Humic Substances: Importance for Agriculture, Affinity and Interactions with Soil Amendments and Pollutants

A special issue of Agronomy (ISSN 2073-4395). This special issue belongs to the section "Soil and Plant Nutrition".

Deadline for manuscript submissions: closed (31 July 2023) | Viewed by 43502

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Guest Editor
Department of Chemistry, Inorganic Chemistry Laboratory, National and Kapodistrian University of Athens, Panepistimiopolis, 157 71 Athens, Greece
Interests: humic substances; aluminosilicate materials; fertilizers; adsorption; soil quality; clays and clay minerals; waste management; perlites
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Special Issue Information

Dear Colleagues,

Humic substances (HSs) constitute a major pool of recalcitrant organic carbon and nitrogen in soils and sediments. HSs have a beneficiary effect on soil aggregation and structure, pH buffering, cation exchange and water retention capacity, they regulate immobilization and transport of nutrients, affect plant metabolism and enzymatic activity related to cell processes acting as promoters, biostimulants and stress relievers; therefore, they play essential roles in sustainable agriculture and in water quality management. The binding of immobile nutrients to HS increases their bioavailability, while the chelation of persistent hydrophobic organic contaminants and toxic metals reduces their toxicity and facilitates pollutant sequestration.

This Special Issue will focus on “Humic Substances: Importance for Agriculture, Affinity and Interactions with Soil Amendments and Pollutants”. Original research articles, review articles, communications, letters and opinions are welcome, providing innovative insights into all related topics of interest that include, but are not restricted to:

  • structural and physicochemical properties of HSs, characterization of fractions;
  • interactions of HSs with chemical compounds related to sustainable agriculture;
  • impact of HSs and HS-containing materials on soils, plants, living organisms, animals;
  • association of HSs with toxic compounds to remove pollutants;
  • green waste management methods, e.g., retention, adsorption, composting, connected to HS fate.

Dr. Maria Roulia
Guest Editor

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Keywords

  • humic substances
  • soil properties/quality/inputs
  • organic/inorganic nutrients
  • metal content
  • plant growth/functionality
  • soil conditioners
  • waste remediation
  • environmental impact
  • detoxification

Published Papers (23 papers)

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Editorial

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6 pages, 202 KiB  
Editorial
Humic Substances: Importance for Agriculture, Affinity and Interactions with Soil Amendments and Pollutants
by Maria Roulia
Agronomy 2024, 14(2), 382; https://doi.org/10.3390/agronomy14020382 - 16 Feb 2024
Viewed by 682
Abstract
Humic substances (HS) represent a key component in both aquatic and terrestrial ecosystems, constituting a major pool of recalcitrant organic carbon and nitrogen [...] Full article

