A stoichiometric approach to estimate sources of mineral-associated soil organic matter.

Mineral-associated soil organic matter (MAOM) is the largest, slowest cycling pool of carbon (C) in the terrestrial biosphere. MAOM is primarily derived from plant and microbial sources, yet the relative contributions of these two sources to MAOM remain unresolved. Resolving this issue is essential for managing and modeling soil carbon responses to environmental change. Microbial biomarkers, particularly amino sugars, are the primary method used to estimate microbial versus plant contributions to MAOM, despite systematic biases associated with these estimates. There is a clear need for independent lines of evidence to help determine the relative importance of plant versus microbial contributions to MAOM. Here, we synthesized 288 datasets of C/N ratios for MAOM, particulate organic matter (POM), and microbial biomass across the soils of forests, grasslands, and croplands. Microbial biomass is the source of microbial residues that form MAOM, whereas the POM pool is the direct precursor of plant residues that form MAOM. We then used a stoichiometric approach-based on two-pool, isotope-mixing models-to estimate the proportional contribution of plant residue (POM) versus microbial sources to the MAOM pool. Depending on the assumptions underlying our approach, microbial inputs accounted for between 34% and 47% of the MAOM pool, whereas plant residues contributed 53%-66%. Our results therefore challenge the existing hypothesis that microbial contributions are the dominant constituents of MAOM. We conclude that biogeochemical theory and models should account for multiple pathways of MAOM formation, and that multiple independent lines of evidence are required to resolve where and when plant versus microbial contributions are dominant in MAOM formation.

Consequences of 33 Years of Plastic Film Mulching and Nitrogen Fertilization on Maize Growth and Soil Quality.

Plastic film mulching and urea nitrogen fertilization are widely used in agricultural ecosystems, but both their long-term use may leave a negative legacy on crop growth, due to deleterious effects of plastic and microplastic accumulation and acidification in soil, respectively. Here, we stopped covering soil with a plastic film in an experimental site that was previously covered for 33 years and compared soil properties and subsequent maize growth and yield between plots that were previously and never covered with the plastic film. Soil moisture was about 5-16% higher at the previously mulched plot than at the never-mulched plot, but NO3- content was lower for the former when with fertilization. Maize growth and yield were generally similar between previously and never-mulched plots. Maize had an earlier dough stage (6-10 days) in previously mulched compared to never-mulched plots. Although plastic film mulching did add substantial amounts of film residues and microplastic accumulation into soils, it did not leave a net negative legacy (given the positive effects of the mulching practice in the first place) for soil quality and subsequent maize growth and yield, at least as an initial effect in our experiment. Long-term urea fertilization resulted in a pH decrease of about 1 unit, which bring a temporary maize P deficiency occurring in early stages of growth. Our data add long-term information on this important form of plastic pollution in agricultural systems.

Dynamics of macroplastics and microplastics formed by biodegradable mulch film in an agricultural field.

Conventional plastic mulch brings agronomic and economic benefits to crop production, but a large amount of plastic waste amasses when the mulch is removed from the fields after harvest. Soil-biodegradable plastic mulch (BDM) has emerged as a promising alternative to conventional plastic mulch as it can be tilled into the soil after harvest, thereby alleviating disposal problems. However, direct evidence on complete degradation of biodegradable mulch under natural conditions is still lacking. We quantified the dynamics of macro- (>5 mm) and microplastics (0.1-5 mm in size) in four years after a one-time application of mulch in a field with monoculture maize. The BDM feedstock was polybutyleneadipate-co-terephthalate (PBAT) and polylactic acid (PLA)-based, and both a clear and black BDM were tested. The BDM plastic mulch films degraded into macro- and micoplastics. Macroplastics disappeared 2.5 years after mulch incorporation. We developed a new extraction method for biodegradable microplastics using a sequential density fractionation approach with a H2O and a ZnCl2 solution. Microplastic concentrations in the soil ranged from 350 to 525 particles/kg after 2.5 years, 175 to 250 particles/kg after 3 years, and 50 to 125 particles/kg after 3.5 year following mulch incorporation. This continuous decrease of detectable plastic particle concentrations in soil suggests that BDMs fragment degrade into smaller and smaller particles, which eventually may biodegrade completely. While we cannot ascertain whether persistent and undetectable nanoplastics may form, macro- and microplastics formed from BDM seem to disappear with time.

