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.