later on when required. A second application of 50 kg N ha1, as urea, was top-dressed in all the plots before first irrigation. Groundnut–rapeseed sequence succeeded the groundnut–mustard sequence for 8 years (1992/ 1993–1999/2000) following the above-described procedure for groundnut–mustard (Table 1). In this, the groundnut crop of 1992 was raised without P treatments and the rates of fertilizer P were further reduced to 0, 20, 30 and 40 kg P2O5 ha1 keeping in view that 40 kg P2O5 ha1 was adequate for mustard and considering the similar requirement of the rapeseed crop. Rapeseed (Brassica napus L.; local name ‘gobhi sarson’) was seeded in the second week of October each year in rows 45 cm apart with a plant-to-plant distance of 10 cm. Data on the crop yields during first 3 and 6 years of the groundnut–wheat rotation were reported earlier (Pasricha et al., 1980; Aulakh et al., 1991). Aulakh and Pasricha (1999) reported crop yields of 5 years (1992/ 1993–1996/1997) of groundnut–rapeseed rotation. Thus, the data for average yields of all three crop rotations are presented here. 2.3. P concentration and uptake by crops To determine the concentration of P in groundnut kernels, rapeseed–mustard seeds, wheat grains, and the vegetative portion of all crops, sub-samples were digested in a 2:1 mixture of HNO3 and HClO4. Total P in the digests was determined by molybdophosphoric acid method (Olsen and Sommers, 1982). P uptake for individual crops in different rotations was reported earlier (Pasricha et al., 1980; Aulakh et al., 1991; Aulakh and Pasricha, 1999). From total P uptake for 25 years, uptake of fertilizer P was calculated by deducting the total 25-year P uptake of 361 kg P ha1 in no-P control plots as follows:
Total crop uptake of fertilizer P ðkg P ha1 Þ ¼ ½total crop uptake in P-fertilized treatment ðkg P ha1 Þ ½total crop uptake in no-P control ðkg P ha1 Þ
(1)
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2.4. Collection and analysis of soils After completing 25 years of experimentation, soil samples from the plough layer (0–15 cm) were collected from the replicates of all treatments in May 2000. For this, four cores were collected with a 5 cm i.d. tube auger and composited separately for each individual plot. Field moist soil samples were thoroughly mixed and representative sub-samples were air-dried, and crushed to pass through 0.9 mm sieve. The Olsen-P content in soils was determined (Olsen and Sommers, 1982), and different P fractions in the soil samples were estimated using the sequential extraction technique outlined by Hedley et al. (1982). In brief, most biologically available P was removed first, and measured in each soil sample by 16 h shaking with an anion exchange resin (Dowex 1 850 > 30 mesh, in bicarbonate form). This was followed by 16 h shaking and extraction with 0.5 M NaHCO3, which is believed to remove Pi and Po absorbed on the colloid surface. Then the same soil was shaken for 16 h with 0.1 M NaOH and centrifuged to remove and measure Pi and Po compounds apparently held more strongly by chemisorption to iron and aluminum components of soil surfaces. Then in the same soil 0.1 M NaOH was added, ultrasonified for 2 min in an ice bath at 75 W, shaken for 16 h and centrifuged to estimate Pi and Po held at the internal surfaces of the soil aggregates. This was followed by 16 h shaking with 1 M HCl that removes and estimates mainly apatite-type minerals and occluded P.
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Inorganic P in extracts was determined colorimetrically with the molybdate–ascorbic acid method (Murphy and Riley, 1962). A portion of the NaHCO3, NaOH and sonified-NaOH extractants were digested by acidified ammonium persulfate (EPA, 1971) and then analyzed by the molybdate–ascorbic acid method to obtain values of total P (Pi þ Po ) in the extracts. The Po content was the difference between total P and Pi. To simplify interpretation of the results, the fractions were grouped into labile P (resin-extractable Pi and NaHCO3-extractable Pi), moderately labile P (NaOH-extractable Pi and sonicated-NaOH-extractable Pi) and non-labile or stable P (HCl-extractable Pi and H2 O2 þ H2 SO4 -extractable P) pools (Aulakh and Pasricha, 1991; Cross and Schlessinger, 1995; Selles et al., 1999). 2.5. Statistical analysis and calculations for phosphorus budget Statistical analysis of crop yields in three groundnut-based cropping systems, and content of P in different soil fractions was carried out by analysis of variance in CRBD (Cochran and Cox, 1950). Mean separation for different treatments was performed using the least significant difference (LSD) test at 0.05 level of probability. Cumulative P fluxes were the sums of annual inputs (fertilizer P) and outputs during the 25-year study. Individual parameters were calculated as
Amount of fertilizer P recovered as Olsen-P in soil ðkg P ha1 Þ ¼ ½amount of Olsen-P in P-fertilized treatment ðkg P ha1 Þ ½amount of Olsen-P in no-P control ðkg P ha1 Þ (2) Total P recovered in soil ðkg P ha1 Þ ¼ ½sum of P recovered in different fractions in the soils ðmg P kg1 soilÞ ð2:325 106 kg ha1 Þ
Finally, the more chemically stable Po and relatively insoluble Pi forms were dissolved by oxidation and acid digestion (H2 O2 þ H2 SO4 ).
(3)
where 2:325 106 kg ha1 is the soil mass of 0–15 cm soil layer computed by using field bulk density of 1.55 g cm3:
Total fertilizer P recovered in soil ðkg P ha1 Þ ¼ ½total P recovered in P-fertilized treatment ðkg P ha1 Þ ½total P recovered in no-P control ðkg P ha1 Þ
(4)
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Fertilizer P present in any specific soil fraction ð%Þ ¼
½Pi or Po in P-fertilized treatment ðkg P ha1 Þ ½Pi or Po in no-P control ðkg P ha1 Þ 100 total fertilizer P recovered in soil ðkg P ha1 Þ
(5)
Overall recovery of fertilizer P ð%Þ ¼
½total crop uptake of fertilizer P in 25yearsðkgPha1 Þ þ ½total Precoveredin soil after 25 years ðkgP ha1 Þ 100 total amount of fertilizer P applied in 25 years ðkg P ha1 Þ (6)
3. Results and discussion 3.1. Crop yields Average yields of different crop rotations revealed large variations in the production capabilities of the summer-grown groundnut crop (Table 2). These were largely a result of changes in crop cultivars (Table 1). For instance, production decreased when M335 replaced groundnut cultivar M13 during the groundnut– mustard rotation, and increased enormously during the groundnut–rapeseed rotation with the replacement of cultivar M335 by SG 84, which had a high yield potential. Groundnut showed small and inconsistent response to applied P and the response of groundnut varied from 20 to 150 kg ha1 in the groundnut–wheat rotation and 83 to 43 kg ha1 in the succeeding groundnut–mustard rotation (Table 2). During the groundnut–rapeseed rotation, however, groundnut responded significantly and consistently up to 20 kg P2O5 ha1 when the preceding winter-grown rapeseed crop did not receive fertilizer P. Application of 20, 30 and 40 kg P2O5 ha1 to groundnut increased its 7-year average yield by 48, 48 and 51%, respectively, suggesting 20 kg P2O5 ha1 as adequate. When the preceding rapeseed received 40 kg P2O5 ha1, the groundnut crop did not respond to groundnut-applied P. In contrast to groundnut, grain yields of wintergrown wheat increased significantly with increasing rate of fertilizer P up to 60 kg P2O5 ha1 (Table 3). The wheat-applied 30, 60 and 90 kg P2O5 ha1 increased wheat grain yield by 1050 (35%), 1510 (50%) and 1640 (54%) kg ha1, respectively. But the increase was only 16–38% with residual P that was applied to preceding groundnut. The significant
P rate frequency interaction indicated lower responses of grain yields to residual P that was applied to the preceding groundnut. However, no significant differences were observed between wheat-applied and Table 2 Pod yield of groundnut from plots receiving different rates and frequencies of fertilizer P in three groundnut-based cropping sequences Treatment (kg P2O5 ha1)
Applied to summer-grown groundnut crop only
Applied to winter-grown crop only
Applied to both crops each year
6-Year (1976–1981) average groundnut yield (kg ha1) in groundnut–wheat rotationa Control 1870 30 1950 1870 1850 60 1930 2020 1980 90 2002 1950 2000 8-Year (1984–1991) average groundnut yield (kg ha1) in groundnut–mustard rotationb Control 691 20 714 709 729 40 710 608 685 60 699 734 711 7-Year (1993–1999) average groundnut yield (kg ha1) in groundnut–rapeseed rotationc Control 968 20 1429 1312 1425 30 1430 1418 1441 40 1466 1457 1451 a
LSD0.05: rate ¼ 130; year ¼ 250; frequency, rate frequency, year frequency, year rate, and year rate frequency ¼ nonsignificant. b LSD 0.05 : year ¼ 200; rate, frequency, rate frequency, year frequency, year rate, and year rate frequency ¼ nonsignificant. c LSD0.05: rate ¼ 300; year ¼ 380; frequency, rate frequency, year frequency, year rate, and year rate frequency ¼ nonsignificant.
