Intact Soil Columns Research Articles

Leaching of antibiotic resistance genes and microbial assemblages following poultry litter applications in karst and non-karst landscapes

Antibiotic resistance is increasingly recognized as a critical challenge affecting human, animal, and environmental health. Yet, environmental dynamics and transport of antibiotic resistance genes (ARGs) and microbial communities in karst and non-karst leachate following poultry litter land applications are not well understood. This study investigates impacts of broiler poultry litter application on the proliferation of ARGs (tetW, qnrS, ermB, sulI, and blaCTX-M-32), class 1 integron (intI1 i), and alterations in microbial communities (16S rRNA) within karst derived soils, which are crucial and under-researched systems in the global hydrological cycle, and non-karst landscapes. Using large, intact soil columns (45 cm diam. × 100 cm depth) from karst and non-karst landscapes, the role of preferential flow and ARG transport in leachate was enumerated following surface application of poultry litter and simulated rain events. This research demonstrated that in poultry litter amended karst soils, ARG (i.e., ermB and tetW) abundance in leachate increased 1.5 times compared to non-karst systems (p < 0.05), highlighting the influence of geological factors on ARG proliferation. Notably, microbial communities in karst soil leachate exhibited increased diversity and abundance, suggesting a potential linkage between microbial composition and ARG presence. Further, our correlation and network analyses identified relationships between leachate ARGs, microbial taxa, and physicochemical properties, underscoring the complex interplay in these environmentally sensitive areas. These findings illuminate the critical role of karst systems in shaping ARG abundance and pollutant dispersal and microbial community dynamics, thus emphasizing the need for landscape-specific approaches in managing ARG dissemination to the environment. This study provides a deeper understanding of hydrogeological ARG dynamics but also lays the groundwork for future research and strategies to mitigate ARG dissemination through targeted manure applications across agricultural landscapes.

Release of phosphorus and metal(loid)s from manured soils to floodwater during a laboratory simulation of snowmelt flooding.

Phosphorus (P) and metal accumulation in manured agricultural soils and subsequent losses to waterways have been extensively studied; however, the magnitudes and the factors governing their losses during spring snowmelt flooding are less known. We examined the P and metal release from long-term manured soil to floodwater under simulated snowmelt flooding with recent manure additions. Intact soil columns collected from field plots located in Randolph, Southern Manitoba, 2 weeks after liquid swine manure treatments (surface-applied, injected, or control with no recent manure addition) were flooded and incubated for 8 weeks at 4± 1°C to simulate snowmelt conditions. Floodwater (syringe filtered through 0.45µm) and soil porewater (extracted using Rhizon-Mom samplers) samples were periodically extracted and analyzed for dissolved reactive phosphorus (DRP), pH, zinc (Zn), manganese (Mn), iron (Fe), magnesium (Mg), calcium (Ca), and arsenic (As). Mean floodwater DRP concentrations (mg L-1) for manure injected (2.0±0.26), surface-applied (2.6±0.26), and control (2.2±0.26) treatments did not differ significantly. Despite manure application, DRP loss to floodwater did not significantly increase compared to the control, possibly due to the elevated residual soil P at this site from the long-term manure use. At the end of simulated flooding, the DRP concentrations increased by 1.5-fold and 5-fold in porewater and floodwater, respectively. Metal(loid) concentrations were not affected by manure treatments in general, except for Zn and Mg on certain days. Unlike DRP, where porewater and floodwater concentrations increased with time, metalloid concentration in porewater and floodwater did not show consistent trends with flooding time.

