Ice-free areas are expanding on the Tibetan Plateau and in its surroundings, and these recently exposed lands are typically deficient in nitrogen (N). Free-living N fixation (FLNF) represents a vital source of N input, especially when symbiotic N-fixing plants are absent. However, we know little about how FLNF changes with ecosystem succession after glacier retreat. In 2018, we measured the potential rates of FLNF in soil profiles at four sites (exposed since 2012, 1980, 1970 and 1930, respectively) along the Hailuogou glacier chronosequence and at a reference site with well-developed Podosols. We also tested the role of nifH gene abundance and resource availability in regulating FLNF rates. The results showed that photosynthetic cyanobacteria likely contributed the majority of FLNF in the biological soil crusts (exposed since 2012), while heterotrophic FLNF may prevail at later successional stages with closed canopies. FLNF rates were the highest in the litter layer of the soil profiles and decreased with soil depth. In the litter layer, FLNF rates were lower at the site dominated by N-fixing shrubs (exposed since 1980) than at the older sites dominated by non-N-fixing trees (exposed since 1970 and 1930, and the reference site). Carbon (C) availability was the major factor explaining variation in heterotrophic FLNF rates measured in the lab without light, while the effect of nifH gene abundance was nonsignificant. Overall, our results highlighted the energetic control on heterotrophic FLNF rates in relatively young ecosystems, which likely resulted in the poor relationship between nifH gene abundance and heterotrophic FLNF rates. The findings will help in understanding and modeling the coupled C and N cycles in glacier forefield ecosystems which are being exposed at accelerated rates.

青藏高原及其周围地区的无冰地区正在扩大，这些最近裸露的土地通常缺乏氮（N）。自由生活的固氮（FLNF）代表了氮输入的重要来源，尤其是在不存在共生固氮植物时。但是，我们对冰川退缩后FLNF如何随着生态系统演替而变化知之甚少。在2018年，我们测量了海螺沟冰川历时序列的四个位置（以及分别在2012年，1980、1970和1930年曝光的土壤位置）和发育良好的Podosols参考点的土壤剖面中FLNF的潜在速率。我们还测试了nifH的作用基因丰度和资源可利用性来调节FLNF发生率。结果表明，光合蓝细菌可能贡献了生物土壤结皮中的大部分FLNF（自2012年接触），而异养型FLNF可能在随后的封闭冠层演替阶段盛行。FLNF率在土壤剖面的垫料层中最高，并随土壤深度而降低。在凋落物层中，以固氮灌木为主的地点（1980年以来暴露）的FLNF发生率低于以非固氮树木为主的较老地点（自1970年和1930年以来以及参考地点暴露）。碳（C）的可用性是解释在无光的实验室中测得的异养FLNF速率变化的主要因素，而nifH的影响基因丰度不显着。总的来说，我们的结果强调了在相对年轻的生态系统中对异养FLNF率的有效控制，这可能导致nifH基因丰度与异养FLNF率之间的关系不佳。这些发现将有助于理解和模拟冰川前场生态系统中以加速速率暴露的C和N循环耦合。