State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi, China.
Key Laboratory of Environmental and Applied Microbiology, Chinese Academy of Sciences, and Environmental Microbiology Key Laboratory of Sichuan Province, Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu, China.
Environ Microbiol. 2022 Nov;24(11):5450-5466. doi: 10.1111/1462-2920.16095. Epub 2022 Jul 18.
Anthropogenic long-term nitrogen (N) deposition may dramatically impact biocrusts due to the overarching N limitation of soil biota in deserts. Even low levels of N may reach a critical loading threshold altering biocrust constituents and function. To identify the impact of chronic and continuous low levels of N deposition on biocrusts, we created a realistic gradient mirroring anthropogenic N addition rate (2:1 NH : NO rates: 0.3, 0.5, 1.0, 1.5, 3 g N m yr ) and measured the response of bacteria and fungi within cyanobacterial-dominated biocrusts over 8 years in a temperate desert, the Gurbantunggut Desert, China. We found that once N deposition reached 1.5 g N m yr biocrust bacterial communities, including diazotrophs, were altered while no such tipping point existed for fungi. Above the threshold, bacterial richness was enhanced, the relative abundance of Chloroflexi, FBP and Gemmatimonadetes was elevated, and diazotrophs shifted from being dominated by Nostocaceae and Scytonemataceae (Cyanobacteria) to free-living Bradyrhizobiaceae (Alphaproteobacteria). Alternatively, the relative recovery of a few fungal species within the Lecanorales, Pleosporales and Verrucariales became either enriched or diminished due to N deposition. The chronic addition of N resulted in a dense and interconnected bacterial co-occurrence network that accentuated a functional shift from networks dominated by phototrophic species within the Nostocaceae, Xenococcaceae, Phormidiaceae and Scytonemataceae (Cyanobacteria) to ammonia-oxidizing species within the Nitrosomonadaceae (Betaproteobacteria) and nitrifying bacteria [i.e. Nitrospiraceae (Nitrospirae)]. Based on structural equation models, the effects of N additions on biocrust constituents were imposed through indirect effects on pH, soil electrical conductivity and ammonium concentrations. In summary, biocrust constituents are generally insensitive to chronic low levels of N depositions until rates reach above 1.5 g N m yr with diazotrophs being the most sensitive biocrust constituents followed by bacteria and finally fungi. Ultimately once the threshold is reached N deposition favours biocrust constituents utilizing inorganic N and other C sources over relying on phototrophic and/or N-fixing cyanobacteria for C and N.
人为长期氮（N）沉积可能会对生物结皮产生巨大影响，因为沙漠土壤生物群落在很大程度上受到 N 的限制。即使低水平的 N 也可能达到改变生物结皮成分和功能的临界负荷阈值。为了确定慢性和持续的低水平 N 沉积对生物结皮的影响，我们创建了一个反映人为 N 添加率（2:1 NH : NO 比率：0.3、0.5、1.0、1.5、3 g N m 年）的现实梯度，并在中国温带沙漠古尔班通古特沙漠测量了 8 年内蓝藻主导的生物结皮中细菌和真菌的反应。我们发现，一旦 N 沉积达到 1.5 g N m 年，生物结皮细菌群落（包括固氮菌）就会发生变化，而真菌则不存在这样的临界点。在阈值以上，细菌丰富度增加，Chloroflexi、FBP 和 Gemmatimonadetes 的相对丰度升高，固氮菌从以 Nostocaceae 和 Scytonemataceae（蓝藻）为主导转变为自由生活的 Bradyrhizobiaceae（α-变形菌）为主导。相反，由于 N 沉积，少数真菌种在 Lecanorales、Pleosporales 和 Verrucariales 中的相对恢复要么丰富，要么减少。N 的慢性添加导致了一个密集且相互关联的细菌共现网络，这加剧了从由 Nostocaceae、Xenococcaceae、Phormidiaceae 和 Scytonemataceae（蓝藻）中的光养生物主导的网络到氨氧化物种的功能转变在 Nitrosomonadaceae（β-变形菌）和硝化细菌[即 Nitrospiraceae（硝化螺旋菌）]内。基于结构方程模型，N 添加对生物结皮成分的影响是通过对 pH、土壤电导率和铵浓度的间接影响施加的。总之，生物结皮成分通常对慢性低水平 N 沉积不敏感，直到沉积率达到 1.5 g N m 年以上，其中固氮菌是最敏感的生物结皮成分，其次是细菌，最后是真菌。最终，一旦达到阈值，N 沉积就有利于利用无机 N 和其他 C 源的生物结皮成分，而不是依赖光养和/或固氮蓝藻获取 C 和 N。
