| Soil CO2 efflux in terrestrial ecosystems is an important consideration in the studies of the global carbon cycle, which contribution to the global carbon budget has been the focus of wide concern. Soil respiration is also crucial for figuring out the "missing sink" of carbon. It is absolutely necessary for terrestrial ecosystem soil respiration study to probe into the role of terrestrial ecosystem in source-sink and missing sink about carbon cycle. With global change, worldwide water distribution will alter greatly, and then soil respiration will change significantly, which will in turn impact on global carbon cycle. Although CO2 efflux plays a critical role in carbon exchange between the biosphere and atmosphere, our understanding of its regulation by soil moisture is rather limited. Drying and wetting cycles in forest soils caused by imbalance of precipitation time distribution is an important process, yet have not received much attention, partly due to methodological limitations for nondestructive monitoring of the soil water content. This phenomenon is familiar in surface soils, yet the mechanisms responsible for producing the CO2 pulse have not been positively identified. The objective of this study was to determine the correlation between forest soil respiration and soil moisture and the mechanism of soil respiration response to moisture. From Jan. 2002, soil respiration, soil moisture, soil temperature and other related environmental factors were studied in a natural forest of Castanopsis kawakamii (NF) and adjacent monoculture plantations of C. kawakamii (CK) and Chinese fir (Cimninghamia lanceolata, CF) in Sanming Nature Reserve, Fujian. In addition, a series of examinations were designed to simulate drying and wetting cycles in field and laboratory. The results were showed as follows:(1) The soil respiration in NF, CK and CF showed distinct seasonal dynamics in the field. Soil respiration was highest in July and lowest in January. The order of soil respiration rate in three stands was NF>CK>CF. Similar to soil respiration dynamics, soil moisture showed pronounced fluctuation. The correlations between total soil respiration, mineral soil respiration, litter respiration and moisture were all linear. Among three stands, the correlation between soil respiration and soil moisture was highest for the NF. Soil respiration was lower when soil temperature below 12C, and soil temperature was limiting factor and independent of soil moisture. When soil temperature was in the range of 12C~23C, soil respiration rate increased with increasing soil moisture and soil temperature. Once soil temperature over 23C, soil respiration declined with increasing soil temperature. When soil moisture under 15.39%, 14.91% and 17.01% in NF, CK and CF respectively, soil respiration rate was low and soil moisture became limitingfactor, so in this case soil respiration independent of soil temperature.(2) The effect of soil rewetting on soil respiration was determined in field for NF, CK, and CF and in laboratory at different temperature of 10C, 19C and 28 C with soil rewetting. The research both in the field and the laboratory, showed an increase of soil respiration and a subsequent decline after reaching a maximum following soil rewetting. This process can be simulated with a time model: R=a t e-b t+c. Higher temperature can increase the response value (E). The response of soil respiration to soil rewetting showed the highest sensitive to temperature in NF, the higher response efficiency with higher temperature. Soil respiration in CF had the highest response efficiency and the highest sensitive to soil water content, but temperature has a negative effect on its response efficiency. It was concluded that the positive effect of soil respiration following soil rewetting could contribute to a ignorable part of total soil CO2 efflux on annual basis, because there are high frequency of rain events in the local area.(3) This study was designed to examine the relationship between soil CO2 efflux and soil moisture in a clear-...
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