Study On Variation Of Soil Organic Carbon In Hainan

The total of global terrestrial soil organic carbon (SOC) is twice as much as the atmosphere carbon, and its small change may affect the balance of the global carbon cycle. Because of special climate conditions in tropical regions, SOC circulation is related closely to the global climate change. Under the impact of strong evaporation, heavy rainfall and aggravating human activity, SOC storage in Hainan had a significant change, which not only affects soil fertility and crop production, but also has impact on regional climate.To explore the variation of SOC content in Hainan, this paper research on SOC in Hainan Island by collecting Hainan historical information, surveying used patterns and administration mode of soil recent years, collecting and analysing soil sample, using ArcGIS to analyse the spatial and temporal variability of SOC in1980s-2000s, and using indoor simulation experiments to analyse the major impact factors, The main research results are as follows:(1) The content of topsoil organic carbon in1980s and2000s changes in the range of2.90g/kg～93.74g/kg and0.25g/kg～44.67g/kg, respectively, which is significant spatial and temporal variability. In the1980s, the high value area of topsoil organic carbon (>16.00g/kg) are mainly distributed in the northern and central mountain region (Qiongzhong Wuzhishan, Baisha, Tunchang), and gradually descend from high values areas to the coastal areas. In the2000s, the high value areas of topsoil organic carbon (8.00g/kg-12.00g/kg) mainly concentrated in Haikou, Chengmai, Danzhou, Wuzhishan, and the junction region of Sanya and Lingshui. In general, SOC content in Hainan has declined by5.86g/kg on average from1980s to2000s. The results of grading SOC content show that it has different changes in diferent periods. It was mainly distributed in third and fourth grade in the1980s, but mainly in fourth and fifth grade in the2000s. Analysing the relationship between the amount of initial organic carbon and annual variation of organic carbon indicate that the higher initial content, the faster the rate of change.(2) Estimating SOC storage by the means of soil type, the average density of topsoil organic carbon and total carbon(0-20cm) in the1980s and2000s is3.45kg·m-2and92.50Tg,1.96kg· m-2and53.93Tg (1Tg=1012g), respectively.This shows that SOC stocks reduced38.57Tg totally,725.66kg per hectare per year equally. In all soil types, the most widely distribution is the laterite which is also the largest carbon storage in the two periods,63.27Tg and37.73Tg, the68.40%,69.95%of total SOC, respectively. In the two periods, volcanic ash soil organic carbon density is highest,4.40kg·m-2and2.27kg·m-2, respectively; but its distribution area is small, its reserves is1.68Tg and0.87Tg, respectively. The different types of topsoil organic carbon storage in Hainan had a great of change in1980s-2000s. Volcanic ash soil and lateritic red soil organic carbon storage had the most obvious decline, which the annual rate of change is approximately-1060kg·hm-2·yr-1and-969.45kg·hm-2-yr-1, respectively.(3) Analyzing the relationship of the average annual rainfall, the average annual temperature, SOC content and average annual variation of SOC in Hainan indicates that SOC content and annual average rainfall were a significant positive correlation, but it was significant negative correlation related with average annual temperature; the average annual precipitation was very significant effect on the decomposition of soil organic carbon, which with the increase in the average annual precipitation, the annual SOC variation also increased, but the average annual temperature is smaller impact on it. The different land use patterns had difference SOC content, SOC content showed:garden> paddy> dry land, and different land use patterns of soil organic carbon has varying degrees of change in the1980s-2000s. As a whole, Paddy soil organic carbon content is larger, because the high rice-cropping index in Hainan, such as three crops a year, so that the prodigious consumption of organic matter, to grow vegetables in winter, water-drought rotation mode, also exacerbated the decomposition of paddy soil organic carbon. In addition, with the reduction of organic fertilizer and increase of chemical fertilizer, SOC content and structure has changed. The downward trend of SOC Hainan is the result of various factors. In the special environment of Hainan, the island with plenty of heat, which not any more the limiting factor of the decomposition of soil organic carbon; the concentrated rainfall and large differences of regional are the dominant factors of temporal and spatial variation of soil organic carbon; coupled with frequent human activities, exacerbating the decline in SOC content.(4) The indoor simulation experiment results show that the paddy soil microbial biomass carbon (SMBC) and readily oxidizable carbon (ROC) were significantly higher than that of dry land. If paddy fields converted to dryland farming, some of ROC is easy to broken down, resulting in the decline of soil organic carbon. Under indoor culture conditions, the rose of temperature conducive to the accumulation of SMBC and ROC; the increased in soil moisture is conducive to the accumulation of SMBC of both soil and ROC of paddy soil; The temperature and moisture showed a significant interaction on soil SMBC and ROC.(5) Based on the date of Hainan soil properties and average annual temperature and precipitation in the2000s, to estimate the density of potential Hainan farmland soil organic carbon sinks which changes in the range of13.42t·hm-2to33.32t hm-2. The density of Hainan potential farmland soil organic carbon sink is spatial differentiation, the highest density SOC sinks in the central region and lowest in the southwest coastal areas. The potential of carbon sequestration of farmland topsoil is average11.44t hm2. When SOC reaches saturation level in Hainan Island, the storage carbon of farmland topsoil can increase about4.80Tg. (6) In the period of1980s-2000s, under the effect of the natural climate and human activities, soil organic carbon is downward trend in Hainan. Forecast in accordance with the current agricultural production management in Hainan, soil organic carbon will continue to reduce. According to the estimation of Hainan agricultural soil carbon sequestration, there is a large potential of carbon sequestration of Hainan soil, if take needful measures can effectively increase soil carbon storage. For the sustainable development of agricultural production in Hainan, to curb the downward trend of soil organic carbon is no time to delay, increase organic fertilizer and return crop stalks to the field, take conservation tillage as a way of land-use management is urgent affairs of Hainan Province.