Effects Of Variable Temperature On Soil Organic Carbon Mineralization And Kinetics Features

Soil sampling in the field and incubation experiment in the laboratory were conducted to investigate the responses of soil organic carbon (SOC) mineralization to variable temperature regimes in the topsoil of limestone soil under forest (SL), limestone soil under dry land (SH) and yellow soil under dry land (HR). Three incubated soils were sampled from the 0-10 cm layers of typical forest land and dry land respectively, which are distributed in Tianlong Mountain area of Puding county, Guizhou province. These soils were incubated at three constant temperatures (15℃, 20℃ and 25℃, referred to as HT-25, HT-20, HT-15 respectively) and a variable temperature (15℃-25℃ diurnal cycle, interval:12 h, referred to as BT-15/25), and the cumulative temperature was the same in the HT-20 and BT-15/25 temperature treatment, and all the treatments were incubated for 56 d. The measured data from the incubation experiment was conducted to a systematic study of the effects of variable temperature on soil organic carbon mineralization and dynamics characteristics, and which were provided basic data and scientific reference for a comprehensive understanding of soil organic carbon mineralization temperature effect and construction of soil organic carbon mineralization model. The main results were as follows:1) The daily mineralization and cumulative mineralization of the three soils were different at the same temperature, where FL>SH>HR, and the differences reached significant level (P<0.05). Within all different temperature treatments, average SOC mineralization rates of each soil in the first 14 d were significantly greater than those during the last 42 d of the incubation. It was found that approximately 60%-68% of total CO2-C evolution was produced during the first 14 d. The result showed that cumulative SOC mineralization during the entire incubation was mostly dictated by mineralization rate in the first 14 d. In this study, the cumulative mineralization and mineralization intensity from the soils incubated at constant temperatures increased significantly with increasing temperature. The BT-15/25 treatment reduced the cumulative mineralization of FL soil and increased the cumulative mineralization of HR soil significantly as compared to HT-20 treatment, and there were no significant difference in the cumulative mineralization between the BT-15/25 and HT-20 treatment. These results indicate that difference in temperature pattern (constant or variable) can significantly influence SOC mineralization, and the responses of SOC mineralization to the variable temperature were obvious among the three soils. Therefore, determining SOC mineralization response to temperature variability is necessary in order to accurately evaluate CO2 emission from soils.2) There was a significantly positive correlation between cumulative mineralization and SOC concentration at the same incubation temperature (P<0.05, P<0.01). The result showed that SOC concentration was an important factor in controlling soil carbon mineralization process. The cumulative mineralization and mineralization intensity of the three soils in the BT-15/25 treatment was always between those of HT-25 and HT-15 treatments. This finding showed that SOC mineralization under periodical fluctuation of temperature was restricted by temperature range. The SOC content and composition were significantly different between SL and two dry land soils affected by vegetation and land use type, these connections between SOC composition, Q10 and mineralization intensity suggested that SOC mineralization response to the variable temperature might be affected by SOC content and composition.3) The mineralization potential of SL soil in the BT-15/25 treatment was significantly (P< 0.05) lower than that in the HT-20 treatment, and the C0/SOC in SL soil was significantly lower than that in SH and HR soils, which indicated that the ability of microbial decomposed soil organic carbon in SL soil were weakened and the efficiency of microbial used carbon source were decreased.4) Duing the incubation period, the mineralization rate constants of the recalcitrant SOC fractions (κs) was gradually increased with the increase in incubation temperature, which showed that wanner temperature could improve the capability of microbes to decompose the recalcitrant SOC in soils. In the incubation period, over 40% of cumulative mineralized SOC came from the recalcitrant SOC pool in the incubated soils, which indicates that the recalcitrant SOC fraction may play an important role in SOC mineralization as the labile SOC fraction. Through a comparison between BT-15/25 and HT-20 temperature treatments, it was found that cyclical variation in temperature influenced SOC mineralization process mainly by affecting the liable SOC pool size and mineralization rate constant of the recalcitrant SOC.5) During the whole incubation period, the content of dissolved organic carbon (DOC) in SL soil with higher organic carbon content was always higher than that in SH and HR soils due to the correlation of dissolved organic carbon and soil organic carbon. The dissolubility of dissolved organic carbon in three soils were different influenced by the difference of soil type and temperature conditions, which were mainly connected with the temperature sensitivity of soil organic carbon and soil microbial community activity under different temperature conditions. In addition, the dissolved organic carbon content of three soils were highly (P<0.01) positively correlated with daily mineralization of soil organic carbon under different temperature treatments respectively, which imply that controlling DOC production was an important way to influence SOC mineralization for temperature factor.6) The microbial biomass carbon in SL soil was higher than that in SH and HR soils due to the correlation of microbial biomass carbon and the soil organic carbon. There were no significant correlation between daily mineralization and MBC contents was found in all incubation treatments, with the exception for the HR soil that was incubated at 25℃ (P< 0.05). During incubation period, SOC mineralization was independent of microbial biomass carbon (MBC) content under different temperature treatments. The results indicated that SOC mineralization was not regulated by the microbial biomass size or microbial population. However, the composition, structure and activity of microbial communities might be affected by temperature change, which can result in the change in microbial ability to utilize SOC. Analyzed by combining with mineralization kinetic data showed that, the change of microbial communities activity were important factor that affecting the SOC mineralization, and the temperature was mainly by changing microbial community activity to influence SOC mineralization.