Regional Assessment of N Saturation using Foliar and Root $$\varvec {\delta}^{\bf 15}{\bf N}$$

Abstract

N saturation induced by atmospheric N deposition can have serious consequences for forest health in many regions. In order to evaluate whether foliar d 15 N may be a robust, regional-scale measure of the onset of N saturation in forest ecosystems, we assembled a large dataset on atmospheric N deposition, foliar and root d 15 N and N concentration, soil C:N, mineralization and nitrification. The dataset included sites in northeastern North America, Colorado, Alaska, southern Chile and Europe. Local drivers of N cycling (net nitrification and mineralization, and forest floor and soil C:N) were more closely coupled with foliar d 15 N than the regional driver of N deposition. Foliar d 15 N increased non-linearly with nitrification:mineralization ratio and decreased with forest floor C:N. Foliar d 15 N was more strongly related to nitrification rates than was foliar N concentration, but concentration was more strongly correlated with N deposition. Root d 15 N was more tightly coupled to forest floor properties than was foliar d 15 N. We observed a pattern of decreasing foliar d 15 N values across the following species: American beech>yellow birch>sugar maple. Other factors that affected foliar d 15 N included species composition and climate. Relationships between foliar d 15 N and soil variables were stronger when analyzed on a species by species basis than when many species were lumped. European sites showed distinct patterns of lower foliar d 15 N, due to the importance of ammonium deposition in this region. Our results suggest that examining d 15 N values of foliage may improve understanding of how forests respond to the cascading effects of N deposition.

Regional patterns in foliar 15N across a gradient of nitrogen deposition in the northeastern US

“…Species differences in foliar d 15 N values have been reported in the northeastern US (Nadelhoffer et al, 1999;Templer, 2001;Pardo et al, 2006;Pardo et al, 2007). These differences in foliar d…”

“…15 N values are typically more closely linked to measures of soil N cycling than are foliar %N measurements, they may be a better early indicator of N saturation than foliar %N alone (Pardo et al, 2006). Combining measurements of foliar d 15 N and %N may provide the best information about N cycling dynamics at a given site, particularly for regional surveys.…”

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

“…In addition to these regional-to global-scale changes, many forests are experiencing local changes in fire regimes, drought, and the introduction of non-native plants and animals while recovering from earlier logging and windthrow. Not only do many of these factors have the potential to interact in their effects on forest processes, but they also have the potential to enhance or dampen N availability, which often limits primary productivity and influences forest species composition (Pepper et al, 2007). For example, experiments have suggested that elevated CO 2 may decrease terrestrial N availability (Johnson, 2006;Luo et al, 2004), while chronic N deposition may increase N availability to temperate forests (Aber et al, 2003).…”

“…Although there are many processes that discriminate against 15 N, stand-level plant δ 15 N generally increases with increasing N availability, here defined as the supply of N to terrestrial plants and soil microorganisms relative to their N demands (Schimel and Bennett, 2004). In forests, high δ 15 N values are found in soils and leaves of ecosystems with high rates of nitrification (Pardo et al, 2006), high rates of denitrification (Houlton et al, 2006), partial nitrification followed by nitrate leaching (Pardo et al, 2002), and low reliance on mycorrhizal fungi for plant N uptake (Hobbie and Colpaert, 2003;Högberg et al, 2011).…”

Changes in nitrogen cycling during the past century in a northern hardwood forest

“…Ecosystems with high N availability, such as those that are fertilized with N, exhibit high values of natural abundance 15 N: 14 N ratios (␦ 15 N) in leaf tissue and soil (18). As with tree leaves (19), enrichment of wood with 15 N occurs with high N availability, and therefore dendroisotopic records have been used to reconstruct past N availability (20,21).…”