Key message

Understory dwarf bamboo mitigated soil N competition with co-existing canopy oak trees by foraging in deeper soils and increasing dependence on N forms that differ from those used by canopy trees.

Abstract

Nitrogen (N) competition among co-existing plant species utilizing different mycorrhiza types was explored through the investigation of N sources of oak trees and dwarf bamboo. Vertical distribution of fine roots, soil N pools, δ¹⁵N of leaves, and possible soil N sources and nitrate reductase activity (NRA) were all quantified. The fine roots of canopy trees were more concentrated in the surface soils than roots of the understory dwarf bamboo. Soil NH₄⁺ and extractable organic N (EON) content (based on unit weight) decreased from the organic horizon (O horizon) to the deep soils, the size of the NH₄⁺ pool per unit volume increased with soil depth, and the EON was approximately constant. Soil NO₃⁻ was not detected at any soil depth or was not significant in value, while NO₃⁻ captured by ion-exchange resin (IER) buried at a 10 cm soil depth and net nitrification were observed via laboratory incubation at all soil depths. The δ¹⁵N of the NH₄⁺ and EON pools increased with soil depth and the δ¹⁵N of NO₃⁻ of IER was lower than that of other N forms, except for the δ¹⁵N of NH₄⁺ in the O horizon. Furthermore, root NRA tended to be lower in canopy trees than in the understory, implying lower dependency on NO₃⁻ by canopy trees. The pattern of root distribution and mycorrhizal fungi association of the understory vegetation (as well as the high root NRA) suggested that dependence on N in deeper soils was higher in understory plants than in canopy trees. These findings indicate that understory vegetation mitigates soil N competition against co-existing canopy trees via the use of alternative N sources.

中文翻译：

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日本北部阔叶林中冠层树和矮竹共存的氮源利用

关键信息

矮化矮竹通过在更深的土壤中觅食并增加对不同于冠层树使用的氮形态的依赖性，减轻了与同时存在的冠层橡树的土壤氮竞争。

抽象

通过调查橡树和矮竹的N种来源，探索了利用不同菌根类型的共存植物物种之间的氮（N）竞争。细根，土壤氮库的垂直分布，Δ ¹⁵ ñ落叶，还有可能土壤氮的来源及硝酸还原酶活性（NRA）全部量化。冠层树的细根比表层矮竹的根更集中在表层土壤中。土壤NH ₄ ⁺和可提取有机氮（EON）含量（基于单位重量）从有机层（O层）到深层土壤逐渐减少，单位体积NH ₄ ⁺池的大小随着土壤深度的增加而增加; EON大致恒定。土壤NO ₃ ^-在任何土壤深度没有检测到或不在值显著，而NO ₃ ^-由埋在10cm的土壤深度和经由实验室培养中观察到在所有土壤深处净硝化离子交换树脂（IER）捕获。的δ ¹⁵的NH N个₄ ⁺和EON池与土壤深度和δ增加¹⁵ NO氮₃ ^- IER的比的其它N-形式下，除了δ ¹⁵ ÑNH的₄ ⁺于O地平线。此外，冠层树的根部NRA倾向于低于林下，这意味着对NO _3-的依赖性较低^。被树冠。林下植被（以及高根NRA）的根系分布和菌根真菌协会的模式表明，林下植物比冠层树对深层土壤对氮的依赖性更高。这些发现表明，林下植被通过使用替代氮源减轻了土壤氮对共存冠层树种的竞争。