Chemical structure of soil organic matter

Selective preservation belongs among the important stabilization mechanisms of soil organic matter (SOM). Conceptually, it is based on non-covalent intermolecular interactions of organic molecules, which leads to a decrease in the Gibbs' energy of the SOM structure. Earlier works suggested that this stabilization of SOM physical structure is supported also by water molecules that form clusters bridging SOM moieties. This article reports results suggesting that water is connected also to stabilization of SOM chemical structure. We analyzed the dynamics and composition of gases evolved during drying of 33 mineral soils, which were exposed to 40% relative humidity prior to the analysis. It was observed that moisture elimination occurring below 100 °C is accompanied by evolution of a small amount of low molecular mass gases representing typical degradation products of organic materials. In particular, analyses revealed the evolution of CO, HCN, NO, and probably traces of NH₃ and CO₂, which implied degradation of N-containing molecules. The peak temperature of evolved CO correlated with the amount of adsorbed water. The amount of evolved CO positively correlated with the amount total organic C and N contents and clay content. On the contrary, the amount of CO evolved during degradation did not correlate with the amount of CO₂ produced during incubation of analyzed soils either at short or longer incubation times. Evolution of gases started and culminated simultaneously with drying. The analysis of soils exposed to higher relative humidity levels resulted in a shift of the CO peaks to higher temperatures. Therefore, the results suggested a possible causality between water desorption at elevated temperature and SOM degradation processes.