Soil Atmosphere Research Articles

Spatio-temporal soil drying in southeastern South America: the importance of effective sampling frequency and observational errors on drydown time scale estimates

ABSTRACT The study of the spatio-temporal dynamics of surface soil moisture (SSM) drydowns integrates the soil response to climatic conditions, drainage and land cover and is key to advances in our knowledge of the soil–atmosphere water exchange. SSM drydowns have also been employed to compare soil moisture spatio-temporal behaviour between different data sources such as satellite-derived data and land–surface models, difficult to compare with standard methodologies. However, the errors introduced by satellite effective sampling frequencies (SF) and by different methodologies employed to define a drydown period have until now not been properly addressed in the literature. Here, SSM from microwave remote sensing products operating at L, C and X frequency bands are analysed together with SSM from a land–surface model in southeastern South America during 2010–2014, at seasonal and annual scales. We use an SSM-based drydown detection methodology and an exponential model to estimate the drydown time scale. The errors generated by the SF and by using SSM instead of precipitation to define the start of the drydown period are examined using a synthetic soil moisture model. Most of the products can detect the negative correlation between aridity conditions and drydown time scales (faster soil drying in the semiarid west and slower – and noisier – towards the wetter east). The Soil Moisture Ocean Salinity (SMOS) L-band product reproduces the smoothest drydown time scale spatial patterns at the annual and seasonal scales and displays large seasonal contrasts, although its error due to SF is the highest among the three products. The Organizing Carbon Hydrology In Dynamic Ecosystems (ORCHIDEE) land–surface model resampled by the SF of each product shows better agreement with SMOS, followed by the X-band product. The agreement is higher over the southern Pampas Plains, a region with high coverage of satellite-derived data and flat topography. SSM observational errors generate higher relative uncertainties for drydown time scales longer than 8 days, while the SF is more relevant for shorter drydowns. Also, the SF has a larger impact than soil depth, particularly in the dry season, when sparse temporal coverage misses short drydowns. Soil texture influence is captured by SMOS and ORCHIDEE, revealing slower drydowns for finer textures at the annual scale. Our results show that the soil drying behaviour is comparable between microwave remote sensing products and a land–surface model and that the observational errors and the SF are important sources of uncertainty to consider when interpreting drydown results.

Depth dependence of temperature sensitivity of soil carbon dioxide, nitrous oxide and methane emissions

Carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) are three important greenhouse gases (GHGs) associated with rising temperature. Their temperature sensitivities (Q10) play important roles in predicting soil–atmosphere gas exchange under global warming. Few studies have simultaneously evaluated the variation in Q10 of CO2, N2O and CH4 emissions across different soil depths. Here, we sampled soils from 13 sites along a climatic transect in temperate grasslands of northern China, including meadow steppe, typical steppe and desert steppe. Soil samples were collected from five depths (0–10, 10–20, 20–40, 40–60 and 60–100 cm) and incubated at four temperatures (5, 15, 25 and 35 °C). We determined GHG emission rates, corresponding biotic (microbial biomass, microbial abundance and composition, and extracellular enzyme activities) and abiotic (soil carbon, nitrogen content, and soil pH) factors. The average Q10 of CO2 and N2O emissions differed non-significantly among steppe types, while the effect of steppe type on Q10 of CH4 emission was strongly affected by measured temperature ranges. Q10 of CO2 and N2O emissions declined with soil depth, but showed no significant differences for CH4. The mechanisms driving the variations in Q10 of GHG emissions among soil depths changed with steppe types. Q10 of CO2 emission was significantly correlated with substrate quality and microbial biomass in meadow steppe and desert steppe, and with microbial abundance and composition in typical steppe. Q10 of N2O emission was significantly influenced by microbial biomass in meadow steppe, soil pH in typical steppe, and substrate quality and microbial biomass in desert steppe, respectively. Q10 of CH4 emission was significantly associated with microbial biomass in typical steppe, and with substrate quality in meadow steppe and desert steppe, respectively. Our findings emphasized the importance of considering steppe type when assessing vertical dynamics of Q10 of GHG emissions.

