Effects Of Water Erosion And Vegetation Rehabilitation On The Soil Organic Carbon Transportation And Distribution In Small Catchments Of The Hilly Loess Region

Soil organic carbon?SOC?,as the primary composition of soil carbon pool,plays an important role in the carbon cycle of terrestrial ecosystems and indicating global climate change.Soil erosion and vegetation rehabilitation have been recognized as the critical driving forces in regional carbon cycle in the fragile hilly-gullied region of the Loess Plateau in China.However,the“carbon source/carbon sink”controversy caused by soil erosion in the global carbon cycle has always been a hot spot for scientists.As an internal factor affecting the stability of SOC,the responses of aggregate distribution and stability to soil erosion and vegetation rehabilitation are the key factors to study the dynamics of SOC.Therefore,two typical dam-controlled small catchments,Nianyangou and Shayangou,were selected by field investigation in the middle of the Loess Plateau.Based on the historical long-term erosive rainfall events data of the catchments,combined with and the samples from the check dam sediments and the different land use types,and then by analyzing the distributions and stability of the water stable aggregates,as well as the contents of aggregate associated SOC and total nitrogen?TN?,we quantified the sources of eroded SOC and evaluated the carbon budget variation in the small catchments,then further clarified the“carbon source/carbon sink”role of water erosion in the catchment-scale carbon cycle.Meanwhile,according to the vegetation rehabilitation under the aim of the“Grain-for-Green?GFG?”project,the comprehensive effects of soil erosion and vegetation restoration on the stability of aggregates and aggregate associated SOC and TN content were elaborated.The specific research work and the main results are as follows:?1?The natural abundance levels of the stable carbon isotope(?¹³C)of bulk organic matter and a two-end-member mixing model were used to discriminate the sources of eroded SOC from different siltation stages retained by the Nianyangou check dam in an intensive agricultural catchment of the Loess Plateau,China.The results showed that the eroded SOC was primarily sourced from sloping cropland,which contributed to 81.3%of the total SOC retained by the check dam,whereas the gully soils contributed to 18.7%during the entire siltation stage.Additionally,the contribution of sloping cropland to eroded SOC increased from 1960 to 1990 as a result of rainfall and anthropogenic activities.?2?We clarified the carbon sequestration function of the check dam and the“carbon souce”effect of SOC migration and redistribution caused by soil erosion in the Nianyangou small catchment scale.The check dam intercepted 98.5 Gg of eroded soil and 172.6 Mg of SOC,which verifies that the check dam served as a carbon storage and sequestration structure for the hilly loess region due to its beneficial conditions for carbon sequestration over broad temporal and spatial distributions;A total of 89.7 Mg SOC was lost during soil erosion processes at a rate of 0.17 Mg ha^-1a^-1 and accounted for approximately 30%of the total eroded SOC exported from the eroding areas,indicating that the migration and redistribution processes of SOC caused by soil erosion is the main“carbon source”of the study area.?3?The erosion process caused by different erosive rainfall events has significant differences in the sorting of aggregates,which further affects the distribution and dynamics of SOC.Based on the microaggregate size distribution and the K-means clustering approach,75 flood couplets were identified in an 11.3-m sediment deposit profile of the check dam in the Nianyangou catchment and classified into three sediment couplet types.Different erosion processes were inferred according to the distribution of microaggregates,with the daily precipitation and rainfall intensity indexes corresponding to different sediment couplet types.The following results were obtained.Sediments with fine microaggregate fractions?Couplet Type I?contained abundant SOC and were mainly associated with low-intensity rainfall conditions,whereas sediments with coarse microaggregate fractions?Couplet Type III?had higher SOC concentrations and were mainly associated with high-intensity rainfall conditions;Couplet Type I sediments were most likely related to sheet and interrill erosion,whereas Couplet Type III sediments were likely associated with rill and gully erosion.?4?The vegetation rehabilitation significantly improved the stability of soil structure and the accumulation of SOC and TN in each aggregate fraction,which was an important“carbon sink”in the regional carbon cycle.At the same time,vegetation rehabilitation significantly reduced the sediment redistribution and land degradation caused by soil erosion.The SOC and TN in the bulk soil and aggregate fractions were measured in soils from rehabilitated grasslands and sloping croplands and in sediments retained by the check dam.The results showed that vegetation rehabilitation led to 78%,27%and 9%average increases in the macroaggregate amount,mean weight diameter?MWD?and mean geometric diameter?MGD?,respectively.In addition,rehabilitation resulted in the highest SOC and TN concentrations and contents in macroaggregates among all the aggregate size fractions.By analyzing the variation of sediment deposition and aggregate associated SOC and TN content before and after“GFG”implementation,the results showed that the farmland revegetation reduced the sediment redistribution during soil erosion,resulting in the decrease of sediment deposition in the check dam.The decrease of SOC and TN content in macroaggregates of the sediments retained by the check dam after the farmland revegetation indicated that vegetation rehabilitation increased the sequestration of SOC and TN in sloping cropland,therefore,reducing the enrichment of SOC and TN in macroaggregates of the sediments.