Soil Humic Substances Research Articles

Insights into solubility of soil humic substances and their fluorescence characterisation in three characteristic soils

Soil humic substances (HS) are involved in almost all biogeochemical processes and functions in soils, thus their extraction from aiming to their characterization is very important. However, many factors that influence HS extraction from soil still need further studies. The aim of this work was to assess and quantify comparatively the solubility of soil HS as a function of extraction time, various extractants, solid to liquid ratio and sequential extraction. In this work three different soils, i.e. a forest, a maize and a paddy soil, were examined to assess the solubility of soil HS based on their fluorescence (excitation-emission matrix, EEM) features and changes in nutrient (NO3−-N, PO43−-P and dissolved Si) contents using multiple extraction approaches (time-dependent, various extractants, solid to liquid ratio, and sequential extraction). Three fluorescent components, i.e. humic acids-like (HA-like), fulvic acids-like (FA-like), and protein-like fluorophores (PLF), were identified by parallel factor (PARAFAC) analysis of EEM spectra of the various soil extracts. The solubility of HS, dissolved organic carbon (DOC) and nutrients were shown to increase with extraction time, except for PLF. The FA-like fraction disappeared completely in KCl extracts of all three soils, suggesting the inefficiency of salt extraction. Conversely, HS and nutrients solubility substantially increased in alkaline extracts, and dissolved Si was correlated significantly with the fluorescent intensities of HA-like and FA-like, thus confirming the well-known typical process of alkaline dissolution of HS bound to phytolith and silicate minerals. The relative solubility of HS and nutrients was higher at lower solid to liquid ratio (1:250–1:100), whereas their maximum yields was achieved at high solid to liquid ratio (1:10) for all three soils. Sequential extraction results showed that the first water extraction step contributed 42–55% of HS, which suggested that a single extraction was insufficient to recover HS. In conclusion, water and alkaline extraction could provide, respectively, the labile and insoluble complexed HS existing in soil.

Humic acids derived from Leonardite to improve enzymatic activities and bioavailability of nutrients in a calcareous soil

Understanding the role of humic substances in soils is important for developing and utilizing organic fertilizers or soil amendments for sustainable agriculture. The objective of this study was to determine the effects of different fractions of humic acids derived from Leonardite on enzymatic activities and bioavailability of nutrients in a soil. The experiment was carried out by mixing different factions of humic acids with a soil and incubated for 70 d. The treatments included five factions of humic acids (HS1 (low molecular weight), HS2 (medium molecular weight), HS3 (large molecular weight, SED (sediment of fractions), HS (mixture of HS1, HS2, and HS3)), raw Leonardite (IM) and a control (no addition of humic acid). Experimental results showed that application of humic acids significantly improved acid and alkaline phosphatase activities, especially with HS1. Humic substances with high molecular weights significantly inhibit urease activity, and the optimal application rate was 600 kg/hm2 of humic substances with the high molecular weights. Concentrations of NH4+-N were decreased with increasing humus applications. All treatments (HSmix, HS1, HS2, HS3, IM, SED) did not affect the soil contents of Ca, although soil concentrations of K, P, Cu, Zn were increase significantly when small molecular weight humus (HS1) was applied. Keywords: humic acid, molecular weight, Leonardite, enzyme, nutrient content DOI: 10.25165/j.ijabe.20201303. Citation: Sun Q, Liu J L, Huo L F, Li Y C, Li X, Xia L R, et al. Humic acids derived from Leonardite to improve enzymatic activities and bioavailability of nutrients in a calcareous soil. Int J Agric & Biol Eng, 2020; 13(3): 200–205.

Impact of tillage practices on soil aggregation and humic substances under double‐cropping paddy field

AbstractThe effects of different tillage practices on soil organic carbon (SOC), soil aggregation, soil fulvic acid (C‐FAF), soil humic acid (C‐HAF), and soil humin (C‐HUM) contents at 0–5, 5–10, and 10–20 cm layers under double‐cropping rice (Oryza sativa L.) fields in southern China were explored. The experiment included four tillage treatments: conventional tillage and crop residues incorporation (CT), rotary tillage and crop residues incorporation (RT), no‐tillage and crop residues incorporation (NT), and rotary tillage without crop residues as a control (RTO). The results showed that SOC content under RT and CT treatments were significantly higher than that of RTO treatment. The percentage of soil aggregates of <0.001 mm, 0.005–0.001 mm, and 0.01–0.005 mm at 0–5, 5–10, and 10–20 cm layers under CT treatment were significantly higher than that of RTO treatment. The C‐HUM, C‐FAF, and C‐HAF contents were increased by combined application of tillage with crop residues practices. The results showed that C‐HUM, C‐FAF, and C‐HAF contents under CT and RT treatments were significantly higher than those under the RTO treatment at 0–5, 5–10, and 10–20 cm layers. As a result, it is an appropriate soil practice for increasing SOC content, soil aggregation, and humic substances under double‐cropping rice fields in southern China under conventional tillage and rotary tillage.

