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Effects of Residue Decomposition on Productivityand Soil Fertility in RiceWheat 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 avail-able in the IGP of India (Sarkar et al., 1999). In theRice (Oryza sativa L.)wheat (Triticum aestivum L.) farmers inIndian Punjab alone, about 12 million Mg of rice strawIndiaburn or remove residues to facilitateseedbed preparation. Incor-are burned annually, which causes about 0.7 million Mgporation of residues before planting of the next crop generally de-

creases yields due to N immobilization. Since a window of about 40 d of N loss. The gaseous emissions from burning of riceis available between rice harvest and wheat planting, the effect of straw are 70% CO2, 7%CO, 0.66% CH4, and 2.09% N2Otime of incorporation on rice residue decomposition and N mineraliza- (Samra et al., 2003). Estimated emissions of greenhousetionimmobilization was studied in 19921993. The mass loss of resi- gases caused by burning of rice straw in the whole IGPduewas25% fora 10-d,35% fora 20-d,and 51%for a 40-d decomposi- of India are thus substantial. Besides contributing totion period before wheat planting. Nitrogen release from residue

the greenhouse effect, the large-scale burning of riceranged from 6 to 9 kg ha1 during the wheat season. The immobiliza-

straw results in serious health hazards as is evident fromtion of urea N decreased when residue was allowed to decomposethe reported increase in respiratory and eye problemsfor 10-d or longer. Based on these studies, a long-term (19932000)among the local population (Grace et al., 2003).experiment was conducted on a sandy loam soil to examine the effect

Where residues have been incorporated immediatelyof time of residue incorporation before sowing wheat when comparedwith burning or removal of residue on yields, N-use efficiency, and before planting the next crop, grain yields were lowersoil fertility. The effect of wheat residue incorporation with green than where residues are removed or burned, resultingmanure (GM, Sesbania cannabina L.) on subsequent rice yields was in N immobilization, a problem that is attributable toalso determined. Residue incorporation for 10 to 40 d had no effect the slow rates of residue decay (Sidhu and Beri, 1989;on wheat yields. Rice yields increased (0.180.39 Mg ha1) when Beri et al., 1995). Other potential problems of residuewheat residue was incorporated with GM. Starter N applied at residue

incorporation just before rice transplanting include ac-incorporation did not influence wheat yields but decreasedN recovery

cumulation of phenolic acids in soil and increased CH4efficiency. Physiological efficiency was higher when rice straw wasemissions under flooded conditions (Grace et al., 2003).incorporated in wheat and when wheat straw plus GM were incorpo-In this case, the timing of incorporation of crop residuesrated in rice than when rice straw was incorporated for 10 d or whenis more important than the amount. Compared with thethe straw was burned. The long-term application of rice residue in-

creased C accumulation in soil. traditional method of wet incorporation shortly beforeplanting of the next rice crop, the potential benefits ofshallow incorporation shortly after crop harvest includeaccelerated aerobic decomposition of crop residuesRicewheat is a major crop rotation in the Indo- (about 50% of the C within 3040 d), leading to in-Gangetic Plains (IGP) of South Asia, spread overcreased N availability (Witt et al., 2000), and reduced13.5 million ha in Bangladesh, India, Nepal, and Paki-CH4 emissions (Wassmann et al., 2000). Early incorpo-stan (Ladha et al., 2000). Effective management of post-ration also allows additional time for phenol degrada-harvest crop residues (straw) is perhaps the foremosttion to occur under aerobic conditions, thereby avoidingchallenge facing the intensive ricewheat-producing re-any adverse effect on germinating seeds and seedlings.gions of the world. The wheat residue is used to feed

Burning of crop residues must be avoided at all coststhe animals. However, the rice residue due to large silicafor environmental reasons. Farmers will probably onlycontent is normally burned. Burning of rice residues isincorporate crop residues if legislation forces them tocost-effective and the predominant method of disposalor if there is a clear yield increase that they cannotin areas under combined harvesting in the IGP (Samraachieve with the application of additional fertilizer. Aet al., 2003). However, disposal of crop residues by burn-window of about 35 to 40 d is available between theing is often criticized for accelerating losses of soil or-harvesting of rice and seeding of wheat, which can beganic matter (SOM) and nutrients, increasing C emis-used for in situ decomposition of rice straw. Similarly,sions, causing intense air pollution, and reducing soila fallow period of 50 to 60 d is available after the wheatmicrobial activity (Biederbeck et al., 1980; Rasmussenharvest and before rice planting; this allows decomposi-et al., 1980; Kumar and Goh, 2000). According to antion of wheat straw and the raising of a GM crop (Yad-estimate, 113.6 million Mg of rice and wheat residues,vinder-Singh et al., 1991, 1994). Apart from enhancingresidue decomposition (Singh, 1993), the GM crop can

Yadvinder-Singh, Bijay-Singh, J.K. Ladha, C.S. Khind, and T.S. supply large amounts of N to the following rice crop.Khera, Dep. of Soils, Punjab Agricultural University, Ludhiana, 141 Although the effect of straw incorporation on N immo-004, India; C.S. Bueno, Crop, Soil, and Water Sciences Division, IRRI

bilization in the soil is well known (Christensen, 1986;DAPO Box 7777, Metro Manila, Philippines. Received 24 Dec. 2002.*Corresponding author ([email protected]).

