Revealing N management intensity on grassland farms based on natural δ15N abundance

High application rates of farmyard manure on grasslands generally results in nitrogen (N) losses and a shift in N isotope composition. The aim of this study was to elucidate to which degree the 15N signatures of miscellaneous N pools at two different levels of management intensity may be used to reproduce the N level of various grassland farms in practise, i.e. beyond the control of experimental plots. We hypothesized that (i) higher δ15N values in soil, plant and animal samples can be found with intensified grassland farming and high N-input management, (ii) that the 15N signature originating from manure application is influenced by application technique and (iii) that it declines with increasing distance from stable to field. To test these hypotheses, we monitored different N pools on nine different farms (dairy, suckler, beef production) in grassland dominated regions of North Rhine-Westphalia, Rheinland-Palatinate (Germany) and Styria (Austria). Samples were taken from 0 to 5 cm soil depth and aboveground biomass at each of five sub-sites on farm, as well as from feed components, fertilizers, and cattle product and tissue like milk, hair, faeces and urine.