Effects Of Shrub-encroached Grassland On The Stability Of Soil Aggregates And Their Cements In Alpine Grassland

Shrub encroachment is an important ecological phenomenon in global grassland ecosystem and an important process affecting soil carbon storage in grassland.By changing habitat heterogeneity,species composition,aboveground biomass,and the content of organic carbon,shrub encroachment affects the structure and function of ecosystems,and thus affects the regional carbon cycle.In recent years,the phenomenon of shrub encroachment in alpine grassland on the Qinghai-Tibet Plateau has become more and more universal.As a storage unit of organic carbon,the stability of aggregates directly affects the storage of organic carbon.In this study,four typical shrub species:Spiraea alpina,Sibiraea angustata,Caragana microphylla,Potentilla fruticosa,and one non-shrub grassland were researched,wet screening and sedimentation siphon method proposed by Cambardella and Elliottt were used to separate and extract aggregates.>0.25 mm aggregate content(R_0.25),mean weight diameter(MWD)and fractal dimension(D)were used to evaluate the stability of aggregates,and the contents of organic cementing matter(soil aggregate organic carbon),inorganic cementing matter(Fe-Al oxides)and organic-inorganic complex(Ca bond organic carbon and Fe-Al bond organic carbon)in aggregates were determined,in order to explore the main contributing factors affecting the stability of aggregates.The results are as follows:1)Shrub encroachment affects pH and soil texture.The influence of shrub encroachment on organic cements is as follows:it significantly increased the pH of the shrub plots except for the shrub plots of Potentilla fruticosa(P<0.05),significantly increased the content of sand in Caragana microphylla plot,and significantly decreased the content of silt and clay in Caragana microphylla plot,(P<0.05).Shrub encroachment has no significant effects on SOC,TN and BD(P>0.05).2)Shrub encroachment changes the content distribution of cementing substances and activation of iron oxides in aggregates.The effects of shrub encroachment on organic cements are as follows:shrub encroachment significantly reduced the content of organic carbon in<0.053 mm microaggregates in Sibiraea angustata plot,and significantly increased the content of organic carbon in<0.053 mm microaggregates in Caragana microphylla plot,(P<0.05).The influence of shrub encroachment on inorganic cementing substances and organic-inorganic complex are as follows:The content of complex iron(Fe_p)in various plots(except Caragana microphylla plot)was significantly increased,while the content of other forms of Fe-Al oxides was decreased(P<0.05).The content of Fe/Al oxide and Fe/Al bound organic carbon(Fe/Al-SOC)and calcium bound organic carbon(Ca-SOC)in 0.25?0.053 mm in Caragana microphylla plots decreased the most(P<0.05).Shrub encroachment significantly reduced the Fe/Al-SOC content of aggregates with different particle sizes in Sibiraea angustata plot(P<0.05).Shrub encroachment significantly reduced activation of iron oxide in>2 mm,2?0.25 mm,0.25?0.053 mm in the Caragana microphylla plot(P<0.05).3)Shrub encroachment changes the distribution and stability of soil aggregates in grassland.Shrub encroachment significantly increased the content of 0.25?0.053 mm microaggregates in Caragana microphylla plots,and significantly decreased the content of 2?0.25 mm and<0.002mm(P<0.05).The results of R_0.25,MWD and D values showed that shrub encroachment significantly decreased the stability of aggregate in Caragana microphylla plot(P<0.05),but had no significant effect on other shrub plots(P>0.05).4)According to the RDA analysis of soil physical and chemical properties and aggregates stability index,the results showed that the silt and soil organic carbon were the main physical and chemical factors that played an important role in the stability of aggregates.5)The activation degree of iron oxide in>2 mm,2?0.25 mm,0.25?0.053 mm size has a significant(P<0.05)or extremely(P<0.01)significant positive correlation with the stability index of the aggregate.BRT results of cementing matter and stability of soil aggregates in shrub patches and grassland patches showed that:Fe_p?amorphous form of iron and aluminum(Fe_o?Al_o)were the main factors affecting the stability of soil aggregates in Spiraea alpina plot,soil aggregate organic carbon(SAOC)and free iron oxides(Fe_d)were the main factors affecting the stability of aggregates in grassland patches and other three shrub patches.The results show that Fe-Al oxides and organic carbon in aggregates may have stronger effects on the stability of aggregates than organic-inorganic complexes.6)To sum up,the stability of aggregates is the result of the interaction of various cementing substances,shrub encroachment mainly decreased the contents of silt and clay,2?0.25 mm aggregates and<0.002 mm aggregates,and cementing substance(except for SAOC)content and activation of iron oxides in aggregates in Caragana microphylla plot,ultimately reduce its stability of aggregates.That may be detrimental to their organic carbon sequestration.