The observation of high losses of bioavailable nitrogen (N) and N richness in tropical forests is paradoxical with an apparent lack of N input. Hence, the current concept asserts that biological nitrogen fixation (BNF) must be a major N input for tropical forests. However, well‐characterized N cycles are rare and geographically biased; organic N compounds are often neglected and soil gross N cycling is not well quantified. We conducted comprehensive N input and output measurements in four tropical forest types of the Congo Basin with contrasting biotic (mycorrhizal association) and abiotic (lowland–highland) environments. In 12 standardized setups, we monitored N deposition, throughfall, litterfall, leaching, and export during one hydrological year and completed this empirical N budget with nitrous oxide (N₂O) flux measurement campaigns in both wet and dry season and in situ gross soil N transformations using ¹⁵N‐tracing and numerical modeling. We found that all forests showed a very tight soil N cycle, with gross mineralization to immobilization ratios (M/I) close to 1 and relatively low gross nitrification to mineralization ratios (N/M). This was in line with the observation of dissolved organic nitrogen (DON) dominating N losses for the most abundant, arbuscular mycorrhizal associated, lowland forest type, but in contrast with high losses of dissolved inorganic nitrogen (DIN) in all other forest types. Altogether, our observations show that different forest types in central Africa exhibit N fluxes of contrasting magnitudes and N‐species composition. In contrast to many Neotropical forests, our estimated N budgets of central African forests are imbalanced by a higher N input than output, with organic N contributing significantly to the input‐output balance. This suggests that important other losses that are unaccounted for (e.g., NO_x and N₂ as well as particulate N) might play a major role in the N cycle of mature African tropical forests.

中文翻译：

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刚果盆地非洲热带森林中的氮通量对比

在热带森林中观察到生物利用氮（N）和氮丰富度大量损失的现象是自相矛盾的，显然缺乏氮输入。因此，目前的观点认为，生物固氮必须是热带森林的主要氮输入。但是，特征明确的N个循环很少见，并且在地理上有偏差。有机氮化合物经常被忽略，土壤总氮循环不能很好地量化。我们在生物环境（菌根协会）和非生物环境（低地-高原）形成对比的情况下，在刚果盆地的四种热带森林类型中进行了全面的N输入和输出测量。在12个标准化设置中，我们在一个水文学年期间监测了N的沉积，贯通，凋落，淋失和出口，并使用一氧化二氮（N ₂O）使用^15个N追踪和数值模型在干湿两季和原地土壤总氮转化中的通量测量活动。我们发现，所有森林都表现出非常紧密的土壤氮循环，总矿化与固定化比率（M / I）接近1，总硝化与矿化比率相对较低（N / M）。这与观察到的最丰富的丛枝菌根相关的低地森林类型的溶解性有机氮（DON）占主要的N损失相一致，但是与所有其他森林类型中溶解性无机氮（DIN）的高损失形成对比。总体而言，我们的观察表明，中部非洲的不同森林类型表现出N个通量，其强度和N种组成具有相反的对比。与许多新热带森林相比，我们估算的中部非洲森林的N预算因N投入高于产出而失衡，有机N显着地影响了投入产出平衡。这表明，其他重要的损失是无法解释的（例如，NO _x和N ₂ 以及颗粒N）可能在成熟的非洲热带森林的N循环中起主要作用。