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

The natural N-15 abundance (delta N-15) of different ecosystem compartments is considered to be an integrator of nitrogen (N) cycle processes. Here we investigate the extent to which patterns of delta N-15 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 delta N-15 values of organic fertilizers compared to mineral fertilizer were reflected by higher delta N-15 values in soils and harvested plant material. Furthermore, delta N-15 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 N-2 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 delta N-15 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 15 N (e.g. nitrification, denitrification, ammonia volatilization) were stimulated by the increased supply of readily available N, leading to loss of the N-15 depleted compounds and subsequent N-15 enrichment of the soils. By contrast, in plots with organic fertilization this correlation was partly due to accumulation of N-15-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 delta N-15 patterns. This study for the first time shows that the natural abundance of N-15 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 delta N-15 is better described by N input-output balances. (c) 2006 Elsevier Ltd. All rights reserved.