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Nitrogen and water use efficiency of fertigated processing potato
agricultural water management 85 (2006) 95–104
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Nitrogen and water use efficiency of fertigated processing potato T.M. Darwish a,*, T.W. Atallah b, S. Hajhasan c, A. Haidar c a
National Council for Scientific Research, P.O. Box 11-8281, Beirut, Lebanon Faculty of Agricultural Sciences, Lebanese University, Lebanon c Lebanese Agricultural Research Institute, Tel-Amara, Lebanon b
article info
abstract
Article history:
In this paper, we studied the response of spring processing potato to different N level by
Accepted 17 March 2006
fertigation and deficit irrigation in the dry Mediterranean Bekaa plain of Lebanon. In the first
Published on line 12 May 2006
season between 5 May and 29 August, the impact of different N level was evaluated. Treatments consisted of 125 kg N ha1 (N125), 250 kg N ha1 (N250), 375 kg N ha1 (N375)
Keywords:
and 500 kg N ha1 (N500) applied continuously by drip fertigation. In the second season
Dry Mediterranean
between 11 April and 16 August, the best N treatment (N125) was set as a standard to study
Fertigation
the constraints of deficit irrigation and the possibility of water saving. After plant establish-
Deficit irrigation
ment, treatments consisted of continuously applying a moderate water stress 80% of ET, a
N recovery
severe deficit 60% of ET, a control 100% of ET and an excess irrigation 120% of ET. Labeled N
Water saving
fertilizers in 2000 and 2001, as well as a neutron probe in 2001, were used to estimate N use efficiency and the actual crop evapotranspiration and water use efficiency. Different N level, at recommended water supply (700 mm), did not lead to a crop response. The lower N treatment (N125) gave a significantly higher N recovery (61% of applied N), despite the clay nature of the soil. In the deficit irrigation, water application varied between 500 and 800 mm with 720 mm for the control. Final yield showed the possibility to save 119 mm of water with no reduction in fresh tuber yield. Imposing severe water deficit considerably decreased tuber yield. Extremes of water input, deficit and excess irrigation, lowered the percentage of marketable yield from 84 to 72%. Regardless of the severity of deficit irrigation, the upper 0.4 m soil layer provided up to 90% of crop water demands (uptake). About 40% of crop ET occurred up to the maximum development stage. Under water stress conditions, the effective ground cover was reduced, which can explain the decrease in N recovery from 40 to 20%. This is probably due to higher evaporation rates and N losses. In both trials, increasing N or water input showed inconsistent impact on dry matter production per unit of applied water. Under deficit irrigation, a yield response factor (relationship between relative yield decrease and relative ET deficit) of 0.8 was obtained, while values below 1 justify the implementation of deficit irrigation. Managing continuous deficit irrigation of processing potato in the dry Mediterranean region with 0.80 ET is possible for water saving. Therefore, it could be safe to deplete the soil water content down to 56.6 mm in the upper 30 cm soil layer, which is equivalent to 40% water depletion. # 2006 Elsevier B.V. All rights reserved.
* Corresponding author. Tel.: +961 4 409845/6; fax: +961 4 409847. E-mail address: [email protected] (T.M. Darwish). 0378-3774/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2006.03.012
96 1.
agricultural water management 85 (2006) 95–104
Introduction
In Lebanon, the share of agriculture exceeds 68% of the total water withdrawal, with 46% reliance on groundwater (FAO, 1998). Seeking additional water resources and improving the efficient use of available water are two parallel strategic lines to address the issue of water shortage. This is particularly relevant to cash crops dependant on irrigation water such as potato, which occupies 15,000 ha, corresponding to 17% of irrigated land (FAO, 1998). Its evapotranspiration (ET) was studied using drainage lysimeters in the central Bekaa valley (Abou-Khaled et al., 1969). Results provided the basis for general recommendations on potato crop coefficient and water demands. A shift from furrow irrigation to macrosprinklers allowed a 50% reduction of water application to potato (FAO, 1969). The constraint of this technique is its low efficiency (Al-Jamal et al., 2001), compared to drip irrigation (Hamdy, 1998). Further water economy was demonstrated by the simultaneous application of water and N through potato fertigation, with 35% water saving and improved N use efficiency (Papadopoulos, 1988; Mohammad et al., 1999; Darwish et al., 2003). Water saving by deficit irrigation presents several climatic and crop constraints. A sensitive crop like potato is particularly difficult to manage (Shock and Feibert, 2002), as it presented a negative response to deficit irrigation (Kovacs et al., 1999; Fabeiro et al., 2001). Various techniques such as furrow irrigation (Iqbal et al., 1999), drip (Kovacs et al., 1999, Yuan et al., 2003) and macro-sprinklers (Nimah, 1984) were studied. Yield reduction was reported in these cases, as well as modest water saving. The calibration of the crop response factor (Ky) to deficit irrigation relies on mean annual climatic data to estimate crop ET (Shock and Feibert, 2002). Because Ky differs with different irrigation and fertilization techniques, additional research to achieve mathematical optimization of deficit irrigation in different pedoclimatic conditions is justified. With the implementation of drip systems, fertigation became an important tool to achieve sound irrigation and fertilization. Previously, nitrogen management was particu-
larly relevant to the dry areas where nitrogen was deficient (Dregne, 1976; Ryan et al., 1980). For processing potato, a careful N and water management is required to ensure regular growth, high dry matter content and marketable tubers (Le Corre et al., 1995). Previous studies addressed the effect of supplemental deficit irrigation on early potato (Onder et al., 2005), that of drip irrigation frequency (Kang et al., 2004) and water rates (Meyer and Marcum, 1998) on the performance of potato grown on loam and sandy loam soils. Implementing continuous water stress to processing potato growing in late spring on clay soil is a challenging issue requiring the monitoring of plant available water in the root zone. The objective of this work was first to establish the optimum N rate for processing potato using 15N, and second, at the optimum N rate study the impact of water stress on crop yield and the efficiency of N and water utilization, under the dry Mediterranean pedoclimatic conditions of the Bekaa plain of Lebanon.
2.
Materials and methods
2.1.
Experimental site
Fertigation trials were conducted in Tel-Amara experimental station (398110 N; 418320 E; 905 m) in the central Bekaa plain, Lebanon. The area, characterized by a dry Mediterranean climate, receives an annual precipitation of 600 mm, occurring between November and March. The average monthly temperature varies between 14.4 8C in April and 23.8 8C in July with relatively low relative humidity (Table 1). The calculated average daily evapotranspiration (ETo-PM) was comparable for the two experimental years and varied between 729 mm in 2000 and 761 mm in 2001. Soils of the experimental site were deep, non-calcareous, clay Eutric Cambisols with an average bulk density of 1.2 g cm3, low organic matter content (0.12 g kg1), moderate total N content (0.012 g kg1) and moderately high CEC value (40 Cmol kg1). Gravimetric moisture content varied between 29% at field capacity and 16.5% at permanent wilting point. Thus, between 0 and 75 cm depth the soil water content varied between 236.3 and
Table 1 – Climatic data obtained at the experimental station in 2000 (Experiment I) and 2001 (Experiment II) and the longterm average Season
Month
Air temperature (8C)
Relative humidity (%)
ETo-PM (mm day1)
2000
April May June July August
14.44 16.75 20.24 24.52 23.77
52.38 51.06 49.83 44.85 46.48
4.10 6.44 7.34 7.28 6.33
2001
April May June July August
14.88 17.22 20.72 23.82 23.53
54.18 53.03 40.50 41.31 50.68
5.78 5.63 7.95 8.88 8.80
30-Year average
May June July August
16.20 19.80 21.80 22.50
50.00 45.00 40.20 35.00
3.94 5.6 5.88 5.416
agricultural water management 85 (2006) 95–104
134.4 mm. Irrigation water, originating from a local well, had a good quality (pH of 7.5 and EC = 0.4 dS m1). Both trials were run as part of a 3-year rotation: legume–potato–wheat.
2.2.
Experimental set-up
2.2.1.
Experiment I
Nutrient requirements of processing potato (cv Santana) were evaluated in 2000 under four nitrogen levels and full water supply. Seeds were sown on 5 May and harvested on 29 August. The experiment was set up on 792 m2 in a randomized block design with four replicates. A buffer zone of 1.5 m separated between blocks to avoid interference. The plot size was 11 m 4.5 m. Each plot consisted of six rows, four middle (harvest) and two border rows. A spacing of 75 cm between rows and 25 cm between plants gave a planting density close to 53 333 plants ha1. Observations and measurements were conducted on the effective rows. During the growth season, the crop fully depended on irrigation. Water application was based on the information received from two precise weighing lysimeters, planted to ryegrass as a reference crop, located on the station. Throughout the growth stages, the crop fraction (Kc) was based on Papadopoulos (1988): 0.4 (emergence: 45 days after sowing)–0.7 (45–63 DAS)–0.9 (63–98 DAS)–0.7 (98–113 DAS). This Kc was lower than that proposed by Allen et al. (1998) for the mid and late development stages. Treatments consisted of applying four levels of nitrogen by fertigation: N125 (125 kg N ha1), N250 (250 kg N ha1), and N375 (375 kg N ha1) and N500 (500 kg N ha1). Differential N application occurred throughout the experiment. Phosphorus and potassium feeding took place by fertigation: P as 94 kg P ha1 from phosphoric acid and K equivalent to 344 kg K ha1 from soluble potassium sulfate. Labeled ammonium sulfate was used for the application of 15 N at an enrichment of 1.5% atom excess (a.e.) for both seasons. Microplots consisting of double rows, of four plants each, were located in the middle part of the macroplots. Both microplots and macroplots were simultaneously fertigated through separate drip irrigation systems. The emitters had 40 cm spacing and a discharge of 4 L h1. Barrels containing 15 N solution were connected to the plants by tubes perforated within the microplots to carry one dripper for each labeled plant.
