Intact Soil Columns Research Articles

Impact of preferential flow pathways on phosphorus leaching from typical plastic shed vegetable production soils of China

Despite its importance, the contribution of preferential flow pathways (PFPs) to subsurface phosphorus (P) leaching from heavily-fertilized plastic shed vegetable production (PSVP) soils in China has remained unexplored. To investigate this, five acid fine-textured Orthic Anthrosols and six alkaline coarse-textured Ustic Cambosols intact soil columns (20 cm depth and 15 cm diameter), from two typical PSVP systems in Jiangsu Province, Southeast China, were subjected to six leaching experiments with collection of effluents over 24 h. Leachate samples were analyzed for their concentrations of particulate P (PP) and dissolved reactive P (DRP). Finally, soil columns were photographed for their 2D distribution of PFPs both vertically (0−20 cm) and horizontally at 5 cm intervals after the application of dye tracer (Brilliant Blue). Overall, alkaline coarse-textured Ustic Cambosols with longer history of PSVP had larger PFPs’ volumes than acid fine-textured Orthic Anthrosols with vertical (0−20 cm) values ranging from 812 to 1260 cm2 and 623 to 1104 cm2, respectively. Of soil properties, pH and sand content had significant positive correlations with PFPs’ density (p < 0.05). Whereas, soil contents of clay and oxalate extractable Al and Fe had significant negative correlations with PFPs’ density (p < 0.05). In terms of subsurface P mobilization, preferential water flow had significant impact on DRP leaching (p < 0.05). In contrast, leachate PP concentrations had no significant relationships with both vertical and horizontal PFPs, indicating that PP leaching might occur mostly through soil matrix. However, field assays are still essential for better understanding of P leaching in relation to PFPs through taking into account the soil physiochemical properties and agricultural management.

Effects of freeze-thaw cycles on soil macropores and its implications on formation of hummocks in alpine meadows in the Qinghai Lake watershed, northeastern Qinghai-Tibet Plateau

The alpine regions are characterized by frequent freeze-thaw cycles, which influence the soil structure remarkably. However, the effects of freeze-thaw cycles on the soil macropore structure and its implications on the formation of hummocks are not well understood. This study aims to quantify the soil macropores of hummocks and the effects of freeze-thaw cycles on soil macropores to reveal its implications on the formation of hummocks in alpine meadows. A total of nine intact soil columns (0–50 cm deep) were excavated from the alpine meadows. Three replicates were collected from the hummocks and three were collected from the interhummocks, and they were scanned by X-ray computed tomography (CT). Another three replicates were excavated from the adjacent non-hummock alpine meadows, which were subjected to successive freeze-thaw cycles (0, 1, 3, and 6) and then scanned using X-ray CT. Soil macropore network of hummocks was more extensive and continuous than that of the interhummocks. The volumetric distribution of soil macropores in hummocks was multiple, while soil macropores of the interhummocks intensively fell in the volume range of 0–400 mm3. The soil macroporosity, macropore number density, surface area density, length density, node density, branch density, and mean macropore size of the hummocks were several times higher than those of the interhummocks. These soil macropore parameters showed a decrease after the first and third freeze-thaw cycle and later increased through the next three cycles. Soil macropores with volume > 1000 mm3 transformed into smaller ones (600–1000 mm3) during the freeze-thaw process. The soil macropore patterns of hummocks and interhummocks differed distinctly. Soil macropores responded significantly to freeze-thaw cycles. The effects of freeze-thaw cycles on the macropore number and size were more significant than on the morphology in alpine meadows. Hummock formation is closely related to the soil macropores during the freeze-thaw cycles, and the soil moisture played a crucial role in the hummock formation process.

