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Published online 9 August 2007
Published in Soil Sci Soc Am J 71:1500-1507 (2007)
DOI: 10.2136/sssaj2006.0325
© 2007 Soil Science Society of America
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SOIL FERTILITY & PLANT NUTRITION

Yield and Phosphorus Transformations in a Rice–Wheat System with Crop Residue and Phosphorus Management

R.K. Guptaa, Yadvinder-Singha, J.K. Ladhab,*, Bijay-Singha, Jagmohan Singha, Gurpreet Singha and H. Pathakb

a Dep. of Soils, Punjab Agricultural Univ., Ludhiana, Punjab, India
b International Rice Research Institute–India Office, NASC Complex, Pusa, New Delhi, India

* Corresponding author: j.k.ladha{at}cgiar.org.


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Phosphorous deficiency limits productivity of rice (Oryza sativa L.)–wheat (Triticum aestivum L.) systems in the Indo-Gangetic Plains. Deterioration of soil and air quality due to straw burning is also a concern. Field experiments were conducted to determine the effects of straw and P management strategies on yield, P balance, and P transformations in soil in a rice–wheat system. Four treatments composed of different combinations of rice and wheat straw removal, burning, and incorporation were the main plots. Subplot treatments were P fertilization to wheat or to both rice and wheat and a no-P control. Wheat yield was similar where straw was burned in situ or removed. Incorporation of straw increased the wheat yield in Year 4. Significant straw x P management interactions, observed after 4 yr, suggested that residues can enhance yield under limited P supply situations. Application of 26 kg P ha–1 to wheat increased grain yield by 6 to 15% compared with no P. Rice yield did not respond to incorporation of residues or P fertilization. The P balance was negative with removal or burning of rice straw, but when both wheat and rice straw were incorporated, the balance was positive at the recommended P level (26 kg P ha–1 to wheat only). Changes in total soil P suggested that the two crops remove significant P from below 15 cm. Incorporation of residues increased soil Olsen, inorganic, and organic P; reduced P sorption; and increased P release. Data show that continuous incorporation of residues substituted for 13 kg inorganic P ha–1 yr–1 and improved system yield.

Abbreviations: IGP, Indo-Gangetic Plains • RW, rice–wheat


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Rice–wheat (RW) is a major cropping system covering about 10.5 million ha in the Indo-Gangetic Plains (IGP) of India and another 2.5 million ha in Bangladesh, Nepal, and Pakistan. The intensified RW system is showing a declining yield trend, particularly in rice (Ladha et al., 2003; Yadvinder-Singh et al., 2004a). Imbalanced nutrition and decreased soil organic matter content are the key factors responsible for the observed declining yield (Ladha et al., 2003). Recently, with the advent of mechanical harvesting of rice and wheat, crop residues left in the field are generally burned in situ by the farmers to avoid their hindrance to tillage and seeding operations for the next crop. Burning of residues is causing a serious waste of nutrient resources besides contributing to air pollution. Recycling of crop residues is being advocated to improve soil health in the RW system (Samra et al., 2003; Yadvinder-Singh et al., 2005; Pathak et al., 2006). Crop residues not only replenish soil organic matter, a key determinant of soil quality, but also supply essential plant nutrients (N, P, K, S, and micronutrients) when mineralized.

Phosphorus is an important plant nutrient needed for realizing optimum yields in the RW system (Yadvinder-Singh et al., 2000). While wheat needs a well-aerated seed zone, rice is grown under waterlogged conditions. The contrasting soil and water conditions required by the two crops make P management strategies different than for upland cropping systems. In most lowland rice soils, P availability initially increases on flooding (Ponnamperuma, 1972), and rice may meet its P requirement from the residual P applied to the preceding wheat. Drainage following submergence induces P deficiency in upland crops such as wheat following rice (Willet, 1979), mainly due to changes in the inorganic fractions. Studies conducted in northwest India have shown that rice can meet its P requirement from native and residual P from the recommended 26 kg P ha–1 applied to wheat in the system (Maskina et al., 1988). Yadvinder-Singh et al. (2000) showed, however, that this P management strategy may not be sustainable for obtaining high yields of rice and wheat on a long-term basis.