Research

Jump to: Editorial, Review

12 pages, 797 KiB  
Article
Alleviation of Water-Deficit Stress on Seed Germination of Barley and Fenugreek in a Sandy Soil Using Superabsorbent Polymer
by Rehab R. S. Ali, Ibrahim N. Nassar, Ahmed Ghallab, Esmat F. Ali, Ahmad I. Alqubaie, Mostafa M. Rady and Ahmed A. M. Awad
Agronomy 2023, 13(9), 2324; https://doi.org/10.3390/agronomy13092324 - 5 Sep 2023
Cited by 2 | Viewed by 941
Abstract
Water deficit is one of the major limiting factors of seed crop germination and productivity. Consequently, superabsorbent polymers (SAPs) are among several technologies that enhance water use efficiency, leading to worthy seed germination. The objective of this study was to investigate the potential [...] Read more.
Water deficit is one of the major limiting factors of seed crop germination and productivity. Consequently, superabsorbent polymers (SAPs) are among several technologies that enhance water use efficiency, leading to worthy seed germination. The objective of this study was to investigate the potential effect of three rates of SAPs (0.0%, 0.5%, and 1.0% w/w) on the seedling emergence of barley and fenugreek sown in a sandy soil using three replicates in a randomized complete block design. Outdoor pot experiments were conducted in Aswan Province, Egypt. The differences in the final germination percentages (FGPs) were not significant for both seeds, while the application of SAPs enhanced the seedling germination index (GI) with significant differences for both crop seeds. The application of SAP at 0.5% gave the highest GI for barley (2.47 day−1) and fenugreek (2.66 day−1) seeds. The sigmoidal function effectively described the cumulative germination percentage rates for both seeds as a function of time under the SAP levels with R2 greater than 0.992. The maximum rates were 69.4 and 64.6 day−1 for barley and fenugreek seeds at SAP 0.5%, respectively. The corresponding rates for 0.0% SAP were 11 and 12 day−1. The water germination efficiencies (WGE) were 27.76 and 30.04 cm−1 for 0.0% and 0.5% SAP for barley, while they were 23.07 and 29.9 cm−1 for fenugreek. Accordingly, SAPs could represent a promising solution for increasing moisture conservation for seed germination in a sand soil. For strategic management, barley is recommended for growth over fenugreek in a semidried soil. Full article
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12 pages, 1199 KiB  
Article
Humate-Coated Urea as a Tool to Decrease Nitrogen Losses in Soil
by Konstantin Korsakov, Alexey Stepanov, Lev Pozdnyakov and Olga Yakimenko
Agronomy 2023, 13(8), 1958; https://doi.org/10.3390/agronomy13081958 - 25 Jul 2023
Cited by 4 | Viewed by 1773
Abstract
Processes of N transformation in soil as affected by application of the three kinds of urea fertilizers, conventional urea (U), humate-coated urea (U_HA), and urea treated with the urease inhibitor NBPT (U_UI), are examined in a model laboratory experiment. Effects of urea fertilizers [...] Read more.
Processes of N transformation in soil as affected by application of the three kinds of urea fertilizers, conventional urea (U), humate-coated urea (U_HA), and urea treated with the urease inhibitor NBPT (U_UI), are examined in a model laboratory experiment. Effects of urea fertilizers on soil chemical (content of water-extractable N-NH4 and N-NO3), and microbiological properties (rate of actual and potential N2O emission, basal and substrate-induced respiration, microbial biomass C, emission of ethylene) are focused to answer the following questions: (i) whether humate-coated urea has the ability to decrease N losses in soil; and (ii) how it affects soil biological activity comparable to synthetic urease inhibitor. The results showed that U_HA demonstrated advantages comparable to U in its ability to decrease N losses in soil: it increased N-NH4 content by 35%, reduced nitrate content by 9%, and decreased N2O emissions by 50%. U_HA promoted basal soil respiration by 10% and the specific activity of the soil microbial community by 7%, providing the highest metabolic quotient qCO2. Comparably to NBPT-treated U, U_HA mainly shows intermediate results between U-UI and conventional U. Considering the low cost of raw humates, U-HA can be regarded as a promising tool to decrease N losses in soils. Full article
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28 pages, 24887 KiB  
Article
Exploring the Role of Novel Biostimulators in Suppressing Oxidative Stress and Reinforcing the Antioxidant Defense Systems in Cucurbita pepo Plants Exposed to Cadmium and Lead Toxicity
by Mostafa M. Rady, Mohamed M. M. Salama, Sebnem Kuşvuran, Alpaslan Kuşvuran, Atef F. Ahmed, Esmat F. Ali, Hamada A. Farouk, Ashraf Sh. Osman, Khaled A. Selim and Amr E. M. Mahmoud
Agronomy 2023, 13(7), 1916; https://doi.org/10.3390/agronomy13071916 - 20 Jul 2023
Cited by 5 | Viewed by 1230
Abstract
The use of bio-stimulants (BSs) has become an important policy in managing many stressed crop plants through the regulation of the balance of phytohormones, osmo-protectors (OPs), antioxidant systems, and gene expression, all of which reflect plant growth and productivity. Garlic + onion extract [...] Read more.
The use of bio-stimulants (BSs) has become an important policy in managing many stressed crop plants through the regulation of the balance of phytohormones, osmo-protectors (OPs), antioxidant systems, and gene expression, all of which reflect plant growth and productivity. Garlic + onion extract (GOE) at a concentration of 2.0–3.0% and diluted bee honey solution (BHs) at a concentration of 1.0–1.5% were applied exogenously to squash (Cucurbita pepo) plants subjected to cadmium (Cd) + lead (Pb) stress (0.3 mM CdCl2 + 0.3 mM PbCl2). The objective was to determine the effects of these treatments on growth characteristics, organic metabolites/biomolecules, and mineral nutrients. Cd + Pb stress significantly increased electrolyte leakage (EL, 103%) and malondialdehyde (MDA, 90%) because of an increase in hydrogen peroxide (H2O2, 145%) and superoxide (O2•−, 152%) levels, and contents of abscisic acid (ABA, 164%), Cd (674–711%), and Pb (754–805%). Consequently, marked increases in the contents of OPs and non-enzymatic antioxidants (28–133%), activities of antioxidant enzymes (48–80%), and expressions of enzyme genes (60–84%) were observed. The administration of Cd + Pb treatment reduced plant growth and development parameters (25–59%), yield components (61–86%), photosynthetic components (27–67%), leaf proportional water content (26%), indole-3-acetic acid (IAA, 44%), gibberellic acid (GA3, 56%), and cyto-kinin (CKs, 49%) contents. Nonetheless, the administration of GOE, BHs, and GOE + BHs attenuated the adverse impacts of Cd + Pb stress. The best treatment was GOE + BHs which significantly decreased EL (52%) and MDA (49%) because of a reduction of O2•– (61%), H2O2 (60%), ABA (63%), Cd (89–91%), and Pb (89–91%) levels. This positive outcome was linked to an increase in the OPs’ (22–46%) and non-enzymatic antioxidant (27–46%) levels, activities of enzymes (26–44%), and enzyme gene expressions (35–40%), all of which contributed to the promoted relative water content (RWC, 37%), pigment contents (47–194%), hormonal levels (82–132%), growth traits (31–149%), yield components (154–626%), and fruit quality traits (31–92%). From these results, it can be concluded that treatment of GOE + BHs is recommended as a foliar application to reduce the adverse effects of Cd + Pb stress treatment in squash. Full article
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17 pages, 1915 KiB  
Article
Impact of Soil-Applied Humic Ameliorative Amendment on the Ligno-Cellulose Quality and Calorific Value of Switchgrass Panicum virgatum L.
by Štefan Tóth, Božena Šoltysová, Štefan Dupľák and Pavol Porvaz
Agronomy 2023, 13(7), 1854; https://doi.org/10.3390/agronomy13071854 - 13 Jul 2023
Cited by 2 | Viewed by 1636
Abstract
The main objective of the paper was to determine the ligno-cellulose quality and calorific value of switchgrass Panicum virgatum L. The impact of nutrition treatments (pre-sowing soil humic amendment HA and/or NPK, with annual dose of N on both the treatments, and untreated [...] Read more.
