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DIVISION S-4—SOIL FERTILITY & PLANT NUTRITION

Effects of Residue Decomposition on Productivity and Soil Fertility in Rice–Wheat Rotation

^a Dep. of Soils, Punjab Agricultural University, Ludhiana, 141 004, India
^b Crop, Soil, and Water Sciences Division, IRRI DAPO Box 7777, Metro Manila, Philippines

^* Corresponding author (j.k.ladha{at}cgiar.org).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Rice (Oryza sativa L.)–wheat (Triticum aestivum L.) farmers in India burn or remove residues to facilitate seedbed preparation. Incorporation of residues before planting of the next crop generally decreases yields due to N immobilization. Since a window of about 40 d is available between rice harvest and wheat planting, the effect of time of incorporation on rice residue decomposition and N mineralization–immobilization was studied in 1992–1993. The mass loss of residue was 25% for a 10-d, 35% for a 20-d, and 51% for a 40-d decomposition period before wheat planting. Nitrogen release from residue ranged from 6 to 9 kg ha^–1 during the wheat season. The immobilization of urea N decreased when residue was allowed to decompose for 10-d or longer. Based on these studies, a long-term (1993–2000) experiment was conducted on a sandy loam soil to examine the effect of time of residue incorporation before sowing wheat when compared with burning or removal of residue on yields, N-use efficiency, and soil fertility. The effect of wheat residue incorporation with green manure (GM, Sesbania cannabina L.) on subsequent rice yields was also determined. Residue incorporation for 10 to 40 d had no effect on wheat yields. Rice yields increased (0.18–0.39 Mg ha^–1) when wheat residue was incorporated with GM. Starter N applied at residue incorporation did not influence wheat yields but decreased N recovery efficiency. Physiological efficiency was higher when rice straw was incorporated in wheat and when wheat straw plus GM were incorporated in rice than when rice straw was incorporated for 10 d or when the straw was burned. The long-term application of rice residue increased C accumulation in soil.

Abbreviations: AE, agronomic efficiency • GM, green manure • IGP, Indo-Gangetic Plains • NH₄OAc, ammonium acetate • PE, physiological efficiency • RE, recovery efficiency • SOM, soil organic matter

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
RICE–WHEAT is a major crop rotation in the Indo-Gangetic Plains (IGP) of South Asia, spread over 13.5 million ha in Bangladesh, India, Nepal, and Pakistan (Ladha et al., 2000). Effective management of postharvest crop residues (straw) is perhaps the foremost challenge facing the intensive rice–wheat-producing regions of the world. The wheat residue is used to feed the animals. However, the rice residue due to large silica content is normally burned. Burning of rice residues is cost-effective and the predominant method of disposal in areas under combined harvesting in the IGP (Samra et al., 2003). However, disposal of crop residues by burning is often criticized for accelerating losses of soil organic matter (SOM) and nutrients, increasing C emissions, causing intense air pollution, and reducing soil microbial activity (Biederbeck et al., 1980; Rasmussen et al., 1980; Kumar and Goh, 2000). According to an estimate, 113.6 million Mg of rice and wheat residues, containing about 1.90 million Mg of nutrients, are available in the IGP of India (Sarkar et al., 1999). In the Indian Punjab alone, about 12 million Mg of rice straw are burned annually, which causes about 0.7 million Mg of N loss. The gaseous emissions from burning of rice straw are 70% CO₂, 7% CO, 0.66% CH₄, and 2.09% N₂O (Samra et al., 2003). Estimated emissions of greenhouse gases caused by burning of rice straw in the whole IGP of India are thus substantial. Besides contributing to the greenhouse effect, the large-scale burning of rice straw results in serious health hazards as is evident from the reported increase in respiratory and eye problems among the local population (Grace et al., 2003).

Where residues have been incorporated immediately before planting the next crop, grain yields were lower than where residues are removed or burned, resulting in N immobilization, a problem that is attributable to the slow rates of residue decay (Sidhu and Beri, 1989; Beri et al., 1995). Other potential problems of residue incorporation just before rice transplanting include accumulation of phenolic acids in soil and increased CH₄ emissions under flooded conditions (Grace et al., 2003). In this case, the timing of incorporation of crop residues is more important than the amount. Compared with the traditional method of wet incorporation shortly before planting of the next rice crop, the potential benefits of shallow incorporation shortly after crop harvest include accelerated aerobic decomposition of crop residues (about 50% of the C within 30–40 d), leading to increased N availability (Witt et al., 2000), and reduced CH₄ emissions (Wassmann et al., 2000). Early incorporation also allows additional time for phenol degradation to occur under aerobic conditions, thereby avoiding any adverse effect on germinating seeds and seedlings.

