Water-filled Porosity Research Articles

Agricultural Drainage Filters. I. Filter Hydro-Physical Properties and Tracer Transport

Drainage filters using porous granular material constitute new innovative technologies for remediating phosphorus (P) from agricultural tile drainage water. In drainage filters where convective velocities are often high, we hypothesize that intragranular diffusion may affect solute transport depending on filter characteristics and flow rate. This was investigated for six drainage filter materials (Leca, Filtralite-P®, granulated limestone, crushed seashells, calcined diatomite earth (CDE), and a poorly ordered Fe oxide aggregate (CFH)) conducting a tritium (3H2O) tracer experiment at low (0.26 cm h−1), medium (23 cm h−1), and high (41 cm h−1) flux densities. The filter materials differed widely with respect to grain-size distribution (D 50 from 1.6 to 3.3 mm), uniformity coefficient (1.7 to 2.2), particle density (1.75 to 2.76 g cm−3), bulk density (0.34 to 1.46 g cm−3), and water-filled porosity (0.39 to 0.73 cm3 cm−3). Measurements of specific surface area (SSA) included both SSABET and SSAEGME to ensure inclusion of the intragranular microporosity, not accounted by N2-BET. SSA varied widely across methods and allowed the differentiation of filters according to the significance of the intragranular porosity. Tritium transport varied from approximately equilibrium transport at all flow rates in Leca, Filtralite-P®, and limestone, to progressive non-equilibrium transport as flow rate increased in Seashells, CDE, and CFH. In general, the filter materials were highly variable in hydro-physical properties. Filters with (approximately) equilibrium transport were, however, all characterized by low specific surface areas. The non-equilibrium transport was explained by an intragranular diffusion in filters with larger specific surface area (Seashells, CDE, and CFH).

Pore-Scale Joint Evaluation of Dielectric Permittivity and Electrical Resistivity for Assessment of Hydrocarbon Saturation Using Numerical Simulations

Summary Complex pore geometry and composition, as well as anisotropic behavior and heterogeneity, can affect physical properties of rocks such as electrical resistivity and dielectric permittivity. The aforementioned physical properties are used to estimate in-situ petrophysical properties of the formation such as hydrocarbon saturation. In the application of conventional methods for interpretation of electrical-resistivity (e.g., Archie's equation and the dual-water model) and dielectric-permittivity measurements [e.g., complex refractive index model (CRIM)], the impacts of complex pore structure (e.g., kerogen porosity and intergranular pores), pyrite, and conductive mature kerogen have not been taken into account. These limitations cause significant uncertainty in estimates of water saturation. In this paper, we introduce a new method that combines interpretation of dielectric-permittivity and electrical-resistivity measurements to improve assessment of hydrocarbon saturation. The combined interpretation of dielectric-permittivity and electrical-resistivity measurements enables assimilating spatial distribution of rock components (e.g., pore, kerogen, and pyrite networks) in conventional models. We start with pore-scale numerical simulations of electrical resistivity and dielectric permittivity of fluid-bearing porous media to investigate the structure of pore and matrix constituents in these measurements. The inputs to these simulators are 3D pore-scale images. We then introduce an analytical model that combines resistivity and permittivity measurements to assess water-filled porosity and hydrocarbon saturation. We apply the new method to actual digital sandstones and synthetic digital organic-rich mudrock samples. The relative errors (compared with actual values estimated from image processing) in the estimate of water-filled porosity through our new method are all within the 10% range. In the case of digital sandstone samples, CRIM provided reasonable estimates of water-filled porosity, with only four out of twenty-one estimates beyond 10% relative error, with the maximum error of 30%. However, in the case of synthetic digital organic-rich mudrocks, six out of ten estimates for water-filled porosity were beyond 10% with CRIM, with the maximum error of 40%. Therefore, the improvement was more significant in the case of organic-rich mudrocks with complex pore structure. In the case of synthetic digital organic-rich mudrock samples, our simulation results confirm that not only the pore structure but also spatial distribution and tortuosity of water, kerogen, and pyrite networks affect the measurements of dielectric permittivity and electrical resistivity. Taking into account these parameters through the joint interpretation of dielectric-permittivity and electrical-resistivity measurements significantly improves assessment of hydrocarbon saturation.

