Abstract Weathering and ecosystem nutrition are intimately linked through the supply of fresh mineral nutrients from regolith and bedrock (the “geogenic nutrient pathway”). However, the prominence of this link is dependent on the efficiency of nutrient recycling from plant litter (the “organic nutrient cycle”). Isotope ratios of strontium (Sr), an element that behaves similarly to Ca in ecosystems, confer two types of information: radiogenic Sr isotopes inform as to the sources of Sr and the degree of weathering, while stable Sr isotopes constrain partitioning between compartments of the Critical Zone (bedrock, water, secondary solids, and plants). To date, however, neither the reactions nor the mass balance between compartments that fractionate Sr isotopes, nor the fractionation factors involved, are well understood. Here, we present geochemical budgets of Sr (using radiogenic and stable Sr isotopes, and Ca/Sr ratios) at four sites along a substantial climate and primary production gradient in the coastal mountains of Chile. We found that Sr release through weathering is isotopically congruent, and released Sr is not strongly isotopically fractionated either during secondary mineral formation or transfer into the exchangeable pool. Despite this, the 88Sr/86Sr ratio of bio-available Sr, which should reflect the ratio of dissolved Sr, is higher than that of rock and regolith. We propose that this offset is caused by plants: while 88Sr/86Sr in plant organs at the four study sites systematically increased from roots towards their leaves, whole-plant Sr isotope compositions indicate preferential uptake of light Sr into plants (with a fractionation of up to −0.3‰ relative to the bio-available pool). Despite this strong biological fractionation, 88Sr/86Sr ratios in bio-available Sr do not covary with biomass production across our study sites, because with greater plant growth Sr is recycled more times after release by weathering – an isotope-neutral process. Rather, the loss of Sr from the ecosystem in solid organic material sets the isotope ratio of dissolved or bio-available Sr. Organic solids thus appear to constitute a significant export path of elements released during weathering, with the removal of solid plant debris reducing the recycling factor of Sr, and possibly that of other mineral nutrients too.

中文翻译：

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锶同位素在临界区沿着气候和植被梯度追踪生物活动

摘要 风化作用和生态系统营养通过来自风化层和基岩的新鲜矿质养分（“地质成因养分途径”）的供应密切相关。然而，这种联系的重要性取决于植物凋落物的养分循环效率（“有机养分循环”）。锶 (Sr) 的同位素比率是一种在生态系统中与 Ca 表现相似的元素，它提供两种类型的信息：放射性 Sr 同位素告知 Sr 的来源和风化程度，而稳定的 Sr 同位素限制了隔间之间的分配临界区（基岩、水、次生固体和植物）。然而，迄今为止，无论是反应还是分离 Sr 同位素的隔室之间的质量平衡，以及所涉及的分离因素，都没有得到很好的理解。这里，我们展示了智利沿海山区气候和初级生产梯度沿线的四个地点的 Sr 地球化学收支（使用放射成因和稳定 Sr 同位素以及 Ca/Sr 比率）。我们发现通过风化释放的 Sr 在同位素上是一致的，并且在次生矿物形成或转移到可交换池中，释放的 Sr 没有强烈的同位素分馏。尽管如此，生物可利用 Sr 的 88Sr/86Sr 比率（应反映溶解的 Sr 比率）高于岩石和风化层的比率。我们认为这种抵消是由植物引起的：虽然四个研究地点的植物器官中的 88Sr/86Sr 从根到叶系统地增加，但全植物 Sr 同位素组成表明光 Sr 优先吸收到植物中（分馏为到-0。3‰ 相对于生物可利用池）。尽管有这种强大的生物分馏，生物可利用 Sr 中的 88Sr/86Sr 比率与我们研究地点的生物质产量没有共变，因为随着植物生长的增加，Sr 在风化释放后会被回收更多次 - 一种同位素中性过程。相反，固体有机材料中 Sr 从生态系统中的流失决定了溶解的或生物可利用的 Sr 的同位素比率。因此，有机固体似乎构成了风化过程中释放的元素的重要输出路径，固体植物残骸的去除减少了Sr 的循环因子，也可能是其他矿质营养素的循环因子。因为随着植物生长的增加，Sr 在通过风化释放后会被回收更多次 - 一种同位素中性过程。相反，固体有机材料中 Sr 从生态系统中的流失决定了溶解的或生物可利用的 Sr 的同位素比率。因此，有机固体似乎构成了风化过程中释放的元素的重要输出路径，固体植物残骸的去除减少了Sr 的循环因子，也可能是其他矿质营养素的循环因子。因为随着植物生长的增加，Sr 在通过风化释放后会被回收更多次 - 一种同位素中性过程。相反，固体有机材料中 Sr 从生态系统中的流失决定了溶解的或生物可利用的 Sr 的同位素比率。因此，有机固体似乎构成了风化过程中释放的元素的重要输出路径，固体植物残骸的去除减少了Sr 的循环因子，也可能是其他矿质营养素的循环因子。