Short-term responses of soil enzyme activities and carbon mineralization to added nitrogen and litter in a freshwater marsh of Northeast China

• Nitrogen and litter addition could affect the stability of soil organic C in marshland.
• Ammonium nitrate addition reduced soil C mineralization while litter increased it.
• Both ammonium nitrate and urea significantly decreased the soil DOC contents.
• Soil urease and invertase activities were accelerated by the added litter.

Soil organic matter decomposition is regulated by nutrient availability. Adding nitrogen (N) and litter could affect the stability of soil organic carbon (SOC). An incubation experiment was conducted to examine the effects of ammonium nitrate (AN), urea (U), and litter amendment on activities of soil microbial extracellular enzyme, soil microbial biomass C (MBC), dissolved organic C (DOC), and C mineralization in freshwater marsh. The results showed that adding N including AN and U decreased soil urease activity, MBC, DOC, and soil pH. However, litter amendment and combined litter and N amendment increased the enzymes activities of urease and invertase, MBC and DOC concentrations. The response of soil C mineralization to N and litter additions was different, being inhibited with AN addition while stimulated with littler addition and the combined litter and N addition, initially stimulated but inhibited thereafter with urea addition. Our results suggest that adding N is helpful for improving marshland soil C stocks via decreasing soil cumulative CO2–C emissions, labile organic carbon concentration of MBC and DOC. However, adding litter could reduce C storage stability by stimulating soil C mineralization and increasing soil labile C fractions and enzyme activities.