Effects Of Soil Moisture And Temperature On Organic Carbon Mineralization Of Typical Soils In The Hydro-fluctuation Belt Of The Three Gorges Reservoir

Soil organic carbon(SOC) pool was the largest carbon pool in terrestrial ecosystem, which played an important role in the global CO2 balance. SOC mineralization, as an important process of soil biochemistry, was not only related to the production of greenhouse gases in the soil, but also affected the release and supply of soil nutrients, and the maintenance of soil quality. The anti-seasonal water level fluctuation has made the hydro-fluctuation belt periodic dry in summer and wet in winter contrary to the alternating wet and dry season before the construction of the Three Gorges Reservoir, and the great hydrothermal environment change would be bound to affect SOC mineralization in the hydro-fluctuation belt. Therefore, two typical soils of purple soil and paddy soil which were widely distributed in the Three Gorges reservoir were selected as the research objects in the paper, and soil sampling in the field and an incubation experiment in the laboratory were conducted to investigate the effects of soil moisture and temperature on SOC mineralization of typical soils in the hydro-fluctuation belt of the Three Gorges Reservoir. The relevant data measured in the incubation experiment was used to analyze the effects of gradients of moisture and temperature on the mineralization characteristics of SOC, and combined with the Two-pool first-order model and temperature sensitivity coefficient to study the dynamic characteristics and the temperature sensitivity of SOC mineralization under different hydrothermal conditions, and analysis of the effect of drying-rewetting alternation on SOC mineralization in the hydro-fluctuation belt of the Three Gorges Reservoir, which provided basic data and scientific reference for the comprehensive understanding of the process of soil carbon cycle in the hydro-fluctuation belt of the Three Gorges Reservoir. The main results were as follows:(1) In 0-10 days, the SOC mineralization rates of two soils in non-flooded treatments were decreased sharply with the increasing of incubation time, while in the flooding treatment which showed a relatively small decline or even a slight rise and fall. After 10 days, the mineralization rates of SOC decreased slowly, and then remained stable when in 40 d of the incubation. In the initial culture period, the mineralization rate maximum of purple soil and paddy soil appeared in 100% water holding capacity(WHC) and 70% WHC treatment respectively and the lowest was in flooding treatment, while the mineralization rate of flooding treatment was maintained at a high level in the culture medium and even appeared higher than other moisture treatments in some periods.(2) In the entire incubation period(66 d), the SOC cumulative mineralization of purple soil at 100% WHC and under flooding condition was not significantly different from that at 70% WHC treatment, but significantly higher than that at 40% WHC treatment at 10℃and 20℃, while the cumulative mineralization of organic carbon at 100% WHC and under flooding condition was significantly higher than that at 70% WHC and 40% WHC at 30℃(P<0.05). The results indicated that compared with 70% WHC treatment, the higher moisture content(100% WHC and flooding condition) had no negative-effects on the organic carbon mineralization of purple soil in the hydro-fluctuation belt even promotion-effects at high temperature(30℃). The SOC cumulative mineralization of paddy soil under 70% WHC, 100% WHC and flooding treatments had no significant difference, and the cumulative mineralization at 40% WHC treatment was significantly lower than that at 70% WHC and 100% WHC treatments but no obvious difference with flooding treatment when at 10℃, while when at 20 and 30℃, the cumulative mineralization at 40% WHC treatment was significantly lower than that at other moisture treatments, which suggested that compared with 70% WHC treatment, 40% WHC treatment had inhibit effects to SOC mineralization of paddy soil in the hydro-fluctuation belt, but higher moisture content was neither obvious inhibition nor promotion effects. In addition, in the range of 10-30℃, the SOC cumulative mineralization of purple soil and paddy soil increased with the increasing of incubation temperature. SOC cumulative mineralization of purple soil and paddy soil was significantly affected by temperature and soil moisture, and there was a significant interaction with temperature and soil moisture in purple soil, but no significant interaction effect in paddy soil.