M.S. Aulakh et al. / Field Crops Research 83 (2003) 283–296 Table 3 Grain yield of winter-grown wheat and seed yield of mustard and rapeseed crops from plots receiving different rates and frequencies of fertilizer P in the groundnut-based cropping sequences Treatment (kg P2O5 ha1)
Applied to summer-grown groundnut crop only
Applied to winter-grown crop only
Applied to both crops each year
6-Year (1976/1977–1981/1982) average wheat yield (kg ha1) in groundnut–wheat rotationa Control 3020 30 3510 4070 4150 60 4270 4530 4600 90 4160 4660 4530 7-Year (1984/1985–1990/1991) average mustard yield (kg ha1) in groundnut–mustard rotationb Control 947 20 1002 1254 1237 40 1238 1451 1444 60 1282 1452 1451 7-Year (1993/1994–1999/2000) average rapeseed yield (kg ha1) in groundnut–rapeseed rotationc Control 1360 20 1566 1801 1717 30 1713 1986 1854 40 1836 2060 2040
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summer, relatively high temperatures (27–41 8C) compared to 6–20 8C in winter, coupled with high soil water status in the rainy season (74–85% of annual rainfall occurs during the July–September period) enhances the solubility of native soil P and residual fertilizer P. Secondly, in the slightly alkaline semiarid subtropical soils, most of the inorganic P exists as calcium-bound P (Pasricha et al., 2002), which legumes often can more effectively utilize along with residual soil P compared to non-leguminous plants (Kalra and Soper, 1968; Hundal and Sekhon, 1976). Legumes have been found very effective in the solubilization of Ca–P and improving available P status of soils (Reddy and Surekha, 1999), presumably due to their genetic abilities such as development of proteoid roots and mobilization of nutrients in the soil with the help of specific root exudates (Gardner et al., 1983; Marschner, 1995). Significantly higher responses of legumes such as pigeon pea and soybean than wheat to residual fertilizer P have been reported under similar semiarid soils (Bahl and Pasricha, 1998; Aulakh et al., 2002). 3.2. Inorganic soil P fractions
a
LSD0.05: rate ¼ 350, frequency ¼ 220, year ¼ 210, rate frequency ¼ 335; year frequency, year rate, and year rate frequency ¼ non-significant. b LSD 0 . 0 5 : rate ¼ 67, frequency ¼ 49, year ¼ 110, rate frequency ¼ 110; year frequency, year rate, and year rate frequency ¼ non-significant. c LSD0.05: rate ¼ 140, frequency ¼ 120, year ¼ 210, rate frequency ¼ 210; year frequency, year rate, and year rate frequency ¼ non-significant.
wheat plus groundnut-applied P treatments. During the succeeding groundnut–mustard rotation, the mustard crop responded significantly (32–53%) to mustard-applied P up to 40 kg P2O5 ha1 rate but there was a much reduced response (6–31%) to groundnutapplied P (Table 3). The results obtained with rapeseed confirmed that the mustard-rapeseed crops require 40 kg P2O5 ha1 for optimum production and there is relatively little response to residual P (groundnutapplied P). For all three winter crops, application of P to both winter and summer crops, however, did not further increase yields. Therefore, the two times application appeared to be excessive and could be dispensed with, without affecting crop yields. During
Table 4 presents data on the contents of different labile, moderately labile and non-labile P fractions in soils after 25 years as extracted by sequential technique. The content of resin-extractable P was 9.1 mg P kg1 in no-P control plots and increased successively with each increment of applied P (Table 4). Averaged over all P rates, the content of resin-P was 13.8 mg P kg1 in summer-applied, 12.8 mg P kg1 in winter-applied and 14.7 mg P kg1 in summer plus winter-applied P treatments. The content of NaHCO3Pi in no-P control was 4.2 mg P kg1, which increased to 10.8, 10.4 and 14.2 mg P kg1 in summer-applied, winter-applied and summer plus winter-applied P treatments, respectively. Resin-P and NaHCO3-Pi fractions that constitute the labile P pool (Mattingly, 1975; Cross and Schlessinger, 1995) equaled 13.3 mg P kg1 in no-P control, 24.6 mg P kg1 in summerapplied, 23.2 mg P kg1 in winter-applied and 28.9 mg P kg1 in summer plus winter-applied P treatments (Table 4) representing 11, 13 and 10% of total residual fertilizer P recovered in soil (Table 5). Hydroxide-extractable and sonified-NaOH-extractable Pi represent the moderately labile P pool as they
Table 4 Phosphorus content in different inorganic (Pi) and organic (Po) fractions in soils after 25 years of fertilizer P applications in groundnut-based cropping systems Total fertilizer P applied in 25 years (kg P ha1) Control
Inorganic P fractions (mg P kg1) Resin-P
NaHCO3-Pi NaOH Pi Sonified- HCl-P NaOH-Pi
NaHCO3-Po NaOH-Po
SonifiedNaOH-Po
5.3
76.2
117.9
1.3
10.6
6.4
237.3
Applied to summer-grown groundnut only 209 11.8 7.8 15.7 387 13.7 9.9 17.6 564 15.8 14.8 18.2
7.2 9.1 10.4
99.9 108.4 116.3
122.3 136.7 167.1
3.4 6.0 7.6
18.4 22.2 23.0
8.5 9.9 10.0
295.0 333.5 383.1
13.8
4.2
H2 O2 þ H2 SO4 -P
Total Pa (mg P kg1)
6.3
Mean
9.1
Organic P fractions (mg P kg1)
10.8
17.2
8.9
108.2
142.0
5.7
21.2
9.5
337.2
Applied to winter crop only 231 11.0 426 12.9 621 14.6
8.3 8.7 14.2
16.4 18.0 18.9
6.2 8.2 10.7
83.8 107.0 99.2
113.8 127.5 161.4
3.7 6.2 6.4
18.2 22.2 22.7
8.6 10.0 9.1
269.8 320.7 357.2
Mean
10.4
17.8
8.4
96.7
134.2
5.4
21.0
9.2
315.9
and winter crop 19.0 9.3 22.0 12.2 25.2 13.4
78.8 125.5 134.5
142.5 170.6 190.0
5.2 7.7 8.6
22.0 24.4 26.8
11.7 14.8 19.9
313.9 405.0 451.7
12.8
Applied both to summer-grown groundnut 440 13.8 11.6 813 13.7 14.1 1185 16.6 16.8 Mean
14.7
14.2
22.1
11.6
112.9
167.7
7.2
24.4
15.5
390.2
LSD0.05 P-rate 1.8 Frequency 2.0 Rate frequency 3.1
1.4 1.8 nsb
2.5 1.9 ns
2.0 2.2 ns
9.0 14.5 ns
20.0 26.3 ns
0.7 0.9 ns
2.0 1.9 ns
4.1 ns ns
21.2 25.0 35.0
a b
Sum of all inorganic and organic P fractions. Non-significant.
Table 5 Transformations of residual fertilizer P in different P fractions in soils after 25 years of fertilizer P applications in groundnut-based cropping systems (values are means of three fertilizer P rates)a P fraction
Percentage of total recoverable fertilizer P P applied to summer-grown groundnut Pi
Resin-P NaHCO3-P
4.7 6.6
Po – 4.4
Total 4.7 11.0
P applied to winter crop Pi 4.7 7.9
Po – 5.2
P applied both to groundnut and winter crop Total 4.7 13.1
Pi 3.7 6.5
Po – 3.9
Total 3.7 10.4
Total labile-P
11.3
4.4
15.7
12.6
5.2
17.8
10.2
3.9
14.1
NaOH-P Sonified-NaOH-P
10.9 3.7
10.6 3.1
21.5 6.7
14.7 3.9
13.3 3.5
28.0 7.4
10.3 4.1
9.0 5.9
19.3 10.0
Total moderately labile P
14.6
13.7
28.3
18.6
16.8
35.4
14.4
14.9
29.3
HCl-P H2 O2 þ H2 SO4 -P
32.0 24.0
– –
32.0 24.0
26.1 20.7
– –
26.1 20.7
24.0 32.6
– –
24.0 32.6
Total non-labile P
56.0
–
56.0
46.8
–
46.8
56.6
–
56.6
a
Pi: inorganic P; Po: organic P.
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are not immediately available to plants but have the potential to become available over a medium period (months to a few years) through biological and physico-chemical transformations (Cross and Schlessinger, 1995; Zhang and MacKenzie, 1997). The content of NaOH-Pi in the no-P control was 6.3 mg P kg1, and on an average of P rates, increased to 17.2 mg P kg1 in summer-applied, 17.8 mg P kg1 in winter-applied, and 22.1 mg P kg1 in summer plus winter-applied P treatments (Table 4). Average content of sonified-NaOH-Pi was 8.9 mg P kg1 in summer-applied, 8.4 mg P kg1 in winter-applied and 11.6 mg P kg1 in summer plus winter-applied P treatments as compared to 5.3 mg P kg1 in no-P control plots. These two moderately labile Pi forms increased consistently with increasing rate and frequency of applied fertilizer P representing 15, 19 and 14% of total residual fertilizer P recovered in soil of summer-applied, winter-applied and summer plus winter-applied P treatments, respectively (Table 5). Phosphorus removed by 1 M HCl and concentrated H2 O2 þ H2 SO4 represents non-labile or stable P compounds such as Ca-bound P and stable humus and humic acid (Syers et al., 1972; Williams et al., 1980; Hedley et al., 1982). The content of HCl-P was 76.2 mg P kg1 in no-P control soil (Table 4). On an average of P rates, it increased to 108.2 mg P kg1 in summer-applied, 96.7 mg P kg1 in winter-applied and 112.9 mg P kg1 in summer plus winter-applied P soils. The presence of a high amount of HCl-P in the no-P control and fertilized soils reconfirmed that the major form of P in semiarid alluvial soils is Ca-bound (Pasricha et al., 2002) and is responsible for the longterm release of P by pedological processes (Smeck, 1973). The share of HCl-P and H2 O2 þ H2 SO4 -P in total fertilizer P recovered in soil was 56, 47 and 57% of total residual fertilizer P recovered in soil of summer-applied, winter-applied and summer plus winter-applied P treatments, respectively (Table 5). These findings suggest that non-labile P is the major sink of excessive added fertilizer P. 3.3. Organic soil P fractions Organic P extracted with NaHCO3 represents the labile Po pool as it is easily mineralizable (Bowman and Cole, 1978) and contributes to plant available P (Beck and Sanchez, 1996). Average NaHCO3-Po
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content in the no-P control, summer-applied, winterapplied and summer plus winter-applied P treatments was 1.3, 5.7, 5.4 and 7.2 mg P kg1, respectively (Table 4). Organic P fractions of the moderately labile pool are represented by the NaOH-Po and sonifiedNaOH-Po. The total content of these two fractions was 17.0 mg P kg1 in no-P control and on an average of P rates, increased to 30.7, 30.2 and 39.9 mg P kg1 in the summer-applied, winter-applied and summer plus winter-applied P treatments, respectively (Table 4). After 8 years of study, the content of labile and moderately labile Po in no-P control soil (0–15 cm) was 30.1 mg P kg1 (Aulakh and Pasricha, 1991). After 25 years, their content was reduced by 39% (18.3 mg P kg1) most likely as a consequence of a large amount of P extraction by plants; the total P uptake by all crops in the no-P control was 361 kg P ha1 in 25 years. In contrast to the control, 11–16% of total residual fertilizer P was present as Po in labile and moderately labile P pools after 8 years (Aulakh and Pasricha, 1991), which increased to 18–22% after 25 years (Table 5) depicting an increasing trend with increasing rate and frequency of applied P, and longevity of P applications. These results confirmed the earlier findings of Zhang and MacKenzie (1997) who observed that soil Po of no-P plots declined by 14% of the initial value over 5 years of maize production. As these mineralizable Po fractions contribute to plant available P especially in low-input systems (Bowman and Cole, 1978; Seeling and Jungk, 1996), our results clearly illustrate that the immobilization–mineralization of P is strongly controlled by the supply of, and plant need for P. 3.4. Transformations of residual fertilizer P in soil over a 25-year period Olsen-P status of soils of the present long-term experiment after 3 and 8 years, as reported earlier by Pasricha et al. (1980) and Aulakh et al. (1991), along with the data recorded after 25 years show large differences in Olsen-P status of soil with different treatments of applied P (Fig. 1). In no-P control plots, the initial level of 11.6 kg P ha1 declined to 10.5, 9 and 7 kg P ha1 after 3, 8 and 25 years, respectively. In a 22-year experiment conducted nearby on a similar semiarid subtropical soil with maize–wheat–cowpea fodder rotation, Benbi and Biswas (1999) reported a
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removed by crops and decrease in Olsen-P in the no-P control plots could be due to several reasons:
Fig. 1. Dynamics of Olsen-P in soil during 25 years in no-P control and with fertilizer P applications to summer-grown groundnut, winter crop and both crops each year. P1 P2, and P3 refer to 30, 60 and 90 kg P2O5 ha1 applied in groundnut–wheat; 20, 40 and 60 kg P2O5 ha1 in groundnut–mustard; 20, 30 and 40 kg P2O5 ha1 in groundnut–rapeseed sequence, respectively.