Reduction and stabilization of Cr(VI) in soil by using calcium polysulfide: Catalysis of natural iron oxides

Cr(VI)-contaminated soils could be remediated by using calcium polysulfide (CPS), while natural iron oxides as a main composition of soil would influence the pathways of the remediation. Through kinetic batch tests, the kinetics of Cr(VI) removal from soil, the effects of the contents of natural iron oxides, soil environmental conditions and mechanisms of Cr(VI) removal by using CPS with the presence of natural iron oxides were investigated. The results show that the removal of Cr(VI) by using CPS in soil fitted the pseudo-second-order model best, and the appearance of goethite increased the apparent rate constant from 0.0002 kg mg−1 h−1 to 0.0005 kg mg−1 h−1. The presence of iron oxides enhanced the removal of Cr(VI) by using CPS, and an extended reductive atmosphere of soil was created. The enhancement of Cr(VI) removal increased with the contents of iron oxides from 0 to 9 g kg−1, and declined from 9 to 12 g kg−1. Acidic environment favored the removal of Cr(VI) from soil by using CPS with or without the iron oxides compared to neutral soil and increased it from 87% to 100% because of proton-consuming reactions and electrostatic attraction. Twenty-nine percent of exchangeable and bound-to-carbonates species of chromium declined after the remediation, while 24% bound-to-iron-and-manganese-oxide species increased simultaneously. The findings of the study indicate that natural iron oxides in soils catalyze the reduction of Cr(VI) in soil and facilitate significantly the remediation of Cr(VI)-contaminated soil by using CPS.

Soil–atmosphere interaction in the overburden of a short-lived low and intermediate level nuclear waste (LLW/ILW) disposal facility

Abstract The long-term safety of shallow disposal facility for short-lived low and intermediate level nuclear waste (LLW/ILW) relies on the performance of engineered barriers (caps and liners). Soil-atmosphere interaction may affect the capping material at shallow depths, due to changes of its coupled hydro-thermal-mechanical behavior under the effect of atmospheric conditions. This study aims to investigate the soil-atmosphere interaction at different time scales in the overburden of an LLW/ILW disposal facility, located in middle France. Actual meteorological data from Valencia El Saler station in Spain, including solar radiation, air temperature, latent heat, rainfall, and actual evaporation, at four different time scales (30 min, daily, weekly and monthly), were applied as the future climate conditions of the studied area based on climate analogues. A numerical approach combining a coupled hydro-thermal model and a soil-atmosphere interaction model was employed. Results show that the employment of meteorological data at a short time scale (30 min) can increase the simulation accuracy by capturing the rainfall events and accelerating the soil-atmosphere interactions. Furthermore, the effect of future climate conditions in the long-term (7 years) on soil hydro-thermal behavior was examined. This study allows a further detailed inspection of the climate’s role in the LLW/ILW storage system.

Background Volumetric Activity of Radon and Thoron in Soil and Surface Atmosphere Observed by a Radon-Recording Seismic Station

The SRS-05 radon-recording seismic station is widely used to monitor the volumetric activity of radon, primarily to detect its increase as an earthquake precursor. The low sensitivity of the sensor does not allow real-time recording of background volumetric activity of radon isotopes 222Rn (radon) and 220Rn (thoron). However, accurate measurement of background volumetric activity of radon isotopes is of great interest, especially in studies of ionization processes and the electric state of the near-surface atmosphere. This paper proposes a method for determining the mean background volumetric activity of radon isotopes based on α-spectra from a radon-recording seismic station over a long period (several days). The determined background volumetric activity of 222Rn and 220Rn in soil and near-surface atmosphere at the Borok Geophysical Observatory are presented.

An Overview of Parameterezations of Heat Transfer over Moss-Covered Surfaces in the Earth System Models

Moss cover plays an important role in shaping the thermodynamic and biogeochemical functioning of ecosystems at high latitudes, where it is the dominant vegetation type. It reduces the heat transfer through the soil–atmosphere interface and between deep levels of the active layer, which leads to a decrease in the thickness of the seasonally thawed layer, and regulates the soil moisture. At the same time, in Earth system models, the heat and moisture exchange in the moss cover is represented by simplified approaches. The purpose of this paper is to summarize up-to-date knowledge on heat-transfer mechanisms acting in moss cover and methods for their quantitative description; we also aim to identify the data missing for constructing physically justified parameterizations and formulate tasks for the future theoretical and experimental work. This paper provides a brief overview of the heat-transfer parameterizations for the moss–lichen cover used in modern Earth system models; presents the results of experimental and theoretical studies of the thermal conductivity of soils and mosses; and strives to encompass available information on the dynamic and thermal roughness of plant communities, including mosses. In conclusion, recommendations are given on the development of parameterizations of thermodynamic processes in the moss cover and design of desirable model and field experiments.

