Thyme travels: 15N isoscapes of Thymus vulgaris L. 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 δ15N 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 δ15N 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 δ15 N, animal faeces cover (as a proxy for grazing intensity) and N2-fixing species cover from inside to outside Thymus vulgaris L. (thyme)-invaded lightly grazed pastoral communities in Central Otago, southern South Island, New Zealand. Mean thyme foliar δ15N 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 δ15N 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 δ15N. Thyme invasion may result in lowered soil δ15N reflecting alterations to N dynamics. Associated invasion-related impacts of animal grazing may also impact soil δ15N. Further studies are required to distinguish the underlying mechanism responsible for the observed patterns of foliar and soil δ15N values across thyme-invaded Central Otago landscapes.