Burning of crop residues must be avoided at all costs for environmental reasons. Farmers will probably only incorporate crop residues if legislation forces them to or if there is a clear yield increase that they cannot achieve with the application of additional fertilizer. A window of about 35 to 40 d is available between the harvesting of rice and seeding of wheat, which can be used for in situ decomposition of rice straw. Similarly, a fallow period of 50 to 60 d is available after the wheat harvest and before rice planting; this allows decomposition of wheat straw and the raising of a GM crop (Yadvinder-Singh et al., 1991, 1994). Apart from enhancing residue decomposition (Singh, 1993), the GM crop can supply large amounts of N to the following rice crop. Although the effect of straw incorporation on N immobilization in the soil is well known (Christensen, 1986; Toor and Beri, 1991; Bhogal et al., 1997; Mary et al., 1996), a few studies have investigated how time of incorporation and starter-N application influence crop residue decomposition, nutrient release, and crop yields (Adachi et al., 1977; Bijay-Singh et al., 2001). More (or improved) knowledge about residue decomposition dynamics is essential for developing effective management strategies. No single residue management practice is superior under all conditions (Kumar and Goh, 2000). In the IGP of South Asia, wheat straw is mainly used as animal feed, but rice straw is disposed of by burning. The objectives of our study were to (i) estimate the long-term effects of different times of incorporation of rice residue before sowing wheat vis-à-vis residue burning and residue removal on crop production in the rice–wheat system, and (ii) investigate crop residue decomposition and N mineralization as a function of time of incorporation of rice straw. To achieve this objective, three sets of studies were performed. The first study investigated the effect of time of incorporation (during the prewheat fallow period) on rice residue decomposition and N release using the litterbag technique under field conditions. The second study investigated the effect of the different times of incorporation of rice straw on fertilizer N mineralization and immobilization and K release dynamics under laboratory conditions. Based on the outcome of these experiments, the third study examined the long-term effects of time of incorporation of rice residue on the yields of rice and wheat, and soil fertility parameters. In addition, the role of starter N and the combination of wheat residue and the leguminous GM (Sesbania cannabina) to enhance the decomposition of rice and wheat residue was also examined.

	MATERIALS AND METHODS

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Site Description
The experimental field was located at the research farm of the Punjab Agricultural University, Ludhiana (30° 56' N, 75° 52' E, 247 m above mean sea level), located in the IGP in the state of Punjab, India. The soil was a Fatehpur sandy loam (Typic Ustipsamment) with the following characteristics: pH 7.2 (1:2 soil/water ratio), electrical conductivity (EC) 0.27 dS m^–1, cation exchange capacity (CEC) 10.6 cmol (p+) kg^–1 (using BaCl₂ solution; Sumner and Miller, 1996), 3.5 g kg^–1 organic C (Walkley and Black method, Nelson and Sommers, 1996), 12.5 mg kg^–1 Olsen P (Olsen et al., 1954), 53.1 mg kg^–1 ammonium acetate (NH₄OAc)-extractable K (Brown and Warncke, 1988), 790 g kg^–1 sand, 101 g kg^–1 silt, and 109 g kg^–1 clay. Under average climatic conditions, the area receives 800 mm of annual rainfall, about 80% of which occurs from June to September. The mean minimum and maximum temperatures during the rice season (July–October) were 18 and 35°C, whereas during wheat season (November–April) the mean temperatures were 6.7 and 22.6°C, respectively. Monthly distribution (averaged for the last 15 yr) of rainfall, minimum and maximum temperatures, and sunshine hours for the experimental site are presented in Fig. 1 .

View larger version (15K): [in this window] [in a new window]   Fig. 1. Monthly distributions (averaged for 15 yr) of (A) maximum and minimum temperatures, (B) sunshine hours, and (C) rainfall at Punjab Agricultural University, Ludhiana, India.

