查看更多>>摘要：? 2022 Elsevier B.V.In saltmarsh soils, humic acids (HA) are involved in geochemically important redox processes. The electron donating capacity (EDC) of HA depends on their molecular structure, but also reflects the intensity of biological reduction in tidal environments. We examined twelve soils in three saltmarshes located along a geographical gradient and applied a specific sequential extraction procedure for the isolation of HA fractions bound (BHA) or not (FHA) to the mineral matrix by Ca2+ bridges, and investigated the relationships of their properties, in particular their EDC, with the biogeochemical characteristics of the soils. Spectroscopic assessment was carried out by UV–vis, FTIR and 13C NMR, quantification and characterization of radicals was performed by electron paramagnetic resonance (EPR) spectroscopy. The EDC was determined by using the 2,2′-azinobis-(3-ethylbenzothiazolinesulfonic acid) (ABTS) decolorization assay and experimental data were fitted to a biphasic model to calculate the contributions of the fast and slow reactions to electron transfer. The results confirmed that the two HA fractions possess different structural characteristics and that BHA present higher EDC values compared to FHA. The EDC of both fractions is strongly related to the geochemical characteristics of soils, and represents an easily measured and highly informative parameter to understand mechanisms affecting redox processes in transitional environments.
查看更多>>摘要：? 2022Decomposition of crop residues may affect soil organic carbon (C) stocks, which are key for soil fertility improvement and mitigation of climate change. Numerous independent studies across the world point to contradictory results but their existence provides an opportunity to conduct a comprehensive analysis of the impact of crop type on residue decomposition. In the present study, data from 394 trials from across the world were used to assess cumulative CO2 emissions from residues of 17 crops during 0–30, 0–90 and 0–120 days (i.e. CR30, CR90 and CR120; 1-[CR30/CR120] ratio as a stability index of C emissions) and to relate the results with residue quality (C, N and lignin concentrations) and selected soil properties (texture, pH, soil organic carbon concentration). At all durations, legumes exhibited the highest CO2 emissions per gram of C added (1003 mg CO2-C g?1C after 120 days) followed by grasses (9 4 7), oilseed crops (9 4 4) and cereals (8 4 6), with the legumes and grasses showing the lowest temporal stability of C emission as pointed out by a 1-[CR30/CR120] of 0.78 and 0.79, respectively, versus 0.82 and 0.83 for cereals and oilseed crops. At all durations, maize residues emitted the least C-CO2 (86, 275 and 495 mg CO2-C g?1C), followed by two other lignin rich crops (cotton and sunflower), while the highest emissions were from Alfalfa residues that produced about 4 times more CO2 (e.g. 359 at CR30 and 1319 at CR120) than maize. Overall, CO2 emissions were positively correlated with soil clay concentration (r > 0.22), residue C concentration (e.g. r = 0.46 at CR90 and r = 0.37 with emission stability, P < 0.05) but negatively to residue N concentration (r = ?0.26 at CR120, P < 0.05). The global trend pointed to decreased CO2 emissions with increasing residue lignin. Contrary to what is generally believed, providing the soil with high lignin and high N concentration may foster C stabilization into soils by soil microbes.
