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The aim of this study was to assess the effect of long-term catch crop application on the structural properties of humin, which is considered the most recalcitrant fraction of soil organic matter. Soil samples from a 30-year field experiment on triticale cultivated with and without catch crops were analysed to determine the total organic carbon content and fractional composition of humic substances. Meanwhile, humin isolated from bulk soil was analysed to determine its elemental composition and spectroscopic properties measured with UV-Vis, fluorescence, and 13C-CPMAS-NMR. It was found that catch crop farming enhanced the formation of highly reactive humus substances, like low-molecular-weight fractions and humic acids, while decreasing the humin fraction. The higher H/C and O/C atomic ratios of humin and the UV-Vis, fluorescence, and 13C-CPMAS-NMR results confirmed a higher share of oxygen-containing functional groups in humin isolated from the soil with catch crop rotation, also corroborating its greater aliphatic nature. Under the conditions of our field experiment, the results indicated that organic residues from catch crops quickly undergo the decay process and are transformed mainly into highly reactive humus substances, which can potentially improve soil health, while mineral fertilisation alone without catch crops favours the stabilisation and sequestration of carbon. [ABSTRACT FROM AUTHOR]

Introduction: Humic substances (HS) are increasingly being applied as crop plant biostimulants because they have been shown to increase plant productivity, especially under environmentally stressful conditions. There has been intense interest in elucidating the HS molecular structures responsible for eliciting the plant biostimulant response (PBR). The polar and weakly acidic carboxylic (COOH) and phenolic hydroxyl (ArOH) functional groups play major roles in the acid nature, pH dependent solubilities, conformation, and metal- and saltbinding capabilities of HS. Reports on the role played by these groups in the PBR of HS found growth parameters being both positively and negatively correlated with COOH and ArOH functionalities. Materials and methods: To investigate the role of COOH and ArOH in HS biostimulant activity we used a humic acid (HA), purified from an oxidized sub bituminous coal to prepare HAs with COOH groups methylated (AHA), ArOH groups acetylated (OHA), and with both COOH and ArOH groups methylated (FHA). The original HA was designated (NHA). The four HAs were subjected to elemental, 13C-NMR, FTIR, and EPR analyses and their antioxidant properties were assessed using the trolox equivalents antioxidant capacity assay (TEAC). 13C-NMR and FTIR analysis revealed significant alkylation/acetylation. To determine the effects of alkylating/acetylating these functional groups on the HA elicited PBR, the HAs were evaluated in a plant bioassay on corn (Zea mays L.) seedling under nutrient and non-nutrient stressed conditions. Treatments consisted of the four HAs applied to the soil surface at a concentration of 80 mg C L-1, in 50 ml DI H2O with the control plants receiving 50ml DI H2O. Results: The HA-treated plants, at both fertilization rates, were almost always significantly larger than their respective control plants. However, the differences produced under nutrient stress were always much greater than those produced under nutrient sufficiency, supporting previous reports that HA can reduce the effects of stress on plant growth. In addition, for the most part, the HAs with the alkylated/acetylated groups produced plants equal to or larger than plants treated with NHA. Conclusion: These results suggests that COOH and ArOH groups play a limited or no role in the HA elicited PBR. Alternatively, the HA pro-oxidant to antioxidant ratio may play a role in the magnitude of the biostimulant response. [ABSTRACT FROM AUTHOR]

The aim of the research was to investigate interactions between pendimethalin (PE), an active substance of plant protection products, and the humin fraction (HM), which is the most recalcitrant component of soil organic matter. The soil material used for the study was collected from the mollic horizon of two Chernozems different in physicochemical properties. HM was obtained in the solid state after discarding humic acid (HA) and fulvic acid (FA) and digesting the mineral fraction in an HF-HCl mixture. Pure PE and two commercial products containing the substance (Penfox 330 EC and Stomp Aqua 455 SC), dissolved in hexane and added to the HM, were used in the research. The batch sorption experiment was carried out in accordance with OECD Guidelines for the Testing of Chemicals No.106. Spectroscopic methods (UV-Vis-nearIR and fluorescence) were used to assess the molecular characteristics of HM exposed to pesticides. The study showed significant differences in the molecular structures of the tested HM as a result of interactions with the pesticides used. The results revealed that PE had a different effect than the commercial products, and proved that the composition of commercial products significantly modified the strength and ability of organic matter to bind pesticides. This is probably related to the presence of adjuvants that are more competitive in forming bonds with the active sorption sites of HM, and thus strengthen or weaken the process of forming pesticide-HM complexes. As a result, the diversity of the molecular structure of HM and the presence of accompanying substances in commercial pesticide products interact, leading to different sorption mechanisms and durability of the resulting products. [ABSTRACT FROM AUTHOR]

Clear‐cutting means forest removal (stems only) and is the most common type of forest harvesting but undoubtedly has a negative impact on the C budget in soils. This work aimed to describe responses of soil organic matter (SOM) in the mixed coniferous forest following the removal of trees under temperate climate conditions in lowland and mountain regions in southwestern Poland. Investigated soils are described as Brunic Arenosols derived from sand with an average content of total organic carbon (TOC) in the Oa‐horizons of 244 g·kg−1 in the lowland and Dystric Cambisols derived from gneiss; the average content of TOC in the Oa‐horizons in the mountain regions was 309 g.kg‐1. Analyzed soils were very acidic in the both regions (pH 2.7–3.5 in the topsoil). In the experiment, two replicated soil profiles were located on each site, both taken after clear‐cutting and without any harvesting. Humic fraction was isolated from Oa‐ and A‐horizons; differences between investigated objects were checked with a significance p < 0.05, (n = 16). Using the instrumental analysis, like electron paramagnetic resonance (EPR), proton nuclear magnetic resonance (1H NMR), and Fourier transformed infrared (FTIR) spectroscopy applied to isolated humic fractions, it has been found that clear‐cutting alters C cycling and accelerates decomposition in the forest floor, leading to loss of humic fractions in the investigated soils. In the mountain forests, the more labile, low‐molecular‐weight fulvic fraction decreased as the effect of harvesting practice by 30% in Oa‐horizon and by 54% in A‐horizon. The analysis of stable radicals by EPR showed a decrease in radical concentration observed for humic acids (HA) from mountain clear‐cut areas compared to the undisturbed forest. Results presented in this article have proved there is less aliphatic character of HA molecules from the lowlands, compared to the mountain forest as the effect of clear‐cutting. Changes in plant cover observed as a consequence of deforestation lead to the decrease in concentration of humic fractions with different solubility properties. [ABSTRACT FROM AUTHOR]