Abbreviations: AE, agronomic efficiency; GM, green manure; IGP,Published in Soil Sci. Soc. Am. J. 68:854864 (2004). Soil Science Society of America Indo-Gangetic Plains; NH4OAc,ammonium acetate;PE, physiological

efficiency; RE, recovery efficiency; SOM, soil organic matter.677 S. Segoe Rd., Madison, WI 53711 USA

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YADVINDER-SINGH ET AL.: EFFECTS OF RESIDUE DECOMPOSITION 855

Toor and Beri, 1991; Bhogal et al., 1997; Mary et al.,1996), a few studies have investigated how time of incor-poration and starter-N application influence crop resi-due decomposition, nutrient release, and crop yields(Adachi et al., 1977; Bijay-Singh et al., 2001). More(or improved) knowledge about residue decompositiondynamics is essential for developing effective manage-

ment strategies. No single residue management practiceis superior under all conditions (Kumar and Goh, 2000).In the IGP of South Asia, wheat straw is mainly usedas animal feed, but rice straw is disposed of by burning.The objectives of our study were to (i) estimate thelong-term effects of different times of incorporation ofrice residue before sowing wheat vis-a-vis residue burn-ing and residue removal on crop production in the ricewheat system, and (ii) investigate crop residue decom-position and N mineralization as a function of time ofincorporation of rice straw. To achieve this objective,three sets of studies were performed. The first studyinvestigated the effect of time of incorporation (duringthe prewheat fallow period) on rice residue decomposi-

tion and N release using the litterbag technique underfield conditions. Thesecond study investigated the effectof the different times of incorporation of rice straw onfertilizer N mineralization and immobilization and Krelease dynamics under laboratory conditions. Basedon the outcome of these experiments, the third studyexamined the long-term effects of time of incorporationof rice residue on the yields of rice and wheat, and soilfertility parameters. In addition, the role of starter Nand the combination of wheat residue and the legumi-nous GM (Sesbania cannabina) to enhance the decom-position of rice and wheat residue was also examined.

MATERIALS AND METHODS

Site Description

The experimental field was located at the research farm ofthe Punjab Agricultural University, Ludhiana (30 56 N, 75 Fig. 1. Monthly distributions (averaged for 15 yr) of (A) maximum52 E, 247 m above mean sea level), located in the IGP in the and minimum temperatures, (B) sunshine hours, and (C) rainfall

at Punjab Agricultural University, Ludhiana, India.state of Punjab, India. The soil was a Fatehpur sandy loam(Typic Ustipsamment) with the following characteristics: pH7.2 (1:2 soil/water ratio), electrical conductivity (EC) 0.27 dS fertilized crop using the litterbag decomposition techniquem1, cation exchange capacity (CEC) 10.6 cmol (p) kg1 (Beare et al., 2002) under different times of incorporation (10,(using BaCl2 solution; Sumner and Miller, 1996), 3.5 g kg

1

20, and 30 d before sowing of the wheat crop) in 19921993.organic C (Walkley and Black method, Nelson and Sommers,

Stems and leaves of mature rice straw that had an initial1996), 12.5 mg kg1 Olsen P (Olsen et al., 1954), 53.1 mg

elemental composition of 5.6 g N kg1 and 410 g C kg1 (C/N,kg1 ammonium acetate (NH4OAc)-extractable K (Brown and 73:1) were collectedfrom the previous rice crop planted beforeWarncke, 1988), 790 g kg1 sand, 101 g kg1 silt, and 109 g

wheat, dried at 60C and cut into 2-cm pieces. A plant samplekg1 clay. Under average climatic conditions, the area receives

of 15 g was placed in nylon mesh bags (10 by 15 cm, 1-mm

800 mm of annual rainfall, about 80% of which occurs from mesh). Litterbags (15 for each treatment, five sampling datesJune to September. The mean minimum and maximum tem-by three replicates) were randomly assigned to treatment

peratures during the rice season (JulyOctober) were 18 andplots. Sealed nylon bags were placed horizontally at the 10-

35C, whereas during wheat season (NovemberApril) theto 12-cm depth in the designated plots in the field (described

mean temperatureswere 6.7 and 22.6C, respectively. Monthlyabove) starting from 8 Oct. 1992. The position of each nylon

distribution (averaged for the last 15 yr) of rainfall, minimumbag in the plots was marked with a nylon thread tied to aandmaximum temperatures, andsunshinehoursfor theexper-wooden stick. All litterbags in the plots were carefully re-imental site are presented in Fig. 1.moved before cultivation for wheat seeding, and stored in thelaboratory for 3 d. The litterbags were returned to the sameRice Residue Decompositionplots just after wheat seeding. The wheat crop received the

and Nitrogen Release In Siturecommended fertilizer doses (120 kg N ha1, 26 kg P ha1,and 50 kg K ha1). The litterbags were collected at regularWe studied the in situ decomposition and N release dynam-

ics for incorporated rice residue collected from an adequately intervals (at wheat sowing and at 35, 72, 122, and 150 d after

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856 SOIL SCI. SOC. AM. J., VOL. 68, MAYJUNE 2004

sowing) from cultivation (15 Nov. 1992) through harvest of NO3 ) was determined in 1M KCl extracts by microKjeldahlsteam distillation.the wheat crop (17 Apr. 1993). The straw remaining on each

sampling date was removed from the litterbag, shaken gentlyover a sieve (1 mm), and spray-rinsed to remove the adhering

Field Experimentsoil. The residue samples were oven-dried in paper bags at60C for 48 h, weighed, and then ground to pass through a A field experiment was conducted from 1993 to 2000 at the

research farm of the Punjab Agricultural University, Ludhi-1-mm sieve.The loss in weightwas assumed to be theamountof residue that decomposed during that period. The amount ana. The experiment was laid out in a randomized complete

block design with three replications. Plots were 10 m long andof N released from the rice residue was calculated as thedifference between initial residue N input (assuming an initial 4.2 m wide. The eight treatments (T1T8) included different

combinations of rice straw management and urea-N manage-residue input of 7.1 Mg ha1 containing 39.8 kg N ha1) andresidue N recovered (calculated from the actual mass of rice ment practices (Table 1). After the harvest of rice in the

first or second week of October, rice straw was allowed tostraw and its N content) at different periods after incorpo-ration. 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