Precipitation of secondary phases of iron and its role in controlling the mobility of potentially toxic elements in soils in a semiarid river basin in Northwest Mexico

Soils have the ability to retain potentially toxic elements (PTEs) through different chemical processes that promote low mobility of these elements, such as the precipitation of secondary phases of Fe, which facilitate the adsorption/co-precipitation of PTEs. The main objective of this study was to evaluate the mobility of PTEs present in an acid solution in two soils with different concentrations of calcite, understanding the role of secondary iron phases in the retention of these elements. To evaluate this phenomenon, intact soil columns of two different types of soils from the Sonora River in Northwest Mexico were exposed to an acid solution with high concentration of dissolved PTEs (mainly Fe, Al, and Cu). The Tinajas soil was free of carbonates while the Bacanuchi soil had more carbonate content than the Tinajas soil. Secondary precipitates corresponding to secondary phases of iron (mainly ferrihydrite and jarosite) were identified by X-ray diffraction. Using scanning electron microscopy, the PTEs retained in the soils were identified. The presence of calcite favored the neutral pH values in the collected leachates in the Bacanuchi soil; consequently, the mobility of the PTEs present in the acid solution was nullified. Furthermore, this process facilitated the retention of the toxic elements in the Bacanuchi soil. The retention of PTEs was 100% in the Bacanuchi soil where the natural acid-neutralizing capacity in this soil was associated with calcite. The formation of secondary phases of Fe, among them ferrihydrite, jarosite, and schwertmannite, mainly in Bacanuchi soil, promoted the retention of Al, As, Cd, Cu, Fe, Mn, and Pb (elements analyzed in this work). Results of this work can provide key insights to improve cleanup and conservation strategies in mining sites.

Lime application to reduce phosphorus release in different textured intact and small repacked soil columns

Phosphorus (P) losses from agricultural fields through leaching are the main contributors to eutrophication of lakes and rivers in North America. Adoption of P-retaining strategies is essential to improve the environmental quality of water bodies. The main objective of this study is to evaluate lime as a soil amendment in reducing phosphorus concentration in the leachate from three common soil textures with neutral to alkaline pH. Phosphorus leaching from undisturbed soil columns (10 cm in diameter and 20 cm deep) as well as small repacked columns was investigated and compared in this study. Lime (high calcium hydrated lime) at the rate of 1% by air-dried soil mass was applied to the topsoil of the columns. Both sets of experiments followed a full factorial design with two factors of soil texture at three levels (sandy loam, loam, and clay loam) and treatment at two levels (control and limed) with three replicates. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy was performed on the control and limed soil samples to confirm the formation of calcium phosphate compounds. For both intact and repacked columns, dissolved reactive phosphorus (DRP) concentrations in the leachates from limed sandy loam and limed loam soil columns was significantly reduced, while DRP in the limed clay loam column leachates was not changed. Elemental mapping demonstrated that in limed sandy loam and loam soils, the calcium loadings on the soil surface were always linked with phosphorus. The formation of calcium phosphate compounds and the increased phosphate adsorption on the soil surface through Ca bridging could be the two main phosphorus-lime retention mechanisms. Total dissolved phosphorus (TDP) in the leachates of limed loam and limed clay loam indoor intact and repacked columns was reduced, while there was no change in that of the sandy loam soil. In finer textured soils, lime can increase TDP retention through the immobilization of organic phosphates. The impact of lime application on DRP and TDP varied with the soil texture. The lime-induced reduction in the DRP and TDP was variable between the intact and repacked columns demonstrating the importance of soil structure on phosphorus and lime interactions in the soil. Overall, lime application at the studied rate can be considered a promising soil amendment in mitigating phosphorus loss from non-calcareous neutral to alkaline soils.

Nitrogen transformation processes and gaseous emissions from a humic gley soil at two water filled pore spaces