Matrix Permittivity Measurements for Rock Powders

Summary Formation water content is one of the key petrophysical quantities provided by dielectric logging. However, to determine water content from formation permittivity measurements, the rock matrix permittivity must be known. Uncertainty in the rock matrix permittivity values translates into uncertainty in the water-content estimate, which is especially important in low-porosity formations or complex lithologies. Matrix permittivity values are not well-known for a number of minerals and can also vary for the same type of mineral in different formations. Thus, a laboratory methodology for the accurate determination of matrix permittivity at dielectric logging frequencies is required to facilitate accurate log interpretation. One can measure matrix permittivity values on solid plugs (Seleznev et al. 2011). However, the plug-based methodology can be challenging in very-low-permeability or unconventional reservoirs because of difficulties with plug drying. In addition, it is not readily applicable to unconsolidated formations. Finally, it may be impossible to cut solid plugs because of limited availability of rock material. Matrix permittivity measurements made on rock powders are capable of addressing all these issues. We introduce a methodology for laboratory measurements of matrix permittivity on rock powders at 1 GHz. The methodology is based on conducting dielectric measurements on mixtures of rock powders and liquids with variable permittivities in a dielectric resonator. The permittivity of the rock matrix is inverted from a series of measurements obtained on pure liquids and powder/liquid mixtures. The methodology was benchmarked on a collection of samples representing common oilfield lithologies with matrix-permittivity values between 4.6 and 8.6. The reference matrix-permittivity values were first measured on solid plugs. Then, the plugs were crushed into powders, and the matrix permittivity values were determined on powders following the proposed methodology. The values obtained on powders matched the ones measured on solid plugs within 0.2 dielectric units, resulting in accuracies better than 1% for the water-filled porosity and better than 1,000 ppm for water salinity. This new methodology was applied to a number of core samples from a carbonate reservoir offshore Sarawak, where dielectric logging was performed along with conventional core analysis. The resulting measured matrix permittivity values were then used to interpret the dielectric log measurement. Results showed a better estimation of water-filled porosity and of the textural MN parameter, equivalent to the Archie's cementation exponent in a water-bearing zone, than would have resulted from using “chartbook” values of matrix permittivity. A consistent and optimized interpretation was obtained in porosities ranging from 5% to more than 30%.

Emission of CO2 from tropical riparian forest soil is controlled by soil temperature, soil water content and depth to water table

Tropical forests play a key role in the global carbon cycle. However, little is known about carbon cycling in the substantial portion of tropical forests that are low-lying, with shallow and fluctuating water tables. This study aimed to determine what factors control emissions of CO2 from soil in a riparian rainforest in Queensland, Australia. Emissions were measured over the course of 1 year, using static chambers. Emission rates were significantly related to soil temperature (0–0.1 m depth), soil water content (0–0.12 m depth) and depth to water table. The most efficient linear model of emissions as a function of measured parameters, which also included soil pH (0–0.1 m depth), had r2 = 0.355. CO2 emissions were highest (5.2–7.5 μmol m–2 s–1) at moderate soil temperature (24−28°C), water table depth (0.2–1.5 m) and soil water-filled porosity (0.25–0.79). They were lowest (<0.5 μmol m–2 s–1) at low soil temperature (<22°C) or when the water table was within 0.15 m of the surface. An additional interaction between temperature and soil water was determined in the laboratory. Incubation of soil cores showed that temperature sensitivity of the heterotrophic component of respiration increased as the soil dried. It is clear that models of soil respiration in lowland tropical forests should take into account depth to water table, which is a key, but hitherto unreported, controller of CO2 emissions in tropical forests.