decline in Olsen-P in the no-P control during the initial 5 years and no change thereafter. The results of the present study further confirmed that Olsen-P showed a little decrease and perhaps stabilized and developed ‘quasi-equilibrium’ as discussed below. Interestingly, the decrease in Olsen-P status of the no-P control soil was so small when the total P removed by crops in 25 years was enormous (361 kg P ha1). The large gap between the amount of P
(a) There may be large-scale transformations of native soil P from moderately labile and nonlabile P fractions to labile forms as a consequence of substantial P extraction by plants (Hedley et al., 1982), and more so due to the inclusion of groundnut in the cropping sequences, as legumes are capable of solubilizing and utilizing Ca–P (Kalra and Soper, 1968; Reddy and Surekha, 1999). Thus relatively small decrease in concentration of Olsen-P could possibly be due to the continued quasi-equilibrium of different Pi pools in soils. (b) Plants and microbes release extracellular phosphate enzymes to facilitate mineralization of soil Po when the concentration of PO4-P in the soil solution is low, but cease their production at higher PO4-P concentrations through a process of feedback inhibition (Sharpley and Smith, 1985; Seeling and Jungk, 1996). Such trends were earlier observed by Zhang and MacKenzie (1997) who found decline in soil Po of non-fertilized plots by 14% of the initial value over 5 years of maize production. Our results indicate the mineralization of soil Po in control plots since the decrease in Po from year 8 (Aulakh and Pasricha, 1991) to year 25 (Table 4) was 39%. (c) Some P absorption by the crops from deeper layers of the soil especially by deep-rooted wheat and rapeseed–mustard crops as the 15–30 cm soil layer had only 4.5 mg P kg-1 of labile P (Aulakh and Pasricha, 1991). (d) Inadvertent mixing of soil from the P-treated plots, which were rich in P, during field operations. However, as every possible care including during ploughing was taken, the chances of such contamination are exceedingly small. Thus the most plausible reasons for the supply of P to plants are from Pi pools other than Olsen-P and through mineralization of Po. Although Olsen-P can also extract some labile organic P compounds, however, these are removed by the addition of activated charcoal during extraction, and thus are not accounted for. Increasing rates of fertilizer P raised the Olsen-P content of the soil in the summer-applied P treatment
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by 3–20, 4–31 and 6–32 kg P ha1 after 3, 8 and 25 years, respectively (Fig. 1). The corresponding increase in Olsen-P was 2–19, 7–34 and 6–32 kg P ha1 in the winter-applied and 5–22, 12–46 and 28–47 kg P ha1 in the summer plus winter-applied P treatments. These results illustrate an increasing build-up of Olsen-P until 8 years but negligible enrichment thereafter, which could be attributed to the reduction in the fertilizer P rates from 30, 60 and 90 kg P2O5 ha1 in the initial groundnut–wheat rotation in succeeding crop rotations as well as raising five crops (three groundnut and two mustard crops) in between rotational changeovers without applying fertilizer P (Table 1). Had we not reduced the fertilizer P rates, further increase in the accumulation of Olsen-P would have most likely continued as has been shown earlier by Benbi and Biswas (1999) in a 22-year study conducted nearby on a similar soil. Thus the magnitude of Olsen-P accumulation was commensurate with the applied fertilizer P rate and frequency during the entire period of 25 years. The relative proportion of residual fertilizer P in the plough layer soil of the winter-applied P treatment after 8 years was 36, 52 and 12% in labile, moderately labile and non-labile P pools, respectively (Aulakh and Pasricha, 1991), which changed to 18, 35 and 47% after 25 years (Table 5). In contrast, corresponding recoveries in the cumulative-applied P soil changed from 29, 45 and 26% after 8 years (Aulakh and
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Pasricha, 1991) to 14, 29 and 57% after 25 years, respectively (Table 5). These results are strongly suggestive of (a) increasing conversion of residual fertilizer P from plant available to less available and permanently stable P forms with increasing frequency of P fertilization (one-time versus two-time application annually), and with increasing period as the content of non-labile P pool increased by 2–4-fold from 8 to 25 years, and (b) apparently non-labile P pool was the major sink of excessive added fertilizer P. 3.5. Phosphorus budget The recovery of fertilizer P by all crops raised during 25 years ranged from 21 to 30% in the summer-applied, 42–54% in the winter-applied and 26–40% in the summer plus winter-applied P treatments suggesting the best utilization of winter-applied P (Table 6). When the P balances were viewed as a function of time, it became evident that during the initial groundnut–wheat rotation, for example, at the highest P rate (Table 1), the crop recovery of the annual input was 38 and 23% in summer or winter-applied P and summer plus winter-applied P treatments, respectively (data not shown). Reduction in P rate according to crop requirement during the succeeding crop rotations enhanced these recoveries to 50 and 30%, thus narrowing the difference between annual input and output.
Table 6 Balance sheet of fertilizer phosphorus after 25 years of applications at different rates and frequencies in groundnut-based cropping systems Total fertilizer P applied in 25 years (kg P ha1)
Total crop uptake of fertilizer P
Total fertilizer P recovered in soil
Total fertilizer P recovered (plant þ soil)
kg P ha1
kg P ha1
% of applied P
kg P ha1
% of applied P
Applied to summer-grown groundnut only 209 44 21 387 115 30 564 154 27
134 224 339
64 58 60
178 338 493
85 88 87
15 12 13
Applied to winter crop only 231 124 426 177 621 263
76 194 279
33 45 45
200 371 542
87 87 87
13 13 13
Applied both to summer-grown groundnut and winter crop 440 177 40 179 813 248 30 390 1185 304 26 498
41 48 42
356 638 802
81 78 68
19 22 32
% of applied P
54 42 42
Fertilizer P unaccounted for (%)
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Substantial amounts of the unutilized fertilizer P accumulated in the soil (33–64% of applied P) (Table 6). The winter-applied P greatly reduced fertilizer P accumulation (44–76% of summer plus winter-applied P treatment). The total apparent recovery of fertilizer P (plant þ soil) ranged from 68 to 88%. The unaccounted for fertilizer P in the plough layer after 25 years was 13% in winter-applied P and 19–32% in summer plus winter-applied P treatments, which was perhaps moved down below plough layer. After 8 years of experimentation, 84–85 and 15–16% of residual fertilizer P accumulated in 0–15 and 15–30 cm soil layers, respectively (Aulakh and Pasricha, 1991). Given the coarse texture of the soil in the present long-term study, irrigation of winter crops throughout the entire study period, and the average annual rainfall of 865 mm received at the site, downward translocation of P may be expected (Weaver et al., 1988; Beck and Sanchez, 1996). Whalen and Chang (2001) demonstrated that 7–15% of P applied for 16 years was not accounted for. They speculated that it moved through soil layers, eventually reaching the groundwater. Results of present study suggest that winter-applied P minimized the potential risk of groundwater P contamination from P movement through such irrigated porous soils. Future studies are, however, needed to determine if P contamination of groundwater is likely in this region.
4. Conclusions and implications for soil testing and fertilizer recommendations Results of this 25-year field study with groundnutbased cropping systems in a semiarid subtropical soil support several conclusions that may have important implications for soil testing and the formulation of recommendations for P fertilization of crops, and for environmental safety:
Our results indicate a marked difference in P fraction changes between fertilized and non-fertilized control plots. In fertilizer plots, residual fertilizer P accumulated in different bio-available forms of P (both Pi and Po) in the soil. Also, the magnitude of accumulation was proportional to the applied fertilizer P rate, frequency and its uptake by the crops. In the no-P control soil, Po mineralized to supply P
to plants with little change in Olsen-P status suggesting that mineralization of soil organic P and immobilization of fertilizer P were strongly controlled by the supply of, and plant need for P. Thus, the Olsen-P test alone cannot reflect such changes.
