Distribution And Mineralization Of Soil Carbon And Nitrogen In The Erosion And Deposition Sites Of Small Watersheds Across China's Loess Plateau

Soil contains the largest pool of carbon?C?in the terrestrial ecosystem,and the dynamics of soil C is closely linked to atmosphere CO₂ concentration and global climate change.Erosion induced C variation is the key part of global C cycling.It has been estimated that soil erosion contributes to about 1 Pg C yr^-1 emission or sequestration.Such great uncertainties of C sink or source in erosion systems arise mainly from the redistribution and stability of C and nitrogen?N?associated with the transportation and deposition of soil particles,and the fate of eroded C and N either in situ soils or in the depositional sediments.Furthermore,climate and soil texture control soil erosion intensity and the mineralization rates of C and N,influencing the distribution and stability of C and N.This study systematically investigated the distribution and stability of soil C and N in the erosion-deposition continuation landform.We selected 5 small watersheds in the Loess Plateau of China.From north to south,these small watersheds are Shenmu,Suide,Ansai,Guyuan and Changwu,with increasing annual mean precipitation and temperature,decreasing erosion intensity and finer soil texture.In each watershed,the erosion site was located in the slope shoulder and deposition site was located in the adjacent check dam.Soil samples were collected at the 0-200 cm soil depth.Soil C and N concentrations in bulk soils,aggregate fractions and dissolved organic matter?DOM?were measured.Laboratory incubation experiment at different moisture and temperature levels were conducted to measure soil C and N mineralization.Redundancy analyses were used to figure out the influence of the chemistry characteristics of soil organic matter,the physical protection effects from aggregates,the chemical stabilization effects of soil particles and the environmental factors on soil C and N mineralization characteristics.Main results are listed below:1.In Ansai,Guyuan and Changwu small watersheds where soil erosion intensities are relatively lower,soil clay content was higher in the deposition site than in the erosion site.Soil clay was not accumulation in the deposition site in Shenmu and Suide small watersheds where erosion intensities are relatively higher.In Shenmu,Suide and Ansai,macro-aggregate was lower while silt and clay fraction was higher of the top 20 cm soils in the deposition site.In Guyuan where soil clay content was relatively higher and erosion intensity was lower,soil particles re-aggregated in the deposition site and aggregate stability was higher in the deposition site than in the erosion site.In Suide,Ansai and Changwu,soil bulk density was higher in the deposition site than in the erosion site,while saturated hydraulic conductivity was in the opposite trend.The accumulation of clay,the disruption and formation of soil aggregation and the variations in ventilation and permeability would further influence the stability of soil C and N.2.For the small watersheds in the Loess Plateau,soil inorganic C storage at the0-200 cm soil depth consisted of 88%of soil total C storage,and was higher in the southern part than the northern part of the Loess plateau.Soil inorganic C storage and its spatial distribution were similar in the erosion and the deposition sites.However,erosion or deposition landform should be considered for the storage of soil organic C?SOC?and total N?TN?.The SOC and TN storage were higher in the south than in the north of the Loess Plateau in the erosion site which was similar with the spatial pattern of clay content,while they were highest in Guyuan and there were no significant relationship between the SOC,TN storage and clay content in the deposition site.In shenmu,Ansai and Guyuan,SOC content was higher in the deposition site(1.95,2.68 and 2.91 g kg^-1,respectively)than in the erosion site(0.80,1.37 and 2.21 g kg^-1,respectively),and TN content was also higher in the deposition site(0.16,0.29 and 0.54 g kg^-1,respectively)than in the erosion site(0.09,0.14 and0.16 g kg^-1,respectively).For these 3 small watersheds,the difference of SOC content between landform was greatest in Shenmu especially at the 0-20 cm soil depth,and the difference of TN content was greatest in Guyuan especially at the deep soils.The accumulation of SOC exceeded the accumulation of TN in Shenmu,but on the opposite in Ansai and Guyuan.In Suide,SOC content in the 0-20 cm soils was higher in the erosion site(2.26 g kg^-1)than in the deposition site(1.40 g kg^-1)and differed not significantly at other layers.In Changwu,SOC and TN contents in the 0-40 cm soils were higher in the erosion site(3.91 and 0.73 g kg^-1)than in the deposition site(2.43 and 0.39 g kg^-1)and were similar at the 40-200 cm depth.Organic matter input from plants replaced the lost SOC and TN in the erosion site in Suide and Shenmu.These results indicated that the