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 δ 15 N values in Quercus alba have been increasing steadily over the past 100 years. In contrast, wood δ 15 N 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 δ 15 N 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.

Reconstructing terrestrial nutrient cycling using stable nitrogen isotopes in wood

“…In some cases, the sharp increase at the heartwood/sapwood boundary is lessened, and rings show a constant increase in [N] through both the heartwood and the sapwood (Poulson et al 1995;McLauchlan and Craine 2012). When the pith is present, it may also exhibit a spike in [N] comparable to, and sometimes exceeding, the spike in the most recent ring (Merrill and Cowling 1966).…”

“…Mixed species within a stand are frequently sampled, making this the appropriate spatial scale for analysis. Tree species do have individualistic effects on forest N cycling (Lovett et al 2004), and when wood  15 N records have been analyzed at the species level, there are some differences in temporal trajectory that indicate partitioning of available soil N (McLauchlan and Craine 2012). Increasing the diversity of taxonomic sampling for wood  15 N analysis could further inform the potential consequences of species-specific nutrient use.…”

“…Numerous studies have documented decreasing wood δ 15 N values toward present during the last century from a wide variety of locations including the United States (McLauchlan and Craine 2012, McLauchlan et al 2007, Poulson et al 1995), Spain (Cuoto-Vázquez and González-Prieto 2010, Peñuelas and Estiarte 1997, and Canada (Härdtle et al 2013, Kranabetter et al 2013. In some cases, decreasing trends were attributed to lower ecosystem N loss and increased reliance of plants on mineralized N or N from fixation by mycorrhizae (CuotoVázquez and González-Prieto 2010, Peñuelas and Estiarte 1997), or a general decline in ecosystem N availability (Kranabetter et al 2013, McLauchlan et al 2007).…”

“…In some cases, decreasing trends were attributed to lower ecosystem N loss and increased reliance of plants on mineralized N or N from fixation by mycorrhizae (CuotoVázquez and González-Prieto 2010, Peñuelas and Estiarte 1997), or a general decline in ecosystem N availability (Kranabetter et al 2013, McLauchlan et al 2007). The key to interpreting temporal trends in these studies was relatively high sample size, because individual trees in the same location showed increasing, decreasing, or flat trends (Kranabetter et al 2013, McLauchlan andCraine 2012).…”

Phenotypic Plasticity Explains Response Patterns of European Beech (Fagus sylvatica L.) Saplings to Nitrogen Fertilization and Drought Events

“…Many ecosystems are currently subject to unprecedented shifts in environmental conditions on both regional and global scales [1]. This is true of forest ecosystems in particular, since trees are characterized by long life-cycles, and growth processes are mediated by the environment over centuries [2].…”