Composition And Distribution Characteristics Of Soil Carbon Of Farmland In The Loess Plateau

Loess Plateau was located at arid and semi-arid area, the soil layers was deep and carbonstorage was rich, which played an important role in the global carbon cycle. In this study, weanalyzed the distribution of soil organic carbon and inorganic carbon, the compositioncharacteristics of soil organic carbon pool，the physical protection mechanism of soil organiccarbon and the main control factors in the wheat/maize cropping system of Farmland in theLoess Plateau.The results were as follows:1. Accumulation and distribution of soil organic carbon and soil inorganic carbon offarmland had some differences in the Loess Plateau. With the increasing of soil depth, soilorganic carbon decreased as a whole, straw returning significantly improved soil organiccarbon of Guanzhong Plain Area, soil organic carbon have obviously accumulationcharacteristics at the soil layer of0-40cm of Guanzhong Plain Area, Loess Platform Area andGully Region of Loess Plateau, there had cumulative characteristics at the upper soll layer ofHilly and Gully Region and Wind Sand Grass Shoal Area；There had no significant changesof the inorganic carbon with the soil depth increased,inorganic carbon had an uniformeddistribution of the profile in the south of the Weihe River and gully region,the other samplingpoints inorganic carbon showed significant differences in the profile；Latitude and altitudehad an important influence on soil organic carbon, with the latitude increased the content ofsoil organic carbon decreased, soil organic carbon was highly significant negatively related toLatitude at each layer of the profile, altitude mainly effected the layer of0-60cm, with thealtitude increased soil organic carbon decreased, soil organic carbon was highly significantnegatively related to Altitude at the layer of0-20cm and20-40cm and was significantnegatively related to altitude at the layer of40-60cm.2. There showed some accumulation and distribution characteristics of soil organiccarbon fractions of farmland in the Loess Plateau.Particulate organic carbon(POC)showedsignificant differences at the layer of0-20cm in the Guanzhong plain area, which rangedfrom1.98±0.62to3.96±0.43g/kg, straw returning significantly improved the content ofparticulate organic carbon, the content of particulate organic carbon in the south of the WeiheRiver significantly lower than that in the north; The content of particulate organic carbon hadsome differences in the Loess Platform Area and Gully Region of Loess Plateau.The content of mineral-associated organic carbon (MOC) were2.1to4.4times、2.4to4.6times and1.8to4.0times the content of POC in the Guanzhong Plain Area、Loess Platform Area and GullyRegion of Loess Plateau respectively. The content of soil organic carbon were extremelysignificant positively related to POC and MOC in the Guanzhong Plain Area, the effects ofMOC on soil organic carbon was more significantly, the content of soil organic carbon had nosignificant related to POC and MOC in the Loess Platform Area, the content of soil organiccarbon were significant positively related to POC and MOC in the Gully Region of the LoessPlateau. The content of MBC and POC had significant difference with the soil layer changed，mainly distributed at0-20cm and20-40cm soil layer, mostly distributed at the surface soillayer in the Loess Plateau of East Gansu Province. The content of MBC decreased with thesoil depth increased in the range of0-60cm soil layer. With the altitude increased,the contentof MBC had an relative increasing tendency at0-20cm soil layer in the range of180.92-282.53mg/kg;The content of POC were in the range of1.67±0.33-2.31±0.14g/kg and0.57±0.05-1.31±0.08g/kg at the soil layer of0-20cm and20-40cm respectively. The contentof MOC were2.8to3.5times and3.5-8.4times the content of POC at the layer of0-20cmand20-40cm respectively, it was generally ranged from10to30times below40cm layers,POC had greater multiples at the layer of extremely lower. With the soil layer increased, thetrend of MBC/SOC and POC/SOC were consistent with that of MBC and POC respectively,MBC/SOC and POC/SOC can be acted as indexes to reflect the soil profile quality changes.3.There had close relationship between soil mechanical composition and soil organiccarbon of farmland in the Loess Plateau.The soil mechanical composition of soil mainlycomposed of clay and silt in the Guanzhong Plain、the Loess Platform Area and the GullyRegion of Loess Plateau, the silt content of lou soil mainly composed of0.002-0.02mmparticle fraction in the Guanzhong Plain, the content of0.002-0.02mm and0.02-0.2mmparticle fraction of dark loessial soil had little difference in the Gully Region of LoessPlateau;The soil mechanical composition of loessial soil mainly composed of silt and sand inthe hilly and gully region; The soil mechanical composition of aeolian sandy soil mainlycomposed of sand in the Wind Sand Grass Shoal Area. The soil organic carbon wassignificantly positively correlate with the content of clay and silt and was significantlynegatively correlated with the content of sand; The percentage of less than0.25mmaggregates at the0-20cm soil layer was the highest(except Gaoqu township of SanYuancounty)of farmland in the Loess Plateau,straw returning significantly improved thepercentage of aggregates lager than2mm in the Guanzhong Plain and the Loess PlatformArea,it also improved the stability of soil aggregates,the stability of soil aggregates weredifference in different regions,in this study,we found the aggregates which were less than 0.25mm and larger than2mm had the biggest contribution to the stability of soil aggregates;Without straw returning,soil organic carbon was mainly distributed in the less than0.25mmaggregates, straw returning reduced the content of soil organic carbon of less than0.25mmaggregates (except Gaoqu township of SanYuan county)in the Guanzhong Plain, meanwhile,it increased the content of soil organic carbon of other particle size aggregates, the content ofsoil organic carbon of larger than5mm、0.25-0.5mm and0.5-1mm aggregates weresignificantly increased.Soil organic carbon mainly distributed in the larger than2mmaggregates in the Loess Platform Area.In the Gully Region of Loess Plateau,soil organiccarbon mainly distributed in the aggregates of2-5mm、0.25-0.5mm and less than0.25mm.