Erratum: How ß-cyclodextrin- loaded mesoporous SiO2 nanospheres ensure efficient adsorption of rifampicin.

[This corrects the article DOI: 10.3389/fchem.2022.1040435.].

The addition of discrimination inhibitors stimulations discrimination potential and N2O emissions were linked to predation among microorganisms in long term nitrogen application and straw returning systems.

Introduction: Ammonia oxidizing archaea (AOA) and ammonia oxidizing bacteria (AOB) have been proven to be key microorganisms driving the ammonia oxidation process. However, under different fertilization practices, there is a lack of research on the impact of interaction between predators and AOA or AOB on nitrogen cycling at the multi-trophic level. Methods: In this study, a network-oriented microscopic culture experiment was established based on four different long-term fertilization practices soils. We used the nitrification inhibitors 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxide-3-oxyl (PTIO) and 3, 4-Dimethylpyrazole phosphate (DMPP) inhibited AOA and AOB, respectively, to explore the impact of interaction between protists and AOA or AOB on nitrogen transformation. Results: The results showed that long-term nitrogen application promoted the potential nitrification rate (PNR) and nitrous oxide (N2O) emission, and significantly increased the gene abundance of AOB, but had no obvious effect on AOA gene abundance. DMPP significantly reduced N2O emission and PNR, while PTIO had no obvious effect on them. Accordingly, in the multi-trophic microbial network, Cercozoa and Proteobacteria were identified as keystone taxa of protists and AOB, respectively, and were significantly positively correlated with N2O, PNR and nitrate nitrogen. However, Nitrososphaerota archaeon as the keystone species of AOA, had an obvious negative linkage to these indicators. The structural equation model (SEM) showed that AOA and AOB may be competitors to each other. Protists may promote AOB diversity through direct trophic interaction with AOA. Conclusion: The interaction pattern between protists and ammonia-oxidizing microorganisms significantly affects potential nitrification rate and N2O emission, which has important implications for soil nitrogen cycle.

How ß-cyclodextrin- loaded mesoporous SiO2 nanospheres ensure efficient adsorption of rifampicin.

In this study, ß-CD@mesoporous SiO2 nanospheres (ß-CD@mSi) were prepared by loading ß-cyclodextrin (ß-CD) onto mesoporous silica nanospheres through an in situ synthesis. This not only solved the defect of ß-CD being easily soluble in water, but also changed the physical structure of the mesoporous silica nanospheres. FTIR and XPS results showed that ß-CD was successfully loaded onto mesoporous silica nanospheres (mSi), while enhancing the adsorption effect. ß-CD@mSi with a monomer diameter of about 150 nm were prepared. At a temperature of 298k, the removal efficiency of a 100 mg/L solution of rifampicin can reach 90% in 4 h and the adsorption capacity was 275.42 mg g-1 at high concentration. Through the calculation and analysis of adsorption kinetics, adsorption isotherms and adsorption thermodynamics based on the experimental data, the reaction is a spontaneous endothermic reaction dominated by chemical adsorption. The electron transfer pathway, structure-activity relationship and energy between ß-CD@mSi and rifampicin were investigated by quantum chemical calculations. The accuracy of the characterization test results to judge the adsorption mechanism was verified, to show the process of rifampicin removal by ß-CD@mSi more clearly and convincingly. The simulation results show that π-π interaction plays a major interaction in the reaction process, followed by intermolecular hydrogen bonding and electrostatic interactions.

The active functional microbes contribute differently to soil nitrification and denitrification potential under long-term fertilizer regimes in North-East China.

