Influence of forest disturbance on stable nitrogen isotope ratios in soil and vegetation profiles
Soil and plant stable nitrogen isotope ratios (15 N) are influenced by atmospheric nitrogen (N) inputs and processes that regulate organic matter (OM) transformation and N cycling. The resulting 15N patterns may be useful for discerning ecosystem differences in N cycling. We studied two ecosystems; longleaf pine wiregrass (LLP) and Appalachian hardwood (AHW) forests in SE USA 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 (N2-fixers). LLP treatments included burning every 2 years with N2-fixers and reference fire exclusion sites without N2-fixers. AHW included 25 year-old clearcut plots with and without N2-fixers, and uncut reference without N2-fixers. We hypothesized that: 1) compared to the reference, OM removal (LLP) would enrich 15N values while OM addition (AHW) would deplete 15 N in soil and vegetation pools; 2) N2-fixers will mitigate 15N enrichment in LLP response and exacerbate 15N depletion in AHW. We examined total 15N in soil profiles, tree increment cores, and foliage. LLP soil and vegetation 15 N values showed no treatment effect. In AHW 15 N values in clearcut subsurface soils (20–60 cm) were lower than the reference but N2-fixers had no effect. Wood 15N differed with treatment; N2-fixers had no effect. Our data suggest that AHW soil profile 15 N patterns may indicate pastdisturbance however, wood and foliar 15 N response is species specific. Additionally, N2-fixing plants respond to ecosystem disturbance, but data suggests they play little role in soil or plant 15 N values.