Effects of crabs on greenhouse gas emissions, soil nutrients, and stoichiometry in a subtropical estuarine wetland

Crabs may elicit effects on wetland carbon (C), nitrogen (N), and phosphorus (P) concentrations and associated ecological stoichiometry. In this study, we assessed effects of crabs on carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) emissions; soil C, N, and P concentrations; and stoichiometry in upper and mid-tidal flats of an estuarine wetland in China. The results showed that averaged CO2, CH4, and N2O fluxes were greater in the upper and mid-tidal flats in the presence of crabs, being 46.4, 66.7, and 69.7% and 53.6, 143, and 73.1% greater than control, respectively. Mixed model analyses showed overall positive relationships between wetland soil CO2 CH4 and N2O emissions (F = 4.65, P = 0.033; F = 42.42, P = 0.042 and F = 10.2, P = 0.0018, respectively) in the presence of crabs, taking into account season, flooding intensity, and plot effects. This may be related to the direct effects of respiration and the indirect effects of feeding, excretion, and disturbance of soil on microorganisms and/or plant roots. There were no effects of crabs on total C or N concentrations, whereas decreased soil total P concentrations, especially in the upper-tidal flats (P = 0.04). Crab presence was positively associated with soil C/P and N/P ratios (P < 0.0001 and P < 0.0001, respectively), taking into account season, flooding intensity, and plot effects. In the upper and mid-tidal flats, soil CO2 emissions were negatively correlated with total soil C; CH4 emissions were positively correlated with ratios of C/N and C/P; and N2O emissions were positively correlated with N content. In general, global warming potential (GWP) of the upper-tidal flats in the presence of crabs increased by 138% compared with the absence of crabs, and GWP of the mid-tidal flats in the presence of crabs increased by 99.3% compared with the absence of crabs. Global warming and associated flooding rise in several coastal wetland areas are favoring benthic fauna number enhancement, and this in turn increases GWP of overall gas emissions further contributing to future warming rise.