Fencing farm dams to exclude livestock halves methane emissions and improves water quality

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.