Soil N supply and plant N uptake by irrigated rice in Tamil Nadu

EL-SEVIER

Field

Field Crops Research

Crops Research 5 1 ( 1997) 55-64

Soil N supply and plant N uptake by irrigated rice in Tamil Nadu T.M. Thiyagarajan a*b,P. Stalin ‘, A. Dobermann b, K.G. Cassman b, H.F.M. ten Berge ‘. * a Tarnif ‘ DLO

Research

Nadu Rtce Rrwarch Institute, ’ Intematronal Rice Research InstituteforAKrohioloK\

and

Tamil Nadu Institute. Soil

Ferti/itv.

Agnculturul L’nrl,ersiR. Aduthurai 6/? P.O. BOU 5-33, 1099 Manrla. Phrltppnrs P.O.

Bar

II.

6700

AA

Wagmnqen.

IO/.

Indk

The Netherlands

Abstract This study was undertaken to assess the N supply capacity of some irrigated rice soils of India and its relationship with N uptake of crops with and without fertilizer N supply and to simulate effects of different soil-N supply patterns on optimizing fertilizer N application using the MANAGE-N model. Soil samples were collected from N,, plots (no fertilizer N applied) of a multilocation field experiment with rice ( OQW sari~a L.) conducted in Tamil Nadu, India. during the southwest monsoon season (July-October 1994) with cultivar IR64. The N supply capacities of soil samples were assessed by static soil test methods and anaerobic incubation with and without K-saturated cation exchange resin. The experiments had a common set of treatments of different N application strategies. Plant samples were collected at different growth stages and N uptake was measured from the biomass and N content. MANAGE-N was used to optimize N application with different soil N supply regimes. The results revealed that soil-N supply capacities assessed by both static and anaerobic incubation methods were related to plant N uptake up to first flowering (r’ = 0.64 to 0.85) but they failed to correlate with grain yield. Actual N uptake rates of the N,, crops during different growth periods ranged from 0.28 to 1.17 kg ha- ’ day ‘. Some of the soils could supply N equal to that of a sufficiently fertilized crop in the initial period. Based on the soil-N supply capacities, the soils could be classified into those not requiring basal N and those requiring basal N. Simulation results revealed that if the soil N supply regime observed at Ambasamudram were available at Paiyur, the grain yield of N, crop would increase by 38% and there would be 7 to 1 I’% increased yield for 100 and 200 kg N applications and less fertilizer N would be required for different yield levels obtained with the Paiyur soil-N supply regime. MANAGE-N generated fertilizer N recommendation curves that identified different optimal timing of N application for different soil N supply regimes.

1. Introduction N availability in irrigated rice soils and the demand of the rice crop for N are both dynamic during a crop cycle. Unlike P. K, Ca and Mg, which have residual values and possible buildup in soil. the N-supplying capacity of soils could not be increased permanently by applying large amounts of fertilizer * Correspondmg author.

N (Olson and Kurtz, 1982). High-yielding rice plants need additional N from the soil to maintain a higher leaf photosynthetic activity for assimilating a large amount of carbohydrate and to supply more nitrogenous compoundsto grains during the ripening period (Murayama, 1979). Because the native soil N supply in intensive irrigated rice systems is not sufficient for higher yields, additional N supply through organic or inorganic fertilizers is an inevitable requirement. Match-

0378.4290/97/$17.00 Copyright ,@Z1997 Elsevier Science B.V. All right\ reserved. P/I SO378-4290(96)01040-4

56

T.M.

