Agricultural practices have created tens of millions of small artificial water bodies (“farm dams” or “agricultural ponds”) to provide water for domestic livestock worldwide. Among freshwater ecosystems, farm dams have some of the highest greenhouse gas (GHG) emissions per m² due to fertilizer and manure run-off boosting methane production – an extremely potent GHG. However, management strategies to mitigate the substantial emissions from millions of farm dams remain unexplored. We tested the hypothesis that installing fences to exclude livestock could reduce nutrients, improve water quality, and lower aquatic GHG emissions. We established a large-scale experiment spanning 400 km across south-eastern Australia where we compared unfenced (N = 33) and fenced farm dams (N = 31) within 17 livestock farms. Fenced farm dams recorded 32% less dissolved nitrogen, 39% less dissolved phosphorus, 22% more dissolved oxygen, and produced 56% less diffusive methane emissions than unfenced dams. We found no effect of farm dam management on diffusive carbon dioxide emissions and on the organic carbon in the soil. Dissolved oxygen was the most important variable explaining changes in carbon fluxes across dams, whereby doubling dissolved oxygen from 5 to 10 mg L^-1 led to a 74% decrease in methane fluxes, a 124% decrease in carbon dioxide fluxes, and a 96% decrease in CO₂-eq (CH₄ + CO₂) fluxes. Dams with very high dissolved oxygen (>10 mg L^-1) showed a switch from positive to negative CO₂-eq. (CO₂ + CH₄) fluxes (i.e., negative radiative balance), indicating a positive contribution to reducing atmospheric warming. Our results demonstrate that simple management actions can dramatically improve water quality and decrease methane emissions while contributing to more productive and sustainable farming.

In April 2021, we sampled farm dams across 400 km of the Australian South West Slopes bioregion in south-eastern New South Wales. The study region has a warm temperate climate, with hot dry summers and cool humid winters (the largest city of Albury has an annual mean temperature of 22°C and annual rainfall of 691 mm). Most of the area is dedicated to livestock grazing (especially beef cattle and sheep) and dryland cropping (mainly cereals and oilseed). We surveyed 64 farm dams located in pastures on 17 farming properties. Within each property, we established two experimental treatments: “unfenced” farm dams and “fenced” farm dams. For each experimental treatment within a farming property, we measured between 1 and 5 dams (depending on availability) on the same day. Unfenced farm dams (N = 33) received no management intervention to improve their ecological condition. Fenced farm dams (N = 31) were either entirely fenced (with a pump delivering water into drinking troughs) or partly fenced (providing water access through a hardened access point) for at least 2 years prior to sampling.

We used the statistical software R version 4.0.3 (R Core Team, 2020) with the packages nlme (Pinheiro et al., 2020) and effects (Fox & Weisberg, 2018, 2019) for the statistical analyses, and dplyr (Wickham et al., 2018), plyr (Wickham, 2011), and ggplot2 (Wickham, 2009) for data manipulation and plotting.