Grazing dominantly regulates top soil organic carbon and nitrogen stocks in grasslands

For the past 17 years the Biodiversity Exploratories (BEs) in Germany have collected detailed data on land use intensity (LUI), climate as well as plant and microbial biodiversity for over 100 grassland sites in three different exploratories across Germany. Since these factors alongside physicochemical processes interactively drive soil nitrogen (SON) and carbon (SOC) cycling and storage, the BEs offer a unique opportunity to gain a mechanistic, process-based understanding of the interactions between soil type, climate, biodiversity and management that drive N and C turnover and storage in grasslands.

We quantified SON and SOC stocks as well as δ15N and δ13C isotopic signatures for 25 grassland plots in each of the three exploratories, thereby covering a wide range of LUI. Currently, the results for the Schwäbische Alb exploratory are available. These data clearly show the importance to distinguish for the individual effects of LUI components (fertilization, mowing and grazing). For example, SOC and TN concentrations and stocks in the top 30 cm of soil tend to increase with LUI, but this increase is largely driven by the individual effect of the grazing component of LUI. The C:N ratio on the other hand was largely impacted by mowing and fertilization, possibly because mowing was a relatively important C loss pathway while fertilization was relatively important for N inputs. The narrower C:N ratio with increasing LUI negatively affected plant biodiversity.

Both δ15N and δ13C were related largely to the overall LUI and plant biodiversity. Topsoil δ15N increased with higher LUI and lower plant biodiversity, likely due to the high δ15N of added organic fertilizer and reduced importance of biological N fixation with its low δ15N signature.

Currently, our results indicate that grazing is the dominating management factor regulating SON and SOC stocks in calcareous grasslands of Southwest Germany, with grazing increasing SOC and SON stocks and associated soil functions. Further measurements and data evaluation will show whether this finding is of more universal importance for grasslands across Germany.