Laboratory Experimentwas simulated, leaving about 30-cm long stubble anchored tothe ground. After removing grains, the straw was uniformlyThe effect of predecomposition of rice residue for 0, 10,

20, and 30 d before N fertilizer application on N and K dynam- distributed in the allocated plots. The amount of incorporatedstraw during different years averaged 7.1 Mg ha1, whichics was studied in a sandy loam soil incubated at 35C for 60 d.

A bulk soil sample (0- to 15-cm depth) was collected from a added 44 kg N ha1 and 2.87 Mg C ha1 (Table 3). At thepredesignated periods, the rice straw was incorporated intononexperimental area of the field experiment (see next section

for description of the soil used). Soil was air-dried, ground, the soil using a moldboard plow. Irrigation was applied at 18 d

after incorporation of rice straw in the 40-d decompositionand sieved through a 2-mm mesh. Before the start of theexperiment, bulk soil was rewetted to about 60% field capacity treatments to enhance straw decomposition and in the 20-ddecomposition treatments to provide optimum moisture con-(determined as water held by soil at 10 kPa water potential)

and incubated at 30C for 7 d. The 7-d incubation period ditions for the ease of straw incorporation. All plots werecultivated three to four times, followed by leveling afterbefore rice straw addition allowed the microbial population

to reach a baseline level after the initial flush of activity from applying presowing irrigation about 10 to 12 d before thesowing of wheat. In T1 and T2, rice was harvested manually,soil drying and rewetting. Following incubation thereafter, a

known weight of soil (subsample) from the bulk soil was leaving about 10-cm long stubble above the ground surface,and rice straw wasremoved from the plots.In the straw burnedweighed into 1-L plastic containers (10 cm i.d.) marked for

different dates of straw incorporation. Rice straw (ground to treatment(T3),dry rice straw in theallocated plots wasburnedin situ at 10 to 12 d after the harvest of rice.1- to 2-mm size, 0.67% N) was mixed into the soil at 3 g kg1

soil at 0, 10, 20, and 30 d before the application of 100 mg N In treatment T8, rice straw (20 d) was incorporated 20 dbefore planting wheat. Forthe following rice crop, loose wheat(as urea) kg1 soil. Fertilizer N was applied to all the pots at

the same time. Control soil (without rice straw added) was straw was uniformly distributed 10 to 12 d after wheat harvest.The field was flood-irrigated and the wheat straw was diskedsimilarly prepared. Each treatment had three replications. Soil

moisture was adjusted at 75% of field capacity as described into the soil under optimum field moisture conditions. Thesame day, Sesbania cannabina L. (GM) seeds previouslyearlier. Soil water in the pots was maintained by making up

the loss in water every fourth day during the study. The soil soaked overnight in water were spread on the soil surface at50 kg ha1 and mixed into the soil with the help of a discsamples were then incubated at 35 1C for 60 d. Soil samples

from the individual pots were collected at 10, 20, 30, 45, and harrow (Table 2). The sesbania crop received four to fiveirrigations. The green biomass of 50- to 55-d-old sesbania was60 d after the application of fertilizer. Inorganic N (NH4 and

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

N addition

Treatment Rice Wheat

Treatment Rice Wheat WR GM Fertilizer RR Fertilizer

kg ha1

T1 FN120 # RR-R FN0 0 0 120 0 0T2 FN120 RR-R FN120 0 0 120 0 120

T3 FN120 RR-B FN120 0 0 120 0 120T4 FN120 RR-I (40 d) FN120 0 0 120 44 120T5 FN120 RR-I (20 d) FN120 0 0 120 44 120T6 FN120 RR-I (10 d) FN120 0 0 120 44 120T7 FN120 RR-I (20 d) FN120 0 0 120 44 120T8 WR-I GM FNx## RR-I (20 d) FN120 33 94 26 44 120

WR Wheat residue. GM Green manure. Inorganic fertilizer. RR Rice residue.# FN Fertilizer N (subscript is kg ha1). 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 ha1 applied at the time of residue incorporation.## 120 kg N ha1 applied through GM (average 94 kg N ha1) and the balance through urea (average 26 kg N ha1).

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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 Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 Year 7

Incorporation of wheat straw 23 Apr 21 Apr 26 Apr 27 Apr 29 Apr 29 AprGreen manure

Seeding 23 Apr 21 Apr 26 Apr 27 Apr 29 Apr 29 AprHarvest and incorporation 12 Jun 18 Jun 18 Jun 20 Jun 23 Jun 21 Jun

RiceFlooding and puddling 16 Jun 13 Jun 11 Jun 19 Jun 21 Jun 24 Jun 22 Jun

Transplanting 17 Jun 14 Jun 12 Jun 20 Jun 22 Jun 25 Jun 23 JunCultivar PR 111 PR 111 PR 111 PR 111 PR 111 PR 114 PR 114First fertilizer N 19 Jun 17 Jun 14 Jun 22 Jun 25 Jun 26 Jun 24 JunSecond fertilizer N 8 Jul 5 Jul 3 Jul 11 Jul 13 Jul 16 Jul 14 JulThird fertilizer N 29 Jul 26 Jul 24 Jul 1 Aug 3 Aug 6 Aug 4 AugHarvest 7 Oct 9 Oct 4 Oct 2 Oct 7 Oct 8 Oct 1 Oct

