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Published in Soil Sci. Soc. Am. J. 68:845-853 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

DIVISION S-4—SOIL FERTILITY & PLANT NUTRITION

Long-Term Effects of Organic Inputs on Yield and Soil Fertility in the Rice–Wheat Rotation

Yadvinder-Singha, Bijay-Singha, J. K. Ladha*,b, C. S. Khinda, R. K. Guptaa, O. P. Meelua and E. Pasuquinb

a Dep. of Soils, PAU, Ludhiana, 1451 004, India
b Crop, Soil, and Water Sciences Division, IRRI, DAPO Box 7777, Metro Manila, Philippines

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


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The sustainability of the rice (Oryza sativa L.)–wheat (Triticum aestivum L.) rotation is important to Asia's food security. Intensive cropping with no return of crop residues and other organic inputs result in the loss of soil organic matter (SOM) and nutrient supply, and is assumed to be nonsustainable. We evaluated seven treatments comprised of various combinations of green manure (GM; Sesbania cannabina L.); wheat straw (WS), farmyard manure (FYM), and urea on yields and yield trends; P and K balance; and soil fertility in a rice–wheat experiment (1988–2000) on a loamy sand in Punjab, India. Rice yields were comparable with GM + urea, WS + GM + urea, and urea alone, but yields were reduced when FYM was supplemented with N. Except during 1 yr, integrated use of FYM and GM produced equal or higher rice yields than other GM based treatments. Wheat straw incorporation reduced average rice yields by 7% compared with WS removal. After 5 yr of continuous application, FYM and WS were at par in increasing rice yields. Organic materials applied to rice had no residual effect on wheat yields except FYM, which increased yield by about 6% compared with urea alone. Rice yield declined by 0.02 to 0.13 Mg ha–1 yr–1 but wheat yields remained unchanged. Soil C increased with the application of WS and FYM. Potassium balance was highly negative. Although the causes of yield decline are unknown, inadequate K applications and changes in the climatic parameters are possible reasons.

Abbreviations: AE, agronomic efficiency • FYM, farmyard manure • GM, green manure • PE, physiological efficiency • RE, recovery efficiency • SOC, soil organic carbon • SOM, soil organic matter • WS, wheat straw


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
RICE–WHEAT ROTATION is the principal agricultural production system in South Asian countries. This system occupies around 13.5 million ha in the Indo-Gangetic plains of Bangladesh, India, Nepal, and Pakistan, and 10.5 million ha in China (Ladha et al., 2000). After a dramatic rise in productivity during the 1970s and early 1980s, attributed to the introduction of high-yielding varieties and the adoption of improved cultural practices, rice and wheat yields have either remained stagnant or declined (ICAR, 1988; Duxbury et al., 2000; Ladha et al., 2003a; Pathak et al., 2003). Deterioration of soil properties and water resources because of puddling (tillage in flooded soil) coupled with inadequate crop and nutrient management and adverse change in climatic parameters are considered as some of the basic causes for the yield decline (Ladha et al., 2003b). Studies by Bhandari et al. (2002) and Regmi et al. (2002) attributed the reduced productivity of the rice–wheat system to declining SOM, decreased soil fertility, occurrence of nutrient imbalances, and inappropriate fertilizer practices.

Soil productivity is closely linked with SOM status. Organic amendments play an important role in the improvement of soil structure and SOM content (Meelu et al., 1994; Ponnamperuma, 1984; Yadvinder-Singh et al., 1995). The use and management of crop residues, FYM and GM, are an increasingly important aspect of environmentally sound sustainable agriculture (Timsina and Connor, 2001). The future sustainability of crop production will greatly depend upon improvements in the soil resource base through its effective management in an environmentally benign manner. Biological N fixation by leguminous plants offers potential to reduce, and sometimes eliminate, the need for N fertilizers for the following crop.

