Peatlands are globally significant carbon sinks, but when disturbed, have the potential to release carbon back to the atmosphere as greenhouse gases. Feral horse populations in the Australian Alps degrade Sphagnum peatlands, which are highly sensitive to disturbance. However, the link between this degradation and peatland carbon cycling is not understood. Here, we compared the autumn daytime carbon dioxide (CO 2) and methane (CH 4) fluxes of 12 alpine and subalpine Sphagnum peatlands in Kosciuszko National Park, Australia. The presence of feral horses at these sites was correlated with higher carbon loss: sites with horses were losing carbon to the atmosphere (4.83 and 8.18 g CO 2 -e m−2 d−1 in areas of Sphagnum moss and bare soil, respectively), whereas sites without horses were removing carbon from the atmosphere (−6.39 g CO 2 -e m−2 d−1). Sites with feral horses also had higher soil bulk density, temperature, and electrical conductivity (EC), and higher water pH, EC, and turbidity, than sites without horses. Our findings suggest that excluding feral horses from peatland areas could reduce rates of carbon loss to the atmosphere, in addition to improving overall site condition, peat soil condition, and water quality. We discuss potential management applications, further research, and restoration opportunities arising from these results. • Horse-present peatlands were net carbon sources; horse-absent peatlands were net carbon sinks. • Net carbon emission was 91% higher at bare soil areas than Sphagnum -covered areas. • Soil bulk density, temperature, and EC were higher at peatland sites with feral horses. • Water pH, EC, and turbidity were higher at peatland sites with feral horses. [ABSTRACT FROM AUTHOR]