中文摘要：

黄土丘陵区生物结皮广泛发育,可通过固氮作用影响土壤氮素水平,但该区生物结皮对土壤氮素水平的影响鲜见报道。本文通过野外调查结合采样分析,研究了黄土丘陵区不同降水量带生物结皮组成、覆盖度差异及其对土壤氮素水平的影响。结果表明,1）黄土丘陵区不同降水量带生物结皮覆盖度无显著差异,但组成有差别;2）不同降水量带土壤氮素含量剖面分布具有明显的分层特征,生物结皮显著增加了结皮层土壤氮素含量,对下层土壤影响较小,结皮层下0—2 cm、2—5 cm、5—10 cm土层中氮素含量差异不显著;3）生物结皮层土壤全氮、碱解氮及微生物氮在不同降水量带差异不显著,而0—2 cm、2—5 cm、5—10 cm土壤全氮、碱解氮及土壤微生物氮含量在200～300 mm降水量带小于300～600 mm降水量带。研究结果揭示了黄土丘陵区生物结皮对土壤氮素的贡献,而不同降水量带生物结皮对土壤氮素的贡献差异不显著的原因有待于进一步研究。

英文摘要：

Biocrusts（biological soil crusts）,as ubiquitous living covers on soil surface of the revegetated grasslands in the hilly Loess Plateau regions,are potential influence factors on soil nitrogen contents.However,studies on the influence of biocrusts on soil nitrogens in this region were fresh.The objective of the paper was to determine the impact of biological soil crusts on soil nitrogen contents.Field investigation was conducted,and the contents of soil total nitrogen,soil alkali-hydrolyzable nitrogen and soil microbial nitrogen in different soil layers from various rainfall regions were analyzed.The results show that the coverage of biocrusts in different rainfall regions have no significant differences,while the compositions of biocrusts are different.The formation of the biocrusts enriched nitrogen（soil total nitrogen,alkali-hydrolyzable nitrogen and soil microbial nitrogen） in the upper layers of soils,and the soil nitrogen contents in the biocrust layers are significantly improved compared with the deep layers.Contents of soil nitrogen in 0-10 cm layers are not significant.There are no significant differences in the contents of soil nitrogen in the biocrust layers of different rainfall regions,while the soil nitrogen contents in 0-10 cm layers of 200-300 mm rainfall regions are significantly lower than those of 300-600 mm rainfall regions.As a whole,our results clarify the contribution of biocrusts to soil nitrogen;however,further study is still necessary to explore why there are no differences in the contents of soil nitrogen in the biocrust layers of different rainfall regions.