The rice–wheat production system generates 10 to 14 Mg ha–1 of crop residues annually. Traditionally, wheat and rice straw have been removed from the fields for use as cattle feed and for several other purposes such as livestock bedding, thatching material for house, and fuel (Samra et al., 2003). Recently, because of the advent of mechanized harvesting, farmers prefer to burn large quantities of residues left in the field in situ as these interfere with tillage and seeding operations for the next crop. However, the disposal of crop residues by burning is often blamed for accelerating SOM losses, increasing C emissions, causing air pollution, and reducing soil microbial activity (Biederbeck et al., 1980; Rasmussen et al., 1980; Kumar and Goh, 2000). Rice transplanted in flooded soils immediately after crop residues are incorporated often grows and yields poorly due to N deficiency and accumulation of organic (phenolic) acids and results in increased CH4 emissions (Dobermann and Witt, 2000; Sidhu and Beri, 1989). Effective mitigation of these problems depends on developing crop residue management strategies that enhance residue breakdown. Compared with the traditional method of residue incorporation at the time of puddling shortly before rice is transplanted, the potential benefits of shallow incorporation of residues shortly after wheat harvest include accelerated aerobic decomposition of crop residues, and increased N availability (Witt et al., 2000), and reduced CH4 emissions (Wassman et al., 2000). Type of organic material (such as cereal residues, GM, composted animal manure, etc.) has a strong influence on N immobilization and mineralization (Yadvinder-Singh et al., 1988) and release of allelopathic or phenolic compounds in soils (Gaur and Pareek, 1974; Nelson, 1996), thereby affecting crop yields.

Burning of crop residues has to be avoided at all costs for environmental reasons. The best alternative will be to recycle residues onto agricultural land to improve soil and crop productivity. But farmers will incorporate crop residues only if there is no yield loss in the short term, and there are long-term benefits such as increased crop yields, reduced production inputs, or both. No single residue management practice is superior under all conditions (Kumar and Goh, 2000). A fallow period of 50 to 60 d occurs between wheat harvest and transplanting of rice, for decomposition of WS and establishment of a GM crop (Yadvinder-Singh et al., 1991). Apart from enhancing residue decomposition, the GM crop can supply large amounts of N to the following rice crop. Therefore, the benefits and detriments of various residue management options must be documented before these practices can be recommended to farmers for adoption. Although the effects of residue incorporation on decomposition and N mineralization are well known (Christensen, 1986; Bhogal et al., 1997; Mary et al., 1996), few studies have investigated the long-term effects of GM, WS, and FYM on the productivity of the rice–wheat system in India. The purpose of the present study was to document the adverse effects and benefits of long-term management strategies of WS, GM, FYM, and urea N on crop yields, nutrient balances, and various soil properties as they relate to the sustainability of the rice–wheat cropping system.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experimental Site
A long-term rice–wheat cropping system experiment was established in April 1988 on a loamy sand (Typic Ustipsamment) (Raj Kumar et al., 2000) at the experimental farm of the Punjab Agricultural University, Ludhiana, India (30°54' N and 75° 8' E, 247 m elevation). Before 1988, the field had been cropped to a maize (Zea mays L.)–wheat system for around 20 yr. The initial soil characteristics (0- to 15-cm layer) were pH 7.6 (1:2 soil/water suspension, w/w); electrical conductivity 0.19 dS m–1 (1:2 soil/water suspension); clay 126 g kg–1, silt 89 g kg–1, sand 785 g kg–1, organic C 3.6 g kg–1; total N 0.44 g kg–1; and cation exchange capacity 6.8 cmol kg–1, Olsen P 10 mg kg–1, and ammonium acetate-extractable K 38 mg kg–1. The area is designated as semiarid subtropical zone. The area receives 800 mm of rainfall annually, of which 80% occurs from June to September. The daily average minimum air temperature during rice growth (July–October) was 18°C and maximum temperature was 35°C, whereas during wheat growth (November–April) the daily average minimum air temperature was 6.7°C and maximum temperature was 22.6°C (Weather station, Punjab Agricultural University, Ludhiana).

