Soil amelioration induced by nurse shrubs in coal mines reclaimed to pastures and their synergistic effects with grazing

Native shrubs facilitate the establishment of oak seedlings in the opencast coal mines rehabilitated to pastures in Northern Spain, under a Mediterranean sub-humid climate. We evaluate soil changes as one of the possible facilitative effects of nurse shrubs. We hypothesize that nurse shrubs on mining soils can improve edaphic properties directly and indirectly by reducing the negative effects of trampling and grazing by ungulates. Thus, we assessed the combined effects of nurse shrubs (plots with vs without shrubs) and grazing (plots with vs without a fence for preventing browsing and trampling) upon soil properties in a reclaimed coal mine. Chemical properties such as electrical conductivity, K+, cation exchange capacity, and C/N ratio reached higher values beneath shrubs' canopies, so did total organic matter, total N, total organic C, total P, available P, and Mg2+ under shrubs but only with grazing. In contrast, pH was higher outside the shrubs; also Ca2+, though only in the absence of grazing. With grazing, Na+ decreased under shrubs. Among physical properties, bulk density increased and porosity decreased only in grazed plots, whereas sand content increased under shrubs in the grazed plots, and clay decreased in such locations. Water holding capacity and available water were the highest in ungrazed areas. Overall, we found that shrubs have a positive direct effect on soil fertility, especially relevant in grazed areas because nurse shrubs and grazing have synergistic effects, and a positive indirect effect on physical properties because they attenuate negative effects of grazing, particularly soil compaction, by reducing livestock and wild ungulates trampling. Therefore, these results demonstrate how nurse shrubs contribute to soil amelioration, helping to facilitate plant establishment in reclaimed mines, which has relevant restoration im-plications for pasture and forest recovery.