Effect of N addition, moisture, and temperature on soil microbial respiration and microbial biomass in forest soil at different stages of litter decomposition

The objective of the present study was to investigate the interactive effects of nitrogen (N) addition, temperature, and moisture on soil microbial respiration, microbial biomass, and metabolic quotient (qCO2) at different decomposition stages of different tree leaf litters. A laboratory incubation experiment with and without litter addition was conducted for 80 days at two temperatures (15 and 25 °C), two wetting intensities (35 and 50 % water-filled porosity space (WFPS)) and two doses of N addition (0 and 4.5 g N m−2, as NH4NO3). The tree leaf litters included three types of broadleaf litters, a needle litter, and a mixed litter of them. Soil microbial respiration, microbial biomass, and qCO2 along with other soil properties were measured at two decomposition stages of tree leaf litters. The increase in soil cumulative carbon dioxide (CO2) flux and microbial biomass during the incubation depended on types of tree leaf litters, N addition, and hydrothermal conditions. Soil microbial biomass carbon (C) and N and qCO2 were significantly greater in all litter-amended than in non-amended soils. However, the difference in the qCO2 became smaller during the late period of incubation, especially at 25 °C. The interactive effect of temperature with soil moisture and N addition was significant for affecting the cumulative litter-derived CO2-C flux at the early and late stages of litter decomposition. Furthermore, the interactive effect of soil moisture and N addition was significant for affecting the cumulative CO2 flux at the late stage of litter decomposition but not early in the experiment. This present study indicated that the effects of addition of N and hydrothermal conditions on soil microbial respiration, qCO2, and concentrations of labile C and N depended on types of tree leaf litters and the development of litter decomposition. The results highlight the importance of N availability and hydrothermal conditions in interactively regulating soil microbial respiration and microbial C utilization during litter decomposition under forest ecosystems.

Fate of Effluent‐Borne Nitrogen in the Mounded Drainfield of an Onsite Wastewater Treatment System

Core Ideas Water filled porosity (WFP) controlled N dynamics in the drainfields. Sand layer was hotspot for nitrification due to 0.07–0.32 WFP. Soil layer was hotspot for denitrification due to 0.55–0.91 WFP. Organic N was mobile in the vadose zone and leached below drainfields. Onsite wastewater treatment systems (OWTS), commonly known as septic systems, are increasingly recognized as a potential source of nitrogen in the shallow groundwater. Our objective was to investigate the effect of hydrological and biogeochemical factors in the vadose zone on the fate of effluent‐borne N in the drainfields of a drip‐dispersal OWTS. Three lysimeters (152.4 cm long, 91.4 cm wide, and 91.4 cm high) were constructed using pressure‐treated wood to mimic OWTS drainfields. Each lysimeter had three distinct layers of gravel–sand mixture, soil, and commercial sand. A drip tube, which was covered with commercial sand before planting St. Augustine grass (Stenotaphrum Trin.) on the top and sides of the lysimeters, dispersed 9 L of septic tank effluent (STE) per day on top of the stacked layers. Each lysimeter was instrumented with 10 multi‐probe sensors to determine the water content, electrical conductivity (EC), and temperature in the center and sides of sand and soil layers. Leachate samples were collected over 67 events, which consisted of one sample every 24 h for 15 d (n = 15) and weekly flow‐weighted composite samples (n = 52). In all events, the pH, EC, and chloride were lower in the leachate than STE. Daily multi‐probe data showed that EC was greater in the center than sides of the lysimeters due to more STE interaction. Sensor water content data were used to calculate water filled porosity (WFP), which was greater in the soil (0.55–0.9) than sand (0.07–0.32) due to the textural differences. Mean total N was 70 mg L−1 in the STE, which reduced to 27.4 mg L−1 in the leachate likely due to the denitrification in the soil layer. The dominance of NOx–N in the leachate (61%) as compared to STE (0.6%) was attributed to the nitrification in the sand layer. Higher proportion of organic N in the leachate (39%) than STE (16%) suggests that organic N was mobile in the vadose zone and leached below the drainfields. We conclude that hydrological and biogeochemical controls in the vadose zone play an important role in N transformations and transport of NOx–N and organic N below OWTS drainfields.