2.2.2.
Experiment II
The best N treatment in Experiment I (N125) served as a standard level for the deficit irrigation trial in 2001. Due to early spring conditions, sowing occurred on 11 April 2001 and harvest on 16 August. Treatments consisted of four levels of irrigation water: an excess irrigation (W120) equivalent to 120% of ET, a control (W100) equivalent to 100% of ET, a moderate deficit irrigation (W80) equivalent to 80% of ET and a severe deficit irrigation (W60) equivalent to 60% of ET. In Experiment II, the control was similar to the water input in Experiment I. Between the planting date and the full establishment phase, 112.6 mm were equally applied to all treatments. Differential water application started at the beginning of the maximum development stage, on the 47th DAS (28 May). The
97
crop coefficient (Kc) was 0.7 (47–71 DAS), 0.9 (71–96 DAS) and 0.7 (96–124 DAS). During the differential water input, plots received an equivalent of 707 mm in W120, 588 mm in W100, 469 mm in W80 and 361 mm in W60. A leaching fraction of 15% was given, therefore 85% of these amounts were considered as effectively applied. Soil potential in each plot was monitored using tensiometers, inserted at 30 and 60 cm depth. Irrigation started when the soil matric potential in the control approached 30 kPa at 0.3 m depth. This threshold corresponded to the recommended soil potential used in the scheduling of dripirrigated potato (Papadopoulos, 1988). In addition, a neutron probe served to assess the soil water contents between 0 and 60 cm soil depth. Access tubes were inserted in three replicates for each treatment to a depth of 75 cm. Nitrate leaching was monitored using tensionics, which are devices consisting of tubes with ceramic chambers, allowing to sample the soil solution. They were inserted in three replicates at 30, 60 and 90 cm depths. Soil solution collected at 90 cm depth allowed monitoring water percolation and nitrates leaching beyond the root zone.
2.3.
Data collection
The observations of crop performance corresponded to the following growth stages: maximum shoot development, physiological maturity and full maturity. These coincided with 63 DAS, 98 DAS and 113 DAS in Experiment I and 83, 117 and 127 DAS, respectively, in Experiment II. At each stage, four plants per plot were sampled. At physiological maturity, the middle protected four plants were sampled from each microplot. Fresh and dry matter (DM) yields were recorded. Tubers were sorted by their size and non-marketable (deformed and diseased) tubers were counted, weighted and discarded. Samples (whole shoots and tubers) were weighted fresh, subsampled by quartering and oven-dried at 70 8C until constant weight. 15N analysis and expression of results were done according to Zapata (1990). Briefly, these were: %Ndff (N derived from fertilizers) = (% 15N a.e. plant 100)/% 15 N a.e. fertilizer. N yield (kg ha1) = (DM yield (kg ha1 %N)/100. Fertilizer N yield (kg ha1) = (N yield (kg ha1) %Ndff)/100. N utilization (%) = (fertilizer N yield 100)/rate of N application. Water removal from different soil layers and during vegetation periods was calculated based on the neutron probe readings. These were taken immediately before and 24 h after each irrigation at 20, 40 and 60 cm depths. Soil moisture content was calculated using the calibration curve of the soil of Tel-Amara station reported previously (Darwish et al., 2002) and equal to uv = 0.2681X 0.2309 (r2 = 0.79), where uv is the volumetric soil water content (%) and X is the count ratio (neutron count rate in a given soil layer/count rate in a standard medium). Actual evapotranspiration was calculated as the difference between soil water contents 24 h after an irrigation event and that just before the following irrigation.
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A mathematical function was established between the gradient of hydraulic heads, measured by tensiometer, and neutron probe measurements within the root zone to aid the estimation of plant available water from simple measurement of the soil head potential. Crop response (Ky) links relative yield decrease to relative evapotranspiration deficit. It was expressed as follows: Ky ¼
1 Ya =Ym 1 ETa =ETm
where Ya and Ym are actual and maximum yield (kg ha1); ETa and ETm are actual and maximum evapotranspiration (Kirda et al., 1999). Irrigation water productivity (IWP) was defined as the final yield per unit of applied water, while water use efficiency (WUE) was defined as the final yield per unit of consumed water (Van Cleemput, 2000). Harvest index was measured as the ratio of tuber DM yield to the total DM yield. For nitrate measurements, soil solution was removed once every 8 days, as recommended (Moutonnet et al., 1993). This interval is enough to establish equilibrium of nitrates concentration in the solution inside and outside the ceramic chamber. Nitrate concentration was analyzed using a specific electrode. Statistical analysis was performed by the analysis of variance (ANOVA) using SigmaStat (Jandel Scientific). Means were compared using the LSD test ( p < 0.05).
3.
Results and discussion
3.1.
Crop response to N level
Fig. 1 – Shoot and tuber dry matter yield of processing potato at physiological maturity, in Experiment I, under different N levels.
suggested by the small slope (y = 0.0011x + 36.35). Among yield parameters, only the tuber dry matter content was negatively affected by the higher nitrogen inputs (Table 2). Based on the intercept in the previous relationship, in the absence of nitrogen (zero nitrogen) a tuber yield of 36 t ha1 could be expected. This was explained by the combined effect of the soil pool and nitrate level in the irrigation water. The use of labeled nitrogen fertilizers allows quantifying the contribution of nitrogen fertilizers. In terms of total N removed by the crop (N yield), there was an important difference between N500 (273 kg N ha1) and N125 (172 kg N ha1) treatments (Table 3). Such a larger removal was partly due to higher nitrogen contents in the shoots and tubers, as well as higher shoots yields. The contribution of fertilizers (Ndff) was, as expected, significantly smaller in the N125 treatment (Table 3). Out of the applied fertilizers, the crop removed a total of 157 kg N ha1 in N500 against 76 kg N ha1 in N125. As a proportion of the N fertilizer recovery, the lowest N rate gave the most satisfactory value of 61.4%. This utilization value meets those found in temperate areas (Saoud et al., 1992) and
Increased nitrogen levels promoted a vigorous plant growth, followed by an additional dry matter accumulation in the shoots, to the contrary of tubers. At physiological maturity, the slope for shoots dry matter was much higher than for the tubers (Fig. 1) confirming the impact of higher nitrogen input on the aboveground growth. At the final yield, the fresh tuber production did not present a link with the nitrogen levels as
Table 2 – Impact of nitrogen or water levels on tuber marketability and dry matter contents at commercial maturity during the two seasons Treatment: N level (kg ha1)
Yield parameter
Experiment I
Non-marketable yield (%) Marketable yield (%) Dry matter (%)
125
250
375
500
23.0 106.3 21.65 a
15.5 111.0 20.55 b
24.4 75.3 20.32 b
12.0 105.5 20.35 b
Yield parameter
Experiment II
Non-marketable yield (%) Marketable tuber weight (g) Dry matter (%) Harvest index
Treatment: ETc (%) W120
W100
W80
W60
14.7 a 154 a 20.39 c 0.78
24.0 b 145 a 20.52 bc 0.82
27.4 b 115 b 20.91 b 0.79
30 b 100 b 21.44 a 0.77
Means within each row followed by the same letter are not significantly different at p < 0.05.
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agricultural water management 85 (2006) 95–104
Table 3 – Response of fertigated spring potato (cv Santana) to different N rates, nitrogen derived from fertilizers (Ndff), N uptake and recovery at physiological maturity N level (kg ha1)
DM yield (kg ha1)
Shoots
125 250 375 500
2692 3803 3123 4212
Tubers
125 250 375 500
6766 6734 5648 7154
b ab ab a
Total N (%) 3.31 3.85 3.99 4.04
b a a a
1.23 1.35 1.42 1.43
N yield (kg ha1) 89.12 146.41 124.61 170.18
c ab b a
83.22 90.91 80.20 102.65
Ndff (%) 47.08 51.54 60.67 57.73 41.81 50.02 52.31 57.33
b a a a
Fertilizer N utilization (%) 33.57 30.18 20.16 19.65
a ab bc c
27.83 18.05 11.06 11.17
a b c c
Means within each column followed by the same letters are not significantly different at p < 0.05.
in the arid and semi-arid Mediterranean climate (Mohammad et al., 1999; Darwish et al., 2003), close to 60%. These results confirm the need to restrict nitrogen input to potato in the dry conditions of the Bekaa plain to 130 kg N ha1. Under these conditions, nitrogen derived from the soil and irrigation water was equivalent to 96 kg N ha1 year1, a significant amount. Studies have demonstrated increased nitrate concentrations in the groundwater table in main Lebanese arable lands (Halwani et al., 1999; Darwish et al., 2005) linked to erratic fertilization practices. Leaching followed by a saturation of nitrates and other more stable elements can explain the current pollution of water resources (Jordan and Rippey, 2003). Water being a scarce resource in the area, it was essential to evaluate water use efficiency for the 2000 season. For a water supply of 700 mm, close to the FAO recommendations for potato in the area (FAO, 1969), irrigation water productivity varied between 7.7 and 8.6 kg fresh tuber per cubic meter of applied water. These values were comparable to those obtained by fertigation of table potato, providing full water requirements (Darwish et al., 2003). It could be possible to reduce water input without affecting the crop performance.
