Trees can have large effects on soil nutrients in ways that alter succession, particularly in the case of nitrogen-(N)-fixing trees. In Hawaiʻi, forest restoration relies heavily on use of a native N-fixing tree, Acacia koa (koa), but this species increases soil-available N and likely facilitates competitive dominance of exotic pasture grasses. In contrast, Metrosideros polymorpha (‘ōhi‘a), the dominant native tree in Hawaiʻi, is less often planted because it is slow growing; yet it is typically associated with lower soil N and grass biomass, and greater native understory recruitment. We experimentally tested whether it is possible to reverse high soil N under koa by adding ‘ōhi‘a litter, using additions of koa litter or no litter as controls, over 2.5 yr. We then quantified natural litterfall and decomposition rates of ‘ōhi‘a and koa litter to place litter additions in perspective. Finally, we quantified whether litter additions altered grass biomass and if this had effects on native outplants. Adding ‘ōhi‘a litter increased soil carbon, but increased rather than decreased inorganic soil N pools. Contrary to expectations, koa litter decomposed more slowly than ‘ōhi‘a, although it released more N per unit of litter. We saw no reduction in grass biomass due to ‘ōhi‘a litter addition, and no change in native outplanted understory survival or growth. We conclude that the high N soil conditions under koa are difficult to reverse. However, we also found that outplanted native woody species were able to decrease exotic grass biomass over time, regardless of the litter environment, making this a better strategy for lowering exotic species impacts.

中文翻译：

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可以通过添加垃圾来改变冠层树对土壤和林下的影响吗？

树木可以通过改变演替的方式对土壤养分产生重大影响，特别是在固氮树木的情况下。在夏威夷，森林恢复在很大程度上依赖于使用本地固氮树Acacia koa (koa)，但该物种增加了土壤有效氮，并可能促进外来牧草的竞争优势。相比之下，Metrosideros polymorpha('ōhi'a) 是夏威夷的主要本土树种，因为它生长缓慢，所以很少种植；然而，它通常与较低的土壤氮和草生物量以及更多的原生林下补充有关。我们通过添加'ōhi'a 垫料，使用添加 koa 垫料或不添加垫料作为对照，在 2.5 年内通过实验测试是否有可能逆转 koa 下的高土壤 N。然后，我们量化了 'ōhi'a 和 koa 垃圾的自然凋落物和分解率，以透视添加的垃圾。最后，我们量化了添加垃圾是否会改变草的生物量，以及这是否对本地外植体产生影响。添加'ōhi'a 枯枝落叶增加土壤碳，但增加而不是减少无机土壤氮库。出乎意料的是，考阿猫砂的分解速度比 'ōhi'a 慢，尽管它每单位垫料释放的氮含量更高。我们没有看到由于添加“ōhi”垫料而导致草生物量减少，并且本地外植林下生存或生长没有变化。我们得出结论，koa 下的高氮土壤条件难以逆转。然而，我们还发现，无论垫料环境如何，外来的原生木本植物都能随着时间的推移减少外来草的生物量，这使其成为降低外来物种影响的更好策略。