The Potential of Controlled Traffic Farming to Mitigate Greenhouse Gas Emissions and Enhance Carbon Sequestration in Arable Land: A Critical Review

The drive toward adoption of conservation agriculture to reduce costs and increase production sustainably causes concern due to the potentially negative effects of increased soil compaction. Soil compaction reduces aeration, water infiltration, and saturated hydraulic conductivity and increases the risk of waterlogging. Controlled traffic farming (CTF) is a system in which: (1) all machinery has the same or modular working and track width so that field traffic can be confined to the least possible area of permanent traffic lanes, (2) all machinery is capable of precise guidance along those permanent traffic lanes, and (3) the layout of the permanent traffic lanes is designed to optimize surface drainage and logistics. Without CTF, varying equipment operating and track widths translate into random traffic patterns, which can cover up to 85% of the cultivated field area each time a crop is produced. Nitrous oxide (N2O) is the greatest contributor to agriculture's greenhouse gas (GHG) emissions from cropping, and research suggests that its production increases significantly under conditions of high (>60%) water-filled porosity when nitrate (mainly from fertilizer N) and carbon (usually from crop residues) are available. Self-amelioration of soils affected by compaction occurs slowly from the surface downward; however, the rate of amelioration decreases with increase in depth. Consequently, all soils in non-CTF systems in mechanized agriculture are prone to some degree of compaction, which compromises water infiltration, increases the frequency and duration of waterlogged conditions, reduces gaseous exchange between soil and the atmosphere, inhibits root penetration and exploitation of nutrients and water in the subsoil, and enhances N2O emissions. Adoption of CTF increases soil porosity in the range of 5% to 70%, water infiltration by a factor of 4, and saturated hydraulic conductivity by a factor of 2. The greater cropping opportunity and enhanced crop growth for given fertilizer and rainfall inputs offered by CTF, coupled with no-tillage, provide potential for enhanced soil carbon sequestration. Reduced need and intensity of tillage, where compaction is avoided, also helps protect soil organic matter in stable aggregates, which may otherwise be exposed and oxidized. There is both circumstantial and direct evidence to suggest that improved soil structural conditions and aeration offered by CTF can reduce N2O emissions by 20% to 50% compared with non-CTF. It is not compaction per se that increases the risk of N2O emissions but rather the increased risk of waterlogging and increase in water-filled pore space. There may be an elevated risk of GHG emissions from the relatively small area of permanent traffic lanes (typically <20% of total cultivated area) if these are not managed appropriately. Quantification of the benefits of compaction avoidance in terms of GHG emissions may be possible through the use of well-developed models.

Effect of residual oil saturation on hydrodynamic properties of porous media

To understand the effect of residual oil on hydraulic properties and solute dispersive behavior of porous media, miscible displacement column experiments were conducted using two petroleum products (diesel and engine oil) and a sandy soil. The effective water permeability, effective water-filled porosity, and dispersivity were investigated in two-fluid systems of water and oil as a function of residual oil saturation (ROS). At the end of each experiment, the distribution of ending ROS along the sand column was determined by the method of petroleum ether extraction-ultraviolet spectrophotometry. Darcy’s Law was used to determine permeability, while breakthrough curves (BTCs) of a tracer, Cl−, were used to calibrate effective porosity and dispersivity. The experimental results indicate that the maximum saturated zone residual saturation of diesel and engine oil in this study are 16.0% and 45.7%, respectively. Cl− is found to have no sorption on the solid matrix. Generated BTCs are sigmoid in shape with no evidence of tailing. The effective porosity of sand is inversely proportional to ROS. For the same level of ROS, the magnitude of reduction in effective porosity by diesel is close to that by engine oil. The relative permeability of sand to water saturation decreases with increasing amount of trapped oil, and the slope of the relative permeability–saturation curve for water is larger at higher water saturations, indicating that oil first occupies larger pores, which have the most contribution to the conductivity of the water. In addition, the reduction rate of relative permeability by diesel is greater than that by engine oil. The dispersivity increases with increasing ROS, suggesting that the blockage of pore spaces by immobile oil globules may enhance local velocity variations and increase the tortuosity of aqueous-phase flow paths.