Rice Treatments and Management
The seven treatments (T) applied to rice consisted of control, 150 kg N ha–1 as urea, GM (sesbania grown in situ) + urea, WS + urea, WS + GM + urea, FYM + urea, and FYM + GM (Table 1). Farmyard manure was applied at 12 Mg ha–1 on a fresh weight basis or 5.8 Mg ha–1 on a dry weight basis. When urea was applied in combination with GM and/or FYM the total N rate from all the sources was 150 kg N ha–1 (Table 1). The seven treatment combinations were arranged in a randomized complete block design with three replications. Plots were 10 m long and 3 m wide. Total dry biomass, C, and nutrient additions from WS, GM, and FYM appear in Table 2. Nitrogen addition through FYM ranged from 75 to 107 kg N ha–1 and the aboveground portion of sesbania ranged from 78 to 129 kg N ha–1. A total dose of 150 kg N ha–1 in T3 to T6 was applied through GM or FYM in different years with the balance through urea N. The optimum N dose of 150 kg N ha–1 is recommended for rice on coarse-textured soils in this region. No fertilizer N was applied in the FYM + GM treatment (T7). The N concentrations of GM, WS, and FYM were determined by microKjeldahl analysis, and used to calculate N content of each material.


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  		Table 1. Treatment combinations and nutrients applied to rice through fertilizer, green manure (GM), farmyard manure (FYM), and wheat straw (WS) in a rice–wheat experiment, Punjab Agricultural University, Ludhiana, India (1988–2000).

 

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  		Table 2. Mean (across years) dry matter and N, P, K, and C additions from wheat straw, GM, and FYM.

 
Within 1 wk after wheat harvest dry WS was disked into the soil. After disking, seeds (50 kg ha–1) of sesbania, soaked in water overnight, were broadcast onto the soil surface, and were incorporated with a disc harrow. Sesbania was irrigated as needed with a total of five to seven irrigations depending on the year. After 52 to 55 d, the aboveground biomass was harvested and fresh biomass was distributed equally in T3, T5, and T7 (Table 1). The GM biomass was incorporated into the soil in the third week of June, 1 to 2 d before transplanting rice seedlings. A known weight of FYM was spread uniformly in all plots of T6 and T7 and incorporated into the soil at 5 to 7 d before planting rice. Plots receiving no FYM or GM were maintained weed free during the fallow period.

The rice nursery (variety PR 106 during 1988–1997 and PR 111 during 1997–2000) was seeded in a separate field adjoining the experimental plots in the second week of May and fertilized with 15 Mg FYM ha–1, 15 kg N ha–1 as urea, 18 kg P ha–1 as diammonium phosphate, and 5 kg Zn ha–1 as zinc sulfate. Two and four weeks after rice nursery was planted, 15 kg N ha–1as urea was topdressed each time. Healthy 35- to 40-d-old seedlings were transplanted (2 hill–1) in the third or fourth week of June and the hill spacing was 20 by 15 cm (33.3 x 104 hills ha–1). In T2 and T5, urea N was applied in three equal amounts at 0, 3, and 6 wk after transplanting rice seedlings. On GM- and FYM-amended plots, urea N was applied on the soil surface, when the floodwater had disappeared, in two equal amounts at 3 and 6 wk after transplanting to synchronize N supply with crop demand (Yadvinder-Singh et al., 1991). As per local recommendation, rice received no inorganic P and K fertilizers (PAU, 2003). The experimental area was bordered with seven rows of nonexperimental rice plants on all sides of the field. The crop was irrigated daily during the first 2 wk and thereafter as needed to prevent the soil surface from being without overlying water for more than 2 d. Irrigation water was used from either a canal or tube well. The concentration of NO3–N in the irrigation water averaged 4 (±0.36) mg N L–1 and that of K averaged 4 (±0.25) mg K L–1. Rice was harvested from a 15.3-m2 area (510 hills) 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. Rice straw was removed from all the plots before sowing of wheat.