Nitrogen (N) cycling microorganisms mediate soil nitrogen transformation processes, thereby affecting agricultural production and environment quality. However, it is not fully understood how active N-cycling microbial community in soil respond to long-term fertilization, as well as which microorganisms regulate soil nitrogen cycling in agricultural ecosystem. Here, we collected the soils from different depths and seasons at a 29-year fertilization experimental field (organic/chemical fertilizer), and investigated the transcriptions of N-cycling functional genes and their contribution to potential nitrification and denitrification. We found that long-term fertilization exerted significant impacts on the transcript abundances of nitrifiers (AOA amoA, AOB amoA and hao) and denitrifiers (narG and nosZ), which was also notably influenced by season variation. The transcriptions of AOA amoA, hao, and narG genes were lowest in autumn, and AOB amoA and nosZ transcript abundances were highest in autumn. Compared to no fertilization, soil potential nitrification rate (PNR) was reduced in fertilization treatments, while soil potential denitrification rate (PDR) was significantly enhanced in organic combined chemical fertilizer treatment. Both PNR and PDR were highest in 0-20 cm among the tested soil depths. Path model indicated active nitrifiers and denitrifiers had significant impact on soil PNR and PDR, respectively. The transcriptions of AOA amoA and nxr genes were significantly correlated with soil PNR (Pearson correlation, r > 0.174, p < 0.05). Significant correlation of napA and nosZ transcriptions with soil PDR (Pearson correlation, r > 0.234, p < 0.05) was also revealed. Random forest analysis showed that SOC content and soil pH were the important factors explaining the total variance of active nitrifers and denitrifiers, respectively. Taken together, long-term fertilization regimes reduced soil PNR and enhanced PDR, which could be attributed to the different responses of active N-cycling microorganisms to soil environment variations. This work provides new insight into the nitrogen cycle, particularly microbial indicators in nitrification and denitrification of long-term fertilized agricultural ecosystems.

Contrasting response of fungal versus bacterial residue accumulation within soil aggregates to long-term fertilization.

Soil microorganisms are critical for soil carbon (C) cycling. They primarily regulate the turnover of the soil organic C (SOC) by adjusting their community structure, and contributing residues with a considerable amount to the resistant SOC. Nevertheless, how long-term fertilization (e.g., the combination of manure and chemical fertilizer) affects the spatial distribution of both living microbial communities and dead microbial residue within soil aggregate fractions remains largely unclear. In this study, we analyzed changes in microbial community (lipid biomarkers) and microbial residue retention (amino sugar biomarkers), and also calculated the contribution of microbial residue to organic C in bulk soil and different soil aggregates (> 2 mm, 1-2 mm, 0.25-1 mm, and < 0.25 mm) in Alfisols treated with 29 years fertilization or no fertilization (control). Our results showed that long-term fertilization significantly increased the mean weight diameter (MWD) of aggregates and organic C contents in all aggregate fractions. The fertilization treatment increased the contents of PLFAs and microbial residue C, but the relative contribution of microbial residue to SOC was higher in the control (56.8% vs. 49.0%), due to the low SOC background caused by much lower level of non-microbially derived C input. These results suggested that long-term fertilization could increase SOC by accumulating both plant- and microbial-derived C, while the C deficient soil is more dependent on the accumulation of microbial residues. Long-term fertilization promoted the enrichment of bacterial-derived muramic acid in micro aggregates, but increased the proportion of fungal-derived glucosamine in macro aggregates. Meanwhile, the contribution of bacterial residue to organic C in the fertilization treatment was higher in micro aggregates (7.6% for > 2 mm vs. 9.2% for < 0.25 mm aggregate), while the contribution of fungal residue was higher in macro aggregate fractions (40.9% for > 2 mm vs. 35.7% for < 0.25 mm aggregate). The above results indicated that long-term fertilization could drive the differentiation of heterogeneous microbial residue accumulation patterns that significantly alter the contribution of fungal- versus bacterial-derived C to organic C within soil aggregate fractions.