Disturbance, carbon physicochemical structure, and soil microenvironment codetermine soil organic carbon stability in oilfields

The stability of soil organic carbon (SOC) is crucial for soil quality, fertility, and natural attenuation processes of pollutants. The physicochemical structures of SOC were believed to control its stability, yet has become controversial. Here we hypothesized that disturbance intensity and variations in the soil environment can also influence the SOC stability, and conducted a case study with oil contaminated soils to quantify the contributions to SOC stability of various factors including contamination level, carbon physicochemical structure, and soil properties. Oil contamination led to increased SOC stability, as suggested by appreciably decreased soil CO2 fluxes, the enrichment of the δ13C in the oil contaminated soils, as well as analysis of soil aggregates and humic substances. Redundancy analysis indicated that overall SOC stability were highly correlated to microaggregate (M2), HA/FA, Fe, soil porosity, EC, pH, and total petroleum hydrocarbon (TPH) in oilfields. Variance partitioning analysis showed that carbon physicochemical structure (S), soil properties (P), and oil contamination (O) could explain the variance of overall SOC stability by up to 90%, while 18% of the variation was explained by S × P and 43% by S × P × O. These results show that multiple factors of the disturbance, carbon physicochemical structure, and soil properties should be essential for future studies of SOC stability.

Microbially reducible extent of solid-phase humic substances is governed by their physico-chemical protection in soils: Evidence from electrochemical measurements

A significant fraction of humic substances (HS) in natural soils is protected due to the interactions with mineral surfaces or occlusion within aggregates and clay microstructures, which could render the solid-phase HS less spatially accessible to microorganisms and thus affect their extracellular electron transfer. However, more diverse and convincing evidence is needed to further verify the link between extracellular electron transfer of solid-phase HS and their physico-chemical protection mechanisms in soils. In this study, soil physical fractionation and mediated electrochemical measurements were combined for the first time to assess the microbially reducible extents of solid-phase HS in mineral soils. The results show that the solid-phase HS in soil contain several pools with different microbially reducible extents, due to the fact that different pools employ distinct protection mechanisms of soil organic matter against microbial attacks; the particulate HS exert the greatest microbially reducible extent, followed by the microaggregate occluded HS and non-aggregated silt associated HS, and non-aggregated clay associated HS exert the lowest microbially reducible extent. Our work provides a new evidence to validate the view that the extracellular electron transfer process of solid-phase HS in natural soils is largely controlled by their physico-chemical protection mechanisms.

Current Understanding of the Use of Alkaline Extractions of Soils to Investigate Soil Organic Matter and Environmental Processes

A better understanding of soil organic matter (SOM) biogeochemistry is critical to meeting the environmental challenges that we face currently with the changing climate. Humic substances obtained from alkaline extractions of soils have played a predominant role in developing our knowledge of SOM chemistry and behavior during the 20th century. Recently, there have been new important developments that, according to some researchers, may be used to argue against the existence of the humic substances in soils as they are presently defined. A debate on the definition and continued use of the term “humic substances” has been taking place at conferences and in the scientific literature. However, because a consensus has not been reached on this important topic, the use of the term “humic substances” in this special issue conforms to the current existing definition of this term. This special issue of the Journal of Environmental Quality includes 10 papers on the subject of “Alkaline Extraction of Soils and Natural Waters to Understand Environmental Processes” and contributes to the ongoing assessment of the role of humic substances in current and future soil, environmental, and ecological studies. The contributed papers are grouped into three broad subject headings: (i) chemistry of humic substances, (ii) humic substances in environmental processes, and (iii) case studies and reviews of humic substances. Continued progress in understanding SOM and C biogeochemistry is essential to achieve long‐term ecological, environmental, and agricultural sustainability through the development of management strategies that conserve soil C and reduce the reliance on chemical inputs for meeting crop nutrient needs.Core Ideas This section aims to provide a wide‐ranging view of current humic substances research. Are humic substances appropriate proxies for soil organic matter? Better understanding of soil organic matter will enable future environmental sustainability.