When P input from different sources is higher than P output due to crop removal, P accumulates in the soil. In less weathered subtropical soils, the excess P from fertilizer tends to accumulate as Ca-P (Aulakh et al., 2003; Guo et al., 2000). When the amount of P removed by crops exceeds the P input from all external sources, the deficit is expected to be met from the indigenous soil P pool if crop productivity is to be sustained. To arrest the decline in indigenous P, the P input should be similar to the P output; however, a better understanding of sources and sinks for P in soil is required to maintain this balance.

The availability of applied P is controlled by the sorption–desorption characteristics of the soil. Studies have shown that the addition of organic residues to acid soils improves P availability due to blocking of exposed hydroxyls on the surface of Fe and Al (Appelt et al., 1975), and competition of organic acids with phosphate ions for adsorbing sites (Misra et al., 1989). Incorporation of crop residues in an alkaline soil, however, results in greater P adsorption with a lower affinity coefficient than when it is removed or burned (Beri et al., 1995). Willet and Higgins (1978) and Phongpan (1989) reported that P adsorption increased with the addition of rice straw in several soils during the first week of incubation under flooded conditions. Nwuke et al. (2004) evaluated several crop residues as sources of P for corn (Zea mays L.) and noted that the effect of organic residues on P availability was variable and depended on soil characteristics as well as on the nature of the organic source. All residues increased resin-P compared with no residue, but the magnitude of increase was greater in soils with low P sorption capacity than in those with high P sorption capacity. Organic residues can also modify the availability of native soil P by the products of decomposition (Yadvinder-Singh et al., 1992; Nwuke et al., 2004). In waterlogged soils under rice, crop residues can increase the availability of indigenous P as a result of intense soil reduction (Yadvinder-Singh et al., 1988, 2005).

Concentrations of P in cereal residues are usually low, resulting in their inability to provide sufficient P for crop growth on incorporation (Yadvinder-Singh et al., 2005). In general, NaHCO3–extractable P decreased following the addition of crop residues with C/P >300 (Black and Reitz, 1972; Qui and Ding, 1986; Stevenson, 1986). Studies by McLaughlin et al. (1988) showed that only 5.4% of legume (Medicago truncatula L.) residue P was recovered by wheat plants and a major fraction (40%) of residue P was assimilated into microbial biomass. In a RW cropping system, Hundal and Thind (1993) reported that incorporation of wheat straw depressed labile P and dissolved P but enhanced organic P content during the initial stages of plant growth. In a soil where rice and wheat residues had been incorporated for 11 yr, the soil maintained a higher concentration of HCO3–extractable P during a 90-d incubation period than soils with residue removed or burned (Beri et al., 1995).

Continuous recycling of crop residues may increase P availability in the soil but how the crop residues affect soil P availability and transformation is largely unclear. This is particularly true for the role of crop residues in P nutrition of the RW cropping system in the IGP of South Asia. The objective of this study was to determine the effects of crop residue and P management strategies on crop yields, selected soil parameters, and P dynamics and balances in soil–plant systems.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experimental Site
A field experiment was conducted for 4 yr (2001–2005) at the experimental 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 area receives an average of 700 mm of annual rainfall, about 80% of which occurs from June to September. The mean minimum and maximum temperatures during the rice growing season (July–October) were 18 and 35°C, whereas during the wheat growing season (November–April), they were 6.7 and 22.6°C, respectively. The climate of the region is subtropical, semiarid. The soil of the experimental field was a Typic Ustochrept loamy sand (110 g kg–1 clay and 810 g kg–1 sand) with pH 7.26, cation exchange capacity of 8.54 cmolc kg–1, 3.8 g kg–1 organic C, 4.3 mg kg–1 of 0.5 mol L–1 NaHCO3–extractable P, and 43 mg kg–1 of 1 mol L–1 NH4OAc-extractable K. Total P content in the soil was 222 mg kg–1. Bulk density of the 0- to 15- and 15- to 30-cm layers, measured using cores (Blake and Hartage, 1986), was 1.59 and 1.64 Mg m–3, respectively. Before the initiation of the experiment, the site had been under a RW cropping system for the previous 20 yr.

In November 2001, the experiment was laid out in a split-plot design with four residue management treatments (S1, rice and wheat straws removed; S2, rice straw burned, wheat straw removed; S3, rice straw incorporated, wheat straw removed; and S4, rice and wheat straws incorporated) in main plots and four P management treatments (P1, no P to either rice or wheat; P2, 26 kg P ha–1 to wheat only; P3, 26 kg P to wheat and 13 kg P ha–1 to rice; and P4, 39 kg P ha–1 to wheat only) in the subplots. The treatments were replicated thrice with a plot size of 9 by 3 m2.