Wind effects on soil thermal properties measured by the dual‐probe heat pulse method

AbstractWhen a dual‐probe heat pulse (DPHP) sensor is installed in soil near the soil–atmosphere interface, the basic assumptions of the infinite line heat source (ILS) model and its improvement, the infinite line heat source model with an adiabatic boundary condition (ILS‐ABC) might not be satisfied because of wind. This study aims to explore the effects of wind on DPHP measurements and to compare the performance of the ILS and ILS‐ABC models at different values of wind velocity (υ) and burial depth (d). Our study shows that the results of laboratory experiments, COMSOL simulations, and field experiments are consistent with each other. For dry sand with d ≤ 4 mm, the effects of wind is non‐negligible at υ ≥ 3.5 m s−1 and the DPHP method does not provide accurate estimations whether the ILS model or the ILS‐ABC model is used. Field experiments are prone to large background temperature fluctuations, which can cause a commonly used linear detrending method to perform poorly. In general, at υ < 2.7 m s−1 in field and d = 5 mm, the ILS‐ABC model provides more accurate estimations with relative errors of <15% in thermal conductivity and relative errors of <9% in heat capacity.

Measurement of plant xylem water pressure using the high-capacity tensiometer and implications for the modelling of soil–atmosphere interaction

The response of shallow geotechnical structures is affected by interaction with the atmosphere. Since the ground surface is very often vegetated, plant transpiration plays a major role in such an interaction. Transpiration in geotechnical applications is generally modelled by way of a transpiration reduction function (e.g. the Feddes function). However, its parameters are generally borrowed from the agricultural literature, where the focus is on crop species and often loosely compacted organic agricultural soils. For the non-crop species in denser soils typically encountered in geotechnical applications, monitoring of the flow taking place in the soil through the xylem up to the leaves can potentially be exploited to characterise the transpiration reduction function. The main challenge is the measurement of the water pressure in the xylem. Techniques currently used include the pressure chamber and thermocouple psychrometer. The pressure chamber is destructive and thus not suitable for continuous monitoring and/or where a relatively small number of leaves is available (as often occurs in laboratory experiments). The thermocouple psychrometer is not accurate at low water tension, is affected by the presence of solutes in the xylem water and is significantly sensitive to temperature. This paper explores a novel application of the high-capacity tensiometer (HCT), initially developed for pore-water pressure measurement in soils. The HCT was installed on the stem or branch of different trees and its measurement validated against pressure chamber measurements over a range of xylem water pressure down to −1300 kPa. In addition, its measurement was used to investigate the response of the soil–plant continuum. Results show that the HCT is a viable and convenient instrument to use for xylem water pressure measurement and can provide field-based data for the modelling of plant transpiration. Installing HCTs on stems and branches is quite straightforward and this will help achieve a step change in testing and modelling the effect of plant transpiration on the soil water regime in the vadose zone.

CO2 Transfer Characteristics of Calcareous Humid Subtropical Forest Soils and Associated Contributions to Carbon Source and Sink in Guilin, Southwest China

In karst landscapes, soil CO2 is a key factor in weathering processes and carbon cycling, where its distribution and migration characteristics directly affect fluxes in carbon source–sink dynamics. We measured the CO2 emission and dissolution rates of carbonate tablets in calcareous soil developed from limestone and red soil developed from clastic rock, in karst and non-karst subtropical forests, in Guilin, southwest China between 2015 and 2018, to analyze their CO2 transfer characteristics and source–sink effects. The results showed similar average soil respiration rates between calcareous soil and red soil, with an average CO2 emission flux of 1305 and 1167 t C km−2 a−1, respectively. Carbonate tablet dissolution rates were bidirectional with increasing depth and were greater in red soil than calcareous soil, averaging 13.88 ± 5.42 and 7.20 ± 2.11 mg cm−2 a−1, respectively. CO2 concentration was bidirectional with increasing soil depth, reaching a maximum at the base of the soil–atmosphere interface (50–60 cm), and the bidirectional gradient was more distinctive in red soil. Change in the carbon isotope value of soil CO2 was also bidirectional in calcareous soils, for which the overall average was 0.87‰ heavier in calcareous than red soil. The carbon sink in calcareous soil in karst regions was estimated to be 11.97 times that of red soil in non-karst regions, whereas its role as a carbon source is just 1.12 times that of red soil, thus indicating the key role of karst soil in the reduction of atmospheric CO2.

