Yield trends, and changes in soil organic-C and available NPK in a long-term rice-wheat system under integrated use of manures and fertilisers

Rice-wheat rotations, practised in 12.5 million ha in Indo-Gangetic Plain r egion (IGPR), are the most important production system for food security of south Asian countries. Recent reports, however, indicate that the system i s under production fatigue as yields have started declining due to continuo us rice-wheat cultivation. We analysed the yield trends and effect of ferti liser NPK application, alone or in combination with farmyard manure (FYM), green manure (GM) or wheat crop residue (CR) incorporation, on the changes in soil organic carbon (OC) and available NPK contents. Data of a long-term experiment conducted at six locations in the IGPR and at one location in t he Central Highlands and Plateau region of India, adjacent to IGPR, reveale d that yields of rice and wheat were constantly greater in all the years wh en complete doses of NPK were applied through fertilisers, or 50% dose of N PK were applied through fertilisers along with organic materials compared t o that in unfertilised-control. At Ludhiana and Pantnagar, where yield leve ls during initial years of the experiment were relatively high, a declining trend in yield was observed over years in all the treatments. At other loc ations, rice or wheat yield trends in the treatments receiving complete dos es of fertiliser NPK were generally not significant, but in the treatments having fertiliser + manure combination, positive and significant trends in yield of rice were observed. Linear regressions fitted to pooled data acros s the locations revealed highly significant (p < 0.01) annual increase in y ield of rice with integrated supply of nutrients through fertilisers and ma nures, indicating thereby the advantage of combined use of manures plus fer tilisers over fertilisers alone in sustaining crop yields. Soil OC decrease d over time at locations where the OC content was greater than 6.5 g kg(-1) at the start of the long-term experiment, but increased at locations havin g initially low (<5.0 g kg(-1)) OC content. Available P content increased w ith P additions through fertilisers or manures at four locations. Temporal changes in available P content, however, appeared to have depended on the c hanges in soil OC. Despite annual K additions at recommended rates through fertilisers and at more than the recommended rates through organic manures, available K content decreased due to continuous cropping at five locations . (C) 2000 Published by Elsevier Science B.V.