Symbiotic nitrogen- (N) fixing trees can influence multiple biogeochemical cycles by fixing atmospheric N, which drives net primary productivity and soil carbon (C) and N accumulation, as well as by mobilizing soil phosphorus (P) and other nutrients to support growth and metabolism. The soil micronutrient molybdenum (Mo) is essential to N-fixation, yet surprisingly little is known of whether N-fixing trees alter soil Mo cycling, and if changes to soil Mo are coupled to soil C, N, and P. We compared how symbiotic N-fixing red alder and non-N-fixing Douglas-fir trees modified surface soil C, N, P, and Mo across variation in climate and other site factors in the Pacific Northwest. We found that after two decades, N-fixing trees drove coupled increases in surface soil C, N, total P, and organic P. Consistent with contributions of N-fixing trees to soil organic matter, increased soil C and N were accompanied by lower δ 13 C in all sites, and lower δ 15 N in sites where non-fixer plots exhibited elevated soil δ 15 N. However, N-fixing trees did not affect surface soil Mo concentrations or fractions, suggesting that different factors control the cycling of P versus Mo over decadal timescales. Random forest analysis revealed that surface soil P was most strongly influenced by factors related to soil C accumulation, whereas surface soil Mo was related primarily to environmental factors, including potential differences in atmospheric Mo deposition across sites. Ratios of surface soil P:Mo were higher in extractable pools than in total soil digests, reinforcing the idea of stronger biotic cycling of P than Mo. Overall, our multi-site, multi-decadal field study found surprisingly small effects of N-fixing trees on soil Mo, despite rapid increases in soil organic C, N, and P. We hypothesize that, rather than direct effects of N-fixing vegetation, abiotic or indirect biotic factors such as soil sorption of atmospheric Mo inputs can link C–N–P–Mo cycles in terrestrial ecosystems on longer timescales.

中文翻译：

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温带固氮树对土壤磷钼循环的年代际解耦

共生固氮 (N) 树木可以通过固定大气 N 来影响多个生物地球化学循环，从而推动净初级生产力和土壤碳 (C) 和 N 的积累，以及通过动员土壤磷 (P) 和其他养分来支持生长和代谢。土壤微量元素钼 (Mo) 对固氮至关重要，但令人惊讶的是，人们对固氮树木是否会改变土壤 Mo 循环以及土壤 Mo 的变化是否与土壤 C、N 和 P 耦合的情况知之甚少。我们比较了如何共生固氮红桤木和非固氮花旗松树改变了太平洋西北部气候变化和其他场地因素的表层土壤 C、N、P 和 Mo。我们发现，20 年后，固氮树驱动了表层土壤 C、N、总 P 和有机 P 的耦合增加。与固氮树木对土壤有机质的贡献一致，土壤 C 和 N 的增加伴随着所有地点的 δ 13 C 降低，而在非固氮地块表现出土壤 δ 15 N 升高的地点，δ 15 N 降低。 然而，固氮树不影响表层土壤 Mo 浓度或分数，表明不同因素控制 P 与 Mo 在十年时间尺度上的循环。随机森林分析表明，表层土壤 P 受与土壤 C 积累相关因素的影响最大，而表层土壤 Mo 主要与环境因素有关，包括不同地点大气 Mo 沉积的潜在差异。可提取池中表层土壤 P:Mo 的比率高于总土壤消化物，强化了 P 比 Mo 更强的生物循环的想法。总的来说，我们的多站点，