Harboring polyextremotolerant microbial topsoil communities, biological soil crusts (biocrusts) occur across various climatic zones, and have been well studied in the terrestrial drylands. However, little is known about the functional metabolic potential of microbial communities involved in the biogeochemical processes during the early succession of biocrusts on the tropical reef islands. We collected 26 biocrusts and bare soil samples from the Xisha Islands and Nansha Islands, and applied a functional gene array (GeoChip 5.0) to reveal nitrogen (N) cycling processes involved in these samples. Both physicochemical measurement and enzyme activity assay were utilized to characterize the soil properties. Results revealed the composition of N-cycling functional genes in biocrusts was distinct from that in bare soil. Additionally, microorganisms in biocrusts showed lower functional potential related to ammonification, denitrification, N assimilation, nitrification, N fixation, and dissimilatory nitrate reduction to ammonium compared to bare soils. Although the abundance of nifH gene was lower in biocrusts, nitrogenase activity was significantly higher compared to that in bare soils. Precipitation, soil physicochemical properties (i.e., soil available copper, soil ammonia N and pH) and soil biological properties (i.e., β-glucosidase, fluorescein diacetate hydrolase, alkaline protease, urease, alkaline phosphatase, catalase and chlorophyll a) correlated to the N-cycling functional genes structure. Nitrate N and ammonia N were more abundant in biocrusts than bare soil, while pH value was higher in bare soil. Our results suggested biocrusts play an important role in N-cycling in coral sand soil, and will be helpful in understanding the development and ecological functions of biocrusts on tropical reef islands.

带有多极端微生物的表土群落，生物土壤结皮（biocrusts）遍布不同的气候带，并且在陆地干旱地区已经得到了很好的研究。然而，关于热带礁岛上生物壳的早期演替过程中参与生物地球化学过程的微生物群落的功能性代谢潜力知之甚少。我们从西沙群岛和南沙群岛收集了26个生物结皮和裸土样品，并应用了功能基因阵列（GeoChip 5.0）来揭示这些样品涉及的氮（N）循环过程。利用物理化学测量和酶活性测定来表征土壤特性。结果表明，生物壳中的N循环功能基因的组成与裸露土壤中的不同。此外，与裸露土壤相比，生物外壳中的微生物显示出与氨化，反硝化，N同化，硝化，N固定和异化硝酸盐还原为铵有关的较低的功能潜能。虽然丰富与裸土相比，生物外壳中的nifH基因含量较低，而固氮酶的活性则较高。与氮相关的降水，土壤理化特性（即土壤中可用的铜，土壤氨氮和pH）和土壤生物特性（即β-葡萄糖苷酶，荧光素二乙酸酯水解酶，碱性蛋白酶，脲酶，碱性磷酸酶，过氧化氢酶和叶绿素a）循环功能基因的结构。生物结皮中的硝酸盐氮和氨氮比裸土更丰富，而裸土中的pH值更高。我们的结果表明，生物壳在珊瑚砂土壤中的N循环中起着重要作用，并将有助于了解热带礁岛上生物壳的发育和生态功能。