A New Method for Sequential Fractionation of Nitrogen in Drained Organic (Peat) Soils

The aim of this study was to assess the transformation of organic matter in organic soils undergoing a phase of secondary transformation, based on a new method of nitrogen compound fractionation. Laboratory tests were carried out for 31 layers of muck (after secondary transformation) and peat (parent material of the soil) of drained organic soils (peat). The new method consists of sequential extraction in the following steps: (1) 0.5 M K₂SO₄ (extraction at room temperature); (2) 0.25 M H₂SO₄ (hot hydrolysis) (3) 3.0 M H₂SO₄ (hot hydrolysis); and (4) concentrated H₂SO₄ (mineralization of the post-extraction residue). As a result of the extraction process, the following fractions (operating forms) were obtained: mineral nitrogen (Nmin), dissolved organic nitrogen (N-DON), readily hydrolyzing organic nitrogen (N-RH), non-readily hydrolyzing organic nitrogen (N-NRH), and non-hydrolyzing organic nitrogen (N-NH). The study demonstrates the usefulness of the applied method for assessing the degree of secondary transformation of drained organic soils. The obtained results of nitrogen fractionation indicate the significant dynamics of nitrogen forms' transformations and a significant relationship between these forms and soil properties. Nitrogen transformation processes during the secondary transformation process after dehydration resulted in an increase in the share of N-DON (on average: 1.47% of N_org for the peat layers and 2.97% of Norg for the muck layers) and in an increase in the share of N_RHON (on average: 20.7% of Norg for the peat layers and 33.5% of Norg for the muck layers). The method of sequential nitrogen fractionation used in our study allowed us to define an index determining the degree of transformation of organic matter in peat after drying. We defined it as the ratio of readily hydrolyzable forms (the fraction is very variable in the secondary transformation process) to non-readily hydrolyzable forms (relatively stable fraction in the secondary transformation process): N-RH/N-NRH. The average value of this index was significantly lower in the peat layers (0.64 on average) than in the muck beds (1.04 on average). The value of this index is significantly correlated with soil properties: bulk density (R² = 0.470); general porosity (R² = 0.503); total carbon content (TC) (R² = 0.425); total carbon to total nitrogen ratio (TC/TN) (R² = 0.619); and share of carbon of humic substances (C-HS) (R² = 0.466). We believe that the method of sequential nitrogen fractionation may be useful for other soils and organic materials.