Impacts Of Land Use Change And Heavy Metal Pollution On Soil Respiration And Organic Carbon Reduction From Paddy Soil

Paddy soils are vital source of rice production and play a fundamental role in food security and socio-economic development of China. These soils are substantial contributor to greenhouse gases and main pool of organic carbon as well. Reduction emission of greenhouse gases from paddy soils has become one of the key isuues in the research on global changes in the background of global warming and national food security. China is facing an increasingly challenges to climate change, paddy soil carbon sinks and greenhouse gas emissions for sustainable development of paddy fields have become an important task. Under China's rapid economic development, land-use change and increasing environmental pollution, organic carbon changes in the future of paddy soil has been an important research aspect in carbon cycle and global changes. Effects of land usage changes and heavy metal pollutions on soil organic carbon loss and greenhouse gas generation and emissions changes were investigated based on the determination of soil organic carbon and soil respiration, and the mechanisms of soil and land ecosystem processes were also discussed, which provided a scientific basis for the analysis of the effects of land use and heavy metal contamination of paddy soil on carbon cycle and changes in carbon sinks.Therefore, both field and laboratory incubation study were carried to understand the effects of plantation on the carbon dynamics and the mechanism involve in soil respiration under heavy metal polluted paddy soil. Wunitu paddy soil, a typical Southeast Chinese soil, was selected as the object of this research. Two adjacent fields of Wunitu paddy soil (one with rice-wheat rotation and another with double corn for 3 years after rice and wheat) were chosen to study the SOC dynamics. Both topsoil and whole profile was sampled. Soil particle size factions (PSFs) were separated using low energy ultrasonic dispersion. C pools of total organic carbon (TOC), dissolved organic carbon (DOC) and microbial biomass carbon (MBC) were determined for bulk soils and PSFs. Selected samples of bulk soil and PSFs from both rice and corn fields were also used for 13C natural abundance measurement. Changes on topsoil organic carbon and soil respiration between the non-polluted and serious polluted soil were studied, compositions of soil organic carbon, microbial biomass and bacterial genetic diversity were also compared. The main results were list as following:1. TOC of topsoil decreased drastically after 3 years of continuous corn cultivation although marked increase of DOC and MBC was observed in the cornfield. This was in coincident with the decrease of SOC in the sand PSF despite no remarkable changes in the other PSFs from the corn filed. Significantly heavier carbon could be detected either in bulk samples or in a single PSF from the cornfield than from rice field. Calculation using the data of 813C%o(PDB) indicated that 80% of young carbon inputted by corn residues was allocated in the topsoil of 0-20cm and mainly found in the coarse PSF as well as in the pools of DOC and MBC. These results indicated that the carbon storage was decreased rapidly as a result of increased decomposition rate after the change of paddy soil into dryland soil. These may contributed to the destruction of the physical protected soil organic carbon due to the tillage, which changed the composition of soil particle fractions.2. Soil basal respiration CO2 flux of heavy metal polluted soil was 81.92 mgCO2-C·m-2·h-1 which was obviously higher than unpolluted soil (71.19 mgCO2-C·m-2·h-1). The paddy soil CO2 emission was significantly increased under heavy metal polluted condition. Compared with unpolluted soil, the soil breath intensity was increased 15% under long term heavy metal polluted condition, and the soil microbial biomass, microbial quotient, soil organic carbon and Cmic/Nmic was significantly decreased. Correlation analysis showed that a more prompt response of CO2 flux to soil temperature was increased under heavy metal polluted condition, and the accumulation and stability of paddy soil organic carbon pool was affected, which was decreased, easily affected by the change of temperature.3. CO2 and CH4 emission of paddy soil under heavy metal polluted condition was significantly lower than unpolluted paddy soil in the whole growing season. The rice growth was affected under heavy metal polluted condition due to inefficient photosynthesis that caused less biomass production ultimately decreasing CO2 and CH4 emission of paddy soil. But soil basal respiration CO2 flux was increased under heavy metal polluted condition. This might be due to the fact that decomposition of soil organic carbon and soil CO2 emission was affected by the change of soil microbial activity under heavy metal polluted condition. Difference of CH4 emissions between rice ecosystem and soil was attributed to the rice plant. CO2 emissions in Wheat field ecosystem were significantly lower than that of the rice ecosystem. At the same time, effects of heavy metals on CO2 emissions in wheat ecosystem were significant lower than rice ecosystem. Heavy metal contamination was found to decrease the soil respiration and increase the CO2 emission in non-plant ecosystem, but no impacts on the flux of CH4 emission.4. The soil organic carbon mineralization rate in heavy metals polluted paddy soil was 0.33 mgC·g-1·OC·d-1, which was significantly higher than non-polluting soil organic carbon mineralization rate 0.29 mgC·g-1·OC·d-1. This difference mainly concentrated on beginning of mineralization, indicated that it's the activated carbon not the inert carbon in soil which was most effected by heavy metals pollution. Soil carbon pools were also influenced by heavy metals pollution with the decreasing of Cmic, DOC, TOC and Kos. Heavy metal pollution has changed the nature of soil organic carbon, on the other hand is a higher activity in the new accumulated organic carbon. Heavy-metal contamination impacted on the growth and stability of paddy soil organic carbon, thus affecting China's long-term potential of paddy soil carbon sequestration.5. Heavy metal pollution has increased the number of bacteria in soil, and a significant reduction in the number of actinomycetes and fungi, and a strong reduction of the ratio of fungi to bacteria in paddy soil. Soil bacterial diversity and structure of community were found to be reduced by DGGE, thus affecting the microbial mineralization of organic carbon. Heavy metal pollution can be seen to change the structure of soil microbial community, thus the function of microbial communities.To sum up, organic carbon in paddy soil was deeply affected by land usage changes and the pollutions of heavy metal, which was related to the compositions of soil aggregates, characterizations of soil organic carbon, community structure and diversity of soil microorganisms. That is, soil carbon cycle and climate changes were interrelated by the interactions of organic carbon, structure and functions of microorganisms and soil biogeochemical process. Researches in this area were needed to develop. It provided a theoretical basis to achieve the benefits of carbon sequestration and reducing greenhouse gas emission in paddy soil in particular the current trend of global warming.