Influence of Forest Disturbance on Stable Nitrogen Isotope Ratios in Soil and Vegetation Profiles

Soil and plant stable N isotope ratios (delta N-15) are influenced by atmospheric N-2 inputs and processes that regulate organic matter (OM) transformation and N cycling. The resulting delta N-15 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 N-2-fixing plants. The LLP treatments included burning every 2 yr with N-2 fixers and reference fire exclusion sites without N-2 fixers. The AHW included 25-yr-old clearcut plots with and without N-2 fixers, and uncut reference without N-2 fixers. We hypothesized that: (i) compared with the reference, OM removal (LLP) would enrich delta N-15 values while OM addition (AHW) would deplete delta N-15 in soil and vegetation pools; and (ii) N-2 fixers would mitigate delta N-15 enrichment in LLP response and exacerbate delta N-15 depletion in AHW. We examined total delta N-15 in soil profiles, tree increment cores, and foliage. The LLP soil and vegetation delta N-15 values showed no treatment effect. In AHW, delta N-15 values in clearcut subsurface soils (20-60 cm) were lower than the reference, but N-2 fixers had no effect. Wood delta N-15 differed with treatment; N-2 fixers had no effect. Our data suggest that AHW soil profile delta N-15 patterns may indicate past disturbance; however, wood and foliar delta N-15 response is species specific. Additionally, N-2-fixing plants respond to ecosystem disturbance, but the data suggest that they play little role in soil or plant delta N-15 values.