Wheat Treatments and Management
After rice harvest, wheat was sown in the second week of November each year. Before seeding, the land was tilled three to four times with a disc and spring-tyne harrow followed by planking. Wheat (variety HD 2329 during 1988–1997 and PBW 343 during 1997–2000) was sown (100 kg seed ha–1) in rows 20 cm apart to a depth of about 5 to 6 cm using a hand-drawn plow. After seeding, a light plank was dragged over the field to cover the seed. As per the local recommendation, a uniform application of 26 kg P ha–1 as single superphosphate and 50 kg K ha–1 as KCl was drilled in all plots at the time of seeding (PAU, 2002). Fertilizer N at a uniform rate (90 kg ha–1 as urea) was applied in two equal splits: broadcast at seeding and 22 to 25 d later (1–2 d after irrigation). Depending on the seasonal rainfall, 2 or 3 more flood irrigations of about 7.5 cm were applied at maximum tillering and flowering, and at the milking stage. Wheat was harvested from a 17.6-m2 area at the center of each plot at physiological maturity in the third week of April. In all treatments, except T4 and T5, wheat was cut at 7 to 8 cm above soil surface and all the WS was removed from the plots. In T4 and T5, combine harvesting of wheat was simulated, leaving about 30-cm long stubble anchored to the ground. After threshing, the top portion of the straw was uniformly distributed in the plots receiving WS (T4 and T5). Grain and straw yields are expressed on a dry weight basis.

Soil and Plant Sampling and Analysis
Soil samples were collected periodically (i.e., every 2 yr) within 10 d after rice harvest from the 0- to 15-cm soil depth using a 5-cm diameter auger. Each sample was a composite from three locations within a plot. The soil samples were mixed thoroughly, air-dried, crushed to pass through a 2-mm sieve, and stored in sealed plastic jars for analysis. Soil organic C (SOC) was determined by Walkley–Black method. Available P (0.5 M NaHCO3 extractable) was analyzed by the method described by Olsen et al. (1954) and ammonium acetate-extractable K was analyzed by the method described by Brown and Warncke (1988).

At crop maturity, subsamples of rice and wheat grain and straw were collected from each plot and dried in a hot-air oven at 65°C for 3 d. Plant samples were ground to pass through a 0.5-mm sieve and analyzed for total N by a microKjeldahl method (Bremner and Mulvaney, 1982). Phosphorus concentration of plant tissues digested in HNO3 and HClO4 was determined by the ammonium molybdate method (Olsen and Sommers, 1982), and that of K by flame photometry.

Calculations
Nitrogen-Use Efficiency
The different measures of N-use efficiencies (recovery efficiency [RE]; physiological efficiency [PE]; and agronomic efficiency [AE]) were calculated as described by Dobermann and Fairhurst, (2000). Total N uptake as used in these terms referred to N uptake by aboveground biomass (grain and straw) only.

Nutrient Budgets
Total nutrient removal in grain and straw of rice and wheat (plant N, P, and K) were calculated from the nutrient concentrations and yield data measured every year during the 12-yr study period (1988–2000). Apparent P and K balances were inputs and outputs measured during the present study or other studies (Mishra, 1980).


[1]

[2]

The P and K contents in mineral fertilizer, FYM, WS, and irrigation water were measured directly in the present study. Rainwater contributions of 0.2 kg P ha–1 yr–1 and 5.0 kg K ha–1 yr–1 were based on data reported by Mishra (1980) for Pantnagar, India. Irrigation water (250 cm ha–1 yr–1) contributed 2.5 kg P ha–1 yr–1 and 100 kg K ha–1 yr–1. The concentrations of P and K added to the soil with rice seedlings (dry weight 76 kg ha–1) and wheat seed (100 kg ha–1) were estimated to be 0.4% P and 3.0% K in seedlings (dry weight) and 0.36% P and 0.4% K in seeds.

We assumed that P would not be lost through leaching or otherwise from the soil–plant system. Leaching loss of K was estimated to be 15% of the K input (Smaling and Fresco, 1993; Yadvinder-Singh, unpublished data, 2002).