Incorporation of rice straw40 d 9 Oct 10 Oct 6 Oct 3 Oct 8 Oct 10 Oct 2 Oct20 d 29 Oct 30 Oct 26 Oct 23 Oct 28 Oct 30 Oct 22 Oct10 d 8 Nov 9 Nov 5 Nov 2 Nov 7 Nov 9 Nov 1 Nov

WheatSeeding 18 Nov 19 Nov 15 Nov 12 Nov 17 Nov 19 Nov 11 NovCultivar HD2329 HD2329 HD2329 PBW343 PBW343 PBW343 PBW343First fertilizer N 18 Nov 19 Nov 15 Nov 12 Nov 17 Nov 19 Nov 11 NovSecond fertilizer N 12 Dec 14 Dec 7 Dec 5 Dec 9 Dec 14 Dec 4 DecHarvest 16 Apr 19 Apr 9 Apr 18 Apr 18 Apr 20 Apr 17 Apr

Not included in the study.

incorporated into the soil using a disc harrow 1 to 2 d before Plant and Soil Sampling and Analysistransplanting rice seedlings (Table 2).

At crop maturity, grain and straw samples were collectedFor wheat (cv HD 2329 during 19931996 and PBW343

from each plot and dried in a hot-air oven at 60C for 3 d.during 19962000), fertilizer N (120 kg ha1) as urea was ap-The N in grain and straw subsamples of rice, wheat, andplied in two equal split doses, 50% at the time of wheat sowingsesbania was determined by the microKjeldahl method. Potas-and the remaining 50% topdressed at 20 to 25 d after sowing,sium was analyzed in di-acid (HNO3 and HClO4) digests by2 to 3 d after the first irrigation (Table 2). A basal dose offlame photometric method. Soil samples were collected at26.2 kg P and 25 kg K ha1 was drill-applied at wheat sowing15-cm depth from three sites in each plot using a 4-cm diameterto all treatments. In T7, 30 kg N ha1 (starter N) was appliedauger for the determination of C, K, and bulk density. Theat the time of straw incorporation. The remaining fertilizer Nentire volume of soil was weighed and mixed thoroughly and(90kgNha1) was applied in two equal split doses as describeda subsample was taken from the mixed soil. Subsamples werefor the other treatments.air-dried and crushed to pass through a 2-mm sieve. Soil CRice (cv PR111 during 19941998 and PR114 during 1998was determined by the WalkleyBlack method (Nelson and1999) was transplanted in the first week of June every year. InSomers, 1996) and NH4OAc-extractable K was analyzed using

treatments T1 through T7, wheat crop was harvested manually the methods described by Olsen et al. (1954) and Brown andand straw was removed from the plots. A uniform dose ofWarncke (1988), respectively. All C and nutrient values were120 kg N ha1 was applied in three equal split doses at trans-converted to kilograms per hectare using soil bulk densityplanting and 3 and 6 wk after transplanting (Table 2). In T8,data simultaneously determined from soil cores.N addition through GM ranged from 75 to 95 kg ha1, total

C addition averaged 1.54 Mg ha1 (Table 3), and the total Napplied (GM urea) to rice was kept at 120 kg N ha1. The Data Analysisbalance of N (2545 kg N ha1) was applied as urea in two

Recovery efficiency (RE) of added N was calculated asequal split doses at 3 and 6 wk after transplanting. Rice washarvested from a 24-m2 area at the center of each plot at RE (%) physiological maturity in the first week of October. Grainyield was expressed on the basis of 140 g kg1 water contentandstrawyield wasexpressed on oven-dry weightbasis. Wheat

Total N uptake (kg N ha1) of the treatment

total N uptake (kg N ha1) of the control 100

Applied N (k N ha1) of the treatment [1]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. 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 P K C addition

Mg ha1 kg ha1 Mg ha1

Wheat residue 7.3 (7.17.9) 33 (3237) 5 (47) 65 (5681) 2.99Rice residue 7.0 (5.57.9) 45 (3556) 10 (812) 165 (127211) 2.87Green manure 3.5 (2.85.0) 94 (79122) 1.54

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 kg1 N uptake) 40-d decomposition period compared with 35% for a20-d decomposition treatment and 25% for a 10-d de-composition treatment (Fig. 2). The relationships be-tween mass of residue remaining and decomposition

Grain yield (kg ha1) of the treatment

grain yield (kg ha1) of the control

Total N uptake (kg N ha1) of the treatment

total N uptake (kg N ha1) of the control [2]period for the three time-of-incorporation treatmentswere nonlinear (quadratic) (Fig. 2). Mass loss showedsimilar trends in the three treatments during the 122-d

Agronomic efficiency (AE) of added N was calculated asperiod after wheat planting. The mass loss was almostsimilar under 20-d and under 10-d decomposition periodAE (kg grain kg1 N applied) during the whole decomposition period after wheatplanting. Theamount of mass loss remained significantlyhigher for the 40-d decomposition period than the 10-d

Grain yield (kg ha1) of the treatment

grain yield (kg ha1) of the control

Applied N (kg N ha1) of the treatment [3] or 20-d period up to 72 d after seeding of wheat. At theend of the study, no significant difference was notedSequestration of added organic C as rice straw, wheat straw,among the three treatments (Fig. 2).and sesbania in SOC was calculated (Aulakh et al., 2001) as

The data for all three decomposition periods couldEfficiency of organic C sequestration in SOC be best described using a single logarithmic equation:

Y 14.9ln(X) 8.74, R 2 0.951 (n 15) [5]OC (Mg C ha1) of the treatment

OC (Mg C ha1) of the control

Added C (Mg C ha1) of the treatment [4] where Y equals the total decomposition period (days)and X equals the percentage mass loss of rice residue.