Laboratory Experiment
The effect of predecomposition of rice residue for 0, 10, 20, and 30 d before N fertilizer application on N and K dynamics was studied in a sandy loam soil incubated at 35°C for 60 d. A bulk soil sample (0- to 15-cm depth) was collected from a nonexperimental area of the field experiment (see next section for description of the soil used). Soil was air-dried, ground, and sieved through a 2-mm mesh. Before the start of the experiment, bulk soil was rewetted to about 60% field capacity (determined as water held by soil at 10 kPa water potential) and incubated at 30°C for 7 d. The 7-d incubation period before rice straw addition allowed the microbial population to reach a baseline level after the initial flush of activity from soil drying and rewetting. Following incubation thereafter, a known weight of soil (subsample) from the bulk soil was weighed into 1-L plastic containers (10 cm i.d.) marked for different dates of straw incorporation. Rice straw (ground to 1- to 2-mm size, 0.67% N) was mixed into the soil at 3 g kg^–1 soil at 0, 10, 20, and 30 d before the application of 100 mg N (as urea) kg^–1 soil. Fertilizer N was applied to all the pots at the same time. Control soil (without rice straw added) was similarly prepared. Each treatment had three replications. Soil moisture was adjusted at 75% of field capacity as described earlier. Soil water in the pots was maintained by making up the loss in water every fourth day during the study. The soil samples were then incubated at 35 ± 1°C for 60 d. Soil samples from the individual pots were collected at 10, 20, 30, 45, and 60 d after the application of fertilizer. Inorganic N (NH⁺₄ and NO^–₃) was determined in 1M KCl extracts by microKjeldahl steam distillation.

Field Experiment
A field experiment was conducted from 1993 to 2000 at the research farm of the Punjab Agricultural University, Ludhiana. The experiment was laid out in a randomized complete block design with three replications. Plots were 10 m long and 4.2 m wide. The eight treatments (T1–T8) included different combinations of rice straw management and urea-N management practices (Table 1). After the harvest of rice in the first or second week of October, rice straw was allowed to decompose for different periods (10, 20, and 40 d) before the sowing of wheat in the second or third week of November (Table 2). Except in T1 and T2, combine harvesting of rice was simulated, leaving about 30-cm long stubble anchored to the ground. After removing grains, the straw was uniformly distributed in the allocated plots. The amount of incorporated straw during different years averaged 7.1 Mg ha^–1, which added 44 kg N ha^–1 and 2.87 Mg C ha^–1 (Table 3). At the predesignated periods, the rice straw was incorporated into the soil using a moldboard plow. Irrigation was applied at 18 d after incorporation of rice straw in the 40-d decomposition treatments to enhance straw decomposition and in the 20-d decomposition treatments to provide optimum moisture conditions for the ease of straw incorporation. All plots were cultivated three to four times, followed by leveling after applying presowing irrigation about 10 to 12 d before the sowing of wheat. In T1 and T2, rice was harvested manually, leaving about 10-cm long stubble above the ground surface, and rice straw was removed from the plots. In the straw burned treatment (T3), dry rice straw in the allocated plots was burned in situ at 10 to 12 d after the harvest of rice.

For wheat (cv HD 2329 during 1993–1996 and PBW343 during 1996–2000), fertilizer N (120 kg ha^–1) as urea was applied in two equal split doses, 50% at the time of wheat sowing and the remaining 50% topdressed at 20 to 25 d after sowing, 2 to 3 d after the first irrigation (Table 2). A basal dose of 26.2 kg P and 25 kg K ha^–1 was drill-applied at wheat sowing to all treatments. In T7, 30 kg N ha^–1 (starter N) was applied at the time of straw incorporation. The remaining fertilizer N (90 kg N ha^–1) was applied in two equal split doses as described for the other treatments.

Rice (cv PR111 during 1994–1998 and PR114 during 1998–1999) was transplanted in the first week of June every year. In treatments T1 through T7, wheat crop was harvested manually and straw was removed from the plots. A uniform dose of 120 kg N ha^–1 was applied in three equal split doses at transplanting and 3 and 6 wk after transplanting (Table 2). In T8, N addition through GM ranged from 75 to 95 kg ha^–1, total C addition averaged 1.54 Mg ha^–1 (Table 3), and the total N applied (GM + urea) to rice was kept at 120 kg N ha^–1. The balance of N (25–45 kg N ha^–1) was applied as urea in two equal split doses at 3 and 6 wk after transplanting. Rice was harvested from a 24-m² area at the center of each plot at physiological maturity in the first week of October. Grain yield was expressed on the basis of 140 g kg^–1 water content and straw yield was expressed on oven-dry weight basis. Wheat was harvested from a 24-m² area at the center of each plot at physiological maturity in the third week of April. Grain and straw yields are expressed on a dry-weight basis.

