Influence of Organic Amendments on Soil Carbon Sequestration Potential of Paddy Soils under Two Irrigation Regimes

Soil organic carbon (OC) is one of the most important soil components regulating soil quality, fertility and agronomic productivity as well as the global carbon (C) cycle. Soil acts as a sink for global C, which can be influenced by the water regime and organic matter (OM) management in field. The aim of this study is to evaluate the effect of the application of different organic amendments on C sequestration in paddy soils under contrasting irrigation regimes. A 4-month pot experiment was conducted under net house conditions and the treatments were composed of two organic amendments: rice straw (RS) and poultry manure (PM); four application rates of amendment: 0 g (control), 2.5 g, 5.0 g and 15.0 g kg−1 soil; and two irrigation regimes: (i) continuous waterlogging condition (CWL) and (ii) alternate wetting and drying (AWD). After the incubation period, soil samples were collected from the pot and isolated into labile (>53 µm) and mineral-associated (<53 µm) OM. Bulk (before and after incubation) and fractionated soil samples were analyzed for OC, total nitrogen (N), C:N ratio; and C sequestration percentage was calculated. Relatively higher amounts of soil OC were present in CWL condition (1.23%) than AWD (1.13%). The C sequestration potential also showed the similar trend (CWL: 47% > AWD: 35%). This was explained by the induced aerobic condition in between the anerobic condition in AWD and the continuous anaerobic condition in CWL which resulted in a difference in OM decomposition. The mineral-associated OM fraction (<53 µm) was higher in the CWL condition than AWD condition which also indicated the importance of the chemical stabilization of OC (OC bound to minerals) in the CWL condition. The application of PM led to a significant increase (45%) in C sequestration potential than RS (37%). This could be attributed to C:N ratio and probable biochemical composition of amendments which resulted in lower decomposability of PM than RS, and also in line with the higher distribution of OC in mineral-bound OM than labile fraction. The application of higher organic amendments did not increase OC content, and declined C sequestration potential in soils as the microbial activity presumably did not match with the amendment amount. Overall, C sequestration potential was higher with 5 g PM kg−1 soil application under CWL-irrigated paddy soil. The findings indicated the need to pay more attention to the selection of the proper type and rate of organic amendments for higher C sequestration in soil under a specific irrigation system for sustainable agriculture.