3.2.
Irrigation schedule
In Experiment II parallel measurements of soil potential at 0.3 m depth, before irrigation cycles showed an average of 22 kPa for W120, 32 kPa for W80 and 50 kPa for W60 against 30 kPa for the control. These soil head potentials measured at 0.3 and 0.6 m gave an acceptable linearity with the available soil water content obtained from the neutron probe measurements (Fig. 2). In fact, this equation is very close to the function obtained previously on the same location (unpublished data) and is quite remarkable, considering that it is a clay soil. Based on the equation, the initiation of irrigation in each of the four treatments coincided with 60 mm of available water in the control and 56.6 mm in W80, 31.6 mm in W60 and 70.5 mm in W120. Therefore, it could be safe to deplete the soil water content down to 56.6 mm in the upper 0.3 m soil layer, which is equivalent to 40% water depletion.
of water input. The restricted input of irrigation water was first observed as a decrease in shoot fresh weight (Fig. 3) towards the full development stage (83 DAS). It was not until the physiological maturity that a negative effect of the deficit irrigation was obvious on tuber dry matter yield with a slope of 10.9 and a correlation coefficient of 0.77 (Fig. 3). The shoots dry matter yield did not have as strong a link with the water application, as the slope was 1.28 and the coefficient of correlation was not significant. Later on at full maturity, both slopes decreased, as they become 7.2 for tubers and 0.57 for shoots indicating a greater discrepancy between the plant parts and a visible impact on tubers production. Deficit irrigation (W60) lowered both the tuber dry matter production and the average weight of the commercial tuber (Table 2), leading to 21% loss in fresh yield. No commercial fresh yield loss was found in the moderately stressed regime (W80), due probably to a higher proportion of commercial tubers. Both extreme treatments, W120 and W60, affected the percentage of commercial tuber by favoring tuber diseases for the first and leading to tuber deformation for the second. Somehow, moderate nitrogen fertilization and a reduced water application allowed about 1% increase in dry matter content (Table 2). However, no significant difference was observed for the harvest index, which varied between 0.77 and 0.82 despite a slight decrease for the W60 treatment. Yield losses obtained in this work were significantly less than the reduction observed when deficit irrigation was applied continuously (Iqbal et al., 1999; Smith et al., 2002) or at tuber bulking stage (Minhas and Bansal, 1991). High losses were also reported for other growth stages, notably at the establishment (Skaff, 1997), flowering (Iqbal et al., 1999; Smith
3.3. Effect of deficit irrigation on potato productivity and dry matter content Water saving can be achieved either by decreasing the frequency of irrigation events or by a systematic reduction
Fig. 2 – Available water (mm) as related to the readings of the head potentials (kPa) in tensiometers placed at 0.30 and 0.60 m in Experiment II.
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agricultural water management 85 (2006) 95–104
Fig. 3 – Shoot and tuber DM production in processing potato at physiological maturity under four irrigation regimes.
et al., 2002) and ripening (Fabeiro et al., 2001) stages. Introducing moderate continuous deficit irrigation (0.80 ET) after the establishment phase of potato could be an alternative means for water saving on clay soils of the dry Mediterranean regions.
3.4.
Deficit irrigation and N utilization
For all treatments, N uptake by tubers was higher compared to shoots. Nitrogen content in the tubers dropped from 1.9% in the control to an average of 1.6% in the water-stressed treatments (Table 4). The total N removal by the crop was close to the first season and fluctuated between 156 and 246 kg ha1. Nitrogen derived from fertilizers (19–24%) decreased in the deficit irrigation. Restricted water input showed a significant difference in N utilization by the tubers and the whole plant, including the root system where total N removal dropped to 3– 3.6 kg N ha1 in W60 and W80 (Table 4). Normal and excess water input resulted in significantly higher N fertilizer recovery (about 44%), while deficit irrigation, regardless of
Fig. 4 – Fresh tuber production (t haS1) of processing potato as related to applied water (m3) or consumed water (m3) under four levels of irrigation water.
its severity, equally affected N utilization keeping it below 21.5%. Lower N use efficiency could be explained by the lower tuber DM yield and DM content observed at physiological maturity in deficit irrigation. Water stress probably interfered with the translocation of DM to the sink. Under the same climatic conditions, water stress in corn caused a higher vapor pressure deficit that had accelerated crop phenology and anticipated silking and maturity dates (Karam et al., 2003).
3.5.
Water consumption
For the period of differential irrigation, plots effectively received (excluding the leaching fraction) 614 mm for W120, 510.6 mm for W100, 407.3 mm for W80 and 313.6 mm for W60. The actual crop evapotranspiration varied between 300 mm for W60 and 405 mm for W120. Extreme water stress (W60) limited the real crop ETc to 300 mm, reflecting on crop development and productivity. Ground coverage was also affected as it dropped to 0.7–0.8. Therefore, evaporation limited the readily available water, to the contrary of the
Table 4 – Response of fertigated spring potato (cv Santana) to different irrigation regimes, N uptake and recovery at physiological maturity Treatment
DM yield (kg ha1)
Shoots W120 W100 W80 W60
3146 3474 2841 2650
Tubers W120 W100 W80 W60
7940 7982 5276 4883
Roots W120 W100 W80 W60
Total N (%) 2.33 2.53 2.52 2.64
a a b b
188.5 239.8 193.07 149.6
1.85 1.90 1.54 1.67
1.67 1.83 1.89 1.94
a a b ab
N yield (kg ha1)
Ndff (%)
74.69 89.43 72.38 71.45
25.58 25.83 20.96 19.38
14.17 17.15 11.03 10.11
145.04 151.67 81.25 81.17
23.57 22.28 18.74 17.61
26.17 25.87 11.66 10.94
3.11 4.48 3.62 2.95
22.52 22.32 20.75 19.89
0.54 0.76 0.57 0.45
Means within each column followed by the same letter are not significantly different at p < 0.05.
Fertilizer N utilization (%)
a a b b
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Table 5 – Water consumption (mm) of fertigated processing potato: removal from different soil layers and proportion (in parenthesis) of total consumption Soil depth (cm)
Treatment
0–20 20–40 40–60 Total (mm)
W120
W100
W80
W60
248.1 a (61.3%) 100.1 a (24.7%) 56.8 a (14.0%) 405.0
247.42 a (66.5%) 68.76 b (18.5%) 55.82 a (15.0%) 372.0
263.0 a (65.6%) 99.0 a (24.7%) 39.0 ab (9.7%) 401.0
227.6 a (75.9%) 43.8 b (14.6%) 28.6 b (9.5%) 300.0
Means within each row followed by the same letters are not significantly different at p < 0.05.
Table 6 – Dynamics of water consumption (mm) of processing potato under four irrigation regimes shown in relation to three phases of crop development Growth phase
Treatment W120
W100
W80
W60
Vegetative 40–61 DAS Full top growth 62–86 DAS Tuber bulking 87–107 DAS
136.0 ab (33.6%) 169.0 a (41.7%) 100.0 a (24.7%)
144.5 a (38.8%) 150.0 a (40.3%) 77.3 ab (20.8%)
129.2 ab (32.2%) 166.1a (41.4%) 105.7a (26.4%)
110.4 b (36.8%) 127.8 a (42.6%) 61.8 b (20.6%)
Total (mm)
405.0
372.0
401.0
300.0
Value in the parenthesis denotes total consumption. Means within each row followed by the same letters are not significantly different at p < 0.05.
treatments W80 and W100, where soil shading was complete. Intermingling leaves of plants on adjacent rows, probably, enhanced transpiration and restricted evaporation, thus promoted more efficient water and N use. Higher evaporation in the deficit irrigation may have caused lower N recovery and reduced water use efficiency. This suggests the need for lower row spacing (<75 cm) to promote a dense canopy in deficit irrigation to reduce the evaporation loss. In other treatments, ETc varied between 371 and 405 mm. During the period of differential irrigation, 98 and 96% of the effectively applied water were recovered in W80 and W60 treatments. Water loss increased steadily with higher water treatments, as demonstrated by the slope of the relationship between the fresh yield and the consumed water or the applied water (Fig. 4). For one unit of applied water (1 m3) the relationship gave 4.8 kg of fresh production, whereas this doubled for consumed water reaching 8.8 kg of fresh tuber. This suggests a much sharper increase in water consumption as compared to water application and a clear possibility of reducing the difference between both values. The depletion of water content in the 0–60 cm depth between the consequent irrigation events was assessed, assuming the absence of water rise from the deep ground-
water. Deficit irrigation did not alter the ability of the crop to remove water from the upper 0.2 m layer (Table 5). Between 61 and 75% of total water used for evapotranspiration was derived from this horizon. This allowed for a speculation on the main root distribution within the upper 0.3 m depth. Significantly, less water was removed from the deeper layers. The analysis of water use in relation to the growth stages (Table 6) showed that over 40% of depleted water occurred during the maximum shoot growth stage (62–86 DAS). This period corresponds to higher water demand of processing potato and possibly higher crop vulnerability to water stress. Significantly higher water removal occurred during this phase in the control in comparison with the W60 treatment. Similarly, at tuber bulking stage (between 87 and 107 DAS), significantly less water was evapotranspired in W60. This water distribution in the profile was obvious elsewhere, in the movement of nitrates. Monitoring the soil solution between the full development and physiological maturity showed an accumulation of restricted amounts of nitrates in W80 (between 10 and 35 mg L1) in the 0.3 m soil layer. In the severe deficit irrigation it was impossible to collect soil solution below 0.6 m depth, which indicated the limitation of wetting front to this depth. Whereas in the control and
Table 7 – Productivity of applied N and water in processing potato in 2000 when N rates were tested and in 2001 with four levels of water input Experiment II (N: 130 kg ha1)
Experiment I (water: 700 mm) N rates (kg ha1)
Productivity of applied N (kg DM/kg N)
IWP (kg DM m3)
Applied water (mm)
Productivity of applied N (kg DM/kg N)
IWP (kg DM m3)
125 250 375 500
54.1 27.0 15.0 14.3
0.96 0.95 0.80 1.02
494 602 721 840
37.3 40.4 61.1 60.8
1.00 1.04 1.06 0.90
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Fig. 5 – Water use efficiency (WUE) and irrigation water productivity (IWP) of processing potato subject to four irrigation levels.
