Isotopic composition of soil, vegetation or cattle hair no suitable indicator of nitrogen balances in permanent pasture

Abstract

Stable isotope signatures of cattle tail switch hair or meadow vegetation have been found to be related to nitrogen (N) surpluses of whole farms and of meadows, respectively. Permanent pastures are more patchy in terms of nutrient inputs and outputs and N balances for the whole plot do not necessarily give correct impressions of losses. We here investigated correlations between isotopic signatures and N balances calculated for different spatial and temporal scales in permanent pastures. N concentrations and δ15N values of cattle tail switch hair, vegetation and soil samples were measured in an experiment with different grazing intensities started in 2002. Results were compared to soil surface balances calculated for the whole plot or for plot areas affected by either dung, urine, grazing without excreta input, or the pasture area without dung pats. There were no significant correlations between plant or cattle hair isotopic signatures and any of the balances. N fixation probably influenced vegetation signatures, making the isotopic values less dependent on soil and more on atmospheric N. The cattle preferred short mixed vegetation with more legume biomass, which also influenced the 15N values of their hair. The 15N signatures of soil samples were the best indicators of partial N balances in these heterogeneous pastures, probably because soil values are most directly influenced by N inputs and outputs. Still, soil signatures only explained between 15 and 35% of the variation in balance results. Thus, none of the tested parameters can be used as a reliable indicator of N balance results in this heterogeneous system with small differences in budgets among treatments and potentially small plot-scale N losses. © 2011 The Author(s).