Effects of Long-term Nitrogen and Phosphorus Supply on Soil Microbial Necromass Carbon and Enzyme Activities in a Coastal Wetland

The variation in nitrogen and phosphorus levels in coastal wetland soils affects the structure and function of coastal wetland ecosystems. While some studies have explored the effects of long-term nitrogen and phosphorus addition on soil microbial communities in coastal wetlands, the mechanisms by which these additions influence soil microbial necromass carbon and enzyme activities in the context of the carbon cycle remain unclear. Our study is based on a seven-year (2014-2021) in situ nitrogen and phosphorus addition field experiment in the Yellow River Delta coastal wetlands. The experiment included three nitrogen and phosphorus supply levels (low, medium and high), with three nitrogen-to-phosphorus ratios (5∶1, 15∶1 and 45∶1, ratios, hereafter) for each supply level. The effects of nitrogen and phosphorus supply levels and ratios on soil microbial necromass carbon, enzyme activities, soil organic carbon (SOC), available nitrogen, available phosphorus and their interrelationships were investigated. The results show that nitrogen and phosphorus supply levels and ratios had no significant impact on total microbial necromass carbon, bacterial necromass carbon, fungal necromass carbon, their ratios to SOC, SOC content, available nitrogen, available phosphorus, or the activities of soil β-1, 4-N-acetylglucosaminidase, leucine aminopeptidase, and alkaline phosphatase. However, nitrogen and phosphorus supply levels and ratios significantly affected β-1, 4-glucosidase content in soil, with high nitrogen and phosphorus supply levels under the 15∶1 ratio reducing activity by approximately 40% compared to the control. Significant correlations were observed between soil microbial necromass carbon, enzyme activities and soil carbon, nitrogen and phosphorus contents under different nitrogen and phosphorus supply levels and ratios. This study highlights the importance of soil microbial residue carbon and enzyme activities in maintaining SOC stocks in coastal wetlands, providing valuable insights for predicting changes in SOC in these ecosystems.