W120. This corresponds to water saving between 16.5 and 31% for a relative yield loss of 7 and 19.5%, respectively (Fig. 6). This crop response to water stress was considered acceptable for use in other agro-climatic zones (Smith et al., 2002). When yield loss by deficit irrigation is less than the saved proportion of applied water, the irrigation water productivity must substantially increase in comparison with the recommended water supply. In successful deficit irrigation of potato the relative water use efficiency in comparison with full water supply was 1.06 for drip and 1.2 for furrow irrigation (Kirda, 2002). Our results gave 1.18 for the moderate deficit irrigation a value close to the highest efficiency which indicates a good response to deficit irrigation using the fertigation techniques.
4.
Fig. 6 – Processing potato yield response to deficit irrigation.
W120, regular sampling of the soil solution from deep horizons (0.8 m depth) was possible. This suggests a possible leaching of excess water carrying nitrates with a concentration varying between 20 and 40 mg L1. These amounts were close to the value of nitrates observed in the soil solution of the fertigated plots with controlled N input (Darwish et al., 2003).
3.6.
Water-N productivity and yield response factor
The control of both trials showed comparable results for tuber DM production per unit of applied N and water (Table 7). Excess N input and severe water stress equally lowered the productivity of applied N. In Experiment II, water use efficiency (WUE) varied between treatments. In the control (W100) and the W120 treatments, evapotranspired water was used more efficiently to produce a unit of fresh yield compared to the moderate deficit irrigation (Fig. 5). However, the irrigation water productivity (IWP) showed a relatively better performance under a moderate stress (W80). The irrigation water productivity (IWP) increased with water reduction then decreased with severe deficit irrigation. The success of deficit irrigation is usually possible when applied to drought-tolerant crops cultivated on finely textured soils (Kirda, 2002). It is difficult to manage deficit irrigation with potato, a very sensitive crop to water stress following tuber set (Shock et al., 1993). Moderately reducing water input to potato saved 119 mm season1 of water in comparison with the control (W100) and 238 mm season1 in comparison with
Conclusion
With the current situation in the semi-arid regions, characterized by uneven rainfall distribution causing seasonal leaching of nitrates and groundwater contamination, the application of 130 kg N ha1 by fertigation is considered sufficient for processing potato. Increased N supply did not improve tuber yield, but reduced the efficiency of applied N. The controlled N application secured significantly higher N recovery (61% from applied fertilizers) by processing potato. Based on field data and measured evapotranspiration, water application of 700 mm in the 2000 season and 721 mm for the control in 2001 was comparable to the recommended value. Potato is sensitive to N management as it affects the crop performance and dry matter accumulation. Values of N yield and N recovery were comparable in both seasons. Considering available N pools and results for table potato in the region, reducing N input for processing potato is a cost-saving practice that prevents the groundwater contamination by nitrates. The neutron probe allowed estimating the actual crop evapotranspiration (ETc) over the whole season, in addition to water depletion with soil depths and throughout the growth stages. Eventually assessing plant available water and the soil head potential allowed for the estimation of available water during the season to manage the moderate water stress. Considering the efficiency of the drip system and leaching fraction, water demand of the spring processing potato in the dry Mediterranean conditions of Lebanon, was around 460 mm, after plant establishment. This is much below the recommended values for the potato. The processing potato removed up to 90% of total water demand from the upper 0.4 m soil layer. Around 40% of water was depleted during the phase of maximum biomass production and tuber bulking, which corresponds to the maximum crop vulnerability to water stress. The controlled deficit irrigation (W80) allowed maintaining tuber yield and DM content with a possibility to save 119 mm of water. The severe deficit irrigation (W60) caused a reduced canopy that possibly lead to higher evaporation and lower N recovery. This reflected on irrigation water productivity in relation to fresh tuber production, which increased with water reduction to a certain level then decreased with severe deficit irrigation. A comparable efficiency of dry matter production gained by a unit of N and water application was achieved by the control of both seasons. While a higher productivity of applied N was
agricultural water management 85 (2006) 95–104
found in the lower N treatment, water stress significantly affected dry matter production per unit of applied N. The crop yield response factor (Ky) of processing potato submitted to relative evapotranspiration deficit in the dry Mediterranean conditions of Lebanon was 0.8. Saved water could then be diverted to other social needs or used to irrigate supplemental lands, compensating for the relative yield loss. The economy of 119 mm per single potato season is significant water saving for the semi-arid and dry areas. Confirming this result and extending this approach to other crops would mean substantial water saving at national and regional levels. Unless creating drought-resistant soils and crops, managing soil water content just above the damaging level is important for sustainable agriculture. Yields decrease because of water shortage not only from insufficient rainfall but also from mismanaged irrigation. In this regard, providing a simple tool to monitor plant available water content in the root zone can help overcoming the lack of flexibility of farming systems to sophisticated water measurements.
Acknowledgments This research was conducted as part of the regional technical co-operation project RAW/5/007: ‘‘Fertigation for improved water use efficiency and crop yield’’, jointly supported by IAEA Department of Technical Co-operation and the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture. It also received the support of the Lebanese National Council for Scientific Research.
references
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FAO, 1998. Project de Recensement Agricole. Direction des Etudes et de la Coordination du Ministe`re de l’Agriculture, Beirut, Lebanon, p. 59. Halwani, J., Ouddane, B., Baoudi, M., Wartel, M., 1999. Contamination par les nitrates des eaux souterraines de la plaine d’Akkar au Liban du Nord. Cahiers Sante´ 9, 219–223. Hamdy, A., 1998. Research needs in irrigated agriculture for developing countries. In: Advanced Course on Farm Water Management Techniques. CIHEAM-BARI, CEC-DGI, IAVMorocco, CII, Rabat, Morocco, pp. 313–338. Iqbal, M.M., Mahmood Shah, S., Mohammad, W., Nawaz, H., 1999. Field response of potato subjected to water stress at different growth stages. In: Kirda, C., Moutonnet, P., Hera, C., Nielsen, D.R. (Eds.), Developments in Plant and Soil Sciences. Kluwer Academic Publishers, Dordrecht, pp. 213–224. Jordan, P., Rippey, B., 2003. Lake sedimentary evidence of phosphorus, iron and manganese mobilisation from intensively fertilised soils. Water Res. 37 (6), 1426–1432. Kang, Y., Wang, F.X., Liu, H.J., Yuan, B.Z., 2004. Potato evapotranspiration and yield under different drip irrigation regimes. Irrig. Sci. 23 (3), 133–143. Karam, F., Breidy, J., Stephan, C., Rouphael, J., 2003. Evapotranspiration, yield and water use efficiency of drip irrigated corn in the Bekaa Valley of Lebanon. Agric. Water Manage. 63, 125–137. Kirda, C., Kanber, R., Tulucu, K., 1999. Yield response of cotton, maize, soybean, sugar beet, sunflower and wheat to deficit irrigation. In: Kirda, C., Moutonnet, P., Hera, C., Nielsen, D.R. (Eds.), Developments in Plant and Soil Sciences. Kluwer Academic Publishers, Dordrecht, pp. 21–39. Kirda, C., 2002. Deficit irrigation scheduling based on plant growth stages showing water stress tolerance. In: Deficit Irrigation Practices, Water Reports 22. FAO, pp. 3–10. Kovacs, T., Kovacs, G., Szito, J., 1999. Crop yield response to deficit irrigation imposed at different plant growth stages. In: Kirda, C., Moutonnet, P., Hera, C., Nielsen, D.R. (Eds.), Developments in Plant and Soil Sciences. Kluwer Academic Publishers, Dordrecht, pp. 224–239. Le Corre, P., Gravouille, M.J., Martin, M., 1995. La culture de la pomme de terre de conservation. Institut Technique des Ce´re´ales et des Fourrages. Meyer, R.D., Marcum, D.B., 1998. Potato yield, petiole nitrogen, and soil nitrogen response to water and nitrogen. Agron. J. 90 (3), 420–429. Minhas, J., Bansal, K., 1991. Tuber yield in relation to water stress at stages of growth in potato. J. Indian Potato Assoc. 18, 1–8. Mohammad, M.J., Zuraiqi, S., Quasmeh, W., Papadopoulos, I., 1999. Yield response and nitrogen utilization efficiency by drip-irrigated potato. Nutr. Cycl. Agroecosyst. 54, 243–249. Moutonnet, P., Pagenel, J.F., Fardeau, J.C., 1993. Simultaneous field measurement of nitrate-nitrogen and matric pressure head. Soil Sci. Soc. Am. J. 57, 1458–1462. Nimah, M., 1984. Integrated irrigation. Lebanese Republic. Annual Report. National Council for Scientific Research, Beirut, pp. 39–40. Onder, S., Cakiskan, M.E., Onder, D., Caliskan, S., 2005. Different irrigation methods and water stress on potato yield and yield components. Agric. Water Manage. 73 (1), 73–86. Papadopoulos, I., 1988. Nitrogen fertilization of trickle irrigated potato. Fert. Res. 16, 157–167. Ryan, J., Musharrafieh, G., Barsumian, A., 1980. Soil fertility characterization at the Agricultural Research and Education Center of the American University of Beirut. Publication 64, 47 pp. Saoud, A.A., Van Cleemput, O., Hofman, G., 1992. Uptake and balance of labelled fertilizer nitrogen by potatoes. Fert. Res. 31, 351–353.
