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Plastic film mulching and fertilization strongly affect soil aggregation and dynamics of the total soil organic carbon (C) pool. However, there is limited information on how these agricultural management practices influence the fate and seasonal dynamics of crop residue-derived C in soil aggregates. Therefore, a better understanding of the fate of C derived from crop residues and their location in soil aggregates is crucial to improve our prediction of C sequestration and stabilization in soil. In this study, an in situ 13C-tracing technique was used to identify the dynamic distribution and accumulation patterns of crop residue-derived C in soil aggregates as affected by long-term plastic film mulching and four fertilization treatments (control, CK; nitrogen, N2; organic manure, M2; nitrogen and organic manure, M1N1). The fate of 13C-labeled maize straw in loam soil was studied over 360 days using in situ incubation of 0.2% equivalent dry straw incorporated into the soil and aggregate size fractionation. Soil samples were separated into four particle-size fractions [large (>2 mm) and small (1–2 mm) macroaggregates; large (0.25–1 mm) and small (<0.25 mm) microaggregates] by dry-sieving. Long-term (27-year) application of fertilizers significantly increased the soil organic carbon (SOC) content in brown earth compared to the no-fertilizer control at the onset of our annual study, with the order of M2 > M1 N1 > N2 > CK. Both the content of straw-derived 13C in aggregate fractions and the proportion of 13C in total soil samples decreased considerably from the microaggregate fractions and increased moderately in the macroaggregate fractions in a manner enhanced by plastic film mulching and by seasonal transitions, specifically from spring (day 0) to summer (day 60). In addition, the decomposition of maize straw and soil aggregation exhibited a direct correlation in which the content of 13C-SOC and mean weight diameters decreased over the course of the 360-day experiment. Our results suggest that certain amounts of straw-derived 13C could accumulate in microaggregates, which play an important role in long-term C sequestration, while a large part of straw-derived C tends to transfer from microaggregates to macroaggregates over time as affected by long-term plastic film mulching coupled with fertilization. This study improves our understanding of the effect of plastic film mulching and different fertilization regimes on the retention and stabilization processes of straw-derived C in soil.

The aims of this study were to investigate the composition of clay minerals in soils derived from different parent materials and to elucidate how parent materials and pedogenic environment affect the distribution of clay minerals and reveal the implications for pedogenetics and taxonomy in Stagnic Anthrosols. Clay mineralogy and physicochemical properties of the Hydragric horizon of Stagnic Anthrosols derived from granite (GR), plate shale (PS), quaternary red clays (QRC), limestone (LS), purple sandy shale (PSS) and fluvial-lacustrine deposit (FLD) located in Hunan Province of China were analysed to explore the relationships between the conditions influencing the formation of the soil and the composition of clay minerals. Results indicated that the composition of clay minerals is closely related to both parent material and type of Stagnic Anthrosols: the soils derived from GR, PS and QRC, which are mostly classified as Fe-accumulic-Stagnic Anthrosols, are dominantly 1:1 type kaolinite and vermiculite and illite/vermiculite mixed layer minerals of widespread distribution. However, soils derived from LS, PSS and FLD were mainly classified as Hapli-Stagnic Anthrosols and are mainly composed of 2:1 type illite/smectite mixed layer minerals, where chlorite is commonly found. Illite is widely distributed and its content varies the least among different parent materials. An extremely significant relationship between pH and kaolinite, chlorite and mixed layer minerals was noted, and the two kinds of mixed layer minerals showed highly significant negative correlation. This study revealed that the types and quantities of clay minerals in the soil are closely related to the types of parent material. This reflected better direction and degree of development in Stagnic Anthrosols, which is related to the physicochemical properties of parent material and can be used as one of the bases for the classification of soil groups and subgroups within the soil family for Stagnic Anthrosols in Chinese Soil Taxonomy.