Conventional tillage for wheat involved two passes with a disk and two passes with a spring-tine harrow followed by leveling. Immediately after rice harvest, straw was incorporated in situ in the designated plots (Treatments S3 and S4) with two passes of a disk followed by irrigation to enhance decomposition. The straw was allowed to decompose for about 20 d before sowing of wheat to offset any adverse effects on N immobilization and seed germination (Yadvinder-Singh et al., 2004b). At the same time, rice straw was burned after spreading on the soil surface of the predesignated plots (Treatment S2). Wheat cultivar PBW 343 was sown in the second week of November and harvested in the third week of April the following year. Nitrogen (120 kg N ha–1) was applied as urea in two equal splits at sowing and at 21 to 25 d after sowing before the application of the first irrigation at crown root initiation stage. Wheat received four to five irrigations. The entire amounts of P as single super phosphate and 25 kg K ha–1 as KCl were drilled below the seed at sowing.

After harvest, wheat straw was removed from all plots except the S4 treatment, where it was incorporated into the moist soil about 4 wk before transplanting of rice. One wet plowing in standing water followed by two harrowings to puddle the soil was done in all plots. Transplanting of rice was done manually in the second week of June. About 30-d-old rice seedlings (cultivar PR 116) were transplanted at 15-cm interrow and 20-cm intrarow spacing. Irrigation was applied to maintain a 4-cm depth of standing water during the first 2 wk after transplanting and between saturation and flooding, with up to 5 cm of standing water thereafter until 10 to 12 d before physiological maturity. Rice received a uniform dose of 120 kg N ha–1 as urea applied in three equal splits at transplanting, and 3 and 6 wk after transplanting. Treatment-dependent rates of P, uniform doses of 25 kg K ha–1, and 5 kg Zn ha–1 (as ZnSO4) were incorporated into the soil at the last puddling operation. At maturity, a net area of 15 m2 from the center of each plot was harvested in the second week of October for recording grain and straw yields.

Soil and Plant Sampling and Analysis
Soil samples were collected periodically (after wheat in 2003/2004 and 2004/2005, and after rice in 2005) from each subplot within 10 d after crop harvest from the 0- to 15-cm soil depth using a 5-cm-diameter auger. Each sample was a composite from three locations in a subplot. The fresh soil samples were mixed thoroughly, air dried, crushed to pass through a 2-mm sieve, and stored in sealed plastic jars for analysis. Soil organic C was analyzed by the Walkley and Black (1934) method and available P was determined using the NaHCO3 extraction method (Kuo, 1996).

At maturity, subsamples of grain and straw were collected from each plot and dried in a hot-air oven at 65°C for 3 d. Aboveground biomass was determined by harvesting a 1-m2 area within selected plots at 92 d after sowing during 2004/2005. Plant samples were ground to pass through a 0.5-mm sieve. This was followed by digestion in a triple acid mixture (HNO3–H2SO4–HClO4). An aliquot of the digest was withdrawn and neutralized with NaOH before P analysis using the procedure of Murphy and Riley (1962).

Phosphorus Balance
The P inputs from irrigation and rainfall, and output from leaching, were considered negligible (Dobermann et al., 1998). Phosphorus in rice and wheat straw was measured in this study. The loss of P during in situ burning of rice straw was taken as 5.5% (Sharma and Mishra, 2001). The P input–output balance was calculated for each crop as

Formula
where FP is fertilizer P input, GP is P accumulation in grain, SP is P accumulation in straw, and SR is the fraction of straw removed from the field. The amount of P uptake in grain and straw was calculated by multiplying the respective yields with P concentrations.

Sorption and Release of Phosphorus
Two single-point soil P sorption characteristics were assessed by adding 20 mL of 0.01 mol L–1 CaCl2 solution containing different amounts (20 and 50 mmol L–1 P kg–1) of P (KH2PO4) solutions to 2 g of air-dried soil in a centrifuge tube (Bache and Williams, 1971). Two drops of toluene were added to inhibit microbial activity. The samples were shaken on a reciprocal shaker for 20 h. Following centrifugation at 3000 rpm for 10 min, the equilibrium P concentration in solution was measured by the method of Murphy and Riley (1962). The P that disappeared from the solution was regarded as the adsorbed P. To estimate P release, soil was equilibrated with 0.01 mol L–1 CaCl2 at 1:3 (w/v) for 1 h. Samples were centrifuged at 1500 rpm for 10 min. The extracts were then filtered through Millipore filter paper (<45 µm) and P was measured by the method of Murphy and Riley (1962).

