Long term effects of inorganic fertilizer inputs on crop productivity in a rice-wheat cropping system

Three levels of N (40,80,120 kg N ha(-1)) and P (0,17.5,35 kg P ha(-1)), and 2 levels of K (0,33 kg K ha(-1)) were tested for 19 years in rice and wheat crops of a rice-wheat cropping system in a fixed layout of 3x2x2 factorial partially confounded design along with one control and 3 replications. From this trial, data of 7 treatments, i.e. 0-0-0, 40-35-33, 80-35-33, 120-35-33, 120-0-0, 120-35-0 and 120-0-33 kg ha(-1) N-P-K respectively were compared for yield trends, changes in response functions, soil organic -C and available N,P,K status. Soil organic - C decreased in unfertilized plots by 62% lover initial value of 0.45%) but increased by 44, 40 and 36% in plots receiving 120-35-33, 120-35-0 and 80-35-33 kg ha(-1) N-P-K respectively. Available N was also greatest in these same three treatments. Available soil P increased by about 5 fold in 15 years in treatments supplied with fertilizer P, but no significant change was detected in treatments without P addition. Yields of rice and wheat exhibited linear declining trend in all treatments. The highest rate of decline (89 kg ha(-1) year(-1) in rice and 175 kg ha(-1) year(-1) in wheat), however, was found when 120 kg ha(-1) N was applied alone. The least rate of decline of 20 kg ha(-1) year(-1) in rice and 58 kg ha(-1) year(-1) in wheat was observed when 40-35-33 kg ha(-1) N-P-K respectively was applied to both the crops. At currently recommended levels of NPK (120-35-33 kg ha(-1)), the rate of decline in yields was 25 kg ha(-1) year(-1) for rice and 62 kg ha(-1) year(-1) for wheat. Possible causes of these yield declines are discussed.