查看更多>>摘要：? 2022 The AuthorsUnderstanding how the chemical composition of dissolved and particulate organic matter (DOM and POM) is affected by environment factors is critical because these labile pools of carbon are involved in an array of biological, chemical and physical processes. In this study, the chemical composition of DOM and POM was measured in 13 forest soils using UV–Vis spectroscopy, fluorescence spectroscopy with parallel factor analysis (PARAFAC modelling) and FT-IR spectroscopy. There were significant differences between the soils for the SUVA indexes, PARAFAC components and relative intensities of different IR bands. Redundancy analysis (RDA) revealed that soil parameters had a strong influence on the chemical composition of DOM and POM with high constrained variability (77.9 and 77.1 %, respectively). The pH of the soils proved to be an important controlling factor for both DOM and POM, regulating the concentration of the C3 PARAFAC component (low-molecular-weight compounds associated with biological activity) and the aromatic compounds of POM (aromaticity, rA1630 and rA1515). The silt content was the other main regulating factor controlling the chemical characteristics of the labile pool, having a strong negative correlation with the specific UV absorbance (SUVA) values of DOM due to the preferential adsorption of hydrophobic moieties. RDA analysis also revealed that there is a strong correlation between the chemical composition of POM and DOM: the relative amount of aromatic compounds (rA1515, rA1630 for aromatic rings and rA1270 for phenolic compounds) in the POM fraction was closely correlated with the SUVA values of the DOM. This clearly indicates a strong biogeochemical link between the two labile organic carbon fractions; the high molecular weight compounds of the DOM fraction may be derived from lignin-like structures of the POM fraction.
查看更多>>摘要：? 2022The amount of unfrozen water reflects the soil water potential and is of importance to explore the water/salt migration in saline soil. Using low-field NMR technology, we measured the amount of unfrozen water content in sodium chloride soil under several initial salt contents. The results reveal that there are two phase transition processes in sodium chloride soil between 20 to ?30 °C, the unfrozen water content decreases as ice crystals form in the first phase transition stage, and further decreases as ice and hydrated salt precipitate in the second phase transition stages. Supercooling phenomenon, caused by ice nucleation, delays the ice formation in the freezing process, thus leads to variation of unfrozen water content in soil. Through distribution of the unfrozen pore water in soils, we find that water in large pores is prone to freeze into ice. Considering the real phase transition process of soil, we propose a theoretical model to evaluate the unfrozen water content of sodium chloride soil at a wide temperature range. Not only unfrozen water content is well predicted in the two phase transition stages, but also the change of eutectic temperature can be explained under different initial salt concentrations. Based on this model, we discuss the influence of our parameter α and the supersaturation degree on our calculated results. This work provides a theoretical reference for investigating the deformation mechanism of saline frozen soil.
查看更多>>摘要：? 2022Chemical fumigation is used to reduce soil-borne diseases in agricultural production systems; however, soil carbon (C) and nitrogen (N) dynamics may also be affected. This study investigated the effects of chemical fumigation and substrate C availability on soil respiration, soil nitrous oxide (N2O) production, and soil inorganic N concentrations under controlled conditions over 128 days. This study consisted of a 3 × 3 factorial arrangement of three fumigant treatments (fumigation with chloropicrin, metam sodium, or no fumigation) and three soil amendment treatments (amendment with young barley, mature barley, or no amendment). In soils amended with young barley, chemical fumigation delayed the maximum rate of soil respiration and N2O production by five days compared to the amended non-fumigated soil. Additionally, chloropicrin fumigation decreased cumulative soil respiration in amended soils, regardless of substrate C availability, compared to non-fumigated soil. Chemical fumigation used alone or combined with young barley amendments significantly inhibited nitrification compared to non-fumigated soil, whereas amendment with mature barley resulted in N immobilization, regardless of chemical fumigation. This study demonstrated that chemical fumigation significantly affected soil C and N dynamics in soils amended with high and low available C substrates, indicating decreased microbial activity and significant implications for soil function.