Statistical Analysis
Analysis of variance across years (Gomez and Gomez, 1984) was handled in a split plot design where year was considered as the main plot and treatments as subplots to determine the effects of year, treatment, and their interaction on yield, SOC, and available P and K contents using IRRISTAT version 92 (IRRI, 1992). The analyses of variance showed significant year by treatment interaction in all of these variables. A mean comparison was made within each year using Duncan's multiple range test (DMRT) at P ≤ 0.05. Unless indicated otherwise, differences were considered significant at P ≤ 0.05. Simple linear regression analyses of grain yields across years were performed to determine trends (slopes). The P values on the slope indicate the level of significance of the observed yield changes. Low yields in 1994 associated with the heavy incidence of stem borer in the rice crop were not considered in the trend analysis. Linear regression analysis was performed to determine the relationship between initial rice yield and rice yield decline across a 12-yr period.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Grain Yields
Analysis of variance across the 12-yr study period showed significant year x treatment interactions on rice yield (Table 3). Rice yield response to N fertilizer was observed each year. Rice yields were similar when (i) all of the N was applied as urea-N (T2), (ii) GM-N made up most of the 150 kg N ha–1 (T3), and (iii) WS and GM were combined (T5). Nitrogen applied as urea (T2) and GM (T3) was equally efficient in increasing rice yield. Wheat straw incorporation without GM (T4) resulted in significantly lower rice yield than when combined with GM (T5) in 5 out of 12 yr of the study period (Table 3). No yield differences between the two treatments were noted in the last 4 yr of the study period. Rice grain yields (averaged across 12 yr) were 5.4 Mg ha–1 in T4 and 5.8 Mg ha–1 in T5 (Table 3). Sidhu and Beri (1989) have also reported that incorporation of WS adversely affected grain yield of the following rice because of N deficiency caused by N immobilization. Our study showed that the integrated use of WS (C/N ratio = 100) and GM (C/N ratio = 16) alleviated the adverse effects of WS when applied with urea N.


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  		Table 3. Long-term effects of organic and inorganic fertilizers on grain yield of rice and wheat.

 
Applying FYM combined with urea N (T6) yielded less rice in 7 out of 12 yr than when urea alone was applied (T2; Table 3). These data suggest that either the amount of N released from FYM was inadequate or there was a lack of synchrony of N release and crop demand during rice growth. Yadvinder-Singh et al. (1995) have shown that FYM N was about 45% as efficient as urea N in increasing yield and N uptake of rice. In T7, the combined use of FYM and GM (with no supplementation of urea N) usually produced the highest numerical rice yields (average of 6.0 Mg ha–1) among all the treatments and significantly out yielded urea alone (T2) in 6 out of 12 yr. Although total N additions in T7 were 186 kg N ha–1, the assumed 45% efficiency of N added through FYM and 100% for GM, total N (urea equivalent) was lower than 150 kg N ha–1 in most cases. The combined use of FYM and GM could meet total N needs of high yielding rice cultivars, and could show synergetic effects in increasing rice yield. In addition, FYM supplied significant quantities of P and K to rice and wheat.

Wheat yields were significantly affected only by the treatment main effect (Table 3). There was no residual effect of the long-term application of GM (T3), or WS (T4) on wheat yields. The irrigated coarse-textured soils in subtropical regions possess very high C turnover rates because of favorable soil moisture and temperature conditions. The GM residue decomposes rapidly in these soils, leaving no residual effects (Meelu et al., 1994). Wheat straw incorporated in rice significantly reduced wheat yields (T4) compared with the combined use of WS and GM (T5), though the magnitude of reduction was small (0.12 Mg ha–1).

The FYM application to rice (T6 and T7) showed significant residual effects in the following wheat crop and produced higher wheat yields than all other treatments. Compared with GM and WS, FYM is more recalcitrant and resistant to microbial decomposition in soil. The higher wheat yields obtained on FYM-amended plots were possibly caused by the better supply pattern of N, P, and K, and improved physical conditions. Maskina et al. (1988), and Yadvinder-Singh et al. (1995) have reported that FYM applied to rice showed residual effects in the next wheat crop.

Nitrogen-Use Efficiency
Total N uptake by rice among different treatments followed trends similar to those obtained for grain yields (data not shown). Recovery efficiency calculated by the difference method varied by more than twofold over the years (Table 4). The mean RE was similar for GM (with or without WS; T3 and T5) and urea-N (T2) treatments, but was lowest in the FYM (T6) and WS (T4) treatments. The low availability of N from FYM as reported by Yadvinder-Singh et al. (1995) resulted in low RE values. The same argument applies for the FYM + GM (T7) treatment, in which, despite high yields, RE was lower than in the urea (T2) and GM (T3) treatments. The low RE in WS treatment (T4) was possibly due to less availability of N from WS (He et al., 1994). The lower RE in treatments in which either WS (T4) or FYM (T6) was combined with urea was due to lower grain yields during the particular year (Table 3). The average values of RE for rice were relatively high (27–39%) and fall in the upper range of 14 to 43% reported by Dobermann and Fairhurst (2000) for irrigated lowland rice fields in Asia.


