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a Dep. of Agronomy, Punjab Agricultural Univ., Ludhiana, India
b Crop, Soil, and Water Science Division, International Rice Research Institute (IRRI), DAPO 7777, Metro Manila, Philippines
c Social Sciences Division, International Rice Research Institute (IRRI), DAPO 7777, Metro Manila, Philippines
d Rice-Wheat Consortium for Indo-Gangetic Plains, NASC Complex, Pusa, New Delhi 110 012, India
* Corresponding author (J.K.LADHA{at}CGIAR.ORG)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: DAS, days after sowing • DAT, days after transplanting • DMRT, Duncan's multiple range test • FYM, farmyard manure • GM, green manure • HI, harvest index • LTE, long-term experiment • PAU, Punjab Agricultural University • T, Treatment • WCS, wheat chopped straw
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Several researchers have studied long-term rice–wheat experiments in South Asia but most studies were restricted to simple yield trend analysis (Nambiar, 1994; Yadav et al., 1998; 2000; Dawe et al., 2000; Duxbury et al., 2000) and periodic soil samples were not obtained. An LTE begun in 1983 at the experimental farm of Punjab Agricultural University (PAU), Ludhiana, India, is, however, one of the few exceptions. In this experiment, soil nutrient levels and crop nutrient removal were determined. Data obtained were used to (i) examine the long-term effects of inorganic fertilizer and organic nutrient sources on yield trends of rice and wheat, nutrient balances, and soil nutrient pools, and (ii) identify reasons for the yield trends observed.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Experimental Design and Treatments
The experiment included two crops per year, rice (July–October) and wheat (November–April), with 11 treatments (T1–T11) arranged in a randomized block design with three replications (Table 1). Plots were 19.2 m long and 5.2 m wide. The treatments comprised application of different combinations of inorganic and organic sources of nutrients to rice and wheat. The recommended levels of N, P, and K were supplemented with N through FYM, WCS, and sesbania as green manure (GM) so that the 100% recommended N dose was available to the rice crop. No organic manure was applied to wheat. When the LTE was initiated, the recommendation for N, P, and K for the area was 120-26-25 kg ha-1, respectively, for each rice and wheat. Starting in 1992, application of P decreased to 13.1 kg ha-1 in rice, whereas in wheat it remained at 26.2 kg ha-1. The P rate was reduced because of excess P may induce Zn deficiency and availability of soil P in flooded rice is likely to be higher than wheat which is grown in aerobic conditions. Based on analysis every fourth year, the various organic materials had the following ranges of N, P, and K contents (g kg-1 on oven-dry basis). Farm yard manure had 8 to 12 N, 2 to 4 P, and 3.5 to 6.5 K; WCS had 4 to 6 N, 0.5 to 1.5 P, and 2 to 4 K; and GM had18 to 24 N, 1 to 3 P, and 15 to 21 K. In Treatments 8 and 9, WCS was applied at 6 and 3 Mg ha-1, which supplied 25 and 12.5% of recommended N, respectively, and the remaining N was applied through urea (Table 1). To obtain 50 and 25% N (in Treatments 10 and 11), in situ-grown sesbania was incorporated at 3.0 and 1.5 Mg ha-1. These rates were obtained by manipulating the planting time of sesbania. Sesbania seeds were sown about 2 wk earlier for Treatment 10 (3.0 Mg ha-1) than for Treatment 11 (1.5 Mg ha-1). Since the application of all organic materials was adjusted based on providing a fixed amount of N, the amounts of total P and K added varied by treatment. Total P application to both rice and wheat ranged from 39 to 57 kg ha-1 yr-1 and total K ranged from 38 to 68 kg ha-1 yr-1 in treatments with 100% N, P, and K (Treatments 5–11).
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Crop Management
Crop residue, if any, of the previous crop was removed from the field in April every year. During June and July, the land was plowed, puddled, and leveled. Two rice seedlings (5 wk old) were transplanted in the puddled lowland field at 20- by 15-cm spacing. All the plots were normally flooded (2–4 cm) until 2 wk before rice harvest. Sesbania was grown in situ for 6 and 8 wk before the rice crop in Treatments 10 and 11, respectively. An appropriate amount of aboveground biomass of sesbania was chopped into 5- to 10-cm pieces, uniformly spread into the plots following flooding of the field and incorporated into the soil with an offset disc harrow while the soil was being puddled for transplanting of rice. Sesbania was incorporated into the soil a day before transplanting. However, FYM and WCS were incorporated into the moist soil 2 wk before transplanting of rice.
