Increased anthropogenic nitrogen (N) inputs can alter the N cycle and affect forest ecosystem functions. The impact of increased N deposition depends among others on the ultimate fate of N in plant and soil N pools. Short-term studies (3–18 months) have shown that the organic soil layer was the dominant sink for N. However, longer time scales are needed to investigate the long-term fate of N. Therefore, the soils of four experimental forest sites across Europe were re-sampled ~ 2 decades after labelling with 15N. The sites covered a wide range of ambient N deposition varying from 13 to 58 kg N ha−1 year−1. To investigate the effects of different N loads on 15N recovery, ambient N levels were experimentally increased or decreased. We hypothesized that: (1) the mineral soil would become the dominant 15N sink after 2 decades, (2) long-term increased N deposition would lead to lower 15N recovery levels in the soil and (3) variables related to C dynamics would have the largest impact on 15N recovery in the soil. The results show that large amounts of the added 15N remain in the soil after 2 decades and at 2 out of 4 sites the 15N recovery levels are higher in the mineral soil than in the organic soil. The results show no clear responses of the isotopic signature to the changes in N deposition. Several environmental drivers are identified as controlling factors for long-term 15N recovery. Most drivers that significantly contribute to 15N recovery are strongly related to the soil organic matter (SOM) content. These findings are consistent with the idea that much of the added 15N is immobilized in the SOM. In the organic soil layer, we identify C stock, thickness of the organic layer, N-status and mean annual temperature of the forest sites as most important controlling factors. In the mineral soil we identify C stock, C content, pH, moisture content, bulk density, temperature, precipitation and forest stand age as most important controlling factors. Overall, our results show that these temperate forests are capable of retaining long-term increased N inputs preferably when SOM availability is high and SOM turnover and N availability are low.

中文翻译：

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温带森林土壤中沉积氮的长期归宿

人为氮 (N) 输入的增加会改变氮循环并影响森林生态系统功能。氮沉积增加的影响取决于氮在植物和土壤氮库中的最终归宿。短期研究（3-18 个月）表明有机土壤层是 N 的主要汇。 然而，需要更长的时间尺度来研究 N 的长期归宿。 因此，四个试验林地的土壤在用 15N 标记后大约 2 年对整个欧洲进行了重新采样。这些站点涵盖了从 13 到 58 kg N ha-1 年-1 不等的各种环境氮沉积。为了研究不同 N 负荷对 15N 恢复的影响，通过实验增加或减少环境 N 水平。我们假设：（1）矿质土壤将在 20 年后成为主要的 15N 汇，(2) 长期增加的 N 沉积会导致土壤中 15N 恢复水平降低，(3) 与 C 动态相关的变量将对土壤中的 15N 恢复产生最大影响。结果表明，2 个十年后，大量添加的 15N 仍保留在土壤中，在 4 个地点中的 2 个地点，矿质土壤中的 15N 回收水平高于有机土壤。结果显示同位素特征对 N 沉积的变化没有明确的响应。几个环境驱动因素被确定为长期 15N 回收的控制因素。大多数对 15N 恢复有显着贡献的驱动因素与土壤有机质 (SOM) 含量密切相关。这些发现与大部分添加的 15N 固定在 SOM 中的想法一致。在有机土壤层中，我们确定了 C 库，有机层厚度、氮状态和林地年平均温度是最重要的控制因素。在矿质土壤中，我们将碳储量、碳含量、pH 值、水分含量、容重、温度、降水和林分年龄确定为最重要的控制因素。总体而言，我们的结果表明，这些温带森林能够在 SOM 可用性高且 SOM 周转率和 N 可用性低时保持长期增加的 N 输入。