Preferential flow of phosphorus and nitrogen under steady‐state saturated conditions

AbstractRepeated broiler litter application on agricultural lands can cause nutrient enrichment of subsurface effluent, especially with the existence of preferential flow through soil macropores. Previous studies quantifying soil macropores have not attempted to establish a connection of soil macropore characteristics with the subsurface nutrient (nitrogen [N] and phosphorus [P]) losses, across different topographical locations in the field. This study investigated the effect of broiler litter application and preferential flow on subsurface nutrient transport (N and P) at different topographical positions (upslope, midslope, and downslope) in a no‐till pasture field located in Alabama, USA. Twelve intact soil columns (150 mm id and 500 mm length) were used, and the nutrient leaching measurements from laboratory experiments were linked to soil macropore characteristics quantified using X‐ray computed tomography image analysis and solute transport modeling. Treatments included surface broadcast broiler litter (5 Mg ha−1, on dry basis) and unamended control. Leachates were analyzed for dissolved reactive P (DRP), total P (TP), and nitrate + nitrite‐N (NO3− + NO2−–N). The bromide breakthrough curves provided evidence of preferential flow in all columns. Litter application significantly increased leachate P concentrations, and average TP and DRP concentrations were significantly higher in the leachate from upslope columns compared to those at downslope location. The NO3−–N concentrations in leachate exceeded the US EPA drinking water standard of 10 mg L−1 in all the treatment columns. The highest flow‐weighted mean concentrations of TP and DRP, at 2.7 and 2.5 mg L−1, respectively, were recorded in the upslope columns. Soil physicochemical properties and nutrient leaching losses varied substantially across topographical positions, indicating a need for variable litter application rates to reduce P build‐up and subsequent leaching in vulnerable locations within the field. The relevance of the effect of topographic position on nutrient leaching found in this study should be further tested by investigating a wider range of slopes and soil types in pastures.

Grass cover and shallow tillage inter-row soil cultivation affecting CO2 and N2O emissions in a sloping vineyard in upland Balaton, Hungary

Greenhouse gas (GHG) emissions can accelerate the current climate change trend, resulting in higher soil temperatures and changes in soil water contents. The aim of the study was to investigate to what extent different cultivation methods and sampling positions on the slope (location along the slope) are affecting soil GHG emissions in vineyards. In the study, different inter-row soil cultivation methods were investigated including shallow soil tillage (ST) and grass cover (GC) inter-rows. Intact soil columns were collected and different soil water contents were applied under a laboratory environment to simulate different precipitation conditions. Our findings showed significant differences in the emitted CO2 and N2O between the cultivation methods, where the GC soil had 2.9 times higher average CO2 and 6.1 times higher N2O emissions compared to the bare, ST soil. The amount of initial and longer term soil water content significantly influenced soil-derived GHG emissions, with the highest averaged values observed for the continuously high water content soil samples (0.377 mgCO2 m−2 s−1 and 0.076 μgN2O m−2 s−1). For the GC inter-rows, we also observed significant differences in the GHG emissions among the different sampling positions of the investigated slopes. We found that besides the different soil water contents, the main causes for these differences could be the variances in the soil physical and chemical properties. The findings of this study can be used to better estimate soil GHG emissions at various water contents, for different inter-row soil management cultivation methods, and potentially mitigate some adverse effects of global warming by reducing the amount of GHG released into the atmosphere.

Conversion of biobased substances into biochar to enhances nutrient adsorption and retention capacity.

Reducing the environmental issues brought on by nutrients especially nitrogen pollution and loss is important. Owing to its unique composition and physico-chemical characteristics, biomass-derived biochar exhibits varying degrees of adsorption and interception for all types of soil nutrients. Thus, a novel way to improve nutrient absorption in the soil is to include biomass derived biochar into it. Various biomass-derived biochar from locally available biobased substances was synthesized through low-cost portable charring kiln. It has been quantified the influence of four biobased substances and three pyrolysis temperature on different morphomineralogical characteristics of biochar for utilizing as low-cost sorbent to manage nutrient adsorption and retention capacity. The morphomineralogical characteristics were principally manipulated by feedstocks rather than pyrolysis temperature. Higher porosity and surface area of biomass-derived biochar illustrated its soil structural modification and nutrient retention capacity along with their utilization for adsorbents. With increase in pyrolysis temperature, the adsorption capacity of biochar for NH4+-N and NO3--N was gradually weakened and gradually enhanced respectively. The adsorption process of ammonia nitrogen and nitrate nitrogen conformed to the Langmuir model and the fitted KL value was less than 1 indicating that the adsorption process was uniform monolayer adsorption and the adsorption of biochar was favorable adsorption. With increase in biochar application rate the leaching of NO3--N decreased having higher at 2.5 t ha-1 application rate followed by 5 t ha-1 and lower at 7.5 t ha-1. In packed soil column, the NH4+-N in leachate was maximum in T7 (18.6), followed by T4 (17.9), T13 (17.3) and minimum in T10 (17.2) at same application rate of manures and biochar. Finally, results also revealed that packed soil column performed better as compared with intact soil column to retain soil nutrient and hence, leaching potential of nutrient was less in packed column than intact soil column. In conclusion, biomass-derived biochar can enhance the amount of nutrient that is absorbed into the soil while decreasing the loss of nutrient from the soil in the form of ammonia and nitrate. To sum up, biomass-derived biochar can increase the adsorption amount of the nitrogen and reduce the loss of ammonia nitrogen and nitrate nitrogen in the soil, thus retaining the nitrogen.

