Long-term livestock exclusion is associated with changes in perennial grass composition and the physical properties of soil in an arid grassland

Background/Question/Methods   Changes in soil and vegetation due to livestock grazing are occurring in arid lands throughout the world. The most extreme cases result in desertification, which is seen as largely irreversible, due to feedback loops that perpetuate vegetation changes and altered soil properties. However, recent work has documented recovery of perennial grasses in long-term livestock exclusions at four desertified sites. At one such site, perennial grass recovery was concomitant with changes in soil physical and chemical properties. These data prompted the proposal of a new model to account for reversal of desertification: in the absence of livestock grazing, there is a slow release of soil compaction, and over time, this allows for greater water infiltration rates and reduced erosion. Greater infiltration rates and decreased erosion promote an accumulation of nutrients in the soil. Given sufficient time, water infiltration rates and soil nutrient levels increase sufficiently to support the re-establishment of perennial grass. However, the changes in soil properties outlined in this proposed model could simply be an artifact of the desertification process, and thus need to be tested in additional long-term exclosures. To address these issues, I measured soil bulk density, water infiltration rates, soil nutrients and perennial grass cover at a long-term grazing exclosure in a non-desertified grassland where grazing has continued outside the fence.

Results/Conclusions   Exclusion of livestock for forty years resulted in lower bulk density and higher water infiltration into the soil, compared to cross-fence grazed locations. Plant available nitrogen levels inside the fence did not differ between shrub and intershrub locations, while samples from the grazed area exhibited the classic island of fertility pattern with nutrients concentrated under shrub canopies. Perennial grass cover and composition also differed across the fence. These data corroborate results from desertified sites and provide further evidence that changes in soil physical and chemical properties associated with livestock removal are important factors promoting differences in perennial grass composition in long-term livestock exclosures.