Soil Phosphorus Fractions
The soil organic P fraction was extracted using a sequential extraction procedure with concentrated H2SO4 and 0.5 mol L–1 NaOH as described by Bowman (1989). For estimation of total P, acid and base extracts were digested in the presence of 1 g of K2SO4 and 2 mL of 5.5 mol L–1 H2SO4 at 150°C for 25 min. For determination of inorganic P in the acid and base extracts, five drops of p-nitrophenol were added to adjust the pH with 2 mol L–1 NaOH until the indicator color changed. The P concentration in the digest or solution was measured following the method of Murphy and Riley (1962). The organic P fraction was calculated as the difference between total P and inorganic P contents.

Data Analysis
The data were analyzed using the split-plot technique with IRRISTAT Version 5.0. Combined analysis was performed across years for grain yield and P uptake of rice and wheat, keeping years as the third component of the analysis. Unless indicated otherwise, differences were considered significant only for P ≤ 0.05.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Grain Yields
Straw management had no effect on wheat yield up to Year 3, although yield tended to increase with incorporation after Year 2 (Fig. 1a ). When yields from treatments with both the straws incorporated (S4) and removed (S1) were compared, the increase was 6, 11, 19, and 17% in the 4 consecutive yr, respectively. In Year 4, yield was 24% higher when only rice straw (S3) was incorporated than when it was removed (S1). Annual additions of P through wheat and rice straw averaged 5 and 8 kg ha–1, respectively (Table 1). The recovery of P from a single application of crop residues with wide C/P ratio is likely to be small. Even in the case of legume residues, recovery of P by wheat was only 5.4% of the residue P (McLaughlin et al., 1988). There was no difference in wheat yield between the treatments with rice straw burned (S2) and removed (S1). These results confirm the findings of Verma and Bhagat (1992) and Yadvinder-Singh et al. (2004b), which showed that incorporation of rice straw for 10 to 40 d before seeding wheat, compared with residue removed or residue burned, did not have any adverse effect of N immobilization or wheat yield depression. Yadvinder-Singh et al. (2005) observed adverse effects of straw incorporation on the yield of a following crop, however, particularly when planting was done immediately after straw incorporation. The grain yield of wheat in the treatment with incorporation of both rice and wheat straw (S4) was similar to that of the treatment with incorporation of rice straw alone (S3). Yadvinder-Singh et al. (2004a) observed no residual effect of wheat straw incorporation in rice on the grain yield of the following wheat.


Figure 1
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  		Fig. 1. Effects of straw management and P application on grain yield of wheat and rice in a rice–wheat system. Treatments were S1, rice and wheat straws removed; S2, rice straw burned, wheat straw removed; S3, rice straw incorporated, wheat straw removed; S4, rice and wheat straws incorporated; P1, no P to either rice or wheat; P2, 26 kg P ha–1 to wheat only; P3, 26 kg P to wheat and 13 kg P ha–1 to rice; P4, 39 kg P ha–1 to wheat only.

 

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  		Table 1. Mean (across years) dry matter, N, P, K, and C additions from wheat and rice straw.

 
Wheat responded to applied P in all 4 yr (Fig. 1b). Application of 26 kg P ha–1 to wheat (P2 and P3) increased grain yield by 6 to 15% compared with no P (P1). Since the soil was deficient in NaHCO3–extractable P, response to P fertilization was expected.

There was no residual effect of rice straw management in the previous wheat crop on the grain yield of rice up to Year 3 of the experiment (Fig. 1c). In Year 4, the treatments with rice straw burned (S2) and only rice straw incorporated (S3) had 11.3 to 14.5% higher yield than with straw removal (S1). Incorporation of both rice and wheat straws (S4), however, resulted in lower yield of rice (Fig. 1c). Yadvinder-Singh et al. (2004a) reported that incorporation of wheat straw adversely affected grain yield of rice because of N deficiency caused by immobilization.

Rice failed to respond to direct or residual P application during the first 3 yr of the study (Fig. 1d). Flooding the soil released sufficient P to meet the P requirements of rice. In Year 4, the treatments with P applied to previous wheat (P2, P3, and P4) had higher yields than the no-P treatment (P1). Continuous rice–wheat cropping without application of P fertilizer to wheat and rice for 3 yr resulted in P deficiency in rice, leading to a significant response to P fertilization.

