| Accurately obtain farmland soil organic carbon impact factor on a local scale is thebasis of exactly estimating farmland soil organic carbon storage. The research of soilorganic carbon storage and sequestration mechanism is great importance to understand thepotentials and means of agricultural carbon sequestration. In this study, based on theregional scale in Northwest Arid Area （A case of Pingluo county）, soil carbon sequestrationand rate were studied;The distribution characteristics of soil, and aggregates organiccarbon contents were studied by using the physical method of grouping, combined withusing XANES Spectroscopy to measure the distributions and chemical compositionstability characteristics of the main SOC functional groups; soil aggregates physicalprotection of organic carbon and chemical stability mechanism were studied, which is toprovide the basis of more accurate estimate the potential of soil carbon sequestration andcarbon sequestration mechanisms. The main results and conclusions were presented asfollows:1. The spatial variability characteristics of topsoil bulk density:The results showed that the bulk density of farmland topsoil in Pingluo ranged from1.079to1.612g·cm-3, with the mean of1.355±0.107g·cm-3and with the variationcoefficient of0.080. The topsoil bulk density is a weak variability and decreases graduallyfrom west to east, but shows the certain north-south zonal distribution pattern in Spatialvariability on county scale. Soil texture and land-use patterns are key factors, which affectthe variability of soil bulk density on county scale. 2. Soil organic carbon storage and sequestration rate:From1980to2011, soil organic carbon and organic carbon density in Pingluo （SOCD）were significantly increased. In the last31years，organic carbon sequestration of farmlandtopsoil increased455,000t; the increment of organic carbon per unit area is6690.38kgC·ha-1, the average rate of SOCD is215.82kg C·ha-1·yr-1. Topsoil carbon sequestrationincreased area rated88.24%, decreased area rated3.17%, with the relative balance of thearea accounted for1.69%. In addition to sierozem, the carbon sequestration of other soiltypes has increased, while irrigating warped soil, saline soil and aquatic soil had greatlyincreased carbon sequestration. In terms of farmland soil factor, total nitrogen is one of theprincipal factors influencing the organic carbon content, followed by total phosphorus,total potassium; In terms of farmland management factors, the amount of root stubble isone of the major factors chief factors affecting the rate of soil organic carbon change,followed by nitrogen fertilizer inputs, the amount of retention stubble.3. The characteristics of soil organic carbon fractions:The content of total soil organic carbon in different soil types （the weighted meanvalue in100cm） were as follows: saline soil （5.37±0.17g/kg）> irrigating warped soil（4.89±0.40g/kg）> alluvial soil （4.06±0.55g/kg）. The content of topsoil organic carbonwere as follows: irrigating warped soil （8.49±0.49g/kg）> saline soil （8.14±0.81g/kg）>alluvial soil （6.52±0.65g/kg）.Under aerobic conditions, the content of heavy fraction organic carbon （HFOC, theweighted mean value in100cm） expressed as follows: irrigating warped soil （5.12±0.65g/kg）> saline soil （4.37±0.48g/kg）> alluvial soil （3.29±0.34g/kg）; the content ofmineral-incorporated organic carbon （MOM） expressed as follows: irrigating warped soil（4.40±0.56g/kg）> saline soil （4.25±0.44g/kg）> alluvial soil （3.12±0.14g/kg）; the contentof particulate organic matter （POC） expressed as follows: irrigating warped soil （1.08±0.44g/kg）> saline soil （0.98±0.41g/kg）> alluvial soil （0.64±0.08g/kg）; the content of light fraction organic carbon （LFOC） expressed as follows: saline soil （0.87±0.18g/kg）>alluvial soil （0.46±12g/kg）> irrigating warped soil （0.37±0.11g/kg）. In paddy fields, itis the highest content of HFOC and MOM in alluvial soil, followed by saline soil,irrigating warped soil. It is the highest content of LFOC and POC in irrigating warped soil,followed by saline soil and alluvial soil. The contents of organic carbon fractions under aerobic conditions were higher than in paddy fields.The content of soil organic carbon fractions （HFOC, MOM, POC） is highest in topsoilof all the soil layers, with depth of profiles increasing showed a clear decreasing trend.POC, MOM, HFOC and total organic carbon content showed a significant positivecorrelation （P<0.01）, while no significant relationship between LFOC and total organiccarbon content.4. Distribution of organic carbon and soil aggregates:1) The content of>0.25mm water-stable aggregate is most highest in the topsoil withthe percentage of63.81%, significantly higher than the parent material （36.15%）;aggregate destruction rate in topsoil is lowest with the percentage of27.71%, while theparent material layer is46.28%. The average of irrigating warped soil aggregatedestruction rate is32.27%, lower than the saline soil （44.36%） and alluvial soil （44.72%）,indicating that aggregate stability of irrigating warped soil is highest. Soil aggregatestability of upland soil is higher than paddy soil, the content of>0.25mm water-stableaggregate in upland soil is51.25%, which is higher than paddy soil （40.60%）; theaggregate destruction rate of upland soil is37.60%,significantly lower than paddy soil（43.30%）. The content of>0.25mm water-stable aggregates and soil organic carbon werepositively correlated, indicating that increasing of soil organic carbon content played animportant role in promoting the formation of aggregates.2) Soil organic carbon are mainly fixed in the aggregates of>0.25mm particle size.The average content of>0.25mm aggregate organic carbon in different soil types showedthat irrigating warped soil （8.35±0.08g/kg）> saline soil （8.05±0.12g/kg）> alluvial soil（6.92±0.09g/kg）. The average content of>0.25mm aggregate organic carbon in uplandsoil is8.13±0.08g/kg, significantly higher than paddy （7.83±0.42g/kg）.3) The content of soil aggregate organic carbon fractions （HFOC, MOM, POC） fixedin the aggregates of>0.25mm particle size. The content of HFOC, MOM and POC in thedifferent size aggregate is significantly higher in irrigating warped soil and saline soil thanaquatic soil; the content of HFOC and POC in the different size aggregate is slightly higherin upland soil than the paddy soils.4) The stable organic carbon （HFOC, MOM） in>0.25mm aggregate showedsignificant positive correlation with aggregate organic carbon（P <0.01）. 5. Characteristics of topsoi organic carbon functional groups:With the aggregate particle size increases, in irrigating warped soil, the relativepercentage of organic carbon labile functional groups （carbonyl-C, O-alkyl-C） haddecreased, while the relative percentage of organic carbon stable functional groups（aromatic-C） had increased. However, in saline soil, the relative percentage of organiccarbon stable functional groups （aromatic-C） had decreased, the relative percentage oforganic carbon labile functional groups （carbonyl-C） had significantly increased.Compared with the upland soil, paddy field could improve the chemical stability of soilorganic carbon. |
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