The conversion of fertilizer P to non-labile P forms is significantly enhanced by increasing rates and frequency of applied P. This process is also a function of time, as the proportion of non-labile P pool in winter-applied P and summer plus winterapplied P treatments was 47 and 57% after 25 years as compared to 12 and 26% after 8 years, respectively.
As compared to cumulative application of fertilizer P, alternate application to the winter crop has immense benefits as it can suffice to meet the P needs of both, the winter and the succeeding summer-grown groundnut. This helps to (a) enhance the fertilizer P-use efficiency and optimize cost effectiveness by saving 50% fertilizer, (b) reduce excessive accumulation of residual fertilizer P in soil and transformations to non-labile P forms, and (c) minimize the potential risk of groundwater contamination with P from P movement through such irrigated porous soils.
Although P-build up to a high fertility range would increase plant available P, albeit at a diminishing rate, continuous high application rates of fertilizer P may also result in P-induced micronutrient deficiencies. Therefore, future studies must address this problem by determining (a) the sink for utilization of residual fertilizer P such as raising crops without applying fertilizer P, and (b) how long residual P could maintain yields equal to those obtained with recommended P rates so that existing fertilizer P recommendations could be modified to more closely match P inputs vis-a`-vis P supplying capacity of soil and crop P requirements.
References Aulakh, M.S., Pasricha, N.S., 1991. Transformations of residual fertilizer P in a semiarid tropical soil under eight-year peanut– wheat rotation. Fert. Res. 29, 145–152. Aulakh, M.S., Pasricha, N.S., 1997. Effect of green manuring and fertilizer N application on enhancing crop productivity in mustard–rice rotation in semiarid subtropical region. Eur. J. Agron. 8, 51–58.
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Kang, B.T., Yususa, M., 1977. Effect of tillage method and phosphorus fertilization on maize in the humid tropics. Agron. J. 69, 291–294. Marschner, H., 1995. Mineral Nutrition of Higher Plants, 2nd ed. Academic Press, San Diego, CA, USA. Mattingly, G.E.G., 1975. Labile phosphate in soils. Soil Sci. 119, 369–375. McCollum, R.E., 1991. Buildup and decline in soil phosphorus: 30-year trends on a Typic Umbraquult. Agron. J. 83, 77–85. Murphy, J., Riley, J.P., 1962. A modified single solution method for the determination of phosphate in natural waters. Anal. Chem. Acta 27, 31–36. Olsen, S.R., Sommers, L.E., 1982. Phosphorus. In: Page, A.L., Miller, R.H., Keeney, D.R. (Eds.), Methods of Soil Analysis, Part 2, 2nd ed. Soil Science Society of America, Madison, WI, pp. 403–448. Pasricha, N.S., Aulakh, M.S., Baddesha, H.S., 1980. Comparative response of groundnut and wheat to phosphorus in a groundnut–wheat rotation. J. Agric. Sci. Camb. 94, 691–696. Pasricha, N.S., Aulakh, M.S., Vempati, R., 2002. Evaluation of available phosphorus soil test methods for peanut in neutral and alkaline soils. Commun. Soil Sci. Plant Anal. 33, 3593–3601. Pushparajah, E., Bachik, A.T., 1987. Management of acid soil in southeast Asia. In: Latham, M. (Ed.), Proceedings of the IBSRAM Conference on Land Development—Management of Acid Soils, vol. 4, Bangkok, Thailand. Reddy, M.N., Surekha, K., 1999. Role of chickpea in enhancing available P in chickpea–rice system in Vertisol. J. Indian Soc. Soil Sci. 77, 805–808. Santhy, P., Muthuvel, P., Murugappan, V., Selvi, D., Dhakshinamoorthy, M., 2001. Land degradation due to P depletion under intensive cropping in inceptisols. Fert. News 46 (5), 65–67. Seeling, B., Jungk, A., 1996. Utilization of organic phosphorus in calcium chloride extracts of soil by barley plants and hydrolysis by acid and alkaline phosphatases. Plant Soil 178, 179–184. Selles, F., Kochhann, R.A., Denardin, J.E., Zentner, R.P., Faganello, A., 1997. Distribution of phosphorus fractions in a Brazilian Oxisol under different tillage systems. Soil Till. Res. 44, 23–34. Selles, F., McConkey, B.G., Campbell, C.A., 1999. Distribution and forms of P under cultivator- and zero-tillage for continuous- and fallow–wheat cropping systems in the semiarid Canadian prairies. Soil Till. Res. 51, 47–59. Sharpley, A.N., Smith, S.J., 1985. Fractionation of inorganic and organic phosphorus in virgin and cultivated soils. Soil Sci. Soc. Am. J. 49, 127–130. Smeck, N.E., 1973. Phosphorus: an indicator of pedogenic weathering processes. Soil Sci. 115, 199–206. Syers, J.K., Smillie, G.W., Williams, J.D.H., 1972. Calcium fluoride formation during extraction of calcareous soils with fluoride. Implications to inorganic phosphorus fractionation schemes. Soil Sci. Soc. Am. Proc. 36, 20–25. Tandon, H.L.S., 1987. Phosphorus Research and Agricultural Production in India. Fertiliser Development and Consultation Organisation, New Delhi, India, pp. 160þxii.
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Weaver, D.M., Ritchie, G.S.P., Anderson, G.C., 1988. Phosphorus leaching in sandy soils. II. Laboratory studies of the long-term effects of the phosphorus source. Aust. J. Soil Res. 26, 191–200. Whalen, J.K., Chang, C., 2001. Phosphorus accumulation in cultivated soils from long-term annual applications of cattle feedlot manure. J. Environ. Qual. 30, 229–237.
Williams, J.D.H., Mayer, T., Nriagu, J.O., 1980. Extractability of phosphorus from phosphate minerals common in soils and sediments. Soil Sci. Soc. Am. J. 44, 462–465. Zhang, T.Q., MacKenzie, A.F., 1997. Changes of phosphorus fractions under continuous corn production in a temperate clay soil. Plant Soil 192, 133–139.
Crop yields and phosphorus fertilizer transformations after 25 years of applications to a subtropical soil under groundnut-based cropping systems Milkha S. Aulakh*, Bachitter S. Kabba, H.S. Baddesha, Gulshan S. Bahl, M.P.S. Gill Department of Soils, Punjab Agricultural University, Ludhiana 141004, Punjab, India Received 20 October 2002; received in revised form 16 February 2003; accepted 4 April 2003
Abstract In subtropical regions where two crops are grown each year, effects of fertilizer P applications to summer, winter or both crops each year were evaluated on the basis of crop yields and transformations of fertilizer P into different soil P pools. Summer-grown groundnut showed small and inconsistent response to fertilizer P. Winter-grown crops responded significantly and consistently suggesting 40 and 60 kg P2O5 ha1 as adequate for rapeseed–mustard and wheat, respectively. Application of P to winter crops produced yields comparable to those when applied to both summer and winter crops. After 25 years of cropping, while Olsen-P decreased slightly in no-P control plots, organic P declined by 39% of the 8-year value. In fertilizer-treated plots, organic P increased by 18–22% of the 8-year value, whereas Olsen-P increased from initial 11.6 to 14–34 kg P ha1 after 3 years, 16–58 kg P ha1 after 8 years and 18–59 kg P ha1 after 25 years. Negligible increase in Olsen-P was observed in between 8 and 25 years. This was probably due to reduction in the applied P rates and raising five crops in between rotational changeovers without applying fertilizer P. Of the total fertilizer P added in 25 years, crops removed 21–54% whereas 33–64% accumulated in plough layer soil leaving 12–32% unaccounted for. This was probably leached to lower soil layers. The residual fertilizer P was transformed into labile (14–18%), moderately labile (28–35%) and non-labile (47–57%) pools of soil P. Results of this study with groundnut-based cropping systems in a subtropical region reveal that (a) long-term applications of fertilizer P to both crops each year significantly increased residual fertilizer P accumulation in soil and accelerated its conversion to non-labile P forms. (b) While application of fertilizer P to winter crops is essential, P requirement of summer-grown groundnut can be met from soilderived and residual fertilizer P; this practice helps enhance fertilizer use efficiency (42–54% as compared to 27–40% in cumulative P treatment), saves costly fertilizer (50%), and reduces P accumulation by 44–76%. (c) The Olsen-P test alone is not adequate to reflect the depletion/accumulation of bio-available P. Therefore, the status of different soil P pools in long-term fertilized soils need to be determined to assess and make use of accumulated fertilizer P for sustainable crop production and environmental safety. # 2003 Elsevier B.V. All rights reserved. Keywords: Inorganic P; Mineral P fertilization; Organic P; Residual P; Groundnut–wheat system; Groundnut–mustard system; Groundnut– rapeseed system; Irrigated semiarid soil; Subtropical region; Fertilizer P-use efficiency; P recovery; Crop yield
* Corresponding author. Tel.: þ91-161-458408; fax: þ91-161-400945 E-mail address: [email protected] (M.S. Aulakh).