查看更多>>摘要：? 2022 Elsevier B.V.Real-time soil moisture content (SMC) monitoring is an important parameter in precision agriculture that can be utilized to enhance soil and water management. Visible and near-infrared (vis-NIR) has been proposed as a promising method for SMC monitoring. However, vis-NIR reflectance response to soil moisture is strongly influenced by soil properties such as texture and organic matter content. Thus it is difficult to develop a general prediction model of vis-NIR that can estimate SMC of different soil types. To solve this problem, this study utilizes the External Parameter Orthogonalization (EPO) method to reduce the effect of soil type. Seven soils (CZ, GA, JN, PDS, PY, WF, and XZ) were collected from the North China Plain. A fiber-optic spectrometer (300–1000 nm) was used to obtain spectral response of seven soils adjusted to several SMC levels. The partial least square regression (PLSR) was used to model the pretreated vis-NIR spectra of the seven soils. The modeling results confirmed the feasibility of using vis-NIR spectroscopy to predict SMC on individual soils. However, the prediction models vary significantly between soil types. To obtain a generalized prediction model of SMC that can be applied to different soils, three soils were selected from the seven soils for calibration, while the rest were used for validation. The results showed that direct PLSR modeling failed to achieve a unified prediction model. To solve this problem, EPO was applied to minimize the influence of soil types. The results demonstrated that EPO-PLSR can realize a generalized SMC prediction model. It is suggested that the soils for building EPO-PLSR models should cover a wide span of soil organic matter (SOM) content. Soil with the least SOM content appeared to be best for developing an EPO transfer matrix.
查看更多>>摘要：? 2022 Elsevier B.V.Wetlands represent about one-third of the global soil organic carbon storage and are the largest natural source of atmospheric methane, a powerful greenhouse gas. Understanding the factors that influence the carbon dynamics in wetlands, and their influence on methane emission, is therefore important. Dissolved organic carbon (DOC) is a major form of carbon export from wetlands and plays an essential role in the aquatic carbon cycling process. However, constraints on spatial and temporal dynamics of DOC in vertical peat layers and their relationship to wetland methane emissions are still rare. Here we investigated spatiotemporal changes in porewater DOC concentration and optical properties (SUVA254, E2/E3, E4/E6) in surface (0–10 cm) and deep (20–30 cm, 50–60 cm, 100–110 cm) peat layers in a subtropical wetland complex, central China, with the aim to explore DOC dynamics and their relationship to methane emissions from wetlands. DOC dynamics and environmental controls were investigated in the context of high-resolution environmental monitoring, including air temperature, rainfall, water table depth, water chemistry, and methane emissions. DOC of surface peat layers generally had higher concentrations, higher aromaticity, lower humification, and greater temporal variations compared to deeper peat layers. During summer floods and high water table conditions, DOC concentrations in the surface peat increased significantly, whilst DOC exports from rivers and lakes also increased synchronously. In contrast, DOC concentrations and spectral properties of deep peat layers remained constant across the sampling months and water table fluctuations. Methane emissions were positively correlated with air temperature and DOC concentration in the porewater of surface peat layers, and negatively correlated with water table depth. Our results suggest that DOC concentration in the porewater of surface peat layers may also have affected the flux of methane emission from wetlands.
查看更多>>摘要：? 2022Fly ash and steel slag can potentially mitigate the cadmium (Cd) and arsenic (As) accumulation in rice grains but their long-term effectiveness and impact on soil health are unclear. By running a four-crop-season field trial, we found that the concentrations of Cd, As and inorganic As in rice grains were significantly reduced by steel slag and (consecutively applied) fly ash. For both amendments, decreased soil extractable Cd by increased pH was crucial in reducing grain Cd, but soil re-acidification diminished their effects. Increased soil extractable silicon played a key role in alleviating grain As accumulation. Steel slag had a more persistent effect on reducing grain Cd than fly ash but the sustainability of their effects on reducing grain As depended on rice cultivars. Steel slag improved soil fertility by increasing soil calcium, magnesium, manganese and zinc but chromium and nickel were also increased; it also enhanced the activities of soil urease and alkaline phosphatase, shifted soil bacterial community composition, and increased bacterial diversity. Fly ash had little effect on soil health. Our results indicated that steel slag had positive and sustainable effects on mitigating grain Cd and As accumulation but its potential negative impact on soil health requires in-depth monitoring.
