Analysis On Temporal-Spatial Variation Of Soil Organic Carbon Content In Typical Cotton Field Of The Manas River Valley

【Objective】:High effective development of oasis agriculture based on soil sustainable development, SOC not only indicated N,P available state and soil physical properties, but also was an important index of farmland system stabilization and sustainability. Manas river valley was the largest planting cotton region, planting cotton area was 60～70% of total arable land area(except piedmont plain ),single planting structure. So this article analyzed the spatial variation law of cotton SOC in the Manas River Valley and treads of SOC in different planting way.【Method】:Based on the data of the second soil survey and sampling in 2005and 2006 .through long located experiments,alternative function,the soil particle and fractal model,DNDC model, this article analyzed temporal-spatial variation of SOC content in typical cotton field of the Manas river valley.【Results】:The total organic carbon stock in soil of Manas river valley was 11354.65×106～11753.06×106kg ,estimated by using the analytical data of the second soil investigation .A case of Shihezi Cotton Arable Soil, from south to north, SOC was decreased , organic carbon stock was alluvial fan> alluvial plain > delta. Simple correlation analysis indicated that the fractal dimensions was significant positive correlation with the contents of SOC. Soil clays Content played an important role in accumulation of SOC with the increase of latitude.There was a decreasing trend in cotton field SOC. The same trends were HU-C and FA-C, There was a maximum of HA-C at edge belt of alluvial fan, and decreased in alluvial plain, then it was increased in delta. The HA/FA ratio and Kos showed increasing treads. SOC was reduced gradually with soil profile depth increased, but the organic carbon compositions content was different in landforms. Cotton field SOC had been descended before the 90's because of only one fertilization method. Cotton field SOC was gained with the rate of straw application expanding after 90's middle. The content of SOC was increased in cotton field during different continuous cropping years in Valley of Manas River because of Cotton straw application, but most of it was HU-C. The HA/FA ratio was decreased and declined to 1 after 8 years continuous cropping. The Kos was decreased before 9 years continuous cropping, and then increased. Rotation of cotton, silage corn and cotton can improve HA/FA and reduce Kos. DNDC model was validated by using long-term experiment in Manas river valley, There was a high similarity between the simulated values and measured values. The results derived from tested simulations for 20years showed that SOC was increased under total or 50%cotton straw application, it was decreased under no cotton straw application at 5.8 g/kg of initial SOC. If initial SOC was 15.0 g/kg, SOC would be decreased under no cotton straw application, or increased under total or 50%cotton straw application before 2 years, than decreased. Rotation of cotton and silage corn year by year can improve SOC, but it was inferior to continuous cropping cotton. Planting silage corn after continuous cropping cotton 8 years, SOC added 10699 kg/hm2 ,it was not only breaking the balance of SOC, but also more than continuous cropping cotton(7921kg/hm2).【Conclusion】:There was a decreasing trend in cotton field SOC from south to north in Manas river valley. The fractal dimensions of particle can be used as a high and low index of SOC of cotton field. The content of SOC was increased in cotton field during different continuous cropping years in Valley of Manas River because of Cotton straw application. the warning time of SOC in continuous cropping cotton field is 8～9 years. Rotation of cotton, silage corn and cotton can improve quality of SOC After continuous cropping 8years, then rotation, not only can improve quality of SOC, but also increase SOC.