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Shock, C., Holmes, Z., Stieber, T., Eldredge, F., Zhang, P., 1993. The effect of timed water stress on quality, total solids and reducing sugar content of potatoes. Am. Potato J. 70, 227–241. Shock, C., Feibert, E., 2002. Deficit irrigation of potato. In: Deficit Irrigation Practices. Water Reports 22. FAO, pp. 47–55. Skaff, C., 1997. Conse´quences d’une contrainte hydrique applique´e a` differents stades phe´nologiques sur le rendement de la pomme de terre Diamant. AUPELF-UREF, DEA Graduation project. Agriculture Durable en milieu Me´diterrane´en. Smith, M., Kivumbi, D., Heng, L., 2002. Use of the FAO CROPWAT model in deficit irrigation studies. In: Deficit Irrigation Practices. Water Reports 22. FAO, pp. 17–27.
Van Cleemput, O., 2000.In: Final Annual Co-ordination meeting of Regional TC Project RAW/5/007 on Fertigation for Improved Water Use Efficiency and Crop Yield. November 20–24. IAEA, Vienna. Yuan, B.Z., Nishiyama, S., Kang, Y., 2003. Effects of different irrigation regimes on the growth and yield of drip-irrigated potato. Agric. Water Manage. 63 (3), 153–167. Zapata, F., 1990. Isotope techniques in soil fertility and plant nutrition studies. In: Harderson, G. (Ed.), Use of Nuclear Techniques in Studies of Soil–Plant Relationships Training Course Series No. 2. International Atomic Energy Agency, pp. 61–82.
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Nitrogen and water use efficiency of fertigated processing potato T.M. Darwish a,*, T.W. Atallah b, S. Hajhasan c, A. Haidar c a
National Council for Scientific Research, P.O. Box 11-8281, Beirut, Lebanon Faculty of Agricultural Sciences, Lebanese University, Lebanon c Lebanese Agricultural Research Institute, Tel-Amara, Lebanon b
article info
abstract
Article history:
In this paper, we studied the response of spring processing potato to different N level by
Accepted 17 March 2006
fertigation and deficit irrigation in the dry Mediterranean Bekaa plain of Lebanon. In the first
Published on line 12 May 2006
season between 5 May and 29 August, the impact of different N level was evaluated. Treatments consisted of 125 kg N ha1 (N125), 250 kg N ha1 (N250), 375 kg N ha1 (N375)
Keywords:
and 500 kg N ha1 (N500) applied continuously by drip fertigation. In the second season
Dry Mediterranean
between 11 April and 16 August, the best N treatment (N125) was set as a standard to study
Fertigation
the constraints of deficit irrigation and the possibility of water saving. After plant establish-
Deficit irrigation
ment, treatments consisted of continuously applying a moderate water stress 80% of ET, a
N recovery
severe deficit 60% of ET, a control 100% of ET and an excess irrigation 120% of ET. Labeled N
Water saving
fertilizers in 2000 and 2001, as well as a neutron probe in 2001, were used to estimate N use efficiency and the actual crop evapotranspiration and water use efficiency. Different N level, at recommended water supply (700 mm), did not lead to a crop response. The lower N treatment (N125) gave a significantly higher N recovery (61% of applied N), despite the clay nature of the soil. In the deficit irrigation, water application varied between 500 and 800 mm with 720 mm for the control. Final yield showed the possibility to save 119 mm of water with no reduction in fresh tuber yield. Imposing severe water deficit considerably decreased tuber yield. Extremes of water input, deficit and excess irrigation, lowered the percentage of marketable yield from 84 to 72%. Regardless of the severity of deficit irrigation, the upper 0.4 m soil layer provided up to 90% of crop water demands (uptake). About 40% of crop ET occurred up to the maximum development stage. Under water stress conditions, the effective ground cover was reduced, which can explain the decrease in N recovery from 40 to 20%. This is probably due to higher evaporation rates and N losses. In both trials, increasing N or water input showed inconsistent impact on dry matter production per unit of applied water. Under deficit irrigation, a yield response factor (relationship between relative yield decrease and relative ET deficit) of 0.8 was obtained, while values below 1 justify the implementation of deficit irrigation. Managing continuous deficit irrigation of processing potato in the dry Mediterranean region with 0.80 ET is possible for water saving. Therefore, it could be safe to deplete the soil water content down to 56.6 mm in the upper 30 cm soil layer, which is equivalent to 40% water depletion. # 2006 Elsevier B.V. All rights reserved.
* Corresponding author. Tel.: +961 4 409845/6; fax: +961 4 409847. E-mail address: [email protected] (T.M. Darwish). 0378-3774/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2006.03.012
96 1.
agricultural water management 85 (2006) 95–104
Introduction
In Lebanon, the share of agriculture exceeds 68% of the total water withdrawal, with 46% reliance on groundwater (FAO, 1998). Seeking additional water resources and improving the efficient use of available water are two parallel strategic lines to address the issue of water shortage. This is particularly relevant to cash crops dependant on irrigation water such as potato, which occupies 15,000 ha, corresponding to 17% of irrigated land (FAO, 1998). Its evapotranspiration (ET) was studied using drainage lysimeters in the central Bekaa valley (Abou-Khaled et al., 1969). Results provided the basis for general recommendations on potato crop coefficient and water demands. A shift from furrow irrigation to macrosprinklers allowed a 50% reduction of water application to potato (FAO, 1969). The constraint of this technique is its low efficiency (Al-Jamal et al., 2001), compared to drip irrigation (Hamdy, 1998). Further water economy was demonstrated by the simultaneous application of water and N through potato fertigation, with 35% water saving and improved N use efficiency (Papadopoulos, 1988; Mohammad et al., 1999; Darwish et al., 2003). Water saving by deficit irrigation presents several climatic and crop constraints. A sensitive crop like potato is particularly difficult to manage (Shock and Feibert, 2002), as it presented a negative response to deficit irrigation (Kovacs et al., 1999; Fabeiro et al., 2001). Various techniques such as furrow irrigation (Iqbal et al., 1999), drip (Kovacs et al., 1999, Yuan et al., 2003) and macro-sprinklers (Nimah, 1984) were studied. Yield reduction was reported in these cases, as well as modest water saving. The calibration of the crop response factor (Ky) to deficit irrigation relies on mean annual climatic data to estimate crop ET (Shock and Feibert, 2002). Because Ky differs with different irrigation and fertilization techniques, additional research to achieve mathematical optimization of deficit irrigation in different pedoclimatic conditions is justified. With the implementation of drip systems, fertigation became an important tool to achieve sound irrigation and fertilization. Previously, nitrogen management was particu-
larly relevant to the dry areas where nitrogen was deficient (Dregne, 1976; Ryan et al., 1980). For processing potato, a careful N and water management is required to ensure regular growth, high dry matter content and marketable tubers (Le Corre et al., 1995). Previous studies addressed the effect of supplemental deficit irrigation on early potato (Onder et al., 2005), that of drip irrigation frequency (Kang et al., 2004) and water rates (Meyer and Marcum, 1998) on the performance of potato grown on loam and sandy loam soils. Implementing continuous water stress to processing potato growing in late spring on clay soil is a challenging issue requiring the monitoring of plant available water in the root zone. The objective of this work was first to establish the optimum N rate for processing potato using 15N, and second, at the optimum N rate study the impact of water stress on crop yield and the efficiency of N and water utilization, under the dry Mediterranean pedoclimatic conditions of the Bekaa plain of Lebanon.
2.
Materials and methods
2.1.
