高寒草地灌丛化对土壤团聚体稳定性及其胶结物质的影响

Effects of Shrub-encroached Grassland on the Stability of Soil Aggregates and Cementing Materials in Alpine Grassland of Qinghai-Tibet Plateau

团聚体是土壤有机碳重要的存储单元,其稳定性直接影响有机碳的固存。为探明草地灌丛化是否影响土壤团聚体稳定性及其胶结物质,本文以青藏高原东缘4种典型灌丛化草地(高山绣线菊Spiraea alpina、窄叶鲜卑花Sibiraea angustata、小叶锦鸡儿Caragana microphylla、金露梅Potentilla fruticosa)为研究对象,分析土壤团聚体稳定性大于0.25 mm团聚体含量、平均重量直径(MWD)和分形维数及其胶结物质(团聚体有机碳、铁铝氧化物、钙键和铁铝键结合的有机碳)的含量,并分析各胶结物质对团聚体稳定性的影响。结果表明,(1)小叶锦鸡儿灌丛化显著降低2~0.25 mm和小于0.002 mm团聚体含量和团聚体稳定性(P<0.05),而其他3种灌丛化草地对土壤团聚体含量和稳定性影响不显著(P>0.05)。(2)小叶锦鸡儿灌丛化和窄叶鲜卑花灌丛化改变了团聚体胶结物质的含量。(3)团聚体胶结物质与MWD的增强回归树分析结果显示,高山绣线菊灌丛化草地土壤团聚体稳定性的主要贡献胶结物质为络合态铁(Fe_(p))和无定形态铁铝(Fe_(o)、Al_(o)),而其他3种灌丛化样地和未灌丛化草地主要贡献因子则为团聚体有机碳(SAOC)和游离态铁(Fe_(d))。灌丛化虽增加了小叶锦鸡儿样地主要胶结物质中小于0.053 mm粒径SAOC的含量,但降低了大团聚体、黏粒以及Fe_(d)的含量,从而降低了团聚体稳定性,这可能不利于其有机碳的固存。本研究可为该区域团聚体稳定性和有机碳库的动态变化提供理论知识。

【Objective】Soil aggregate is an important storage unit of soil organic carbon,and its stability directly affects the sequestration of organic carbon.This study explored the effects of shrub encroachment on the stability of soil aggregates and their cementing materials in grassland.【Method】The content of soil aggregates and their cementing materials(soil aggregate organic carbon,Fe-Al oxides,Ca bond organic carbon,and Fe-Al bond organic carbon)and the stability of aggregates(the content of aggregates>0.25 mm,mean weight diameter/MWD and fractal dimension)were determined at four typical shrub-encroached grasslands(Spiraea alpina,Sibiraea angustata,Caragana microphylla,Potentilla fruticosa)in the eastern margin of the Qinghai-Tibet Plateau.【Result】The results show that shrub encroachment significantly decreased the aggregate content of 2~0.25 mm and<0.002 mm and aggregate stability in the Caragana microphylla plot.However,there was no significant effect on the other three shrub grasslands.Shrub encroachment mainly changed the content of cementing substances in the aggregates of the Caragana microphylla and Sibiraea angustata plot.The results of boosted regression tree analysis between cementing materials and MWD show that the main contributing factors to the stability of aggregates in Spiraea alpina polt were complexed iron(Fe_(p))and amorphous iron and aluminum(Fe_(o),Al_(o)).Al_(so),the major contributors to grassland and other shrub plots were soil aggregate organic carbon(SAOC)and free iron oxides(Fe_(d)).【Conclusion】Although shrub encroachment increases the content of main cementing material—SAOC in<0.053 mm in Caragana microphylla plot,it reduced the content of large aggregates and clay,as well as the content of Fe_(d) in the Caragana microphylla plot.Thus,the stability of aggregates is reduced,which may be detrimental to the retention of organic carbon.This study provides theoretical knowledge for the stability theory of aggregates and the dynamic change of the organic carbon pool in this region.