N2-fixing Shrubs as Nature-based Solutions to Store Soil Nutrients in Grazed Post-mining Pastures of Northern Spain

Abstract

Grazing can adversely affect soil physicochemical properties and, despite its potential benefits on restored areas, may ultimately lead to the ecosystem degradation. Given that shrubs can partially buffer these effects, we investigate their potential as a Nature-based Solution to enhance soil nutrient storage in grazed, post-mining pastures. We assessed the combined influence of legume shrubs (plots with vs. without shrubs) and grazing (plots with vs. without a fence to prevent browsing and trampling) on soil C: N:P stoichiometry, as well as on C-N-P and exchangeable cations’ stocks in a rehabilitated coal mine. The natural colonization of legume shrubs in post-mining Mediterranean pastures maintained C: N:P stoichiometry and nutrient stocks at levels comparable to those in ungrazed areas. Specifically, the presence of shrubs increased total soil organic carbon, nitrogen, and phosphorus stocks by approximately 15%, 18%, and 12%, respectively, compared to grazed areas without shrubs. In addition, exchangeable potassium (K+) and magnesium (Mg+ 2) stocks increased by approximately 20% and 16%, respectively, under shrub canopies. In contrast, exchangeable calcium (Ca+ 2) and sodium (Na+) stocks exhibited divergent trends. In Mediterranean post-mining grasslands with low grazing pressure, native leguminous shrubs can effectively replicate the benefits of grazing exclusion by significantly enhancing soil nutrient storage. Variations in soil organic matter and structural properties accounted for 75% and 76% of the variability in nutrient stocks, respectively, underscoring their key mediating roles. These findings highlight the importance of integrating shrub management with grazing practices to support nutrient cycling and soil restoration in degraded grassland ecosystems.