0378-4290/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0378-4290(03)00078-9
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1. Introduction India is the second largest oilseed-growing country with an area of 26.2 m ha, out of which groundnut and rapeseed–mustard crops constitute more than 50% (FAI, 2001). Moreover, groundnut and rapeseed–mustard oils constitute 85% of total edible oils consumed in the country and therefore, are the most important oilseed crops. Development of improved cultivars and production technology during last few decades has led to several fold increase in the production of oilseeds. However, India is still not self-reliant and imports 3.5 m t of edible oil to meet the requirement of its ever-increasing population. Clearly, the production level of oilseed crops should be enhanced. Since the farmers of the region are largely cash-limited which restricts their capacity to buy fertilizers, enhancing the nutrient use efficiency in the cropping system is indispensable. While groundnut can meet a major portion of its N requirement through biological N2 fixation, rapeseed–mustard needs 100 kg N ha1 for optimum seed and oil production (Aulakh and Pasricha, 1997). However, phosphorus (P) deficiency has been identified as one of the major constraints in crop production in soils under intensive irrigated cropping systems in subtropical regions of south Asia (Tandon, 1987; Dev, 1994; Santhy et al., 2001). It has been established that crops often utilize only one-quarter to one-third of inorganic fertilizer P while large amounts of residual P can accumulate in soil. Most of such studies have focused primarily on the Olsen-P (plant available inorganic P only as the organic P components are removed during extraction by the addition of activated charcoal) reporting P accumulation with increasing amounts of P applications (Kang and Yususa, 1977; Pasricha et al., 1980; McCollum, 1991; Selles et al., 1999; Benbi and Biswas, 1999; Whalen and Chang, 2001). Only a few attempts have been made, however, to identify the changes in the amount of individual inorganic P (Pi) and organic P (Po) compounds in soil as induced by the long-term applications, despite the fact that plants are able to utilize substantial amounts of P from the less labile Pi and Po pools (Sharpley and Smith, 1985; Beck and Sanchez, 1994, 1996; Selles et al., 1999). Aulakh et al. (1991) reported that P fertilization increased Olsen-P up to 58 kg P ha1 and the total P increased up to 410 kg P ha1 in a soil receiving P applications for 8
years. After 16 years of P applications, Olsen-P accounted for 55–66% of the total P accumulated in soils (Whalen and Chang, 2001). The large difference between the Olsen-P pool and total P pool could present a quantifiable pool of P with remarkable value for plant use (Zhang and MacKenzie, 1997). Thus, besides recovery of fertilizer P by crops, the evaluation of residual fertilizer P in different soil pools illustrating the P budget could lead to a better understanding of P dynamics and hence to develop crop management systems for efficient use of fertilizer P and minimizing accumulation of unutilized fertilizer P in soil to avoid leaching and contamination of groundwater. For precise monitoring of changes in soil that impact on soil fertility and productivity, long-term field experiments provide the best avenue. Several long-term field experiments have been in operation in different temperate regions. For instance, based on the results of longterm experiments at Rothamsted, UK, Jenkinson (1991) reported that the yields could be sustained in monocultures by using adequate and right inorganic fertilizers. However, Hedley et al. (1982) reported that 65 years of wheat–wheat–fallow rotation on black chernozemic soil of Canada reduced total P content of soil by 29% when compared to permanent pasture. Few long-term studies have also been reported from the tropics depicting transformations of residual fertilizer P in different P pools of soil (Pushparajah and Bachik, 1987; Beck and Sanchez, 1994, 1996; Selles et al., 1997). Here we summarize crop responses in the groundnut-based cropping systems in the subtropics where irrigated groundnut–wheat, groundnut–mustard and groundnut–rapeseed rotations were followed in succession. We studied the transformations of fertilizer P into different Pi and Po pools and constructed a P budget after 25 years of fertilizer P applications. Using the data collected after 3, 8 and 25 years, we have attempted to establish the trends of conversion of residual fertilizer P from one pool to another so that efficient fertilizer P management technology could be developed for semiarid subtropical regions. 2. Materials and methods 2.1. Experimental site and soil characteristics A field study was conducted for 25 consecutive years (1975/1976–1999/2000) on a semiarid, irrigated
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Tolewal sandy loam soil (Typic Ustochrepts) at Punjab Agricultural University Research Farm, Ludhiana, India. Ludhiana is in the subtropical region, situated at 308540 N and 758480 E and is 247 m above mean sea level. Subtropical regions have summer and winter crop-growing seasons where summer is characterized by high temperature and rainfall (i.e. monsoons) and winter is often dry with low temperature, which is suitable for growing field crops such as wheat and rapeseed–mustard under irrigated conditions. During the experimental period, the mean monthly minimum and maximum temperatures fluctuated from 6 and 20 8C in January to 27 and 41 8C in June, respectively. The annual rainfall ranged from 600 to 1200 mm of which 74–85% occurred during the July–September period. The soil (0–15 cm) characteristics at the beginning of study were pH 8.7, electrical conductivity (1:2 soil:water ratio) 0.21 dS m1, organic carbon 3.8 g kg1 and Olsen-P 11.6 kg ha1.
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2.2. Cropping sequences and treatments Table 1 reports the cropping sequences, crop cultivars, and rates of fertilizer P used. For all crop sequences, treatments consisted of four rates of fertilizer P applied each year in three frequencies as follows: (a) applied to summer-grown groundnut (Arachis hypogaea) only: direct for groundnut and residual for succeeding winter-grown crop; (b) applied to winter crop only: direct for winter crop and residual for succeeding summer-grown groundnut; (c) applied both to groundnut and winter crop: two times a year (cumulative). The experimental design was a completely randomized block (CRBD) where 12 treatments (four P rates three P frequencies) were randomized within a block and there were three blocks. Treatments were applied each year on the permanent plots of 3 m 8 m size. Non-experimental plants of same crop on all sides surrounded the experimental area.
Table 1 Groundnut-based cropping sequences, crop cultivars (cv.), and rates and frequencies of fertilizer P followed during 25 years (1975–2000) Year(s)
Summer-grown crop
Groundnut–wheat rotation for 7 years (1975/1976–1981/1982) 1975/1976 Groundnut (cv. M 13)
1976/1977–1981/1982
Groundnut (cv. M 13)
Winter-grown crop
Fertilizer P rates and frequencies
Wheat (cv. WL 357)
Groundnut: general crop without P Wheat: with P (0, 30, 60 and 90 kg P2O5 ha1) to create residual and cumulative P plots for succeeding groundnut crop.
Wheat (cv. WL 357 in 1976/1977) (cv. WL 711 in 1977/1978–1981/1982)
0, 30, 60 and 90 kg P2O5 ha1 applied to groundnut, wheat or to both crops each year
Groundnut–mustard rotation for 10 years (1982/1983–1991/1992) 1982/1983 and 1983/1984 Groundnut (cv. M 13) Mustard (cv. RLM 619) 1984/1985–1991/1992
Groundnut (cv. M 13 in 1984–1987) (cv. M 335 in 1988–1991)
Mustard (cv. RLM 619)
Groundnut–rapeseed rotation for 8 years (1992/1993–1999/2000) 1992/1993 Groundnut (cv. SG 84) Rapeseed (cv. GSL 1)
Without P. 0, 20, 40 and 60 kg P2O5 ha1 applied to groundnut, mustard or to both crops each year Groundnut: without P. Rapeseed: with P (0, 20, 30 and 40 kg P2O5 ha1).
1993/1994–1999/2000
Groundnut (cv. SG 84)
Rapeseed (cv. GSL 1)
0, 20, 30 and 40 kg P2O5 ha1 applied to groundnut, rapeseed or to both crops each year
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For groundnut–wheat sequence, the initial crop of groundnut was seeded as a general crop without fertilizer P in June 1975. At maturity, groundnut was harvested and wheat was seeded in November 1975. To create both residual and cumulative treatments for the succeeding groundnut crop, P was applied to the wheat crop. Thereafter, six cycles of the groundnut–wheat sequence were completed during 1976/1977–1981/1982 with four P rates (0, 30, 60 and 90 kg P2O5 ha1) and three frequencies as described above. Groundnut was seeded in June each year with a presowing irrigation in rows 30 cm apart, with a plant-to-plant distance of 22.5 cm (two kernels per hill). Fertilizer P as triple superphosphate (TSP) was drilled in appropriate treatment plots. A uniform dose of 20 kg S ha1 as gypsum was drilled whereas basal 15 kg N ha1 as urea was broadcast. The crop was irrigated when required. The groundnut crop was harvested in October each year, and pod and total biomass yields were recorded. For wheat (Triticum aestivum L.), fertilizer P as TSP was drilled in the appropriate plots before seeding wheat in November each year in rows 22.5 cm apart. A uniform dose of 120 kg N ha1 in the first year and 150 kg N ha1 in subsequent years as urea was applied in three dressings—at seeding, and with first and second irrigation. All of the aboveground biomass on each plot was harvested in April each year and the yields of grain and total biomass were measured. After discontinuing the groundnut–wheat rotation, the groundnut–mustard sequence was followed for a subsequent 10 years (Table 1). For the initial 2 years (1982/1983–1983/1984), both groundnut and mustard crops were grown without P applications. The rates of fertilizer P were reduced to 0, 20, 40 and 60 kg P2O5 ha1 as P requirement of mustard was lower than wheat. After groundnut, mustard (Brassica juncea L.; local name ‘raya’) was seeded in the first week of November each year in rows 30 cm apart with a plant-to-plant distance of 10 cm. Fertilizer P was applied as diammonium phosphate (DAP). Basal applications of 50 kg N ha1 (26.5–42.2 kg N as urea
after compensating 7.8–23.5 kg N ha1 added in DAP in different treatments) and 25 kg zinc sulfate ha1 were broadcast, and 20 kg S ha1 as gypsum was drilled in all plots. The crop was irrigated 4 weeks after sowing and later on when required. A second application of 50 kg N ha1, as urea, was top-dressed in all the plots before first irrigation. Groundnut–rapeseed sequence succeeded the groundnut–mustard sequence for 8 years (1992/ 1993–1999/2000) following the above-described procedure for groundnut–mustard (Table 1). In this, the groundnut crop of 1992 was raised without P treatments and the rates of fertilizer P were further reduced to 0, 20, 30 and 40 kg P2O5 ha1 keeping in view that 40 kg P2O5 ha1 was adequate for mustard and considering the similar requirement of the rapeseed crop. Rapeseed (Brassica napus L.; local name ‘gobhi sarson’) was seeded in the second week of October each year in rows 45 cm apart with a plant-to-plant distance of 10 cm. Data on the crop yields during first 3 and 6 years of the groundnut–wheat rotation were reported earlier (Pasricha et al., 1980; Aulakh et al., 1991). Aulakh and Pasricha (1999) reported crop yields of 5 years (1992/ 1993–1996/1997) of groundnut–rapeseed rotation. Thus, the data for average yields of all three crop rotations are presented here. 2.3. P concentration and uptake by crops To determine the concentration of P in groundnut kernels, rapeseed–mustard seeds, wheat grains, and the vegetative portion of all crops, sub-samples were digested in a 2:1 mixture of HNO3 and HClO4. Total P in the digests was determined by molybdophosphoric acid method (Olsen and Sommers, 1982). P uptake for individual crops in different rotations was reported earlier (Pasricha et al., 1980; Aulakh et al., 1991; Aulakh and Pasricha, 1999). From total P uptake for 25 years, uptake of fertilizer P was calculated by deducting the total 25-year P uptake of 361 kg P ha1 in no-P control plots as follows:
Total crop uptake of fertilizer P ðkg P ha1 Þ ¼ ½total crop uptake in P-fertilized treatment ðkg P ha1 Þ ½total crop uptake in no-P control ðkg P ha1 Þ
(1)
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2.4. Collection and analysis of soils After completing 25 years of experimentation, soil samples from the plough layer (0–15 cm) were collected from the replicates of all treatments in May 2000. For this, four cores were collected with a 5 cm i.d. tube auger and composited separately for each individual plot. Field moist soil samples were thoroughly mixed and representative sub-samples were air-dried, and crushed to pass through 0.9 mm sieve. The Olsen-P content in soils was determined (Olsen and Sommers, 1982), and different P fractions in the soil samples were estimated using the sequential extraction technique outlined by Hedley et al. (1982). In brief, most biologically available P was removed first, and measured in each soil sample by 16 h shaking with an anion exchange resin (Dowex 1 850 > 30 mesh, in bicarbonate form). This was followed by 16 h shaking and extraction with 0.5 M NaHCO3, which is believed to remove Pi and Po absorbed on the colloid surface. Then the same soil was shaken for 16 h with 0.1 M NaOH and centrifuged to remove and measure Pi and Po compounds apparently held more strongly by chemisorption to iron and aluminum components of soil surfaces. Then in the same soil 0.1 M NaOH was added, ultrasonified for 2 min in an ice bath at 75 W, shaken for 16 h and centrifuged to estimate Pi and Po held at the internal surfaces of the soil aggregates. This was followed by 16 h shaking with 1 M HCl that removes and estimates mainly apatite-type minerals and occluded P.