The artificial drainage of heavy textured gley soils is prevalent on pasture. Drainage of a soil profile reduces the water filled pore space (WFPS) in the upper soil horizons with consequences for N2 and N2O emissions, the fate of nitrogen (N), transformational processes and microbial and bacterial communities. The present intact soil column study with isotopically enriched fertiliser investigates all these aspects simultaneously under two WFPS treatments (80% (HS) and 55% (LS) saturation). Results showed significant differences in nitrous oxyde (N2O) emissions, in both pattern and amount, with maxima at 11.97 mg N2O-N/m2h for HS and at 1.64 for LS. Isotopic enrichment data showed a significant predominance (74.8–97.2%) of nitrification in LS, with a possible reduction in NH4+ but a higher concentration of nitrate (NO3−) in N losses. Denitrification dominated in HS (72.5–73.4%), possibly leading to high ammonium (NH4+) losses. Enrichment values showed differential apportionment patterns. A high component of N2O emission derived from denitrification in HS (6.0% HS; 0.4% LS) with a significant amount of N2O (62.9%) transformed to N2 (3.7% LS). A higher percentage of 15N was retained in LS soil. HS showed a lower amount of unaccounted N highlighting lower losses. Differences in gene copy concentrations (GCC) were found across most analysed genes (16S, nirS, nirK, nosZ1, nosZ2, amoA and nrfA). Both HS and LS treatments showed similar potentials for N2O production and its reduction to N2, but a reduced potential for nitrification and dissimilatory nitrate reduction to ammonium (DNRA) in HS. This study explained the effect of drainage and rewetting on gaseous emissions providing an explanation in terms of community switches. On the current soil type, structures to manage watertable heights would push the system towards complete denitrification with only N2 production but may present risks in terms of ammonia (NH3) and NH4+ losses.

Leaching of organic carbon from grassland soils under anaerobiosis

The projected increase of extreme precipitation and freeze-thawing events may lead to frequent occurrence of anaerobiosis in upland soils, which has significant impacts on biogeochemical processes affecting soil carbon loss. However, compared to mineralization, the impacts of anaerobiosis (potentially accompanied by fermentation) on soil organic carbon (SOC) leaching is limited. Here we conducted microcosm and intact soil column incubation experiments to examine processes influencing SOC leaching from four typical Chinese grassland soils under simulated anaerobiosis. Compared to aerobiosis, non-fermenting anaerobiosis increased the pH, dissolved phenol concentrations and aromaticity of soil leachates. In contrast, fermenting anaerobiosis induced acetate accumulation, lowered pH, stimulated phenol oxidative activity and generally decreased aromaticity in soil leachates in both microcosm and soil column experiments relative to aerobiosis. Both anaerobiosis potentially induced a strong release of dissolved organic carbon (DOC) accompanied by iron and nitrate reduction, especially with fermentation. However, DOC in soil leachates decreased in alpine subsoils under fermentation relative to aerobiosis. This interesting phenomenon was mainly attributed to (i) minimal iron reduction and dissolution in the alpine subsoils and (ii) enhanced DOC oxidation by elevated phenol oxidative activity in the fermentation relative to aerobiosis treatments. These results collectively indicate that anaerobiosis may increase SOC leaching and its magnitude is dependent on the extent of iron reduction and pH variations. Fermentation-enhanced release of ferrous iron and acetate may have an even stronger influence on the downstream biogeochemistry. Hence, temporary anaerobiosis warrants better recognition and investigation in the Mongolian (relative to Qinghai-Tibetan) grasslands that show high soil iron reduction potentials and are predicted to experience increased extreme precipitation in the future.

Greenhouse gas emissions from intact riparian wetland soil columns continuously loaded with nitrate solution: a laboratory microcosm study.

In this study, we aimed at determining greenhouse gas (GHG) (CO2, CH4, and N2O) fluxes exchange between the soil collected from sites dominated by different vegetation types (Calamagrostis epigeios, Phragmites australis, and Carex schnimdtii) in nitrogenous loaded riparian wetland and the atmosphere. The intact soil columns collected from the wetland were incubated in laboratory and continuously treated with [Formula: see text]-enriched water simulating downward surface water percolating through the soil to become groundwater in a natural system. This study revealed that the soil collected from the site dominated by C. epigeios was net CO2and N2O sources, whereas the soil from P. australis and C. schnimdtii were net sinks of CO2and N2O, respectively. The soil from the site dominated by C. schnimdtii had the highest climate impact, as it had the highest global warming potential (GWP) compared with the other sites. Our study indicates that total organic carbon and [Formula: see text] concentration in the soil water has great influence on GHG fluxes. Carbon dioxide (CO2) and N2O fluxes were accelerated by the availability of higher [Formula: see text] concentration in soil water. On the other hand, higher [Formula: see text] concentration in soil water favors CH4 oxidation, hence the low CH4 production. Temporally, CO2 fluxes were relatively higher in the first 15 days and reduced gradually likely due to a decline in organic carbon. The finding of this study implies that higher [Formula: see text] concentration in wetland soil, caused by human activities, could increase N2O and CO2 emissions from the soil. This therefore stresses the importance of controls of [Formula: see text] leaching in the mitigation of anthropogenic N2O and CO2 emissions.

