Landform and vegetation patch type moderate the effects of grazing-induced disturbance on carbon and nitrogen pools in a semi-arid woodland

Background/Question/Methods

Because of their extensive distribution, drylands contain a large portion of the global soil organic carbon (C) pool and are crucial as both a source and sink for atmospheric carbon. Grazing by exotic species has caused substantial degradation in drylands around the world, but the effects of grazing-induced disturbance on soil C pools in these systems have been difficult to generalize. We hypothesized that some of the difficulty generalizing was due to spatial heterogeneity characteristic of many dryland systems. We examined the importance of heterogeneity at vegetation and landform scales on the influence of livestock grazing on soil and litter C and nitrogen (N) pools. In a semi-arid woodland in eastern Australia, litter and soil C and N pools were quantified in different vegetation microsites (tree, shrub, open) and landform elements (dune, swale) within three areas that had different management histories and varying degrees of grazing-induced disturbance (low, moderate, high).

Results/Conclusions

We found that vegetation, landform, and grazing disturbance affected litter and soil C and N pools singly and through interactions. First, litter and soil C and N were diminished by increased grazing disturbance, but only in certain vegetation microsites-landform combinations. Litter pools decreased with greater grazing disturbance beneath trees (P < 0.01) and in swales (P < 0.01). Soil C and N concentrations decreased with greater grazing beneath trees and, to a lesser degree, beneath shrubs in swales, but were unaffected in dunes and open microsites (P < 0.05). Second, resource pools were not distributed evenly across vegetation microsites or landforms. Litter and soil C and N pools were greatest beneath trees in swales (P < 0.01). Overall, our results suggest that predicting changes in C and N pools in drylands will require an understanding of the distribution of different vegetation and landform patch types and the interactions between these and disturbance.  Reliable estimates of landscape-wide resource pools will therefore require rigorous, across-landscape sampling and careful characterization of spatial heterogeneity.