Organic Carbon Sequestration in Soil Humic Substances As Affected by Application of Different Nitrogen Fertilizers in a Vegetable-Rotation Cropping System

Molecular Weight Distribution of Humic Acids Isolated from Calcic Cryosol in Central Yakutia, Russia

The transition of soils into fallow state has a significant impact on the accumulation and transformation of soil organic matter (SOM). However, the issue of SOM transformation as a result of soil transition to fallow state in cryolithozone conditions is insufficiently studied. The aim of this study is to investigate the molecular weight (MW) distribution of humic acids (HAs) isolated from soils of central Yakutia. Native, fallow and agricultural soils in the vicinity of Yakutsk city were studied. MW distributions of HA preparations were obtained on an AKTAbasic 10 UPS chromatographic system (Amersam Biosciences, Uppsala, Sweden) using a SuperdexTM 200 10/300 GL column (with cross-linked dextran gel, fractionation range for globular proteins 10-600 kDa). The data on the molecular-mass distribution of HAs of fallow and agricultural soils of Central Yakutia were obtained for the first time. According to the obtained data, it was found that the highest carbon content in the structure of HAs was observed in agricultural soils (52.56%), and is associated with soil cultivation and fertilizer application. Among the HAs of fallow soils, we note that those soils that are in the process of self-vegetation have a relatively high carbon content in the HAs (45.84%), but the highest content was observed in fallow soils used as hayfields (49.98%), indicating that the reinvolvement of agriculture in fallow soils leads to an increase in the carbon content of HAs. According to the data of the MW distribution of HAs, it was found that the highest content of a high MW fraction of HAs was recorded in native soil (18.8%); this is due to the early stages of humification and the low maturity of organic matter. The highest content of a low MW fraction of HAs was recorded in agricultural soil (73.3%); this is due to the formation of molecular complexes of a "secondary" nature, which are more stable in the environment than the primary transformation products of humification precursors. The molecular composition of the HAs of fallow soils in the process of self-overgrowing is characterized by values closer to the HAs of native soils, which indicates their transformation towards HAs of native soils. The obtained results indicate that the reinvolvement of fallow soils leads to the transformation of the molecular composition of HAs towards HAs of agricultural soils, and to an increase in the resistance of SOM to biodegradation.

Response of electron transfer capacity of humic substances to soil microenvironment

The humic substances (HS) - mediated electron transfer process is of great significance to the reduction and degradation of pollutants and the improvement of soil quality. Different soil conditions lead to different characteristics of HS, resulting in differences in the electron transfer capacity (ETC) of HS. It is unclear how the environmental conditions in soil affect the ETC by affecting on HS. In this study, the response relationship of soil microenvironment, HS and ETC has been studied. The results show that the ETC follows the descending order of: Langshan > Nanchang > Anqing > Beijing > Guilin. There were significant differences in ETC in soil HS in different regions. There were significant differences in electron-donating capacity (EDC) in soil HS in different regions and depths. EDC in soil was higher than electron-accepting capacity (EAC), and on average, are 22.4 times higher than the EAC. The HS components of soils in different regions are different. The most significant differences were in tyrosine-like substances and soluble microbial by-products (SMPs). The five components of the soil HS from Langshan were the most different from those in other regions. There were differences in SMPs and humic-like substances in soils of different depths in Anqing and Guilin. ETC can be affected by the composition of HS components in different regions. The composition of HS at different soil depths in the same regions had little effect on ETC. SMPs can promote ETC and EDC, and tyrosine-like substance can promote EDC. Moisture content, pH and TOC are the main factors affecting the composition of HS components. This results can provide a research basis for the sustainable and safe utilization of agricultural soil.

Effects of continuous and rotational cropping practices on soil fungal communities in pineapple cultivation

Background

Rotational cropping practices can change the fungal structure and diversity of cropping soil, and these changes can promote crop development. However, only a few studies have explored the effects of rotational cropping of pineapple on soil fungal diversity.

Methods

In this study, we investigated fungal diversity in continuous and rotational cropping soil of pineapple in Xuwen and Leizhou of China in summer and winter through high throughput sequencing of the fungal internal transcribed spacer region.

Results

The diversity and richness of the fungal community were observed to be significantly increased after rotational cropping in Xuwen and Leizhou in summer, whereas no changes were observed in winter. Furthermore, Ascomycota, Basidiomycota, Zygomcota, and Chytridiomycota were the dominant phyla, and Chaetomium, Penicillium, Fusarium, Trichoderma, and Cryptococcus were the dominant genera in the continuous and rotational cropping soil of pineapple, respectively, in both summer and winter. Chytridiomycota at phylum level and Gibberella at genus level were observed in rotational cropping soil; however, Ascomycota at the phylum level and Chaetomium at the genus level were the most abundant fungi, and their abundance dramatically decreased in continuous cropping soil. Redundancy analysis revealed that rotational cropping reduced the correlation between environmental parameters and the fungal community in winter. In addition, several fungal biomarkers were found in Xuwen in both continuous and rotational cropping soil samples, including Sporobolomyces, Aspergillus, Corynascus sp JHG 2007, and Corynascus at the genus level, Penicillium and fungal sp p1s11 at the species level in rotational cropping soil, and ales family Incertae sedis and Sordariomycetes at the class level in continuous cropping soil. These results revealed the changes in the structure and diversity of fungal community in continuous and rotational cropping practices for pineapple cultivation, which may be associated with crop yield and quality.