(3) During the whole incubation period, the content of dissolved organic carbon(DOC) and microbial biomass carbon(MBC) of purple soil and paddy soil in the hydro-fluctuation under different hydrothermal treatments had no obvious change rule. It was found that there was no significant correlation between mineralization rate and DOC content in all treatments in addition to T3M2 and T3M4 treatments of paddy soil, and there was no significant correlation between MBC content and SOC mineralization rate of two soils in all hydrothermal treatments(except T1M1 treatment of purple soil and T2M3 treatment of paddy soil). The results showed that DOC content and MBC content of soils did not reflect the difference of SOC mineralization between different hydrothermal treatments, and soil moisture and temperature could not affect the SOC mineralization by changing the content of DOC and MBC in soils.(4) The contents of labile SOC fraction of purple soil in each moisture treatment were consistent with the change of SOC cumulative mineralization, and both showed the maximum at 100% WHC treatment, and that of 40% WHC treatment was the minimum. In paddy soil, the minimum of labile SOC fraction was appeared at 40% WHC treatment, and the change trend of labile SOC fraction at different moisture treatment was similar to the cumulative mineralization. While in the same moisture conditions, the contents of labile SOC fraction increased with the increasing of incubation temperature in two soils. The results indicated that the differences of SOC cumulative mineralization between treatments caused by the soil moisture and temperature affecting the contents of labile SOC fraction. During the whole incubation period, the ratio of recalcitrant SOC fraction and SOC cumulative mineralization was larger, especially under high temperature the ratio was nearly 50%, which showed that the recalcitrant SOC fraction also played an important role in the process of SOC mineralization. With increase of incubation temperature, the mineralization rate of recalcitrant SOC fraction(h) of each moisture treatment in purple soil increased gradually, and in paddy soil when the soil moisture content was not less than 70% WHC, the mineralization rate of recalcitrant SOC fraction also increased with increasing temperature. At the same time, the influence of moisture content on the mineralization rate of recalcitrant SOC fraction was the most prominent under high temperature condition. It suggested that soil moisture and temperature could change the capability of microorganism to decompose the recalcitrant SOC, especially under high temperature conditions the mineralization rate of recalcitrant SOC fraction would significantly increase. Through the comparative analysis of parameters in organic carbon mineralization dynamics model of purple soil and paddy soil, it is found that temperature and moisture mainly through changing the content of labile SOC fraction and the mineralization rate of recalcitrant SOC fraction to influence SOC mineralization process.(5) With the increase of incubation temperature, the temperature sensitivity of SOC mineralization was significantly decreased at 40% WHC treatment in purple soil, but there was no obvious difference when the soil moisture content was not less than 70% WHC. While in paddy soil, the temperature sensitivity in flooding condition was significantly decreased with increasing incubation temperature, but in other moisture treatments did not appear obvious difference.(6) Drying-rewetting alternation could significantly stimulate the organic carbon mineralization of purple soil and paddy soil in the hydro-fluctuation belt of the Three Gorges Reservoir. From flooding to the drying stage, the SOC mineralization rates of two soils appeared the rapidly rising phenomenon from the lower point, and in the first few days of drying incubation reached its peak. From drying to flooding stage, the SOC mineralization rate were reduced to a minimum in the last one day of drying incubation period and after flooding the mineralization rate rose sharply, and reached the maximum in the first few days of flooding incubation. In the three drying-rewetting cycles, the excitation effects of the drying-rewetting alternation on the SOC mineralization rate gradually weaken with the cycles increase. During the whole incubation period, the cumulative mineralization of organic carbon at drying-rewetting alternation condition was significantly higher than that in the constant moisture treatments(70% WHC and flooding condition). It indicated that drying-rewetting alternation was a key factor to promote the organic carbon mineralization of typical soils in the hydro-fluctuation belt of the Three Gorges Reservoir.