Soil enzyme activities in heavily manured and waterlogged soil cultivated with ryegrass (Lolium multiflorum)

Extended waterlogging (WL) conditions can alter soil enzyme activities and their role in maintaining healthy soils. We assessed the effects of soil moisture regimes (field capacity [FC] and WL) and phosphorus (P) rates (0, 15, 30, 45 kg available P ha–1) on ( i) soil enzymes and microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and microbial biomass P (MBP); and ( ii) dissolved organic carbon (DOC) and total dissolved nitrogen (TDN). The treatments were tested in a 4-month greenhouse experiment using intact soil columns under annual ryegrass ( Lolium multiflorum). WL decreased the activity of β-glucosidase and acid phosphomonoesterase but increased N-acetyl-β-glucosaminidase in soils. These changes were associated with changes in MBC, DOC, MBN, and TDN, but not MBP. Anoxic conditions in WL soil promote the activity of anaerobes and contribute to the reduction of Fe oxyhydroxides and the release of DOC, TDN, and P in the soil solution. The activity of the extracellular enzymes decreased in WL with additions of slurry indicating adequate supply of C, N, and P. Our results also showed that both enzyme activities and microbial biomass were restricted in the upper soil layer with limited downward movement along the soil profile. We can conclude that since these enzymes control the hydrolysis of cellulose, phosphomonoester, and chitin, soil moisture influences the direction and magnitude of C, N, and P in manured and waterlogged soil cultivated with ryegrass.

Response of the nitrogen processing bacterial community to water level fluctuation and nitrate availability in an intact marsh soil column.

Wetlands are known to experience fluctuations in water levels and receive exogenous nitrogen inputs that affect various organisms, including soil microorganisms. To study the impact of these factors on microbial diversity, we collected intact soil columns from a Phragmites australis-dominated site in the Qixing River National Nature Reserve in Northeast China. In a laboratory experiment, we simulated water level fluctuations and exogenous nitrogen inputs to the soil columns and examined the associated changes in the relative abundance of 51 bacterial genera involved in nitrogen cycling processes. Our findings revealed that different bacterial genera exhibited varying relative abundances across treatments. Specifically, Massilia showed the highest total relative abundance at the genus level, while Planctomyces had the second highest, and Campylobacter had the lowest abundance. The DESeq2 model, based on negative binomial distribution, revealed that the tags of bacterial genera were significantly correlated with soil depth, but not with water levels or nitrogen concentrations. However, the addition of a 30mg/L nitrate solution caused a decrease in the relative abundances of bacterial genera with decreasing water levels, while a 60mg/L concentration of nitrogen resulted in a decrease and then an increase in the relative abundances of bacterial genera with decreasing water levels. Our study provides valuable insights into the response of nitrogen-cycling bacteria to changes in different environmental conditions.