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  		Table 4. Nitrogen use (recovery, physiological, and agronomic) efficiency of fertilizer N, GM-N, and FYM-N applied in rice (1988–1999).

 
Physiological efficiency also varied greatly over the years, but no trend could be established (data not reported). The average values of PE were similar (34–36 kg grain kg N uptake –1) among treatments, suggesting that it was not affected by the application of GM, WS, or FYM. Like RE and PE, AE also varied markedly among years (Table 4), and followed a trend similar to that obtained for RE. Like for RE, the average values of AE were lower for FYM (T6) and WS + urea N (T4). The average values of AE (9–13) obtained in the study fall in the range of 4 to 17 reported for irrigated lowland rice in Asia (Dobermann and Fairhurst, 2000). When N-use efficiency was calculated considering 45% efficiency of FYM as urea N, AE values for FYM and FYM + GM turned out to be the highest (data not reported).

Apparent Nutrient Balance
The apparent P balance at the system level was negative in all treatments except for those containing FYM (Table 5). The P balance in T1 was near zero because of less P removal by rice. The negative P balance in T2 averaged 10.4 kg P ha–1 yr–1, suggesting that the current fertilizer P recommendations are not adequate to maintain long-term soil-supplying capacity.


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  		Table 5. Apparent P balance during 1988–2000 in a long-term rice–wheat experiment, Punjab Agricultural University, Ludhiana, India.

 
The apparent K balance was also negative in all treatments (Table 6). The average K balance ranged from –78 kg in the control (T1) to –151 kg K ha–1 yr–1 in urea N (T2). When WS or FYM was recycled (T4–T7), the K balance was still negative, though less so. Despite substantial inputs from irrigation water, the K balance was negative. The total K uptake by the rice–wheat system averaged 285 kg K ha–1 yr–1 in T2, which is being maintained at a relatively high rate of K uptake even though insufficient amounts of K were applied. However, the large negative K balance suggests that the system will not be able to sustain the K supply in the long run. The major fraction of K uptake, however, remains in rice straw; thus, recycling of straw will dramatically change the K balance and would keep it within reasonable limits.


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  		Table 6. Apparent K balance during 1988–2000 in a long-term rice–wheat experiment, Punjab Agricultural University, Ludhiana, India.

 
Soil Fertility Parameters
Organic Carbon
Analysis of variance showed significant year x treatment interactions in SOC content (Table 7). Soil organic C did not change with time in T1, and T2, but increased considerably in the WS and FYM treatments (T4–T7; Table 7). Soil organic C in T3 increased after 9 yr in 1997 compared with the initial value in 1988. In T5 and T7 significant increase in SOC was noted 2 yr after the study, whereas in T4 and T6 an increase over initial value was observed after 4 yr (i.e., in 1991) of applying organic sources. These data showed that GM (T3) resulted in a small increase in SOC, and the increase over T2 was significant only in 1993 and 1995. Leguminous GMs decompose rapidly upon incorporation into the soil under the experimental conditions characterized by high temperature and alternate aerobic-anaerobic conditions (Yadvinder-Singh et al., 1992). Therefore, only a small fraction of GM-C was converted into stable soil humus. In fact, the effects of GM on SOM are governed by soil texture, climate, and SOC status (Yadvinder-Singh et al., 1992). Continuous application of GM for 12 yr sequestered 1.0 Mg C ha–1 in the 0- to 15-cm layer (assuming a bulk density of 1.65 Mg m–3), compared with the inorganic fertilizer treatment (T2), which is only about 5% of the added C (measured as amount of total SOC in the treated plot minus total C in the T2, divided by the total C added in the treatment, and multiplied by 100). Incorporation of GM with WS or FYM resulted in greater increase in SOC than when applied alone. On average, soil C was 37% higher with WS (T4 and T5) and 52% higher with FYM (T6 and T7) than in the fertilizer N (T2) plot. Wheat straw (T4) sequestered 3 Mg C ha–1 (10% of the added C) and FYM (T6) 4.7 Mg C ha–1 (20% of the added C). Cereal residues and FYM are known to have a higher humification coefficient than GM and lead to greater C sequestration in soil (Beri et al., 1995; Yadvinder-Singh et al., 1995). The cumulative C sequestered after the 1999 rice season was 9% for WS + GM and 14% for FYM + GM.


