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

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.