Biologically influenced gas fluxes revealed by high‐resolution monitoring of unsaturated soil columns

AbstractModulations of advective gas fluxes at the soil–atmosphere interface were investigated using an enhanced experimental setup developed to perform tracer gas percolation experiments through unsaturated soil columns under well‐controlled conditions associated with long‐term and high‐resolution monitoring. The setup design includes the effect of watering and evaporation cycles, barometric pressure fluctuations, variations in the injection pressure, and plant metabolism. Although injected at a constant flux at the base of the columns, SF6 surface fluxes varied on a timescale of hours to days. These modulations are controlled by (a) barometric pressure, (b) water content and distribution, and (c) plant metabolism. All three mainly act on the pressure gradient. Surface gas fluxes decrease under drying conditions, which increases gas porosity and the relative gas permeability and lowers the pressure gradient. Respiration of plant roots is shown to be responsible for daytime–nighttime oscillations of the tracer flux. During nighttime, O2 consumption and CO2 production locally lowers the pressure gradient up to the root zone due to the higher solubility of CO2 in pore water, resulting in an increased SF6 flux at the surface. During daytime, enhanced water loss by evapotranspiration associated with photosynthesis dominated the respiration effect and resulted in decreasing surface gas fluxes, as generally shown for drying conditions. Surface gas fluxes are therefore controlled by combined physical, chemical, and biological processes. This has important consequences, notably when discrete flux measurements are integrated in space and/or in time to quantify emissions or when used for detecting, identifying, or monitoring underground gas sources.

Evaporation from undulating soil surfaces under turbulent airflow through numerical and experimental approaches

AbstractEvaporation from undulating soil surfaces is rarely studied due to limited modeling theory and inadequate experimental data linking dynamic soil and atmospheric interactions. The goal of this paper is to provide exploratory insights into evaporation behavior from undulating soil surfaces under turbulent conditions through numerical and experimental approaches. A previously developed and verified coupled free flow and porous media flow model was extended by incorporating turbulent airflow through Reynolds‐averaged Navier–Stokes equations. The model explicitly describes the relevant physical processes and the key properties in the free flow, porous media, and at the interface, allowing for the analysis of coupled exchange fluxes. An experiment aiming to simultaneously collect the data of the boundary layer and soil evaporation in the area around the soil–atmosphere interface was conducted using a wind tunnel integrated with a soil tank. The turbulent boundary layer above the undulating soil surface was captured using high‐resolution hot‐wire anemometry, confirming the presence of recirculation zones in the undulating valleys and locally low evaporative flux. Experimental data were used to validate the extended model, and modeling results demonstrate that turbulent airflow enhances evaporation and shortens the duration of Stage I. The surface geometry significantly affects the local evaporative flux by influencing the vapor distribution, concentration gradient, and water availability at the soil surface, especially when recirculation zones form in the valleys. As a joint result of turbulence and surface undulations, the influence of wind speed on both the local and system‐level evaporation rate is restricted.

Analysis of water vapor adsorption in soils by means of a lysimeter and numerical modeling

AbstractDaily temperature oscillations can cause adsorption (and desorption) of atmospheric water vapor by soils. The resulting daily fluctuations in the amount of liquid water in the soil can be measured by high‐precision weighing lysimeters. We analyzed the data of a lysimeter in a sandy dune sediment in southern Spain using Codebright, a thermohydraulic numerical model for unsaturated flow that takes into account water, vapor, and heat transport in the soil, as well as soil–atmosphere interactions such as precipitation, evaporation, and solar radiation. The analysis shows that daily temperature oscillations, psychrometrics, and soil water retention can explain the fluctuations of the amount of liquid water in the soil. The retention curve, especially its driest part, is essential for the existence of these fluctuations. The fluctuations could not be reproduced by a model using the van Genuchten retention curve with a constant residual saturation. On the other hand, satisfactory results could be obtained by models using retention curves that at their driest part still show a change of saturation with suction. Moreover, the models suggest within the top few decimeters of the soil a pattern of alternating bands of condensation and evaporation, which follows the daily temperature oscillations that fade out deeper in the soil.