The main objective of the paper was to determine the ligno-cellulose quality and calorific value of switchgrass Panicum virgatum L. The impact of nutrition treatments (pre-sowing soil humic amendment HA and/or NPK, with annual dose of N on both the treatments, and untreated control UC) and years were evaluated as main effects within a pilot experiment with seven cultivars tested during 2018–2022. Two data sets of acid detergent fiber (ADF), acid detergent lignin (ADL), crude cellulose (CE), hemicellulose (HEM), neutral detergent fiber (NDF), and high heating value (HHV) were evaluated, the primary one in terms of quality content and the secondary one in terms of quality yield. The average ADF content of the switchgrass was 43.94% (range 30.15–50.91), while the average contents of ADL, CE, HEM, NDF, and HHV were 9.21% (6.02–12.41), 34.73% (17.98–40.08), 30.49% (21.34–38.41), 74.43% (59.20–81.15%), and 17.206 kJ g−1 (16.579–17.799), respectively. An adequate value of ADF yield was 4.17 Mg ha−1 (0.01–29.31), while for ADL, CE, HEM, NDF and HHV this was 0.79 Mg ha−1 (0.00–5.39), 3.37 Mg ha−1 (0.01–23.92), 2.79 Mg ha−1 (0.01–17.66), 6.96 Mg ha−1 (0.01–46.93), and 1.466 hGJ ha−1 (0.003–10.603), respectively. In terms of the both quality sets the cultivar was confirmed to be the most important factor followed by the year, with nutrition having the least impact. This impact order of the main effects was valid for each of the parameters. Moreover, in terms of quality yield the formation of homogeneity groups corresponded with dry matter yield and therefore with the order of cultivars (EG 1101 ˃ BO Master ˃ EG 1102 ˃ Kanlow ˃ Alamo ˃ Carthage ˃ NJ Ecotype), the years (2021 ˃ 2020 ˃ 2022 ˃ 2019 ˃ 2018), and the treatments (HA ˃ NPK ˃ UC). Full article
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13 pages, 1670 KiB  
Article
Enhancing Soil Remediation of Copper-Contaminated Soil through Washing with a Soluble Humic Substance and Chemical Reductant
by Lina Wang, Jing Wei, Lu Yang, Yun Chen, Mengjie Wang, Liang Xiao and Guodong Yuan
Agronomy 2023, 13(7), 1754; https://doi.org/10.3390/agronomy13071754 - 28 Jun 2023
Cited by 2 | Viewed by 1458
Abstract
The bioavailability and mobility of copper (Cu) in soil play a crucial role in its toxicity and impact on soil organisms. Humic substances, with their abundant functional groups and unique pore structure, have demonstrated the ability to effectively mitigate the toxic effects of [...] Read more.
The bioavailability and mobility of copper (Cu) in soil play a crucial role in its toxicity and impact on soil organisms. Humic substances, with their abundant functional groups and unique pore structure, have demonstrated the ability to effectively mitigate the toxic effects of heavy metals in soil. This study explores the potential of a soluble humic substance (HS) derived from leonardite for Cu removal from contaminated soils. The effects of various washing conditions, such as concentration and washing cycles, on removal efficiency were assessed. The results showed that a single washing with HS solution achieved an optimal removal efficiency of 37.5% for Cu in soil, with a subsequent reuse achieving a removal efficiency of over 30.5%. To further enhance Cu removal efficiency, a two-step soil washing approach using a chemical reductant NH2OH·HCl coupled with an HS solution (NH2OH·HCl + HS) was employed, resulting in an increased removal efficiency to 53.0%. Furthermore, this approach significantly reduced the plant availability and bioaccessibility of Cu by 13.6% and 11.4%, respectively. Compared to a single washing with NH2OH·HCl, both HS and NH2OH·HCl + HS increased the soil pH and organic matter content. These findings suggest that the two-step soil-washing approach using NH2OH·HCl + HS effectively removed Cu from polluted soil. This study demonstrates the potential of humic substances as environmentally friendly materials for remediating heavy metal-polluted soil, promoting green and sustainable applications in soil remediation practices. Full article
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21 pages, 4756 KiB  
Article
Temperature Dependences of IR Spectral Bands of Humic Substances of Silicate-Based Soils
by Mikhail A. Proskurnin, Dmitry S. Volkov and Olga B. Rogova
Agronomy 2023, 13(7), 1740; https://doi.org/10.3390/agronomy13071740 - 28 Jun 2023
Cited by 2 | Viewed by 973
Abstract
Temperature-dependent IR spectroscopy capable of revealing specific changes in the band intensities, positions, and shape was used to increase the information on humic substances (HS) from soils. Temperature dependences of IR spectra of HS isolated from silicate-based soils differing in the contents and [...] Read more.
Temperature-dependent IR spectroscopy capable of revealing specific changes in the band intensities, positions, and shape was used to increase the information on humic substances (HS) from soils. Temperature dependences of IR spectra of HS isolated from silicate-based soils differing in the contents and nature of organic matter, chernozem and soddy podzolic soil, were investigated by attenuated total internal reflection FTIR in the mid-IR region (4000–400 cm−1) in the air within the moderate temperature range of 25–215 °C (298–488 K) with a step of 2.5 °C. The dependences of shifts in frequency (wavenumber) corresponding to band maxima and integrated band intensities were built for all major bands. Observed reversible frequency shifts upon heating and cooling can be interpreted as forming structures in the dry state. The behavior of integrated intensities of bands assigned to C–H and C–C vibrations, characteristic vibrations of polyaromatic compounds, carboxylic acids, and carboxylates were shown, and similar behavior for the same group (symmetric and antisymmetric stretches) were revealed. Differences in the temperature trends in chernozem and soddy podzolic soils due to different structures (aliphatic and aromatic) and functional groups (carboxylic and carboxylate) are shown. The different behavior of the bands corresponding to carboxylic groups and skeletal vibrations differentiates soil types with different organic matter. The temperature trends of band maximum and intensity shifts are less prone to measurement conditions and may serve as qualitative parameters characterizing the composition of soil humic substances. Full article
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14 pages, 4877 KiB  
Article
Spectroscopic and Microscopic Analysis of Humic Acid Isolated from Stabilized Leachate HSs Fractionation
by Zaber Ahmed, Mohd Suffian Yusoff, Nurul Hana Mokhtar Kamal, Hamidi Abdul Aziz and Maria Roulia
Agronomy 2023, 13(4), 1160; https://doi.org/10.3390/agronomy13041160 - 19 Apr 2023
Cited by 4 | Viewed by 1944
Abstract
Refractory humic substances (HSs), which include humic and fulvic acid as well as hydrophilic portion, are the prime pollutants of stabilized landfill leachate with a concentrated color and chemical oxygen demand (COD). Spectroscopic and microscopic analysis of humic acid remaining in stabilized leachate [...] Read more.
Refractory humic substances (HSs), which include humic and fulvic acid as well as hydrophilic portion, are the prime pollutants of stabilized landfill leachate with a concentrated color and chemical oxygen demand (COD). Spectroscopic and microscopic analysis of humic acid remaining in stabilized leachate as a pollutant contributor were conducted in this study. Microfiltration and centrifugation processes were applied to fractionate the humic acid from the HSs of stabilized leachate. The three-stage isolation process recovered a maximum of 1412 ± 2.5 mg/L (Pulau Burung leachate), 1510 ± 1.5 mg/L (Alor Pongsu leachate) at pH 1.5 and 1371 ± 2.5 mg/L (PBLS), and 1451 ± 1.5 mg/L (APLS) of humic acid (about 42% of the total COD concentration) at pH 2.5, which eventually indicates the efficiency of the process. The spectroscopic analysis of isolated humic acid through scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) significantly indicates the existence of identical elements in the recovered humic acid. The subsequent reduction (around 37%, 36%, and 39%) in ultra-violet absorbance values (UV254, UV280), COD, and color in the humic acid isolated leachate indicates the acid’s significant contribution as a toxic pollutant through aromaticity and conjugated double-bond compounds. Full article
