DOI: 10.5061/dryad.gf1vhhmmd
title: Thermodynamics of soil organic matter decomposition in semi-natural oak (Quercus) woodland in southwest Ireland

description: 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.
description: 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 http://map.geohive.ie/mapviewer.html 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 (<i>L</i>) and fermented (<i>F</i>) layer within the quadrat at each sampling point. Thereafter, using a hammering head and stainless steel rings of 100 cm<sup>3</sup> volume (Eijkelkamp Soil and Water B.V., The Netherlands), bulk density (<i>BD</i>) samples of the humic (<i>H</i>) layer was collected. After removing the entire <i>H</i> layer, the same process was used to collect one <i>BD</i> sample of the top 5 cm of the mineral layer (<i>M</i> 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 <i>L/F</i>,<i> H</i> and <i>M</i> 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 (<i>TG</i>) (<i>TGA-DSC1</i> Mettler Toledo). For <i>TG </i>analysis, samples were placed in 100 mL open aluminium pans under a dry air flow of 50 mL min<sup>-1</sup> with a temperature ramp from 50 to 1000<i> ºC</i> at 10<i> ºC</i> min<sup>-1</sup>.  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.      
description: 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.