Characterization of Root Architectures and Soil Macropore Networks Under Different Ecosystems Using X-ray CT Scanning in the Qinghai Lake Watershed, NE Qinghai–Tibet Plateau

The objectives of this study were to visualise and quantify the soil macropore networks and root architectures within intact soil columns under different ecosystems and to investigate the influences of roots on soil macropore network characteristics. Intact soil columns with diameters of 110 mm and lengths of 400 mm were taken from alpine Kobresia meadow, interspace meadow patches of Potentilla fruticosa shrubs, Potentilla fruticosa shrubs and Artemisia sphaerocephala shrubs in the Qinghai Lake watershed of the NE Qinghai–Tibet Plateau. A Philips medical X-ray CT scanner was used to simultaneously visualise and quantify the soil macropore networks and root architectures. The results showed that macropores and taproots were concentrated in the top 0–150 mm. The root volume density of the P. fruticosa shrub was greater than that of the interspace meadow patches of P. fruticosa shrub and that of the alpine Kobresia meadow. The macroporosity and the root volume density of the soils under the P. fruticosa shrub patches were significantly greater than those in the soils under the other sites. The root volume density was significantly correlated with the macroporosity for the A. sphaerocephala shrubs (R2 = 0.88) and the P. fruticosa shrubs (R2 = 0.73). There were significant positive correlations between the volume density, branch density and mean angle of the roots and those of the macropores. Shrub roots invasion played an important role in the formation of macropores. The root volume density was the most important factor for the quantified characteristics and distributions of the macropores.

A Large Undisturbed Column Method to Study Flow and Transport in Macropores and Fractured Media.

Intact soil columns can bridge the gap between field studies and idealized laboratory investigations of flow and transport in macropores and fractured media. However, the value of intact column studies is often hampered by shortcomings such as lack of column intactness, small column size, and column rim flow, which can cause serious artifacts and hamper system understanding. The flexible-wall pressurized large undisturbed column (LUC) method overcomes these limitations and is a valuable approach to analyze fluid flow and solute transport in macroporous and fractured geological formations. The method investigates subsurface processes in complex media, mimicking in situ conditions and facilitating the control of system boundary conditions including effective stress. In recent years, considerable experience has been gained through different applications of the LUC approach. Modeling tools have also been developed for a detailed interpretation of flow and transport processes in LUC systems. This paper describes the steps of the LUC method from column excavation in the field to experimental setup in the laboratory. The description encompasses the key features of the sampling of LUCs in field excavations, the laboratory setup, the procedure for hydraulic and transport experiments, as well as practical challenges and potential issues during operation of an LUC system. Application examples with a fully three-dimensional numerical model of LUC tracer experiments are also presented to illustrate the quantitative interpretation of transport processes in macroporous clayey tills.

Laboratory study on nitrate removal and nitrous oxide emission in intact soil columns collected from nitrogenous loaded riparian wetland, Northeast China.

