Grazing enhanced spatial heterogeneity of soil dehydrogenase activity in arid shrublands of Patagonia, Argentina

Abstract

Purpose: The impacts of sheep grazing on soil microorganisms and enzyme activities are not yet fully understood in arid and semi-arid regions, despite the fact that most grazed lands are drylands. Therefore, this study aimed to analyze the temporal (seasonal samplings in a two-year study) and spatial (plant-covered patches vs. inter-patches) variability of soil dehydrogenase (DHA) as an indicator of soil microbiological activity in two sites under different grazing managements in the arid Patagonian Monte. Materials and methods: We selected two sites with different grazing managements: site G is currently under grazing disturbance and has been grazed by sheep since the beginning of the 20th century, while site NG used to be grazed by sheep until 2003 (six years before the first sampling) when sheep were gradually removed. Soil samples were collected at plant-covered patches (P) and inter-patches (IP) within G and NG at seven sampling dates (distributed seasonally along a two-year study), and in each sample we determined soil DHA. We further analyzed environmental properties that could have influenced the observed patterns of DHA by measuring soil moisture, soil penetration and gravel content, and plant litter biomass.Results and discussion: In the grazed site, there was a significantly higher soil DHA activity in P than in IP along the whole study timeframe, while no significant differences were observed in soil DHA between P and IP in the non-grazed site. In both sites, soil DHA activity positively correlated with plant litter biomass. In addition, a significant positive correlation between DHA and soil penetration, and a significant negative correlation between DHA and gravel content were detected only in the grazed site. Although significant differences in soil DHA among dates were detected in both G and NG, this variable did not show a seasonal pattern.Conclusions: These results suggest that grazing may increase soil DHA spatial heterogeneity, possibly associated with sheep-induced soil compaction through trampling and increased gravel content through erosion. The lack of significant differences in soil DHA activity between patches and inter-patches in the non-grazed site could be associated with vegetation not showing a clear impact on soil DHA in the time-frame elapsed since sheep-removal.