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17 pages, 1962 KiB  
Article
Quantification of Lignosulfonates and Humic Components in Mixtures by ATR FTIR Spectroscopy
by Evgeniya A. Karpukhina, Dmitry S. Volkov and Mikhail A. Proskurnin
Agronomy 2023, 13(4), 1141; https://doi.org/10.3390/agronomy13041141 - 17 Apr 2023
Cited by 4 | Viewed by 1494
Abstract
The existing techniques for lignosulfonate (LS) in humate fertilizers lack selectivity to humic substances (HS) as the main component; they involve labor- and time-consuming sample preparation to separate the components at the level of detectable LS concentrations. The procedure based on attenuated total [...] Read more.
The existing techniques for lignosulfonate (LS) in humate fertilizers lack selectivity to humic substances (HS) as the main component; they involve labor- and time-consuming sample preparation to separate the components at the level of detectable LS concentrations. The procedure based on attenuated total reflectance (ATR) FTIR spectroscopy with simple sample preparation for directly quantifying lignosulfonates in aqueous solutions and lignosulfonates and HS in aqueous solutions of preparations based on HS of coal origin (Sigma Aldrich, Powhumus, and Life Force) was developed. Lignosulfonate quantification is possible by exploiting the bands at 1266, 1192, 1093, and 1042 cm−1 with limits of detection of 0.4–2 g/L. Quantifying LS in a mixture with humates includes centrifugation of prepared solutions to separate interfering silicate impurities. LS quantification in the range of 10–100 g/L against HS (up to a 2-fold excess) with an error of up to 5% is possible based on the spectral absorptions at 1093 and 1042 cm−1. Simultaneous quantification of humate in the mixture with an error of up to 10% is possible by exploiting the bands at 1570 and 1383 cm−1 (carboxylates). The study shows the possibility of determining lignosulfonate against an HS background several times higher than lignosulfonate. The developed technique is applicable for analyzing fertilizers of simple composition and quality control of pure humates used for plant growth. Obtaining the most accurate results needs calibration solutions from the same brands that make up the test mixture. Full article
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18 pages, 2690 KiB  
Article
Effect of a Soil-Applied Humic Ameliorative Amendment on the Yield Potential of Switchgrass Panicum virgatum L. Cultivated under Central European Continental Climate Conditions
by Štefan Tóth and Štefan Dupľák
Agronomy 2023, 13(4), 1095; https://doi.org/10.3390/agronomy13041095 - 11 Apr 2023
Cited by 4 | Viewed by 2763
Abstract
The yield potential of switchgrass was verified by testing seven cultivars in a small-scale experiment carried out from 2018 to 2022 on Gleyic Fluvisol under Central European conditions. The treatments are as follows: pre-sowing (i/HA) soil humic preparation Humac AGRO (based on leonardite) [...] Read more.
The yield potential of switchgrass was verified by testing seven cultivars in a small-scale experiment carried out from 2018 to 2022 on Gleyic Fluvisol under Central European conditions. The treatments are as follows: pre-sowing (i/HA) soil humic preparation Humac AGRO (based on leonardite) with an ameliorative dose of 1000.0 kg ha−1 or (ii/NPK) basic nutrition with a dose of 220.0 kg ha−1 NPK; an annual dose of 70 kg ha−1 N was applied to both treatments (HA, NPK) and compared to (iii/UC) untreated control. A dry matter (DM) yield of 9.02 t ha−1 was achieved in the total average, which varied from 0.05 t ha−1 to 60.64 t ha−1. The yield was affected mainly by years (F-ratio 106.64), then by nutrition (F-ratio 79.03), followed by cultivars (F-ratio 56.87), and finally by replications (F-ratio 0.00). Switchgrass productivity increased according to the utility year; however, the driest year (2020—388 mm, 2021—372 mm, and 2022—288 mm) changed the order of the three full utility years (12.44, 19.13, and 7.73 t ha−1 DM, respectively). HA gives the highest DM yield of 13.69 t ha−1 on average with values of 9.19 and 4.19 for NPK and UC, respectively. The cultivars order was EG 1101 ˃ BO Master ˃ EG 1102 ˃ Kanlow ˃ Alamo ˃ Carthage ˃ NJ Ecotype (21.15, 12.48, 8.14, 7.70, 6.34, 4.47, and 2.89 t ha−1 DM when ranking average yield or 60.64, 45.20, 29.92, 29.15, 17.87, 9.86, and 5.93 t ha−1 DM when ranking maximal yields). Full article
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18 pages, 1799 KiB  
Article
Impacts of Partial Substitution of Chemical Fertilizer with Organic Manure on the Kinetic and Thermodynamic Characteristics of Soil β–Glucosidase
by Ruixiao Dong, Djido Abdelkerim-Ouba, Danyang Liu, Xianfa Ma and Shuang Wang
Agronomy 2023, 13(4), 1065; https://doi.org/10.3390/agronomy13041065 - 6 Apr 2023
Cited by 3 | Viewed by 1522
Abstract
To study the characteristics of the β–glucosidase enzymatic reaction in wheat field soil under the condition of reducing the application of chemical fertilizer, five fertilization treatments were established, including no fertilizer (CK), chemical fertilizer (F), organic fertilizer (OF), 25% organic fertilizer plus [...] Read more.
To study the characteristics of the β–glucosidase enzymatic reaction in wheat field soil under the condition of reducing the application of chemical fertilizer, five fertilization treatments were established, including no fertilizer (CK), chemical fertilizer (F), organic fertilizer (OF), 25% organic fertilizer plus 75% chemical fertilizer (25% OF), and 50% organic fertilizer plus 50% chemical fertilizer (50% OF). The activity of β–glucosidase and its kinetic and thermodynamic characteristics were analyzed by using microplate p–nitrophenol colorimetry. The results showed that the Vmax values of soil β–glucosidase in the organic substitution of chemical fertilizer treatment were higher than those in the chemical fertilizer and no fertilizer treatments, and the Km values were lower than those in the chemical fertilizer and no fertilizer treatments at the different growth stages. The Vmax value in the 25% OF treatment was the highest at the jointing stage and that of the OF treatment was the highest at the booting stage; the Km value in the 50% OF treatment was the lowest at the different growth stages. Compared with the chemical fertilizer and no fertilizer treatments, the application of organic fertilizer effectively reduced thermodynamic parameters such as Ea, Q10, ∆H, ∆G, and ∆S at the jointing and booting stages of wheat. The thermodynamic parameters in the 25% OF treatment were the lowest at the jointing stage and those in the OF treatment were the lowest at the booting stage. A reasonable amount of organic fertilizer is more beneficial to enzymatic reactions and improves the soil quality and the ability to supply nutrients to wheat cultivation. Full article
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25 pages, 2480 KiB  
Article
Exploring the Macroscopic Properties of Humic Substances Using Modeling and Molecular Simulations
by Yerko Escalona, Drazen Petrov, Edgar Galicia-Andrés and Chris Oostenbrink
Agronomy 2023, 13(4), 1044; https://doi.org/10.3390/agronomy13041044 - 1 Apr 2023
Cited by 3 | Viewed by 1397
Abstract
Soil organic matter (SOM) is composed of a complex and heterogeneous mixture of organic compounds, which poses a challenge in understanding it on an atomistic level. Based on the progress of molecular dynamics simulations and our efforts to create molecular systems that resemble [...] Read more.
