Migration Of Soil Aggregate Organic Carbon Under Splash Erosion

The redistribution of soil organic carbon（SOC）caused by water erosion disturbs the global carbon cycle（GCC）and affects global climate change.As an important part of the carbon cycle,the SOC redistribution caused by erosion has become one of the most important environmental problems all over the world.Raindrop splash erosion is the first key mechanism of soil erosion by water,and the SOC migration and redistribution caused by splash erosion is an important part of the carbon cycle in terrestrial ecosystems.However,the effect of splash erosion on SOC dynamics is not clear at present.In this study,three cultivated soils（black soil,purple soil,and red soil）with different SOC contents in China were used as research objects.The indoor simulated raindrop splash erosion experiments were conducted to explore the breakdown characteristics of aggregate,analyze the distribution,selectivity,and migration of the aggregate SOC,and establish a SOC migration model based on splash erosion.The main conclusions are as follows:（1）The distribution and migration characteristics of splashes under the different raindrop diameters were further investigated,and the breakdown,migration,distribution,and sorting characteristics of the splash aggregates were clarified.The experimental results showed that the amount of splash erosion always increased first and then decreased with the increase of the raindrop diameter,and there was a critical value between the raindrops with a diameter of 2.91-3.53 mm,which made the amount of splash erosion reach the maximum.Meanwhile,the amount of splash erosion of loam soil（black soil and purple soil）was significantly higher than that of silty clay loam（red soil）,but for the amount of splash erosion per unit area,loam soil first increased and then decreased with the increase of the splash distance,and linearly decreased in silty clay loam.Both the total amount of splash erosion and the amount of splash erosion per unit area were determined by soil texture.During the splash erosion,the soil aggregates were separated,broken,and transported by splash erosion,which could be expressed by the mass percentage,mean weight diameter（MWD）,and enrichment ratio（ER_A）of the aggregates.For the mass percentage of the aggregates,the mass percentage of microaggregates（<0.25 mm）increased with increasing splash distance,and the mass percentages of small（0.25-0.5 mm）and large macroaggregates（>0.5 mm）decreased.However,due to the influence of the clay content and rainfall kinetic energy,there were differences in the mass percentage of aggregates in different soils.For example,the mass percentage of large aggregates in red soil was always higher than that in black soil and purple soil.For the MWD of the aggregates,with the increase of the splash distance,the MWD of the three soils all showed a linearly decreasing trend,and the relative difference among the three soils decreased,indicating that the stability of the splash aggregates decreased with the increase of the splash distance.The stability of aggregates increased with the increase of raindrop diameter,but the larger the diameter of raindrops,the greater the decrease of aggregate stability with the increase of splash distance.Meanwhile,red soil had the highest aggregate stability,purple soil was second,and black soil had the lowest aggregate stability.For the ER_A,the ER_A of microaggregates and small macroaggregates increased with the increase of splash distance,while the ER_A of large macroaggregates decreased.There are differences in the ER_A of each size aggregates within different splash distances.（2）The characteristics of distribution and enrichment of aggregate SOC under different raindrop diameters were clarified,and the relationship between SOC enrichment and aggregate enrichment was analyzed.The results showed that there was a significant difference in SOC content in different-size aggregates during black soil and red soil（P<0.05）due to the higher specific surface area and charge density of smaller aggregates in the same splash distance.Purple soil had higher sand content and no significant difference in the SOC content of each size aggregates.Due to the limited kinetic energy of raindrops,the relative aggregate size of the large macroaggregates decreased,and the SOC content of the large macroaggregates significantly increased with the increase of the splashing distance,especially in the black soil and purple soil.SOC content of large macroaggregates increased slightly because of the high stability of the aggregates,while the SOC content of the small macroaggregates and microaggregates fluctuated changes,and there was no significant difference.Meanwhile,there was no significant difference in the aggregate SOC content of the same soil under different raindrop diameters（P>0.05）.During the splash erosion,the selective transport of aggregates affected the selective migration of SOC in each size aggregate.Whereby,the SOC of large macroaggregates of black soil and purple soil has higher selectivity than that of small macroaggregates and microaggregates,and the increase was greater as the distance increased.Compared with runoff erosion,splash erosion had a weaker effect on SOC sorting.The reason is that the raindrop energy during the splash erosion was mainly used to separate aggregates,and the energy used for transportation was relatively lower,while the kinetic energy of runoff erosion was mainly used for the transportation of soil aggregates.Additionally,the transportation of large macroaggregates had a significant negative effect on SOC enrichment（p<0.05）because many large macroaggregates diluted SOC content.（3）The SOC migration characteristics under the splash erosion were revealed,and the SOC migration model under the splash erosion was established.In the black soil and purple soil,microaggregates were used as carriers to migrate SOC,and the transport contribution of large macroaggregates and small macroaggregates was very small;red soil migrated SOC mainly through the transport of large macroaggregates,and the contribution of small macroaggregates and microaggregates to SOC migration cannot be ignored.The SOC migration model was constructed by obtaining the data of splash mass（M）,splash distance（S）,mean weight diameter of aggregate（MWD）,and kinetic energy of rainfall（E）,and the equation was as follows:SOC=e^0.011M^0.967S^-0.788MWD^-1.555E^0.262...