Thyme travels: 15N isoscapes of Thymus vulgarisL. invasion in lightly grazed pastoral communities

Alterations to ecosystem nitrogen (N) cycling by introduced plant species may increase the invasibility of habitat providing a positive feedback for the introduced species to become invasive. Spatial patterns of foliar and soil [delta]15N ratios reflect variation in rates and process of N-cycling across invaded landscapes and provide insight into N-source uptake and utilization strategies of invasive plant species. To evaluate invasion-associated changes in soil and foliar [delta]15N at different scales: regional (among different sites), local (between north- and south-facing aspect at the same site), and microsite (within populations in the same community), we measured foliar and soil [delta]15 N, animal faeces cover (as a proxy for grazing intensity) and N2-fixing species cover from inside to outside Thymus vulgarisL. (thyme)-invaded lightly grazed pastoral communities in Central Otago, southern South Island, New Zealand. Mean thyme foliar [delta]15N were near-zero across the invaded landscape, and did not change across the advancing edge of invasion or with aspect. There was no evidence that associations with N2-fixing species provide a potential N source. Soil [delta]15N was lower inside of thyme compared to at the edge or outside of thyme and was varied between aspects at some sites. Animal faeces cover as a proxy for grazing intensity explained only 23% of this observed variation of soil [delta]15N. Thyme invasion may result in lowered soil [delta]15N reflecting alterations to N dynamics. Associated invasion-related impacts of animal grazing may also impact soil [delta]15N. Further studies are required to distinguish the underlying mechanism responsible for the observed patterns of foliar and soil [delta]15N values across thyme-invaded Central Otago landscapes.