Managing agricultural grazing to enhance the carbon sequestration capacity of freshwater wetlands

Abstract

Freshwater wetlands are important carbon sinks with an estimated ~ 450 gigatons of carbon stored in their sediments. However, when disturbed they can become significant sources of carbon emissions. Understanding carbon dynamics in freshwater wetlands is a research priority to maximise carbon drawdown opportunities through effective management. Grazing can have considerable impacts on the health of freshwater wetland functionality. Fencing to exclude livestock from freshwater wetlands is one approach used to reduce grazer impacts; however, this is not always feasible. Managing grazing intensity is one option that is amenable to most farmers and resource managers; however, to date there have been no studies to evaluate its efficacy. The impacts of seasonal grazing intensity (by sheep) on carbon sequestration by rain-filled freshwater wetlands were investigated in south eastern Australia (Wimmera region). Over a 1-year study duration, this research compared: a) the effects of continuous grazing (sheep present across all seasons); b) crash grazing (sheep present within the spring); and c) exclusion plots (controls; no sheep access) on above-ground plant biomass, soil carbon concentration, and fluxes across seasons. Results suggest that above-ground biomass was 6.4% higher within exclusion plots compared to grazed plots in both continuous and crash grazing regimes. Exclusion plots had 30% higher soil carbon concentration compared to grazed plots. Open-continuously grazed plots had higher fluxes (8.69 ± 10.61 g CO2 m−2 day−1) compared with exclusion plots (6.92 ± 15.86 g CO2-e m−2 day−1). This study shows that exclusion plots had higher soil carbon concentration and lower carbon emissions. Further, excluding grazing from wetlands within the Wimmera region would increase above-ground biomass and soil carbon stock by 545,300 tons of carbon while reducing CO2 emissions by 782,412 tons of CO2 year−1.