Comparative Study Of Two Soil Fractionation Methods With Three Eroding Soils In China

Soil erosion is a serious environmental and agricultural threat worldwide.The redistribution of a large amount of eroded sediment and associated nutrients like C and N across landscapes,their potential effects on biogeochemical cycling and consequently on global climate change are hot topics globally.Soil transports from eroded site to site of deposition or into water bodies in the form of aggregates rather than primary particles.However,the size distribution of eroded soil,as well as associated substances across landscapes,is far from fully understood.Therefore,in this study,we selected China Black soil,Loess soil and Red soil from three typically eroded regions of Heilongjiang,Shaanxi and Jiangxi provinces of China respectively.We fractioned each soil into six aggregated sizes >500,250-500,125-250,63-125,32-63 and <32 by the two methods: 1)Settling Tube Method and 2)Ultrasonic dispersion Method.In ultrasonic dispersion method,ultrasonically dispersed soil was fractionated through wet sieving method.For each fraction of soil,we analyzed SOC and TN content and calculated the mass proportion.Our results showed that there is a significant difference between the two methods,in term of size redistribution,SOC and TN mass.For settling tube method,the majority of the aggregate size accumulated in size 63 ?m to 500 ?m,contributed 86.7%,76.2%,75.4% in Black,Loess and Red soil,respectively but for the Ultrasonic dispersion method,the fraction of size <32 ?m is the predominate soil fraction,which accounts for more than 70% in Black soil,60% in Loess soil,and 48% in Red soil.In settling tube method,for size greater than 63?m contribute the major proportion of the total mass,this part was 4.77,4.15,2.65,times higher when compared with the Ultrasonic dispersion method,while for size less than 63 ?m(Clay + silt)the fraction received through settling tube was 4.72,4.16 and 6.69 times lower for Black,Loess and Red soil respectively.In settling tube method of SOC mass accumulate for size greater than 63 ?m are 82%,85%,and 78%,while for ultrasonic dispersion method only 17%,19% and 44% of SOC mass were recorded for Black soil,Loess soil and Red soil respectively.In case of size <63 ?m SOC mass for Black soil,Loess soil and Red soil,settling tube method contributed 19%,25%,and 13%,while for ultrasonic dispersion method it was increased to 82%,80%,and 64% respectively.In our results of settling tube method,the TN mass accumulation for size greater than 63 ?m is 82.04%,83.54%,and 86.25%.On the other hand,ultrasonic dispersion method exhibited only 18%,14% and 8% of TN mass for Blacksoil,Loesssoil and Red soil respectively.In case of aggregate size(<63 ?m)TN mass,settling tube method contributed 9.18%,16.46%,and 13.75%,while for ultrasonic dispersion method it was 82%,86% and 92%.The C: N ration follow the order: Black soil > Loess soil > Red soil.In Red soil for all fraction size of the ultrasonic dispersion method have significantly higher C: N than settling tube method,while in Black soil only size less than 32 ?m is higher.For settling tube method highest C: N ratio in Loess soil was recorded for size 250-500 ?m.while for Red soil highest C: N was found for size greater than 500 ?m and <32 ?m but for micro-aggregates showed no significant difference.Our study suggests that settling velocity of intact aggregates can more precisely predict the redistribution of soil and associated nutrients like SOC and TN than ultrasonic disperse soil(soil texture).