Long-term changes of the δ15N natural abundance of plants and soil in a temperate grassland

Tracing back the N use efficiency of long-term fertilizer trials is important for future management recommendations. Here we tested the changes in natural N-isotope composition as an indicator for N- management within a long-term fertilization lysimeter experiment in a low mountain range pasture ecosystem at Rengen (Eifel Mountains), Germany. Cattle slurry (delta N-15 = 8.9 +/- 0.5aEuro degrees) and mineral fertilizers (calcium ammonium nitrate; delta N-15 = -1.0 +/- 0.2aEuro degrees) were applied at a rate between 0 and 480 kg N ha(-1) yr(-1) throughout 20 years from 1985 onwards. In 2006, samples were taken from different grass species, coarse and fine particulate soil organic matter, bulk soil and leachates. Total soil N content hardly changed during fertilization experiment. As also N leaching has been small within the stagnant water regime, most N was lost through the gaseous phase beside plant uptake and cutting. Unlike N uptake by plants, the process of N volatilization resulted in strong discrimination against the N-15 isotope. As a consequence, the delta N-15 values of top soil samples increased from 1.8 +/- 0.4aEuro degrees to 6.0 +/- 0.4aEuro degrees and that of the plants from -1.2 +/- 1.3aEuro degrees to 4.8 +/- 1.2aEuro degrees with increasing N fertilizer rate. Samples receiving organic fertilizer were most enriched in delta N-15. The results suggest that parts of the fertilizer N signal was preserved in soils and even discovered in soil organic matter pools with slow N turnover. However, a N-15/N-14 isotope fractionation of up to 1.5aEuro degrees added to the delta N-15 values recovered in soils and plants, rendering the increase in delta N-15 value a powerful indicator to long-term inefficient N usage and past N management in the terrestrial environment.