Background As the pioneer groups extensively developed in arid and semiarid areas, biocrusts have a variety of ecological functions including essential weathering and pedogenic effects on the surface soil. These biocrust effects on soil weathering and formation are of great importance but still have not yet been intensively investigated, especially for the primitive soil such as loess soil and aeolian sandy soil.

Methods In the northern Chinese Loess Plateau, the biocrusts developed on loess and aeolian sandy soils for about 30 years were sampled by field investigation. The major elements geochemical characteristics of the soil within biocrust layer were explored by laboratory analysis, and the different weathering and pedogenic indicators that indicated pedogenic processes were quantified by formula calculation. The correlations among the weathering and pedogenic indicators and their interpretations were also revealed by correlation and factors analyses.

Results 1) The contents of K₂O, Na₂O, CaO, and MgO in the soil within biocrust layer were significantly lower than those in un-crusted soil, while Al₂O₃ and Fe₂O₃ were comparatively enriched in biocrust layer. 2) The silica-alumina-iron ratio, silica-alumina ratio, leaching index, weathering and leaching coefficient, and de-alkalization coefficient of the soil within biocrust layer were averagely 40.0% lower than those of un-crusted soil, whereas the weathering index, residual index, plagioclase alteration index, and mafic alteration index were averagely 1.6 times higher than those of un-crusted soil. These results indicated that biocrusts greatly promoted the leaching process of soil base cations as compared with un-crusted soil, and the degree of alternation from primary minerals to secondary clay minerals in the soil within biocrust layer was higher than that in un-crusted soil. 3) In comparison to un-crusted soil, biocrusts promoted weathering process of soil minerals. More specifically, the chemical alteration index of soil within biocrust layer was 48.5 on average, indicating that it was transitioning from the primary to the moderate weathering stage. 4) According to the results of correlations and factors analyses, the chemical alteration index and plagioclase alteration index can be utilized as typical and representative indexes to indicate weathering and pedogenic processes of soil.

Conclusions All the above findings indicated that biocrusts made a significant contribution to the weathering and pedogenic processes of primitive soils(loess and aeolian sandy soils). The development of biocrusts greatly promoted the base leaching, as well as the weathering and pedogenic degree of surface soil, which is of positive significance to soil formation and ecological restoration of the fragile ecological area in the Loess Plateau. It is, therefore, essential to pay high attention to biocrust effects on weathering and pedogenesis of soil in drylands.