Effects of Residue Decomposition on Productivity and Soil ... - ENVIS

Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.

Effects of Residue Decomposition on Productivity and Soil Fertility in Rice–Wheat Rotation Yadvinder-Singh, Bijay-Singh, J. K. Ladha,* C. S. Khind, T. S. Khera, and C. S. Bueno ABSTRACT

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;

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.

R

ice–wheat is a major crop rotation in the IndoGangetic 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, Yadvinder-Singh, Bijay-Singh, J.K. Ladha, C.S. Khind, and T.S. Khera, Dep. of Soils, Punjab Agricultural University, Ludhiana, 141 004, India; C.S. Bueno, Crop, Soil, and Water Sciences Division, IRRI DAPO Box 7777, Metro Manila, Philippines. Received 24 Dec. 2002. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 68:854–864 (2004).  Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA

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

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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-a`-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 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.

Rice Residue Decomposition and Nitrogen Release In Situ We studied the in situ decomposition and N release dynamics for incorporated rice residue collected from an adequately

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.

fertilized crop using the litterbag decomposition technique (Beare et al., 2002) under different times of incorporation (10, 20, and 30 d before sowing of the wheat crop) in 1992–1993. Stems and leaves of mature rice straw that had an initial elemental composition of 5.6 g N kg⫺1 and 410 g C kg⫺1 (C/N, 73:1) were collected from the previous rice crop planted before wheat, dried at 60⬚C and cut into 2-cm pieces. A plant sample of 15 g was placed in nylon mesh bags (10 by 15 cm, 1-mm mesh). Litterbags (15 for each treatment, five sampling dates by three replicates) were randomly assigned to treatment plots. Sealed nylon bags were placed horizontally at the 10to 12-cm depth in the designated plots in the field (described above) starting from 8 Oct. 1992. The position of each nylon bag in the plots was marked with a nylon thread tied to a wooden stick. All litterbags in the plots were carefully removed before cultivation for wheat seeding, and stored in the laboratory for 3 d. The litterbags were returned to the same plots just after wheat seeding. The wheat crop received the recommended fertilizer doses (120 kg N ha⫺1, 26 kg P ha⫺1, and 50 kg K ha⫺1). The litterbags were collected at regular intervals (at wheat sowing and at 35, 72, 122, and 150 d after


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sowing) from cultivation (15 Nov. 1992) through harvest of the wheat crop (17 Apr. 1993). The straw remaining on each sampling date was removed from the litterbag, shaken gently over a sieve (1 mm), and spray-rinsed to remove the adhering soil. The residue samples were oven-dried in paper bags at 60⬚C for 48 h, weighed, and then ground to pass through a ⬍1-mm sieve. The loss in weight was assumed to be the amount of residue that decomposed during that period. The amount of N released from the rice residue was calculated as the difference between initial residue N input (assuming an initial residue input of 7.1 Mg ha⫺1 containing 39.8 kg N ha⫺1) and residue N recovered (calculated from the actual mass of rice straw and its N content) at different periods after incorporation.

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 (NH4⫹ and

NO3⫺) 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. In treatment T8, rice straw (20 d) was incorporated 20 d before planting wheat. For the following rice crop, loose wheat straw was uniformly distributed 10 to 12 d after wheat harvest. The field was flood-irrigated and the wheat straw was disked into the soil under optimum field moisture conditions. The same day, Sesbania cannabina L. (GM) seeds previously soaked overnight in water were spread on the soil surface at 50 kg ha⫺1 and mixed into the soil with the help of a disc harrow (Table 2). The sesbania crop received four to five irrigations. The green biomass of 50- to 55-d-old sesbania was

Table 1. Treatment details and rates of N added through wheat residue, rice residue, green manure, and fertilizer N. N addition Treatment Treatment

Rice

Rice Wheat

WR†

GM‡

Wheat Fertilizer§

RR¶

Fertilizer

0 0 0 44 44 44 44 44

0 120 120 120 120 120 120 120

ha⫺1

T1 T2 T3 T4 T5 T6 T7 T8

FN120 # FN120 FN120 FN120 FN120 FN120 FN120 WR-I ⫹ GM ⫹ FNx##

RR-R FN0†† RR-R FN120 RR-B FN120‡‡ RR-I (40 d) FN120§§ RR-I (20 d) FN120 RR-I (10 d) FN120 RR-I (20 d) FN120¶¶ RR-I (20 d) FN120

