Effects Of Land Use Types On Soil Aggregation And Microbial Community In Northern Agro-pastoral Ecotone

Cultivation and grazing are the two main human activity in northern agro-pastoral ecotone ecosystem,which induce serious grassland degradation because the unreasonable utilization over a long time.How to improve those problems for current production and theory are urgently needed.Cultivation and grazing significantly effect on soil aggregate,nutrient and microorganisms of grassland soils.Yet,how patterns of variation in soil aggregate,nutrient and microorganisms under grassland conversion and grazing remains obscure.In this study,we conducted experiments in the northern agro-pastoral ecotone areas.To evaluate the impacts of grassland cultivation on soil microbial communities(based on phospholipid fatty acids,PLFAs)and the links between soil microbes and physicochemical properties,we performed a paired field experiment following the conversion from native grasslands to>30-year-old cropland in the agro-pastoral ecotone of northern China.The grazing experiment adopts the long-term different grazing regimes(Enclosure:UG;Winter grazing:WG;Over grazing:OG)observation experiment.Furthermore,we explored the effects of the land use(Enclosure;Cropland;Overgrazing)on soil characteristics.To explored the changes of the soil aggregate stability and soil microbial community and further understand the underling mechanism,the statistical methods including t-test,ANOVA,regression,principal component analysis(PCA)and structural equation model(SEM)were performed.The results showed that:(1)Grassland conversion significantly reduced soil moisture and conductivity,while the soil pH showed inconsistent results among different sites(Guyuan county,Kangbao county,and Huade county).Moreover,grassland converted to cropland decreased the proportion of>1 mm aggregations,while increased the<1 mm aggregations at all three sites.There was no significant difference(P>0.05)for mean weight diameter(MWD)of soil aggregation among different sampling sites,while land use and soil depth to MWD exist significant effect(P<0.05).There was no interaction effect between the sampling location with land use and soil depth(P<0.05).(2)The concentrations of soil organic carbon(SOC),soil total nitrogen(STN)and soil microbial biomass consistently decreased with grassland conversion to cropland.Grassland conversion affected the microbial communities mainly through soil nutrient quality and soil moisture rather than soil aggregation and pH.These findings revealed that cultivation-induced soil nutrient loss may enhance soil microbe depletion and affect microbial community and is also likely to degrade microbial function,such as SOC decomposition and soil C cycling,under the scenarios of grassland conversion into cropland.This meant that the grassland farming should be treated with caution in semi-arid ecosystems.(3)Over grazing significantly decreased the above-/below-ground biomass,litter biomass,soil moisture and>1 mm soil aggregate(P<0.05),but significantly increased the soil bulk density(P<0.05).Overgrazing significantly reduced the mean weighted diameter(MWD)of soil aggregation in 0-10 cm and 10-20 cm soil layers.Soil MWD showed positive linear relationship with soil moisture,bacteria and AMF microbial biomass respectively(P<0.05).(4)Winter grazing significantly increased SOC,STN,MBC,MBN and microbial biomass,but overgrazing significantly decreased them(P<0.05).Overgrazing significantly increased the fungi biomass,but decreased the total microbial biomass,gram-positive/negative bacteria,actinomyces and AMF biomass.Grazing-induced higher fungal bacterial ratio(F:B ratio),and conversely,lower gram-positive/negative bacteria ratio(Gram+/Gram-ratio).Structural equation model(SEM)analysis showed that soil bulk density and the nutrient level directly affect the soil microbial community structure,while soil aggregation,moisture and pH indirectly affect it.(5)Both cultivation and grazing changed the distribution of soil aggregates and reduced the stability of soil aggregates.Compared with enclosed site,the>1 mm soil aggregation was significantly lower than overgrazing site and cropland,but the<1 mm soil aggregation was significantly higher.Cultivation and overgrazing decreased surface soil moisture content,nutrient concentration,microbial biomass and changed microbial community structure.Compared to overgrazing,cultivation has a more serious impact on soil characteristics.The stability of soil aggregates,water content,nutrient and microbial biomass in cropland are significantly lower than enclosure and overgrazing sites.