Influence of forest disturbance on stable nitrogen isotope ratios in soil and vegetation profiles

Soil and plant stable N isotope ratios (δ¹⁵N) are influenced by atmospheric N₂ inputs and processes that regulate organic matter (OM) transformation and N cycling. The resulting δ¹⁵N patterns may be useful for discerning ecosystem differences in N cycling. We studied two ecosystems, longleaf pine (Pinus palustris Mill.)–wiregrass (Aristida stricta Michx.) (LLP) and Appalachian hardwood (AHW) forests in the US Southeast under different management regimes. In LLP, burning removes OM. In AHW, clearcutting creates large OM pulses of logging residue. Although burning removes OM and clearcutting creates a pulse addition of OM, both management regimes increase soil N availability and N₂–fixing plants. The LLP treatments included burning every 2 yr with N₂ fixers and reference fire exclusion sites without N₂ fixers. The AHW included 25-yr-old clearcut plots with and without N₂ fixers, and uncut reference without N₂ fixers. We hypothesized that: (i) compared with the reference, OM removal (LLP) would enrich δ¹⁵N values while OM addition (AHW) would deplete δ¹⁵N in soil and vegetation pools; and (ii) N₂ fixers would mitigate δ¹⁵N enrichment in LLP response and exacerbate δ¹⁵N depletion in AHW. We examined total δ¹⁵N in soil profiles, tree increment cores, and foliage. The LLP soil and vegetation δ¹⁵N values showed no treatment effect. In AHW, δ¹⁵N values in clearcut subsurface soils (20–60 cm) were lower than the reference, but N₂ fixers had no effect. Wood δ¹⁵N differed with treatment; N₂ fixers had no effect. Our data suggest that AHW soil profile δ¹⁵N patterns may indicate past disturbance; however, wood and foliar δ¹⁵N response is species specific. Additionally, N₂–fixing plants respond to ecosystem disturbance, but the data suggest that they play little role in soil or plant δ¹⁵N values.