responses of SOC and TN to soil erosion were dependent on site and soil depth at the small watershed scale.3.In Shenmu,Ansai and Guyuan small watersheds,soil dissolved organic C?DOC?content was higher in the deposition site than the erosion site,and dissolved organic N?DON?content was higher in the deposition site than the erosion site in Ansai.However,for these 3 watersheds,DOC/SOC was lower in the erosion site?4.50,3.10 and 3.61%,respectively?than in the erosion site?10.47,4.90 and 4.62%,respectively?,and also DON/TN was lower in the erosion site?4.91,3.30 and 2.20%,respectively?than in the erosion site?11.35,4.42 and 2.21%,respectively?,and the differences of DOC/SOC and DON/TN between landforms were greater in Shenmu than in Ansai and Guyuan.In Suide and Changwu small watersheds,ultraviolet A?UVA?-like humus and ultraviolet C?UVC?-like humus in DOM at the 0-20 cm depth was greater in the erosion site than the deposition site;and DOM in the erosion site was more exogenous.In the deep soil layers?60-80,120-140 and 180-200 cm?of the5 small watersheds,DOM in the deposition site was with higher levels of aromaticity,hydrophobicity and molecular weight and had greater UVA-like and UVC-like humus.DOM in deep soils was less decomposable in the deposition site than in the erosion site for the 5 small watersheds.These results indicated that the labile part of SOC and TN was lower and C and N were more stable in the deposition than in the erosion site in Shenmu,Ansai and Guyuan small watersheds;DOM of top soils in Suide and Changwu small watersheds was mainly derived from plants.4.In Shenmu and Ansai small watersheds,SOC and TN contents associated in micro-aggregate and silt and clay fraction were higher in the deposition site than in the erosion site,while SOC and TN contents associated in macro-aggregate were similar between landforms.In Guyuan,SOC and TN contents were greater in the deposition site than in the erosion site in each aggregate fraction.In these 3 small watersheds,the accumulated SOC and TN between the erosion and deposition site were mainly accumulated in the micro-aggregate and the silt and clay fraction,and were thus less decomposable.The replaced SOC in Suide erosion site was mainly in the micro-aggregate fraction,the replaced SOC and TN in Changwu erosion site were mainly in the macro-aggregate fraction;and SOC and TN contents associated in the silt and clay fraction were similar between landforms in these two small watersheds.These results indicated that the accumulated SOC and TN were less decomposable in the deposition site in Shenmu,Ansai and Guyuan,and the replaced SOC and TN may lost with the disruption of aggregates in the erosion site in Suide and Changwu.5.In the 5 small watersheds on the Loess Plateau,the cumulated SOC mineralization and the cumulated net N mineralization were similar in the erosion and deposition sites.In Shenmu,Ansai and Guyuan,the specific SOC mineralization was higher in the erosion site(32.93,11.87 and 9.02 mg g^-1 SOC)than in the deposition site(14.40,7.35 and 5.66 mg g^-1 SOC),which indicated that SOC in the erosion site was more biodegradable.The chemistry properties of soil organic matter explained 51%of the variations in soil C and N mineralization,followed by the environment factors which explained 48%of the variations.DOC/SOC explained the most variation of C and N mineralization characteristics?29%?in top soils and DON/TN explained the most?22%?in deep soils.Among environmental factors,moisture and non-capillary porosity were the most important factors in top and deep soils,explained 19%and 11%of the variations in C and N mineralization,respectively.These results indicated that the soil organic matter was less accessible to microorganisms in the deposition site in Shenmu,Ansai and Guyuan small watersheds,and the labile part of C and N mostly accounted for soil C and N mineralization characteristics.Results from this study stressed the importance of erosion/deposition landform in determining the stocks and contents of SOC and TN on the regional scale.The effect of landform position on the distribution and mineralization of SOC and TN was dependent on site and soil depth at the small watershed scale.In Shenmu,Ansai and Guyuan where soils were more sandy and erosion intensities were relatively greater,SOC and TN accumulated in the deposition sites and they were more stable and less biodegradable especially for Shenmu and for deep soil layers.In Suide and Changwu,vegetation input replaced and even exceeded the eroded C and N in the erosion sites and the effect was greater in top soils.This study clarified the importance of the labile part of C and N in explaining the mineralization of C and N in the erosion-deposition system.Results from this study contributed to understand the response of soil C and N to erosion at small watershed scale,aided in the construction and regulation of soil erosion-C-N model and benefited the evaluation of soil erosion and C sink or source relationship.