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Inorganic P in extracts was determined colorimetrically with the molybdate–ascorbic acid method (Murphy and Riley, 1962). A portion of the NaHCO3, NaOH and sonified-NaOH extractants were digested by acidified ammonium persulfate (EPA, 1971) and then analyzed by the molybdate–ascorbic acid method to obtain values of total P (Pi þ Po ) in the extracts. The Po content was the difference between total P and Pi. To simplify interpretation of the results, the fractions were grouped into labile P (resin-extractable Pi and NaHCO3-extractable Pi), moderately labile P (NaOH-extractable Pi and sonicated-NaOH-extractable Pi) and non-labile or stable P (HCl-extractable Pi and H2 O2 þ H2 SO4 -extractable P) pools (Aulakh and Pasricha, 1991; Cross and Schlessinger, 1995; Selles et al., 1999). 2.5. Statistical analysis and calculations for phosphorus budget Statistical analysis of crop yields in three groundnut-based cropping systems, and content of P in different soil fractions was carried out by analysis of variance in CRBD (Cochran and Cox, 1950). Mean separation for different treatments was performed using the least significant difference (LSD) test at 0.05 level of probability. Cumulative P fluxes were the sums of annual inputs (fertilizer P) and outputs during the 25-year study. Individual parameters were calculated as
Amount of fertilizer P recovered as Olsen-P in soil ðkg P ha1 Þ ¼ ½amount of Olsen-P in P-fertilized treatment ðkg P ha1 Þ ½amount of Olsen-P in no-P control ðkg P ha1 Þ (2) Total P recovered in soil ðkg P ha1 Þ ¼ ½sum of P recovered in different fractions in the soils ðmg P kg1 soilÞ ð2:325 106 kg ha1 Þ
Finally, the more chemically stable Po and relatively insoluble Pi forms were dissolved by oxidation and acid digestion (H2 O2 þ H2 SO4 ).
(3)
where 2:325 106 kg ha1 is the soil mass of 0–15 cm soil layer computed by using field bulk density of 1.55 g cm3:
Total fertilizer P recovered in soil ðkg P ha1 Þ ¼ ½total P recovered in P-fertilized treatment ðkg P ha1 Þ ½total P recovered in no-P control ðkg P ha1 Þ
(4)
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Fertilizer P present in any specific soil fraction ð%Þ ¼
½Pi or Po in P-fertilized treatment ðkg P ha1 Þ ½Pi or Po in no-P control ðkg P ha1 Þ 100 total fertilizer P recovered in soil ðkg P ha1 Þ
(5)
Overall recovery of fertilizer P ð%Þ ¼
½total crop uptake of fertilizer P in 25yearsðkgPha1 Þ þ ½total Precoveredin soil after 25 years ðkgP ha1 Þ 100 total amount of fertilizer P applied in 25 years ðkg P ha1 Þ (6)
3. Results and discussion 3.1. Crop yields Average yields of different crop rotations revealed large variations in the production capabilities of the summer-grown groundnut crop (Table 2). These were largely a result of changes in crop cultivars (Table 1). For instance, production decreased when M335 replaced groundnut cultivar M13 during the groundnut– mustard rotation, and increased enormously during the groundnut–rapeseed rotation with the replacement of cultivar M335 by SG 84, which had a high yield potential. Groundnut showed small and inconsistent response to applied P and the response of groundnut varied from 20 to 150 kg ha1 in the groundnut–wheat rotation and 83 to 43 kg ha1 in the succeeding groundnut–mustard rotation (Table 2). During the groundnut–rapeseed rotation, however, groundnut responded significantly and consistently up to 20 kg P2O5 ha1 when the preceding winter-grown rapeseed crop did not receive fertilizer P. Application of 20, 30 and 40 kg P2O5 ha1 to groundnut increased its 7-year average yield by 48, 48 and 51%, respectively, suggesting 20 kg P2O5 ha1 as adequate. When the preceding rapeseed received 40 kg P2O5 ha1, the groundnut crop did not respond to groundnut-applied P. In contrast to groundnut, grain yields of wintergrown wheat increased significantly with increasing rate of fertilizer P up to 60 kg P2O5 ha1 (Table 3). The wheat-applied 30, 60 and 90 kg P2O5 ha1 increased wheat grain yield by 1050 (35%), 1510 (50%) and 1640 (54%) kg ha1, respectively. But the increase was only 16–38% with residual P that was applied to preceding groundnut. The significant
P rate frequency interaction indicated lower responses of grain yields to residual P that was applied to the preceding groundnut. However, no significant differences were observed between wheat-applied and Table 2 Pod yield of groundnut from plots receiving different rates and frequencies of fertilizer P in three groundnut-based cropping sequences Treatment (kg P2O5 ha1)
Applied to summer-grown groundnut crop only
Applied to winter-grown crop only
Applied to both crops each year
6-Year (1976–1981) average groundnut yield (kg ha1) in groundnut–wheat rotationa Control 1870 30 1950 1870 1850 60 1930 2020 1980 90 2002 1950 2000 8-Year (1984–1991) average groundnut yield (kg ha1) in groundnut–mustard rotationb Control 691 20 714 709 729 40 710 608 685 60 699 734 711 7-Year (1993–1999) average groundnut yield (kg ha1) in groundnut–rapeseed rotationc Control 968 20 1429 1312 1425 30 1430 1418 1441 40 1466 1457 1451 a
LSD0.05: rate ¼ 130; year ¼ 250; frequency, rate frequency, year frequency, year rate, and year rate frequency ¼ nonsignificant. b LSD 0.05 : year ¼ 200; rate, frequency, rate frequency, year frequency, year rate, and year rate frequency ¼ nonsignificant. c LSD0.05: rate ¼ 300; year ¼ 380; frequency, rate frequency, year frequency, year rate, and year rate frequency ¼ nonsignificant.
M.S. Aulakh et al. / Field Crops Research 83 (2003) 283–296 Table 3 Grain yield of winter-grown wheat and seed yield of mustard and rapeseed crops from plots receiving different rates and frequencies of fertilizer P in the groundnut-based cropping sequences Treatment (kg P2O5 ha1)
Applied to summer-grown groundnut crop only
Applied to winter-grown crop only
Applied to both crops each year
6-Year (1976/1977–1981/1982) average wheat yield (kg ha1) in groundnut–wheat rotationa Control 3020 30 3510 4070 4150 60 4270 4530 4600 90 4160 4660 4530 7-Year (1984/1985–1990/1991) average mustard yield (kg ha1) in groundnut–mustard rotationb Control 947 20 1002 1254 1237 40 1238 1451 1444 60 1282 1452 1451 7-Year (1993/1994–1999/2000) average rapeseed yield (kg ha1) in groundnut–rapeseed rotationc Control 1360 20 1566 1801 1717 30 1713 1986 1854 40 1836 2060 2040
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summer, relatively high temperatures (27–41 8C) compared to 6–20 8C in winter, coupled with high soil water status in the rainy season (74–85% of annual rainfall occurs during the July–September period) enhances the solubility of native soil P and residual fertilizer P. Secondly, in the slightly alkaline semiarid subtropical soils, most of the inorganic P exists as calcium-bound P (Pasricha et al., 2002), which legumes often can more effectively utilize along with residual soil P compared to non-leguminous plants (Kalra and Soper, 1968; Hundal and Sekhon, 1976). Legumes have been found very effective in the solubilization of Ca–P and improving available P status of soils (Reddy and Surekha, 1999), presumably due to their genetic abilities such as development of proteoid roots and mobilization of nutrients in the soil with the help of specific root exudates (Gardner et al., 1983; Marschner, 1995). Significantly higher responses of legumes such as pigeon pea and soybean than wheat to residual fertilizer P have been reported under similar semiarid soils (Bahl and Pasricha, 1998; Aulakh et al., 2002). 3.2. Inorganic soil P fractions
a
LSD0.05: rate ¼ 350, frequency ¼ 220, year ¼ 210, rate frequency ¼ 335; year frequency, year rate, and year rate frequency ¼ non-significant. b LSD 0 . 0 5 : rate ¼ 67, frequency ¼ 49, year ¼ 110, rate frequency ¼ 110; year frequency, year rate, and year rate frequency ¼ non-significant. c LSD0.05: rate ¼ 140, frequency ¼ 120, year ¼ 210, rate frequency ¼ 210; year frequency, year rate, and year rate frequency ¼ non-significant.