Leaching of soil legacy nitrogen in intact soil columns and significance of soil macropore structure

Excessive application of chemical fertilizers accumulates nitrogen in soil (soil legacy nitrogen), and its release has a long-term impact on environmental quality. The information regarding leaching of soil legacy nitrogen and role of soil pore structure is scarce. Fifteen undisturbed soil cores with a depth of ~200 mm were collected from five paddy fields in a subtropical area of China, and soil pore structure was characterized with X-ray computed tomography. The batch leaching column experiments were conducted to investigate the nitrogen leaching from the soil cores. In the leachate, inorganic nitrogen (NO3−–N and NH4+–N) accounted for 73–85 % of total dissolved nitrogen (TDN), which showed small variations among the five sampling sites. The NO3−–N, NH4+–N and TDN concentrations in the leachate over the number of leaching times could be well fitted with the Gaussian, exponential or linear models, suggesting that different nitrogen forms showed variable leaching dynamics. NO3−–N and TDN leaching losses continued to increase with the number of leaching times, whereas NH4+–N leaching from the soil had a threshold value. Combination of changes in soil nitrogen content after leaching, the lagging effect of soil legacy nitrogen may be partly attributed to the continuous transformation of NH4+–N and organic nitrogen into NO3−–N in the soils. Besides the nitrogen content in the soil, the most important factor of soil pore structure controlling NO3−–N and TDN leaching loss was the fractal dimension; NH4+–N was mainly controlled by connectivity. The present study highlighted the importance of soil pore structure and nitrogen format transformation in the leaching of soil legacy nitrogen.

Phosphorus Mobility in Heavily Manured and Waterlogged Soil Cultivated with Ryegrass (Lolium multiflorum)

Extended waterlogging (WL) conditions in heavily manured soils can change soil phosphorus (P) dynamics. We assessed the effects of soil moisture regimes (field capacity (FC) and WL) and P rates on (i) dry matter (DM) yield and P offtake of ryegrass, (ii) changes in soil Fe3+, Fe2+, and soil P, and (iii) risk of P leaching. The treatments were tested in a four-month greenhouse experiment using intact soil columns and annual ryegrass (Lolium multiflorum). The DM yield and P offtake were lower under WL compared with FC. The concentration of Fe3+ was 1984 mg kg−1 (0–30 cm) under FC, but 1213 mg kg−1 at 0–5 cm and 2024 mg kg−1 at 25–30 cm depth under WL. The concentration of Fe2+ was 244 mg kg−1 (0–30 cm) under FC, but 2897 at 0–5 cm and 687 mg kg−1 at 25–30 cm under WL. The water extractable P (Pw) was 12.7 mg kg−1 at 0–5 cm and 9.5 mg kg−1 at 25–30 cm under FC, but 8.6 mg kg−1 at 0–5 cm and 10.5 mg kg−1 at 25–30 cm under WL. The P saturation index (PSI) was 27.2% at 0–5 cm and 17.4% at 25–30 cm under FC, but averaged 11.9% at 0–30 cm under WL. We can conclude that extended WL associated with flooding creates reducing conditions in the soil, thus decreasing the concentration of Fe3+, but increasing the concentrations of Fe2+ and the solubility of P which can exacerbate the risk of P loss with runoff and leaching.

Mobility of arsenic and vanadium in waterlogged calcareous soils due to addition of zeolite and manganese oxide amendments.

Addition of manganese(IV) oxides (MnO2 ) and zeolite can affect the mobility of As and V in soils due to geochemical changes that have not been studied well in calcareous, flooded soils. This study evaluated the mobility of As and V in flooded soils surface-amended with MnO2 or zeolite. A simulated summer flooding study was conducted for 8 weeks using intact soil columns from four calcareous soils. Redox potential was measured in soils, whereas pH, major cations, and As and V concentrations were measured biweekly in pore water and floodwater. Aqueous As and V species were modeled at 0, 4, and 8 weeks after flooding (WAF) using Visual MINTEQ modeling software with input parameters of redox potential, temperature, pH, total alkalinity, and concentrations of major cations and anions. Aqueous As concentrations were below the critical thresholds (<100μgL-1 ), whereas aqueous V concentrations exceeded the threshold for sensitive aquatic species (2-80μgL-1 ). MnO2 -amended soils were reduced to sub-oxic levels, whereas zeolite-amended and unamended soils were reduced to anoxic levels by 8 WAF. MnO2 decreased As and V mobilities, whereas zeolite had no effect on As but increased V mobility, compared to unamended soils. Arsenic mobility increased under anoxic conditions, and V mobility increased under oxic and alkaline pH conditions. Conversion of As(V) to As(III) and V(V) to V(IV) was regulated by MnO2 in flooded soils. MnO2 can be used as an amendment in immobilizing As and V, whereas the use of zeolite in flooded calcareous soils should be done cautiously.

