Effects of four different restoration treatments on the natural abundance of 15N stable isotopes in plants

delta N-15 signals in plant and soil material integrate over a number of biogeochemical processes related to nitrogen (N) and therefore provide information on net effects of multiple processes on N dynamics. In general little is known in many grassland restoration projects on soil-plant N dynamics in relation to the restoration treatments. In particular, delta N-15 signals may be a useful tool to assess whether abiotic restoration treatments have produced the desired result. In this study we used the range of abiotic and biotic conditions provided by a restoration experiment to assess to whether the restoration treatments and/or plant functional identity and legume neighborhood affected plant delta N-15 signals. The restoration treatments consisted of hay transfer and topsoil removal, thus representing increasing restoration effort, from no restoration measures, through biotic manipulation to major abiotic manipulation. We measured delta N-15 and %N in six different plant species (two non-legumes and four legumes) across the restoration treatments. We found that restoration treatments were clearly reflected in delta N-15 of the non-legume species, with very depleted delta N-15 associated with low soil N, and our results suggest this may be linked to uptake of ammonium (rather than nitrate). The two non-legume species differed considerably in their delta N-15 signals, which may be related to the two species forming different kinds of mycorrhizal symbioses. Plant delta N-15 signals could clearly separate legumes from non-legumes, but our results did not allow for an assessment of legume neighborhood effects on non-legume delta N-15 signals. We discuss our results in the light of what the delta N-15 signals may be telling us about plant-soil N dynamics and their potential value as an indicator for N dynamics in restoration.