In 2004/2005, wheat plant samples collected at 36 d after sowing contained a higher P concentration (3.7 g kg–1) in the S4 treatment than in S2 (3.5 g kg–1) or S1 (3.0 g kg–1) (data not shown). The P concentration increased from 2.7 g kg–1 in the no-P control (P1) to 3.6 to 3.8 g kg–1 in P-fertilized treatments. There were significant straw x P fertilizer effects on the P concentration of wheat plants. The P concentration in wheat was higher in S3 than in S2, except with no P fertilizer, where the reverse was true. The data on P concentration in wheat plants support our hypothesis that regular incorporation of crop residues increases P availability in soils.

Wheat yield was highest in 2001/2002 (5.0 Mg ha–1) followed by 2004/2005 (4.7 Mg ha–1) (Fig. 1a). Yields were lower in 2002/2003 and 2003/2004 (4.1 Mg ha–1). The statistical analysis across all years showed that year x P fertilizer and straw x P fertilizer interactions were significant (Table 2). The effects of straw and P fertilizer on wheat yield averaged across the 4 yr were also significant. Grain yield was high when either rice straw alone or both rice and wheat straw were incorporated. Yield in the treatment with rice straw burned (S2) was higher than that in the straw-removal treatment (S1), but was lower than the treatment with rice straw incorporated (S3). Wheat yield (averaged across four seasons) increased with the application of P fertilizer (Table 2, Fig. 2 ). The straw management effect differed with P fertilizer treatment, thus giving a significant straw x P fertilizer interaction. In the P4 treatment, wheat yield did not differ among straw management options (Fig. 2). Treatments with straw incorporation (S3 and S4), however, gave higher wheat yields than those with straw removal or burning at low P levels (P1 and P2). Incorporation of rice and wheat straw (S4) along with the application of 26 kg P ha–1 produced wheat yields similar to those obtained with the application of 39 kg P ha–1 (P3 and P4) when both the straws were removed from the field (Fig. 2). This suggested that continuous return of both rice and wheat straws to the field can save 13 kg P ha–1 in the RW system.


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  		Table 2. Combined statistical analysis across all years for grain yield and total P uptake by rice and wheat at harvest.

 

Figure 2
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  		Fig. 2. Interaction effects (pooled across 4 yr) of straw management and P application on grain yield of wheat in a rice–wheat system. Treatments were P1, no P to either rice or wheat; P2, 26 kg P ha–1 to wheat only; P3, 26 kg P to wheat and 13 kg P ha–1 to rice; P4, 39 kg P ha–1 to wheat only.

 
Phosphorous Uptake
Total uptake of P by wheat ranged from 14.5 to 21.5 kg ha–1 and was not influenced by the straw management options during the first 2 yr (Fig. 3a ). Incorporation of rice straw in wheat (S3 and S4), however, increased P uptake compared with straw removal (S1) in Years 3 and 4. Phosphorus application increased P uptake from 15.7 kg ha–1 with no P (P1) to 18.4 kg ha–1 in the P2 treatment (Fig. 3b). Wheat yield and plant P uptake generally followed similar trends for straw and P fertilizer treatments within each year (Fig. 1 and 3). Despite low wheat yields in 2003/2004, however, plant P uptake was similar to that in 2001/2002.


Figure 3
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  		Fig. 3. Effects of straw management and P application on total P uptake at harvest of rice and wheat in a rice–wheat system. Treatments were S1, rice and wheat straws removed; S2, rice straw burned, wheat straw removed; S3, rice straw incorporated, wheat straw removed; S4, rice and wheat straws incorporated; P1, no P to either rice or wheat; P2, 26 kg P ha–1 to wheat only; P3, 26 kg P to wheat and 13 kg P ha–1 to rice; P4, 39 kg P ha–1 to wheat only.

 
Rice P uptake, ranging from 21.0 to 30.4 kg ha–1, was not influenced by straw management in the initial 3 yr (Fig. 3c). Rice yield and plant P uptake followed similar trends for straw treatments (Fig. 1 and 3), but fertilizer P significantly increased P uptake without a corresponding increase in yield, particularly during 2002 and 2004. In Year 4, total P uptake was higher with rice straw incorporation (S3) than with straw removal (S1). Phosphorous application to the preceding wheat crop increased P uptake by rice significantly in 2 out of 4 yr. Application of 13 kg P ha–1 to rice (P3) did not result in a further increase in P uptake.