Th~yagarapn

et al./

Fwld

ing the seasonal demand of the crop with the supply from soil would be the ideal N management and this requires understanding of the dynamics of crop requirement and native N supply. Recent field studies indicated that improvement in recovery efficiency is possible if farmers adjusted N rates with regard to the N-supplying capacity of their soils (De Datta et al., 1988; Cassman et al., 1993). Fertilizer recommendations for crops are made using tests such as the concentration of an available form of nutrient in the soil, or soil incubation tests, tissue tests, counts of tiller density, records of cropping history, or by applying test strips of nutrients to a crop. The recommendation of fertilizer N application rate based on soil N supply should be made from the individual prediction of soil N-supplying capacity for each cropping season but there could be deviations in the prediction of soil N supply. Zhu (1989) attributed those to contributions of subsoil and nonsoil N. Several methods and indices are used to estimate and represent the soil N supply capacity and these include soil tests for alkaline KM&-N (Subbiah and Asija, 19X), organic carbon, total N, NH, released by microdiffusion with NaOH (Lu, 1981), initial exchangeable NH, (Schoen et al., 19851, exchangeable NH d after anaerobic incubation, NH, desorption, N uptake by plants grown on non-Nfertilized plots, and improved anaerobic incubation methods using ion-exchange resins (Saeed, 1995: Dobermann et al., 1994). Modelling has now been included in the array of methods to generate fertilizer recommendations because models integrate nutrient information with information on other factors that affect yield and its response to added nutrients (Angus et al., 1993). Comparison of several models involving N revealed, however, that simulation of below-ground processes generally limited model performance (Groot et al., 1991). Van Keulen (1982) also concluded that variation in the external efficiency of N uptake was far greater than the variation in the efficiency of N within the crop. A simple dynamic model ORYZA-0 was introduced by Ten Bege et al. (1994); Ten Berge et al. (1996a)) to optimize N application to irrigated rice and this model is now included in a user-friendly package called MANAGE-N (Riethoven et al., 1995;

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51 (19971

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Ten Berge et al., 1996a; Ten Berge et al., 1996b). The model simulates biomass production taking into consideration N uptake, partitioning of N to the leaf canopy, and utilization of leaf N in converting daily incident global radiation into dry matter. The availability of N for uptake by the crop is determined by native soil-N supply, applied amounts of fertilizer N and the time course of potential recovery of applied fertilizer N. In the model, soil-N supply represents the rate at which N released from mineralization is taken up by the crop and is estimated from N uptake of unfertilized crops (N,). Assessing the seasonal pattern of soil N supply is thus an important requirement. To generate fertilizer management recommendations for farmers, which are to be site-specific, N uptake from non-N-fertilized crop cannot be practically obtained from every field. Other simple methods are therefore needed to assess the soil-N supply capacity. Patterns of N supply from certain irrigated rice soils were studied by observing N uptake by unfertilized crops and comparing these with various soil-N tests. We also evaluated the recovery of fertilizer N applied at different crop growth stages and the significance of the seasonal soil N supply in optimizing N application using the simulation tool MANAGE-N.

2. Materials

and methods

A common experiment with rice (Oryza satiua L.) was conducted in 10 rice-growing research centers of Tamil Nadu (9) and Union Territory of Pondicherry (11, India, during the southwest monsoon season (July-October 1994). All sites were located between 8”42’ and 12”30’ N, and 76”30’ and 79”OO’ E. At the nine sites of Tamil Nadu, the test crop was IR64 and at Karaikkal (Union Territory of Pondicherry), the cultivar was ADT36. The seed source, date of sowing (10 June 1994) and seedling age (27 d) were identical at all sites in Tamil Nadu, whereas at Karaikkal, the crop was grown 1 month later. The treatments with different N application strategies were conducted in a randomized block and replicated four times. The treatments included a control (N,) with no fertilizer N and another one (N,,,) with 200 kg N ha-’ applied in four splits of 50 kg N haa’ at transplanting (TP), active tillering (AT),