Chemical and spectroscopic characterization of humic substances from sediment and riparian soil of a highly polluted urban river (Suquía River, Córdoba, Argentina)

The Suquía River, the largest urban river in Córdoba (Argentina), has been severely polluted for decades. Actions must be taken to restore its environmental quality by managing riparian zones for increased water-self purification. The current study aimed to characterize organic matter (OM) dynamics and humic substances (HS) spectrochemical properties along the lower-middle basin of the Suquía River. Riparian soil (0–20 cm) and sediment (0–10 cm) samples were collected from a reference location (S1) and four polluted sites (S2–S5) during a low-flow period. The contents of soil and sedimentary OM and HS fractions were analyzed by wet oxidation, as well as HS Fourier transform infrared (FT-IR) and ultraviolet–visible (UV–Vis) spectrochemical properties. The OM and HS fractions from riparian soil were high upstream of Córdoba City (S1 and S2, 50.2–50.4 g/kg OM) and within a 50 km downstream location (S5, 30.9 g/kg OM) owing to a surplus of fresh plant biomass-carbon (C) inputs. Highly heterogeneous sediment samples did not show any significant differences among sites (P > 0.05). The lowest values of the ratio of absorbances at 465 and 665 nm (E4/E6) (1.78) and the Δ log K (0.15) coefficient (a measure of HS maturity degree) were obtained downstream of Córdoba City, for both riparian soil and sediment, indicating that HS were enriched by more condensed aromatic structures within highly degraded portions of the river. All samples exhibited similar IR spectra, implying overlapping recalcitrant-C structures at the functional group level, but with different absorbance intensity. Data from the current study constitute a baseline for understanding the chemical nature of HS from sediment and riparian soil along the Suquía River and can be used as a reference for future studies tracking OM compositional changes over time.

Pyrolysis characteristics of soil humic substances using TG-FTIR-MS combined with kinetic models

The incorporation and cycling of pyrogenic organic matter in soil is a potential carbon sink, while the pyrolysis behaviors of soil organic matter are still lacking. Pyrolysis characteristics of soil fulvic acid (FA) and humic acid (HA) were investigated using thermogravimetry combined with Fourier transform infrared spectrometer-mass spectrometer (TG-FTIR-MS) and kinetic models. Four reaction stages corresponding to four pseudo-components were distinguished for both FA and HA. FA exhibited greater transformation contributions of hemicellulose-like and cellulose-like pseudo-components, while HA exhibited greater transformation contributions of lignin-like pseudo-components. Compared to HA, higher levels of heat-resistant aromatic compounds, phenolic groups, and carboxylic groups were recognized in FA. Values of both activation energy (Ea, 246.13–661.40 kJ·mol−1) and pre-exponential factor (lnA, 53.49–107.16 min−1) of FA were greater than corresponding Ea (241.74–466.70 kJ·mol−1) and lnA (51.99–74.36 min−1) values of HA determined by Flynn-Wall-Ozawa method and Distributed Activation Energy Model. The main pyrolysis reaction mechanisms of both FA and HA closely matched with the order-based model corresponding to 2nd and 3rd order random nucleation on an individual particle. The evolved gas species of H2, CH4, H2O, and CO2 were dominant for FA and HA pyrolysis. Generally, the total H2/CO2 and CH4/H2O releases were relatively larger for FA and HA pyrolysis, respectively. TG-FTIR-MS is shown to be an effective method to provide valuable and qualitative analysis of the gaseous volatile species evolved during HS pyrolysis. Findings from this systematic study of soil organic matter responding to pyrolysis will be critical for predicting the changes of soil systems or carbon cycle affected by future climate and fire regimes.

Photochemical Degradation of Organic Pollutants in Solutions of Soil Humic Acids

Chernozem humic acid (HA) fractions with nominal molecular sizes (MS) >100, 30–100, 5–30 and <5 kDa were obtained, using a combination of low-pressure preparative size exclusion chromatography with analytical polyacrylamide gel electrophoresis or multistep ultrafiltration. The initial HA and fractions were tested for the ability to destroy model aromatic compounds 2,4,6-trimethylphenol and furfurol in aqueous solution when illuminated with polychromatic light in the range 300–450 nm under laboratory conditions. It has been found that chernozem HA fractions with MS less than 5 kDa had the maximum ability to decompose the above-mentioned substances, while fractions with MS above 30 kDa practically did not perform this function. The data obtained allow us to explain partially the mechanisms of the photoinduced transformation of many organic pollutants by soil humic substances.

