Soil Organic Carbon Dynamics Of Wetlands And Croplands

Soil Organic Carbon is one of the key properties which have closely relationship with ecosystem functions and plant growth. Soil organic carbon pool, which is 2 to 3 times larger than the pool s of atmosphere and vegetation, is the most active pool in terrestrial ecosystems, and its dynamic will affect future climate change trends directly. The dynamic of soil carbon pool and its process mechanisms under human disturbance, has become the key research areas of ecology, soil science and other disciplines.The dynamic of soil organic carbon pool and its quantification are important contents for studying the carbon cycling and carbon sequestration. Domestic and international researches showed that soil carbon pool in cropland was disturbed strongly by agricultural managements, while it could be adjusted by human in a short time. Therefore, there will be an important scientific significance in carbon sequestration strategies and the options of sustainable agricultural development paths of China for the studies of SOC dynamics and its sequestration potential under landuse changes and agricultural managements. The studies of SOC dynamics under different scales of geographical regions have become the research focuses of soil science, ecology and agricultural science.This thesis studied the SOC dynamics of wetlands and croplands in the Yangtze River region in Lu’an city, Anhui province, and also analyzed the SOC variability of spatial scale, in order to explore the factors which impacted storage and accumulation of SOC and provided the scientific basis for SOC sequestration, land use sustainable and agricultural managements in this region. The main results are as follows:1. The soil in the regions of Yangtze River lost SOC seriously when the wetlands were reclaimed as croplands. Yangtze River wetlands, which were reclaimed deeply and had low SOC contents, were distributed widely. Compared with the original wetland soils, the soils under different cultivation methods have different decrements of SOC contents, and these decrements increased with the increments of the utilization years. However, there was a little increment in SOC when the wetlands were reclaimed as rice paddies. The SOC contents were increased by 35%-67% when the wetlands in Jianghan Plain and Wanjiang Plian wre reclaimed as the rice paddies. Therefore, wetland is reclaimed as rice paddy is one of the well approaches in carbon sequestration.2. The SOC contents in Wanjiang Plain wetland distributed between 11.30 to 27.83 g·kg-1, and there were still much SOC in deep soil. The SOC in wetlands reclaimed showed a decrease trend, and its distribution among the regions and soil deeps showed more variability than other wetlands. After reclaimed as croplands, the soils lost the SOC as 18 tC·hm-2 in topsoil and 30 tC·hm-2 in total soils, respectively. The SOC density of wetlands cultivated were only 40% of original. The loss of SOC storage estimated as 40-60Tg in recent 50 years, and 0.15PgC in history.3. The SOC content variation with the landuse and spatial changes in Lu’an was studied in this study. The results show that the SOC content (12.27±2.90g-kg-1) and coefficient of variation (23.63%) in uncultivated soil was highest in all the soils of Lu’an City; SOC content of paddy soil (11.50±1.08 g·kg-1) was significantly higher than that of dryland soil (8.11±1.17 g·kg-1), and the coefficient of variation (9.39%) is less than the latter. The topsoil SOC in different landscape units were Mountain (14.60±3.14 g·kg-1)> hill (13.20±3.26 g·kg-1)> hillock (10.22±1.00 g·kg-1)> Plain Wei Fan (10.05±1.36 g·kg-1). Under county scale, the coefficient of variation was between 24.95% and 5.31%,3.80% to 16.75% under township scale,20.1% in the whole range of Lu’an city. It indicated that there was a high degree of spatial variability in SOC content in this region, which mainly due to land use, topography, agricultural activities and the complexity of other factors. Topography factor affected more obviously than others under the larger landscape scale (county), but agricultural activities had the important influence in smaller scale;4. There were some differences in SOC content among soil classifications. The coefficient of variation in SOC within soil species (4.29%～83.78%, average 44.04%) is greater than that among soil types (32.00%); the soil organic carbon is a weighted average of 10.22 (g·kg-1), and coefficient of variation was 43.03% among soil classifications. Coefficient of variation in SOC under sub-class as a unit may be greater than that under lower taxa. Coefficient of variation (95.37%) among natural soils (e.g. widely distributed in the mountainous sub-soil and thick bone) showed larger than others, and also with the highest SOC (16.95±16.17 g·kg-1). The soil SOC of human use soils showed a lower coefficient of variation, with 11.97% only by rinse paddy soils sub-class. The coefficient of variation among sub-class was 58.75%. The coefficient of variation in SOC among soil types was also great, with the highest in mountain acid brown type (95.42%), and with the smallest in mound lime concretion black soil type (18.92%). Therefore, in taxa statistics, coefficient of variation of soil organic carbon content had the rising trend with the changes of taxa levels, which was determined by the topography of the study area, joint decisions and land use. Because the land use impact is reflected in the high-level unit, therefore, the SOC among the high-level taxa had a very high variability;5. We studied the changes of SOC content with the different scales in Huoshan County. The results showed that the SOC in the county’s cropland showed an increase trend in recent 20 years, which indicated that soil organic carbon accumulated in this region’s croplands, while the overall coefficient of variation of SOC showed a reduced trend. For example, the coefficient of variation of SOC content among species decreased from 48.62% in 1985 to 14.64% in average between 2005 and 2008. Within the county, the SOC content coefficient of variation was between 4.53%～14.91%, which induced by different scales of space. The results also showed that among the village group (villages) scale, variability of SOC was max; variability of SOC among township was lower than that among villages. Therefore, from the county scale for the quantification of cropland SOC, the town scale was more reliability than the village-scale. The impact factors of the changes of SOC content and its variability in county were mainly agricultural use and farmland capital construction. The soils planted tea and rice has the higher SOC. The SOC was effected significantly by agricultural management under county scale.Therefore, soil organic carbon changes with land use and agricultural management due to different space and time, the relationship between soil organic carbon dynamic, ecosystem function and agricultural productivity is still a need for advanced. The change of the regional soil carbon pool and the effects of human activities will continue to be the subject of future research under future agricultural development and climate change. In particular, the efforts of Characterization of quantitative analysis, climate change, land use change, human activity intensity and tillage management on soil organic carbon needs full attention in the future research of areas scale.