T.M. Thiy+wajan

et al. /Field

panicle initiation (PI) and first flowering (FF) stages. Phosphorus (26 kg haa’ as P,O,) and zinc and sulphur (10 and 5 kg ha-‘, respectively. as ZnSO,) were applied basally. Potassium (50 kg ha-’ as K,O) was applied in two equal splits (basal and at panicle initiation stage). The crops were grown under puddled and irrigated conditions. The plot size varied between sites and ranged from 20 to 40 m’. Plant samples (5 hills from each plot) were collected at TP, AT, PI, FF harvest stages as per the procedure described by Thiyagarajan et al. (1995). In each plot, one half of the area (10 to 20 m’, always on one side of the field) was marked for plant sample collection and the other half was left for final harvest. The plant samples were separated into roots, stems, leaves and panicles and dried at 80°C. Grain yield, biomass and N content of individual plant organs (roots, stems, leaves and panicles) were measured. Before transplanting, soil samples (O-15 cm depth) were collected from six random sites of each replicate control plot. After mixing the samples thoroughly into one bulk sample for each replicate, the soil was air dried in shade and crushed gently with a wooden mallet to pass through a 2 mm sieve. Soil properties such as total N, organic C, alkaline KMnO,-N and initial NH,-N were determined on a composite sample of each site following standard procedures (Van Reeuwijk, 1992; Ponnamperuma et al., 1981; Subbiah and Asija, 1956). For the anaerobic incubation studies with Kf-saturated cation exchange resin (Saeed, 1995) the replicate soil samples of a site with less than 20% coefficient of variation (CV) in grain yield were combined into one composite sample (Aduthurai (ADT), Ambasamudram (ASD), Coimbatore (CBE), Karaikkal (KKL), Madurai (MDU) and Tirurkupam (TKM)), whereas samples of all other sites were retained as replicates (Paiyur (PYR), Sirugamani @GM) and Thanjavur (TNJ)). In the anaerobic incubation studies with K resin, NH,-N was determined after 0, 14, 35 and 56 days incubation, and in the anaerobic incubation without K resin, NH,-N was measured on 0 and 14 d after incubation. NH,-N concentration was determined calorimetrically by the salicylate method (Kempers and Zweers, 1986). To estimate the potential soil-N supply rate, the NH,-N mineralized by anaerobic incubation with K resin was computed on a per hectare basis assuming a plow layer (15 cm)

Crops Research

51 (19971 55-64

51

soil bulk density of 0.9 g crnm3 for the heavy-textured soils (ADT, CBE, KIU and SGM); 1.O g cmm3 for the medium-textured soils (MDU and TKM) and 1.1 g crne3 for the light-textured soils (ASD, PYR and TNJ). Data on N uptake rates and fertilizer N recovery presented in this paper pertain to ASD, CBE, PYR and TKM, as complete data could not be obtained at other sites. Parameters for N optimization by simulation with MANAGE-N were derived from crop data (data not presented). The effects of different seasonal patterns of soil N supply on the differences in grain yields for selected N application levels were studied, as well as the corresponding changes in the N recommendation curves.

3. Results 3.1. Soil N

.wppl.v

The soils of the experimental sites ranged from sandy loam to clay in texture and were neutral to alkaline in reaction. The clay content of the soils varied from 13 to 58%. Soils at ASD were slightly acidic (pH 6.5) while the soils at TNJ were neutral (pH 6.9). The other soils were alkaline (pH 7.6-8.4). The soils of ASD, PYR and TN3 had very low CEC (6.3-9.0 cmol kg-i ). The organic carbon content was around 1.0% in the soils of ADT, ASD, CBE, MDU and SGM and around 0.5% in the soils of PYR, TNJ and TKM (Table 1). Total N ranged from 0.043% (TNJ) to 1.09% (ADT) and the C-N ratio ranged from 8.32 (ADT) to 10.3 (TNJ). The available N status as indicated by KM&NH,-N ranged from 75 mg kg- ’ to 110 mg kg- ’ . Initial NH,-N ranged from 8 to 47 mg kg- ’ (Table 2) and had a much higher CV than alkaline KMnO,NH,-N (60 to 110 mg kg-‘); organic C (4.4-10.9 g kgg’) or total N (0.4-1.3 g kgg’). The net NH,-N mineralized within 14 d anaerobic incubation without K resin ranged from 7 mg kg ’ to 6 1 mg kg- ’ and for the corresponding period with K resin, the values ranged from 27 mg kg- ’ to 93 mg kg- ’ (Table 2). Net NH,-N measured without K resin (O-14 d) was in most cases 30 less than net NH,-N with K resin. In to40mgkgg’ some of the soils (KKL. PYR, TKM) this corre-

58

ZM.

