Local vegetation and hydroperiod influence spatial and temporal patterns of carbon and microbe response to wetland rehabilitation

Wetlands have a major influence on the global carbon cycle, with capacity to act as carbon ‘sinks’ or ‘sources’. The source-sink capacity of wetlands is governed by microbially-mediated biogeochemical processes, which are furthermore regulated by environmental conditions. With growing interest in nature-based climate solutions, policymakers and resource managers seek information on ways how wetlands can be managed to maximize carbon drawdown. Here, we explored how rehabilitation (i.e., fencing and grazing removal) influences greenhouse gas (GHG) emissions and soil biogeochemistry (microbial communities and soil quality) from semi-arid, rain-filled freshwater wetlands in south-eastern Australia. Specifically, we investigated the carbon and microbes response under major local environmental factors, such as vegetation type (graminoids vs eucalyptus tree) and seasonal hydroperiod (spring vs autumn), across wetlands that are currently used for grazing or cropping and those that have been under fencing rehabilitation for up to 20 years. We found that rehabilitation did not reduce CO2 and CH4 emissions, rather CH4 flux rates were elevated when the wetlands were flooded (spring). Anaerobic conditions from spring waterlogged soils also increased microbial diversity by 2.6-fold, and influenced the relative abundance of putative methane oxidizers (Nitrososphaerales and Mixococcales) and methanogenic archaea (Methanomicrobia, Methanobacteria). Organic matter quality (measured as C:N ratio) was reduced by the removal of grazing pressure in eucalyptus-dominated sites only. Soil quality, influenced by vegetation type, also had a significant impact on relative abundance of putative nitrogen and carbon cyclers. Overall, our results suggest that fencing rehabilitation in semi-arid rain-filled wetlands had a minor impact on microbial dynamics and carbon processes, overshadowed by the influence of the water table and vegetation type. Focusing future research on spatial and temporal patterns of carbon and microbe rehabilitation responses will help managers devise the most effective rehabilitation practice within a particular geographical area.