Wheat (100 kg seed ha-1) was sown in rows 22.5 cm apart in November every year. Prior to seeding, the land was plowed two times to about a 20-cm depth with a cattle (Bos taurus)-drawn plow. After seeding, a plank was dragged over the field to cover the seed. In wheat, three to four irrigations were given at sowing, crown root initiation (21 d after sowing, DAS), maximum tillering (55 DAS), and flowering (100 DAS). All P and K and a half dose of N were drilled at sowing. The remaining N was topdressed 21 DAS with irrigation.
Dates of transplanting and sowing and the varieties used for rice and wheat are presented in Table 2. The varieties of both rice and wheat changed during the course of the experiment. Once in every 3 yr, ZnSO4 (60 kg ha-1) was applied to all plots before the rice crop. Irrigation water was applied from a deep tube well and also by canal. Weeds, pests, and diseases were controlled as required.
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The crops were harvested at ground level at maturity. Grain and straw yields were determined from a 60-m2 harvested area at the center of each plot. Grains were separated from straw using a plot thresher, dried in a batch grain dryer, and weighed. Grain moisture was determined immediately after weighing and subsamples were dried in an oven at 65°C for 48 h. Grain weights for rice and wheat were expressed at 150 and 120 g kg-1 water content, respectively, whereas straw weights were expressed on an oven-dry basis (65°C). Subsamples of grain and straw were dried at 70°C, ground to pass through a 0.5-mm sieve, and analyzed for total N by a micro-Kjeldahl method (Yoshida et al., 1976). Phosphorus and K were analyzed in di-acid (HNO3 and HClO4) digests by the colorimetric and flame photometric method, respectively (Yoshida et al., 1976). The nutrients in grain plus those in straw were taken as the measure of total uptake. The same methods were used for analysis of nutrient contents in manures.
Nutrient Budgets
Apparent P and K balances were estimated for the last three years (1995–97) in T1 (without N, P, or K), T5 (100% N-P-K), T6 (50% N-P-K + 50% N through FYM), T8 (50% N-P-K + 50% N through WCS), and T10 (50% N-P-K + 50% N through GM) by considering different inputs and outputs measured during either the present study or other studies. Phosphorus and K balances were calculated as:
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The P and K contents in mineral fertilizer, organic manure, and irrigation water were measured directly in the present study. Phosphorus and K contributions of 0.2 and 5.0 kg ha-1 yr-1 with rainwater were based on the data reported by Mishra (1980) for Pantnagar, India (29°N lat. and 79°5'E long. and altitude 244 m above mean sea level). Irrigation water (250 cm yr-1) contributed 2.5 and 62.6 kg ha-1 yr-1 P and K, respectively. The quantities 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 4.0, and 30.0 g kg-1 yr-1 in seedlings (dry weight) and 3.6 and 4.0 g kg-1 yr-1 in seeds, respectively (Mishra, 1980).
Plant P and K were measured in the present study. We assumed P would not be lost through leaching or otherwise from the soil system. Leaching loss of K was taken to be 150 g kg-1 of K input (Smaling and Fresco, 1993; Bijay-Singh, personal communication, 2000).
Data Analyses
Simple linear regression analyses of grain yields over the years were done to determine trends (slopes). The P values, on the slopes indicate the level of significance of the observed yield changes. Analysis of variance across years (Gomez and Gomez, 1984) was done to determine the effects of treatment, year, and their interaction on yield using IRRISTAT version 92 (International Rice Research Institute, Philippines). Mean comparison was done for each year using the Duncan's multiple range test (DMRT) at the 5% level of significance. Rice and wheat yield response to different levels of N-P-K application for the initial and final 3 yr of the experiment were compared by doing simple linear regression analyses on each year's data of grain yield and N, P, and K applied. The slopes and y-intercepts were compared based on their 95% confidence intervals.
The change in total and available soil P and K was determined by getting the difference between values obtained in 1988 and 1997 in each replicate block for the five treatments. The standard deviation of the difference for each treatment was computed to assess the statistical significance of observed changes.
In the first year (1983), rice and wheat yields were substantially lower than those of three subsequent years and therefore were not considered in the trend analysis or the economic analysis. This was assumed to be related to the change in land management from a maize–wheat to rice–wheat rotation.