Table 1 Static soil test parameters Location

Organic

ADT ASD CBE KKL MDU PYR SGM TNJ TKM SEd

I .09 1.01 0.94 0.59 0.96 0.53 0.94 0.44 0.58 0.25

Thiygarajan

of the soils (dry C (“/cl

et al. / Fzeld Crops Reseurch

basis) at the different

Total N (%‘c)

C-N

0.13 0.10 0.10 0.06 0.11 0.06 0.10 0.04 0.07 0.03

8.3 10.0 9.9 9.5 9.1 9.1 9.4 10.3 8.6 0.7

Location

NH,-N

production

without

K resin

(O-14 ADT ASD CBE

61 42 43 7 57 9 33 17 8 21

MDU PYR SCM TNJ TKM SEd na:

not available.

d)

anaerobic plots

(mg kg

55-64

locations

ratio

KMnO,-NH,-N

(mg kgg’)

Initial

110 93 91 60 95 74 91 76 71 15

sponded to large relative differences. Net NH,-N with K resin during 14 to 35 d and 35 to 56 d periods of incubation ranged from 9 to 73 mg N kg-’ and 1 to 19 mg kg-‘, respectively. Net NH,-N mineralized during the 0 to 14 d were 40 to 78%; that during 14 to 35 d were 19 to 44%; and that during 35 to 56 d were 2 to 14% of total NH,-N mineralized up to 56 d. Potential soil-N supply rates calculated with the NH,-N measured with K resin ranged from 3.2 to 10 kg ha-’ dd’ during the 0 to 14 d incubation period; from 0.6 to 3.5 kg haa’ dd’ during 14 to 35 d and from 0.1 to 1.4 kg ha- ’ d- ’ during the 35 to 56 d period (Fig. 1). Except for the late-phase N release (35-56 d). correlations between standard soil N tests and anaerobic incubation parameters were highly significant Table 2 Net NH,-N production measured during grain yield (14% moisture) in unfertilized

51 (1997)

incubation

NH,-N

(Table 3). The net NH,-N without K resin provided higher correlation with plant N uptake and static soil tests than net NH,-N with K resin, which we cannot explain at present. 3.2. Grain yield and plant N uptake The grain yields in the N, crops ranged from 1.1 to 5.7 t haa’ while the crop N uptake at ‘first flowering’ stage (FF) ranged from 24 to 107 kg ha-‘. Crop N uptake at harvest were not available for all locations because crops at ADT, SGM and TNJ were affected by pests and harvest sampling at ASD and MDU was hampered due to rains. There was positive correlation between crop N uptake at FF and all soil-N test parameters except the late-phase

with and without

K resin: crop N uptake at first flowering

’)

NFF

with K resin

(kg N ha

O-14d

14-35

79 74 80 37 93 66 6X 27 38 23

42 36 44 9 35 25 55 29 23 13

d

(mg kg- ‘J

47 34 21 17 15 15 26 15 8 12

35-56 2 2 9 19 5 3 II 10 6

d

stage (N,,) Yield

‘J

(kg ha- ’ )