wheat plus groundnut-applied P treatments. During the succeeding groundnut–mustard rotation, the mustard crop responded significantly (32–53%) to mustard-applied P up to 40 kg P2O5 ha1 rate but there was a much reduced response (6–31%) to groundnutapplied P (Table 3). The results obtained with rapeseed confirmed that the mustard-rapeseed crops require 40 kg P2O5 ha1 for optimum production and there is relatively little response to residual P (groundnutapplied P). For all three winter crops, application of P to both winter and summer crops, however, did not further increase yields. Therefore, the two times application appeared to be excessive and could be dispensed with, without affecting crop yields. During
Table 4 presents data on the contents of different labile, moderately labile and non-labile P fractions in soils after 25 years as extracted by sequential technique. The content of resin-extractable P was 9.1 mg P kg1 in no-P control plots and increased successively with each increment of applied P (Table 4). Averaged over all P rates, the content of resin-P was 13.8 mg P kg1 in summer-applied, 12.8 mg P kg1 in winter-applied and 14.7 mg P kg1 in summer plus winter-applied P treatments. The content of NaHCO3Pi in no-P control was 4.2 mg P kg1, which increased to 10.8, 10.4 and 14.2 mg P kg1 in summer-applied, winter-applied and summer plus winter-applied P treatments, respectively. Resin-P and NaHCO3-Pi fractions that constitute the labile P pool (Mattingly, 1975; Cross and Schlessinger, 1995) equaled 13.3 mg P kg1 in no-P control, 24.6 mg P kg1 in summerapplied, 23.2 mg P kg1 in winter-applied and 28.9 mg P kg1 in summer plus winter-applied P treatments (Table 4) representing 11, 13 and 10% of total residual fertilizer P recovered in soil (Table 5). Hydroxide-extractable and sonified-NaOH-extractable Pi represent the moderately labile P pool as they
Table 4 Phosphorus content in different inorganic (Pi) and organic (Po) fractions in soils after 25 years of fertilizer P applications in groundnut-based cropping systems Total fertilizer P applied in 25 years (kg P ha1) Control
Inorganic P fractions (mg P kg1) Resin-P
NaHCO3-Pi NaOH Pi Sonified- HCl-P NaOH-Pi
NaHCO3-Po NaOH-Po
SonifiedNaOH-Po
5.3
76.2
117.9
1.3
10.6
6.4
237.3
Applied to summer-grown groundnut only 209 11.8 7.8 15.7 387 13.7 9.9 17.6 564 15.8 14.8 18.2
7.2 9.1 10.4
99.9 108.4 116.3
122.3 136.7 167.1
3.4 6.0 7.6
18.4 22.2 23.0
8.5 9.9 10.0
295.0 333.5 383.1
13.8
4.2
H2 O2 þ H2 SO4 -P
Total Pa (mg P kg1)
6.3
Mean
9.1
Organic P fractions (mg P kg1)
10.8
17.2
8.9
108.2
142.0
5.7
21.2
9.5
337.2
Applied to winter crop only 231 11.0 426 12.9 621 14.6
8.3 8.7 14.2
16.4 18.0 18.9
6.2 8.2 10.7
83.8 107.0 99.2
113.8 127.5 161.4
3.7 6.2 6.4
18.2 22.2 22.7
8.6 10.0 9.1
269.8 320.7 357.2
Mean
10.4
17.8
8.4
96.7
134.2
5.4
21.0
9.2
315.9
and winter crop 19.0 9.3 22.0 12.2 25.2 13.4
78.8 125.5 134.5
142.5 170.6 190.0
5.2 7.7 8.6
22.0 24.4 26.8
11.7 14.8 19.9
313.9 405.0 451.7
12.8
Applied both to summer-grown groundnut 440 13.8 11.6 813 13.7 14.1 1185 16.6 16.8 Mean
14.7
14.2
22.1
11.6
112.9
167.7
7.2
24.4
15.5
390.2
LSD0.05 P-rate 1.8 Frequency 2.0 Rate frequency 3.1
1.4 1.8 nsb
2.5 1.9 ns
2.0 2.2 ns
9.0 14.5 ns
20.0 26.3 ns
0.7 0.9 ns
2.0 1.9 ns
4.1 ns ns
21.2 25.0 35.0
a b
Sum of all inorganic and organic P fractions. Non-significant.
Table 5 Transformations of residual fertilizer P in different P fractions in soils after 25 years of fertilizer P applications in groundnut-based cropping systems (values are means of three fertilizer P rates)a P fraction
Percentage of total recoverable fertilizer P P applied to summer-grown groundnut Pi
Resin-P NaHCO3-P
4.7 6.6
Po – 4.4
Total 4.7 11.0
P applied to winter crop Pi 4.7 7.9
Po – 5.2
P applied both to groundnut and winter crop Total 4.7 13.1
Pi 3.7 6.5
Po – 3.9
Total 3.7 10.4
Total labile-P
11.3
4.4
15.7
12.6
5.2
17.8
10.2
3.9
14.1
NaOH-P Sonified-NaOH-P
10.9 3.7
10.6 3.1
21.5 6.7
14.7 3.9
13.3 3.5
28.0 7.4
10.3 4.1
9.0 5.9
19.3 10.0
Total moderately labile P
14.6
13.7
28.3
18.6
16.8
35.4
14.4
14.9
29.3
HCl-P H2 O2 þ H2 SO4 -P
32.0 24.0
– –
32.0 24.0
26.1 20.7
– –
26.1 20.7
24.0 32.6
– –
24.0 32.6
Total non-labile P
56.0
–
56.0
46.8
–
46.8
56.6
–
56.6
a
Pi: inorganic P; Po: organic P.
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are not immediately available to plants but have the potential to become available over a medium period (months to a few years) through biological and physico-chemical transformations (Cross and Schlessinger, 1995; Zhang and MacKenzie, 1997). The content of NaOH-Pi in the no-P control was 6.3 mg P kg1, and on an average of P rates, increased to 17.2 mg P kg1 in summer-applied, 17.8 mg P kg1 in winter-applied, and 22.1 mg P kg1 in summer plus winter-applied P treatments (Table 4). Average content of sonified-NaOH-Pi was 8.9 mg P kg1 in summer-applied, 8.4 mg P kg1 in winter-applied and 11.6 mg P kg1 in summer plus winter-applied P treatments as compared to 5.3 mg P kg1 in no-P control plots. These two moderately labile Pi forms increased consistently with increasing rate and frequency of applied fertilizer P representing 15, 19 and 14% of total residual fertilizer P recovered in soil of summer-applied, winter-applied and summer plus winter-applied P treatments, respectively (Table 5). Phosphorus removed by 1 M HCl and concentrated H2 O2 þ H2 SO4 represents non-labile or stable P compounds such as Ca-bound P and stable humus and humic acid (Syers et al., 1972; Williams et al., 1980; Hedley et al., 1982). The content of HCl-P was 76.2 mg P kg1 in no-P control soil (Table 4). On an average of P rates, it increased to 108.2 mg P kg1 in summer-applied, 96.7 mg P kg1 in winter-applied and 112.9 mg P kg1 in summer plus winter-applied P soils. The presence of a high amount of HCl-P in the no-P control and fertilized soils reconfirmed that the major form of P in semiarid alluvial soils is Ca-bound (Pasricha et al., 2002) and is responsible for the longterm release of P by pedological processes (Smeck, 1973). The share of HCl-P and H2 O2 þ H2 SO4 -P in total fertilizer P recovered in soil was 56, 47 and 57% of total residual fertilizer P recovered in soil of summer-applied, winter-applied and summer plus winter-applied P treatments, respectively (Table 5). These findings suggest that non-labile P is the major sink of excessive added fertilizer P. 3.3. Organic soil P fractions Organic P extracted with NaHCO3 represents the labile Po pool as it is easily mineralizable (Bowman and Cole, 1978) and contributes to plant available P (Beck and Sanchez, 1996). Average NaHCO3-Po
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content in the no-P control, summer-applied, winterapplied and summer plus winter-applied P treatments was 1.3, 5.7, 5.4 and 7.2 mg P kg1, respectively (Table 4). Organic P fractions of the moderately labile pool are represented by the NaOH-Po and sonifiedNaOH-Po. The total content of these two fractions was 17.0 mg P kg1 in no-P control and on an average of P rates, increased to 30.7, 30.2 and 39.9 mg P kg1 in the summer-applied, winter-applied and summer plus winter-applied P treatments, respectively (Table 4). After 8 years of study, the content of labile and moderately labile Po in no-P control soil (0–15 cm) was 30.1 mg P kg1 (Aulakh and Pasricha, 1991). After 25 years, their content was reduced by 39% (18.3 mg P kg1) most likely as a consequence of a large amount of P extraction by plants; the total P uptake by all crops in the no-P control was 361 kg P ha1 in 25 years. In contrast to the control, 11–16% of total residual fertilizer P was present as Po in labile and moderately labile P pools after 8 years (Aulakh and Pasricha, 1991), which increased to 18–22% after 25 years (Table 5) depicting an increasing trend with increasing rate and frequency of applied P, and longevity of P applications. These results confirmed the earlier findings of Zhang and MacKenzie (1997) who observed that soil Po of no-P plots declined by 14% of the initial value over 5 years of maize production. As these mineralizable Po fractions contribute to plant available P especially in low-input systems (Bowman and Cole, 1978; Seeling and Jungk, 1996), our results clearly illustrate that the immobilization–mineralization of P is strongly controlled by the supply of, and plant need for P. 3.4. Transformations of residual fertilizer P in soil over a 25-year period Olsen-P status of soils of the present long-term experiment after 3 and 8 years, as reported earlier by Pasricha et al. (1980) and Aulakh et al. (1991), along with the data recorded after 25 years show large differences in Olsen-P status of soil with different treatments of applied P (Fig. 1). In no-P control plots, the initial level of 11.6 kg P ha1 declined to 10.5, 9 and 7 kg P ha1 after 3, 8 and 25 years, respectively. In a 22-year experiment conducted nearby on a similar semiarid subtropical soil with maize–wheat–cowpea fodder rotation, Benbi and Biswas (1999) reported a
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removed by crops and decrease in Olsen-P in the no-P control plots could be due to several reasons:
Fig. 1. Dynamics of Olsen-P in soil during 25 years in no-P control and with fertilizer P applications to summer-grown groundnut, winter crop and both crops each year. P1 P2, and P3 refer to 30, 60 and 90 kg P2O5 ha1 applied in groundnut–wheat; 20, 40 and 60 kg P2O5 ha1 in groundnut–mustard; 20, 30 and 40 kg P2O5 ha1 in groundnut–rapeseed sequence, respectively.
