Running Title : Reducing carbon emissions from farm dams 5 6

Agricultural practices have created tens of millions of small artificial water bodies (“farm 35 dams” or “agricultural ponds”) to provide water for domestic livestock worldwide. Among 36 freshwater ecosystems, farm dams have some of the highest greenhouse gas (GHG) 37 emissions per m 2 due to fertilizer and manure run-off boosting methane production – the 38 second most potent GHG. However, management strategies to mitigate the substantial 39 emissions from millions of farm dams remain unexplored. We tested the hypothesis that 40 installing fences to exclude livestock could reduce nutrients, improve water quality, and 41 lower aquatic GHG emissions. We established a large-scale experiment spanning 400 km 42 across south-eastern Australia where we compared unfenced (N = 33) and fenced farm dams 43 (N = 31) within 17 livestock farms. We found that fenced farm dams generated 32% less 44 dissolved nitrogen, 39% less dissolved phosphorus, 22% more dissolved oxygen, and 56% 45 less diffusive methane emissions than unfenced dams. The higher dissolved oxygen and the 46 lower dissolved nitrogen underpinned the reduced CO 2 -eq (methane + carbon dioxide) carbon 47 fluxes of fenced dams. Dams with very high dissolved oxygen (>10 mg L -1 ) showed net 48 uptake of carbon dioxide that exceeded CO 2 -eq. methane emissions (i.e., negative radiative 49 balance), contradicting the general paradigm that farm dams are invariantly contributing to 50 atmospheric warming. We also found a positive association between sediment organic carbon 51 and CO 2 -eq fluxes. Our results demonstrate that simple management actions can dramatically 52 improve water quality and decrease GHG emissions while contributing to more productive 53 and sustainable farming.