Cadmium (Cd) and Nickel (Ni) Distribution on Size-Fractioned Soil Humic Substance (SHS).

Soil humic substances (SHS) are heterogeneous, complex mixtures, whose concentration, chemical composition, and structure affect the transport and distribution of heavy metals. This study investigated the distribution behavior of two heavy metals [cadmium (Cd) and nickel (Ni)] in high molecular weight SHS (HMHS, 1 kDa–0.45 μm) and low molecular weight SHS (LMHS, <1 kDa) extracted from agricultural soils. The HMHS mass fractions were 45.1 ± 19.3%, 17.1 ± 6.7%, and 57.7 ± 18.5% for dissolved organic carbon (DOC), Cd, and Ni, respectively. The metal binding affinity, unit organic carbon binding with heavy metal ratios ([Me]/[DOC]), were between 0.41 ± 0.09 μmol/g-C and 7.29 ± 2.27 μmol/g-C. Cd preferred binding with LMHS (p < 0.001), while Ni preferred binding with HMHS (p < 0.001). The optical indicators SUVA254, SR, and FI were 3.16 ± 1.62 L/mg-C/m, 0.54 ± 0.18 and 1.57 ± 0.15, respectively for HMHS and 2.65 ± 1.25 L/mg-C/m, 0.40 ± 0.17, and 1.68 ± 0.12, respectively for LMHS. The HMHS contained more aromatic and lower FI values than LMHS. Multilinear regression showed a significant positive correlation between the measured predicted [Me]/[DOC] ratios (r = 0.52–0.72, p < 0.001). The results show that the optical indices can distinguish the chemical composition and structure of different size SHS and predict the binding ability of Me-SHS.

The Interaction Effect of Humic Substances and Mineral Phosphorus Fertilization on Forage Yield and some Macronutrients Uptake of Triticale under different Soil Salinity Levels.

Salinity becomes one of the most important and serious agricultural problems. Furthermore, it is an ever-present impendence to crop productivity, especially in the countries where irrigation is a determining factor for agricultural activity. It is known that crop matching is one of the ways to overcome the problem of soil salinity, such as triticale plant which was recognized as a crop suitable for growth in saline soils. Two field trials were conducted to study the interaction effect of soil salinity levels, Humic substances and mineral phosphorus fertilization, it can further illustrate how triticale cultivation in saline soils is developed by studying the impact on plant height, forage fresh and dry weight, total NPK uptake in dry forage. All privies parameters were depressed with increased salinity levels. While the order of Humic substances for their influences on triticale growth parameters and total N, P and K uptake were as follows: Falvic soil, Falvic foliar, Humic soil, Humic foliar, without Humic substances. While, applying 140 kg P ha-1 gave the highest results of previous parameters. While the interaction effect shows that when using 70 kg p ha-1 in low soil salinity (SL), or when using 140 kg p ha-1 in moderate soil salinity (SM), the effect of Humic substances will become clear on fresh and dry forage yield and total N, P and K uptake of triticale. Therefore, it is preferred that applying Humic substances and mineral phosphorus fertilization to the improvement of triticale production with increasing soil salinity.

Analysis of 129I and 127I in soils of the Chernobyl Exclusion Zone, 29 years after the deposition of 129I.

