| Organic carbon pool of agricultural soil is the most activist part of the pool in land ecosystem. It is significantly changed in the serious effect of human activity. The storage of organic carbon pool of agricultural soil and the capacity of carbon sequestration are one of the bases that are used to evaluate recent potentiality of greenhouses gases emission reduction. It has been admitted by Kyoto Protocol, and increase in organic carbon pool of agricultural soil is double-win between stabilized productivity of agriculture soil and the response of climate change. A lot of studies on the storage and potentiality of agricultural soil organic carbon (SOC) sequestration in geography region as well as land utilization such as studies on changes of soil organic carbon (SOC) and potentiality of soil carbon sequestration. However, for some special region, studies of the changes of SOC under different land utilization and parent materials and the effect mechanisms on the capacity of C sequestration are relative rare.In this study, the articles on Chinese Academic Journal Web about the contents of soil organic matter/carbon (SOM/SOC) were collected and analyzed. The changes of SOC in agriculture soils since1980were analyzed. In addition, soil samples were collected in Jianghan plain, the middle reaches of Yangtze River, and Poyang Lake plain and the materials of the second general survey of soil in China were analyzed at the same time. The content of SOC in regional agricultural soil and the changes since the second general survey of soil in China were studied. While, the changes of contents of total organic carbon (TOC) and the fractions of agricultural soils under different soil utilization and parent materials were also researched. In addition, the distribution and stability of SOC in particle size fractions (PSFs) were studied too. Influences on the contents of SOC in soils and recent saturated capacity of SOC of agricultural soils, and potentiality of carbon sequestration in the future were discussed in this study. The main contents were as follows: (1) Trends of changes of the contents of SOC in agricultural topsoil, ChinaThe study supported the realization that the contents of SOC increased in agricultural topsoil since1985, and it is higher in paddy field than in dry farmland, and it is higher in eastern than in northern, but lost in northeast. In recent30years, the contents of SOC of paddy field and dry farmland decreased and then increased. The highest contents of SOC of paddy field were in2003to2009years, but it was highest before1985in dry farmland. It showed that the contents of SOC of dry farmland were lost compared to that before1985. It was the main problem that the lower contents of SOC of agricultural soil.(2) Contents of SOC and its changes under different land use and soil origin of croplands in Jianghan plain and Poyang Lake plainThe contents of SOC of agricultural topsoil under different land use and soil origin, a case study of agricultural topsoil in Jianghan plain and Poyang Lake plain, was studied through closely sampling. The saturated capacity of C in agricultural topsoil and potentiality of C sequestration in the future were also discussed according to the changes of SOC since the second general survey of soil in China and under different land use and soil origin of croplands. The results showed that contents of SOC of agricultural topsoil increased than the second general survey of soil in China for recent30years in the study regions. Increase in SOC was founded in paddy soils and dry farmland in different level, while, increase in Jianghan region was higher than other regions relatively. It showed that C pool increased and C accumulation effect was significant. In addition, a serious percentage of content of SOC in the region was lower than the lack standard curve of paddy soil, such as16.27%in Jianghan plain and5.71%in Poyang Lake plain. It showed there was significant patentability of C sequestration in these regions. The saturated capacity of SOC under different soil origin was higher originated from Jianghan plain wetland and Poyang Lake plain wetland which was26.83g kg-1and26.79g kg-1, respectively. However, it was lower in Jianghan Plain and red soil region of Poyang Lake plain which was23.41g kg-1and20.96g kg-1, respectively. The saturated capacity level of topsoil of dry farmland decreased significantly compared to paddy soil. In addition, the highest saturated SOC, existing in red soil in Jianghan plain, then was in farmland originated from Poyang Lake plain and Jianghan plain wetland, but the lowest farmland originated from red soil was14.19g kg-1. In this study, C sequestration under different soil origin was significant and indicated that the potentiality of C sequestration was significant higher in paddy soil than in dry farmland. The highest potentiality of C sequestration was8.49g kg-1and5.93g kg-1in paddy soil and dry farmland originated from Jianghan plain wetland, but the lowest was5.31g kg-1and4.00g kg-1in paddy soil and dry farmland derived from red soil.(3) Distribution of soil C and N in profile and changes of C density under different land use and soil origin in Jianghan plain and Poyang Lake plainThe study on contents of SOC of nine soil layers in0～100cm profile paddy soil, cotton field soil and wetland and soils derived from wetland in Jianghan plain, red paddy soil, reclaimed lake paddy soil and red paddy soil. The results showed that there was significantly difference of SOC in profile distribution under various land use. It was higher in paddy soils than in wetland and cotton field soils, and it decreased with the deep of soil layer. The content of SOC decreased quickly in cotton field soils and trended to be stable soon, but it unsteadily decreased. There was a very significant correlation between SOC and total nitrogen (TN). However, the correlation between C and N was lower in nature ecosystem than in agriculture ecosystem (for wetland R2=0.9043, cotton field soil R2=0.9903, paddy soil R2=0.9646, P<0.001). The contents of SOC were higher in reclaimed lake in Jiangxi and wetland derivation in Hubei, the lowest was red paddy soil. The content of SOC in profile decreased quickly and trended to be stable soon in red paddy soil. It decreased unsteadily in paddy soil originated from wetland. The highest density of SOC was in paddy soil not only in0-20cm but also in0-100cm. However, the highest density of SOC was in paddy soil originated from Jiangxi wetland in different soil origination both in0-20cm and in0～100cm, then was originated from Hubei wetland and the lowest was paddy soil originated from red soil. Compared to C density in0-100cm, the percentage of C density in0～20cm was from27.72%to50.86%. Thus, non-determinacy was higher when the C density was evaluated through topsoil.(4) Changes of content of SOC under different land use in Jianghan plain wetland and cultivated farmlandThe changes of SOC and distribution in PSFs under paddy soil, cotton field, orange field and wetland which was derived from wetland of Jianghan plain were studied. The results showed that the contents of SOC of paddy soils in topsoil were significantly higher than in other soils. Stable fractions of SOC and its ratio were higher than other physical fractions, especially chemical binding stable fractions of SOC and its ratio of paddy sols. The physical protection of SOC was decreased in dry farmland utilization, but the protection of SOC by PSFs was promoted in paddy soils. Thus, chemical binding stabilization and physical protection of SOC maybe the main problem of significant difference of SOC between paddy field and dry farmland.When Jianghan plain wetland was cultivated, SOC did not lost quickly, but SOC was increased in natural wetland under paddy field environment. It showed that it was a good way for land use to keep SOC pool well by cultivating sandy parent material wetland as paddy field in the middle and lower reaches of Yangtze River.(5) Changes of fractions of SOC and distribution in Jianghan plainThe study on fractions and stability of SOC in agricultural topsoil under different land use and soil origin and discussed the organic-mineral chemical complex manner in Jingzhou, Jianghan plain. The result showed that the contents of SOC would be affected by different land use and soil origin, and for comparison, the agriculture utilization affected the agricultural SOC more significantly. The ratio of stable fractions to total organic carbon (TOC) was higher in paddy field than in dry farmland, and it was significant higher in farmland originated from red soil than farmland originated from wetland. The clay protection was significant taken into effect in alkaline wetland origination soils. However, the chemical combining stabilization of SOC related to free iron oxide protection. There were a lot of free iron oxide in paddy field originated from red soil, and it made significant contribution to C protection. Thus, clay contents was not the only factors to affect C sequestration in paddy soils, free iron oxide take an important effect on C chemical stabilization in paddy field. |
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