Nitrate pollution of surface and groundwater systems is a major problem globally. For some time now wetlands have been considered potential systems for improving water quality. Nitrate dissolved in water moving through wetlands can be removed through different processes, such as the denitrification process, where heterotrophic facultative anaerobic bacteria use for respiration, leading to the production of nitrogen (N2) and nitrous oxide (N2O) gases. Nitrate removal and emission of N2O in wetlands can vary spatially, depending on factors such as vegetation, hydrology and soil structure. This study intended to provide a better understanding of the spatial variability and processes involved in removal and emission of N2O in riparian wetland soils. We designed a laboratory experiment simulating surface water flow through soil columns collected from different sites dominated by different plant species within a wetland. Water and gas samples for and N2O analyses were collected every 5 days for a period of 30 days. The results revealed significant removal of in all the soil columns, supporting the role of riparian wetland soils in removing nitrogen from surface runoff. Nitrate removal at 0 and 10cm depths in sites dominated by Phragmites australis and Carex schnimdtii was significantly higher than in the site dominated by Calamagrostis epigeio. Nitrous oxide emissions varied spatially and temporally with negative flux observed in sites dominated by P. australis and C. schnimdtii. These results reveal that in addition to the ability of wetlands to remove , some sites within wetlands are also capable of consuming N2O, hence mitigating not only agricultural nitrate pollution but also climate change.

Remarkable N2O emissions by draining fallow paddy soil and close link to the ammonium-oxidizing archaea communities

Fallow paddies experience natural flooding and draining water status due to rainfall and evaporation, which could induce considerable nitrous oxide (N2O) emissions and need to be studied specially. In this study, intact soil columns were collected from a fallow paddy field and the flooding-draining process was simulated in a microcosm experiment. The results showed that both N2O concentrations in the soil and N2O emission rates were negligible during flooding period, which were greatly elevated by draining the fallow paddy soil. The remarkable N2O concentrations in the soil and N2O emission/h during draining both had significant relationships with the Arch-amoA gene (P < 0.01) but not the Bac-amoA, narG, nirK, nirS, and nosZ genes, indicating that the ammonium-oxidizing archaea (AOA) might be the important players in soil N2O net production and emissions after draining. Moreover, we observed that N2O concentrations in the upper soil layers (0–10 cm) were not significantly different from that in the 10–20 cm layer under draining condition (P > 0.05). However, the number of AOA and the nitrification substrate (NH4+-N) in the 0–10 cm layer were significantly higher than in the 10–20 cm layer (P < 0.01), indicating N2O production in the 0–10 cm layer might be higher than the measured concentration and would contribute considerably to N2O emissions as shorter distance of gas diffusion to the soil surface.

Linking saturated hydraulic conductivity and air permeability to the characteristics of biopores derived from X-ray computed tomography

The different types of soil macropores (e.g., biopores and non-biopores) vary in the conductivity of water or air due to the difference in the 3D pore characteristics. The objectives of this study were to reveal which types of macropores and which macropore characteristics played the most important roles in regulating water or air flow. Intact soil columns sampled from the subsoil of a long-term fertilization experiment were scanned by medical X-ray computed tomography (CT), and subsequently, saturated hydraulic conductivity (Ks) and air permeability at −12 cm water potential (Ka12) were measured. The 3D characteristics of macropores were then analyzed with image analysis. The biopores and the percolating biopores that connected the top and the bottom of a soil column were separated for the biopore-dominated samples (with percolating biopores) and their 3D characteristics were also quantified. Our results showed that the mean macropore diameter of the limiting layer (MDLL) presented the best relationships with Ks and Ka12 compared with the other macropore characteristics for all the samples. The biopores and percolating biopores contributed 27.8–74.5% and 3.26–64.4% of the volume of total macropores, respectively, for the biopore-dominated samples. The hydraulic radius, mean diameter, compactness, global and local connectivities, and MDLL of biopores, especially those of percolating biopores, were generally larger than those of total macropores. The global connectivity (Γ) of biopores performed very well for estimating Ks and Ka12. The MDLL of percolating biopores could predict Ks much better than the MDLL of biopores and total macropores. Moreover, the performance of MDLL for estimating Ka12 was as good as the MDLL of biopores but was much better than the MDLL of total macropores. The findings of this study reveal that the MDLL is a more useful parameter in estimating saturated hydraulic conductivity and air permeability at low water potential than the other CT-derived pore characteristics.