The Chernobyl Exclusion Zone (CEZ) represents a unique natural laboratory that received significant 129I contamination across a range of soils and land-use types in a short time period in 1986. Data are presented on 129I and 127I in soil samples collected from highly contaminated areas in the CEZ in 2015. The geometric mean (GM) total concentration of stable iodine (127I) was 6.7 × 10-7 g g-1 and the (GM) total concentration of 129I was 2.39 × 10-13 g g-1, equivalent to 1.56 mBq kg-1. GM total 127I concentration is below the European average soil concentration of 3.94 × 10-6 g g-1, while 129I is significantly higher than the pre-Chernobyl activity concentration for 129I of 0.094 mBq kg-1. Significant differences were found in the extractability of native, stable 127I and 129I almost 30 years after the introduction of 129I to the soils. Both 127I and 129I were predominantly associated with alkaline-extractable soil organic matter, established using a three-step sequential extraction procedure. Whereas 127I was significantly correlated with gross soil organic matter (measured by loss on ignition), however, 129I was not. The ratio of 129I/127I was significantly lower in extracts of soil organic matter than in more labile (soluble and adsorbed) fractions, indicating incomplete equilibration of 129I with native 127I in soil humic substances after 29 years residence time in the CEZ soils. The initial physico-chemical form of 129I in the CEZ soils is unknown, but the widespread presence of uranium oxide fuel particles is unlikely to have influenced the environmental behaviour of 129I. Our findings have implications for long-term radiation dose from 129I in contaminated soils and the use of native, stable 127I as a proxy for the long-term fate of 129I.

Effects of litter addition on the dynamics of soil humic substances during freeze-thaw events in a subalpine forest.

During soil formation, the accumulation of humic substances such as humic acid and fulvic acid is an important way to maintain soil fertility and nutrient cycling, which is regulated by soil substrate quality, litter, and environmental factors. In a laboratory incubation experiment, we exa-mined the effects of litter addition on soil humic substances accumulation in freeze-thaw environment by controlling the freeze-thaw cycles and litter additions in soils from the typical coniferous forest, mixed forest and broadleaved forest in a subalpine forest in western Sichuan. The freeze-thaw events significantly increased the content of humic substances in the coniferous forest soils but decreased those in the mixed forest soil and broadleaved forest soil. Litter addition had no significant effect on the content of soil humic substances. Freeze-thaw events increased the content of humic acid, with the net accumulation of humic acid following the order of mixed forest > coniferous forest > broad-leaved forest. Freeze-thaw events decreased the content of fulvic acid in the three forest soils during the early stage of incubation, and the degree of fulvic acid degradation was broadleaved forest > mixed forest > coniferous forest. Litter addition had no significant effect on the content of soil humic acid and fulvic acid. With prolonged incubation, the content of humic acid and fulvic acid in the three types of forest soils all declined. These results indicated that litter had effects on soil humic substances, which were related to soil substrate quality and affected by the duration of soil freeze-thaw events in winter.

Characterization of organic matter fractions in the top layer of soils under different land uses in Central‐Eastern Europe

AbstractThe objective of this study was to investigate differences in organic matter fractions, such as dissolved organic carbon and humic substances, in soils under different land uses. Soil samples were collected from the upper layer of arable lands and grasslands. Humic substances (HS) were chemically fractionated into fulvic acids (FA), humic acids (HA) and humins (HUM), and based on the separated fractions, the humification index (HI) and the degree of HS transformation (DT) were calculated. Dissolved organic carbon (DOC) was determined by cold (CWE) and hot water (HWE) extractions. Regardless of land use, the results indicated significant differences in soil organic carbon (SOC) and HS composition, with HA and HUM as the dominant fractions. Total SOC was higher in grassland (median = 17.51 g kg−1) than arable soils (median = 9.98 g kg−1); the HI and DT indices did not differ significantly between land uses (HI = 0.3–10.3 and DT = 0.2–6.2 for grasslands, p &gt; 0.05; HI = 0.3–3.9 and DT = 0.2–20.1 for arable lands, p &gt; 0.05). This indicates the relatively high stability of organic carbon and efficient humification processes in both land uses. Additionally, in arable soils lower CWE‐C (0.75 g kg−1) and higher HWE‐C (2.59 g kg−1) than in grasslands (CWE‐C = 1.13 g kg−1, HWE‐C = 1.60 g kg−1) can be related to farming practice and application of soil amendments. The results showed that both labile and humified organic matter are better protected in grassland soils and are consequently less vulnerable to mineralization.

The (Bio)Chemistry of Soil Humus and Humic Substances: Why Is the “New View” Still Considered Novel After More Than 80 Years?

