Tracing nitrate uptake by plants using the natural stable isotopes

硝态氮（NO_3^-）是植物必需的营养物质。一般认为植物只能通过根组织获得NO_3^-。然而，由于大气中大量的NO_3^-(NO_3^-_<atm>)通过湿沉降和干沉降的方式沉积在地表，部分植物在一定程度上可以通过叶片组织获得NO_3^-。然而，在以往的研究中，对NO_3^-_<atm>的直接同化难以明确和量化。三氧同位素组成(Δ^<17>O≦Δ^<17>O -0。NO_3^-的52×δ^<18> O)可以作为一种有用的示踪剂来量化NO_3^-_<atm>在植物总NO_3^- - （NO_3^-_<植物>）中的混合比例。这是因为NO_3^-_<atm>是巨大的^<17> O异常（Δ^<17O>>）的唯一来源，这是由于^<17> O富集臭氧对O原子的贡献。土壤中生物再矿化NO_3^- (NO_3^-_<soil>)的^<17> O异常较小（Δ^<17> O=0）。此外，在沉积后同位素分馏过程中，Δ^<17> O值是稳定的，而NO_3^-_<植物>的Δ^< 15> N和Δ^< 18> O值是可变的。因此，通过测定NO_3^-_<plant>的theΔ^<17> O值，可以估计NO_3-_<atm>在植物各组织中的贡献，验证植物是否同化NO_3^-_<atm>。为了明确NO_3^-的直接吸收作用，我们在北海道大学校园森林地面采集的莎草各组织（叶、茎、根）中NO_3^-_<植物>的determinedΔ^<17> O值。在叶片中发现了较大的^<17> O异常（Δ^<17> O= +2 ~ +13‰），而在茎和根中发现了较小的^<17> O异常（Δ^<17> O= 0 ~ +6‰）。因此，我们认为叶片中含有一定程度的NO_3^-_<atm>。此外，虽然theΔ^<17>的O值在茎和根中是稳定的，但^<18>的O浓度大于10‰表明NO_3^-_<土壤>在茎和根中有同化作用。相反，在叶片中观测到的betweenΔ^<17> 0和δ<18>^ 0 0的线性相关值表明叶片内NO_3^-<atm>和NO_3^-_<soil>之间存在混合。此外，生长季节的^<18> o富集趋势表明叶片内同化的进展。综上所述，我们在植物体内发现了NO_3^-_<atm>，说明植物在一定程度上吸收了通过叶片获得的NO_3^-_<atm>。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