| Biological soil crusts are widely distributed in karst rocky desertification areas.They are one of the crucial factors affecting soil physicochemical properties,enzyme activities and microbial communities in the preliminary stage of revegetation.In this paper,we took Huajiang karst canyon,the typical karst rocky desertification area in Guizhou,China,as the research area.By measuring nutrients,physical properties,enzyme activities and microbial communities of the soils both adhered(crust layer,0?1 cm depth)and overlaid(sub-crust layer,1?3 cm depth)by different types of biocrusts(algal crusts?mixed crusts?moss crusts),we analyzed the variation characteristics and the impact factors of soil nutrients and microbial communities between different successional stages of biocrusts;through in situ buried tube incubation experiments,we obtained data on soil nitrogen mineralization rates of different types of biocrusts,and revealed the influence laws of biocrusts on soil nitrogen transformation in karst rocky desertification areas.The main results are as follows:(1)Total N(5.07±0.75 g·kg-1),total P(1.83±0.19 g·kg-1)and ammonium N(1.83±0.78 g·kg-1)contents in soils showed numerical trend of bare ground-1),sucrase activities(50.29±0.65 U·g-1)were highest in moss crusts and nitrate reductase(16.66±1.02 U·g-1)activities were highest in algal crusts.Total nitrogen and ammonium nitrogen contents and urease,sucrase and nitrate reductase activities were significantly higher in the crust layer soil than in the sub-crust layer soil.The results indicate that the development and succession of biocrusts accumulate increasingly higher soil nutrients and enzyme activities,till the moss crusts at the later successional stage reach a significant effect;however,this accumulation effect tends to be manifested only in the most superficial soils,especially in the crust layer.Biocrusts can promote the accumulation of nutrients in the surface soil of karst rocky desertification areas,which has positive significance for local vegetation restoration.(2)With the succession of biocrusts,the Sobs index and Chaol index of soil bacterial community gradually increased,while the fungal community richness indices did not change significantly;the Shannon diversity index of either bacterial or fungal communities did not change significantly.The soil bacterial and fungal richness indices were significantly higher in the biocrust layer than in the sub-crust layer,while no significant difference was found in bacterial or fungal species diversity between the two soil layers.In addition,the community composition of soil bacteria or fungi did not differ significantly between different types of biocrusts,while the differences were significant between crust layer and sub-crust layer;both the soil bacterial and fungal co-occurrence networks were more complex in the crust layer than in the sub-crust layer.The two-factor network analysis of the co-occurrence network and Mantel test analysis showed that total nitrogen and urease had significant effects on both bacterial and fungal communities and the co-occurrence network in the soil.The results indicated that the bacterial community abundance was gradually improved with the succession of biocrusts,,but the biocrusts didn't play a significant role in bacterial abundance until the appearance of moss(mixed crusts and moss crusts);the microbial community abundance and co-occurrence network complexity were significantly higher in the crust layer than in the sub-crust layer due to the significant influence of nitrogen and urease.Biocrusts have promoted the growth and development of soil microorganisms in karst rocky desertification areas,enriched local microbial resources,and made significant contributions to soil health.(3)The contents of ammonium nitrogen,nitrate nitrogen and inorganic nitrogen in moss crusts,mixed crusts and bare ground soil showed noticeable seasonal changes.The levels in spring and summer were higher than those in autumn and winter,with the highest in summer and the lowest in winter;in spring,summer and winter,the levels of the three forms of nitrogen were highest in moss crusts,followed by mixed crusts and the lowest in bare ground.In summer,the net ammonification rate of soil covered by moss crusts was significantly higher than that by mixed crust and bare ground;in each of the other three seasons,there was no significant difference in the net ammonification rate of soils among the three crust types,and the rate was negative in winter.In spring and summer,the net soil nitrification rate of moss crusts was significantly higher than that of mixed crust and bare ground;in autumn and winter,there was no significant difference in the net nitrification rate among the three crust types.The net soil nitrogen mineralization rate of moss crust was significantly higher than mixed crusts and bare ground in spring and summer,while there was no significant difference between the three crust types in autumn and winter.Soil temperature and moisture were the critical environmental factors affecting nitrogen transformation:soil temperature was the main factor for nitrogen transformation in autumn and winter,and soil moisture was the main factor in spring and summer.The results indicate that moss crusts can effectively promote the nitrogen mineralization process in karst rocky desertification areas and are essential for nutrient storage.In summary,with the succession and development of biocrusts,moss crusts at the late successional stage can significantly improve soil nutrient levels and microbial abundance,and the surface layer of biocrusts can significantly accumulate more soil nutrients and promote the development of microbial communities;in addition,among different types of biocrusts,moss crusts have the most vital nitrogen mineralization capacity and are essential components of the nitrogen cycle in karst rocky desertification areas.This study not only enriches the ecological studies of biocrusts under climate change scenarios,but also provides theoretical support for ecological restoration practices and construction management in karst rocky desertification areas. |
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