Northern forests are characterized by low temperatures that play a key role in the whole ecosystem functioning. However, Northern forests are expected to experience the largest temperature increase of all forest biomes in the next decades, which could affect central ecosystem processes, such as carbon (C) mineralization and N2 fixation. Aiming to clarify the temperature-dependence of non-symbiotic N2 fixation and C mineralization in Northern forest soils, we quantified the rates of both processes in soils of Scots Pine (Pinus sylvestris) forests located along a temperature gradient in Sweden in laboratory incubations at different temperatures (5, 12 and 20 °C). Our results show that N2 fixation by free-living bacteria in the organic layer of these forest soils ranges between 2 and 10 kg N ha−1 yr−1 which highlights the importance of non-symbiotic N2 fixation in Northern forest soils. We found a positive correlation between non-symbiotic N2 fixation per area and mean annual temperature (MAT). This relationship was caused by the positive relationship between the organic layer stock and MAT rather than by the direct effect of temperature on the process rate. In contrast, C mineralization per g of soil was negatively related to MAT. Furthermore, our results show that C mineralization is more sensitive to changes in incubation temperature (it increased by a factor of 2.2 from 5 to 12 °C as well as from 12 to 20 °C) than non-symbiotic N2 fixation that was not significantly affected by incubation temperature. Taken together, while N2 fixation responded little to changes in incubation temperature, our results suggest that the higher N2 fixation rate per area at sites with higher MAT is beneficial for primary production and organic matter inputs to soil leading to larger organic layer stocks. Hence, there is a positive, temperature-dependent feedback among non-symbiotic N2 fixation, primary production, and the organic layer formation in Northern forests.

中文翻译：

---

在北方森林土壤中，非共生 N2 固定对温度变化的敏感性低于碳矿化


北方森林的特点是低温，这在整个生态系统的运作中起着关键作用。然而，预计未来几十年北部森林将经历所有森林生物群落中最大的温度上升，这可能会影响中心生态系统过程，例如碳 （C） 矿化和 N2 固定。为了阐明北部森林土壤中非共生 N2 固定和 C 矿化的温度依赖性，我们量化了位于瑞典温度梯度沿线的苏格兰松 （Pinus sylvestris） 森林土壤中这两个过程的速率在不同温度 （5、12 和 20 °C） 下的实验室培养中。我们的结果表明，自由生活的细菌在这些森林土壤的有机层中对 N2 的固定范围在 2 到 10 kg N ha-1 yr-1 之间，这突出了非共生 N2 固定在北部森林土壤中的重要性。我们发现每个区域的非共生 N2 固定与平均年温度 （MAT） 之间存在正相关。这种关系是由有机层原料和 MAT 之间的正关系引起的，而不是由温度对加工速率的直接影响引起的。相比之下，每克土壤的 C 矿化与 MAT 呈负相关。此外，我们的结果表明，C 矿化对孵育温度的变化更敏感（从 5 °C 到 12 °C 以及从 12 °C 到 20 °C 增加了 2.2 倍），而不是不受孵育温度显着影响的非共生 N2 固定。 综上所述，虽然 N2 固定对孵化温度的变化反应不大，但我们的结果表明，在具有较高 MAT 的地点，较高的单位区域 N2 固定率有利于初级生产和土壤有机质输入，从而导致更大的有机层储量。因此，在北方森林的非共生 N2 固定、初级生产和有机层形成之间存在正的、温度依赖性的反馈。