Combined analysis of plant P uptake data across years showed significant year effects in both rice and wheat (Table 2). Mean P uptake by wheat ranged from 18.0 kg ha–1 in 2002/2003 to 20.3 kg ha–1 in 2001/2002; in rice, it ranged from 24.7 kg ha–1 in 2003 to 26.7 kg ha–1 in 2002 and 2005. Straw management significantly influenced P uptake by wheat but no effect was observed in rice. Application of P fertilizer increased plant P uptake in both wheat and rice. Plant P uptake (mean for 4 yr) in wheat increased from 16.8 kg ha–1 in the no-P control (P1) treatment to 20.7 kg ha–1 in the P4 treatment.

Soil Fertility
Phosphorus Balance, Bicarbonate-Extractable Phosphorus, and Soil Phosphorus Fractions
In the treatment with removal of both rice and wheat straws (S1) and application of recommended P (P2), average P removal was 23.0 kg ha–1 yr–1 in rice and 15.3 kg ha–1 yr–1 in wheat, which exceeded the P input by 12.3 kg P ha–1 yr–1 (Table 3). Net removal of P in this treatment, compared with the treatment with no fertilizer P, represented a small percentage of the fertilizer P added (7%) during the period of this study. In the P1 treatment, P removed by the two crops was 36.5 kg P ha–1 yr–1. Thus a soil containing total P of 222 mg kg–1 in the surface 0- to 15-cm layer was able to supply a consistent amount of P to rice and wheat during the 4-yr period. The continuous P deficit would, however, eventually deplete soil P in the surface layer, resulting in a poor crop yield. In the P3 and P4 treatments, P removal by the two crops exceeded the amount of P applied by about 3 kg P ha–1 yr–1. When rice straw was burned (S2) in situ, the P balance changed substantially compared with straw removal (S1).


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  		Table 3. Effect of residue management and P fertilizer on P balance in a rice–wheat system.

 
In Treatment S3 at the P level of P2, the P balance was negative (-8.5 kg P ha–1 yr–1), which further decreased to -12.3 kg P ha–1 yr–1 when both straws were removed (S1) (Table 3). In the P3 and P4 treatments, P additions exceeded the removal by 3 to 4 kg P ha–1 yr–1. When both wheat and rice straws were incorporated (S4), the P balance at the recommended P level (P2) became only slightly negative. At the P3 and P4 levels, P additions exceeded removal by about 10 kg P ha–1 yr–1. The calculated P balance suggested that the recommended P level of 26 kg P ha–1 in wheat and no P in rice (P2) may not sustain the present yield level of 4.5 Mg ha–1 wheat and 7.0 Mg ha–1 rice, and we need to apply higher P levels (P3), particularly when wheat straw is completely removed from the fields (S1, S2, and S3). To meet the food needs of an increasing population in Asia, however, our aim will be to enhance yields from the present level, requiring even higher inputs of P. Attainable 6 Mg ha–1 wheat and 8.0 Mg ha–1 rice yields would require a total P uptake of about 50 kg P ha–1 yr–1. In a previous study, Yadvinder-Singh (2000) concluded that despite inconsistent P response by rice, the strategy of no P application to rice is likely to be nonsustainable.

The net P balance after four cropping cycles followed similar trends but the magnitude varied widely (Table 3). The changes in total P in the surface (0–15 cm) soil was measured, which showed a large positive P balance compared with the P balance calculated from plant P uptake data. The most plausible reason could be uptake of a large amount of soil P from layers below 15 cm by rice and wheat. While rice roots can penetrate up to 40 or 50 cm, wheat roots are known to penetrate up to 2 m in these alluvial soils of the IGP (Gajri and Prihar, 1985). The changes in total P in the surface soil layer therefore may not be close to the P balance computed from plant P uptake and addition from fertilizers. Another source of P addition in the surface layer could be the canal water used for irrigation. It carries a significant amount of silt, particularly during the monsoon season when rice is grown. Rice alone requires 20 to 25 irrigations, of which more than half are with canal water.

Bicarbonate-extractable soil P was influenced by straw management and P fertilization (Table 4). It was higher when rice straw was either burned (S2) or incorporated (S3 and S4), compared with its removal (S1) after 4 yr. The NaHCO3–extractable P increased by 40% when 26 kg P ha–1 was applied to wheat (P2), compared with the no-P treatment (P1); however, small but significant increases in P availability in the soil did not result in yield increases. Contrary to our results, Surekha et al. (2003) observed no effect of rice straw management on available soil P after 2 yr.


