Tracing nitrate uptake by plants using the natural stable isotopes

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Nitrate(NO_3^-) is an essential nutrient for plants. In general, it is assumed that plants obtained NO_3^-only through their root tissues. However, some plants could obtain NO_3^-though leaf tissues to some extent, because significant quantities of atmospheric NO_3^-(NO_3^-_<atm>) deposit onto earth surface through wet and dry deposition. However, it was difficult to clarify and to quantify the direct NO_3^-_<atm> assimilation in past studies. The triple oxygen isotopic composition(Δ^<17>O≦δ^<17> O-0. 52×δ^<18> O) of NO_3^-can be a useful tracer to quantify the mixing ratios of NO_3^-_<atm> within total NO_3^-in plants(NO_3^-_<plant>). This is because NO_3^-_<atm> is the only source of the large^<17> O anomaly(Δ^<17O>> 0) due to the O atom contribution from^<17> O-enriched ozone. In contrast, re-mineralized NO_3^-produced biologically in a soil(NO_3^-_<soil>) shows little^<17> O anomaly(Δ^<17> O=0). Furthermore,Δ^<17> O values are stable during the post-depositional isotopic fractionat … More ion processes such as assimilation, whereas bothδ^<15> N andδ^<18> O values of NO_3^-_<plant> are variable. Therefore, the determination on theΔ^<17> O values of NO_3^-_<plant> allows us to estimate the contribution of NO_3-_<atm> in each plant tissue and to verify whether plants assimilate NO_3^-_<atm> or not. To clarify the direct NO_3^-assimilation, we determinedΔ^<17> O values of NO_3^-_<plant> in each tissue(leaf, stem and root) of Sasa, which was collected at forest floors located in campuses of Hokkaido University. We found large^<17> O anomaly in leaves(Δ^<17> O=from+2 to+13‰), whereas little^<17> O anomaly in stems and roots(Δ^<17> O=from 0 to+6‰). Therefore, we concluded the leaves contained NO_3^-_<atm> to some extent. Besides, while theΔ^<17> O values were stable in the stems and roots, the^<18> O-enrichments more than 10‰suggest the assimilation of NO_3^-_<soil> in the stems and roots. In contrast, the linear correlation betweenΔ^<17> O andδ<18>^O0values observed in leaves suggest mixing between NO_3^-<atm> and NO_3^-_<soil> within leaves. Furthermore, the^<18> O-enrichment trend during the growth seasons suggests the progress of assimilation within leaves. In conclusion, we found NO_3^-_<atm> within the plant and clarified that the plants assimilate NO_3^-_<atm> obtained through leaves to some extent. Less