Effect of N and water management on greenhouse gas emission and N availability in paddy soils of Bangladesh

Soil moisture and nitrogen (N) are the two important factors determining growth and yield of rice along with their vital influence on paddy soil emission of CH4, N2O and CO2, N mineralization and possibly micronutrient availability. Rice cultivation by alternate wetting and drying (AWD), instead of conventional/continuous flooding (CF), is expanding in South-East Asia due to shortage of irrigation water. However, studies comparing the influence of irrigation management on GHG emission, soil N availability, biochemical soil parameters and available content of micronutrients is still lacking for young floodplain soils in Bangladesh. We assessed the consequence of AWD and direct seeded (DS) rice on CH4, N2O and CO2 emissions, N mineralization, and micronutrients availability compare to CF in a paddy field experiment at the Bangladesh Agricultural University in the 2014 Boro season. The cultivated rice variety (BRRI dhan28) was grown for 14 weeks with (T2: N120) or without N fertilizer (T1:N0) (subplot) supply under three water management practices (CF, AWD and DS). Global warming potentials (GWPs) based on cumulative emissions of CH4, CO2 and N2O were compared. In addition dissolved organic carbon (DOC), Fe, Mn and Zn content in soil solution, collected at two depths (0-6cm and 6-12cm) were measured biweekly. Grain and straw yield parameters were recorded during harvest. Water management practices had no large influences on the evolution of mineral N (NH4-N+NO3-N) (mg kg-1) over time. On the opposite, CH4 fluxes (mg m-2 h-1) over time varied widely between the water management practices. As expected, CH4 fluxes in all the continuously flooded (CF) soils were higher compared to AWD and DS plots. Also differences between CH4 emissions of the N0 and N120 treatments were high with CF, low with AWD and negligible with DS management. The N applied CF and AWD plots had higher CH4 fluxes compared to those of the nil N application objects (N0). Analyses of released CO2 and N2O and fixed NH4 are still on-going. The DOC content in pore water increased gradually during 12 weeks of rice growing regardless water management, N fertilization and soil depth. At any time the DOC content was higher under CF and AWD compare to DS management. The Fe content in soil pore solution was higher and the Mn content was lower in CF compare to AWD, suggesting more Fe reduction in the former case. The content of Zn was unaffected by irrigation and N management. The average grain yield (t ha-1) in N fertilized AWD plots was 9 and 17% higher than those of CF and DS plots, respectively. Based on all these results we conclude that rice growing through AWD decreased CH4 emissions without any influence on grain yield, despite limiting the availability of Fe in subtropical paddy soils.