Assessing passive rehabilitation for carbon gains in rain-filled agricultural wetlands

Wetland ecosystems have a disproportionally large influence on the global carbon cycle. They can act as carbon sinks or sources depending upon their location, type, and condition. Rehabilitation of wetlands is gaining popularity as a nature-based approach to helping mitigate climate change; however, few studies have empirically tested the carbon benefits of wetland restoration, especially in freshwater environments. Here we investigated the effects of passive rehabilitation (i.e. fencing and agricultural release) of 16 semi-arid rain-filled freshwater wetlands in southeastern Australia. Eight control sites were compared with older (>10 year) or newer (2-5 year) rehabilitated sites, dominated by graminoids or eucalypts. Carbon stocks (soils and plant biomass), and emissions (carbon dioxide - CO₂; and methane - CH₄) were sampled across three seasons, representing natural filling and drawdown, and soil microbial communities were sampled in spring. We found no significant difference in soil carbon or greenhouse gas emissions between rehabilitated and control sites, however, plant biomass was significantly higher in older rehabilitated sites. Wetland carbon stocks were 19.21 t C_org ha^-1 and 2.84 t C_org ha^-1 for soils (top 20 cm; n = 137) and plant biomass (n = 288), respectively. Hydrology was a strong driver of wetland greenhouse gas emissions. Diffusive fluxes (n = 356) averaged 117.63 mmol CO₂ m² d^-1 and 2.98 mmol CH₄ m² d^-1 when wet, and 124.01 mmol CO₂ m² d^-1 and -0.41 mmol CH₄ m² d^-1 when dry. Soil microbial community richness was nearly 2-fold higher during the wet phase than the dry phase, including relative increases in Nitrososphaerales, Myxococcales and Koribacteraceae and methanogens Methanobacteriales. Vegetation type significantly influenced soil carbon, aboveground carbon, and greenhouse gas emissions. Overall, our results suggest that passive rehabilitation of rain-filled wetlands, while valuable for biodiversity and habitat provisioning, is ineffective for increasing carbon gains within 20 years. Carbon offsetting opportunities may be better in systems with faster sediment accretion. Active rehabilitation methods, particularly that reinstate the natural hydrology of drained wetlands, should also be considered.