Total 123 112 133 47 14x 96 127 67 71 34

n.a. 64.0 42.6 23.7 na. 41.2 n.a. n.a. 38.5 8.5

3.0 4.5 4.4 2.5 5.7 3.9 2.1 4.0 4.5 0.4

and

T.M. Thiygarajan

ADT

ASD

CBE

et al. /Field

Crops Research

KKL

MDU

51 119971 5.5-64

PYR

59

TNJ

SGM

TKM

Locations Fig. 1. Potential soil N supply rates of soils dunng different periods after incubatton. Calculated from the net NH,-N mineralized during 0 to 14. 14 to 35 and 35 to 56 d after incubation. The initial NH,-N was included in the O-14 d period. To convert the values on a per hectare basis for the 15 cm soil layer, a bulk density of 0.9 g cm-j was assumed for the heavy-textured soils at Aduthurai (ADT), Coimbatore (CBE). Karaikkal (KKL) and Situgamani (SGM): 1.0 g cm ~’ for medium-textured soils at Madurai (MDU) and Tirurkupam (TKM); and 1.1 g cm-’ for the light-textured soila at Ambassamudram (ASD). Paiyur (PYR) and Thanjavur (TNJ).

(35-56 d) net NH,-N release (Table 3). Grain yield was only correlated highly (I’ = 0.67) with the latephase (35-56 d) net NH,-N release, however. Correlations between soil-N tests and most anaerobic incubation method parameters were also highly significant. A positive relationship between these parameters is well-established (Sahrawat, 1982; Saeed. 1995) Crop N uptake rates in the N,, and N,,,, crops during transplanting to active tillering (TP-AT), ac-

tive tillering to panicle initiation (AT-PI), and panicle initiation to first flowering (PI-FF) periods are presented in Fig. 2. The N uptake rates in the N, crop at ASD and TKM were lowest during PI-FF period while at CBE and PYR the N uptake rates were lowest during TP-AT period. When fertilizer N was applied (200 kg ha- ’ >, the highest uptake rates were observed only during the PI-FF period and the values ranged from 1.89 to 4.7 1 kg ha- ’ d- ’ across the four locations. The maximum N uptake rate in

Table 3 Correlation

stage (FF)

matrix

of parameters

with plant N uptake Plant N uptake

Plant N uptake at FF Total N Organic C Alkaline KMnO,-NH,-N Initial NH,-N Net NH,-N without K resin Net NH ,-N with K resin O-14d 14-35 d 35-56 d O-56 d

0.77 0.72 0.85 0.8.5 0.80

Ls * .’ * ’ . - * . _. ’

0.51 * 0.53 * 0.25 0.62 * * *

at FF

at first flowennp Total N

0.99 . . 0.86 _ * 0.74 _ . 0.86 *

with static soil tests and anaerobtc

Organic

* * *

0.75 . ’ * 0.61 . . . - 0.24 0.80 * *

0.86 0.72 0.x7

C

. - * * * _* *

0.74 _ * 0.64 ’ * ~ 0.23 0.81 . * *

KMnO,-NH,

0.75 0.92

N

I ’ * _. *

0.67 _ _ * 0.62 r + * - 0.07 0.78 * . *

incubation Initial

methods

NH,N

0.73 * * 0.48 * 0.51 * - 0.45 0.52 *

60

T.M. Thi.vagarajan

TP-AT 5

1

AT-PI

et al. /Field

Crops Research

51 (1997)

55-64

P 1-F F

CBE

TP

I

AT

PI

FF

AT

PI

FF

AT

PI

FF

PI

FF

200 1

TP-AT 5

AT-P

I

P I-F F

PYR

TP 200 ,

v f

Y c 3 4 z

PYR 150

I 100 50 0 TP 200 TKM

P 5 150

TP-AT

AT-PI

Crop

growth

P I-F F

5. p 100 3 4 50 z 0

intervals

Fig. 2. Actual crop N uptake rates in N, (Tl) and Nzoa (T2) crops during different growth periods at Ambasamudram (ASD), Coimbatore (CBE), Paiyur (PYR) and Tirurkupam (TKM). TP-AT: transplanting to active tillering, AT-PI: active Meting to panicle initiation; PI-FF: panicle initiation to first flowering stage.

