Nitrogen transformation and carbon sequestration in wetland paddy field of Bangladesh

Rice-based cropping systems in Bangladesh have received little consideration of changing nitrogen (N) and carbon (C) in soil. Therefore, this study was conducted at the Research Field of Bangabandhu Sheikh Mujibur Rahman Agricultural University, Bangladesh, in monsoon to determine release patterns of ammonium–N (NH 4 + –N) and nitrate–N (NO 3 − –N), C contents of different sized soil fractions and to quantify the rates of C sequestration as influenced by organic amendments and N fertilizer in wetland paddy fields. Rice straw, vermicompost, rice husk biochar, cow dung and poultry manure were applied using 2 t C ha −1 with 0, 100 and 150 N kg ha −1 in a factorial randomized complete block design. Application of different organic materials attributed maximum amount of NH 4 + –N and NO 3 − –N in soils at 45–60 days after transplanting, while N fertilizer application considering 150 kg N ha −1 provided the maximum amounts of available N. Rice grain yield was found statistically similar between 100 and 150 kg N ha −1 . Carbon contents in soils of different sized fractions inconsistently varied with organic and inorganic fertilization and time elapsed. The highest amount of organic C was sequestered in cow dung-treated plots followed by rice straw, rice husk biochar, poultry manure and vermicompost. Nitrogen fertilizer enhanced decomposition of organic materials, and therefore, C sequestration was significantly lower with higher N rates. The study recommends continuous application of organic fertilizers and optimization of N in the tropical and subtropical regions which ultimately either contribute to maintain or increase C sequestration in crop fields.