Effects of soil amendments on fractions and stability of soil organic matter in saline-alkaline paddy.

Soil amelioration is an effective practice to alleviate the adverse effects of soil salinization. However, increasing the fertility of salt-affected soils has been challenging, particularly in coastal saline-alkaline paddy soils. Here, we carried out a 45-day incubation experiment to evaluate the impacts of soil amendments on fractions and stability of soil organic matter (SOM) in a saline-alkaline paddy. The experiment simulates the flooding-draining practice and consists of CaCO, gypsum and biochar amendments using different fertility soils. We measured dissolved organic carbon (DOC) and nitrogen (DON) in supernatant liquids, water-soluble cations, water extractable organic carbon (WEOC) and nitrogen (WEON), and microbial biomass carbon (MBC) and nitrogen (MBN) in soils after the incubation. Results showed that water soluble sodium (Na) was significantly decreased under all amendments (by 17%-32%), except in high fertility soil. We found a significant decrease in DOC (by 36%-47%) under gypsum treatment, but in DON (by 18%-59%) under biochar treatment. However, there was no significant effect on DOC or DON under CaCO treatment. Gypsum treatment led to decreased WEOC content (by 0.067%-5.4%), but increased MBC (by 0.16%-44%) and MBN (by 8.3%-37%) in all soils. Biochar treatment caused a decrease in the ratios of WEOC to soil organic carbon (SOC) and WEON to total nitrogen (TN), and an increase in MBC:SOC and MBN:TN ratios. These results suggest that gypsum and biochar amendments can enhance SOM stability in the saline-alkaline paddy. However, SOM stability was not enhanced under CaCO treatment, probably due to the presence of a large amount of Na in these soils. Our study highlights that soil amelioration has different effects on soil carbon and nitrogen cycles in the saline-alkaline paddy soils, which is associated with water-logged condition.