Three years ago, a novel “soil continuum model” was proposed, in which soil organic matter was suggested to be of heterogeneous composition and to consist of a continuum of organic fragments of all sizes. A search of the literature reveals that this model is identical to several similar conceptualizations proposed about 15 years ago, and that it corresponds closely with the description of humic substances given in Waksman’s (1936) remarkably thorough book on the topic, that also emphasized the intimate connections existing between humic substances and soil microorganisms. Several historical reasons, reviewed in this Perspective article, may explain why Waksman’s viewpoint might still be considered novel more than 80 years later. Here we argue that the key reason for the agonizingly slow rate of progress in the field is linked to the extreme compartmentalization of research and education in soil science, which has been organized along distinct subdisciplines, with the result that interdisciplinary efforts that are desperately needed to understand the dynamics of soil humic substances are very hard to launch. To meet growing demands on soils we need to understand the mechanisms that underpin their many functions. To gain this understanding and finally make badly needed progress, we must reorganize funding and educational efforts to support exploration of “Waksman’s frontier”, which includes the microscale where the microbial, physical and biochemical processes governing organic matter turnover occur.

Changes in soil organic matter and soil humic substances following the afforestation of former agricultural lands in the boreal-nemoral ecotone (Latvia)

Abandonment of agricultural land is a widespread process in Northern Europe, which is associated with changes in content and distribution of organic matter in soil, including C stocks. There is insufficient information on changes in soil organic matter properties during afforestation of these lands. The aim of the study was to determine and describe the influence of afforestation on organic carbon (CORG) content and properties of soil organic matter (SOM) in mineral topsoil in the boreo-nemoral ecotone in Latvia. We studied soils in 4 model territories that represented abandoned farmlands and territories where mixed or coniferous forests were established at different times on former agricultural lands. Soil samples from mineral topsoil (genetic A or AE horizons) were obtained during the summer period in two model territories (11 sampling sites) with loamy texture soil, and two model territories with higher sand content of soil (9 sampling sites). The soil samples were analysed for organic carbon (CORG) and total nitrogen (NTOT) content. Soil alkali (NaOH) extracts were used to determine properties of soil organic matter – alkali extractable carbon (CHS) content, composition and humification index (HIX) of soil humic substances (HS) and spectroscopic (UV–Vis, fluorescence emission spectra, FTIR) properties of the soil humic acid fraction (HA). CORG content gradually increased with forest land age. Gradual decrease in the humic acid (HA) to fulvic acid (FA) ratio (CHA/CFA) after land-use change indicated that SOM was transported to mineral topsoil mainly through leaching of the soluble FA fraction. FTIR spectra revealed that the HA fraction from all studied soils had relatively similar chemical structure and that SOM in mineral topsoil was degraded to relatively similar compounds. However, HAs extracted from forest soils had higher abundance of –OH groups, more aromatic CC structures, and relatively lower abundance of NH groups in amides. UV–Vis and fluorescence emission spectroscopic analysis of HA fraction along with PCA analysis showed that soil properties also may have affected chemical properties of soil organic matter during the afforestation. Molecular weight of the HA fraction gradually increased with age of the forest land in sandy soils while there were no significant changes in sandy loam soils. Fluorescence emission analysis also showed that HAs extracted from EAh horizons in Podzol and Anthrosol soils in old forest lands had the highest aromaticity.The results suggest that sandy soils have the most dynamic changes in SOM properties after afforestation. Therefore, further studies should focus on whether and how these changes affect SOM fate in soils, physical stabilisation with mineral soil and SOM leaching into subsoil.

Soil organic carbon fractions and humic substances are affected by land uses of Caatinga forest in Brazil

Caatinga is a Brazilian dry ecosystem that occupies around 1 million km2 and is one of the largest tropical dry forests of the world. About 46% of the area that was originally covered has been deforested. Land use can cause pronounced reduction in soil carbon stocks that play a major role in the global carbon cycle. The objective of this study was to improve our understanding of the effect of land use on oxidizable carbon fractions, total carbon stocks and humic substances in different layers of soil in a Brazilian semi-arid region. We analyzed soils from tropical dry forest (TDF), forest succession with Anadenanthera falcata (ANA), with Tabebuia alba (TAB), secondary scrubby regeneration (SCR), and non-irrigated maize (MS). Forests showed larger fractions of more labile carbon, except for TDF. The most recalcitrant fraction of carbon stock, humin fraction stock, with the different land use decreased by 38–53% compared to TDF. Oxidizable carbon fractions, carbon stocks, and humic fraction stocks were able to differentiate the successional land uses and agricultural cover from TDF, mainly in the 0–5 cm layer. Our results show that changes in land use, especially with ANA forest succession, showed a larger labile carbon fraction, indicating easy decomposition and loss. Our results provide an alternative tool for the management of deforested areas in tropical dry caatinga ecosystems. This would contribute to the conservation of dry forest systems and could serve as guideline for sustainable management of agriculturally impacted caatinga areas.