Depth-specific transport of bacteriophages MS2 and ΦX174 in intact soils

Soil geochemical and structural heterogeneities are coupled in influencing the fate and transport of viruses in soil profiles. Only the soil layer at a certain depth, where viruses have least mobility, determines the total flux and distance of virus transport. However, few studies have addressed the dependence of virus transport on soil depths. This dependence is mostly due to the gradients of geochemistry, soil structure, and solution chemistry along the soil profile. We employed intact soil columns (5 cm in length, 4.5 cm in inner diameter) from three depths (5–10 cm, 35–40 cm, and 65–70 cm) to investigate the transport of two very different model viruses: 1) MS2; a hydrophobic, negatively charged, less aggregated phage of E. coli, and 2) ΦX174; a hydrophilic, positively charged (pH=6.58 ± 0.21), highly aggregated phage of E. coli as a function of depth. The breakthrough of both viruses increased with soil depth in both 10 mM and 40 mM NaCl solutions. The mobility of MS2 was greater than ΦX174 at 5–10 cm and 35–40 cm depths, but the mobility of ΦX174 increased much faster than MS2 with soil depth and surpassed MS2 mobility in the soil of 65–70 cm depth. The effect of ionic strength on the mobility of nanosized MS2 decreased with soil depth because of decreased soil organic matter (SOM) content. The decreased SOM lessened ionic strength sensitivity via a hydrophobicity-reduced Lewis acid-base attraction mechanism. In comparison, ΦX174 viruses showed an overall weaker response to ionic strength change since they were aggregated into microsized particles in the experimental solutions and thereby subjected to mechanical straining in the intact soils. The elevated straining effect counteracted the electrostatically facilitated transport of ΦX174. These findings suggest that soil amendments with organic matter might be low cost, effective measures for reducing virus transport through soil profiles.

Initial soil moisture conditions affect the responses of colloid mobilisation and associated cadmium transport in opposite directions

The effects of initial soil moisture on colloid-associated transport are still poorly understood given the well-recognized significance of colloid-facilitated transport of strongly-sorbing contaminants. In this study, Cd leaching was sequentially conducted in an intact soil column under three initial moisture conditions (near saturation, field capacity and dryness). Soil colloids were always the dominant carriers for Cd. However, upon the lowering of initial soil moisture, increased transport of colloids (96.2→101.0→168.2 mg) was observed, surprisingly, along with decreased transport of colloid-associated Cd (C-Cd) (23.9→10.7→8.2 µg) and enrichment factor (248.4→105.9→48.8 mg/kg) of Cd on colloids, resulting from pH reduction which increased Cd desorption and colloid size increase and/or ζ-potential decrease that showed lower affinity for Cd. Correlation, redundancy analysis and structural equation modelling revealed the dominantly positive role of colloids, EC plus cations (Ca2+ and Mg2+) in the release of C-Cd and dissolved Cd (D-Cd), respectively, under initial moistures of near saturation and field capacity. Under initially dry conditions, soil water potential showed dominantly negative effects on the transport of both C-Cd and D-Cd. These findings highlighted the critical role of initial moisture conditions in modulating colloid-facilitated Cd mobilisation, providing insights into the environmental risk assessment of heavy metals in other leaching scenarios.