Soil organic matter (SOM) is composed of a complex and heterogeneous mixture of organic compounds, which poses a challenge in understanding it on an atomistic level. Based on the progress of molecular dynamics simulations and our efforts to create molecular systems that resemble SOM, in this work, we expanded our knowledge of SOM through the use of humic substances (HSs). Specifically, we studied the standardized samples of HS of the International Humic Substances Society (IHSS). This society provided the elemental and organic composition used as input parameters for our Vienna Soil Organic Matter Modeler 2 (VSOMM2). We modeled and simulated different HS samples from various sources, including soil, peat, leonardite, and blackwater river. In order to compare between different HS, we reduced the organic composition information to two principal components, which are associated principally with the amount of carboxyl and aromatic groups in the HS, denominated as PCacid and PCarom, respectively. We performed a plethora of analyses to characterize the structure and dynamics of the systems, including the total potential energy, density, diffusion, preferential solvation, hydrogen bonds, and salt bridges. In general terms, at the water content value of 0.2, we observed that most properties depend on the carboxyl group protonation state. The Coulombic interactions from this ionic specie and the interaction with cations determine the overall behavior of the studied systems. Furthermore, the type of cations and the pH influence those properties. This study exemplifies the importance of molecular dynamics to explain macroscopic properties from the structure and dynamics of the molecules modeled, such as the interaction network, i.e., hydrogen bonds or salt bridges of molecules presented in the system and their mobility. Full article
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15 pages, 1679 KiB  
Article
Impact of Care and Nutrition Methods on the Content and Uptake of Selected Mineral Elements in Solanum tuberosum
by Iwona Mystkowska, Krystyna Zarzecka, Marek Gugała, Agnieszka Ginter, Anna Sikorska and Aleksandra Dmitrowicz
Agronomy 2023, 13(3), 690; https://doi.org/10.3390/agronomy13030690 - 26 Feb 2023
Cited by 2 | Viewed by 1228
Abstract
The aim of the study was to evaluate the content and uptake of macronutrients (P, Ca, Mg, K) in Solanum tuberosum tubers and the effect of care with biostimulants from a three-year field experiment conducted at the Agricultural Experimental Station in Zawady, Poland. [...] Read more.
The aim of the study was to evaluate the content and uptake of macronutrients (P, Ca, Mg, K) in Solanum tuberosum tubers and the effect of care with biostimulants from a three-year field experiment conducted at the Agricultural Experimental Station in Zawady, Poland. The experiment was set up in a split-plot arrangement with three replications. The first factor was two varieties of edible potato (Oberon and Malaga), and the second factor was five treatments with herbicide and biostimulants: (I) control facility, (II) herbicide (chlomazone + metribuzin), (III) herbicide and biostimulant PlonoStart, (IV) herbicide and biostimulant Aminoplant, (V) herbicide and biostimulant Agro-Sorb Folium. Biostimulants and herbicide increased the concentration of P, Mg, Ca, and K compared to tubers harvested from the control facility. The Oberon variety had the highest macro-nutrient uptake capacity. The application of herbicide with biostimulants increased the uptake of the mentioned mineral nutrients compared to the control variant. Climatic conditions affected the content and uptake of selected elements. Full article
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20 pages, 4590 KiB  
Article
Adsorption of As(V) at Humic Acid-Kaolinite-Bacteria Interfaces: Kinetics, Thermodynamics, and Mechanisms
by Min Xiao, Jingwen Guo, Shan Zhao and Shifeng Li
Agronomy 2023, 13(2), 611; https://doi.org/10.3390/agronomy13020611 - 20 Feb 2023
Cited by 2 | Viewed by 1634
Abstract
The immobilization and transformation of arsenic at the mineral-organic interface in soil environments ultimately depend on the soil components and their interactions. Herein, the effect of humic acid (HA) and a typical bacterium (a Gram-positive Bacillus subtilis) coating on the adsorption of [...] Read more.
The immobilization and transformation of arsenic at the mineral-organic interface in soil environments ultimately depend on the soil components and their interactions. Herein, the effect of humic acid (HA) and a typical bacterium (a Gram-positive Bacillus subtilis) coating on the adsorption of arsenate As(V) to kaolinite (Kao) mineral was investigated. The As(V) adsorption reaction kinetics, isotherms, thermodynamics, and mechanism on the clay mineral-organic composites of kaolinite-Bacillus subtilis (Kao-B.s) and humic acid-kaolinite-Bacillus subtilis (Kao-HA-B.s) were investigated. The As(V) adsorption on the composites was better fitted to pseudo-second-order kinetics and the Freundlich model. The adsorption capacity of As(V) followed the order of Kao-HA-B.s > Kao-B.s > B.s > Kao-HA > Kao. The positive ΔH (31.44, 5.87 kJ mol−1) and ΔG (0.10–0.96 kJ mol−1) values confirmed that the adsorption of As(V) by all composites was nonspontaneous and endothermic in character at room temperature. The FT-IR, XRD, and thermodynamic results revealed that the adsorption mechanism of As(V) on the kaolinite–organic interfaces could be attributed to the electrostatic forces between the terminal aluminum or silanol groups of kaolinite and As(V) and the complexation between HA, bacteria, and As(V), which formed an inner-sphere complex and surface complex, respectively. The experimental results showed that the adsorption of As(V) on the Kao-HA or Kao-bacteria system was accompanied by significant additive interactions, while the ternary Kao-HA-bacteria system had a significant inhibitory effect on As(V) binding at a higher HA content due to the shielding effect, with the promotion effect shown at a lower concentration for dispersion effect for HA on the kaolinite particles. Full article
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20 pages, 2207 KiB  
Article
Salt-Stressed Coriander (Coriandrum sativum L.) Responses to Potassium Silicate, Humic Acid and Gamma Irradiation Pretreatments
by Raifa A. Hassanein, Omaima S. Hussein, Iman A. Farag, Yousra E. Hassan, Amal F. Abdelkader and Mohamed Ibrahim
Agronomy 2022, 12(10), 2268; https://doi.org/10.3390/agronomy12102268 - 22 Sep 2022
Cited by 5 | Viewed by 2088
Abstract
The application of biostimulants has great potential in preserving plants against abiotic or biotic stresses and is integrated into promoting tolerance and acclimating of coriander against salinity stress. Deciphering the morphological, physiological and molecular responses underpinning the ameliorative aspects of ecofriendly biostimulants is [...] Read more.
The application of biostimulants has great potential in preserving plants against abiotic or biotic stresses and is integrated into promoting tolerance and acclimating of coriander against salinity stress. Deciphering the morphological, physiological and molecular responses underpinning the ameliorative aspects of ecofriendly biostimulants is indispensable to link and overlap the ameliorative responses of seed priming. Hereby, the ameliorative responses of seed priming against salinity stress using potassium silicate, humic acid, and gamma irradiation were evaluated. Salinity stress generally diminishes vegetation, productivity, and metabolic activities. However, abscisic acid (ABA) levels and soluble sugars were elevated. Pretreatments with potassium silicate or humic acid, followed by gamma rays, alleviated and promoted growth parameters, yield components, and vital metabolic processes in salinity-stressed coriander. This promotion was concurrent with an increase in growth promotors, chlorophyll a/b, carbohydrates, antioxidants (compounds and enzymes), and upregulation of RuBisCO large subunit protein expression. Collectively, potassium silicate and humic acid were the best at alleviating the adverse effects of saline conditions. Triggered pretreatments might be engaged in maintaining metabolic activities toward deleterious salinity impacts. Thus, it was suggested that seed priming by potassium silicate and humic acid is an effective regime benefitting salinized along with nonsalinized plants that sustain coriander productivity. Full article
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19 pages, 3514 KiB  
Article
Foliar Applications of Humic Substances Together with Fe/Nano Fe to Increase the Iron Content and Growth Parameters of Spinach (Spinacia oleracea L.)
by Metin Turan, Melek Ekinci, Raziye Kul, Ayhan Kocaman, Sanem Argin, Anastasia M. Zhirkova, Irina V. Perminova and Ertan Yildirim
Agronomy 2022, 12(9), 2044; https://doi.org/10.3390/agronomy12092044 - 27 Aug 2022
