Influence (o)f nitrogen fertilization on the net ecosystem carbon budget in a temperate mono-rice paddy

Abstract

In temperate rice paddy fields, mono-rice is cultivated under flooding for < 120 days during summer, and thereafter, soil is maintained under the dried upland condition during the cold fallow season. In this region, rice is generally only cultivated by chemical fertilization without organic amendment. Furthermore, almost all rice straw is removed as a livestock feeding material, and then, soil organic carbon (SOC) stock is rapidly depleted with cropping practices. However, the seasonal and annual variations of the soil C balance have not been evaluated in this paddy field. To investigate the nitrogen (N) fertilization effect on SOC stock changes in mono rice paddy fields, urea was applied at different levels (0-180 kg N ha(-1)) as a N fertilizer and the annual C balances were determined by analyzing the net ecosystem C budget (NECB) for 2 years. The annual NECB was a negative value (minus 1192 to minus1434 kg C ha(-1) year(-1)), irrespective of the N fertilization rates. This means that these levels of SOC stock could be depleted with cropping practices. The negative NECB was mainly influenced by the highly mineralized C loss during the fallow season and harvest removal during the rice cultivation season. More specifically, the seasonal NECB was largely negative (minus 1679 to minus 1969 kg C ha(-1)) during the dried fallow season, but slightly positive (375-661 kg C ha(-1)) during flooded rice cultivation. However, the annual and seasonal NECBs were changed by N fertilization according to a quadratic response model. The annual and seasonal NECBs increased with the increasing N fertilization level, maximized at 113-127 kg N ha(-1), and thereafter, decreased. This indicates that the optimum level, not the excess level, of N fertilization is favorable to increase the soil C stock in a temperate mono-rice paddy soil.