The natural abundance of ¹⁵N in plant and soil‐available N indicates a shift of main plant N resources to NO from NH along the N leaching gradient

Abstract

To investigate which of ammonium (NH(4)(+)) or nitrate (NO(3)(-)) is used by plants at gradient sites with different nitrogen (N) availability, we measured the natural abundance of (15)N in foliage and soil extractable N. Hinoki cypress (Chamaecyparis obtusa Endlicher) planted broadly in Japan was selected for use in this study. We estimated the source proportion of foliar N (NH(4)(+) vs. NO(3)(-)) quantitatively using mass balance equations. The results showed that C. obtusa used mainly NH(4)(+) in N-limited forests, although the dependence of C. obtusa on NO(3)(-) was greater in other NO(3)(-)-rich forests. We regarded dissolved organic N (DON) as a potential N source because a previous study demonstrated that C. obtusa can take up glycine. Thus we added DON to our mass balance equations and calculated the source proportion using an isotope-mixing model (IsoSource model). The results still showed a positive correlation between the calculated plant N proportion of NO(3)(-) and the NO(3)(-) pool size in the soil, indicating that high NO(3)(-) availability increases the reliance of C. obtusa on NO(3)(-). Our data suggest the shift of the N source for C. obtusa from NH(4)(+) to NO(3)(-) according to the relative availability of NO(3)(-). They also show the potential of the foliar delta(15)N of C. obtusa as an indicator of the N status in forest ecosystems with the help of the delta(15)N values of soil inorganic and organic N.

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

“…As a result, there was no strong evidence that preference for ammonium, for instance, was retained irrespective of its relative availability. This finding is in agreement with previous observations that plants track nitrate availability in lower latitude ecosystems (Houlton et al 2007;Wang and Macko 2011;Kahmen et al 2008;Takebayashi et al 2010).…”

δ15N constraints on long-term nitrogen balances in temperate forests

“…1) suggests that the proportion of root N uptake from forest floor versus mineral soil did not shift across our sites, as suggested to occur elsewhere (Compton et al 2007). Finally, shifts in reliance on different soil N forms may influence foliar d 15 N, particularly as nitrate availability increases in N-rich sites (Takebayashi et al 2010). We found that foliar d 15 N and soil water d 15 NO 3 -converged to similar values at the most N-rich sites (-1%, Figs.…”

Drivers of spatial variability in urban plant and soil isotopic composition in the Los Angeles basin

“…The depletion of EF at 80-90% net nitrification may be influenced by plant preferences for IN forms, while the relatively more enriched EF value at very high % net nitrification may indicate the influences of other N sources that supported soil nitrification such as atmospheric N deposition. Takebayashi et al (2010) reported a shift of primary plant N sources from NH 4 + -N to NO 3 − -N, which was more δ 15 N-depleted, as the relative availability of NO 3 − -N increased, and a subsequent decrease in plant δ 15 N. Plants at our sites might also preferentially use the dominant NO 3 − -N first when % nitrification is very high. However, future studies are needed to distinguish between the influences of N deposition and gaseous NO 2 on plant δ 15 N. Apart from N availability, plant preferences for N forms, and N isotopic discrimination during N uptake, mycorrhizal processes could also influence plant δ 15 N. Evans et al (1996) reported highly enriched foliar δ 15 N compared to roots of tomato with NO 3 − -N uptake, but no intra-plant δ 15 N differences with NH 4 + -N. Kolb and Evans (2003) found increased N isotopic discrimination in barley during N uptake at high N concentrations rather than low concentrations, whether the source was NH 4 + -N or NO 3 − -N. Hobbie et al (2000) also pointed out that plant-mycorrhizal interactions during N cycling, such as the transfer of N from fungi to plants, could be a significant source of foliar δ 15 N variation.…”