An economic profitability analysis was performed on the use of FYM, WCS, and GM in Treatments 6, 8, and 10. Compared with the full mineral N-P-K treatment (T5), the difference in rice and wheat yields in the FYM, WCS, and GM treatments over the full length of the experiment was valued at the average price of rice and wheat prevailing on world markets from 1993 to 1998 ($295 Mg-1 for rice with a milling ratio of 0.65 for conversion from paddy, $160 Mg-1 for wheat). The reduction in inorganic fertilizer input in the FYM, WCS, and GM treatments was valued at the average prices of N, P, and K prevailing on world markets from 1993 to 1998 ($340 Mg-1 for N from urea, $339 Mg-1 for P from (NH4)2HPO4, and $189 Mg-1 for K from KCl). The difference in the value of rice and wheat yields was added to the savings in mineral fertilizer costs to give the net benefit of using organic nutrient sources. This net benefit was divided by the quantity of organic manure to give a break-even cost per megagram for FYM, WCS, and GM (in $ Mg-1). If the per megagram costs of applying organic manure (labor costs for gathering and application) are less than this break-even cost, application of organic manure is financially profitable.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The integrated use of manures and fertilizers proved to be as efficient as 100% inorganic N-P-K (T5) in the productivity of the rice–wheat system in the long term. The total yield of rice and wheat was similar among Treatments 5, 6, 7, 10, and 11 (Table 3), suggesting that 50% of N can be replaced by FYM or GM without yield reduction. With WCS (T8 and T9), total productivity of the rice–wheat system was lower by 1.08 to 1.25 Mg ha-1 yr-1 compared with the N-P-K treatment (T5) (Table 3).
Linear regression analyses of rice yield from 1984 to 1997 showed downward trends, with the decline ranging from 0.07 to 0.13 Mg ha-1 yr-1 except in the control, in which the yield decline (0.05 Mg ha-1 yr-1) was not significant at P < 0.05 (Fig. 1). The decline in rice yield across treatments was correlated with initial yields (Fig. 2) . Wheat yields were more stable (Table 3). In wheat, significant (P < 0.05) declining trends were observed only with the applications of 75% (T4) and 100% (T5) NPK fertilizer. On the other hand, the control treatment recorded a nonsignificant yield increase. Total yield of rice and wheat also declined in all the treatments except in the control (T1), in which the decline was not significant (P < 0.05). Total productivity declined from 0.10 to 0.17 Mg ha-1 yr-1 over 14 yr of the LTE (Table 3).
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Apparent Nutrient Balance
The apparent K balances were negative for the rice–wheat system, ranging from 34 to 153 kg ha-1 yr-1. However, the magnitude varied among the treatments (Table 5). The GM treatment (T10) had the largest outflux of K. Unlike the K balance, the apparent P balance was positive (4–21 kg ha-1 yr-1) in all except the unfertilized treatment (T1), which had a negative balance of 8 kg P ha-1 yr-1.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Monitoring of soil nutrient pools suggested a gradual decline of total soil N and available pools of P and K in all the treatments except with FYM, where total N remained unchanged, and available and total P pools increased. These declines occurred with continuous cultivation from 1988 to 1997 in treatments with recommended N, P, and K when supplemented with or without crop residues. While the gradual depletion of one or more nutrients may have collectively contributed to the yield decline, we hypothesize that depletion of total soil N is likely to have played a major role in this experiment. Total soil N declined markedly at 0.03, 0.02, and 0.02 g kg-1 yr-1 in treatments with N, P, and K, GM, and WCS, respectively.
A contrasting trend of total soil C and N was observed in this rice-wheat LTE. We assume that N losses were responsible for the decline in total soil N despite relatively large N inputs from fertilizer and organic sources in some treatments. Tropical wetlands with rice-upland rotations are prone to large N losses because of alternate soil wetting (anaerobic) and drying (aerobic) (Kundu and Ladha, 1995). It is postulated that the pattern of N and C dynamics and balances in these rice–wheat systems was expected to be similar to that of tropical rice-upland crops, but the temperature during the dry season will affect the magnitude of these processes. In a well-drained sandy rice-wheat soil with high pH where leaching and volatilization are major pathways (Katyal et al., 1985; Bijay-Singh et al., 1991), it is likely that N is readily lost but C is retained. The total soil N pool however, was maintained in the treatment where FYM partially replaced inorganic N, P, and K, suggesting that application of organic matter such as FYM is critical for maintaining soil fertility in the sandy soils of Punjab. It is also important to note that the quality of organic matter is equally critical (Becker et al., 1994; Clement et al., 1995). Green manure with very low C/N behaving similarly to mineral fertilizer was unable to immobilize N and therefore not able to arrest a soil N decline.