Plant and Soil Sampling and Analysis
At crop maturity, grain and straw samples were collected from each plot and dried in a hot-air oven at 60°C for 3 d. The N in grain and straw subsamples of rice, wheat, and sesbania was determined by the microKjeldahl method. Potassium was analyzed in di-acid (HNO₃ and HClO₄) digests by flame photometric method. Soil samples were collected at 15-cm depth from three sites in each plot using a 4-cm diameter auger for the determination of C, K, and bulk density. The entire volume of soil was weighed and mixed thoroughly and a subsample was taken from the mixed soil. Subsamples were air-dried and crushed to pass through a 2-mm sieve. Soil C was determined by the Walkley–Black method (Nelson and Somers, 1996) and NH₄OAc-extractable K was analyzed using the methods described by Olsen et al. (1954) and Brown and Warncke (1988), respectively. All C and nutrient values were converted to kilograms per hectare using soil bulk density data simultaneously determined from soil cores.

Data Analysis
Recovery efficiency (RE) of added N was calculated as

[1]

Physiological efficiency (PE) of added N was calculated as

[2]

Agronomic efficiency (AE) of added N was calculated as

[3]

Sequestration of added organic C as rice straw, wheat straw, and sesbania in SOC was calculated (Aulakh et al., 2001) as

[4]

Analysis of variance (ANOVA) for randomized complete block design was performed to determine the effects of treatment, year, and their interaction on grain yield of wheat using the PROC GLM procedure in SAS (SAS Institute, 1989). To account for the difference between years, a repeated measures model was used with time as the repeated variable. A probability level 0.05 was considered significant. Duncan's multiple range test at the 5% level of significance was done using IRRISTAT version 92 (IRRI, 1992) to compare treatment means for RE, AE, PE, SOC, and available K data. A separate ANOVA was performed for each sampling date for residue decomposition and N mineralization data.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Rice Residue Decomposition Dynamics under Field Conditions
Decomposition of Rice Residue In Situ
Time of incorporation had a large effect on the decomposition of rice residue during the fallow phase (October–November) after rice harvest (Fig. 2) . At wheat seeding, the mass loss of rice residue was 51% for a 40-d decomposition period compared with 35% for a 20-d decomposition treatment and 25% for a 10-d decomposition treatment (Fig. 2). The relationships between mass of residue remaining and decomposition period for the three time-of-incorporation treatments were nonlinear (quadratic) (Fig. 2). Mass loss showed similar trends in the three treatments during the 122-d period after wheat planting. The mass loss was almost similar under 20-d and under 10-d decomposition period during the whole decomposition period after wheat planting. The amount of mass loss remained significantly higher for the 40-d decomposition period than the 10-d or 20-d period up to 72 d after seeding of wheat. At the end of the study, no significant difference was noted among the three treatments (Fig. 2).

View larger version (13K): [in this window] [in a new window]   Fig. 2. Effect of decomposition period on the mass remaining of litterbag rice residue. Values of LSD (0.05) for comparing treatment means at each sampling period are 6.9 for 0 d, 4.3 for 35 d, 5.4 for 72 d, 3.9 for 122 d, and NS, nonsignificant for 150 d after wheat planting. Each data point represents mean of three replicates.

[5]

where Y equals the total decomposition period (days) and X equals the percentage mass loss of rice residue.

The above equation accounted for 95% of the variation and may prove useful in predicting residue decomposition under soil and environmental conditions similar to that in the present study.

Since mass loss was measured at 10, 20, and 40 d after incorporation, rates of residue decomposition could be measured more accurately for the 0- to 10-, 10- to 20-, and 20- to 40-d periods. Averaged across the three treatments, rice straw decaying for 0- to 10-d lost 2.45% of its initial mass each day (Fig. 2). By comparison, the values were 1.0% d^–1 for 10 to 20 d and 0.8% d^–1 for 20 to 40 d during fallow. During the wheat-cropping phase, rates of straw decomposition were similar under the three treatments, except during the first sampling at 35 d after seeding wheat. When considered over the whole cropping season, the rate of mass loss under the three treatments averaged 0.41% d^–1. Not much is known on in situ decomposition of rice straw in tropical countries. Although Mishra et al. (2001) studied the decomposition of rice straw incorporation before wheat; the influence of different times of straw amendment was not investigated. Crop residue decomposition, being a function of time and site conditions, has to be studied under a particular set of conditions. The rate of mass loss was much slower after 35 d during the cropping season. This was mainly due to low winter temperatures during December–January (35–72 d) (Fig. 1) and to a deficiency of soil moisture in the surface soil layers in the latter part of the wheat-cropping season (February–April) when soil temperature increases (Fig. 1) and the surface layer surrounding the litterbags rapidly dries. Irrespective of time of incorporation, about 70% of rice residue was decomposed during the 5 to 6 mo that covers the wheat-growing season.

