Stable nitrogen isotope patterns of trees and soils altered by long-term nitrogen and phosphorus addition to a lowland tropical rainforest

Abstract

Foliar nitrogen (N) isotope ratios (d 15 N) are used as a proxy for N-cycling processes, including the ''openness'' of the N cycle and the use of distinct N sources, but there is little experimental support for such proxies in lowland tropical forest. To address this, we examined the d 15 N values of soluble soil N and canopy foliage of four tree species after 13 years of factorial N and P addition to a mature lowland rainforest. We hypothesized that N addition would lead to 15 Nenriched soil N forms due to fractionating losses, whereas P addition would reduce N losses as the plants and microbes adjusted their stoichiometric demands. Chronic N addition increased the concentration and d 15 N value of soil nitrate and d 15 N in live and senesced leaves in two of four tree species, but did not affect ammonium or dissolved organic N. Phosphorus addition significantly increased foliar d 15 N in one tree species and elicited significant N 9 P interactions in two others due to a reduction in foliar d 15 N enrichment under N and P co-addition. Isotope mixing models indicated that three of four tree species increased their use of nitrate relative to ammonium following N addition, supporting the expectation that tropical trees use the most available form of mineral N. Previous observations that anthropogenic N deposition in this tropical region have led to increasing foliar d 15 N values over decadal time-scales is now mechanistically linked to greater usage of 15 N-enriched nitrate.

The interaction between nitrogen and phosphorous is a strong predictor of intra-plant variation in nitrogen isotope composition in a desert species

“…In addition, reported a positive correlation for root tips. These positive correlations, which were all interrather than intra-plant in nature, are consistent with the reported experimental finding that an increase in soil nitrogen availability tends to lead to an increase in δ 15 N of non-Nfixing plants (Wigand et al, 2007;Mayor et al, 2014). A hypothesis based on plant-mycorrhizal fungus interactions has been advanced to explain this positive relationship (Hobbie et al, 2000;Craine et al, 2009;Hobbie and Högberg, 2012).…”

“…For example, Major et al (2014) showed that long-term additions of both N and P reduced foliar δ 15 N compared with N or P additions alone in a lowland tropical rainforest. Studies such as this are the basis for the suggestion that δ 15 N could be used as an indicator of ecosystem P limitation (McKee et al, 2002;Wigand et al, 2007;Inglett et al, 2007;Mayor et al, 2014).…”

“…For this reason, δ 15 N is often considered an integrator of terrestrial N cycling, and numerous studies have analyzed natural variations in plant δ 15 N across disturbance and successional stages (e.g., Hobbie et al, 2000;Wang et al, 2007;Resco et al, 2011;Hyodo et al, 2013), climate and topoedaphic gradients (e.g., Austin and Sala, 1999;Schulze et al, 1998;Martinelli et al, 1999;Amundson et al, 2003;Craine et al, 2005Craine et al, , 2009Bai et al, 2009), species (e.g., Cernusak et al, 2009;Gubsch et al, 2011), and types of mycorrhizal fungi Hobbie and Hög-berg, 2012). Other studies have used δ 15 N as an indicator of relative N and phosphorus (P) availability and limitation on plant growth (McKee et al, 2002;Wigand et al, 2007;Inglett et al, 2007;Mayor et al, 2014). These studies have demonstrated the power of using natural variations in δ 15 N to understand physical and biological processes controlling N cycling in terrestrial ecosystems.…”

Two tropical conifers show strong growth and water-use efficiency responses to altered CO2concentration

“…Podocarpus had a significantly higher foliar d 15 N than the remaining species across all levels of c a , potentially suggesting a greater preference of organic N or ammonium over nitrate as an N source (Högberg, 1997;Mayor et al, 2014) when compared with Araucaria and the angiosperm taxa. Consistent with the d 15 N data, soil nitrate concentrations were much higher in pots in which Podocarpus seedlings were grown.…”

Stable Nitrogen Isotopic Changes in Winter Wheat (<i>Triticum aestivum</i> L.) Induced by its Growth Temperature

“…During nitrogen surging of the plant, its δ 15 N values correlate with changes in the physiological characteristics of the plant (Mariotti et al, 1980;1982;Evans et al, 1996), the sources of nitrogen and the surrounding environment (Högberg, 1997;Handley et al, 1997;Austin and Vitousek, 1998;Yoneyama, 1996;Amundson et al, 2003;Pardo et al, 2006;Aranibar et al, 2008;Mayor et al, 2014). Consequently, the plant's δ 15 N composition can be used as a marker to evaluate direction and rate of ecological processes related to isotope fractionation (Tiunov, 2007).…”