Microwave (MW) heating has been identified as a potential cost-effective technique to remediate hydrocarbon-polluted soils; however, the soil texture and properties could have a great impact on its full-scale treatment. In addition, very limited energy and economical data on MW treatment are available, and this lack makes its real application very limited. In this work, a first experimental phase was performed simulating a MW of several hydrocarbon-polluted soils. Obtained data were elaborated for a techno-economic analysis. Four soil textures, corresponding to medium, fine silica sand (at different soil moistures), silt as silica flour and clay as kaolin, were artificially contaminated with diesel fuel and irradiated by MWs using a bench scale apparatus. Soil samples were treated applying four specific power values at different times. At the end, soil temperature was measured, whereas residual contaminant concentrations were measured and fitted considering and exponential decay kinetic model. Temperature data, as well as kinetic parameters obtained, were used for the techno-economic analysis. The changing of the internal electric field was calculated for all the soils and operating conditions, then considering initial contamination values ranging from 750 to 5000 mg kg−1, the minimal remediation time, specific energy and costs for the remediation were assessed. At low powers, MW effectiveness is limited by low soil moistures or fine soil textures due to a limitation of the electric field penetration, whereas when high powers are used soil properties have a limited effect. Remediation time, as a function of the initial contamination level, follows a linear trend, except for dry soils, for which an exponential trend was observed. For powers higher than 30 kW Kg−1, remediation times lower than about 100 min are needed, for all the moisturized soils, in order to treat a contamination of 5000 mg kg−1. The variation of soil moisture or soil texture results in the range 20–160 € ton−1, and doubled costs are required for the treatment of clayey soils respect to sandy soils. The analysis performed suggests that soil layers lower than 70 cm should be considered for ex situ remediation. MW has been shown as a quick technique also for high hydrocarbon concentrations; however, for energy saving, the application of some powers should be avoid. Unmoisturized or fine texture soil treatment results in higher costs; however, a maximum cost of 160 € ton−1 generally makes MW heating a quick and cost-effective ex situ technique.

Although micronutrients are essential to higher plants, it remains unclear whether the projected future climate change would affect their availability to plants. The objective of this study was to investigate the effect of carbon dioxide (CO2) enrichment and warming on soil micronutrient availability and plant uptake. This study was conducted in an open field experiment with CO2 enrichment and plant canopy warming. Four treatments were included: (1) free-air CO2 enrichment up to 500 ppm (CE); (2) canopy warming by plus 2 °C (WA); (3) CO2 enrichment combined with canopy warming (CW), and (4) ambient condition as control. Plant and soil samples were collected, respectively, at the jointing, heading, and ripening stage over the whole wheat growing season in 2014. The micronutrient concentrations both in soil and plant were both analyzed, and the accumulated uptake by wheat harvest was assessed. Both CO2 enrichment and warming increased the availability of most soil micronutrients. The availability of Fe, Mn, Cu, and Zn under CO2 enrichment increased by 47.7, 22.5, 59.8, and 114.1 %, respectively. Warming increased the availability of Fe, Cu, and Zn by 60.4, 23.8, and 15.3 %, respectively. The plant growth induced changes in soil pH and in soil microbial biomass carbon (MBC) accounted to the changes in soil micronutrient availability. The enrichment of CO2 and warming had significant effects on micronutrient uptake by wheat. The enrichment of CO2 decreased the concentration of Fe by 9.3 %, while it increased the concentrations of Mn and Zn by 18.9 and 8.1 % in plant shoot, respectively. Warming increased the concentration of Fe and Cu by 24.3 and 7.6 % in plant shoot, respectively. The increase in soil micronutrient availability did not always lead to the increase in micronutrient uptake. The element types and crop growth stage affected the uptake of micronutrients by wheat under CO2 enrichment and warming. Additionally, CO2 enrichment decreased the translocation of Fe and Zn by 25.3 and 10.0 %, respectively, while warming increased the translocation of Fe, Mn, Cu, and Zn across stages. Our results demonstrated that CO2 enrichment and warming would improve availability of some micronutrients and their uptake by wheat. However, it is still unclear whether a net removal of micronutrient through crop straw harvest would occur under CO2 enrichment and warming.

Dissolved organic matter (DOM) plays an important role in soil C and N cycling. However, the driving mechanisms of DOM concentration and composition under the background of N deposition remains ill-defined. Through experimental N addition, we explored the influence and contribution of microbial community characteristics on DOM concentration and composition. Field experiments with different N-addition levels were set up in the Daiyunshan Nature Reserve, Fujian, China. Using water extraction and DAX-8 resin separation and phospholipid fatty acids as well as 96-well microplate methods, we mainly probed into the response of DOM concentration and composition, microbial community structure, and extracellular enzyme activity at two soil depths (0–10 and 10–20 cm) to N addition, separately. Furthermore, structural equation models (SEMs) were constructed to explore the effects of microbial community characteristics on the DOM concentration and composition. The dissolved organic carbon and hydrophilic matter (HIM) decreased significantly with N addition. Moreover, low N addition significantly increased the abundance of gram-negative bacteria and microbial biomass nitrogen, and the activities of urease and cellobiohydrolase in soil. SEMs revealed that the contribution rates of microbial community characteristics to dissolved organic matter (DOM) concentration and HIM were 65% and 60%, respectively. This study focused on the microbial regulation paths for the DOM concentration and composition, including mechanical decomposition, extracellular enzyme secretion, and absorption assimilation. Short-term N addition stimulated microbial communities to regulate soil DOM concentration and composition via three pathways in the subtropical Pinus taiwanensis forest. Under short-term N addition, the absorption and utilization of unstable hydrophilic DOM by microbial communities is the main regulation process, which eventually leads to soil C loss in the form of water-soluble organic C.

