A prevalent strategy to mitigate the negative impacts of soil compaction or sealing on urban trees involves installing aeration tubes, aeration holes or near surface aeration horizons to enhance gas exchange between the soil air and the atmosphere. Despite their widespread use, there is currently no scientific evidence confirming their effectiveness. In this study, gas exchange between the atmosphere and soils that were aerated using different methods was modelled and evaluated in the laboratory and in the field. Both the laboratory and field measurements could be modelled well with the simulation model. The research showed that the effectiveness of aeration tubes and holes is highly dependent on the air-filled porosity of the soil. The more gas exchange can take place via the soil pores, the lesser the influence of the aeration equipment. Thus, the use of aeration tubes is not necessary when using tree substrates in unsealed tree pits but could mitigate disturbances in soil aeration in compacted and fine-grained soils with low air capacity. However, modelling shows that the effect of aeration tubes and holes is less than expected and that near-surface aeration layers are generally more effective than vertical aeration systems. With the help of the modelling of gas exchange as presented here, it is possible to optimise the level of aeration of the soil depending on the existing degree of compaction and/or the planned surface sealing.
Extensive usage of agricultural plastic film correspondingly leads to excessive residues of microplastics (MPs). MP accumulation alters soil hydraulic properties and water flow. However, little is known about the combined effects of concentration and particle size on soil hydrological properties, and a numerical approach for modelling infiltrated flow has not been well developed. Hence, we determined soil hydraulic properties and infiltrated flow affected by MP concentration and particle size and established a water flow model suitable for MP-contaminated soils. Quantitative findings indicated that the saturated conductivity for soil–MP mixture was 10.8%–50.0% smaller than that for pure soil, which decreased and increased with the increase in MP concentration and size, respectively. The MP concentration always had significant influences on saturated conductivity; in contrast, the MP particle size always generated significant influences under the condition of small particle size. Besides, higher concentration or size of MPs led to weaker soil water-holding capacity, and the saturated and residual water content decreased by 0.6% – 41.5% and 0.2% – 11.6%, respectively. Furthermore, the presence of MPs inhibited water infiltration, with the wetting front migration rate and cumulative infiltration decreased by 7.1% – 29.4% and 4.7% – 21.7%, respectively, with the increase in the MP concentration and size. Correlation analysis indicated that MP particle size was negatively correlated with saturated/residual moisture, wetting front migration and cumulative infiltration; in addition, MP concentration was negatively correlated with saturated conductivity, residual moisture, wetting front migration and cumulative infiltration; compared with the MP particle size (15.63%), the MP concentration (46.28%) played a major role in the response of soil hydraulic properties and water movement to changes in the external environment. A two-dimensional numerical approach was proposed by considering the Richards equation and hydraulic parameters of soil–MP mixture, and a model based on finite element theory was further employed and validated through comparing experimental observations with numerical simulations, which indicated that the proposed model had a high accuracy in simulating the infiltration process in MP-contained soils. Our findings elucidate the influence of MP concentration and size on soil hydraulic properties and water flow and confirm the potential of using simulations to predict water infiltration in MP-contained soils.
Rice grown in California constitutes 20% of total U.S. rice production and is typically grown in a continuous rice monoculture system. In recent years, growers have been forced to fallow their lands often due to winter droughts leading to water restrictions or spring rains leading to prevented planting. Increased soil aeration due to fallowing creates knowledge gaps in soil nitrogen (N) availability. A two-year field study was conducted to evaluate differences in crop N uptake between rice cultivation following a fallow season, fallow rice (FR) and continuous rice (CR) systems. Crop uptake of soil N (N uptakesoil) and fertiliser N (N uptakefertilizer) were quantified using 15N-enriched ammonium sulfate applied in microplots as a preplant (150 kg N ha−1) or topdress (30 kg N ha−1) application. In both seasons when N was applied as a preplant fertiliser, the FR treatment had a higher grain yield than did the CR treatment, with yield differences of 2.3 Mg ha−1 in 2021 (p < 0.05) and 1.7 Mg ha−1 in 2022 (p < 0.05). Examining the sources of crop N uptake for preplant applied N, on average, N uptakesoil in the FR treatment was 16.7 kg N ha−1 higher than the CR treatment at maturity (p < 0.05). In contrast, N uptakefertilizer was similar between treatments. Additionally, comparable soil and crop fertiliser N recoveries in CR and FR preplant N suggested that the pathways and magnitudes of fertiliser N losses were similar in both systems. These results indicate that N uptakesoil was primarily responsible for lower N uptake in CR. Similar results were found when N was applied as a topdress, where FR had increased N uptakesoil in both years. We further investigated the reason for lower rates of N uptakesoil in CR. Soil phenols, which have been documented to accumulate in continuously flooded rice systems and stabilise soil N, were quantified in the field study. Complementing the rigorous field study, a regional survey study that incorporated nine paired fields was conducted to quantify regional phenol levels. In both the field and the regional survey studies, soil phenols were higher in CR than in FR fields. Together, higher phenol levels and lower N uptakesoil in CR provide mechanistic evidence that the introduction of a season-long fallow to continuous rice systems enhances soil N availability by reducing organic substrate recalcitrance. Future work should identify the duration needed for soil phenol accumulation to impair soil N cycling under continuous rice cultivation, as well as any roles of soil microbial populations in these soil N cycling patterns.
