THERMODYNAMIC STABILITY OF HUMIC ACID AND FULVIC ACID IN SOIL AND ITS DRIVING FACTORS
From the aspect of process, formation and transformation of humus are essentially microbe-based biochemical processes, while from that of difference in energy level between the initial and final stages, these processes could also be considered as an issue of the thermodynamic stability. In the soil there are quite a number of factors affecting these processes, such as type and content of clays, vegetation, microorganisms, soil moisture, temperature, air composition, chemical composition and concentration of soil solution, acidity, status of redox, etc. But from the angle of thermodynamics, in order to calculate reaction equilibrium constant (logK_R) and Gibbs energies (ΔG_f~θ) at a given temperature of 25℃, those soil conditions could be simplified into 3 parameters: water activity ([H_2O]), oxygen partial pressure (P_(O_2)) and carbon dioxide partial pressure (P_(CO_2)), since all organic substances are composed of these three elements and would eventually be decomposed into H_2O and CO_2 that contain these three elements. According to this new idea and principle mentioned above, this paper cited black soil as an example to explore approaches to the study on factors driving formation and transformation and thermodynamic stability of HA and FA, and to the calculation of ΔG_f~θ, logK_R and range of thermodynamic stability of HA and FA with "the elemental composition-soil condition parameter method". Meanwhile, a simulated incubation in the lab of soil with organic matter was conducted to study effect of mono-environmental factors like O_2 and CO_2 on accumulation of HA and FA. Results show that the molecular formula of HA and FA is nC_(21)H_(21)O_9N and nC_(24)H_(33)O_(17)N, respectively. In soils low in O_2, high in moisture and high in CO_2 concentration, FA was relatively stable whereas HA was on the contrary. In the incubation experiment, high CO_2 concentration and low O_2 concentration would result in higher FA/HA ratio. This method and the findings in this study could help explain and speculate the rule of spatial variability of the composition of soil humus, and provide guidance to regulation of soil fertility and soil carbon sequestration.
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