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


Barros, Nieves (2020), Thermodynamics of soil organic matter decomposition in semi-natural oak (Quercus) woodland in southwest Ireland, Dryad, Dataset,


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.


This work pretends to provide a way to characterize themodynamically some of the reactions involved in the decomposition of soil organic matter (SOM) using soil samples at different degree of decomposition obtained by collecting them at different depths in three different sampling sites.

Sampling sites can be located in this link by introducing the following coordinates:

Derrycunnihy (491537.228, 580411.229) (DC samples)

Glengarriff (492662.632, 556212.602) (G samples)

Uragh (482978.239, 563077.816) (K samples)

At each study site, between three and five sampling points were identified with each point located mid-way between the tree stem and the maximum crown radius of a Sessile oak tree. Firstly, a rectangular quadrat of 0.25 × 0.25 m was used to collect all loose litter (L) and fermented (F) layer within the quadrat at each sampling point. Thereafter, using a hammering head and stainless steel rings of 100 cm3 volume (Eijkelkamp Soil and Water B.V., The Netherlands), bulk density (BD) samples of the humic (H) layer was collected. After removing the entire H layer, the same process was used to collect one BD sample of the top 5 cm of the mineral layer (M samples). In addition, three extra cores were taken at each location for further chemical, thermal and calorespirometric analysis. Within each stand, samples from the five locations were mixed together and bulked samples were prepared for all laboratory analysis. In all cases, depth of the different L/F, H and M layers were recorded and sampling produced minimum disturbance and compaction. Each of these soil layers contains SOM at different degree of decomposition.

Although all samples were subdued to different analysis, the essential ones to obtain the triad of functions defining thermodynamically every system, were Carbon content, Thermal Analysis and Calorespirometry. By thermal analysis the thermodynamic properties of the SOM can be determined by the procedure detailed in the paper.

For thermal analysis soil samples must be air dried at about 22 ºC (lab temperature), grinded and sieved (mesh size 2 x 2 mm for mineral soils). Thermal properties of the samples were studied by thermogravimetry (TG) (TGA-DSC1 Mettler Toledo). For TG analysis, samples were placed in 100 mL open aluminium pans under a dry air flow of 50 mL min-1 with a temperature ramp from 50 to 1000 ºC at 10 ºC min-1.  The DSC must be previously calibrated following the technical instructions of the instrument.

TG and DSC provides TG traces and DSC curves that allow to determine the enthalpy of combustion of the soil organic matter. This enthalpy change yields the degree of reduction of the organic matter, the Gibbs energy change and the entropy change of the SOM combustion in the DSC. Attached the Excell file for the DSC and DTG raw data of figure 1 in the paper. In the future it can be useful if these methods start to be applied for the thermodynamic characterization of SOM. Scientists could use this material to add more samples to the ones that they collected. Also for other studies to provide the thermal properties of SOM.




Usage Notes

DSC are already corrected to the base line to be integrated to obtain the heat of combustion of the samples in kJ / g SOM. It should be plotted versus time in seconds to obtain that value.

DTG is ready to give plots representing the SOM profiles defining their thermal properties.

All the quantitive data linked to these plots to proceed with some of the calculations in the paper is attached too.