The legacy effect of biochar application on soil nitrous oxide emissions

AbstractExisting studies suggest that biochar application can reduce soil nitrous oxide (N2O) emissions, mainly based on short‐term results. However, it remains unclear what the effects (i.e., legacy effects) and underlying mechanisms are on N2O emissions after many years of a single application of biochar. Here, we collected intact soil columns from plots without and with biochar application in a subtropical tea plantation 7 years ago for an incubation experiment. We used the N2O isotopocule analysis combined with ammonia oxidizer‐specific inhibitors and molecular biology approaches to investigate how the legacy effect of biochar affected soil N2O emissions. Results showed that the soil in the presence of biochar had lower N2O emissions than the control albeit statistically insignificant. The legacy effect of biochar in decreasing N2O emissions may be attributed to the reduced effectiveness of the soil substrate, nitrification and denitrification activities, and the promotion of the further reduction of N2O. The legacy effect of biochar reduced the relative contribution of nitrifier denitrification/bacterial denitrification, nitrification‐related N2O production, and the relative abundance of several microorganisms involved in the nitrogen cycle. Our global meta‐analysis also showed that the reduction of N2O by biochar increased with increasing application rate but diminished and possibly even reversed with increasing experimental time. In conclusion, our findings suggest that the abatement capacity of biochar on soil N2O emissions may weaken over time after biochar application, but this remains under further investigation.

Waterlogging effects on N2O and N2 emissions from a Stagnosol cultivated with Silphium perfoliatum and silage maize

European policy recommends that biomass production occur on marginal land, such as poorly draining Stagnosols. Compared to annual cropping, perennial crops may better mitigate N2O emissions at such sites, through more complete denitrification. To test that hypothesis, we compared N2 and N2O fluxes from the soils of a perennial crop (cup plant, Silphium perfoliatum L.) and an annual crop (silage maize, Zea mays L.). Intact soil columns (35 cm height, 14.4 cm diameter) were incubated for 37 days. The soils were fertilized with 60 or 120 kg N ha−1 and exposed to successive phases of waterlogging: free drainage, waterlogging of 1/3-, and waterlogging of 2/3- of the column. Source-specific N2O and N2 fluxes were measured using the 15 N gas flux method. Denitrification was higher in cup plant than maize soil and total N losses from denitrification were dominated by emissions from the third phase. Cup plant soil emitted 33.6 ± 78.1 mg N m−2 and 95.8 ± 64.4 mg N m−2 more N2O than maize soil in the low and high N treatments, respectively. The product ratio of denitrification (N2Oi = N2O/(N2 + N2O)) increased with waterlogging in maize soil, while remaining stable in cup plant soil. Emissions from the top 10 cm dominated the N2Oi rather than N2 fluxes from the saturated soil. This study did not show N2O mitigation in cup plant soil, instead highlighting the complexity of plant-soil effects on denitrification. We clearly showed that the application of a general N2Oi for agricultural soils across annual and perennial cropping is not recommended.

Identification of water flow through non‐root soil macropores and along roots in shrub‐encroached grassland

AbstractIdentification of water flow through non‐root soil macropores and along roots using X‐ray computed tomography (CT) is still challenging because of limited methods to quantify roots and pore structures. This study selected shrub‐encroached grassland consisting of a mosaic of shrub patches and interspace grass patches and the original grassland as a control treatment. Twelve intact soil columns (0–30 cm deep) were excavated, which included four replicates each from the soils under the original grassland, the shrub patches and interspace grass patches. Three‐dimensional networks of soil macropores and roots were quantified using CT scanning and image analysis techniques, and active flow pathways were identified through KI tracing experiment using CT scanning. The results showed that the tracer depths of soils under the shrub patches and interspace grass patches were deeper than those of the original grassland, which was attributed to the well‐developed root system. The tracer volume density was significantly and positively correlated with soil macropore node density and root volume density (p &lt; 0.05), and significantly and negatively correlated with root mean angle (p &lt; 0.05). Soils under the shrub patches had lower tracer volume density than that under the interspace grass patches, which was attributed to the horizontal‐oriented roots under the shrub patches. The relative contribution of water flow along roots was larger than that through non‐root soil macropores 35 min after dye application. This provided direct evidence that shrub roots were the main pathway of macropore flow resulting in water flow into deep soil layers in the shrub‐encroached grassland.Highlights The tracer depths of shrub‐encroached grassland were deep due to well‐developed root system. Shrub patches had low tracer volume density due to the horizontal‐oriented roots. The contribution of water flow along roots was larger than that through non‐root soil macropores. Shrub roots were the main pathway of water flow.