Extreme soil surface temperatures reflect need to rethink agronomic management

AbstractThe soil surface exists at the soil–atmosphere interface and is critical in regulating transfers of water, energy, and nutrients in agroecosystems. Soil surface temperature plays an important role in the rate and quantity at which these resources cross the interface, as temperature regulates numerous physical, chemical, and biological processes. Many current management practices manipulate soil surface temperatures, especially through tillage or residue management, to create conditions suitable for early‐season crop growth. However, these conditions persist beyond plant establishment and have the potential to develop extremely high temperatures, even in historically mild climates. We show the potential for extreme maximum soil surface temperatures in Iowa after crop establishment but before canopy closure and discuss possible consequences of those temperatures. Given the conditions required to develop extreme maximum temperatures, we contend that soil surface temperatures may be an indicator of conditions that are inconsistent with highly productive, sustainable, efficient agroecosystems.

Atmosphere-soil-structure interaction: Isolated building foundation movements induced by environmental loads

This paper presents a simplified method for the study of the soil-structure interaction problem for isolated footings on soils subjected to water content-related volume changes. The method is based on the 2D modelling of thermo-hydro-mechanical processes occurring in the soil and the soil–atmosphere interface, the calculation of the soil-footing system stiffness and the modelling of the structure. The idealization of soil-structure interaction from an equivalent springs model, obtained from the above characteristics, allows the movements changes in time of the foundations to be obtained. These in turn can be used for evaluation of building damage related to angular distortion among foundations.

Long-term effects of forest fires on soil greenhouse gas emissions and extracellular enzyme activities in a hemiboreal forest

Fire is the most important natural disturbance in boreal forests, and it has a major role regulating the carbon (C) budget of these systems. With the expected increase in fire frequency, the greenhouse gas (GHG) budget of boreal forest soils may change. In order to understand the long-term nature of the soil–atmosphere GHG exchange after fire, we established a fire chronosequence representing successional stages at 8, 19, 34, 65, 76 and 179 years following stand-replacing fires in hemiboreal Scots pine forests in Estonia. Changes in extracellular activity, litter decomposition, vegetation biomass, and soil physicochemical properties were assessed in relation to carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) emissions. Soil temperature was highest 8 years after fire, whereas soil moisture varied through the fire chronosequences without a consistent pattern. Litter decomposition and CO2 efflux were still lower 8 years after fire compared with pre-fire levels (179 years after fire). Both returned to pre-fire levels before vegetation re-established, and CO2 efflux was only strongly responsive to temperature from 19 years after fire onward. Recovery of CO2 efflux in the long term was associated with a moderate effect of fire on enzyme activity, the input of above- and below-ground litter carbon, and the re-establishment of vegetation. Soil acted as a CH4 sink and N2O source similarly in all successional stages. Compared with soil moisture and time after fire, soil temperature was the most important predictor for both GHGs. The re-establishment of overstorey and vegetation cover (mosses and lichens) might have caused an increase in CH4 and N2O effluxes in the studied areas, respectively.

Volatile-mediated antagonism of soil bacterial communities against fungi.

SummaryCompetition is a major type of interaction between fungi and bacteria in soil and is also an important factor in suppression of plant diseases caused by soil‐borne fungal pathogens. There is increasing attention for the possible role of volatiles in competitive interactions between bacteria and fungi. However, knowledge on the actual role of bacterial volatiles in interactions with fungi within soil microbial communities is lacking. Here, we examined colonization of sterile agricultural soils by fungi and bacteria from non‐sterile soil inoculums during exposure to volatiles emitted by soil‐derived bacterial communities. We found that colonization of soil by fungi was negatively affected by exposure to volatiles emitted by bacterial communities whereas that of bacteria was barely changed. Furthermore, there were strong effects of bacterial community volatiles on the assembly of fungal soil colonizers. Identification of volatile composition produced by bacterial communities revealed several compounds with known fungistatic activity. Our results are the first to reveal a collective volatile‐mediated antagonism of soil bacteria against fungi. Given the better exploration abilities of filamentous fungi in unsaturated soils, this may be an important strategy for bacteria to defend occupied nutrient patches against invading fungi. Another implication of our research is that bacterial volatiles in soil atmospheres can have a major contribution to soil fungistasis.