Experimental site
Fertigation trials were conducted in Tel-Amara experimental station (398110 N; 418320 E; 905 m) in the central Bekaa plain, Lebanon. The area, characterized by a dry Mediterranean climate, receives an annual precipitation of 600 mm, occurring between November and March. The average monthly temperature varies between 14.4 8C in April and 23.8 8C in July with relatively low relative humidity (Table 1). The calculated average daily evapotranspiration (ETo-PM) was comparable for the two experimental years and varied between 729 mm in 2000 and 761 mm in 2001. Soils of the experimental site were deep, non-calcareous, clay Eutric Cambisols with an average bulk density of 1.2 g cm3, low organic matter content (0.12 g kg1), moderate total N content (0.012 g kg1) and moderately high CEC value (40 Cmol kg1). Gravimetric moisture content varied between 29% at field capacity and 16.5% at permanent wilting point. Thus, between 0 and 75 cm depth the soil water content varied between 236.3 and
Table 1 – Climatic data obtained at the experimental station in 2000 (Experiment I) and 2001 (Experiment II) and the longterm average Season
Month
Air temperature (8C)
Relative humidity (%)
ETo-PM (mm day1)
2000
April May June July August
14.44 16.75 20.24 24.52 23.77
52.38 51.06 49.83 44.85 46.48
4.10 6.44 7.34 7.28 6.33
2001
April May June July August
14.88 17.22 20.72 23.82 23.53
54.18 53.03 40.50 41.31 50.68
5.78 5.63 7.95 8.88 8.80
30-Year average
May June July August
16.20 19.80 21.80 22.50
50.00 45.00 40.20 35.00
3.94 5.6 5.88 5.416
agricultural water management 85 (2006) 95–104
134.4 mm. Irrigation water, originating from a local well, had a good quality (pH of 7.5 and EC = 0.4 dS m1). Both trials were run as part of a 3-year rotation: legume–potato–wheat.
2.2.
Experimental set-up
2.2.1.
Experiment I
Nutrient requirements of processing potato (cv Santana) were evaluated in 2000 under four nitrogen levels and full water supply. Seeds were sown on 5 May and harvested on 29 August. The experiment was set up on 792 m2 in a randomized block design with four replicates. A buffer zone of 1.5 m separated between blocks to avoid interference. The plot size was 11 m 4.5 m. Each plot consisted of six rows, four middle (harvest) and two border rows. A spacing of 75 cm between rows and 25 cm between plants gave a planting density close to 53 333 plants ha1. Observations and measurements were conducted on the effective rows. During the growth season, the crop fully depended on irrigation. Water application was based on the information received from two precise weighing lysimeters, planted to ryegrass as a reference crop, located on the station. Throughout the growth stages, the crop fraction (Kc) was based on Papadopoulos (1988): 0.4 (emergence: 45 days after sowing)–0.7 (45–63 DAS)–0.9 (63–98 DAS)–0.7 (98–113 DAS). This Kc was lower than that proposed by Allen et al. (1998) for the mid and late development stages. Treatments consisted of applying four levels of nitrogen by fertigation: N125 (125 kg N ha1), N250 (250 kg N ha1), and N375 (375 kg N ha1) and N500 (500 kg N ha1). Differential N application occurred throughout the experiment. Phosphorus and potassium feeding took place by fertigation: P as 94 kg P ha1 from phosphoric acid and K equivalent to 344 kg K ha1 from soluble potassium sulfate. Labeled ammonium sulfate was used for the application of 15 N at an enrichment of 1.5% atom excess (a.e.) for both seasons. Microplots consisting of double rows, of four plants each, were located in the middle part of the macroplots. Both microplots and macroplots were simultaneously fertigated through separate drip irrigation systems. The emitters had 40 cm spacing and a discharge of 4 L h1. Barrels containing 15 N solution were connected to the plants by tubes perforated within the microplots to carry one dripper for each labeled plant.
2.2.2.
Experiment II
The best N treatment in Experiment I (N125) served as a standard level for the deficit irrigation trial in 2001. Due to early spring conditions, sowing occurred on 11 April 2001 and harvest on 16 August. Treatments consisted of four levels of irrigation water: an excess irrigation (W120) equivalent to 120% of ET, a control (W100) equivalent to 100% of ET, a moderate deficit irrigation (W80) equivalent to 80% of ET and a severe deficit irrigation (W60) equivalent to 60% of ET. In Experiment II, the control was similar to the water input in Experiment I. Between the planting date and the full establishment phase, 112.6 mm were equally applied to all treatments. Differential water application started at the beginning of the maximum development stage, on the 47th DAS (28 May). The
97
crop coefficient (Kc) was 0.7 (47–71 DAS), 0.9 (71–96 DAS) and 0.7 (96–124 DAS). During the differential water input, plots received an equivalent of 707 mm in W120, 588 mm in W100, 469 mm in W80 and 361 mm in W60. A leaching fraction of 15% was given, therefore 85% of these amounts were considered as effectively applied. Soil potential in each plot was monitored using tensiometers, inserted at 30 and 60 cm depth. Irrigation started when the soil matric potential in the control approached 30 kPa at 0.3 m depth. This threshold corresponded to the recommended soil potential used in the scheduling of dripirrigated potato (Papadopoulos, 1988). In addition, a neutron probe served to assess the soil water contents between 0 and 60 cm soil depth. Access tubes were inserted in three replicates for each treatment to a depth of 75 cm. Nitrate leaching was monitored using tensionics, which are devices consisting of tubes with ceramic chambers, allowing to sample the soil solution. They were inserted in three replicates at 30, 60 and 90 cm depths. Soil solution collected at 90 cm depth allowed monitoring water percolation and nitrates leaching beyond the root zone.
2.3.
Data collection
The observations of crop performance corresponded to the following growth stages: maximum shoot development, physiological maturity and full maturity. These coincided with 63 DAS, 98 DAS and 113 DAS in Experiment I and 83, 117 and 127 DAS, respectively, in Experiment II. At each stage, four plants per plot were sampled. At physiological maturity, the middle protected four plants were sampled from each microplot. Fresh and dry matter (DM) yields were recorded. Tubers were sorted by their size and non-marketable (deformed and diseased) tubers were counted, weighted and discarded. Samples (whole shoots and tubers) were weighted fresh, subsampled by quartering and oven-dried at 70 8C until constant weight. 15N analysis and expression of results were done according to Zapata (1990). Briefly, these were: %Ndff (N derived from fertilizers) = (% 15N a.e. plant 100)/% 15 N a.e. fertilizer. N yield (kg ha1) = (DM yield (kg ha1 %N)/100. Fertilizer N yield (kg ha1) = (N yield (kg ha1) %Ndff)/100. N utilization (%) = (fertilizer N yield 100)/rate of N application. Water removal from different soil layers and during vegetation periods was calculated based on the neutron probe readings. These were taken immediately before and 24 h after each irrigation at 20, 40 and 60 cm depths. Soil moisture content was calculated using the calibration curve of the soil of Tel-Amara station reported previously (Darwish et al., 2002) and equal to uv = 0.2681X 0.2309 (r2 = 0.79), where uv is the volumetric soil water content (%) and X is the count ratio (neutron count rate in a given soil layer/count rate in a standard medium). Actual evapotranspiration was calculated as the difference between soil water contents 24 h after an irrigation event and that just before the following irrigation.
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A mathematical function was established between the gradient of hydraulic heads, measured by tensiometer, and neutron probe measurements within the root zone to aid the estimation of plant available water from simple measurement of the soil head potential. Crop response (Ky) links relative yield decrease to relative evapotranspiration deficit. It was expressed as follows: Ky ¼
1 Ya =Ym 1 ETa =ETm
where Ya and Ym are actual and maximum yield (kg ha1); ETa and ETm are actual and maximum evapotranspiration (Kirda et al., 1999). Irrigation water productivity (IWP) was defined as the final yield per unit of applied water, while water use efficiency (WUE) was defined as the final yield per unit of consumed water (Van Cleemput, 2000). Harvest index was measured as the ratio of tuber DM yield to the total DM yield. For nitrate measurements, soil solution was removed once every 8 days, as recommended (Moutonnet et al., 1993). This interval is enough to establish equilibrium of nitrates concentration in the solution inside and outside the ceramic chamber. Nitrate concentration was analyzed using a specific electrode. Statistical analysis was performed by the analysis of variance (ANOVA) using SigmaStat (Jandel Scientific). Means were compared using the LSD test ( p < 0.05).
3.
Results and discussion
3.1.