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  		Table 7. Long-term effects of inorganic and organic sources on soil organic C content.

 
Available Phosphorus
Analysis of variance showed significant year x treatment interactions in NaHCO3 extractable P concentration (Table 8). The NaHCO3–extractable P in soil decreased from its initial value in all treatments, except where FYM was applied (T6 and T7, Table 8). The decrease in available P from its initial value was observed during 1997 in T1, in 1991 in T2 and during 1989 in T3 through T5 (Table 8). Phosphorus removal by rice and wheat crops was higher than that added through fertilizers, resulting in a depletion of P from the available soil pool. Application of FYM (T6 and T7) resulted in an increase in available P at 4 yr after continuation application during 1991. No effect of FYM on NaHCO3–extractable P was, however observed thereafter. There was no positive effect of GM on the availability of P in the soil as reported in the literature (Yadvinder-Singh et al., 1992). Incorporation of WS (T4 and T5) resulted in a small increase in the availability of soil P as compared with T2, though the differences were only significant during 1995 and 1997. Olsen P increased with the application of FYM in T6 and T7. The soil P increases in the FYM treatments were expected as the FYM contained high organic P (35 kg P ha–1 yr–1). Although the P balance in FYM-treated plots is highly positive, the increase in available P content in the soil was relatively small. The major P fraction added through FYM is in the organic pool, which mineralizes slowly with time.


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  		Table 8. Long-term effects of inorganic and organic sources on available P and K contents in soil.

 
Available Potassium
Analysis of variance showed significant year x treatment interactions in available K concentration (Table 8). The available pool of K in the soil showed a declining trend with continuous rice–wheat cropping in the urea (T1–T2) and GM (T3) treatments (Table 8). A decrease in available K from its initial value was first observed in 1997 in T1, in 1993 in T2, and in 1995 in T3 (Table 8). Available K content in T4 and T5 did change significantly from its initial value. A significant increase in available K content in soil was observed at 4 yr (1991) after regular application of FYM, compared with the initial value (Table 8). Available K content in soil decreased significantly thereafter, suggesting that FYM application is not sufficient to sustain soil K fertility under highly exhaustive rice–wheat system where crop residues, particularly rice straw, are removed from the fields. Wheat straw incorporation helped in maintaining the initial contents of available K in the soil, showing only a small increase in the available K content over the urea or GM treatments. After 2 yr of rice–wheat rotation, Prasad et al. (1999) noted an increase of 7 mg K kg–1 only when WS was incorporated. The application of FYM resulted in a larger increase in the availability of K than WS. At the end of the 12-yr cropping cycle, the available K in the FYM-amended plots increased from the initial concentration of 38 mg kg–1 to an average of 42 mg kg–1. The available K in soil in all treatments were always below the critical limit of 55 mg K kg–1 recommended for these soils.

