Management effects on nitrate and ammonium leaching in temperate grasslands

Grassland management can influence nitrogen leaching by changing respective inputs (like fertilization) and outputs (e.g. harvest) from the ecosystem but also through changes in plant and microbial communities and their interactions with the soil. Yet the mechanisms of these processes and their importance for nitrate (NO₃^-) and ammonium (NH₄⁺) are poorly researched. Although temperate grassland management can cause a high leaching risk, studies measuring annual soil nitrogen fluxes and covering a high number of sites are limited. Using a resin method, we measured annual leaching fluxes of NO₃-N and NH₄-N in 150 grassland sites in three German regions from spring 2018 to spring 2019 at a depth of 10 cm. We used Structural Equation Modeling to identify direct and indirect management effects on nitrogen leaching. Both fertilization and grazing intensities increased NO₃-N leaching to almost the same extent. Fertilization intensity increased NO₃-N leaching directly and indirectly i) by decreasing the negative effect of plant richness on NO₃-N leaching and ii) by increasing the positive effect of legume cover on NO₃-N leaching. Similarly, grazing intensity increased NO₃-N directly and indirectly i) by increasing the positive effect of legume cover on NO₃-N leaching and ii) by decreasing the negative effect of grass cover on NO₃-N leaching. Microbial biomass (nitrogen) increased NO₃-N leaching and it was not controlled by management. Fertilization strongly increased NH₄-N leaching, both by a direct effect and indirectly by increasing the positive effect of plant nitrogen on NH₄-N. Grazing intensity and plant richness had no direct effects on NH₄-N leaching. We have shown that grassland management considerably influences inorganic nitrogen leaching in the annual basis, and that fertilization intensity is more important than grazing, especially for NH₄-N leaching. Moreover, we found that plant diversity decreases NO₃-N leaching, but this effect is controlled by fertilization leading to increased leaching risks. Thus, reducing the management intensity, both in meadows and pastures, can be used as a mitigation tool for inorganic nitrogen leaching.