Suffusion is a typical form of internal erosion and a major cause of the degradation and failure of hydraulic structures. Suffusion-induced particle movement is a random process because of the statistical distribution of pore flows in soil. However, few studies have focused on the application of the statistical distribution of pore flow in the suffusion model. This study evaluates four statistical distributions of conduit flows to equate pore flow in calculating the soil particles movement rate in the suffusion model for cohesionless gap-graded soils. Multiple experimental data are collected to comprehensively evaluate the four distribution-based models with multiple variable changes during suffusion. The results show that the uniform distribution of the equivalent conduit flows is more accurate and reliable for gap-graded soils than the other three distributions are. Compared with previous studies, this article directly provides an applicable statistical distribution of pore flows, which simplified calculation steps and improved efficiency by explicitly calculating the particle movement velocity. Moreover, the uniform distribution has a concise formula; thus, it is recommended to estimate suffusion in unstable gap-graded soil. The accurate evaluation of statistical conduit flow is critical for the suffusion model because of its complete synchronicity with the particle movement rate. More attention should further be paid to the statistical distribution of pore flows, as the uniform distribution-based model may also generate deviations during the later period of the simulation.
Studying the soil organic and inorganic carbon (SOC and SIC) dynamics is essential to assess the carbon (C) sequestration potential of calcareous soils. Isotopic signatures (δ13C) are used to assess the C origin of SOC or SIC. However, as measuring SOC and SIC contents, measuring δ13CSOC and δ13CSIC on a non-pretreated aliquot remains a challenge. Thermal analyses, like the Rock-Eval (RE) analysis, are promising to quantify SOC and SIC in a single analysis, but, to our knowledge, no development was conducted to assess δ13CSOC and δ13CSIC. We coupled a RE device to an isotopic gas analyser (Picarro) to continuously measure δ13CCO2 and approach δ13CSOC and δ13CSIC. We hypothesised that different carbonate mineralogies and/or crystal sizes in SIC involve fluctuations of the δ13CCO2. Two calcareous soils, a lithogenic (calcite) and a biogenic (snail shell) carbonate, and five calcite/shell mixes were analysed with the RE-Picarro setup. Two distinct δ13CCO2 values were obtained before and after 650°C and were consistent with the δ13CSOC and δ13CSIC obtained by EA-IRMS. The fluctuations of δ13CCO2 above 650°C were higher with calcite/shell mixes than with pure carbonates. A δ13CCO2 fluctuation > ± 0.2‰ could be a pertinent indicator to detect mixes of carbonate with different δ13C in soils. The RE-Picarro setup is promising to assess SOC and SIC contents, δ13CSOC and δ13CSIC and detect mixes of carbonate with different origin on a non-pretreated aliquot. Development is needed (i) on more soil and carbonate samples and (ii) to improve the precision and accuracy of the RE-Picarro setup.
The response of soils to extreme weather events will become increasingly important in the future as more frequent and severe floods and droughts are expected to subject soils to drying and rewetting cycles as a result of climate change. These extreme events will be experienced against a backdrop of overall warming. Farmers are adopting cover cropping as a sustainable management practice to increase soil organic matter and benefit soil health. Cover crops may also increase the resilience of soils to help mitigate the impacts of climate change. We examined the legacy of warming and cover crops on the response of soil microbial function to repeated drying and rewetting cycles. We introduced open-top chambers to warm the soil surface of a field plot experiment in which cover crops (single-species monocultures and 4-species polycultures) were grown over the summer after harvest and before planting autumn sown cash crops in a cereal rotation. Soil samples were collected from warmed and ambient areas of the experimental plots in spring, before harvesting the cereal crop. Warming significantly increased, and cover crops significantly decreased, the abundance of genes encoding fungal β-glucosidase. We quantified respiration (a measure of soil microbial function) with high-frequency CO2 flux measurements after 0, 1, 2, 4 or 8 wet/dry cycles imposed in the laboratory and the addition of barley grass powder substrate at a rate of 10 mg g−1 soil. We observed lower cumulative substrate-induced respiration in soils previously planted with cover crop mixtures than expected from the average of the same species grown in monoculture. Repeated drying and rewetting cycles increased the cumulative substrate-induced respiration rate observed, suggesting that repeated perturbations selected for a community adapted to processing the barley shoot powder more quickly. When we calculated the cumulative respiration after 8 wet/dry cycles, relative to cumulative respiration after 0 wet/dry cycles (which we infer represents the extent to which microbial communities adapted to repeated drying and rewetting cycles), our data revealed that the legacy of warming significantly reduced soil microbial community adaptation, but the legacy of cover crops significantly increased, soil microbial community adaptation. This adaptation of the soil microbial community was positively correlated with the concentration of water-extractable organic carbon in the soils before imposing the drying and rewetting cycles and/or adding the substrate. We conclude that cover crops may enhance the ability of the soil microbial community to adapt to drought events and mitigate the impact of warming, possibly due to the provision of labile organic carbon for the synthesis of osmolytes which then prime the decomposition of labile plant material upon rewetting.