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  		Table 4. Effect of crop residues and P fertilizer on bicarbonate-extractable soil P after wheat in 2003/2004 and 2004/2005, and P fractions in soil after wheat in 2004/2005.

 
The P fractionation study revealed that H2SO4–extractable inorganic P constituted the majority of the total P (75%) in the surface layer (Table 4). In the less weathered, subtropical soils used in this study, the primary Ca-P mineral tends to be predominant and could be a source of available P (Aulakh and Pasricha, 1991; Guo et al., 2000). Inorganic P content in the soil increased with rice and wheat straw incorporation (S4) and P fertilization (Table 4). As expected, incorporation of both rice and wheat straw (S4) caused the maximum increase in inorganic P, followed by either rice (S3) or wheat straw incorporated (S4) or burned (S2). In the P-fertilized plots, the increase in inorganic P was related to the level of P application.

In unfertilized plots, soil organic P remained unchanged, indicating a small contribution toward NaHCO3–extractable P. This could be due to either the soil already containing a high amount of P from past fertilization or a highly recalcitrant nature of organic P in the soil. Soil organic P increased with straw incorporation (S3 and S4), compared with straw removal (S1) or burning (S2), after 4 yr (Table 4).

A significant reduction in inorganic P was accentuated in the surface layer from an initial level of 170 mg kg–1 to a level of 136 mg kg–1 when both rice and wheat straws were removed (S1) and no fertilizer P was applied to rice and wheat (P1). The reduction in inorganic P in the surface soil with insufficient P fertilization relative to the P uptake of crops could be perceived as Ca-P being a source of available P in this soil. In essence, NaOH-extractable inorganic P acted as a source of available P in P-deficit situations and as a sink in P-sufficient situations. Total P increased with continuous use of fertilizer P at the P3 level. Crop residue incorporation also resulted in increased total P content in the soil (Table 4).

Phosphorous Fixation and Release
From Langmuir P sorption isotherms of several Swedish soils, Borling et al. (2001) demonstrated that two indices derived from single-point P additions were well correlated with maximum P sorption capacity. The values of P sorption at both 20 mmol P kg–1 (PSI-1) and 50 mmol P kg–1 (PSI-2) were significantly reduced with the incorporation of crop residues compared with removal of residues (Table 5). Rice straw burning (S2) had small effects on P sorption. The P sorption was higher at PSI-1 than at PSI-2, irrespective of straw management practices. Phosphorous sorption was lower in P-fertilized than unfertilized treatments because P already occupied some of the sorption sites in the former treatment. The magnitudes of P sorption in this study were three to four times higher than those reported by Borling et al. (2004) for Swedish soils. The high P sorption capacity may be explained by the continuous intensive cropping or the presence of high contents of Fe and Al hydroxides resulting from alternate wetting and drying of the soil. The labile P determined by extraction with 0.5 mol L–1 NaHCO3 was quite low (4–7 mg kg–1) in the soil.


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  		Table 5. Effect of crop residues and P fertilizer on sorption and release of P following addition of P at two concentrations.

 
Phosphorous release was 21 to 32% higher with straw incorporated (S3 and S4) or burned (S2) than when it was removed (S1) (Table 5). The P release rate did not differ between the treatments with incorporation of rice straw alone and both rice and wheat straws, suggesting that rice straw played a major role in enhancing P release. The P release rates were several times higher in P-fertilized treatments (P2 and P3) than in the unfertilized treatment (P1).


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
This study showed that short-term incorporation of rice and wheat residues has no negative effect on crop yield while the trends point toward long-term positive effects through gradual improvement in the availability of macro- and micronutrients. An integrated use of residue and mineral P fertilizer improved soil P dynamics and maintained a positive soil P balance. Changes in total soil P suggested that the two crops remove a significant amount of P from soil layers below 15 cm. Incorporation of residues increased soil Olsen, inorganic, and organic P; reduced P sorption; and increased P release, thereby supplying a considerable amount of P to the crops. The continuous incorporation of crop residues will help save at least 13 kg P ha–1 yr–1 in a RW system. Moreover, avoidance of residue burning will have an immediate positive effect by reducing the adverse environmental effect and decreasing the load of greenhouse gases in the atmosphere.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication September 15, 2006.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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