Methane and nitrous oxide fluxes across an elevation gradient in the tropical Peruvian Andes

Abstract. Remote sensing and inverse modelling studies indicate that the tropics emit more CH4 and N2O than predicted by bottom-up emissions inventories, suggesting that terrestrial sources are stronger or more numerous than previously thought. Tropical uplands are a potentially large and important source of CH4 and N2O often overlooked by past empirical and modelling studies. To address this knowledge gap, we investigated spatial, temporal and environmental trends in soil CH4 and N2O fluxes across a long elevation gradient (600–3700 m a.s.l.) in the Kosñipata Valley, in the southern Peruvian Andes, that experiences seasonal fluctuations in rainfall. The aim of this work was to produce preliminary estimates of soil CH4 and N2O fluxes from representative habitats within this region, and to identify the proximate controls on soil CH4 and N2O dynamics. Area-weighted flux calculations indicated that ecosystems across this altitudinal gradient were both atmospheric sources and sinks of CH4 on an annual basis. Montane grasslands (3200–3700 m a.s.l.) were strong atmospheric sources, emitting 56.94 ± 7.81 kg CH4-C ha−1 yr−1. Upper montane forest (2200–3200 m a.s.l.) and lower montane forest (1200–2200 m a.s.l.) were net atmospheric sinks (−2.99 ± 0.29 and −2.34 ± 0.29 kg CH4-C ha−1 yr−1, respectively); while premontane forests (600–1200 m a.s.l.) fluctuated between source or sink depending on the season (wet season: 1.86 ± 1.50 kg CH4-C ha−1 yr−1; dry season: −1.17 ± 0.40 kg CH4-C ha−1 yr−1). Analysis of spatial, temporal and environmental trends in soil CH4 flux across the study site suggest that soil redox was a dominant control on net soil CH4 flux. Soil CH4 emissions were greatest from habitats, landforms and during times of year when soils were suboxic, and soil CH4 efflux was inversely correlated with soil O2 concentration (Spearman's ρ = −0.45, P &lt; 0.0001) and positively correlated with water-filled pore space (Spearman's ρ = 0.63, P &lt;0.0001). Ecosystems across the region were net atmospheric N2O sources. Soil N2O fluxes declined with increasing elevation; area-weighted flux calculations indicated that N2O emissions from premontane forest, lower montane forest, upper montane forest and montane grasslands averaged 2.23 ± 1.31, 1.68 ± 0.44, 0.44 ± 0.47 and 0.15 ± 1.10 kg N2O-N ha−1 yr−1, respectively. Soil N2O fluxes from premontane and lower montane forests exceeded prior model predictions for the region. Comprehensive investigation of field and laboratory data collected in this study suggest that soil N2O fluxes from this region were primarily driven by denitrification; that nitrate (NO3−) availability was the principal constraint on soil N2O fluxes; and that soil moisture and water-filled porosity played a secondary role in modulating N2O emissions. Any current and future changes in N management or anthropogenic N deposition may cause shifts in net soil N2O fluxes from these tropical montane ecosystems, further enhancing this emission source.

