Spatial patterns of soil resources under different land use in Prosopis woodlands of the Monte desert

Abstract

Changes in the spatial distribution of resources constitute an indicator of degradation of arid grazing lands. In arid and semi-arid ecosystems, the distribution of soil resources has been commonly associated with the structure and the spatial arrangements of the vegetation. Although the formation of “fertile islands” beneath vegetation patches is well documented, much less is known about the changes induced by grazing systems on the distribution of soil resources. We examine how pastoralist settlements are affecting the spatial distribution of soil resources and the soil nutrient balance in central-western woodlands of Argentina. We analyzed the distribution of soil water, chloride, nitrate, total nitrogen, and organic matter at increasing distances from livestock corrals and in undisturbed woodlands, at different soil depths. We also calculated variation indexes of soil organic matter and total nitrogen produced by livestock settlements, as an indicator of degree of deterioration or improvement of the soils. The transects located in pastoralist settlements demonstrated an increasing centripetal gradient in availability of soil water and nutrients compared to transects outside of these disturbed areas. Livestock corrals create local hotspots of nutrient enrichment, but when we analyzed the effects of livestock settlements at a higher spatial scale, we found net losses of soil organic matter and total nitrogen. We conclude that the coupling between nutrient and patch dynamics is disrupted by the pastoralist settlements, which caused a redistribution of soil resources, controlled by the location from the livestock corrals. The processes that promote nutrient losses, such as ammonium volatilization, denitrification, nitrate leaching, organic matter oxidation, manure exports, and soil erosion, are relatively higher than the extra inputs of dung and urine. Therefore, this study emphasizes the role of grazing systems as modulators of water and nutrient fluxes, and soil nutrient balance.