Bacterial necromass as the main source of organic matter in saline soils

Soil salinity poses a major threat to crop growth, microbial activity, and organic matter accumulation in agroecosystems in arid and semiarid regions. The limitations of carbon (C) accrual due to salinity can be partly mitigated by the application of organic fertilizers. Although microorganisms are crucial for soil organic carbon (SOC) stabilization, the relationships between living and dead microbial C pools and the community features of SOC accrual in saline soils are not known. A two-year field experiment was conducted to examine the effects of organic fertilizers on the microbial regulatory mechanisms of C sequestration in saline soil (chloride-sulfate salinity). Compared to manure addition alone, manure plus commercial humic acid increased SOC stock by 11% and decreased CO2 emissions by 10%, consequently facilitated soil C sequestration. We explain these results by greater bacterial necromass formation due to the dominance of r-strategists with faster turnover rate (growth and death), as well as larger necromass stability as supported by the increased aggregate stability under the addition of humic acids with manure. Humic acids increased the abundance of bacterial phylum Proteobacteria (copiotrophs) and decreased Acidobacteria (oligotrophs) compared with straw, indicating that r-strategists outcompeted K-strategists, leading to bacterial necromass accumulation. With larger C/N ratio (88), straw increased leucine aminopeptidase to mine N-rich substrates (i.e., from necromass and soil organic matter) and consequently reduced SOC stock by 8%. The decreased salinity and increased organic C availability under straw with manure addition also led to a 13% higher CO2 flux compared with manure application alone. Thus, humic acids added with manure benefited to SOC accumulation by raising bacterial necromass C and reducing CO2 emissions.