Nitrogen Concentration Versus Mass Loss
The N concentration of the rice residue increased continuously during the 190-d decomposition period (data not shown), indicating loss of C as CO₂ and/or N immobilization in the residue by microorganisms, which build up new microbial protein from plant and soil N. The N content of rice straw at the time of incorporation was 5.6 g kg^–1 (or 39.8 kg ha^–1), which increased to 14.8 g kg^–1 at the end of the study period. At the time of wheat sowing, residue N was significantly lower (7.0 g kg^–1) in the 10-d decomposition treatment than in the 20-d (7.9 g kg^–1) or 40-d (8.1 g kg^–1) treatment. After long periods of residue decomposition (122 d), differences between 10- and 20-d treatments became nonsignificant, while residue in the 40-d decomposition treatment still had a significantly higher N concentration (13.8 g kg^–1) than in the 10- and 20-d treatments (12.5–12.9 g kg^–1). At the end of the study (150 d after wheat sowing), the N concentration in rice residue was observed to be similar (14.8 g kg^–1) for the three treatments.

The N concentration in rice residue during the decomposition period followed a quadratic trend given below:

[6]

where Y = total N concentration (g kg^–1) in rice residue and X = total decomposition period (days).

The relationship between N concentration and mass loss was curvilinear (quadratic) (Fig. 3) . Burgess et al. (2002) also reported a curvilinear relationship between N concentration and mass loss for barley (Hordeum vulgare L.) straw. However, for wheat and sorghum (Sorghum bicolor Pers.) residues (with N content lower than rice residues), Schomberg et al. (1994) observed an inverse linear relationship between dry mass remaining and N concentration. These observations imply that the relationship between N and mass loss of residues is likely to be influenced by residue type, soil and environmental conditions, and duration of the study.

View larger version (8K): [in this window] [in a new window]   Fig. 3. Relationship between N concentration and mass loss of litterbag rice residue. Each data point represents mean of three replicates.

View this table: [in this window] [in a new window]   Table 4. Amount of N release during rice residue decomposition under field conditions (1992–1993).

View larger version (12K): [in this window] [in a new window]   Fig. 4. Relationship between N release and mass loss of litterbag rice residue. Each data point represents mean of three replicates.

View larger version (18K): [in this window] [in a new window]   Fig. 5. Effect of predecomposition period of rice straw on mineral N (NH4 + NO3) dynamics in soil amended with 100 mg N kg–1 and incubated at 75% field capacity moisture regime at 30°C. Values of LSD (0.05) for comparing treatment means at each sampling period are 5.94 for 10 d, 7.14 for 20 d, 5.46 for 30 d, 4.77 for 45 d and 4.25 for 60 d. Each data point represents mean of three replicates.

The results from the decomposition and N mineralization studies suggested that rice residue is likely to have little adverse effects on N availability in the soil when it is allowed to decompose under aerobic conditions for at least 10 d before sowing of the next crop. Release of K from the rice straw occurs quickly after incorporation into the soil.

