Short-term effects of tidal flooding on soil nitrogen mineralization in a Chinese tidal salt marsh

Abstract Tidal flooding is an important control of nitrogen biogeochemistry in wetland ecosystems of Yellow River Delta, China. Variations in hydrology could change soil redox dynamics and conditions for microorganisms living. A tidal simulation experiment was designed to extract tidal flooding effect on nitrogen mineralization of salt marsh soil. Inorganic nitrogen and relevant enzyme were measured during the 20-day incubation period. Considering the variation of both inorganic N and enzymes, nitrogen mineralization process in tidal salt marsh could be divided into 2 phases of short term response and longtime adaption by around 12th incubation day as the inflection point. Soil ammonium nitrogen (NH 4 + -N) and volatilized ammonia (NH 3 ) occupied the mineralization process since nitrate nitrogen (NO 3 − -N) was not detected over whole incubation period. NH 4 + -N varied fluctuant and increased significantly after 12 day's incubation. Released NH 3 reached to peak value of 14.24 mg m −2 d −1 at the inflection point and declined thereafter. Inorganic nitrogen released according to net nitrogen mineralization rate (R M ) under the tidal flooding condition without plant uptake except first 2 days. However, during the transitional period of 6–12 days, R M decreased notably to almost 0 and increased again after inflection point with the value of 0.182 mg kg −1 d −1 . It might be due to the change of microbial composition and function when soil shifted from oxic to anoxic, which were reflected by arylamidase, urease and fluorescein diacetate. Fluorescein diacetate hydrolysis and arylamidase had the similar variation of U style with decreasing activities before 12 days' incubation. All the enzymes measured in this experiment increased after inflection point. Whereas, urease activity kept constant from 2 to 12 days. Alternant oxidation reduction condition would increase N loss through denitrification and ammonia volatilization during the transitional period, while more inorganic nitrogen would be available in reductive environment of long-term tidal flooding. Therefore, hydrological process regulation has great influence on nitrogen cycling and further influence on wetland productivity.