Both rice and wheat maintained a relatively high uptake of K (data not shown) even though insufficient amounts of K were applied. This was possible partly because of a fairly large K supply from the irrigation water and therefore explains the maintenance of the total pool of soil K. However, the declining available pool and negative apparent balance suggest that the system will not be able to sustain the K supply in the long run. The fact that the magnitude of the rice yield decline across treatments was correlated with initial yields provided further support for the hypothesis that higher N and K removals by the crops may have led to a decline in indigenous N and K supply. This suggests that the present rate of N and K application was inadequate for sustainable production of the rice–wheat system. In a rice-rice-wheat LTE in the Indo-Gangetic plains of Nepal, Regmi et al. (unpublished data, 2000) also observed a gradual decline of soil K resulting in a yield decline of rice and wheat. These results are contrary to the general belief that K is rarely a yield-limiting factor because most soils of the alluvial floodplains of Asia are high in K, and additional K is supplied from irrigation water (De Datta and Mikklesen, 1985; Bajwa, 1994).
In general, the magnitude of the yield decline in wheat was less than that in rice. The differential response of rice and wheat appeared to be associated with increased HI (kilogram grain per kilogram plant biomass) of the wheat cultivars used in the later years of the experiment. Wheat cultivar WH 542 used in 1995 through 1998 had a higher HI (0.44) than ‘PBW 154’ (0.38) and ‘PBW 120’ (0.36), used during 1986 through 1988 and 1989 through 1994, respectively. This could be because of continuous improvements in grain-yielding abilities of wheat as reported from India and elsewhere (Nagarajan, 1998; Rajaram, 1998). For wheat in northwestern India, it is reported that a genetic gain of 16 g kg-1 yr-1 has been made in yield since the mid-1960s (Morris et al., 1994; Nagarajan, 1998). The improvements were largely because of the higher number of grains per spike and grains per squared meter (Sankaran et al., 2000), thereby increasing HI. With rice, yield potential has remained at the same level as for ‘IR8’ and ‘Jaya’ released in the late 1960s, though disease and insect resistance and yield stability have improved (Aggarwal et al., 1997; Peng et al., 1999, 2000).
In wheat, treatments with FYM did not show any significant yield change at P < 0.05, unlike the inorganic N, P, and K treatments, which showed significant yield declines at P < 0.025 and P < 0.033 for T5 and T4, respectively. This could be associated with other benefits from organics apart from N, P, and K supply such as improvement in soil physical properties; improvement in microbial activities; better supply of nutrients such as S and B, which are not supplied by inorganic fertilizers; and less losses of nutrients from the soil (Abrol et al., 1997; Yadav et al., 2000).
The effects of organic amendments, however, did not appear to be sufficiently important in this particular experiment to be profitable for farmers. It should be noted, though, that similar economic analyses of the other LTEs often show better financial returns than were evident in this LTE (D. Dawe et al., unpublished data, 2001). It is also possible that there will be better financial returns to the use of FYM if the experiment is continued for a longer period of time, since rice yields in the treatment with FYM appeared to decline more slowly than in the full N-P-K treatment. The addition of FYM increased total soil C, N, and P by 9, 6, and 18%, respectively. This may result in higher yields or less input requirement in the long run. Our analysis did not account for possible environmental benefits of organic amendments, such as enhanced C sequestration.
The effects of weather, pest infestation, and adverse soil conditions were ruled out as the causes of yield decline in the present study. We analyzed the weather data from 1985 to 1996 to examine whether there was any significant change in temperature, sunshine hours per day, and rainfall pattern, which can affect the potential yield of crops. Based on linear regression analysis, we found no appreciable change in weather variables during the study period. Pest and disease incidences were properly controlled in the experiment. Analysis of soil samples showed no measurable change in electrical conductivity (EC) or pH after 14 yr of cropping.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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It is often difficult to maintain or increase the organic matter and N in cultivated soil (perhaps with an exception for the rice–rice cropping system in lowlands) unless a cover crop is included in the rotation or a heavy application of manures and crop residues is frequently applied (Stevenson, 1982). A potential solution is therefore to regulate the timing of N application to a crop. Replenishment of other nutrients such as K can also be difficult if farmers lack knowledge about the soil-supplying capacity and beneficial effects of K or face credit constraints when attempting to purchase K fertilizer. A fertilizer management strategy is needed that ensures (i) high and stable overall productivity with optimum economic return and (ii) sufficient nutrient supply for potential yield increases with minimal leakage of nutrients into the environment. The total input of N, P, and K should be close to optimal to ensure sufficient nutrient supply for higher yields.
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ACKNOWLEDGMENTS |
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Received for publication June 5, 2000.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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