Mountains are particularly vulnerable to climate change, as they are warming at a rate that exceeds the global average, significantly impacting cold-adapted ecosystems. In these environments, soil organic matter (SOM) stocks are often considerably larger than at lower elevations. These stocks are therefore highly susceptible to global warming and the associated risk of greenhouse gas (GHG) (CO₂, CH₄, N₂O) emissions driven by temperature-induced increases in SOM mineralisation. In order to quantify these emissions and the change of mineralisation rates under warming, it is necessary to gain an understanding of the annual mineralisation balance. We investigated how warming impacts the duration and intensity of mineralisation in different seasons. The main aim of this study is to quantify alpine SOM mineralisation rates and GHG production under a range of seasonal conditions, including those associated with warming. An in vitro approach was employed to expose alpine topsoils (0–10 cm) to the conditions of key seasonal periods: snow cover, growing season and rainfall/snowmelt. This was achieved by experimentally varying temperature and inflow of precipitation water. Additionally, the soil samples were subjected to a temperature increase of 4°C. The short-term responses of carbon (C), nitrogen (N) and phosphorus (P) mineralisation and GHG production were monitored. The results demonstrated that alpine soil respiration rates exhibited a twofold increase with a 4°C warming, while the relative proportion of labile SOM demonstrated a decline with rising temperatures. Water saturation from simulated rain and snowmelt played a crucial role in organic matter mineralisation and increased the mineralisation of carbon (+12% to +53%), nitrogen (+20% to +80% of net ammonification) and phosphorus (+50% of net phosphate production). This suggests that nutrients present in the snowpack or the rain were added to the soil. In contrast, soil–water saturation decreased net nitrate production by between 10% and 90%. The results of this study highlight the potential for alpine soil warming to release labile SOM and demonstrate the influence of the snow regime on nutrient and carbon fluxes.
Peatlands occur globally, and peat's microbial biodiversity is dominated by nutrient cycling. Peat soils derive acidity through carbon cycling, and the role of sulfur-utilising bacteria is less understood. A montane peatland within the Greater Blue Mountains World Heritage Area in New South Wales, Australia, was investigated to understand microbial controls on sulfide acidity. Peat soils were mildly acidic, with sulfides present in the transition and reduced zones. Sulfur-reducing bacteria, including from the Desulfobaccaceae and Syntrophobacteraceae families, were present in the transition and reduced zones of the soil profile. The key drivers of the microbial biodiversity were acid extractable sulfate in the oxidised zone (0–20 cm) and chromium reducible sulfur potential acidity in the transition to reduced (60–150 cm) zones. This peatland is unique as it reflects a montane, freshwater landscape with sulfides at depth, which exert a distinct control on soil microbial biodiversity. The formation and presence of sulfides place the site at risk for acidity generation, where landscape dehydration may occur.
Electromagnetic induction (EMI), by Geonics EM38, was used to characterise the volumetric magnetic susceptibility (MS) of soils on 12 farms in southwestern Ontario, Canada. Three different points on lower, middle and upper slope positions were selected at each farm to represent poorly-, moderate- and well-drained soil. Soil core samples were collected for each measurement point, from which soil redoximorphic conditions (gleying and mottling) were characterised at 5 cm depth increments. The volume MS, mass-specific MS and frequency dependence (FD) of MS of soil samples were carried out using Bartington MS2C and MS2B sensors, respectively. The impact of heating the samples to 400°C and 700°C on soil MS was also investigated. Results show that at each farm, the lowest volume MS values belong to soils at the lower slope position, which is poorly drained, and the highest volume MS values belong to the soils at the upper slope position, which is well drained. The inverted models from apparent MS data, measured by EM38, seem to be good representatives of volume MS readings attained from core samples. Results show that at most of the selected points, while the FD is higher in poorly drained points than in moderate and well-drained ones, the mass-specific MS shows an opposite behaviour, which can be used as attributes to characterise poorly and well-drained soil conditions.
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