TP

Fig. 4. applied crop N (ASD).

of actual crop N uptake

-

rates in N, crop at Ambasamudram

growth

stages

Contribution of soil N and fertilizer N (200 kg N ha-’ in four equal splits at TP, AT, PI and FF stages) in the uptake at different crop growth stages at Ambasamudram Coimbatore (CBE), Paiyur (PYR) and Tirurkupam (TKM).

2

Fig. 3. Time course (TIChO.

AT

Crop

(ASD),

Coimbatore

Ambasamudram

(CBE),

Paiyur

(PYR)

and Tirurkupam

TM.

ASD

Thiwgarajan

er al. / Field Crops Rerearch

61

51 (19971 55-64

soil N supply

6000

*

PYR

soil

N supply

150

100

N application

level

(kg/ha)

Fig. 5. Simulated grain yields for different levels of optimized N-application at Paiyur Ambasamudram. The weather and crop parameters were those pertainmg to Paiyur only.

the N, crop was only about 1 kg ha- ’ d- ’ and this peak occurred during different growth periods at the different locations (Fig. 3). Soil N contribution to the N uptake in the N,,,, crop was greatest at CBE and least at PYR and the percentage contribution during different growth periods varied among the four locations (Fig. 4).

with

soil N supply

data from

Paiyur

and

tions, for 200 kg N ha-’ level (Fig. 6) reveal the shifts in the optimized N application pattern at both locations as a result of the different soil-N supply patterns. At PYR, with ASD soil-N supply, fertilizer N application was delayed due to higher initial soil-N supply. At ASD, if CBE soil-N supply pattern was

3.3. Implications of different soil N supply regimes: simulation study The parameters derived from the experiments (details not presented here) were used to generate optimized time schemes for fertilizer N application. This was done with the help of the model MANAGE-N. The results presented here are meant to demonstrate the significance of the soil-N supply capacity to fertilizer management. Simulating crop growth at PYR (low soil N supply regime) with soil-N supply data of ASD (higher soil-N supply regime) resulted in 11% increased grain yields for 200 kg N ha- ’ (Fig. 5). When crop growth at ASD was simulated with CBE soil-N supply data (laboratory soil tests revealed similar soil-N supply regimes in both sites but there were differences in the time course of N uptake rate by unfertilized crops), grain yield was reduced by 11% for the same N level. Grain yields of the unfertilized crops increased by 38% at PYR with ASD soil-N supply and were reduced by 23% at ASD with CBE soil-N supply. The recommended cumulative N application curves for the above situa-

a

0 IO

0

20

30

40

50

60

50

60

200 .-s ;;j p Bmm 2 Q .-$5 x7 E a

0

/f/p

PYR sotl N supply

150 // 100 50

ASD soll N supply

0 0

10

30

20

Days

after

40

transplanting

Fig. 6. Optimized cumulative N application curves basamudram with ASD and CBE soil N supply; Paiyur with PYR and ASD soil N supply.

(A) for Amand (B) for

62

TM.

Thiygarajan

et al. /Field

substituted, fertilizer N addition was advanced according to these calculations.

4. Discussion Anaerobic incubation with K resin provides information on the release pattern of NH,-N over time and a general pattern of decreasing mineralization was observed for the soils of all locations. The potential soil N supply rates during O-14 d and 14-35 d periods were much higher than actual crop N uptake rates even with sufficiently fertilized crops. Differences between net NH,-N with and without K resin were significantly correlated with soil pH (r2 = 0.684) and exchangeableCa (r’ = 0.661, confirming possible loss of NH, due to volatilization in calcareoussoils during anaerobic incubation without K resin as found by Singh and Pasricha (1977). The higher correlation found between crop N uptake and net NH,-N without resin indicates, however, that this or a related loss processoccurs also in the field. Hence, net NH,-N without resin might still give a better estimateof plant-available N than net NH,-N with K resin. The current study of the soil N supply assessed with standard tests, anaerobic incubation methods and field observations indicated that the soils could be classified as those that would require basal N (KKL, PYR, TNJ and TKM) and those that would not (ADT, ASD, CBE, MDU and SGM). Table 4 ranks the soils of the study according to the different N supply parameters. ADT, MDU, CBE and ASD

