Effect of organic and inorganic fertilizers and rice straw on carbon sequestration and soil fertility under a rice–rice cropping pattern

Rice–fallow–rice, the dominant cropping system in Bangladesh, has received little attention regarding soil organic carbon (SOC) changes through organic amendments. Understanding the contributions of organic amendments in C sequestration is important for carbon budgeting. This study determined the effect of organic amendments on CO2 emission to the atmosphere and C sequestration in soil. A series of field experiments in five consecutive rice seasons were conducted during 2010–2012 at the research farm of Bangabandhu Sheikh Mujibur Rahman Agricultural University, Bangladesh, using five treatments – control, cow dung (CD), poultry manure (PM), rice straw (RS) and soil test-based fertilizer (STB). The carbon application rate from CD, PM and RS was 2 t C ha−1 season−1. Carbon dioxide production from rice fields was measured through NaOH absorption followed by HCl titration. The difference in the amount of cumulative CO2 evolution and SOC accretion between the control and organic treatments gave apparent C balance and C sequestration. CD, PM and RS contributed to the positive soil nutrient balance. Application of CD, PM and RS resulted in 36, 28 and 37% loss of applied C through emission, respectively. The application of organic C through RS, CD and PM accounted for 10, 30 and 49% sequestration, respectively. There were 34, 23 and 53% unaccounted amounts of applied C from CD, PM and RS, respectively, which may be attributed to anaerobic decomposition where CO2 was not produced, or escaping of the produced CO2 through aerenchyma channels to the leaf surface. Poultry manure was found to be efficient in increasing carbon and other nutrients in soils, and contributed to a higher grain yield of rice compared to RS and CD. Due to STB fertilization, microbial activities might be enhanced and favored better growth of root biomass, which contributed to slight sequestration of SOC in the rice–rice cropping system.