Study On The Features Of Soil Aggregates Organic Carbon And Nitrogen Under Different Vegetation Cover Types In Typical Karst

The soil organic carbon is abundant. It is an important medium for carbon transfer in the karst ecosystem and also a main way for carbon circulation.The composition of organic carbon in the soil can release CO2 into the atmosphere after being used by microbes. It may be as the sources and sinks of atmospheric CO2. Its tiny change may lead to the change of CO2 concentration in the atmosphere.In Karst area, the land use and cover change, climate, environment and human activity’s interference and so on can cause the changes of soil physical and chemical properties, which affect the soil organic carbon cycle. This study took 5 different vegetation types (Abandoned land, Grassland, Shrubs, Shrub-forest, and Forest) in Karst Valley in Zhong Liang Mountain of Chongqing as the research object to show five different stages of vegetation restoration. Through field investigation and indoor analysis, this research studied the content change of organic carbon,its fractions, nitrogen and available nitrogen of different soil aggregates in different vegetation cover types, and analyzed the relationship of carbon and nitrogen, and discussed the carbon sink effect. Besides, It also explored the soil’s carbon sequestration ability in karst area. These studies would provide theoretical reference for further revealing the driving mechanism of karst dynamic system. The main experimental results are as follows:(1) The clay particle with <0.001 mm diameter account for 27.47%-43.98%, which is high proportion. The soil pH value is 5.12-7.12, increasing with the depth increase. It shows weak acidic or neutral on the whole. The TOC content of each soil layer between 6.83～30.49 g/kg, decreasing with soil depth increase.The range of CEC is between 13.55～28.68 cmol(+)/kg, increasing with the soil depth increase. The TN, AN, TP and AP’s content in each soil layer of different vegetation types all reduce with the soil depth increase. Except Forest, the content of TCa and ECa both increase with the soil depth increase. The relationship between TN and AN, TP and AP, TCa and ECa all present very significant positive correlation.(2)The TOC content in different vegetation cover types ranges from 16.75 g/kg to 32.16 g/kg. the soil TOC content order from large to small is Forest> Shrub-forest> Shrubs Grassland> Abandoned land. The TOC contents present an obvious rising trend in the process of forward recovery. Among the same size of particle aggregates, the content change of SOC, EOC and LFOC are in line with the TOC. In different vegetation cover types, most of the soil aggregates are>0.25mm, which accounts for 97.03%-97.88%.The SOC, EOC and LFOC’s content are significant different in different size soil aggregates, increasing with the decrease of soil aggregates’diameter. The content of SOC, EOC and LFOC in micro soil aggregates (<0.25 mm) are largest, but their contribution to the TOC are lowest. While the SOC, EOC and LFOC content in the soil aggregates of 10-5mm are smallest, but their contribution to the TOC are biggest. This proves the big soil aggregates(>0.25 mm)is the main contribution of the TOC carrier. For the soil aggregates with 5-0.25mm diameter, the contribution rate of SOC, EOC and LFOC to TOC are respectively between 34.29%～62.4%,6.37%～10.7% and 6.22%～9.19%. The organic carbon and the fractions show very significant positive correlation and show very strong carbon sink effect. Therefore, the 5-0.25mm soil aggregates can be considered as the characteristics aggregates for soil carbon sequestration in this region. The relationship between SOC and TOC, EOC and TOC, LFOC and TOC are all very significant positive correlation.(3) The content of TN and AN for different vegetation cover types are different. For the same size grade soil aggregates, except the TN content in 2-1 mm soil aggregates, the TN and AN content orders are consistent with the TOC. Except the TN content in Abandoned land and the AN content in Grass Land, the TN and AN content in other vegetation cover types are increasing with the decreasing of aggregates’size. The C/N in soil aggregates of different vegetation cover types are different. The value of C/N ranges from 10.28 to 14.40, which is relative low. the C/N’s variable coefficient varies from 0.76% to 12.27%, belonging to weak variation level. The difference of vegetation cover types, diameter size and the interaction between them all have significant effects on C/N. The SOC and TN, SOC and AN, SOC and C/N all show very significant positive correlation. The organic carbon and nitrogen’s content change in soil aggregates both have significant impacts on soil carbon and nitrogen cycle and can effectively indicate the changes of soil environment in the area.(4) The formation of soil aggregates is often thought to be the important mechanism of the soil carbon sequestration. The physical protection of soil aggregates protects the organic matter from decomposition and fixes the new carbon. The soil nitrogen are stored in organic matter mainly in the form of organic nitrogen. The relationship between SOC and TN is stable. The accumulation of nitrogen means the increase of SOC.The difference between vegetation’s biological properties and the organic matter input amount lead to the changes of C and N content in the soil. As the vegetations recover gradually, the increase of organic matter input promotes the soil C and N accumulation. The accumulation of soil SOC constitutes the largest carbon stock in karst ecosystem and plays an important role in "carbon sink" in karst area. The forest is beneficial to the recovery of soil organic carbon and has strong ability of carbon sequestration. Therefore, reducing human interference, at the same time trying to prolong the time of abandoning and fallowing and increasing vegetation coverage appropriately and promoting the vegetation recovery help to strengthen the soil "carbon sink" effect.