0 0 0 0 0 0 0 33

0 0 0 0 0 0 0 94

kg 120 120 120 120 120 120 120 26

† WR ⫽ Wheat residue. ‡ GM⫽ Green manure. § Inorganic fertilizer. ¶ RR ⫽ Rice residue. # FN ⫽ Fertilizer N (subscript is kg ha⫺1). †† RR-R ⫽ Rice residue removed. ‡‡ RR-B ⫽ Rice residue burned. §§ RR-I ⫽ Rice residue incorporated (values in parentheses indicate the predecomposition period before seeding wheat, in d). ¶¶ 30 kg fertilizer N ha⫺1 applied at the time of residue incorporation. ## 120 kg N ha⫺1 applied through GM (average 94 kg N ha⫺1) and the balance through urea (average 26 kg N ha⫺1).

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Table 2. Calendar of different field operations, crop cultivars, and incorporation of green manure and crop residues in rice-wheat rotation.


Operation Incorporation of wheat straw Green manure Seeding Harvest and incorporation Rice Flooding and puddling Transplanting Cultivar First fertilizer N Second fertilizer N Third fertilizer N Harvest Incorporation of rice straw 40 d 20 d 10 d Wheat Seeding Cultivar First fertilizer N Second fertilizer N Harvest

Year 1

Year 2

Year 3

Year 4

Year 5

Year 6

Year 7

–†

23 Apr

21 Apr

26 Apr

27 Apr

29 Apr

29 Apr

– –

23 Apr 12 Jun

21 Apr 18 Jun

26 Apr 18 Jun

27 Apr 20 Jun

29 Apr 23 Jun

29 Apr 21 Jun

16 Jun 17 Jun PR 111 19 Jun 8 Jul 29 Jul 7 Oct



19 Jun 20 Jun PR 111 22 Jun 11 Jul 1 Aug 2 Oct




9 Oct 29 Oct 8 Nov

10 Oct 30 Oct 9 Nov

6 Oct 26 Oct 5 Nov

3 Oct 23 Oct 2 Nov

8 Oct 28 Oct 7 Nov

10 Oct 30 Oct 9 Nov

2 Oct 22 Oct 1 Nov

18 Nov HD2329 18 Nov 12 Dec 16 Apr



12 Nov PBW343 12 Nov 5 Dec 18 Apr




† Not included in the study.

incorporated into the soil using a disc harrow 1 to 2 d before transplanting rice seedlings (Table 2). 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

RE (%) ⫽ Total N uptake (kg N ha⫺1) of the treatment ⫺ total N uptake (kg N ha⫺1) of the control ⫻ 100 Applied N (k N ha⫺1) of the treatment

[1]

Physiological efficiency (PE) of added N was calculated as

Table 3. Mean (across years) dry matter biomass and N, P, K, and C additions from wheat residue, rice residue, and green manure. Nutrient addition Organic material

Dry matter biomass

N

ha⫺1

Wheat residue Rice residue Green manure

Mg 7.3 (7.1–7.9)‡ 7.0 (5.5–7.9) 3.5 (2.8–5.0)

P

K

C addition†

65 (56–81) 165 (127–211) –

Mg ha⫺1 2.99 2.87 1.54

ha⫺1

33 (32–37) 45 (35–56) 94 (79–122)

kg 5 (4–7) 10 (8–12) –§

† Carbon additions calculated by assuming 41% C in wheat straw and rice straw and 44% in green manure (Aulakh et al., 2001). ‡ Values in parentheses are ranges for different years of the study. § P and K additions through GM not considered.


