Biological soil crusts (biocrusts) are complex communities of micro- and macro-organisms dwelling at the soil surface in dryland regions worldwide, which contribute to important ecological functions in these areas. Biocrusts may reach different developmental stages, associated to autogenic succession of specific phototrophic organisms. However, notwithstanding the large amount of existing literature, little is known regarding the relationship between biocrust stages of development and soil nutrient dynamics. In this opinion paper, we specifically focused on soil nitrogen, and compared the total nitrogen content and potential nitrogenase activity of different developmental types of biocrust (-covered) soils. Based on published reports, we looked at a possible relationship between nitrogen content at the soil surface and biocrust development, discussing the potential mechanisms leading to the observed soil nitrogen content heterogeneity. The results showed that a higher nitrogen content remained associated to the presence of moss-dominated biocrusts, and this biocrust morphotype was characterized by a lower potential nitrogenase activity compared to the cyanobacteria- and/or lichen-dominated biocrusts. We hypothesized that these seemingly contradictory findings might be attributed to three potential mechanisms: nitrogen inheritance, nutrient retention and nutrient transfer, which we hereby examined one by one. Altogether, our opinion supports the theory of biocrust succession from incipient cyanobacteria-dominated stages to more ‘mature’ stages dominated by mosses. We stress how the heterogeneous distribution of soil nitrogen, which is closely related with biocrust development and community types, eventually affects regional and even global nitrogen dynamic and storage.

中文翻译：

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旱地土壤氮含量异质性与生物结皮发育相关的潜在机制

生物土壤结皮（生物结皮）是居住在世界各地旱地地区土壤表面的微生物和宏观生物的复杂群落，对这些地区的重要生态功能做出了贡献。生物结皮可能达到不同的发育阶段，与特定光养生物的自生演替相关。然而，尽管现有文献大量，但人们对生物结皮发育阶段与土壤养分动态之间的关系知之甚少。在这篇意见论文中，我们特别关注土壤氮，并比较了不同发育类型生物结皮（覆盖）土壤的总氮含量和潜在固氮酶活性。根据已发表的报告，我们研究了土壤表面氮含量与生物结皮发育之间的可能关系，讨论了导致观察到的土壤氮含量异质性的潜在机制。结果表明，较高的氮含量仍然与以苔藓为主的生物结皮的存在有关，并且与蓝藻和/或地衣为主的生物结皮相比，这种生物结皮形态类型的特点是潜在固氮酶活性较低。我们假设这些看似矛盾的发现可能归因于三种潜在机制：氮遗传、养分保留和养分转移，我们在此一一进行研究。总而言之，我们的观点支持生物结皮演替理论，从早期的蓝藻细菌主导阶段到更“成熟”的苔藓主导阶段。我们强调土壤氮的异质分布与生物结皮发育和群落类型密切相关，最终影响区域乃至全球的氮动态和储存。