Natural 15N abundance of plants and soils under different management practices in a montane grassland

The natural 15N abundance (δ15N) of different ecosystem compartments is considered to be an integrator of nitrogen (N) cycle processes. Here we investigate the extent to which patterns of δ15N in grassland plants and soils reflect the effect of different management practices on N cycling processes and N balance. Investigations were conducted in long-term experimental plots of permanent montane meadows with treatments differing in the amount and type of applied fertilizer (0–200 kg N ha−1 yr−1; mineral fertilizer, cattle slurry, stable manure) and/or the cutting frequency (1–6 cuts per season). The higher δ15N values of organic fertilizers compared to mineral fertilizer were reflected by higher δ15N values in soils and harvested plant material. Furthermore, δ15N of top soils and plant material increased with the amount of applied fertilizer N. N balances were calculated from N input (fertilization, atmospheric N deposition and symbiotic N2 fixation) and N output in harvest. ‘Excess N’—the fraction of N input not harvested—was assumed to be lost to the environment or accumulated in soil. Taking fertilizer type into account, strong positive correlations between δ15N of top soils and the N input–output balance were found. In plots receiving mineral N fertilizer this indicates that soil processes which discriminate against 15N (e.g. nitrification, denitrification, ammonia volatilization) were stimulated by the increased supply of readily available N, leading to loss of the 15N depleted compounds and subsequent 15N enrichment of the soils. By contrast, in plots with organic fertilization this correlation was partly due to accumulation of 15N-enriched fertilizer N in top soils and partly due to the occurrence of significant N losses. Cutting frequency appeared to have no direct effect on δ15N patterns. This study for the first time shows that the natural abundance of 15N of agricultural systems does not only reflect the type (organic or mineral fertilizer) or amount of annual fertilizer amendment (0–200 kg ha−1 yr−1) but that plant and soil δ15N is better described by N input–output balances.