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PE (kg grain kg⫺1 N uptake) ⫽ Grain yield (kg ha⫺1) of the treatment ⫺ grain yield (kg ha⫺1) of the control Total N uptake (kg N ha⫺1) of the treatment ⫺ [2] total N uptake (kg N ha⫺1) of the control Agronomic efficiency (AE) of added N was calculated as

AE (kg grain kg⫺1 N applied) ⫽ Grain yield (kg ha⫺1) of the treatment ⫺ grain yield (kg ha⫺1) of the control Applied N (kg N ha⫺1) of the treatment

[3]

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

Efficiency of organic C sequestration in SOC ⫽ OC (Mg C ha⫺1) of the treatment ⫺ OC (Mg C ha⫺1) of the control Added C (Mg C ha⫺1) of the treatment

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). The data for all three decomposition periods could be best described using a single logarithmic equation: Y ⫽ 14.9ln(X) ⫺ 8.74, R 2 ⫽ 0.951 (n ⫽ 15)

[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 ⱕ0.05 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 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

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

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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: Y ⫽ ⫺0.0001X2 ⫹ 0.067X ⫹ 6.16, R2 ⫽ 0.969 (n ⫽ 15)

[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. Rice residue appeared to have a brief initial period of N release after incorporation (Table 4). The amount of N release was 2 to 4 kg N ha⫺1 when residues were decomposed for 10 to 20 d, which further increased to 7.9 kg N ha⫺1 (19.8% of the residue N applied) when decomposition period was extended to 40 d. The total amount of N released under different decomposition treatments during the whole period of study (190 d) ranged from 6 to 9 kg N ha⫺1. Residues go through several phases in their decomposition, with N dynamics related to stage or extent of mass loss. Even low N residues appeared to have a brief initial period of N release after placement, in agreement with data presented in other studies (Christensen, 1986; Burgess et al., 2002). Since a certain fraction of initial residue components is water-soluble, some initial N (and C) losses can occur because of leaching (Christensen, 1986; Parsons et al., 1990) if the water-soluble portion is exposed to sufficient precipitation or soil water movement. However, under field conditions with intact (rather than ground) residue, not all such material is necessarily available for leaching (Havis and Alberts, 1993). The initial period of N loss was generally followed by a period of increasing or relatively unchanging residue N

Fig. 3. Relationship between N concentration and mass loss of litterbag rice residue. Each data point represents mean of three replicates.

content (Table 4), and this trend has also been reported earlier by Christensen (1986). Although N concentration in residue increased with time, N release remained nearly unchanged during the course of the study. The rice residue either did not seem to have decomposed sufficiently to have considerable N release or, after an N release from the residue, immobilization of soil N occurred. Thus, net apparent N release measured was low. In the Canterbury region of New Zealand, Beare et al. (2002) reported that ⬍5 kg N ha⫺1 was released from decaying barley straw over a period of 18 mo. Using 15N-labeled rice residue, Yoneyama and Yoshida (1977) reported that 8% of the N in the rice leaf sheath (8 g N kg⫺1) was mineralized in 30 d at 30⬚C under upland conditions in the laboratory. Schomberg et al. (1994) and Burgess et al. (2002) reported net N release by low-N residues, wheat (and maize [Zea mays L.]) after 50 to 60% mass loss. In our study, despite a substantial mass loss of 69% residue, N release was small. Nitrogen release from rice residue during the decomposition period was linearly related to mass loss (Fig. 4). The relationship showed that for every 10% increase in mass loss, there was about 2.75% (1.07 kg N ha⫺1) release from the applied residue N. The actual values of N release from the rice residue may differ from that calculated in the present study as part of the N may be assimilated by soil microbial biomass. Table 4. Amount of N release during rice residue decomposition under field conditions (1992–1993).† Time of Incorporation (DBS)‡ 40 20 10

During fallow phase 7.9 a§ 3.5 b 2.3 b

During wheat cropping phase (after seeding) 0–35 8.6 a 4.6 b 5.5 b

0–72 kg 7.7 a 4.5 b 4.0 b

0–122

0–150

ha⫺1 6.7 a 5.0 b 6.3 a

9.2 a 7.3 b 5.9 b

† Total rice residue input assumed to be 7.1 Mg ha⫺1, which contained 39.8 kg N ha⫺1 at the time of incorporation. ‡ DBS ⫽ days before seeding wheat. § Means in a column followed by the same letter are not significantly different by Duncan’s multiple range test (P ⫽0.05).


860


Fig. 4. Relationship between N release and mass loss of litterbag rice residue. Each data point represents mean of three replicates.

