Impacts of feral grazers and unseasonal summer flooding on floodplain carbon dynamics: A case study

Wetlands are amongst the world's most effective natural carbon sinks, and also have the potential to emit large quantities of stored carbon back into the atmosphere if disturbed. In the Murray-Darling Basin, high consumptive demand for water has led to widespread wetland loss and degradation. Remaining wetlands are commonly under additional pressure from grazing and trampling by feral herbivores or livestock and unseasonal flooding in summer to meet peak irrigation demand. Here, during a summer flood in Barmah National Park, we compared soil carbon and nitrogen and carbon emissions within, and outside of, a feral herbivore exclusion plot. The sustained global warming potential from the whole wetland (i.e., both within- and outside of the exclusion plot), totalled from both carbon dioxide (CO2) and methane (CH4) emissions, averaged 132.27 ± 9.88 g CO2-e m−2 d−1, an estimated 28- to 89-fold higher than the global average for natural wetlands. The high warming potential of the wetland was driven strongly by CH4 emissions, which made up over 90% of total CO2-equivalents, with CO2 contributing <10%. We expect these values represent peak CH4 emissions from the site due to high water and soil temperatures, and low water levels, at the time of sampling. Soil carbon and nitrogen did not differ significantly within and outside of the exclusion plot, but surface soil samples indicated recent condition improvement in the absence of feral herbivores. We suggest that an emissions avoidance incentive could be developed around unseasonal, summer watering of forested floodplains where flood timing can be controlled.