اثرات مدیریت پسماند بر خواص فیزیکی خاک تحت سیستم برنج، گندم در پنجاب هند

Sustainability of the rice–wheat rotation is important to Asia's food security. Intensive cropping with no return of crop residues and other organic materials results in loss of soil organic matter and is not sustainable. We evaluated effect of eight treatments comprised of various combinations of green manure (GM), wheat straw (WS), rice straw (RS), farmyard manure (FYM) and urea alone (control) on physical and hydraulic properties of soil in a rice–wheat experiment (1988–2001) on a loamy sand in Punjab, India. After rice harvest, organic carbon (OC) content in the FYM (0.51%), WS (0.56%) and WS + RS (0.59%) treatments were significantly greater as compared to control (0.42%). With addition of GM to all these treatments; FYM + GM (0.59%), WS + GM (0.60%), WS + RS + GM (0.64%) and GM (0.47%), organic carbon content further increased significantly. Increased OC content of the soil in turn improved its aggregation status, infiltration rate and decreased the bulk density, dispersion ratio and soil strength correspondingly. After wheat harvest mean effects of these organic treatments continued, but their magnitude decreased. The differences in rice yield were not significant among urea, GM and WS applied alone or in combination. However, FYM + GM + urea produced highest yield. There was no residual effect of the long-term application of GM, WS and RS incorporation in wheat yields, but FYM application to rice showed significant residual effects on wheat.

Rice–wheat is a major cropping system in north-west India because of the availability of high-yielding rice and wheat varieties and favourable climate conditions together with subsidized inputs and price support. The area under rice cultivation in the Punjab (India) increased from 0.29 million ha in 1965–1966 to 2.53 million ha in 2002–2003. In rice–wheat cropping system, puddling (working the soil in wet conditions) the soil for rice growing damages the soil structure and affects the physical and hydraulic properties of the soil (Meelu et al., 1979). Furthermore, ready availability of low cost inorganic fertilizers has increased the use of inorganic fertilizers and pushed apart the use of organic manures in rice–wheat system. Yaduvanshi (2001) and Khan et al. (1986) have reported that continuous cropping of rice–wheat with inorganic fertilizers has decreased the organic carbon content of the soil. Because of deteriorated physical and hydraulic properties and low organic carbon content of the soil, the present productivity potential of rice–wheat cropping system in this region is under threat despite increased use of inorganic fertilizers. The soil productivity is linked with soil organic carbon (SOC). Reduced productivity of rice–wheat system due to decline in SOC has been reported by Bhandari et al. (2002) and Regmi et al. (2002). For maintaining the productive potential of these structurally damaged soils which are low in organic matter content, it is imperative to improve the organic carbon content and restore structural status of the soil through the integrated use of organic manures and inorganic fertilizers in the rice–wheat cropping system. The present study was undertaken to assess the magnitudes of change in organic carbon content, physical and hydraulic properties of the soil with long-term applications of green manure, farmyard manure and crop residue in rice–wheat cropping system.

From the results it can be conclusively stated that in rice–wheat cropping system, use of green manure, farmyard manure and crop residues restore the damaged soil structure (due to puddling in rice) by increasing its organic carbon content, size and stability of aggregates, water retention and infiltration; and decreasing bulk density, dispersion ratio and soil strength. The beneficial effects confines to 10 cm with green manure, farmyard manure individually and to 15 cm in combination with crop residues. Incorporation of residues of the both crops together with green manure is most effective to maintain the structure of the plough layer soil in rice–wheat cropping system.