Effects of salinity on microbial utilization of straw carbon and microbial residues retention in newly reclaimed coastal soil

Abstract

Soil salinity greatly influences soil organic carbon (SOC) stocks. However, how soil salinity affect SOC formation by changing microbial utilization of newly input organic C and subsequent microbial residues retention remains unclear. In this study, high salinity (HS, EC1:5 20.04 dS m−1) and low salinity (LS, EC1:5 0.90 dS m−1) soils were collected in a newly reclaimed coastal area in eastern China. Soil salt content decreased rapidly in the LS site due to irrigation and drainage management. The effect of soil salinity on microbial transformation of straw was investigated by adding 13C-labelled ryegrass straw into the HS and LS soils in a 180-day incubation experiment. Results showed that straw-derived dissolved organic C was higher in the HS soil. The higher availability of labile substrate in the HS soil offset the negative effect of high salinity on Gram-negative bacteria and fungi, but did not on Gram-positive bacteria. Microbial biomass C (MBC) and microbial C use efficiency (CUE) of straw were higher in the HS soil at 3 days due to the higher assimilation of straw C by Gram-negative bacteria and fungi. Straw-derived MBC and microbial CUE decreased over time and were similar between both soils at 60 and 180 days. Thus, high salinity did not decreased quantity and efficiency of microbial utilization of straw C. However, newly formed amino sugars were lower in the HS soil at 60 and 180 days suggesting that high salinity was not beneficial to the retention of microbial residues after microbial cells died. In conclusion, high salinity reduced microbial-derived SOC formation primarily by affecting stabilization and recycling of microbial necromass rather than by decreasing microbial utilization of newly input organic carbon in the studied coastal soil.