Cited by 12 | Viewed by 3679
Abstract
Iron deficiency, which severely decreases the plant yield and quality, is one of the major problems of calcareous soils. Foliar applications of humic substances and/or Fe fertilizers are environmentally friendly methods to cope with Fe deficiency. The aim of this study was to [...] Read more.
Iron deficiency, which severely decreases the plant yield and quality, is one of the major problems of calcareous soils. Foliar applications of humic substances and/or Fe fertilizers are environmentally friendly methods to cope with Fe deficiency. The aim of this study was to investigate the combined effect of Fe/nano Fe and humic/fulvic acid-based biostimulant foliar applications on the Fe content and plant growth parameters of spinach. Treatment solutions were prepared either by mixing a common Fe fertilizer, FeSO4·7H2O, with different commercial biostimulants (Fulvic-based: Fulvagra®, Fulvagra®WSG; Humic-based: HS300®, Humin Fe® and Liqhumus®, Grevenbroich, Germany) or by mixing nano ferrihydrite with different ratios of fulvic substance (FA-50, FA-75, and FA-100) and humic acid (Nano Iron). Growth parameters (plant fresh and dry weights, plant dry matter, root fresh and dry weights, root dry matter, leaf number per plant, and leaf area); chlorophyll reading value (SPAD); chlorophyll (a,b, and total) and carotenoid contents; and leaf and root mineral contents (N, P, K, Ca, Mg, S, Cu, Mn, Zn, B, active Fe, and total Fe) of samples were determined. Our results showed that foliar application of biostimulants together with Fe sources improved the nutrient uptake, chlorophyll contents, growth characteristics, and yield; however, not all humic substances had the same effect. When all parameters were considered, Fulvagra treatment—which contained 17% fulvic acid and microorganisms in its content together with 20 mM FeSO4·7H2O—was the most effective application, followed by FA100 treatment containing fulvic acid and 20 mM nano ferrihydrite. This finding indicates that fulvic acid containing biostimulants is more effective in foliar applications than humic-based biostimulants against Fe deficiency due to their low molecular weight which enables better penetration into the leaves. In conclusion, foliar applications of fulvic substances together with Fe fertilizers can be used to increase the Fe uptake of crops and the yields under Fe-deficient conditions. Full article
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9 pages, 581 KiB  
Communication
The Positive Effects of Humic/Fulvic Acid Fertilizers on the Quality of Lemon Fruits
by Xiaoying He, Hanqi Zhang, Jinxue Li, Fan Yang, Weifeng Dai, Cheng Xiang and Mi Zhang
Agronomy 2022, 12(8), 1919; https://doi.org/10.3390/agronomy12081919 - 15 Aug 2022
Cited by 13 | Viewed by 3183
Abstract
Humic acid (HA) is a kind of organic substance that has shown good effects in regard to promoting crop growth. In the current study, the influences of three kinds of fertilizers mainly containing humic/fulvic acids, including water-soluble fertilizer containing humic acid (WHA), fulvic [...] Read more.
Humic acid (HA) is a kind of organic substance that has shown good effects in regard to promoting crop growth. In the current study, the influences of three kinds of fertilizers mainly containing humic/fulvic acids, including water-soluble fertilizer containing humic acid (WHA), fulvic acid potassium (FAP), and fulvic acid distillate (FAD), on the qualities of ‘Eureka’ lemon fruits were investigated systematically at their different harvest times. As demonstrated by the results, the indexes used to assess the quality of lemon fruit showed that all lemons treated with those three humic/fulvic acid fertilizers at different harvest times improved in quality. Notably, the single fruit weight, edible rate, and juice yield of lemon fruit in the WHA- and FAP-treated groups, as well as the contents of vitamin C, total acid, total sugar, and total soluble solid, were higher than those in the blank control (CK) group (p < 0.05). Furthermore, the contents of total flavonoids and phenols in the peels, pulps, and seeds of lemons in humic/fulvic acid fertilizer–treated groups were all higher than those in the CK group at the three harvest times (p < 0.05). This result indicated that humic/fulvic acid fertilizers produced positive effects on the quality of lemon fruits and could be used in lemon planting to improve the quality and added value of lemons. Full article
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13 pages, 743 KiB  
Article
Copper Distribution and Binding Affinity of Size-Fractioned Humic Substances Taken from Paddy Soil and Correlation with Optical Characteristics
by Wei-Hsiang Huang, Tzu-Che Lin, Chao-Min Huang, Ting-Chien Chen and Yi-Lung Yeh
Agronomy 2022, 12(7), 1689; https://doi.org/10.3390/agronomy12071689 - 16 Jul 2022
Cited by 3 | Viewed by 1380
Abstract
Soil organic matter (SOM) is heterogeneous and a complex mixture, whose concentration, chemical composition, and structure are strongly associated with the binding behavior of heavy metals (HMs) in soil. The HM-SOM binding affinity affects the HM biotoxicity and bioavailability in the environment. This [...] Read more.
Soil organic matter (SOM) is heterogeneous and a complex mixture, whose concentration, chemical composition, and structure are strongly associated with the binding behavior of heavy metals (HMs) in soil. The HM-SOM binding affinity affects the HM biotoxicity and bioavailability in the environment. This study investigated Cu’s distribution and binding affinity on five size-fractioned alkaline-extracted soil organic matters (AEOMs) taken from paddy fields. The fractioned AEOMs were M-A (100 kDa −0.45 μm), M-B (3–100 kDa), M-C (1–3 kDa), M-D (0.3–1.0 kDa), and M-E (<0.3 kDa). The average organic carbon (OC) mass percentages were 10.0, 40.3, 6.3, 5.0, and 38.4%, and Cu mass percentages were 9.8, 66.7, 4.7, 4.7, and 14.1% for fractioned M-A, M-B, M-C, M-D, and M-E solutions, respectively. The Cu and AEOM binding affinity, [Cu]/[DOC] ratios, ranged from 3.1 to 127.6 μmol/g-C with 41-fold variation. The ratio order was 63.5 ± 32.3 (M-B) > 32.8 ± 12.8 (M-A) > 28.0 ± 10.5 (M-D) > 24.25 ± 10.7 (M-C) > 12.5 ± 6.0 (M-E) μmol g-C−1. Cu preferred binding with size-fractioned AEOMs ranging from 3 to 100 kDa. The specific ultraviolet absorbance at 254 nm (SUVA254), the fluorescence index (FI), and the biological index (BIX) were significantly correlated with the [Cu]/[DOC] ratios. In each site, the combined bulk and the five size-fractioned AEOMs, the selected optical indicators had a significant correlation with the corresponding [Cu]/[DOC] ratios. Cu-AEOM binding affinity was enhanced by AEOM rich in aromaticity. However, high microbial and autochthonous AEOM origins decreased the binding affinity. Full article
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12 pages, 1247 KiB  
Article
Deciphering the Effectiveness of Humic Substances and Biochar Modified Digestates on Soil Quality and Plant Biomass Accumulation
by Jiri Holatko, Tereza Hammerschmiedt, Oldrich Latal, Antonin Kintl, Adnan Mustafa, Tivadar Baltazar, Ondrej Malicek and Martin Brtnicky
Agronomy 2022, 12(7), 1587; https://doi.org/10.3390/agronomy12071587 - 30 Jun 2022
Cited by 4 | Viewed by 1591
Abstract
The effective use of digestate as exogenous organic matter to enhance soil carbon sequestration depends on the balance between labile and recalcitrant organic carbon, which is influenced by the type of feedstock, the fermentation process, and the fraction (liquid, solid) of the fermented [...] Read more.