decline in Olsen-P in the no-P control during the initial 5 years and no change thereafter. The results of the present study further confirmed that Olsen-P showed a little decrease and perhaps stabilized and developed ‘quasi-equilibrium’ as discussed below. Interestingly, the decrease in Olsen-P status of the no-P control soil was so small when the total P removed by crops in 25 years was enormous (361 kg P ha1). The large gap between the amount of P
(a) There may be large-scale transformations of native soil P from moderately labile and nonlabile P fractions to labile forms as a consequence of substantial P extraction by plants (Hedley et al., 1982), and more so due to the inclusion of groundnut in the cropping sequences, as legumes are capable of solubilizing and utilizing Ca–P (Kalra and Soper, 1968; Reddy and Surekha, 1999). Thus relatively small decrease in concentration of Olsen-P could possibly be due to the continued quasi-equilibrium of different Pi pools in soils. (b) Plants and microbes release extracellular phosphate enzymes to facilitate mineralization of soil Po when the concentration of PO4-P in the soil solution is low, but cease their production at higher PO4-P concentrations through a process of feedback inhibition (Sharpley and Smith, 1985; Seeling and Jungk, 1996). Such trends were earlier observed by Zhang and MacKenzie (1997) who found decline in soil Po of non-fertilized plots by 14% of the initial value over 5 years of maize production. Our results indicate the mineralization of soil Po in control plots since the decrease in Po from year 8 (Aulakh and Pasricha, 1991) to year 25 (Table 4) was 39%. (c) Some P absorption by the crops from deeper layers of the soil especially by deep-rooted wheat and rapeseed–mustard crops as the 15–30 cm soil layer had only 4.5 mg P kg-1 of labile P (Aulakh and Pasricha, 1991). (d) Inadvertent mixing of soil from the P-treated plots, which were rich in P, during field operations. However, as every possible care including during ploughing was taken, the chances of such contamination are exceedingly small. Thus the most plausible reasons for the supply of P to plants are from Pi pools other than Olsen-P and through mineralization of Po. Although Olsen-P can also extract some labile organic P compounds, however, these are removed by the addition of activated charcoal during extraction, and thus are not accounted for. Increasing rates of fertilizer P raised the Olsen-P content of the soil in the summer-applied P treatment
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by 3–20, 4–31 and 6–32 kg P ha1 after 3, 8 and 25 years, respectively (Fig. 1). The corresponding increase in Olsen-P was 2–19, 7–34 and 6–32 kg P ha1 in the winter-applied and 5–22, 12–46 and 28–47 kg P ha1 in the summer plus winter-applied P treatments. These results illustrate an increasing build-up of Olsen-P until 8 years but negligible enrichment thereafter, which could be attributed to the reduction in the fertilizer P rates from 30, 60 and 90 kg P2O5 ha1 in the initial groundnut–wheat rotation in succeeding crop rotations as well as raising five crops (three groundnut and two mustard crops) in between rotational changeovers without applying fertilizer P (Table 1). Had we not reduced the fertilizer P rates, further increase in the accumulation of Olsen-P would have most likely continued as has been shown earlier by Benbi and Biswas (1999) in a 22-year study conducted nearby on a similar soil. Thus the magnitude of Olsen-P accumulation was commensurate with the applied fertilizer P rate and frequency during the entire period of 25 years. The relative proportion of residual fertilizer P in the plough layer soil of the winter-applied P treatment after 8 years was 36, 52 and 12% in labile, moderately labile and non-labile P pools, respectively (Aulakh and Pasricha, 1991), which changed to 18, 35 and 47% after 25 years (Table 5). In contrast, corresponding recoveries in the cumulative-applied P soil changed from 29, 45 and 26% after 8 years (Aulakh and
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Pasricha, 1991) to 14, 29 and 57% after 25 years, respectively (Table 5). These results are strongly suggestive of (a) increasing conversion of residual fertilizer P from plant available to less available and permanently stable P forms with increasing frequency of P fertilization (one-time versus two-time application annually), and with increasing period as the content of non-labile P pool increased by 2–4-fold from 8 to 25 years, and (b) apparently non-labile P pool was the major sink of excessive added fertilizer P. 3.5. Phosphorus budget The recovery of fertilizer P by all crops raised during 25 years ranged from 21 to 30% in the summer-applied, 42–54% in the winter-applied and 26–40% in the summer plus winter-applied P treatments suggesting the best utilization of winter-applied P (Table 6). When the P balances were viewed as a function of time, it became evident that during the initial groundnut–wheat rotation, for example, at the highest P rate (Table 1), the crop recovery of the annual input was 38 and 23% in summer or winter-applied P and summer plus winter-applied P treatments, respectively (data not shown). Reduction in P rate according to crop requirement during the succeeding crop rotations enhanced these recoveries to 50 and 30%, thus narrowing the difference between annual input and output.
Table 6 Balance sheet of fertilizer phosphorus after 25 years of applications at different rates and frequencies in groundnut-based cropping systems Total fertilizer P applied in 25 years (kg P ha1)
Total crop uptake of fertilizer P
Total fertilizer P recovered in soil
Total fertilizer P recovered (plant þ soil)
kg P ha1
kg P ha1
% of applied P
kg P ha1
% of applied P
Applied to summer-grown groundnut only 209 44 21 387 115 30 564 154 27
134 224 339
64 58 60
178 338 493
85 88 87
15 12 13
Applied to winter crop only 231 124 426 177 621 263
76 194 279
33 45 45
200 371 542
87 87 87
13 13 13
Applied both to summer-grown groundnut and winter crop 440 177 40 179 813 248 30 390 1185 304 26 498
41 48 42
356 638 802
81 78 68
19 22 32
% of applied P
54 42 42
Fertilizer P unaccounted for (%)
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Substantial amounts of the unutilized fertilizer P accumulated in the soil (33–64% of applied P) (Table 6). The winter-applied P greatly reduced fertilizer P accumulation (44–76% of summer plus winter-applied P treatment). The total apparent recovery of fertilizer P (plant þ soil) ranged from 68 to 88%. The unaccounted for fertilizer P in the plough layer after 25 years was 13% in winter-applied P and 19–32% in summer plus winter-applied P treatments, which was perhaps moved down below plough layer. After 8 years of experimentation, 84–85 and 15–16% of residual fertilizer P accumulated in 0–15 and 15–30 cm soil layers, respectively (Aulakh and Pasricha, 1991). Given the coarse texture of the soil in the present long-term study, irrigation of winter crops throughout the entire study period, and the average annual rainfall of 865 mm received at the site, downward translocation of P may be expected (Weaver et al., 1988; Beck and Sanchez, 1996). Whalen and Chang (2001) demonstrated that 7–15% of P applied for 16 years was not accounted for. They speculated that it moved through soil layers, eventually reaching the groundwater. Results of present study suggest that winter-applied P minimized the potential risk of groundwater P contamination from P movement through such irrigated porous soils. Future studies are, however, needed to determine if P contamination of groundwater is likely in this region.
4. Conclusions and implications for soil testing and fertilizer recommendations Results of this 25-year field study with groundnutbased cropping systems in a semiarid subtropical soil support several conclusions that may have important implications for soil testing and the formulation of recommendations for P fertilization of crops, and for environmental safety:
Our results indicate a marked difference in P fraction changes between fertilized and non-fertilized control plots. In fertilizer plots, residual fertilizer P accumulated in different bio-available forms of P (both Pi and Po) in the soil. Also, the magnitude of accumulation was proportional to the applied fertilizer P rate, frequency and its uptake by the crops. In the no-P control soil, Po mineralized to supply P
to plants with little change in Olsen-P status suggesting that mineralization of soil organic P and immobilization of fertilizer P were strongly controlled by the supply of, and plant need for P. Thus, the Olsen-P test alone cannot reflect such changes.
The conversion of fertilizer P to non-labile P forms is significantly enhanced by increasing rates and frequency of applied P. This process is also a function of time, as the proportion of non-labile P pool in winter-applied P and summer plus winterapplied P treatments was 47 and 57% after 25 years as compared to 12 and 26% after 8 years, respectively.
As compared to cumulative application of fertilizer P, alternate application to the winter crop has immense benefits as it can suffice to meet the P needs of both, the winter and the succeeding summer-grown groundnut. This helps to (a) enhance the fertilizer P-use efficiency and optimize cost effectiveness by saving 50% fertilizer, (b) reduce excessive accumulation of residual fertilizer P in soil and transformations to non-labile P forms, and (c) minimize the potential risk of groundwater contamination with P from P movement through such irrigated porous soils.
Although P-build up to a high fertility range would increase plant available P, albeit at a diminishing rate, continuous high application rates of fertilizer P may also result in P-induced micronutrient deficiencies. Therefore, future studies must address this problem by determining (a) the sink for utilization of residual fertilizer P such as raising crops without applying fertilizer P, and (b) how long residual P could maintain yields equal to those obtained with recommended P rates so that existing fertilizer P recommendations could be modified to more closely match P inputs vis-a`-vis P supplying capacity of soil and crop P requirements.
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