Effects of Residue Decomposition on Productivity and Soil Fertility in Rice–Wheat Rotation

containing about 1.90 million Mg of nutrients, are available in the IGP of India (Sarkar et al., 1999). In the Rice (Oryza sativa L.)–wheat (Triticum aestivum L.) farmers in Indian Punjab alone, about 12 million Mg of rice straw India burn or remove residues to facilitate seedbed preparation. Incorare burned annually, which causes about 0.7 million Mg poration of residues before planting of the next crop generally decreases yields due to N immobilization. Since a window of about 40 d of N loss. The gaseous emissions from burning of rice is available between rice harvest and wheat planting, the effect of straw are 70% CO2, 7% CO, 0.66% CH4, and 2.09% N2O time of incorporation on rice residue decomposition and N mineraliza(Samra et al., 2003). Estimated emissions of greenhouse tion–immobilization was studied in 1992–1993. The mass loss of resigases caused by burning of rice straw in the whole IGP due was 25% for a 10-d, 35% for a 20-d, and 51% for a 40-d decomposiof India are thus substantial. Besides contributing to tion period before wheat planting. Nitrogen release from residue the greenhouse effect, the large-scale burning of rice ranged from 6 to 9 kg ha 1 during the wheat season. The immobilizastraw results in serious health hazards as is evident from tion of urea N decreased when residue was allowed to decompose the reported increase in respiratory and eye problems for 10-d or longer. Based on these studies, a long-term (1993–2000) 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 immediately of time of residue incorporation before sowing wheat when compared with burning or removal of residue on yields, N-use efficiency, and before planting the next crop, grain yields were lower soil fertility. The effect of wheat residue incorporation with green than where residues are removed or burned, resulting manure (GM, Sesbania cannabina L.) on subsequent rice yields was in N immobilization, a problem that is attributable to also 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 (0.18–0.39 Mg ha 1) when Beri et al., 1995). Other potential problems of residue wheat residue was incorporated with GM. Starter N applied at residue incorporation just before rice transplanting include acincorporation did not influence wheat yields but decreased N recovery cumulation of phenolic acids in soil and increased CH4 efficiency. Physiological efficiency was higher when rice straw was emissions under flooded conditions (Grace et al., 2003). incorporated in wheat and when wheat straw plus GM were incorpoIn this case, the timing of incorporation of crop residues rated in rice than when rice straw was incorporated for 10 d or when is more important than the amount. Compared with the the straw was burned. The long-term application of rice residue increased C accumulation in soil. traditional method of wet incorporation shortly before planting of the next rice crop, the potential benefits of shallow incorporation shortly after crop harvest include accelerated aerobic decomposition of crop residues R is a major crop rotation in the Indo(about 50% of the C within 30–40 d), leading to inGangetic Plains (IGP) of South Asia, spread over creased N availability (Witt et al., 2000), and reduced 13.5 million ha in Bangladesh, India, Nepal, and PakiCH4 emissions (Wassmann et al., 2000). Early incorpostan (Ladha et al., 2000). Effective management of postration also allows additional time for phenol degradaharvest crop residues (straw) is perhaps the foremost tion to occur under aerobic conditions, thereby avoiding challenge facing the intensive rice–wheat-producing reany 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 costs the animals. However, the rice residue due to large silica for environmental reasons. Farmers will probably only content is normally burned. Burning of rice residues is incorporate crop residues if legislation forces them to cost-effective and the predominant method of disposal or if there is a clear yield increase that they cannot in areas under combined harvesting in the IGP (Samra achieve with the application of additional fertilizer. A et al., 2003). However, disposal of crop residues by burnwindow of about 35 to 40 d is available between the ing is often criticized for accelerating losses of soil orharvesting of rice and seeding of wheat, which can be ganic matter (SOM) and nutrients, increasing C emisused for in situ decomposition of rice straw. Similarly, sions, causing intense air pollution, and reducing soil a fallow period of 50 to 60 d is available after the wheat microbial activity (Biederbeck et al., 1980; Rasmussen harvest and before rice planting; this allows decomposiet al., 1980; Kumar and Goh, 2000). According to an tion of wheat straw and the raising of a GM crop (Yadestimate, 113.6 million Mg of rice and wheat residues, vinder-Singh et al., 1991, 1994). Apart from enhancing residue 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 immo004, 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. 68:854–864 (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