Evaluating the ability of standardised leaching tests to predict metal(loid) leaching from intact soil columns using size-based elemental fractionation

Laboratory-based leaching tests are frequently used for in situ risk assessments of contaminant leaching to groundwater and surface waters. This study evaluated the ability of three standardised leaching tests to assess leaching of lead (Pb), zinc (Zn), arsenic (As) and antimony (Sb) from four intact soil profiles, by considering particulate (0.45–8 μm; percolation test), colloidal (10 kDa–0.45 μm) and truly dissolved (<10 kDa) fractions of these elements. Deionised water was used as the percolation test leachant, while either deionised water or 1 mM CaCl2 was used in batch tests. Data from an irrigation experiment were used as reference. The results indicated that in percolation tests, leachate should be collected at a liquid:solid ratio (L/S) range of 2–10, instead of 0–0.5 or 0.5–2. Even at L/S = 2–10, the percolation test overestimated total Pb concentration, mainly because of greater mobilisation of particle-bound Pb, but appeared suitable for categorising soils into high/low risk with respect to mobilisation of particulate and colloidal contaminants. The batch test performed better with CaCl2 than with deionised water when standard membrane filtration (0.45 μm) was used, as the high Ca2+ concentration reduced colloidal mobilisation, avoiding overestimation of concentrations of elements such as Pb. However, the higher Ca2+ concentration and lower pH could result in overestimated concentrations of weakly sorbed elements, e.g. Zn.

Transport of Potential Manure Hormone and Pharmaceutical Contaminants through Intact Soil Columns.

Although adding manure to agricultural soils is a commonly practiced disposal method and a means to enhance soil productivity, potential environmental contamination by any associated chemicals of emerging concern (CECs) such as hormones and pharmaceuticals is not well understood. Our objective was to provide field-relevant predictions of soil transport and attenuation of 19 potential manure CECs using undisturbed soil columns irrigated under unsaturated conditions. The CEC concentrations in leached water were monitored for 13 wk using high performance liquid chromatography-time of flight-mass spectrometry (HPLC-TOF-MS), after which time soil in the cores was removed and sampled for extractable CECs. Compounds quantified in column leachate included all four of the added sulfonamide antibiotics and the nonsteroidal, anti-inflammatory drug flunixin. Only trace amounts of several of the seven hormones, five remaining antibiotics, and two antimicrobials leached from the columns from exogenous soil additions. Soil residues of all 19 compounds were detected, with highest extractable amounts for 17α-hydroxyprogesterone > triclosan (antimicrobial) > flunixin > oxytetracycline. Those CECs with the highest recoveries as calculated by summing leached and extractable amounts were flunixin (14.5%), 17α-hydroxyprogesterone (5.3%), triclosan (4.6%), and sulfadimethoxine (4.8%). Manure management to prevent CEC contamination should consider the potential environmental problems caused by negatively charged compounds with the greatest mobility (flunixin and sulfadimethoxine) and those that have long residence times in soil (triclosan, 17α-hydroxyprogesterone, flunixin, and oxytetracycline). Flunixin is particularly important given its mobility and long residence time in soil.

Soil Column Simulation of Natural Nutrient Flux after Short‐term Inundation

Core Ideas Intact soil column flood studies inaccurately capture short‐term nutrient flux. Filter paper creates barrier to physical disturbance of soil into water column. Filters are permeable to dissolved nutrients to chemically flux into water column. Cone filters prevent floating soil and organic materials from entering water column. Intact soil column studies have been widely used as laboratory simulations in determining the effects of long‐term inundation on nutrient flux in soils, but preliminary studies on total Kjeldahl N (TKN) have shown that physical disturbance associated with current flooding techniques causes an overestimation of TKN flux after short‐term inundation. Therefore, the objective of this study was to create a laboratory protocol that would minimize the effect of physical disturbance associated with current flooding procedures. To determine physical disturbance, total P concentrations were measured as a proxy for soil particles. Within 3 h of inundation, results found that total P concentrations in the water column were significantly reduced when using flat or cone‐shaped filter paper on the soil surface compared with blanks. By adapting the Collins‐filter barrier technique to short‐term flooding simulations, we were able to more accurately predict TKN flux to represent a natural real‐world response to short‐term inundation.

