Abiotic and biotic factors mediate the decomposition of crop straw in saline farmland soils

• Straw apparent carbon mineralization increased with increasing salinity. • The high-salinity soils had more residual nitrate, which was greatly depleted due to straw addition. • High salinity increased access to available nitrogen and recalcitrant substance. • Straw addition induced the rapid proliferation of copiotrophic bacterial groups. Crop straw addition is a well-established measure that promotes soil carbon (C) sequestration and increases soil quality. However, the effects and driving mechanisms of soil salinity on straw decomposition in saline farmland soils are poorly understood. In this study, we obtained five salinity gradient soils (namely, NS, LS, MS, HS, and SS corresponding to non-saline soil, low-salinity soil, moderately saline soil, highly saline soil, and severely saline soil, respectively) from saline farmland and conducted a 63- day indoor constant temperature incubation experiment with wheat and maize straw as C sources. We found a linear increase in the apparent C mineralization (C min ) with increasing salinity. The two types of straw added to the SS exhibited similar C min . However, in the other saline soils, maize straw addition had greater effects on C min than did wheat straw addition. The soil salinity significantly increased the dissolved organic C content under the straw addition. The high-salinity soils contained more residual NO 3 – -N, which was greatly depleted due to straw addition. The soil salinity also increased access to available N and recalcitrant substances by stimulating protease and polyphenol oxidase activities. Analysis of the soil bacterial communities revealed a decrease in both the richness and diversity indices in the high-salinity soils and the bacterial communities became simpler. Straw addition induced the rapid proliferation of copiotrophic bacterial groups, which outcompeted oligotrophic bacterial groups in saline farmland soils. These copiotrophic bacteria had to invest more energy in survival due to salt stress and therefore exhibited a greater microbial metabolic quotient. Overall, our study provides an interesting insight that straw addition to saline farmland has the potential to increase the straw mineralization rates and reduce NO 3 – -N accumulations. In the future, organic C regulation in saline farmland should consider N management and salinity reduction.