A new climatic chamber for studying soil–atmosphere interaction in physical models

A new climatic chamber at the Universidad de Los Andes in Bogotá, Colombia has been designed and built to simulate atmosphere. It has been instrumented to measure various environmental variables, including relative humidity (RH), wind velocity, radiation and temperature. The climatic chamber has been calibrated so that it properly simulates each environmental variable as well as the heat-transfer mechanisms that affect desiccation in soil layers. First, a potential evaporation test was performed in a container filled with water. The weight of the water evaporated was measured, and the interaction with the artificial atmosphere was studied. Then, an actual evaporation test was performed on a soil layer, and the relations among environmental variables and soil properties such as soil temperature, water content and suction were determined. The principal results show the existence of a gradient of RH at the soil–atmosphere interface. Also, a comparison between potential and actual evaporation indicates that suction is the main soil property that affects the actual evaporation rate.

Some aspects of water and energy budget of a pyroclastic cover

This paper investigates the soil–atmosphere interaction for a pyroclastic soil. With this aim, a physical model has been developed, consisting of a pyroclastic layer exposed to the atmosphere and extensively monitored to record the various terms of the soil–atmosphere energy balance and layer mass balance. The work interprets a long-term (4-year) monitoring data set in order to estimate the range of variation of the different quantities characterising soil–atmosphere interaction. The physical quantities are obtained from measurements or estimated from mass and energy balance. The study provides estimations on a monthly scale, by averaging over the monitoring period. In addition, mean seasonal values are provided for the energy terms. The paper adopts and compares two different approaches to estimating sensible heat flux and latent heat flux, distinguishing between the different components of short-wave radiation and determining average monthly values for the layer albedo. The main goal of the paper is to provide the range of variation of physical quantities, which are often unusual or in any case are poorly characterised for pyroclastic soils.

Prediction of soil suction using measured climatic data

This paper presents calibration and validation of a one-dimensional soil–atmosphere model created using the SVFlux software package for predicting soil suction changes based on the measured climate data at two Oklahoma Mesonet sites. The parameters for the soil water characteristic curves and the hydraulic conductivity function were obtained from published calibration studies conducted using laboratory test results on soil samples extracted from these sites. The plant cover quality, the root depth and the root uptake distribution function were first calibrated using 1-year data at one site. While a rectangular root uptake distribution function overpredicted suction at deeper layers, the triangular uptake distribution function overpredicted suction at shallower depths. In addition, reducing the root depth to 1 m improved the predictions at surface layers and led to overestimation of the suction at deeper layers. Based on the root mean squares of the differences between the simulated and the measured values and weighting the shallower depths more, a rectangular root uptake distribution function and a root depth of 1 m were selected to predict suction values at the calibration site and the second site for another year. Reasonable predictions of soil suction values at shallower depths were obtained at both sites.

Influence of climatic conditions on evaporation in soil samples

Evaporation rates can be successfully modelled for soil–atmosphere interaction problems by means of a convective vapour flux at the soil surface equal to the product of a driving force (difference between the vapour density at the soil surface and that of the surroundings) and a mass transfer coefficient characterising the resistance of a boundary layer where the transfers take place. While the vapour density at the soil surface can be deduced from solving the coupled heat and moisture transfer equations for the soil below the surface, the determination of the mass transfer coefficients under varying weather conditions is rarely investigated. Laboratory-scale drying tests are performed on a compacted silt specimen, focusing on the influence of atmospheric conditions (relative humidity and wind speed) on the evaporation kinetics and the mass transfer coefficient. Two experimental techniques are used and compared: a drying chamber with relative humidity controlled through saline solutions and a convective microdryer. New insights are given into the prediction of mass transfer coefficients from mass transfer theories. Finally, the results highlight the necessity of testing specimens that are large enough to define relevant mass transfer coefficients for geotechnical applications.