Analysis of variance (ANOVA) for randomized complete The above equation accounted for 95% of the varia-block design was performed to determine the effects of treat- tion and may prove useful in predicting residue decom-ment, year, and their interaction on grain yield of wheat using position under soil and environmental conditions similarthe PROC GLM procedure in SAS (SAS Institute, 1989). To

to that in the present study.account for the difference between years, a repeated measures

Since mass loss was measured at 10, 20, and 40 d aftermodel wasused with time as the repeated variable.A probabil-incorporation, rates of residue decomposition could beity level 0.05 was considered significant. Duncans multiplemeasured more accurately for the 0- to 10-, 10- to 20-,range test at the 5% level of significance was done usingand 20- to 40-d periods. Averaged across the three treat-IRRISTAT version 92 (IRRI, 1992) to compare treatment

means for RE, AE, PE, SOC, and available K data. A separate ments, rice straw decaying for 0- to 10-d lost 2.45% ofANOVA was performed for each sampling date for residue its initial mass each day (Fig. 2). By comparison, thedecomposition and N mineralization data. values were 1.0% d1 for 10 to 20 d and 0.8% d1 for

20 to 40 d during fallow. During the wheat-croppingphase, rates of straw decomposition were similar underRESULTS AND DISCUSSIONthe three treatments, except during the first sampling

Rice Residue Decomposition Dynamics at 35 d after seeding wheat. When considered over theunder Field Conditions whole cropping season, the rate of mass loss under thethree treatments averaged 0.41% d1. Not much isDecomposition of Rice Residue In Situknown on in situ decomposition of rice straw in tropical

Time of incorporation had a large effect on the de-countries. Although Mishra et al. (2001) studied the

composition of rice residue during the fallow phase (Oc-decomposition of rice straw incorporation before wheat;

toberNovember) after rice harvest (Fig. 2). At wheatthe influence of different times of straw amendment

seeding, the mass loss of rice residue was 51% for awas not investigated. Crop residue decomposition, beinga function of time and site conditions, has to be studiedunder a particular set of conditions. The rate of massloss was much slower after 35 d during the croppingseason. This was mainly due to low winter temperaturesduring DecemberJanuary (3572 d) (Fig. 1) and to adeficiency of soil moisture in the surface soil layers inthe latter part of the wheat-cropping season (FebruaryApril) when soil temperature increases (Fig. 1) and thesurface layer surrounding the litterbags rapidly dries.Irrespective of time of incorporation, about 70% of riceresidue was decomposed during the 5 to 6 mo that coversthe wheat-growing season.

Nitrogen Concentration Versus Mass LossFig. 2. Effect of decomposition period on the mass remaining of lit-terbag rice residue. Values of LSD (0.05) for comparing treatment The N concentration of the rice residue increasedmeans at each sampling period are 6.9 for 0 d, 4.3 for 35 d, 5.4 for

continuously during the 190-d decomposition period72 d, 3.9 for 122 d, and NS, nonsignificant for 150 d after wheatplanting. Each data point represents mean of three replicates. (data not shown), indicating loss of C as CO2 and/or N

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immobilization in the residue by microorganisms, whichbuild up new microbial protein from plant and soil N.The N content of rice straw at the time of incorporationwas 5.6 g kg1 (or 39.8 kg ha1), which increased to14.8 g kg1 at the end of the study period. At the timeof wheat sowing, residue N was significantly lower (7.0 gkg1) 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. Afterlong periods of residue decomposition (122 d), differ-ences between 10- and 20-d treatments became nonsig-nificant, while residue in the 40-d decomposition treat-ment still had a significantly higher N concentration(13.8 g kg1) than in the 10- and 20-d treatments(12.512.9 g kg1). At the end of the study (150 d afterwheat sowing), the N concentration in rice residue wasobserved to be similar (14.8 g kg1) for the three

Fig. 3. Relationship between N concentration and mass loss of lit-treatments. terbag rice residue. Each data point represents mean of three rep-

The N concentration in rice residue during the decom- licates.position period followed a quadratic trend given below:

content (Table 4), and this trend has also been reportedY 0.0001X2 0.067X 6.16, R2

earlier by Christensen (1986).

0.969 (n 15) [6] Although N concentration in residue increased withtime, N release remained nearly unchanged during the

where Y total N concentration (g kg1) in rice residue course of the study. The rice residue either did not seemand X total decomposition period (days). to have decomposed sufficiently to have considerable

The relationship between N concentration and mass N release or, after an N release from the residue, immo-loss was curvilinear (quadratic) (Fig. 3). Burgess et al. bilization of soil N occurred. Thus, net apparent N re-(2002) also reported a curvilinear relationship between lease measured was low. In the Canterbury region ofN concentration and mass loss for barley (Hordeum New Zealand, Beare et al. (2002) reported that 5 kgvulgare L.) straw. However, for wheat and sorghum N ha1 was released from decaying barley straw over a(Sorghum bicolorPers.) residues (with N content lower period of 18 mo. Using 15N-labeled rice residue, Yone-than rice residues), Schomberg et al. (1994) observed an yama and Yoshida (1977) reported that 8% of the N ininverse linear relationship between dry mass remaining the rice leaf sheath (8 g N kg1) was mineralized inand N concentration. These observations imply that the 30 d at 30C under upland conditions in the laboratory.relationship between N andmass loss of residues is likely Schomberg et al. (1994) and Burgess et al. (2002) re-

to be influenced by residue type, soil and environmental ported net N release by low-N residues, wheat (andconditions, and duration of the study. maize [Zea mays L.]) after 50 to 60% mass loss. In our