Temporal Dynamics of Bacterial Communities in Soil and Leachate Water After Swine Manure Application.

Application of swine manure to agricultural land allows recycling of plant nutrients, but excess nitrate, phosphorus and fecal bacteria impact surface and drainage water quality. While agronomic and water quality impacts are well studied, little is known about the impact of swine manure slurry on soil microbial communities. We applied swine manure to intact soil columns collected from plots maintained under chisel plow or no-till with corn and soybean rotation. Targeted 16S-rRNA gene sequencing was used to characterize and to identify shifts in bacterial communities in soil over 108 days after swine manure application. In addition, six simulated rainfalls were applied during this time. Drainage water from the columns and surface soil were sampled, and DNA was extracted and sequenced. Unique DNA sequences (OTU) associated with 12 orders of bacteria were responsible for the majority of OTUs stimulated by manure application. Proteobacteria were most prevalent, followed by Bacteroidetes, Firmicutes, Actinobacteria, and Spirochaetes. While the majority of the 12 orders decreased after day 59, relative abundances of genes associated with Rhizobiales and Actinomycetales in soil increased. Bacterial orders which were stimulated by manure application in soil had varied responses in drainage waters over the course of the experiment. We also identified a “manure-specific core” of five genera who comprised 13% of the manure community and were not significantly abundant in non-manured control soils. Of these five genera, Clostridium sensu stricto was the only genus which did not return to pre-manure relative abundance in soil by day 108. Our results show that enrichment responses after manure amendment could result from displacement of native soil bacteria by manure-borne bacteria during the application process or growth of native bacteria using manure-derived available nutrients.

Efficacy of mitigation measures for reducing greenhouse gas emissions from intensively cultivated peatlands

Drained and cultivated fen peats represent some of the world's most productive soils, however, they are susceptible to degradation and typically exhibit high rates of greenhouse gas (GHG) emission. We hypothesised that GHG losses from these soils could be reduced by manipulating water table depth, tillage regime, crop residue application or horticultural fleece cover. Using intact soil columns from a horticultural peatland, emissions of CO2, N2O and CH4 were monitored over a six-month period, using a closed-chamber method. Concurrent measurements of soil properties allowed identification of the key controls on GHG emissions. Raising the water table to the soil surface provided the strongest reduction in global warming potential (GWP100; 25 ± 6 kg CO2-e ha−1 d−1), compared to a free-draining control (80 ± 1 kg CO2-e ha−1 d−1), but this effect was partially negated by an emission pulse when the water table was subsequently lowered. The highest emissions occurred when the water table was maintained 15 cm below the surface (168 ± 11 kg CO2-e ha−1 d−1), as this stimulated N2O loss. Placement of horticultural fleece over the soil surface during spring had no significant effect on GWP100, but prolonged fleece application exacerbated GHG emissions. Leaving lettuce crop residues on the surface increased soil GWP100 (105 ± 4 kg CO2-e ha−1 d−1) in comparison to when residues were incorporated into the soil (85 ± 4 kg CO2-e ha−1 d−1), however, there was no evidence that this promoted positive priming of native soil organic matter (SOM). For maximum abatement potential, mitigation measures should be applied during the growing season, when GHG emissions are greatest. Our results also suggest that introduction of zero- or minimum-till practices may not reduce GHG emissions. Maintaining a high water table was the only option that reliably reduced GHG emissions, however, this option is impractical to implement within current horticultural systems. We conclude that alternative strategies or a major change in land use (e.g., conversion from horticulture/arable to wetland) should be explored as a means of preserving these soils for future generations.