Yield Trends
The rice yield of all treatments decreased with time during the study period (Table 9). The values of the slope were significantly (P < 0.05) different from zero in T3, T5, and T7. Linear regression analysis of rice yield from 1988 to 1999 showed downward trends, with the decline ranging from 0.02 to 0.13 Mg ha–1 yr–1 (Fig. 1) . The rate of decline in rice yield was the lowest in the control (T1) and FYM (T6) treatments, but it was not significant at P < 0.05 (Table 9). The decline in rice yield across treatments was correlated with initial yield (Fig. 2) . These observed trends in the rice yield decline in the rice–wheat system are similar to those reported by other researchers (Bhandari et al., 2002; Regmi et al., 2002). The decline in rice yield was explained by the gradual decline in soil K supply by these researchers. The application of FYM and WS increased SOM compared with the plots treated with urea N (Table 7). Factors other than SOM therefore appeared to be limiting rice yields. Recently, we reported that during 1995–1999 average solar radiation (27.3 MJ m–2 d–1) during the rice season decreased at 0.12 MJ m–2 d–1 yr–1, minimum temperature (23.4°C) increased significantly by 0.05°C yr–1, and maximum temperature (33.0°C) increased by 0.04°C yr–1 at the experimental site (Pathak et al., 2003). These adverse changes in the climatic parameters may have caused a significant decrease in potential yields by 0.07 Mg ha–1 yr–1 and on-farm yields by 0.05 Mg ha–1 yr–1 (Pathak et al., 2003). These data suggest that the major cause of the rice yield decline could be due to adverse climatic changes. Part of the yield decline (>0.05 Mg ha–1 yr–1) may be due to the gradual decline in soil K supply and some unknown factors.


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  		Table 9. Average yields, yield trends, and analyses of variance of rice and wheat yields in a long-term rice–wheat experiment, PAU, Ludhiana, India (1988–1999).

 


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  		Fig. 1. Yield trends of rice in the long-term experiment, Punjab Agricultural University, Ludhiana, India, 1988 through 1999 with selected treatments. See Table 1 for treatment details and results of linear regression analysis in Table 9.

 


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  		Fig. 2. Relationship between first year (1988) rice yield and rice yield declines over a 12-yr period for all the treatments.

 
Wheat yields were stable as no declining trends were noted (Table 9). The lack of a significant yield trend in wheat may be due to the use of a low N rate (90 kg N ha–1). The maximum yield therefore, could not be achieved in wheat. The problem of a yield decline is often not witnessed at low productivity levels with suboptimal fertilizer use. The optimum N rate used for wheat in this region is 120 kg N ha–1. The suboptimal N rate used in this study was to document residual effects of organic inputs applied to rice. A contrasting trend of rice and wheat yields could also be due to a few other reasons. Evidence suggests that continuing efforts in selection and breeding have led to improvement in the genetic potential of new cultivars, particularly wheat cultivars. A genetic gain of 16 kg ha–1 yr–1 has been reported in wheat yield since the mid-1960s (Ladha et al., 2002b). The potential yield of rice, however, has remained at the same level since the late 1960s. In addition, the wheat crop is less prone to the incidence of pest and disease attack. Pathak et al. (2002) have also reported that climatic changes during the wheat season had no adverse effect on the yield stability of wheat in the region.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Our study demonstrates that rice yields and N-use efficiency decreased with WS incorporation when compared with straw removal. Wheat straw can be incorporated concurrently with GM with no adverse effects on rice yields. Nitrogen from GM and urea was equally efficient in increasing yield and N-use efficiency in rice. Organic manuring with FYM plus GM without the application of any inorganic fertilizers proved quite effective in producing high rice yields. The regular incorporation of crop residues and FYM increased (though at a slow rate) SOM contents. Though the available K supply may be maintained from the nonexchangeable soil pools and from irrigation water for some time, a reduced supply of K will eventually occur due to a K deficiency. The results clearly reveal that current fertilizer recommendations for P and K are inadequate in the long run. The total input of N, P, and K should be optimal to ensure a sufficient nutrient supply for higher yields. Rice yields in this long-term rice–wheat experiment declined with the application of urea as well as organic materials. These data seem to rule out the possibility of decline in SOM as a reason of negative yield trends. The adverse changes in climatic parameters along with a decreased soil supply of available K may be the possible reasons associated with the yield decline. The effect of decreased soil K supply as a result of large negative K balance on yield decline was, however, not apparent in the present study. Wheat yields were more stable during 1988–2000. Further studies are needed to understand the causes of yield decline in rice–wheat system. Regular monitoring of climatic factors would help in predicting problems in achieving high yields and allow measures to be taken to improve productivity.

Received for publication November 13, 2002.


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




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R.K. Gupta, Yadvinder-Singh, J.K. Ladha, Bijay-Singh, J. Singh, G. Singh, and H. Pathak
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