Rice residue appeared to have a brief initial period study, despite a substantial mass loss of 69% residue,of N release after incorporation (Table 4). The amount N release was small. Nitrogen release from rice residueof N release was 2 to 4 kg N ha1 when residues were during the decomposition period was linearly related todecomposed for 10 to 20 d, which further increased to mass loss (Fig. 4). The relationship showed that for7.9 kg N ha1 (19.8% of the residue N applied) when every 10% increase in mass loss, there was about 2.75%decomposition period was extended to 40 d. The total (1.07 kg N ha1) release from the applied residue N.amount of N released under different decomposition The actual values of N release from the rice residuetreatments during the whole period of study (190 d) may differ from that calculated in the present study asranged from 6 to 9 kg N ha1. Residues go through part of the N may be assimilated by soil microbialseveral phases in their decomposition, with N dynamics biomass.related to stage or extent of mass loss. Even low Nresidues appeared to have a brief initial period of N Table 4. Amount of N release during rice residue decomposition

under field conditions (19921993).release after placement, in agreement with data pre-sented in other studies (Christensen, 1986; Burgess et During wheat cropping phase

(after seeding)al., 2002). Since a certain fraction of initial residue com-Time of Incorporation During

ponents is water-soluble, some initial N (and C) losses (DBS) fallow phase 035 072 0122 0150can occur because of leaching (Christensen, 1986; Par-

kg ha1sons et al., 1990) if the water-soluble portion is exposed

40 7.9 a 8.6 a 7.7 a 6.7 a 9.2 ato sufficient precipitation or soil water movement. How- 20 3.5 b 4.6 b 4.5 b 5.0 b 7.3 b

10 2.3 b 5.5 b 4.0 b 6.3 a 5.9 bever, under field conditions with intact (rather thanground) residue, not all such material is necessarily Total rice residue input assumed to be 7.1 Mg ha1, which contained

39.8 kg N ha1 at the time of incorporation.available for leaching (Havis and Alberts, 1993). The DBS days before seeding wheat.initial period of N loss was generally followed by a Means in a column followed by the same letter are not significantly

different by Duncans multiple range test (P0.05).period of increasing or relatively unchanging residue N

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position periods than under the no-straw treatment atall sampling times. These data clearly demonstrated thatincorporation of rice straw at 20 d or more before wheatsowing will minimize any adverse effects on crop growthdue to N immobilization after straw incorporation(Fig. 5). Starting at 30 d after fertilizer application, min-eral N in the 10-d predecomposition treatment was also

higher than in the 0-d decomposition period or in theno-straw treatment (Fig. 5). Starting at 45-d period afterfertilizer application, the initial soil mineral N did notdiffer from that of the control (no-straw) treatment,suggesting remineralization of immobilized N (Fig. 5).Our study suggested a lower amount of N immobiliza-tion by rice straw containing 6.7 g N kg1 when allowedto decompose for 20 or 30 d before fertilizer N applica-tion compared with that reported for wheat and barleystraw by Mary et al. (1996). Bhogal et al. (1997)observed

Fig. 4. Relationship between N release and mass loss of litterbag rice that straw that had been incorporated before fertilizerresidue. Each data point represents mean of three replicates. application for cumulative thermal days 1200, does

not cause an appreciable immobilization of fertilizerNitrogen and Potassium Mineralization N. Nitrogen release from rice straw, measured from

Dynamics for Rice Residue litterbag decomposition in situ, was much lower thanunder Laboratory Conditions that observed in the laboratory incubation study. Al-

though no reasons are obvious, the most plausible expla-Soil mineral N (NH4 NO3) at 10 d after fertilizernation could be the differences in soil-straw micro sitesapplication was significantly lower in treatments inand loss of released N through various means, particu-which rice straw was incorporated at 0 and 10 d beforelarly under field conditions.application of fertilizer than in the no-straw treatment

The release of K from rice straw occurred at a fast(Fig. 5). This suggests immobilization of fertilizer N withrate and within 10 d after incorporation. Available soilstraw incorporation. The rice straw used in the presentK contents increased from 50 mg K kg1 in the untreatedstudy had a C/N ratio of 60:1 and several laboratorycontrol to 66 mg K kg1 in all straw-amended treatmentsstudies have reported immobilization of soil and fertil-(data not shown). Rice straw contains about 65% ofizer N after the incorporation of organic materials withtotal K in water-soluble form (Yadvinder-Singh et al.,much lower C/N ratios (Paul and Clark, 1989; Yad-2004), and it is readily released in the soil upon incorpo-vinder-Singh et al., 1988, 1992; Toor and Beri, 1991).ration. The predecomposition period had no marked

The magnitude of immobilized N was influenced by effect on K release from rice straw, except at 30 d afterthe decomposition period of rice straw before fertilizerfertilizer application (data not shown).application. Interestingly, mineral N in the treatment

The results from the decomposition and N mineraliza-where fertilizer N was applied concurrently with strawtion studies suggested that rice residue is likely to haveincorporation (0 d) always remained lower than thelittle adverse effects on N availability in the soil whentreatment without straw (Fig. 5). Mineral N in the soilit is allowed to decompose under aerobic conditions forwas significantly higher under the 20- and 30-d predecom-at least 10 d before sowing of the next crop. Release ofK from the rice straw occurs quickly after incorporationinto the soil.