查看更多>>摘要：? 2022Phosphorus (P) limitation to microorganisms is increasingly recognized in soils, but how the limitation mediates the metabolic processes of microbes driving soil carbon (C) release remains unclear. Here, we performed a 60-day incubation experiment adding two 13C-labeled organic C sources (glucose and straw) at five inorganic P addition levels in loess with low available P from the Loess Plateau, China. The nutrient limitations of microbes were quantified by enzymatic vector analysis, associated with soil respiration, microbial metabolic quotient (qCO2), C use efficiency (CUE) and priming effect (PE) at both early (10 days) and late (60 days) stages of incubation. Results showed that reducing microbial P limitation increased CO2 release from soils by 19–26% and from labeled glucose and straw by 12% and 29%, respectively. This indicated that soil P limitation overall constrains rather than promotes microbial C metabolism. A negative relationship between relative C and P limitations at the first 10-day incubation further indicated that added P (decreased P-acquiring enzyme activities) stimulated microbial C metabolism (increased C-acquiring enzyme activities) under enough C source. Whereas a positive relationship at 60-day incubation suggested that high microbial heterotrophic respiration under high P addition alleviate their C limitation. Furthermore, both multiple regression and partial least squares path models indicated that an increase in CO2 release with P and C additions at early incubation was due to two processes, i.e., increasing available P promoted decomposition of native soil organic C due to PE as well as decay of added organic C by increasing qCO2 and decreasing CUE. At late incubation, however, P addition increasing decomposition of native soil C via PE is the dominated control on CO2 release under C limitation. We conclude that microbes are dominant by maintenance rather than growth metabolism in loess with low phosphorus availability, whereas the pathways of the metabolism driving C release depend on soil C availability. Our findings suggest that microbial P limitation has considerable positive effects on soil C sequestration in these ecosystems with low soil P availability.
查看更多>>摘要：? 2022 Elsevier B.V.Soil has the ability to sequester carbon (C) and mitigate anthropogenic enrichment of carbon dioxide, however numerous variables influence its C storage potential. Climate, soil properties, plant species composition, and livestock management practices, may all influence C storage in rangeland soils. The purpose of this study was to examine the relative importance of these factors in predicting organic and inorganic soil C in a semi-arid rangeland in northern Arizona USA. Sampling sites at 60 locations within the 40,469 ha Diablo Trust Ranches were selected using a stratified random method to encompass five distinct soil series across a precipitation gradient (230 mm–623 mm mean annual precipitation) with fence-lines that separate actively grazed areas with adjacent areas that have excluded livestock for at least 20 years. A total of 240 soil samples were collected from two depths (0–5 cm and 20–25 cm) in grazed and adjacent ungrazed sides of fences. Soil texture, bulk density, plant community composition, root biomass, soil organic carbon (SOC) and soil inorganic carbon (SIC) were measured. Across the precipitation and soil gradient, SOC was highest (up to 28.6 Mg ha?1) in the surface layer of clay-rich soil derived from basalt and SIC was highest (up to 74.7 Mg ha?1) in subsurface soil derived from limestone. General linear models showed that grazing had no significant impact on total soil C, instead, soil depth and soil texture were the main predictors of SOC and SIC. Precipitation was positively correlated with SOC and negatively correlated with SIC. Soil texture and precipitation were highly autocorrelated, so only soil texture was included in structural equation models (SEMs). Separate SEMs for surface and subsurface soils showed that soil texture (and autocorrelated precipitation) was the strongest predictor of SOC, SIC and plant community composition at both soil depths. The relative abundance of C4 grasses was higher in grazed plots and was positively correlated with root biomass at the soil surface. Also, surface root biomass was weakly positively related to SOC while both surface and subsurface roots were negatively related to SIC. This study shows the importance of interactions among multiple factors in predicting soil C stocks and that SIC can be a substantial C reservoir in dryland soils, especially those derived from sedimentary parent material. We could not detect an influence of livestock grazing on either SOC or SIC in this landscape scale study. This suggests that in this semi-arid rangeland it may be difficult to increase total soil C storage through livestock management.
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