Table 4 Ranking of locations Location

Organic (5%)

ADT MDU CBE ASD SCM TNJ PYR TKM KKL

1 3 4 2 5 9 8 7 6

per soil test method C

(1 for highest,

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51 (19971 55-64

ranked highest for almost all parameters. Consistently low were the soils at TNJ, PYR, TKM and KKL. Accordingly, the soils at ASD and CBE would fall under no-basal-N category and the soils of PYR and TKM would fall under basal-N-neededcategory. The field experiment also demonstratedthat fertilizer N recovery during TP-AT was lessat ASD and CBE than at PYR and TKM. Simulated grain yields at PYR using the soil-N supply data of ASD demonstrated the higher yield potential of the soil at ASD, given a favorable radiation regime. The optimized cumulative N-application curves also revealed that for soils such as that at ASD, with higher initial soil-N supply, N application must be shifted to a later date. As shown here, such simulation studiesenable not only the identification of optimal N-application patterns, but also the separateassessmentof soil and climate effects on yield formation.

5. Conclusions Some of the soils considered in this study were able to supply N equal to the demand of a sufficiently fertilized crop during the initial crop growth period, suggestingthat basal N application could be skipped in those soils. The laboratory studies also demonstrated that these soils have higher soil N supply regimes. The highest soil N uptake rate of unfertilized crops during important growth periods was around 1 kg ha-’ dd ’ and the occurrence of this peak varied

9 for lowest test values)

Total N (0)

C:N ratio

KMnO,-NH,-N

Initial h-H,-N

Net NH,-N without resin O-14d

Net NH,-N with K resin O-14 d

Net NH,-N with K resin, total

1 2 4 3 5 9 8 6 7

9 6 3 2 5 1 7 8 4

1 2 4 3 5 6 7 8 9

1 6 4 2 3 8 7 9 5

1 2 3 4 5 6 7 8 9

3 1 2 4 5 9 6 7 8

4 1 2 5 3 8 6 7 9

T.M.

Thiwgurajan

et al./

Field

with locations. Immobilization of mineralized N during the early periods and later remineralization could be one of the reasons for this variation. Potential N-supply rates as measured by anaerobic incubation with K resin were much higher during the O-14 d and 14-35 d incubation periods than actual N uptake rates of unfertilized crops. Volatilization of mineralized NH, in the field may have caused the relatively low uptake, if the root system was insuffciently developed to capture the mineralized nitrogen. Plant N uptake in the unfertilized plots correlated better with total N, organic C, alkaline KMnO,-N, initial NH,-N and net NH,-N from incubation without K resin, than with net NH,-N from incubation with K resin. Simulation results revealed that if the soil N supply regime observed at ASD was available at PYR, the grain yield of the N, crop would increase by 38% and there would be 7 to 11% increased yield for 100 and 200 kg N applications, and less fertilizer N would be required for different yield levels obtained now with the PYR soil-N supply regime. MANAGE-N-generated fertilizer-N recommendation curves indicated the effect of different soil N supply regimes on optimal timing of N application,

Acknowledgements The field experiments were conducted in the different locations by many scientists as part of a training program on “Systems Approaches for Nitrogen Application to Rice (SANAR)” organized by the Tamil Nadu Rice Research Institute, Aduthurai. India. The dedicated involvement of these scientists and the support from the vice-Chancellor and other senior administrators of Tamil Nadu Agricultural University, Coimbatore, India, are gratefully acknowledged.

References Angus. J.F.. Bowden. J.W. and Keating. B.A., 1993. Modellmg nutrient responses in the field. Plant Soil. 155-156: 57-66. Cassman, K.G.. Kropff, M.J.. Gaunt. J. and Peng, S. 1993.

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