Crop response to N level
Fig. 1 – Shoot and tuber dry matter yield of processing potato at physiological maturity, in Experiment I, under different N levels.
suggested by the small slope (y = 0.0011x + 36.35). Among yield parameters, only the tuber dry matter content was negatively affected by the higher nitrogen inputs (Table 2). Based on the intercept in the previous relationship, in the absence of nitrogen (zero nitrogen) a tuber yield of 36 t ha1 could be expected. This was explained by the combined effect of the soil pool and nitrate level in the irrigation water. The use of labeled nitrogen fertilizers allows quantifying the contribution of nitrogen fertilizers. In terms of total N removed by the crop (N yield), there was an important difference between N500 (273 kg N ha1) and N125 (172 kg N ha1) treatments (Table 3). Such a larger removal was partly due to higher nitrogen contents in the shoots and tubers, as well as higher shoots yields. The contribution of fertilizers (Ndff) was, as expected, significantly smaller in the N125 treatment (Table 3). Out of the applied fertilizers, the crop removed a total of 157 kg N ha1 in N500 against 76 kg N ha1 in N125. As a proportion of the N fertilizer recovery, the lowest N rate gave the most satisfactory value of 61.4%. This utilization value meets those found in temperate areas (Saoud et al., 1992) and
Increased nitrogen levels promoted a vigorous plant growth, followed by an additional dry matter accumulation in the shoots, to the contrary of tubers. At physiological maturity, the slope for shoots dry matter was much higher than for the tubers (Fig. 1) confirming the impact of higher nitrogen input on the aboveground growth. At the final yield, the fresh tuber production did not present a link with the nitrogen levels as
Table 2 – Impact of nitrogen or water levels on tuber marketability and dry matter contents at commercial maturity during the two seasons Treatment: N level (kg ha1)
Yield parameter
Experiment I
Non-marketable yield (%) Marketable yield (%) Dry matter (%)
125
250
375
500
23.0 106.3 21.65 a
15.5 111.0 20.55 b
24.4 75.3 20.32 b
12.0 105.5 20.35 b
Yield parameter
Experiment II
Non-marketable yield (%) Marketable tuber weight (g) Dry matter (%) Harvest index
Treatment: ETc (%) W120
W100
W80
W60
14.7 a 154 a 20.39 c 0.78
24.0 b 145 a 20.52 bc 0.82
27.4 b 115 b 20.91 b 0.79
30 b 100 b 21.44 a 0.77
Means within each row followed by the same letter are not significantly different at p < 0.05.
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Table 3 – Response of fertigated spring potato (cv Santana) to different N rates, nitrogen derived from fertilizers (Ndff), N uptake and recovery at physiological maturity N level (kg ha1)
DM yield (kg ha1)
Shoots
125 250 375 500
2692 3803 3123 4212
Tubers
125 250 375 500
6766 6734 5648 7154
b ab ab a
Total N (%) 3.31 3.85 3.99 4.04
b a a a
1.23 1.35 1.42 1.43
N yield (kg ha1) 89.12 146.41 124.61 170.18
c ab b a
83.22 90.91 80.20 102.65
Ndff (%) 47.08 51.54 60.67 57.73 41.81 50.02 52.31 57.33
b a a a
Fertilizer N utilization (%) 33.57 30.18 20.16 19.65
a ab bc c
27.83 18.05 11.06 11.17
a b c c
Means within each column followed by the same letters are not significantly different at p < 0.05.
in the arid and semi-arid Mediterranean climate (Mohammad et al., 1999; Darwish et al., 2003), close to 60%. These results confirm the need to restrict nitrogen input to potato in the dry conditions of the Bekaa plain to 130 kg N ha1. Under these conditions, nitrogen derived from the soil and irrigation water was equivalent to 96 kg N ha1 year1, a significant amount. Studies have demonstrated increased nitrate concentrations in the groundwater table in main Lebanese arable lands (Halwani et al., 1999; Darwish et al., 2005) linked to erratic fertilization practices. Leaching followed by a saturation of nitrates and other more stable elements can explain the current pollution of water resources (Jordan and Rippey, 2003). Water being a scarce resource in the area, it was essential to evaluate water use efficiency for the 2000 season. For a water supply of 700 mm, close to the FAO recommendations for potato in the area (FAO, 1969), irrigation water productivity varied between 7.7 and 8.6 kg fresh tuber per cubic meter of applied water. These values were comparable to those obtained by fertigation of table potato, providing full water requirements (Darwish et al., 2003). It could be possible to reduce water input without affecting the crop performance.
3.2.
Irrigation schedule
In Experiment II parallel measurements of soil potential at 0.3 m depth, before irrigation cycles showed an average of 22 kPa for W120, 32 kPa for W80 and 50 kPa for W60 against 30 kPa for the control. These soil head potentials measured at 0.3 and 0.6 m gave an acceptable linearity with the available soil water content obtained from the neutron probe measurements (Fig. 2). In fact, this equation is very close to the function obtained previously on the same location (unpublished data) and is quite remarkable, considering that it is a clay soil. Based on the equation, the initiation of irrigation in each of the four treatments coincided with 60 mm of available water in the control and 56.6 mm in W80, 31.6 mm in W60 and 70.5 mm in W120. Therefore, it could be safe to deplete the soil water content down to 56.6 mm in the upper 0.3 m soil layer, which is equivalent to 40% water depletion.
of water input. The restricted input of irrigation water was first observed as a decrease in shoot fresh weight (Fig. 3) towards the full development stage (83 DAS). It was not until the physiological maturity that a negative effect of the deficit irrigation was obvious on tuber dry matter yield with a slope of 10.9 and a correlation coefficient of 0.77 (Fig. 3). The shoots dry matter yield did not have as strong a link with the water application, as the slope was 1.28 and the coefficient of correlation was not significant. Later on at full maturity, both slopes decreased, as they become 7.2 for tubers and 0.57 for shoots indicating a greater discrepancy between the plant parts and a visible impact on tubers production. Deficit irrigation (W60) lowered both the tuber dry matter production and the average weight of the commercial tuber (Table 2), leading to 21% loss in fresh yield. No commercial fresh yield loss was found in the moderately stressed regime (W80), due probably to a higher proportion of commercial tubers. Both extreme treatments, W120 and W60, affected the percentage of commercial tuber by favoring tuber diseases for the first and leading to tuber deformation for the second. Somehow, moderate nitrogen fertilization and a reduced water application allowed about 1% increase in dry matter content (Table 2). However, no significant difference was observed for the harvest index, which varied between 0.77 and 0.82 despite a slight decrease for the W60 treatment. Yield losses obtained in this work were significantly less than the reduction observed when deficit irrigation was applied continuously (Iqbal et al., 1999; Smith et al., 2002) or at tuber bulking stage (Minhas and Bansal, 1991). High losses were also reported for other growth stages, notably at the establishment (Skaff, 1997), flowering (Iqbal et al., 1999; Smith
3.3. Effect of deficit irrigation on potato productivity and dry matter content Water saving can be achieved either by decreasing the frequency of irrigation events or by a systematic reduction
Fig. 2 – Available water (mm) as related to the readings of the head potentials (kPa) in tensiometers placed at 0.30 and 0.60 m in Experiment II.
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Fig. 3 – Shoot and tuber DM production in processing potato at physiological maturity under four irrigation regimes.
et al., 2002) and ripening (Fabeiro et al., 2001) stages. Introducing moderate continuous deficit irrigation (0.80 ET) after the establishment phase of potato could be an alternative means for water saving on clay soils of the dry Mediterranean regions.
3.4.
Deficit irrigation and N utilization
For all treatments, N uptake by tubers was higher compared to shoots. Nitrogen content in the tubers dropped from 1.9% in the control to an average of 1.6% in the water-stressed treatments (Table 4). The total N removal by the crop was close to the first season and fluctuated between 156 and 246 kg ha1. Nitrogen derived from fertilizers (19–24%) decreased in the deficit irrigation. Restricted water input showed a significant difference in N utilization by the tubers and the whole plant, including the root system where total N removal dropped to 3– 3.6 kg N ha1 in W60 and W80 (Table 4). Normal and excess water input resulted in significantly higher N fertilizer recovery (about 44%), while deficit irrigation, regardless of
Fig. 4 – Fresh tuber production (t haS1) of processing potato as related to applied water (m3) or consumed water (m3) under four levels of irrigation water.
its severity, equally affected N utilization keeping it below 21.5%. Lower N use efficiency could be explained by the lower tuber DM yield and DM content observed at physiological maturity in deficit irrigation. Water stress probably interfered with the translocation of DM to the sink. Under the same climatic conditions, water stress in corn caused a higher vapor pressure deficit that had accelerated crop phenology and anticipated silking and maturity dates (Karam et al., 2003).
3.5.
Water consumption
For the period of differential irrigation, plots effectively received (excluding the leaching fraction) 614 mm for W120, 510.6 mm for W100, 407.3 mm for W80 and 313.6 mm for W60. The actual crop evapotranspiration varied between 300 mm for W60 and 405 mm for W120. Extreme water stress (W60) limited the real crop ETc to 300 mm, reflecting on crop development and productivity. Ground coverage was also affected as it dropped to 0.7–0.8. Therefore, evaporation limited the readily available water, to the contrary of the
Table 4 – Response of fertigated spring potato (cv Santana) to different irrigation regimes, N uptake and recovery at physiological maturity Treatment
DM yield (kg ha1)
Shoots W120 W100 W80 W60
3146 3474 2841 2650
Tubers W120 W100 W80 W60
7940 7982 5276 4883
Roots W120 W100 W80 W60
Total N (%) 2.33 2.53 2.52 2.64
a a b b
188.5 239.8 193.07 149.6
1.85 1.90 1.54 1.67
1.67 1.83 1.89 1.94
a a b ab
N yield (kg ha1)
Ndff (%)
74.69 89.43 72.38 71.45
25.58 25.83 20.96 19.38
14.17 17.15 11.03 10.11
145.04 151.67 81.25 81.17
23.57 22.28 18.74 17.61
26.17 25.87 11.66 10.94
3.11 4.48 3.62 2.95
22.52 22.32 20.75 19.89
0.54 0.76 0.57 0.45
Means within each column followed by the same letter are not significantly different at p < 0.05.
