Thermodynamics of soil organic matter decomposition in semi-natural oak (Quercus) woodland in southwest Ireland

The evolution of soil terrestrial ecosystems is a subject with
difficulties to define their maturity and evolutionary state. In the last
century, thermodynamics was one of the options considered by ecologists
for that goal. Difficulties in quantifying the thermodynamic parameters
needed by the evolutionary theories caused that this subject has been
practically locked since the end of the last century. Application of
thermodynamics needs reactions and one of the main reactions in soil
ecosystems are those involved in the decomposition of the soil organic
matter. This paper aims to provide an initial step to study those
reactions from a thermodynamic perspective. With that goal in mind,
thermal analysis and isothermal calorespirometric measurements were made
on soil samples collected at three depths in semi-natural oak woodlands at
three different sites in southwest Ireland. It is assumed that the organic
matter evolves from a less to a higher mature state as soil depth
increases. The maturity state could be chemically defined by the redox
state. The proposed methods yield the enthalpy change, Gibbs energy
change, and entropy change for the microbial catabolism and combustion
reactions of the soil organic matter. The degree of reduction was
calculated by the enthalpy changes. Results show the soil organic matter
becomes more reduced from the soil organic surface to mineral soils. The
top layer is characterized by high carbon content, organic materials with
low energy content per Cmole, and fast biodegradation rates. Mineral soils
are characterized by low carbon content, organic materials with high
energy content per Cmole, and slow biodegradation rates. Values obtained
for the entropy change were sensitive to these differences among the
different soil layers. These results contribute to unlock the
thermodynamics of the soil reactions and to develop the bioenergetics of
soil ecosystems.