黄土高原生物结皮对土壤养分的表层聚集与吸附固持效应

  【目的】  生物结皮的广泛发育可显著影响表层土壤养分状况，在土壤养分积累和循环中发挥重要作用。通过淋溶实验和吸附试验研究其作用效果和机制。  【方法】  以黄土高原质地不同的风沙土和黄绵土为对象，分别选取有生物结皮覆盖和无结皮覆盖的地块，分析生物结皮层、结皮层下0—2、2—5和5—10 cm土层土壤的有机质、全碳、全氮、全磷含量，研究生物结皮对土壤养分含量的影响及其随土壤深度的变化规律。以Cl?、K+、Ca2+为示踪离子开展土壤淋溶实验，分析其淋出土壤的特征；淋溶实验结束后，测定包括结皮层在内的各土层离子吸附解析量。  【结果】  1) 生物结皮层 (约2 cm厚) 养分含量是无结皮土壤的0.43～10.51倍。生物结皮覆盖下0—10 cm土壤的养分含量均高于对应深度的无结皮土壤，有机质、全碳、全氮、全磷含量比无结皮土壤增加了1.4%～184.9%。2) 生物结皮层的养分含量比其下层土壤提高了38.2%～557.1%，而无结皮的表层 (0—2 cm) 土壤养分含量仅比其下层土壤提高了13.4%～213.9%，这表明生物结皮增强了土壤养分的表层聚集。3) 生物结皮覆盖土壤中添加的养分在相同条件下相较于无结皮土壤更难以淋出；除易淋溶的Cl?全部淋出外，K+和Ca2+未被全部淋出，且在生物结皮覆盖土壤中的累积淋出量比无结皮土壤低21.9%～47.4%。淋溶实验结束后结皮层的Cl?、K+、Ca2+含量均显著高于无结皮 (8.8%～340.4%) 和结皮下层土壤 (14.5%～62.7%)。4) 生物结皮显著增加了土壤对Cl?、K+、Ca2+的吸附量，其增加幅度为27.8%～118.1%，且生物结皮层对不同离子吸附能力的强弱依次为Ca2+>K+>Cl?。  【结论】  与无结皮土壤相比，生物结皮能够增加土壤养分含量并促进土壤养分的表层聚集，同时提升土壤对养分的吸附与固持能力，因而有利于退化贫瘠土壤的养分积累，在干旱和半干旱地区土壤肥力提升与生态环境改善方面发挥着至关重要的作用。

Biocrusts increase soil nutrient levels by increasing the nutrient retention ability of surface soil on the Loess Plateau

  【Objectives】  Biocrusts affect the nutrient content in the topsoil, playing a crucial role in the nutrient accumulation and cycling. Here, we studied these effects through a leaching experiment.  【Methods】  In the Loess Plateau of China, aeolian and loess soils with and without biocrusts on the surface were sampled at every 2 cm layer in a 0?10 cm soil depth profile. The soil organic matter content, total C, N, and P were measured to determine biocrust effects. Soil leaching experiments were conducted in the four soil samples using Cl?, K+, and Ca2+ as tracers. On completion of the leaching experiments, the ion content in each soil layer was measured to explore the effects of biocrust on surface soil’s nutrient retention and adsorption abilities of surface soil.  【Results】  1) The nutrient contents in biocrusts layer were 0.43?10.51 times as compared with the uncrusted soil. And the nutrient contents at a soil depth of 0?10 cm under biocrusts were higher than those without biocrusts, the organic matter, total C, N, and P in soils with biocrusts increased by 1.4%?184.9% compared with the same layer of the uncrusted soil. 2) The nutrient contents in the biocrusts layer were 38.2%?557.1% higher than that in the underlying soil, while the nutrient contents in the surface layer of uncrusted soil were only 13.4%?213.9% higher than that in the underlying soil. These results indicated nutrient accumulation in the surface soil caused by biocrusts. 3) The nutrients in the soils with biocrusts recorded lower leaching than those in uncrusted soil under the same conditions. Cl? was completely leached out of the soils with and without biocrusts in our experiments, while the K+ and Ca2+ were only partly leached out, with 21.9%?47.4% lower in the biocrusts covered soil than the uncrusted soil. Moreover, Cl?, K+, and Ca2+ contents in the biocrust layer were (P<0.05) higher by 8.8%?340.4% than those in the uncrusted soil, and by 14.5%?62.7% in the soil under the biocrusts layer after the leaching experiments. 4) Biocrusts significantly increased the adsorption or retention ability of surface soil for Cl?, K+, and Ca2+ by 27.8%?118.1% compared to the uncrusted soil; the adsorption ability of the biocrusts layer for the tested ions was in the order Ca2+>K+>Cl?.  【Conclusions】  Our findings show that biocrusts can enrich soil nutrients in the surface soil while also increasing the surface soil's nutrient retention ability, which is favourable for nutrient accumulation in degraded soil.