Complete annual CO2, CH4, and N2O balance of a temperate riparian wetland 12 years after rewetting

Abstract

Drained riparian wetlands have been rewetted and restored in recent decades to remove nutrients, increase biodiversity, and mitigate soil carbon dioxide (CO2) emissions. Yet, few studies have documented the long-term effects of rewetting on complete greenhouse gas (GHG) balances including emissions of CO2, methane (CH4), and nitrous oxide (N2O). Here, we report the complete annual GHG balance of an extensively managed riparian wetland, dominated by creeping bentgrass (Agrostis stolonifera), 12 years after rewetting. Net ecosystem exchange (NEE) of CO2 was measured by transparent closed chambers, and fluxes were partitioned into gross primary production (GPP) and ecosystem respiration (ER) for modelling and extrapolation to annual emissions based on photosynthetically active radiation, ratio vegetation index and temperature. Fluxes of CH4 and N2O were monitored with opaque chambers. Groundwater table (GWT) was close to soil surface for most of the growing period, whereas the site was inundated during winter. Biomass was cut in late summer (8.5 Mg dry weight ha−1), but left on-site according to current management in the area. Annual ER (1360 g CO2-C m−2) exceeded GPP (–1140 g CO2-C m−2), and the ecosystem was a net source of CO2 with NEE of 220 g CO2-C m−2 yr−1. However, fluxes of CH4 (53 g CH4 m−2 yr−1) dominated the annual GHG balance with 405 g CO2-Ceq m−2 yr−1 which contributed 60% to the total GHG balance. Fluxes of N2O were primarily found at times of changing GWT with annual emission of 0.7 g N2O m−2 (50 g CO2-Ceq m−2) equal to 7% of the complete GHG balance. With proper management, rewetting and restoration of wetlands is expected to eventually resume the carbon sink function of natural wetlands, but this was not found in the present study as net fluxes of both CO2 and CH4 were positive. This was mainly attributed to on-site deposition of biomass which apparently stimulated both CO2 and CH4 emissions and partly reduced GPP by acting as a mulch layer. Future studies should focus on managements that increase CO2 uptake and biomass yield, and at the same time reduce CH4 emissions; such managements should avoid on-site deposition of aboveground biomass at rewetted sites.