Visible\u2013Near-Infrared Spectroscopy can predict Mass Transport of Dissolved Chemicals through Intact Soil

The intensification of agricultural production to meet the growing demand for agricultural commodities is increasing the use of chemicals. The ability of soils to transport dissolved chemicals depends on both the soil’s texture and structure. Assessment of the transport of dissolved chemicals (solutes) through soils is performed using breakthrough curves (BTCs) where the application of a solute at one site and its appearance over time at another are recorded. Obtaining BTCs from laboratory studies is extremely expensive and time- and labour-consuming. Visible–near-infrared (vis–NIR) spectroscopy is well recognized for its measurement speed and for its low data acquisition cost and can be used for quantitative estimation of basic soil properties such as clay and organic matter. In this study, for the first time ever, vis–NIR spectroscopy was used to predict dissolved chemical breakthrough curves obtained from tritium transport experiments on a large variety of intact soil columns. Averaged across the field, BTCs were estimated with a high degree of accuracy. So, with vis-NIR spectroscopy, the mass transport of dissolved chemicals can be measured, paving the way for next-generation measurements and monitoring of dissolved chemical transport by spectroscopy.

Particle- and colloid-facilitated Pb transport in four historically contaminated soils - Speciation and effect of irrigation intensity

Due to the low solubility of lead (Pb) in many soils, colloidal and particulate transport may have large effects on Pb leaching. However, the role of colloidal and particulate transport varies considerably between soils and the mechanisms controlling mobilisation are complex and poorly known. Furthermore, increased frequency of high-intensity rainfall events is expected in some parts of Europe and North America in response to climate change, which might increase the mobilisation of particles and colloids. In this work, we investigated transport of particulate (>0.45 μm), colloidal (10 kDa-0.45 μm) and truly dissolved (<10 kDa) Pb in an irrigation experiment on intact soil columns from four historically contaminated soils. We also investigated the effect of irrigation intensity (2–20 mm h−1) on Pb leaching in these fractions. The mechanism binding Pb on particles and colloids was evaluated by extended X-ray absorption fine structure (EXAFS) spectroscopy and geochemical modelling. A 10-fold increase in irrigation intensity brought about at most a three-fold change in leached particulate and colloidal Pb concentrations. In contrast, the fraction of leached Pb associated with particles and colloids varied by one order of magnitude between soils. Hence, the results suggest that it is more important to consider soil type than potential future increases in rainfall. For one soil with high concentrations of both arsenic (As) and Pb, geochemical modelling indicated that mimetite, Pb5(AsO4)3Cl(s), was the major Pb species in the colloidal and particulate fractions. For the other three soils, EXAFS of Pb on isolated particles and colloids indicated that ferrihydrite was a major phase-sorbing Pb and this was supported by geochemical equilibrium modelling. Thus geochemical modelling can be used to indicate the speciation of Pb in particles and colloids leached in intact soils.

Effects of tillage and liming on macropore networks derived from X‐ray tomography images of a silty clay soil

AbstractSoil structure influences water infiltration, aeration and root growth and, thereby, also the conditions for sustainable crop production. Our objective was to quantify the effects of different soil management methods and land uses on the topsoil structure of a silty clay soil. We sampled 32 intact soil columns (18 cm high, 12.7 cm diameter) from an experimental silty clay field with four treatments: conventional tillage (CT), conventional tillage followed by liming (CTL), reduced tillage (RT) and unfertilized fallow (UF). The columns were analysed using 3‐D X‐ray tomography. The samples were taken in autumn after harvest, 7 yr after quick lime was applied to the CTL plots. Despite a relatively large number of replicates per treatment (8, 8, 8 and 6 (two UF samples were excluded), respectively), there were no significant differences between any of the investigated macropore network properties related to tilled treatments. The UF treatment, in contrast, exhibited more vertically oriented macropores, which were also better connected compared to the other treatments. This confirms previous findings that tillage may disrupt the vertical continuity of macropore clusters. The impact of liming on soil pore network properties may have been limited to pores smaller than the resolution in our X‐ray images. It is also possible that the effects of lime on soil structure were limited to a few years which means that any effect would have diminished by the time of this study. These matters should be further investigated in follow‐up studies to understand better the potential of lime amendments to clay soil.