Natural and artificial humic substances to manage minerals, ions, water, and soil microorganisms

The chemistry of humic substances (HSs) occurs hidden from our sight, but is of key importance to agriculture and the environment, and nowadays even to medicine and technology. HSs are nowadays not only natural, but extracted and engineered, and in the past 20 years such products have been widely used in soil improvement and environment governance. In this review, we collate and summarize the applications and working principles of such HSs in agriculture and environmental ecology, mainly to elaborate the multiple roles of this functional polymer along with physical chemical quantification. Then several of the latest synthesis technologies, including hydrothermal humification technology (HTH), hydrothermal carbonization technology (HTC) and hydrogen peroxide oxidation technology (HOT) are presented, which were introduced to prepare artificial humic substances (A-HSs). The availability of reproducible and tunable synthetic A-HSs is a new chemical tool, and effects such as solubilization of insoluble phosphorus minerals, recovery of phosphorus, improvement of soil fertility for crop growth and reduction of toxicity of typical pollutants, can now be analyzed in detail and quantified. As a result, we can provide an effective chemical technology for utilizing biomass side products ("biowaste") to generate A-HSs of different types, thus realizing improvement in agricultural production and control of environmental pollution by the macro-synthesis of A-HSs-.

Water-soluble mercury induced by organic amendments affected microbial community assemblage in mercury-polluted paddy soil

Mercury (Hg) pollution or organic amendments (OA) may individually induce changes in the microbial community of paddy soils. However, little is known regarding the interaction of Hg and OA and the effect of different OA applications on the microbial community assemblage in Hg-polluted paddy soil. A soil incubation experiment was performed by applying three organic amendments (OA), namely a food-waste compost (FC), and its HA and FA, into an Hg-polluted paddy soil to examine the changes in the microbial community and merA/merB gene abundance. The results showed that the OA treatments promoted total (SOC) and dissolved organic carbon (DOC) in soils, which may harbor copiotrophic bacteria. The HA and FA treatments decreased microbial diversity and richness along with an increase of water-soluble Hg (WHg) through the complexation of DOC to Hg, which may be mainly attributed to the enhanced Hg biotoxicity to soil microbiome induced by the increased WHg under these two treatments. Additionally, the WHg enhancement also contributed to the increase of Hg-resistant bacteria and merA/merB gene abundance, and consequently, induced changes in the microbial community. These results indicated the interaction of Hg and different OA induced the variation of WHg fraction in paddy soil, which played a fundamental role in the distinct responses of the microbial community assemblage. Collectively, the application of FA and HA to Hg-polluted soil should be limited considering Hg risk to microbiome, and FC can be an alternative.

Organic amendments affect dissolved organic matter composition and mercury dissolution in pore waters of mercury-polluted paddy soil

Organic amendments (OA) have been applied in many mercury (Hg)-polluted paddy soils to meet increasing food demands with scarce land resources. However, little is known on the effects of different OAs on Hg dissolution and the composition of dissolved organic matter (DOM) in soil pore waters, both of which may be associated with Hg mobility. Consequently, DOM composition and Hg release levels were investigated in soil pore waters after applying food waste compost (FC), fulvic acids (FA) and humic acids (HA) to Hg-polluted paddy soils. FA and HA treatments promoted increased abundances of humic- and fulvic-like substances in pore water DOM while FC amendment increased soluble microbial by-products. FA amendment and high levels of both HA and FC amendments greatly promoted Hg dissolution in pore waters that could be attributed to the complexation of Hg with different DOM components. However, among all DOM components, only UVA fulvic and visible humic-like substances were positively correlated with Hg release levels and total organic carbon. These results indicate that discrepant DOM compositions are induced by different OA. Further, these differences may be associated with differential Hg dissolution in pore waters. Consequently, FA amendment and high level of FC or HA amendments should be limited to reduce potential Hg release into pore waters.

Demonstration of Chemical Distinction among Soil Humic Fractions Using Quantitative Solid-State 13C NMR

Humic substances (HS) are vital to soil fertility and carbon sequestration. Using multiple cross-polarization/magic-angle spinning (multiCP/MAS) NMR combined with dipolar dephasing, we quantitatively characterized humic fractions, i.e., fulvic acid (FA), humic acid (HA), and humin (HM), isolated from two representative soils (upland and paddy soils) in China under six long-term (>20 years) fertilizer treatments. Results indicate that each humic fraction showed chemical distinction between the upland and paddy soils, especially with much greater aromaticity of upland HMs than of paddy HMs. Fertilizer treatment exerted greater influence on chemical natures of upland HS than of paddy HS, although the effect was less than that of soil type. Organic manure application especially decreased the percentages of aromatic C in the upland HAs and HMs compared with the control. We concluded that humic fractions responded in chemical nature to environmental conditions, i.e., soil type/cropping system/soil aeration and fertilizer treatments.