Tracing nitrate uptake by plants using the natural stable isotopes

硝酸盐(NO_3~-)是植物必需的营养物质。一般认为，植物只通过根组织获得NO_3^-。然而，由于大气中大量的NO_3^-(NO_3^-)通过干湿沉降沉积到地表，一些植物可以通过叶组织获得一定程度的NO_3~-。然而，在以往的研究中，很难澄清和量化直接的NO_3同化作用。三重氧同位素组成(Δ^&lt；17&gt；O≦δ^&lt；17&gt；O-0)。NO_3^-的52×δ^；18&gt；O)是一种有效的示踪剂，可以定量测定NO_3^-在植物体内的混合比例。这是因为，由于富氧臭氧对氧原子的贡献，NO_3^-17&t；ATM；是大的氧异常(Δ^&lt；17O&gt；0)的唯一来源。相反，在土壤中生物产生的再矿化NO_3^-几乎没有表现出异常(Δ^&lt；17&gt；O=0)。此外，Δ^&lt；17&gt；O值在…沉积后同位素分馏过程中是稳定的更多的离子过程，如同化，而δ^&lt；15&gt；N和δ^&lt；18&gt；O值都是可变的。因此，测定植物体内NO_3~(3-)的Δ和O值，可以估计植物体内NO_3~-的贡献，从而验证植物是否吸收NO_3~(2-)。为了阐明硝酸根的直接同化作用，我们测定了采自北海道大学校园森林地面的硝酸根、茎和叶等组织中硝酸根和植物的Δ^&Gt；O值。在叶片中发现了较大的^&lt；17&gt；O异常(Δ^&lt；17&gt；O=From+2~+13‰)，而在茎和根中发现了较小的^&lt；17&gt；O异常(Δ^&lt；17&gt；O=From 0~+6‰)。因此，我们得出结论，烟叶中含有一定程度的NO_3~-。此外，茎和根中的Δ^&lt；17&gt；O值稳定，但茎和根中的^&lt；18&gt；O-富集值超过10‰，说明茎和根中的土壤对NO_3的同化作用。相反，叶片中的Δ^&lt；17&gt；O与δ&lt；18&gt；00之间的线性相关关系表明，叶片内的NO_3^-&lt；atm&gt；和土壤&gt；此外，生长季节的~(18)~-O-富集化趋势表明了叶片内同化的进展。综上所述，我们在植物体内发现了NO_3~-；ATM&gt；，说明植物在一定程度上同化了通过叶片获得的NO_3~+。较少

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