National Natural Science Youth Fund Project(41801086)
生物结皮在干旱半干旱地区生态系统中起着重要的作用。目前，有关生物结皮的存在对土壤酶活性的影响已有大量研究，而对生物结皮基本特征对土壤酶活性的研究较少。以流沙为对照，研究了不同组成生物结皮（藻结皮、地衣结皮、地衣-藓结皮、藓结皮）中以结皮厚度、叶绿素a含量、胞外多糖、胞外蛋白质及结皮中养分为特征的变化规律及其对结皮下土壤酶活性的影响。结果表明：（1）不同组成生物结皮的结皮厚度、叶绿素a、胞外多糖、胞外蛋白质、C、N、P、C/N、C/P、N/P的变化范围从藻结皮到藓结皮分别为：2.28-8.29 mm、1.79-8.05 μg/cm2、14.60-20.43 mg/kg、13.16-19.37 mg/kg、5.54-51.42 g/kg、0.44-3.36 g/kg、0.23-0.54 g/kg、9.62-15.32、24.07-94.98、1.92-6.23，且上述结皮特征均沿着藻结皮、地衣结皮、地衣-藓结皮、藓结皮方向显著增加（P<0.001），但胞外多糖和胞外蛋白质在地衣结皮、地衣-藓结皮和藓结皮间无显著差异（P>0.05）；（2）相比于C和N，结皮的P含量具有较小的变异系数，且不同组成生物结皮的N/P均小于14，说明不同组成生物结皮均易受到N限制；（3）结皮下土壤过氧化氢酶、蔗糖酶、脲酶、碱性磷酸酶、蛋白酶活性在不同组成生物结皮呈增加趋势，但均随土壤深度的增加而减小；（4）生物结皮特征与结皮下土壤酶活性呈显著正相关。冗余分析显示，生物结皮的胞外蛋白质、C/N和P是影响结皮下土壤酶活性的关键影响因子，且胞外蛋白质含量的影响最大（解释量为92.2%）。
Biocrusts play an important role in the ecosystem of arid and semi-arid regions. At present, lots of studies have concerned on the influence of the existence of biocrusts on soil enzyme activity. However, there are few studies focusing on the relationship between basic characteristics of biocrusts and soil enzyme activity. In this study, five types of biocrusts, including shifting sand, algal crust, lichen crust, lichen-moss crust, and moss crust, were studied in the Hongwei natural vegetation zone of the Shapotou Desert Research and Experiment Station of the Chinese Academy of Sciences. The variation patterns of the thickness, chlorophyll a, exopolysaccharide, extracellular protein, and C, N and P contents as the main characteristics of the biocrusts in different compositions of the biocrusts and their effects on soil enzyme activity beneath the crusts layer were investigated. Results showed that:1) thickness and contents of chlorophyll a, exopolysaccharide, extracellular protein, C, N, and P, and C/N, C/P, and N/P from algal crust to moss crust ranged from 2.28-8.29 mm, 1.79-8.05 μg/cm, 14.60-20.43 mg/kg, 13.16-19.37 mg/kg, 5.54-51.42 g/kg, 0.44-3.36 g/kg, 0.23-0.54 g/kg, 9.62-15.32, 24.07-94.98, and 1.92-6.23, respectively. Meanwhile, the above-mentioned crusts characteristics were significantly increased in different compositions of biocrusts (P<0.001), while exopolysaccharide and extracellular protein did not have significant differences among lichen crust, lichen-moss crust, and moss crust (P>0.05); 2) C, N and P contents and their stoichiometric characteristics increased significantly in different compositions of biocrusts, while C and N had a larger coefficient of variation compared to P. N/P in different compositions of biocrusts was less than 14, indicating that different compositions of biocrusts are more easily restricted by N; 3) The soil enzyme activity beneath crusts showed significant differences in different compositions of biocrusts and characteristics of biocrusts were positively correlated with soil enzyme activity. Soil invertase, urease, alkaline phosphatase and protease activity beneath algal crust, lichen crust, lichen-moss crust and moss crust were significantly different between 0-5 cm and 5-10 cm (P<0.05). With the increasing of soil depth, soil enzyme activity decreased. The two-factor analysis of variance showed that the interaction of different composition of biocrusts and soil depth had significant effects on the activity of catalase, invertase, urease, alkaline phosphatase and protease (P<0.01); 4) All of the above crusts characteristics had significant influence on soil enzyme activity. The individual effects in the redundancy analysis showed that the importance of the biocrusts characteristics on soil enzyme activity beneath the crusts was as follows:extracellular protein > exopolysaccharide > C/N > P > C/P > N/P > chlorophyll a > N > crust thickness > C, explaining the variation of enzyme activity by 92.2%, 90%, 75%, 68.9%, 62.8%, 62.3%, 59.6%, 57%, 56.8%, 52.1%, respectively. However, the conditional effects analysis showed that extracellular protein, C/N and P in biocrusts were the main influencing factors on soil enzyme activity with the explanation of 92.2%, 4% and 2.2%, respectively. The results of the study will help to further clarify the contribution of different composition of biocrusts to soil enzyme activity and will enrich the understanding of biocrusts for its ecological importance in the restoration of desert soil and the improvement of soil fertility.