High rates of biological nitrogen fixation (BNF) are commonly reported for tropical forests, but most studies have been conducted in regions that receive substantial inputs of molybdenum (Mo) from atmospheric dust and sea-salt aerosols. Even in these regions, the low availability of Mo can constrain free-living BNF catalyzed by heterotropic bacteria and archaea. We hypothesized that in regions where atmospheric inputs of Mo are low and soils are highly weathered, such as the southeastern Amazon, Mo would constrain BNF. We also hypothesized that the high soil acidity, characteristic of the Amazon Basin, would further constrain Mo availability and therefore soil BNF. We conducted two field experiments across the wet and dry seasons, adding Mo, phosphorus (P), and lime alone and in combination to the forest floor in the southeastern Amazon. We sampled soils and litter immediately, and then weeks and months after the applications, and measured Mo and P availability through resin extractions and measured BNF by the acetylene reduction assay. The experimental additions of Mo and P increased their availability and the lime increased soil pH. While the combination of Mo and P increased BNF at some time points, BNF rates did not increase strongly or consistently across the study as a whole, suggesting that Mo, P, and soil pH are not the dominant controls over BNF. In a separate short-term laboratory experiment, BNF did not respond strongly to Mo and P even when labile carbon was added. We postulate that high nitrogen (N) availability in this area of the Amazon, as indicated by the stoichiometry of soils and vegetation and the high nitrate soil stocks, likely suppresses BNF at this site. These patterns may also extend across highly weathered soils with high N availability in other topographically stable regions of the tropics.

中文翻译：

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钼、磷和 pH 值不限制亚马逊东南部热带森林的固氮作用

热带森林的生物固氮率 (BNF) 普遍较高，但大多数研究都是在从大气尘埃和海盐气溶胶中获得大量钼 (Mo) 输入的地区进行的。即使在这些地区，钼的低可用性也会限制由异养细菌和古生菌催化的自由生活 BNF。我们假设在大气中 Mo 的输入量较低且土壤风化程度较高的地区，例如亚马逊东南部，Mo 会限制 BNF。我们还假设亚马逊盆地的高土壤酸度会进一步限制钼的有效性，从而限制土壤 BNF。我们在雨季和旱季进行了两次田间试验，将钼、磷 (P) 和石灰单独和组合添加到亚马逊东南部的森林地面。我们立即对土壤和垫料进行采样，然后在施用后数周和数月进行采样，并通过树脂提取测量 Mo 和 P 的有效性，并通过乙炔还原测定法测量 BNF。实验性添加 Mo 和 P 增加了它们的有效性，石灰增加了土壤的 pH 值。虽然 Mo 和 P 的组合在某些时间点增加了 BNF，但 BNF 率在整个研究中并未强烈或持续增加，这表明 Mo、P 和土壤 pH 值并不是 BNF 的主要控制因素。在一个单独的短期实验室实验中，即使添加了不稳定的碳，BNF 对 Mo 和 P 也没有强烈反应。我们假设亚马逊这一地区的高氮 (N) 可用性，如土壤和植被的化学计量以及高硝酸盐土壤库存所示，可能会抑制该地点的 BNF。