1) Species loss is often associated with a decline in ecosystem functions. Globally, digging mammals (or ecosystem engineers) are functionally important, altering soil processes at local scales. However, their effects on the process of decomposition are poorly understood, particularly at larger scales, where the environment may moderate the magnitude of effects. 2) We tested the landscape-scale effects of reintroducing ecologically extinct digging mammals on two aspects of nutrient cycling over a large environmental gradient in Australia, where many digging mammals became extinct or ecologically extinct following the arrival of Europeans. 3) We measured the impacts of digging mammals on soil organic matter content and plant litter decomposition over a 3000 km transect, where annual average rainfall varied between 166 and 877 mm. We set up paired study plots (n = 8-10) inside and outside five reintroduction reserves. We took soil samples to assess soil organic matter content and set up litter bags to measure plant litter decomposition over 4 and 12 months. We used macro-invertebrate exclusion and control treatments to determine the relative importance of macroinvertebrates in decomposition with and without digging mammals. 4) Soil organic matter was greater in re-introduction areas, but the magnitude of the effect was driven by productivity (average annual rainfall as a proxy), with little effect of digging activity at the wettest sites. Short-term plant matter decomposition was greater in the presence of digging mammals, and their effect was dependent on the amount of rain that fell during the study period. Long-term litter decomposition increased with annual rainfall, independent of digging mammals. Unexpectedly, macroinvertebrate exclusion increased decomposition rates over 12 months. 5) Reintroduction of digging mammals substantially alters soil processes and organic matter decomposition, but impacts are rainfall-dependent. Restoring native digging mammals to their historical distribution is likely to reverse degradation of ecosystem processes, but the magnitude of this effect depends on the environment.