Characterization of Humic Substances in the Soils of Ophiocordyceps sinensis Habitats in the Sejila Mountain, Tibet: Implication for the Food Source of Thitarodes Larvae.

Humic substances in soil are considered to be an alternative food to the tender plant roots for Thitarodes larvae in the habitats of Ophiocordyceps sinensis in the Qinghai-Tibetan Plateau. However, there is no report involving the evaluation of their potential as a food source from the composition and structure of habitat soils. In this work, the composition and structure of humic substances in habitat soils from the Sejila Mountain, Tibet were characterized by diverse techniques for evaluating the nutritional value and possibility of humus as the food source for Thitarodes larvae. Fourier transform infrared spectroscopy revealed that humic acid may possess superior ability to provide the molecular segments for biosynthesizing lipids more than other humic fractions. Combining with the analysis of solid-state 13C nuclear magnetic resonance spectrum, the fractions of hydrophobic fulvic acid and hydrophilic fulvic acid are further considered as a potential food source for Thitarodes larvae. Overall, humic substances in habitat soils are rich in the molecular segments for biosynthesizing lipids and other important nutrients, which may provide the energy and material sources for maintaining the survival of Thitarodes larvae in the absence of tender plant roots, particularly in the annual cold winter. Combining with the evidence of physico-chemical parameters of habitat soils and stable carbon isotopic composition of major tender plant roots in the Sejila Mountain, the composition and structure of humic substances in habitat soils may provide a novel idea for the eco-friendly and semi-wild cultivation of Thitarodes larvae with low cost.

Evolution over years of structural characteristics of humic acids in Black Soil as a function of various fertilization treatments

Humic substances are the relatively stable and recalcitrant pool of soil organic carbon in the agricultural system. Humic acid (HA) is an active part of soil humic substances. However, little information exists on how structural characteristics of HA evolved with fertilization years in Black Soil. Soil samples were collected in 1997, 2002, 2008, and 2012 from the Long-term Fertilization Station of Black Soil. The methods of 13C-nuclear magnetic resonance spectroscopy, elemental composition analysis, and infrared spectroscopy were employed to analyze the structure of HA. Long-term fertilization had no significant (P > 0.05) effect on the ratio of O and S to C in HA. The ratio of H to C in HA increased with fertilization years after the application of organic manure (M) single or combined with chemical NPK fertilizers. The ratios of aliphatic C to aromatic C, and alkyl C to O-alkyl C in soil HA increased by 7.72% and 20.3%, respectively, in MNPK and M treatments, whereas NPK treatment declined the ratio of aliphatic C to aromatic C by 2.67% compared with CK treatment. The HA structure tends to become aliphatic and simplified with years of organic manure combined with NPK fertilizers. To save costs, organic manure was only applied once within the period of crop rotation and was combined with NPK every year to satisfy crop demands for soil nutrients.

Long-term fertilization decreases chemical composition variation of soil humic substance across geographic distances in subtropical China

Many previous research have investigated the spatial distribution of the content of soil organic matter (SOM) and soil humic substance (HS), while few work focused on the chemical composition variation of HS across geographic distance. Paddy soils, collected from different long-term fertilization treatments across a wide geographic area in subtropical China, were used to evaluate the spatial variation of HS chemical composition. The 3-dimensional fluorescence spectroscopy combined with parallel factor analysis was used to evaluate the chemical composition of the soil extractable humic substance (EHS, including humic acid and fulvic acid). Analysis of distance-decay relationship (DDR) was introduced here to study the spatial variation of the chemical compositions of these EHSs. It was found that geographic location impose greater impact on the compositional variation of soil EHS than fertilization practice. Fertilization treatments including NPK (mineral N, P and K fertilizers) and NPKS (NPK fertilizers plus straw return) exhibited significant flattened DDR slopes as compared to CK (no fertilizers), which indicated fertilization (NPK and NPKS) decreased the variation of EHS chemical composition in paddy soils across geographic distance. A positive correlation was found between variations of soil bacterial community and EHS chemical composition, and fertilization (NPK and NPKS) enhanced their relationships. This work demonstrated that although the geographic location mainly dominated the spatial variation of the soil EHS chemical composition across geographic distance, the same long-term fertilization practice could gradually domesticate the paddy soils to display more similar EHS composition.