Nitrogen and Potassium Mineralization Dynamics for Rice Residue under Laboratory Conditions Soil mineral N (NH4 ⫹ NO3) at 10 d after fertilizer application was significantly lower in treatments in which rice straw was incorporated at 0 and 10 d before application of fertilizer than in the no-straw treatment (Fig. 5). This suggests immobilization of fertilizer N with straw incorporation. The rice straw used in the present study had a C/N ratio of 60:1 and several laboratory studies have reported immobilization of soil and fertilizer N after the incorporation of organic materials with much lower C/N ratios (Paul and Clark, 1989; Yadvinder-Singh et al., 1988, 1992; Toor and Beri, 1991). The magnitude of immobilized N was influenced by the decomposition period of rice straw before fertilizer application. Interestingly, mineral N in the treatment where fertilizer N was applied concurrently with straw incorporation (0 d) always remained lower than the treatment without straw (Fig. 5). Mineral N in the soil was significantly higher under the 20- and 30-d predecom-

position periods than under the no-straw treatment at all sampling times. These data clearly demonstrated that incorporation of rice straw at 20 d or more before wheat sowing will minimize any adverse effects on crop growth due to N immobilization after straw incorporation (Fig. 5). Starting at 30 d after fertilizer application, mineral N in the 10-d predecomposition treatment was also higher than in the 0-d decomposition period or in the no-straw treatment (Fig. 5). Starting at 45-d period after fertilizer application, the initial soil mineral N did not differ from that of the control (no-straw) treatment, suggesting remineralization of immobilized N (Fig. 5). Our study suggested a lower amount of N immobilization by rice straw containing 6.7 g N kg⫺1 when allowed to decompose for 20 or 30 d before fertilizer N application compared with that reported for wheat and barley straw by Mary et al. (1996). Bhogal et al. (1997) observed that straw that had been incorporated before fertilizer application for cumulative thermal days ⬎1200, does not cause an appreciable immobilization of fertilizer N. Nitrogen release from rice straw, measured from litterbag decomposition in situ, was much lower than that observed in the laboratory incubation study. Although no reasons are obvious, the most plausible explanation could be the differences in soil-straw micro sites and loss of released N through various means, particularly under field conditions. The release of K from rice straw occurred at a fast rate and within 10 d after incorporation. Available soil K contents increased from 50 mg K kg⫺1 in the untreated control to 66 mg K kg⫺1 in all straw-amended treatments (data not shown). Rice straw contains about 65% of total K in water-soluble form (Yadvinder-Singh et al., 2004), and it is readily released in the soil upon incorporation. The predecomposition period had no marked effect on K release from rice straw, except at 30 d after fertilizer application (data not shown). 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

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 30ⴗC. 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.

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 ⫻ 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

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Table 5. Effect of rice straw management practices on grain yield of wheat.


Wheat yield Treatment T1 T2 T3 T4 T5 T6 T7 T8

1993–1994 2.90 c† 5.30 a 5.32 a 4.96 ab 5.17 a 4.70 b 4.82 ab –‡

Source of variation Treatment T1 vs. others T2 vs. T4–T7 T8 vs. T4–T7

1994–1995 2.61 4.85 5.10 5.02 5.09 4.88 4.78 5.16

** ** * –

b a a a a a a a

1995–1996 2.30 c 4.97 b 5.29 b 5.27 b 5.29 b 5.02 b 5.07 b 5.75 a

** ** NS§ NS

1996–1997

Rice yield

1997–1998

Mg ha⫺1 2.33 b 2.76 b 4.64 a 4.63 a 4.50 a 4.66 a 4.60 a 4.90 a 4.75 a 5.01 a 4.42 a 4.55 a 4.52 a 4.69 a 4.79 a 4.89 a Analysis of variance ** ** NS NS

** ** NS *

1998–1999 2.10 4.48 4.80 4.96 4.82 4.84 4.70 4.98

c b ab a ab ab ab a

** ** NS NS

1999–2000 2.42 5.88 6.05 6.45 6.41 6.21 6.09 6.55

Mean (1993–2000)

c b ab a a ab ab a

** ** NS NS

2.49 4.94 5.10 5.17 5.22 4.95 4.97 5.35

Mean (1994–1999) 6.13 6.19 6.25 6.34 6.29 6.33 6.29 6.52

c bc bc bc bc b bc a

** ** * NS

† Means in a column followed by the same letter are not significantly different by Duncan’s multiple range test (P ⫽ 0.05). ‡ Treatment not included in the study during 1993–1994. * Significant at the 0.05 probability level. ** Significant at the 0.01 probability level. § NS, not significant.

