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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. 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. |
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