Preferential transport of phosphorus from surface‐applied poultry litter in soils from karst and nonkarst landscapes

AbstractPreferential transport of P in manure‐amended soils through soil macropores can be a substantial source of nonpoint P pollution, although studies quantifying P losses from intact soil columns are lacking. Large, intact soil columns (45 cm diam. by 100‐cm height) collected from karst and nonkarst landscapes (three replicates per soil) were used to evaluate the role of preferential flow in P leaching from surface‐applied poultry litter. Losses of P in leachate, dissolved P, and total P were quantified following poultry litter application (5.6 Mg ha–1) and three successive simulated rain events (3.9 cm h–1 for 2 h). Dissolved P constituted the major fraction of leachate‐P loss as more than 80% of TP, which peaked (5.61 mg L–1) following the first irrigation event after poultry litter application and decreased to about half (2.88 mg L–1) by the third irrigation event. Soils from the karst landscape had greater (p &lt; .05) leachate volume and total leached P, with approximately six times greater losses occurring than nonkarst soils. Results indicate that the top 1 m of soil obtained from karst landscapes is vulnerable to vertical P transport into deeper soil layers. Therefore, soils with substantial preferential flow pathways may increase legacy P in subsurface soil and pose a threat to surface and groundwater over time.

Evidence of the mineral ZnHAsO4·H2O, koritnigite, controlling As(V) and Zn(II) solubility in a multi-contaminated soil

Assessing element speciation and solubility control mechanisms in multi-contaminated soils poses great challenges. In this study, we examined the speciation and mechanisms controlling the solubility of As and Zn in a soil historically contaminated with As, Cu, Cr, and Zn salts used for wood preservation. The leaching behavior of dissolved species, particles, and colloids was studied in an irrigation experiment with intact soil columns. Batch experiments were used to study the solubility of dissolved species as a function of pH (2–8). The speciation of As and Zn in bulk soil and leached particles was studied with microscale X-ray fluorescence (μ-XRF) and extended X-ray absorption fine structure (EXAFS) spectroscopy. Chemical speciation and solubility were evaluated by geochemical modelling. μ-XRF of bulk soil and particles showed that As and Zn were correlated in space. Bulk- and μ-EXAFS of As and Zn, in combination with calculated ion activity products of possible As-Zn minerals, suggested a koritnigite (ZnHAsO4·H2O) phase controlling the dissolved fraction of As(V) and Zn with an apparent log Ksp of −21.9 ± 0.46. This phase lowered the solubility of As by almost two orders of magnitude in soil at pH > 5, and could therefore be of great importance at other multi-contaminated sites.

Preferential flow under high‐intensity short‐duration irrigation events in soil columns from a karst and nonkarst landscape

AbstractPreferential flow through soil macropores is a major cause of rapid transport of contaminants through soils to drainages systems. Knowledge of the extent of macropore flow in different soil systems is crucial for the effective implementation of nutrient management plans. The objective of this study was to determine the differences in preferential flow in soil columns characterized by varying contents of coarse fragments with depth and obtained from a regional, pasture‐based karst and a nonkarst landscape. Intact soil columns (40 × 100 cm; two replicates per treatment) were subjected to two Brˉ leaching experiments (BL1 and BL2) which included a pulse application of Brˉ tracer solution (3.02 and 1.15 g L−1 during BL1 and BL2, respectively), each followed by five successive irrigation events (5.67 mm min−1 for 2 min in 30‐min intervals). Each soil column rested on a stainless‐steel grid (consisting of 100 cells, each measuring 3.81 cm × 3.81 cm), which facilitated partitioning of the soil bottom and collection of drainage from each grid. Significant differences in the drainage losses of water and Brˉ through the 1‐m‐deep soil profiles were observed with karst‐derived stony soils draining approximately five times greater water and Brˉ, compared with nonkarst‐derived soil. These results suggest that soils with high fraction of coarse fragments are more susceptible to leaching losses than soils with less coarse fragments. Further studies under different flow regimes and encompassing different soil units may be beneficial in understanding the potential leaching risk in these areas and for implementing best management practices for landscape‐based nutrient management.