Long-Term Field Experiment in theRiceWheat System

Grain YieldsWheat grain yields without fertilizer N ranged from

2.1 to 2.9 Mg ha1 from 1993 to 2000 (Table 5). FertilizerN (120 kg N ha1) application significantly increasedyield regardless of straw management treatment. Therewere significant year treatment effects on the yieldof wheat. During the first year, wheat yield was lower

Fig. 5. Effect of predecomposition period of rice straw on mineral N in T6, (rice residue incorporated 10 d before wheat(NH4 NO3) dynamics in soil amended with 100 mg N kg

1 and sowing) than in T2 (residues removed), suggesting im-incubated at 75% field capacity moisture regime at 30C. Values mobilization. The other residue treatments, however,of LSD (0.05) for comparing treatment means at each sampling

were not significantly different from T2 from 1993 toperiod are 5.94 for 10 d, 7.14 for 20 d, 5.46 for 30 d, 4.77 for 45 d and4.25 for 60 d. Each data point represents mean of three replicates. 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 Rice yield

Mean MeanTrea tmen t 19 93 1 99 4 1 99 4 19 95 19 95 1 99 6 1 99 6 19 97 19 97 19 98 19 98 1 99 9 1 99 9 20 00 (199 3 20 00 ) (1994 1 99 9)

Mg ha1

T1 2.90 c 2.61 b 2.30 c 2.33 b 2.76 b 2.10 c 2.42 c 2.49 6.13 cT2 5.30 a 4.85 a 4.97 b 4.64 a 4.63 a 4.48 b 5.88 b 4.94 6.19 bc

T3 5.32 a 5.10 a 5.29 b 4.50 a 4.66 a 4.80 ab 6.05 ab 5.10 6.25 bcT4 4.96 ab 5.02 a 5.27 b 4.60 a 4.90 a 4.96 a 6.45 a 5.17 6.34 bcT5 5.17 a 5.09 a 5.29 b 4.75 a 5.01 a 4.82 ab 6.41 a 5.22 6.29 bcT6 4.70 b 4.88 a 5.02 b 4.42 a 4.55 a 4.84 ab 6.21 ab 4.95 6.33 bT7 4.82 ab 4.78 a 5.07 b 4.52 a 4.69 a 4.70 ab 6.09 ab 4.97 6.29 bcT8 5.16 a 5.75 a 4.79 a 4.89 a 4.98 a 6.55 a 5.35 6.52 a

Analysis of varianceSource of variationTreatment ** ** ** ** ** ** **T1 vs. others ** ** ** ** ** ** **T2 vs. T4T7 * NS NS NS NS NS *T8 vs. T4T7 NS NS * NS NS NS

Means in a column followed by the same letter are not significantly different by Duncans multiple range test (P 0.05). Treatment not included in the study during 19931994.* 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 yields were lowest in plots where rice residue was re-moved and no fertilizer N was applied to the precedingwheat compared with straw being allowed to decompose

for 20 d or more before planting of wheat could be the wheat (T1) but were at par with that in T2 through T5and T7. There was no residual effect of rice residuegreater losses of fertilizer N via nitrificationdenitrifica-

tion. Significantly greater yields were observed in T4 incorporation in wheat on the grain yield of the follow-ing rice crop. The co-incorporation of wheat straw (C/Nin 19981999 and 19992000 and in T5 in 19992000

compared with T2 where rice residues were removed. ratio of 60) and sesbania GM (C/N ratio of 16) into riceshowed no adverse effect on rice, as reported earlierWheat yields were higher in plots (T8) where GM and

wheat straw were incorporated in the preceding rice by Meelu et al. (1994) who observed that wheat strawincorporation had an adverse effect on rice yield butthan when rice straw was incorporated (mean of T4T8)

during 19961997 only. Compared with residue removal that GM incorporation along with wheat straw helped(T2) or residue burning (T3), incorporation of rice resi- mitigate these effects. Nitrogen supplied through GMdue 10 to 40 d before seeding wheat (T4T7) did not proved as efficient as urea N in increasing the grainshow any adverse effect on wheat yield, but on average yield of rice. In another study at the same site, Yad-

significantly increased yield in 19931994 and 1999 vinder-Singh et al. (1990) reported a similar N-use effi-2000. Wheat yields in the 20- and 40-d treatments were, ciency for GM N and urea N.however, very close to yields in T8. These results areconsistent with conclusions drawn from other decompo- Nitrogen Uptake and Nitrogen-Use Efficiencysition and N mineralization studies. The application of in Wheat25% of fertilizer N as starter N at the time of residue

Total N uptake by wheat was not significantly affectedincorporation (T7) showed some depression (0.10.4by rice residue incorporation compared with straw re-Mg ha1) in wheat yield in all years compared with T5moval or burning (data not shown). Recovery efficiency(Table 5), although the differences were not significant.by the difference method decreased with applicationThe results obtained from the laboratory incubationof starter N (T7) compared with all other treatmentsstudy clearly showed that N applied concurrently with(Table 6). The trends in RE were similar to those ob-straw incorporation gets immobilized and does not re-served for total N uptake by wheat (Table 6). The REmineralize easily. Bijay-Singh et al. (2001) reported avalues for wheat ranged from 49 to 54% in differentdecrease in 15N recovery by wheat when 25% of the

