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

The results indicated a considerable variation in δ15N values of the top soil (1.47‰ to 7.91‰) and of harvested plant material (−2.18‰ to 6.79‰). On average, δ15N values of samples from high N-input grasslands were elevated by 2.8 delta units relative to those of low N-input grasslands. For the soil and plant samples, the δ15N values were thus closely correlated with the overall N balance as well as with stocking rate and fertilizer input (r2 = 0.71 to 0.85). Respective trends for the isotopic signatures in milk, hair and faeces were also evident but less apparent. Furthermore, low emission application techniques of organic fertilizer and increasing distances from the stable to the field exhibited lower δ15N values of top soils and plants, but only in the low input system. We conclude that high N-input on grassland farms systematically changes the δ15N values of soils and above ground biomass and thus also the N signature in animal tissues. Application of N isotope technique to these N pools thus allows for tracing back intensity of fertilizer management regardless of high natural δ15N variations in terrestrial environments.