Modeling nitrate leaching and organic‐C build‐up under semi‐arid cropping conditions of N India

Nitrate leaching, overall N balance, and organic‐C build‐up in a semi‐arid agro‐ecosystem in NW India was estimated from the results of a long‐term manurial trial with farmyard manure (FYM) and mineral‐N fertilizer in operation since 1967 at the Research Farm of CCS Haryana Agricultural University, Hisar, India. The model LEACHN was calibrated for the wheat‐growing period November 2000 to April 2001 and the leaching of nitrate during this period was predicted to 48 kg N ha^–1 without mineral‐N fertilization and 59 kg N ha^–1 with addition of 120 kg mineral‐N fertilizer, both with the addition of 15 t ha^–1 FYM. The N balance for the simulation period showed that the 120 kg N ha^–1–mineral N fertilization compared to zero mineral N, both plus FYM, resulted in only slightly higher crop uptake, leaching losses, and NH₃ volatilization, and a negligible increase of N in organic matter. The largest amount remains as an additional build‐up of mineral N in the profile (84.3 kg N ha^–1) which is prone to losses as ammonia or nitrate. The model was used to simulate organic‐C build‐up with FYM and a decrease of organic C without FYM for a period of 33 y (1967–2000). The simulated C build‐up to about 0.1 g kg^–1 agreed very well with the measured values and showed that additional mineral‐N fertilization will not have any significant effect on organic‐C content. Simulations with the assumption of no FYM application showed a gradual decrease of organic C from its starting value of 0.046 g kg^–1 in 1967 down to almost half of this. This agreed well with the observed organic‐C values of 0.028 g kg^–1 as measured for unmanured plots.