Effects Of Microtopography On Spatial Heterogeneity Of Soil Particles And Mineralization Of Organic Carbon On Long Gentle Slope In The Black Soil Area

Erosion processes spatially redistribute soil particles and the associated soil organic carbon?SOC?across landscapes.The spatial redistribution pattern is governed by the transport distances of individual displaced soil particles,which is not only dependent on their settling velocity,but also affected by micro-topography.However,when it comes to soil erosion process and carbon cycling,most researchers put more attention on carbon loss in field scale,and often ignore the spatial heterogeneity of SOC caused by micro-topography and soil connectivity within a field.In fact,the spatial heterogeneity of soil fertility and environmental conditions would directly affect crop growth,organic carbon dynamic replacement,as well as carbon balance in a slope system.In other words,the changeable micro-topography would lead to differential micro-environment along a slope,which may further impact the carbon dynamic balance between land and atmosphere.In this study,we investigated a long gentle slope in the Northeast China.In the field,images of the slope were gained with the help of an unmanned aerial vehicle?UVA?,from which we could build both digital elevation models?DEMs?and orthophoto maps to extract topographic information of the slope.In addition,48 soil samples were collected evenly on the whole slope to specify the influence of topography on the spatial distribution of SOC.Furthermore,we chose two kinds of sub-topography,i.e.,concave slope and convex slope,to figure out how erosion process redistributes soil particles and changes SOC mineralization.For these two slopes,topsoil at ten typical locations was collected and then classified into four size fractions by the method of sedimentation velocity:>250,63-250,20-63 and<20?m.SOC and?¹³C content of the four fractions were measured,as well as their CO₂ emission rate at six temperature gradients:5°C,10°C,15°C,20°C,25°C and 30°C.Finally,the temperature sensitivity of SOC mineralization of the four size fractions along the slope can be compared.The main results were as follow:1. The slope becomes higher and the curvature becomes larger from light erosion area to intensive erosion area,while in the sedimentary area the terrain tends to be gentler.SOC shows an increasing trend from intensive erosion area to light erosion area,and finally enriches in deposition area.slope is significant correlated with the distribution of soil organic in terms of the whole research area,and the correlation is different between concave and convex slope.2. The influence of micro-topography on the redistribution of soil particles and SOC mineralization shows in the following results:1)soil particles along the convex slope firstly shows a coarsening trend and then a fining trend from uphill to downhill,whilst the soil particles in concave slope remains a relatively uniform component;2)SOC content in the convex slope is less than that in the concave slope,yet the?¹³C content is generally higher in convex slope.3. In the convex slope,CO₂ emission rate is higher compared with that in the concave slope,but both are significantly increased in comparison with unfractionated soil.The temperature sensitivity also differed in the two topo-sequences.The above results indicate that a varying terrain would lead to the change of terrain factors,impacting the environmental conditions and the mineralization of SOC.Micro-topographic variation has a major influence on both lateral and vertical carbon fluxes,which can be demonstrated by the redistribution of soil aggregates,differences of SOC content and the differed temperature sensitivity.However,these are often unnoticed due to low-fidelity DEMs with coarse grid size and average sediment delivery ratios.As a result,factors like micro-topography,distribution and stabilization of SOC are supposed to be properly accounted for when calculating slope-scale carbon balance.