Soil Fertility Improvement and Carbon Sequestration through Exogenous Organic Matter and Biostimulant Application

One of the main tasks in the search for environmentally friendly crop-growing methods is to increase soil fertility by improving its physical, chemical and biological parameters. The aim of this study was to determine the effect that the long-term annual application of different types of soil fertility agents (exogenous organic matter: 1. manure, 2. straw in combination with nitrogen fertilization and liming and 3. the addition of biostimulants) had on organic matter properties, including humic acid (HAs) properties. The research was carried out on the basis of soil samples from a ten-year pot experiment which was set up as single-factor pot experiment with four replications. PVC pots with perforated bottoms were filled with soil samples taken from the tilled layer of an arable field where winter wheat was grown in monoculture. The pots were exposed directly to the weather and were left without vegetation. The soil samples were assayed for the content of total organic carbon (TOC), total nitrogen and fractional composition of humus. HAs were extracted with the Schnitzer method and analyzed for the elemental composition, spectrometric parameters in the FT-IR and UV-VIS range and hydrophilic and hydrophobic properties. In addition, EPR spectra were produced. The results showed that the content of organic matter compared to soil without additives increased with the use of manure and the use of straw in the CaO variant and in the form of a mulch. The content of dissolved organic carbon (DOC) ranged from 124.6 to 286.1 mg kg⁻¹ and had strong positive correlation with TOC content. The values of the ratio of carbon content in humic acids to carbon content in fulvic acids (CHAs/CFAs) ranged from 0.71 to 0.99. The use of a biostimulator—with or without the addition of straw—increased carbon sequestration in humic acid molecules, as well as their oxidation level and their share of hydrophobic fractions with the longest retention time. Thus, the addition of UGmax intensifies humification processes, leading to the formation of stable humic acid molecules.