Species-specific trajectories of nitrogen isotopes in Indiana hardwood forests, USA

Abstract

Abstract. Humans have drastically altered the global nitrogen (N) cycle, and these alterations have begun to affect a variety of ecosystems. In North America, N deposition rates are highest in the central US, yet there are few studies that examine whether N availability has been increasing to different tree species in the forests of the region. To determine the species-specific trajectories of N availability in secondary temperate forests experiencing high N deposition, we measured the N concentrations and composition of stable N isotopes in wood of four tree species from six hardwood forest remnants in northern Indiana, USA. Annual nitrogen deposition rates averaged 5.8 kg ha−1 from 2000 to 2008 in this region. On average, wood δ15N values in Quercus alba have been increasing steadily over the past 100 years. In contrast, wood δ15N values have been declining in three other hardwood species – Acer saccharum, Carya ovata, and Fagus grandifolia – over the same time period. The species-specific trends suggest a change in the partitioning of ammonium and nitrate among species, due to an increase in nitrification rates over time. With no apparent net change in wood δ15N over the past century at the stand level, there is currently little evidence for consistent trends in stand-level N availability over time in the Indiana forests.