The effective use of digestate as exogenous organic matter to enhance soil carbon sequestration depends on the balance between labile and recalcitrant organic carbon, which is influenced by the type of feedstock, the fermentation process, and the fraction (liquid, solid) of the fermented product used. In this work, in order to change the ratio of labile to stable carbon in the resulting fertiliser, the digestate was mixed with organic carbon-rich supplements: biochar and Humac (a humic acid-rich substance). The pot experiment was carried out under controlled conditions with maize (Zea mays L.) in soil amended with the digestate (D), which was incubated with Humac (H), biochar (B), or a combination of both (D + B + H) before the application. Digestate enriched with Humac showed improved short-term nutrient (carbon, phosphorus, nitrogen) transformation, as indicated by soil enzyme activity and the highest maize biomass production of. Total carbon content, C:N ratio, short-term respiration activity, and nitrification were most enhanced by digestate enriched with either biochar or combined biochar + Humac). Long-term nitrogen mineralization was mostly enhanced by digestate + Humac, as indicated by amino-acid-induced respiration and urease activity. Short-term positive effects of digestate + biochar (eventually + Humac) on catabolism were proven, whereas their long-term effects on nutrient mineralization were negative (i.e., biochar-mediated immobilization, sequestration), which should be the focus of further research in future. Full article
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17 pages, 1137 KiB  
Article
Changes in Relationships between Humic Substances and Soil Structure following Different Mineral Fertilization of Vitis vinifera L. in Slovakia
by Vladimír Šimanský, Elżbieta Wójcik-Gront, Jarmila Horváthová, Dorota Pikuła, Tomáš Lošák, Agnieszka Parzych, Martin Lukac and Elena Aydın
Agronomy 2022, 12(6), 1460; https://doi.org/10.3390/agronomy12061460 - 17 Jun 2022
Cited by 7 | Viewed by 1795
Abstract
The quantity and quality of soil organic matter (SOM) depending on many edaphic and environmental factors may change in response to agriculture-related practices. The SOM humification process can be supported by the application of mineral fertilizers, but in the production vineyards, such information [...] Read more.
The quantity and quality of soil organic matter (SOM) depending on many edaphic and environmental factors may change in response to agriculture-related practices. The SOM humification process can be supported by the application of mineral fertilizers, but in the production vineyards, such information is lacking. NPK fertilizer alters the soil quality, and therefore, the aim of this study was to: (1) assess the extent and dynamics of different NPK (control—no fertilization; 1st NPK; and 3rd NPK levels) rates to the soil with grass sward cover in a productive vineyard on changes in SOM, humic substances (HS), and soil structure, and (2) identify relationships between SOM, HS, and soil structure. Results showed that the share of humic acids in soil organic carbon decreased only in NPK1 compared to control and NPK3 treatments. The color quotient of humic substance values in NPK1 and NPK3 increased by 4 and 5%, respectively, compared to control. Over a period of 14 years, the content of soil organic carbon increased by 0.71, 0.69, and 0.53 g kg−1 year−1 in the control, NPK1, and NPK3, respectively. The content of HS increased linearly with slight differences due to NPK application. The vulnerability of the soil structure decreased due to fertilization—more at the higher NPK level. The rate of formed soil crust was decreasing in the following order: control > NPK1 > NPK3. In the control treatment, the relations between SOM, HS, and soil structure were most abundant and with the greatest significance, while with the increasing level of NPK, these relations lost their significance. Full article
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8 pages, 4113 KiB  
Communication
Calcined Oyster Shell-Humic Complex as Soil Amendment to Remediate Cd- and As-Contaminated Soil
by Guodong Yuan, Dongxue Bi, Jing Wei and Liang Xiao
Agronomy 2022, 12(6), 1413; https://doi.org/10.3390/agronomy12061413 - 12 Jun 2022
Cited by 3 | Viewed by 2025
Abstract
Soil acidification in the subtropical region reduces soil productivity and increases the bioavailability of heavy metal(loid)s in soil. Here 800 °C-calcined oyster shell and leonardite-derived humic substance were combined to form a Ca-humic complex to remediate an acidic soil contaminated by cadmium (Cd) [...] Read more.
Soil acidification in the subtropical region reduces soil productivity and increases the bioavailability of heavy metal(loid)s in soil. Here 800 °C-calcined oyster shell and leonardite-derived humic substance were combined to form a Ca-humic complex to remediate an acidic soil contaminated by cadmium (Cd) and arsenic (As) for safe vegetable production. In the pot experiment, the complex was added to soil as a soil amendment at 0%, 0.5%, 1.0%, 2.0%, and 5.0%. A 2.0% dose reduced Cd and As contents of the vegetable bok choy by 98% and 71%, respectively, alleviating the human health risk of Cd and As from the food chain. The unified bio-accessibility method (UBM) was followed to estimate the bio-accessibility of Cd and As. The complex at a 2.0% dose lowered soil Cd and As bio-accessibility in simulated gastric (BioG) and intestine (BioGI) compartments by 15.5% for BioG-Cd, 39.2% for BioGI-Cd, 28.8% for BioG-As, and 45.0% for BioGI-As, thus reducing the human health risk of Cd and As from soil ingestion. The use of oyster shell waste to produce Ca-humic complex as a soil amendment is a practical means of achieving dual agronomic and environmental benefits, from the remediation of soil contamination to the safe disposal of shell waste. Full article
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17 pages, 2952 KiB  
Article
Effect of Humic Acids on Soybean Seedling Growth under Polyethylene-Glycol-6000-Induced Drought Stress
by Renata Matuszak-Slamani, Romualda Bejger, Małgorzata Włodarczyk, Danuta Kulpa, Mariola Sienkiewicz, Dorota Gołębiowska, Elżbieta Skórska and Aleksandra Ukalska-Jaruga
Agronomy 2022, 12(5), 1109; https://doi.org/10.3390/agronomy12051109 - 2 May 2022
Cited by 9 | Viewed by 2730
Abstract
Humic substances (HS) are the most important natural biostimulant of plants. However, the relationship between their structure and biological activity in plants is still not well recognized. The objective of this paper was to assess the influence of molecular fractions of humic acids [...] Read more.
Humic substances (HS) are the most important natural biostimulant of plants. However, the relationship between their structure and biological activity in plants is still not well recognized. The objective of this paper was to assess the influence of molecular fractions of humic acids (HA) (HA < 30 kDa and HA > 30 kDa) on reducing negative effects of drought stress in soybean (Glycine max (L.) Merr.) seedlings of Progres and Nawiko cultivars. Drought stress was induced in laboratory conditions by the addition of polyethylene glycol 6000 (PEG 6000) to make a water potential of −0.5 MPa. HA were extracted according to the International Humic Substances Society procedure, and then were separated into two molecular fractions by membrane filtration. The following physiological and biometric parameters were determined: chlorophyll content, photosynthesis activity, electrical conductivity, fresh and dry mass of overground and roots, and plant length. The enzyme activity and ion contents were also measured. Differences in response to drought stress with the addition of HA < 30 kDa and HA > 30 kDa or not to the Hoagland’s solution were observed among studied cultivars. Drought stress caused a decrease in the most physiological parameters and increase in peroxidase activity in the case of both studied cultivars. However, the results of biometric measurements showed that the Progres cultivar appears to have better tolerance to drought stress. The significant influence of water deficit on most macroelement content in dry matter leaves of both studied cultivars was not observed, while its effect on microelement uptake by soybean plants was concluded. In the case of the Progres cultivar, the results showed a significant decrease in microelement content in the dry matter of leaves, whereas in the leaves of Nawiko cultivar there was a significant increase. The influence of HA > 30 kDa and HA < 30 KDa fractions on physiological features of both studied cultivars was varied. HA > 30 kDa fraction better up-regulated the antioxidant defense system. Unfortunately, no effect of either HA fraction on the macro- and micronutrients uptake system of both studied cultivars was observed. Full article
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Review

