| The Zhujiawan small watershed is located in the northwest of the Sichuan Basin and is an important water source conservation and water and soil conservation area in the upper reaches of the Yangtze River.In recent years,due to the continuous increase in human activities,the disturbance of soil under different terrain conditions in the basin has become increasingly intensified,which not only affects the water and soil conservation of the basin,but also poses a potential threat to the ecological security of the middle and lower reaches of the Yangtze River.This paper uses the Zhujiawan small watershed as the research object.Through the combination of field survey and indoor analysis,the dry sieving method is used to classify the soil,obtaining >5mm,5-2mm,2-1mm,1-0.5mm,0.5-0.25 mm,<0.25 mm size agglomerates,and determination of mass fraction of aggregates,organic carbon content,easily oxidized organic carbon,particulate organic carbon,soluble organic carbon,and analysis of various particle sizes under different terrain conditions The stability of soil aggregates,and reveal the influence of soil organic carbon and its components on the stability of aggregates and the interaction mechanism of the two,in order to provide a reference for the sustainable use of soil resources and vegetation restoration in small watersheds.The main findings are as follows:(1)The particle size distribution of soil aggregates under all topographical conditions is the highest for aggregates >5 mm,followed by 5-2 mm aggregates,and the least for0.5-0.25 mm aggregates.In the shady slope,>5mm agglomeration content is in the middle reaches>downstream>upstream,and the ratio is higher than 53%;in the slope of sunny slope,>5mm agglomeration content is in the upstream>downstream>in the middle reaches,and the ratio is higher than 42 %.In the case of shady slopes and sunny slopes,in the channel locations,the midstream>upstream>downstream accounts for 62%,53%,and 42%,respectively.In the slope position,uphill>middle slope>downhill,accounting for 69%,62% and 42%,respectively.(2)Among the total organic carbon content in soil aggregates,the organic carbon content in the shady soil aggregates was distributed in the range of 9.31-34.37g·kg-1;the organic carbon content in the soil aggregates on the sunny slope was distributed in7.11-21.98g·kg-1.The content of total organic carbon in soil aggregates of 0-20 cm soil layers in all plots was higher than that of 20-40 cm soil layers.In the five soil aggregates,the organic carbon content first decreased and then increased.Among them,the organic carbon content peaked in <0.25 mm aggregates,and it showed: upstream> downstream>midstream in the channel position,and In the slope position,the upstream slope and the middle slope decreased in order,and the downstream slope showed the opposite law.(3)In this study,the oxidizable organic carbon content in shady slope soil aggregates was between 0.26-8.42 g·kg-1,accounting for 15.3%-53.6% of the aggregate organic carbon content in the aggregates;The content of easily oxidizable organic carbon was between 0.87-7.49g·kg-1,accounting for 19.9%-44.8% of the total organic carbon content of soil aggregates.In the 0-20 cm soil layer,the oxidizable organic carbon content in the soil aggregates mostly appeared in the upper reaches of the shady slope,and the content of the uphill slope was significantly higher than that in the middle slope and the down slope;in the 20-40 cm soil layer The content of easily oxidized organic carbon in the soil aggregates of various parts of shady slopes appeared mostly in the middle and lower reaches,and the contents of the uphill and middle slopes were significantly higher than those of the down slopes,and the maximum value appeared on the sunny slopes.There is no significant change in the position of the slope and the channel.(4)The organic carbon in the aggregates of 0-20 cm soil layer and 20-40 cm soil layer on Yinpo surface was distributed between 1.27-9.85g·kg-1,accounting for of total organic carbon in soil aggregates.22.3%-77.1%;Yangpo noodles between 1.33-9.35g·kg-1,accounting for 13.6%-45.1% of total organic carbon in soil aggregates.In the 0-20 cm soil layer,the maximum content of particulate organic carbon in the shady soil aggregates appears on the upslope,and the content of particulate organic carbon in the soil aggregates upstream of the channel position is the highest,and the sunny slope is located downstream of the channel position.The content of particulate organic carbon in the soil aggregates was the highest;in the 20-40 cm soil layer,the shady slopes conformed to the 0-20 cm soil law,and the sunny slope showed the highest content in the middle reaches of the channel location.(5)The content of soluble organic carbon in soil aggregates was between 0.47-1.22 g· kg-1,and the content of soluble organic carbon in soil aggregates on sunny slope was between 0.17-1.07 g · kg-1.It was concluded that the content of soluble organic carbon in soil aggregates conforms to the rule that the total organic carbon content in soil aggregates is higher than that on the sunny slope.At the same time,the content of soluble organic carbon in soil aggregates in 0-20 cm soil layers is higher than 20-In the 40 cm soil layer,the content of soluble organic carbon in the 0.5-0.25 mm soil aggregates is the highest among the 5 aggregates.The mean value of organic carbon in the aggregates of the dry-sieved dry-sievingaggregates at each particle size showed a tendency to increase with decreasing particle size in the 0-20 cm soil layer.In the 20-40 cm soil layer,the mean value of organic carbon in the aggregates of each particle size showed a trend of decreasing first and then increasing with decreasing particle size.The contribution rate of soil aggregates to organic carbon in0-20 cm soil layer shows a decreasing trend with the decrease of particle size.In the20-40 cm soil layer,the average contribution rate of soil aggregates to organic carbon showed a tendency to decrease with decreasing particle size. |
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