The agricultural use of peatlands, the stabilization of the substrate for building or road construction or for increasing the capacity of soil to support heavy machinery for industrial activities (peat and petroleum extraction), harvesting to provide peat for energy, and the growing media and initiation of chemical processes must be preceded by drainage. As a consequence of drainage, peat underwent an irreversible conversion into moorsh (secondary transformation of the peat). The object of the study was to investigate comparatively the organic matter composition and molecular structure of humic acids (HAs) in the raised bog, fen, and peat-moorsh soils developed in various compositions of botanical cover, peat-forming species, and oxic and anoxic conditions as a result of the oscillation of ground water during drainage as well as to evaluate the vulnerability of soil organic matter (SOM) to decomposition. Drainage was shown to be the principal factor causing the various chemical compositions and physicochemical properties of HAs. Large and significant differences in chemical composition of peat and the properties of HAs were found to be related to the degree of decomposition. The HAs from drained peatlands were less chemically mature. In contrast, the HAs from fen and raised bog were found to be more mature than that of the corresponding drained peatlands. The above findings showed the distinguishable structure of HAs within the soil profile created by the plant residue biodegradation and formed in both oxic and anoxic conditions. The analytical methods of thermal analysis together with the optical densities and paramagnetic behaviour are suitable and effective tools for studying structure–property relationships characterizing the origin and formation process of HAs in various environmental conditions.

Laboratory studies were conducted to evaluate the effect of moisture (field-moist and 15% moisture) and nitrogen concentrations (field content and after the addition of nitrogen in the form of 0.25 and 0.5% urea, respectively), pH and organic matter on the changes in nitrate reductase activity in soils under shelterbelt and an adjoining cultivated field. Shelterbelt consists mainly of Robinia pseudoaccacia. A first-order kinetics reaction model was fitted to the experimental changes in nitrate reductase activity over time data at different moisture and nitrogen content. Under shelterbelt, an increase in the soil moisture content from field-moist to 15% led to a 1.13-fold increase in the first-order reaction rate constant. However, under the adjoining cultivated field, an increase in the soil moisture content from field-moist to 15% increased the first-order reaction rate constant significantly more than in soil under shelterbelt. Changes in nitrate reductase activity for a 15% moisture content in soil under shelterbelt and in field-moist soil in the adjoining cultivated field were similar. Moreover, the addition of 0.25% nitrogen to the adjoining cultivated field with field-moist soil led to a 3.65-fold increase in the first-order reaction rate constant.