Substrate quality overrides soil salinity in mediating microbial respiration in coastal wetlands

Abstract

As productive and essential ecosystems, coastal wetlands have experienced increased environmental impacts such as saltwater intrusion and eutrophication, resulting in significant shifts in microbially mediated ecosystem functions, such as carbon sequestration and nutrient transformations. The soil microbial respiration, a primary process in the transfer of carbon from soil to the atmosphere, is susceptible to environmental changes. However, studies on how salinity affects soil microbial respiration in coastal wetlands have not been fully explored. Soil samples were systematically collected from divergent sampling sites covering medium- and extremely-saline wetlands along a river-estuary-coast continuum to investigate mechanisms controlling soil microbial respiration in coastal wetlands. According to the results, the microbial biomass and carbon-related extracellular enzyme activities were significantly lower in extremely saline (ECe >15 ds m−1, ES) than medium and highly saline soils (ECe <15 ds m−1, MHS) (p < 0.05), indicating a suppressive effect of salinity on soil microbiota. Meanwhile, high-salinity soils had lower vector length and soil microbial respiration rates, suggesting that soils with low carbon limitation might cause less carbon loss under higher salinity environments. Moreover, it was showed that increased available phosphorus could alleviate microbial carbon limitations. Changes in the microbial functional community demonstrated that the microbial community in favor of metabolic mediates and secondary metabolites substrates (regarded as labile substrates) were more sensitive to salinity. The partial least square path modeling further confirmed that microbial nutrient limitation and microbial biomass contribute more directly to promoting soil microbial respiration. These results have substantial implications for elucidating carbon dynamics in coastal wetlands ecosystems under increased nutrient discharge and sea-level rise.