Potential effect of nitrate applied during flood period on N2O emissions during subsequent drainage in a paddy soil

Application of nitrate (NO3−) in flooded soil promotes denitrification activity and N2O emissions, accompanied by NO3− depletion. However, whether the stimulating effect of NO3− on flooded soil still affects the response of nitrifiers and denitrifiers to subsequent water drainage remains unclear, as does the N2O emission dynamics. We conducted a microcosm experiment using intact paddy soil columns with three levels of NO3− treatment under flooded conditions: the application of NO3− solutions with 0, 60 and 200 kg KNO3-N per hectare (N0, N60, and N200, respectively). The N2O emission rate was measured using a closed chamber method, and the community composition and abundance of nitrifiers and denitrifiers were determined using the terminal restriction fragment length polymorphism (T-RFLP) and real-time quantitative polymerase chain reaction (qPCR) methods, respectively. The results showed that all the added NO3− was exhausted during the flood period and below 0.50 mg kg−1 at the end of flooded incubation. Arch-amoA and narG gene abundance were significantly elevated by NO3− application before drainage. Then water drainage promoted the narG gene numbers of N60- and N200-treated soils increase by 109 copies g−1 more than N0 treatment, and the community compositions of narG- and nirK-containing denitrifiers differed significantly among the three NO3− treatments. In addition, draining resulted in clear shifts in the community structures of Arch-amoA- and Bac-amoA-containing nitrifiers in the N60-treated soils, and also resulted in higher increase of Arch-amoA gene abundance in the N60- and N200-treated soils than in N0 treatment. These results indicate that NO3− application on flooded soil enhance the response of Arch-amoA and narG gene abundance to subsequent water drainage. However, the differences in the denitrifier community had few correlations with N2O emissions during draining (p > 0.05), while the Arch-amoA gene abundance in the N0- and N60-treated soils increased about 6.45 and 3.70 times respectively after drainage (p < 0.05) and strongly correlated with higher N2O emissions (r > 0.92, p < 0.01). Considering the NH4+-N consumption and the NO3−-N production, we could speculate that the changes in the ammonia-oxidizing archaea community after drainage are only important regulators of N2O emission rates in this paddy soil columns.

Effects of bio-subsoilers on subsoil pore-system functionality: Case study with intact soil columns

Field traffic-induced subsoil compaction poses an escalating problem in relation to key soil functions because of the use of increasingly heavier machinery. Natural recovery from subsoil compaction is slow and therefore there is a need for measures such as bio-subsoilers to stimulate the recovery. This study thus investigated the influence of potential bio-subsoilers on pore functionality and morphology of a sandy loam soil, with an emphasis on the transition between the topsoil and compacted subsoil and on the compacted subsoil per se, in an intact soil column experiment. The control crop was spring barley, and potential bio-subsoilers included chicory, lucerne, fodder radish, tall fescue and kernza. The intact soil columns (Ø = 0.20 m, h = 0.50 m) were X-ray CT-scanned prior to and two years after crop establishment. Undisturbed subsamples (246 cm3) were collected one and two years after crop establishment in five replicates at 0.10, 0.25 and 0.35-m depth from the soil columns. The laboratory measurements and derived indices showed that after one-year non-restrictive air-filled porosity (εa = 0.13 m3 m−3) and relative gas diffusivity (Ds/Do = 0.0059) was reached by lucerne in the Transition layer (0.25 m). In the second year, lucerne also showed a significantly higher εa (0.09 m3 m−3), and a non-significantly higher Ds/Do (0.0029) compared to barley in the Compacted layer (0.30 m). Pore geometry indices derived from the relationship between εa, Ds/Do and air permeability indicated the formation of less tortuous and more connected small pores (at −100 hPa matric potential) throughout the Transition and Compacted layers by chicory, lucerne and tall fescue. A significantly higher CT-number of pores < 1 mm3 was found for lucerne and a higher CT-volume of pores < 1.5 mm in diameter for lucerne and chicory in the Compacted layer. Root number density was significantly higher for chicory, lucerne and tall fescue compared to barley from the first year, and also for kernza from the second year. Lucerne had the greatest subsoil compaction mitigation potential in our experiment in intact columns. Further studies in real field situations are needed.