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. Seven-year means of rice grain yield are shown because the treatment ⫻ year interaction was not significant (Table 5). Rice grain yields were significantly higher in plots treated with wheat residue and GM (T8) than plots exposed to other treatments (T1–T7). Rice

yields were lowest in plots where rice residue was removed and no fertilizer N was applied to the preceding wheat (T1) but were at par with that in T2 through T5 and T7. There was no residual effect of rice residue incorporation in wheat on the grain yield of the following rice crop. The co-incorporation of wheat straw (C/N ratio of 60) and sesbania GM (C/N ratio of 16) into rice showed no adverse effect on rice, as reported earlier by Meelu et al. (1994) who observed that wheat straw incorporation had an adverse effect on rice yield but that GM incorporation along with wheat straw helped mitigate these effects. Nitrogen supplied through GM proved as efficient as urea N in increasing the grain yield of rice. In another study at the same site, Yadvinder-Singh et al. (1990) reported a similar N-use efficiency for GM N and urea N.

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

862


Table 6. Recovery, physiological, and agronomic efficiencies of fertilizer N applied to wheat during 1993–2000. Recovery efficiency


Treatment no.

Range

Physiological efficiency

Mean

Range

52 56 54 56 53 49 58

kg grain kg⫺1 N uptake 36–48 40 ab 35–47 39 b 34–49 41 ab 34–52 41 ab 34–46 39 b 35–48 42 a 35–48 42 a

% T2 T3 T4 T5 T6 T7 T8

43–60 43–70 41–68 37–73 35–70 33–63 44–76

Agronomic efficiency

bc† ab ab ab bc c a

Mean

Range

Mean

kg grain kg⫺1 N applied 16–29 21 c 16–30 22 bc 17–34 22 c 19–33 23 b 14–32 21 c 16–31 21 c 18–34 25 a

† Means in a column followed by the same letter are not significantly different by Duncan’s multiple range test (P ⫽ 0.05).

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).

Soil Organic Carbon and Available Potassium Rice residue incorporation increased organic C content of the soil more significantly than straw burning or removal (Table 7). The increase in SOC was maximized when GM and wheat residues were incorporated in the preceding rice crop (T8). Soil organic C showed an increasing trend with time in all the residue incorporation treatments. Carbon sequestration in the soil from rice residue applied at 7.1 Mg ha⫺1 annually for 7 yr averaged 14.6% (Table 7). The amount of C sequestered from the application of rice residue in wheat and GM ⫹ wheat residue in rice decreased to 7.6%. These values of C sequestration are lower than those reported by Aulakh et al. (2001) from a 3-yr study at a similar location. The relative increase in C sequestration in T4 through T7 versus T8 suggests a nonlinear relationship between the amount of residue C added and amount of C sequestered in SOC. The C sequestration derived from changes in soil C content may overestimate net C sequestration because part of the sequestered C from straw may compensate for the loss from soil C. Although SOC increased with increasing amounts of organic material, the relative proportion of C sequestered in SOC decreased because the SOC equilibrium is controlled by climate and cultural practices. 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 residueamended 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 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

Table 7. Effect of rice straw management practices on organic C, C sequestration, and available K in soil. Soil organic C after wheat Treatment no.

1993–1994

1996–1997

Added residue C sequestered 1999–2000

kg⫺1

T1 T2 T3 T4 T5 T6 T7 T8 LSD (0.05)

3.47 b† 3.53 b 3.50 b 3.89 ab 3.97 ab 4.37 a 4.50 a –‡

g 3.70 3.65 3.84 4.53 4.77 4.63 5.02 4.83

b b b a a a a a

1993–2000

Available K after wheat 1996–1997

% of added C 3.73 3.75 3.91 4.91 5.02 5.14 5.05 5.30

b b b a a a a a

13.0 14.1 16.1 15.1 7.6 2.52

† Means in a column followed by the same letter are not significantly different by Duncan’s multiple range test (P ⫽0.05). ‡ Treatment not included in the study during 1993–1994.

1999–2000 mg

40.1 38.5 40.7 44.6 43.7 45.8 47.0 44.9

b b b a a a a a

kg⫺1 33.4 c 34.7 c 41.4 b 45.3 a 44.2 a 46.0 a 45.6 a 49.8 a



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