Jump to: Editorial, Research

20 pages, 3104 KiB  
Review
Macromolecular Size and Architecture of Humic Substances Used in the Dyes’ Adsorptive Removal from Water and Soil
by Panagiota G. Fragouli, Maria Roulia and Alexandros A. Vassiliadis
Agronomy 2023, 13(12), 2926; https://doi.org/10.3390/agronomy13122926 - 28 Nov 2023
Cited by 3 | Viewed by 1175
Abstract
Humic substances are naturally occurring materials composed of complex biogenic mixtures of substituted aromatic and aliphatic hydrocarbon core materials derived from the degradation and decomposition of dead plant and animal matter. They are ubiquitous in both terrestrial and aquatic systems constituting biotic pools [...] Read more.
Humic substances are naturally occurring materials composed of complex biogenic mixtures of substituted aromatic and aliphatic hydrocarbon core materials derived from the degradation and decomposition of dead plant and animal matter. They are ubiquitous in both terrestrial and aquatic systems constituting biotic pools and are characterized by unique properties; they are amphiphilic redox compounds with exceptional chelating features. Humic substances play a crucial role in both agriculture and the environment as carbon sequestrators, soil improvers, plant health promoters, as well as stabilizers of soil aggregates and regulators of organic/inorganic nutrients bioavailability. This review article attempts to summarize current knowledge about the molecular nature and characterization techniques employed for the study of humic substances worldwide as the chemistry of their components differs markedly and depends on natural processes, several abiotic and biotic factors, the origin of the organic matter, and their complexation with inorganic, e.g., metal-ion, compounds. This work is equally concerned with the association of humic substances with dyes, a notorious pollutant, responsible for various environmental issues generally arising from the discharge of untreated effluents into soils and water bodies. Azo dyes, in particular, negatively affect soil microbial communities, as well as plant germination and growth. The aim is to feature the potential contribution of humic substances as novel materials for environment-friendly and sustainable processes. Full article
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