Predicting response of microbial communities to pollution requires an underlying understanding of the linkage between microbial community structure and geochemical conditions. Yet, there is scarce information about microbial communities in polycyclic aromatic hydrocarbons (PAH)-contaminated riverbank sediments. The aim of this study was to characterize bacterial communities in highly PAH-contaminated sediments and establish correlations between bacterial communities and environmental geochemistry of the sediments. Sediment core samples were collected from a highly PAH-contaminated site for (1) analysis of geochemical parameters including total nitrogen, total organic matter, moisture, total carbon, sulfate, pH, and PAH concentrations and (2) bacterial enumeration, 16S rDNA-based terminal restriction fragment length polymorphism analysis and sequencing. Non-metric dimensional scaling analyses revealed that bacterial community composition was strongly influenced by PAH concentration. Sulfate, organic matter, pH, and moisture were also related to community composition. A diverse microbial community was identified by the large number of operational taxonomic units recovered and by phylogenetic analyses. δ-Proteobacteria, firmicutes, and bacteriodetes were the dominant groups recovered. We also observed a high number of phylotypes associated with sulfate-reducing bacteria, some of which have been previously described as important in PAH degradation. Our study suggests that, despite intense pollution, bacterial community composition did exhibit temporal and spatial variations and were influenced by sediment geochemistry. Significant relationships between bacterial community composition and PAHs suggest that, potentially, extant microbial communities may contribute to natural attenuation and/or bioremediation of PAHs.

In flooded paddy soils, quantifying and discerning nitrous oxide (N2O) production by four biological pathways (nitrifier nitrification (NN), nitrifier denitrification (ND), nitrification-coupled denitrification (NCD), and heterotrophic denitrification (HD)) are essential for developing innovative strategies to mitigate the greenhouse effect. Soils were collected from Shenyang Experimental Station of the Institute of Applied Ecology, Liaoning Province, China, and were sealed and incubated in the dark at 25 °C and submerged for 48 h. The amount of N2O produced by each of the four pathways and the abundance of corresponding functional genes were determined by dual-isotope (15N-18O) labeling technique combined with quantitative PCR (qPCR). In our incubation experiment, 15N isotope tracing showed that not urea but paddy soil was the largest contributor of N2O within 48 h after urea application. Combined application of urea and mixed inhibitors (N-(n-butyl) thiophosphoric triamide (NBPT) + phenylphosphorodiamidate (PPD) + 3,4-dimethylpyrazole phosphate (DMPP)) could reduce total N2O production by 26.69%. Through dual-isotope (18O-15N) labeling technology, it was found that N2O production mainly came from HD pathway, accounting for 77% of total N2O production, and N2O produced by ammonia oxidation (NN, ND, and NCD) after urea application accounted for 23% of total N2O production. The largest proportion of N2O production among the ammonia oxidation pathways was the ND pathway (0–23.00%), followed by the NN pathway (0–19.21%) and NCD pathway (0–3.79%). Application of mixed inhibitors significantly reduced the N2O produced by the HD pathway by more than 15%; reduced the N2O produced by the ammonia oxidation pathway from 23.00 to 10.39%; and reduced the N2O produced by the NN, ND, and NCD pathways by more than 50%. This is probably caused by the decreased ammonium level and reduced gene copies of AOB amoA, narG, and nirK, which are key N2O producing genes. Incubation experiment showed that ammonia oxidation pathway is also an important pathway for N2O production in flooded paddy soil. Mixed inhibitors have inhibitory effects on N2O produced by NN, ND, NCD, and HD pathways. The future development of mixed inhibitor application strategies suitable for paddy fields is of great significance for mitigating the global greenhouse effect.