Long-Term Field Experiment in the Rice–Wheat System
Grain Yields
Wheat grain yields without fertilizer N ranged from 2.1 to 2.9 Mg ha^–1 from 1993 to 2000 (Table 5). Fertilizer N (120 kg N ha^–1) application significantly increased yield regardless of straw management treatment. There were significant year x treatment effects on the yield of wheat. During the first year, wheat yield was lower in T6, (rice residue incorporated 10 d before wheat sowing) than in T2 (residues removed), suggesting immobilization. The other residue treatments, however, were not significantly different from T2 from 1993 to 1998. Another possible reason for the lower rice yield with rice straw incorporation at 10-d before sowing of wheat compared with straw being allowed to decompose for 20 d or more before planting of wheat could be the greater losses of fertilizer N via nitrification–denitrification. Significantly greater yields were observed in T4 in 1998–1999 and 1999–2000 and in T5 in 1999–2000 compared with T2 where rice residues were removed. Wheat yields were higher in plots (T8) where GM and wheat straw were incorporated in the preceding rice than when rice straw was incorporated (mean of T4–T8) during 1996–1997 only. Compared with residue removal (T2) or residue burning (T3), incorporation of rice residue 10 to 40 d before seeding wheat (T4–T7) did not show any adverse effect on wheat yield, but on average significantly increased yield in 1993–1994 and 1999–2000. Wheat yields in the 20- and 40-d treatments were, however, very close to yields in T8. These results are consistent with conclusions drawn from other decomposition and N mineralization studies. The application of 25% of fertilizer N as starter N at the time of residue incorporation (T7) showed some depression (0.1–0.4 Mg ha^–1) in wheat yield in all years compared with T5 (Table 5), although the differences were not significant. The results obtained from the laboratory incubation study clearly showed that N applied concurrently with straw incorporation gets immobilized and does not remineralize easily. Bijay-Singh et al. (2001) reported a decrease in ¹⁵N recovery by wheat when 25% of the total N dose was incorporated at the same time as rice straw incorporation. In our study, annual additions of 40 to 50 kg N ha^–1 through rice residue for 7 yr did not influence grain yield of wheat as the recommended split application of 120 kg N ha^–1 (one-half drilled at sowing and the remaining half topdressed at 21–25 d after sowing) was already applied to all the treatments in wheat.

Nitrogen Uptake and Nitrogen-Use Efficiency in Wheat
Total N uptake by wheat was not significantly affected by rice residue incorporation compared with straw removal or burning (data not shown). Recovery efficiency by the difference method decreased with application of starter N (T7) compared with all other treatments (Table 6). The trends in RE were similar to those observed for total N uptake by wheat (Table 6). The RE values for wheat ranged from 49 to 54% in different treatments, which are well within the acceptable range reported in the literature (Katyal et al., 1987). Physiological efficiency was not affected by different treatments, except that it was significantly higher in T7 (25% of the total N dose applied at residue incorporation) and T8 (GM and wheat residue incorporated in rice) than in T3 (residue burned) and T6 (residue incorporated at 10 d before wheat seeding) (Table 6). The higher PE values in T7 were due to the decrease in N uptake, which was not accompanied by a decrease in wheat yield. In T8, fertilizer N uptake was more efficiently translated into grain yield than in T2, T3, and T6. Like RE and PE, AE was significantly higher in T8 than in all other treatments and was lowest in T2, T6, and T7 (Table 6).

The incorporation of rice residue caused a smaller but more significant increase in available K content in the soil than did the residue removal treatments (Table 7). On average, rice residue added about 175 kg K ha^–1 annually in T4 through T8 and wheat residue added 65 kg K ha^–1 in T8. Despite such large additions, the increase in K availability in the soil was small. Our laboratory results predicted a large increase in K availability in soil amended with rice straw. One possible reason for the small increases observed in residue-amended plots may be the loss through leaching of a significant proportion of residue K during rice cultivation on this permeable soil. From a column study, Yadvinder-Singh et al. (2004) observed that up to 25% of the rice straw K can be leached below the 90-cm depth after 14 irrigations applied on sandy loam soil.

	CONCLUSIONS

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Because of serious environmental effects, crop residue burning is not desirable. Farmers will incorporate crop residues only if there are no yield losses or if there is a clear yield advantage over residue burning in the long run. The 7-yr data demonstrate that rice and wheat productivity is not adversely affected when rice residue is incorporated for at least 10 d and preferably 20 d before the establishment of the succeeding crop. Our study showed rapid rice residue decomposition in sandy soils of the IGP. Rice residue decomposition of about 25% during the prewheat fallow period was sufficient to avoid any detrimental effects on rice and wheat yields. The laboratory study shows no immobilization of fertilizer N when rice residues are incorporated at 20 d or more before fertilizer application. The incorporation of GM (narrow C/N) in combination with wheat residue (wide C/N) can be advantageous in mitigating the adverse effects of wheat residue on rice due to N immobilization and can increase yield and N-use efficiency in the following wheat crop. In addition, residue incorporation resulted in considerable increases in soil organic matter content and soil K supply, which would lead to favorable nutrient balances and improved yields. There is a need to fine-tune nutrient management for rice and wheat grown on soils into which crop residues are continuously incorporated. The adoption of this practice by farmers of the region will prove immensely useful because of reduced air pollution and recycling of nutrients.

Received for publication December 24, 2002.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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