treatments, which are well within the acceptable rangetotal N dose was incorporated at the same time as ricereported in the literature (Katyal et al., 1987). Physio-straw incorporation. In our study, annual additions oflogical efficiency was not affected by different treat-40 to 50 kg N ha1 through rice residue for 7 yr did notments, except that it was significantly higher in T7 (25%influence grain yield of wheat as the recommended splitof the total N dose applied at residue incorporation)application of 120 kg N ha1 (one-half drilled at sowingand T8 (GM and wheat residue incorporated in rice)and the remaining half topdressed at 2125 d after sow-than in T3 (residue burned) and T6 (residue incorpo-ing) was already applied to all the treatments in wheat.rated at 10 d before wheat seeding) (Table 6). TheSeven-year means of rice grain yield are shown be-higher PE values in T7 were due to the decrease in Ncause the treatment year interaction was not signifi-uptake, which was not accompanied by a decrease incant (Table 5). Rice grain yields were significantlywheat yield. In T8, fertilizer N uptake was more effi-higher in plots treated with wheat residue and GM (T8)

than plots exposed to other treatments (T1T7). Rice ciently translated into grain yield than in T2, T3, and

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Table 6. Recovery, physiological, and agronomic efficiencies of fertilizer N applied to wheat during 19932000.

Recovery efficiency Physiological efficiency Agronomic efficiency

Treatment no. Range Mean Range Mean Range Mean

% kg grain kg1 N uptake kg grain kg1 N applied

T2 4360 52 bc 3648 40 ab 1629 21 cT3 4370 56 ab 3547 39 b 1630 22 bcT4 4168 54 ab 3449 41 ab 1734 22 c

T5 3773 56 ab 3452 41 ab 1933 23 bT6 3570 53 bc 3446 39 b 1432 21 cT7 3363 49 c 3548 42 a 1631 21 cT8 4476 58 a 3548 42 a 1834 25 a

Means in a column followed by the same letter are not significantly different by Duncans multiple range test (P 0.05).

T6. Like RE and PE, AE was significantly higher in T8 the soil than did the residue removal treatmentsthan in all other treatments and was lowest in T2, T6, (Table 7). On average, rice residue added about 175 kgand T7 (Table 6). K ha1 annually in T4 through T8 and wheat residue

added 65 kg K ha1 in T8. Despite such large additions,the increase in K availability in the soil was small. OurSoil Organic Carbon and Available Potassiumlaboratory results predicted a large increase in K avail-

Rice residue incorporation increased organic C con-ability in soil amended with rice straw. One possible

tent of the soil more significantly than straw burning orreason for the small increases observed in residue-

removal (Table 7). The increase in SOC was maximized amended plots may be the loss through leaching of awhen GM and wheat residues were incorporated in thesignificant proportion of residue K during rice cultiva-

preceding rice crop (T8). Soil organic C showed an in-tion on this permeable soil. From a column study, Yad-

creasing trend with time in all the residue incorpora-vinder-Singh et al. (2004) observed that up to 25% oftion treatments.the rice straw K can be leached below the 90-cm depthCarbon sequestration in the soil from rice residueafter 14 irrigations applied on sandy loam soil.applied at 7.1 Mg ha1 annually for 7 yr averaged 14.6%

(Table 7). The amount of C sequestered from the appli-cation of rice residue in wheat and GM wheat residue CONCLUSIONSin rice decreased to 7.6%. These values of C sequestra-

Because of serious environmental effects, crop resi-tion are lower than those reported by Aulakh et al.due burning is not desirable. Farmers will incorporate(2001) from a 3-yr study at a similar location. The rela-crop residues only if there are no yield losses or if theretive increase in C sequestration in T4 through T7 versusis a clear yield advantage over residue burning in theT8 suggests a nonlinear relationship between the amountlong run. The 7-yr data demonstrate that rice and wheat

of residue C added and amount of C sequestered in productivity is not adversely affected when rice residueSOC. The C sequestration derived from changes in soilis incorporated for at least 10 d and preferably 20 dC content may overestimate net C sequestration be-before the establishment of the succeeding crop. Ourcause part of the sequestered C from straw may compen-study showed rapid rice residue decomposition in sandysate for the loss from soil C. Although SOC increasedsoils of the IGP. Rice residue decomposition of aboutwith increasing amounts of organic material, the relative25% during the prewheat fallow period was sufficientproportion of C sequestered in SOC decreased becauseto avoid any detrimental effects on rice and wheat yields.the SOC equilibrium is controlled by climate and cul-The laboratory study shows no immobilization of fertil-tural practices.izer N when rice residues are incorporated at 20 d orThe incorporation of rice residue caused a smaller

but more significant increase in available K content in 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 aft er wheat Added r es idue C seques tered Available K af ter wheat

Treatment no. 19931994 19961997 19992000 19932000 19961997 19992000

g kg1 % of added C mg kg1

T1 3.47 b 3.70 b 3.73 b 40.1 b 33.4 cT2 3.53 b 3.65 b 3.75 b 38.5 b 34.7 cT3 3.50 b 3.84 b 3.91 b 40.7 b 41.4 bT4 3.89 ab 4.53 a 4.91 a 13.0 44.6 a 45.3 aT5 3.97 ab 4.77 a 5.02 a 14.1 43.7 a 44.2 aT6 4.37 a 4.63 a 5.14 a 16.1 45.8 a 46.0 aT7 4.50 a 5.02 a 5.05 a 15.1 47.0 a 45.6 aT8 4.83 a 5.30 a 7.6 44.9 a 49.8 aLSD (0.05) 2.52

Means in a column followed by the same letter are not significantly different by Duncans multiple range test (P0.05). Treatment not included in the study during 19931994.

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partnership research. p. 131. IRRI Discuss. Pap. Ser.40. IRRI,GM (narrow C/N) in combination with wheat residueManila, Philippines.(wide C/N) can be advantageous in mitigating the ad-

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