Fertilizer N utilization (%)
a a b b
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Table 5 – Water consumption (mm) of fertigated processing potato: removal from different soil layers and proportion (in parenthesis) of total consumption Soil depth (cm)
Treatment
0–20 20–40 40–60 Total (mm)
W120
W100
W80
W60
248.1 a (61.3%) 100.1 a (24.7%) 56.8 a (14.0%) 405.0
247.42 a (66.5%) 68.76 b (18.5%) 55.82 a (15.0%) 372.0
263.0 a (65.6%) 99.0 a (24.7%) 39.0 ab (9.7%) 401.0
227.6 a (75.9%) 43.8 b (14.6%) 28.6 b (9.5%) 300.0
Means within each row followed by the same letters are not significantly different at p < 0.05.
Table 6 – Dynamics of water consumption (mm) of processing potato under four irrigation regimes shown in relation to three phases of crop development Growth phase
Treatment W120
W100
W80
W60
Vegetative 40–61 DAS Full top growth 62–86 DAS Tuber bulking 87–107 DAS
136.0 ab (33.6%) 169.0 a (41.7%) 100.0 a (24.7%)
144.5 a (38.8%) 150.0 a (40.3%) 77.3 ab (20.8%)
129.2 ab (32.2%) 166.1a (41.4%) 105.7a (26.4%)
110.4 b (36.8%) 127.8 a (42.6%) 61.8 b (20.6%)
Total (mm)
405.0
372.0
401.0
300.0
Value in the parenthesis denotes total consumption. Means within each row followed by the same letters are not significantly different at p < 0.05.
treatments W80 and W100, where soil shading was complete. Intermingling leaves of plants on adjacent rows, probably, enhanced transpiration and restricted evaporation, thus promoted more efficient water and N use. Higher evaporation in the deficit irrigation may have caused lower N recovery and reduced water use efficiency. This suggests the need for lower row spacing (<75 cm) to promote a dense canopy in deficit irrigation to reduce the evaporation loss. In other treatments, ETc varied between 371 and 405 mm. During the period of differential irrigation, 98 and 96% of the effectively applied water were recovered in W80 and W60 treatments. Water loss increased steadily with higher water treatments, as demonstrated by the slope of the relationship between the fresh yield and the consumed water or the applied water (Fig. 4). For one unit of applied water (1 m3) the relationship gave 4.8 kg of fresh production, whereas this doubled for consumed water reaching 8.8 kg of fresh tuber. This suggests a much sharper increase in water consumption as compared to water application and a clear possibility of reducing the difference between both values. The depletion of water content in the 0–60 cm depth between the consequent irrigation events was assessed, assuming the absence of water rise from the deep ground-
water. Deficit irrigation did not alter the ability of the crop to remove water from the upper 0.2 m layer (Table 5). Between 61 and 75% of total water used for evapotranspiration was derived from this horizon. This allowed for a speculation on the main root distribution within the upper 0.3 m depth. Significantly, less water was removed from the deeper layers. The analysis of water use in relation to the growth stages (Table 6) showed that over 40% of depleted water occurred during the maximum shoot growth stage (62–86 DAS). This period corresponds to higher water demand of processing potato and possibly higher crop vulnerability to water stress. Significantly higher water removal occurred during this phase in the control in comparison with the W60 treatment. Similarly, at tuber bulking stage (between 87 and 107 DAS), significantly less water was evapotranspired in W60. This water distribution in the profile was obvious elsewhere, in the movement of nitrates. Monitoring the soil solution between the full development and physiological maturity showed an accumulation of restricted amounts of nitrates in W80 (between 10 and 35 mg L1) in the 0.3 m soil layer. In the severe deficit irrigation it was impossible to collect soil solution below 0.6 m depth, which indicated the limitation of wetting front to this depth. Whereas in the control and
Table 7 – Productivity of applied N and water in processing potato in 2000 when N rates were tested and in 2001 with four levels of water input Experiment II (N: 130 kg ha1)
Experiment I (water: 700 mm) N rates (kg ha1)
Productivity of applied N (kg DM/kg N)
IWP (kg DM m3)
Applied water (mm)
Productivity of applied N (kg DM/kg N)
IWP (kg DM m3)
125 250 375 500
54.1 27.0 15.0 14.3
0.96 0.95 0.80 1.02
494 602 721 840
37.3 40.4 61.1 60.8
1.00 1.04 1.06 0.90
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Fig. 5 – Water use efficiency (WUE) and irrigation water productivity (IWP) of processing potato subject to four irrigation levels.
W120. This corresponds to water saving between 16.5 and 31% for a relative yield loss of 7 and 19.5%, respectively (Fig. 6). This crop response to water stress was considered acceptable for use in other agro-climatic zones (Smith et al., 2002). When yield loss by deficit irrigation is less than the saved proportion of applied water, the irrigation water productivity must substantially increase in comparison with the recommended water supply. In successful deficit irrigation of potato the relative water use efficiency in comparison with full water supply was 1.06 for drip and 1.2 for furrow irrigation (Kirda, 2002). Our results gave 1.18 for the moderate deficit irrigation a value close to the highest efficiency which indicates a good response to deficit irrigation using the fertigation techniques.
4.
Fig. 6 – Processing potato yield response to deficit irrigation.
W120, regular sampling of the soil solution from deep horizons (0.8 m depth) was possible. This suggests a possible leaching of excess water carrying nitrates with a concentration varying between 20 and 40 mg L1. These amounts were close to the value of nitrates observed in the soil solution of the fertigated plots with controlled N input (Darwish et al., 2003).
3.6.
Water-N productivity and yield response factor
The control of both trials showed comparable results for tuber DM production per unit of applied N and water (Table 7). Excess N input and severe water stress equally lowered the productivity of applied N. In Experiment II, water use efficiency (WUE) varied between treatments. In the control (W100) and the W120 treatments, evapotranspired water was used more efficiently to produce a unit of fresh yield compared to the moderate deficit irrigation (Fig. 5). However, the irrigation water productivity (IWP) showed a relatively better performance under a moderate stress (W80). The irrigation water productivity (IWP) increased with water reduction then decreased with severe deficit irrigation. The success of deficit irrigation is usually possible when applied to drought-tolerant crops cultivated on finely textured soils (Kirda, 2002). It is difficult to manage deficit irrigation with potato, a very sensitive crop to water stress following tuber set (Shock et al., 1993). Moderately reducing water input to potato saved 119 mm season1 of water in comparison with the control (W100) and 238 mm season1 in comparison with
Conclusion
With the current situation in the semi-arid regions, characterized by uneven rainfall distribution causing seasonal leaching of nitrates and groundwater contamination, the application of 130 kg N ha1 by fertigation is considered sufficient for processing potato. Increased N supply did not improve tuber yield, but reduced the efficiency of applied N. The controlled N application secured significantly higher N recovery (61% from applied fertilizers) by processing potato. Based on field data and measured evapotranspiration, water application of 700 mm in the 2000 season and 721 mm for the control in 2001 was comparable to the recommended value. Potato is sensitive to N management as it affects the crop performance and dry matter accumulation. Values of N yield and N recovery were comparable in both seasons. Considering available N pools and results for table potato in the region, reducing N input for processing potato is a cost-saving practice that prevents the groundwater contamination by nitrates. The neutron probe allowed estimating the actual crop evapotranspiration (ETc) over the whole season, in addition to water depletion with soil depths and throughout the growth stages. Eventually assessing plant available water and the soil head potential allowed for the estimation of available water during the season to manage the moderate water stress. Considering the efficiency of the drip system and leaching fraction, water demand of the spring processing potato in the dry Mediterranean conditions of Lebanon, was around 460 mm, after plant establishment. This is much below the recommended values for the potato. The processing potato removed up to 90% of total water demand from the upper 0.4 m soil layer. Around 40% of water was depleted during the phase of maximum biomass production and tuber bulking, which corresponds to the maximum crop vulnerability to water stress. The controlled deficit irrigation (W80) allowed maintaining tuber yield and DM content with a possibility to save 119 mm of water. The severe deficit irrigation (W60) caused a reduced canopy that possibly lead to higher evaporation and lower N recovery. This reflected on irrigation water productivity in relation to fresh tuber production, which increased with water reduction to a certain level then decreased with severe deficit irrigation. A comparable efficiency of dry matter production gained by a unit of N and water application was achieved by the control of both seasons. While a higher productivity of applied N was
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found in the lower N treatment, water stress significantly affected dry matter production per unit of applied N. The crop yield response factor (Ky) of processing potato submitted to relative evapotranspiration deficit in the dry Mediterranean conditions of Lebanon was 0.8. Saved water could then be diverted to other social needs or used to irrigate supplemental lands, compensating for the relative yield loss. The economy of 119 mm per single potato season is significant water saving for the semi-arid and dry areas. Confirming this result and extending this approach to other crops would mean substantial water saving at national and regional levels. Unless creating drought-resistant soils and crops, managing soil water content just above the damaging level is important for sustainable agriculture. Yields decrease because of water shortage not only from insufficient rainfall but also from mismanaged irrigation. In this regard, providing a simple tool to monitor plant available water content in the root zone can help overcoming the lack of flexibility of farming systems to sophisticated water measurements.
Acknowledgments This research was conducted as part of the regional technical co-operation project RAW/5/007: ‘‘Fertigation for improved water use efficiency and crop yield’’, jointly supported by IAEA Department of Technical Co-operation and the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture. It also received the support of the Lebanese National Council for Scientific Research.
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