Advanced aquifer characterization for optimization of managed aquifer recharge

Managed aquifer recharge (MAR) will play an increasingly important role in solving water scarcity. The performance of MAR systems depends primarily upon local hydrogeology. Greatest opportunities for improvement in the implementation of MAR systems lie in the targeted application of conventional and advanced technologies to improve aquifer characterization. Surface geophysics (e.g., VES, TDEM, and seismic reflection and refraction) generally provide low resolution, but areal extensive data on subsurface hydrogeology. Times series of relative microgravity measurements have been used to map changes in both vadose and phreatic zone storage and to augment monitoring well systems. Surface nuclear magnetic resonance (NMR) has the potential to provide quantitative data on water-filled porosity and pore size distribution and, in turn, an estimate of hydraulic conductivity. Standard borehole geophysical logging techniques can provide coarse-scale data on aquifer heterogeneity. Advanced borehole logging techniques, such as NMR, microresistivity imaging, and gamma ray spectroscopy, have been used in MAR projects in the USA and UAE to provide fine-scale petrophysical data (e.g., porosity, porosity-types, and pore-size distribution). Groundwater modeling is used in MAR investigations to evaluate system feasibility and to optimize system design and operation. Opportunities to improve groundwater modeling exist through the application of advanced reservoir simulation platforms that allow for the processing and integration of available lithological, geophysical, and aquifer testing data and their subsequent incorporation into groundwater flow models. Advanced groundwater modeling programs are available that allow for the simulation of complex aquifers such as dual-porosity systems and variable density.

Soil moisture and soil-litter mixing effects on surface litter decomposition: A controlled environment assessment

Recent studies suggest the long-standing discrepancy between measured and modeled leaf litter decomposition in drylands is, in part, the result of a unique combination of abiotic drivers that include high soil surface temperature and radiant energy levels and soil-litter mixing. Temperature and radiant energy effects on litter decomposition have been widely documented. However, under field conditions in drylands where soil-litter mixing occurs and accelerates decomposition, the mechanisms involved with soil-litter mixing effects are ambiguous. Potential mechanisms may include some combination of enhanced microbial colonization of litter, physical abrasion of litter surfaces, and buffering of litter and its associated decomposers from high temperatures and low moisture conditions. Here, we tested how soil-litter mixing and soil moisture interact to influence rates of litter decomposition in a controlled environment. Foliar litter of two plant species (a grass [Eragrostis lehmanniana] and a shrub [Prosopis velutina]) was incubated for 32 weeks in a factorial combination of soil-litter mixing (none, light, and complete) and soil water content (2, 4, 12% water-filled porosity) treatments. Phospholipid fatty acids (PLFAs) were quantified one week into the experiment to evaluate initial microbial colonization. A complementary incubation experiment with simulated rainfall pulses tested the buffering effects of soil-litter mixing on decomposition.Under the laboratory conditions of our experiments, the influence of soil-litter mixing was minimal and primarily confined to changes in PLFAs during the initial stages of decomposition in the constant soil moisture experiment and the oscillating soil moisture conditions of the rainfall pulse experiment. Soil-litter mixing effects on CO2 production, total phospholipid concentrations, and bacterial to total PLFA ratios were observed within the first week, but responses were fairly weak and varied with litter type and soil moisture treatment. Across the entire 32-week incubation experiment, soil moisture had a significant positive effect on mass loss, but soil-litter mixing did not. The lack of strong soil-litter mixing effects on decomposition under the moderate and relatively constant environmental conditions of this study is in contrast to results from field studies and suggests the importance of soil-litter mixing may be magnified when the fluctuations and extremes in temperature, radiant energy and moisture regimes common dryland field settings are in play.

Interpretation of porosity and fluid constituents from well logs using an interactive neutron-density matrix scale

Neutron and density logs are important borehole measurements for estimating reservoir capacity and inferring saturating fluids. The neutron log, measuring the hydrogen index, is commonly expressed in apparent water-filled porosity units assuming a constant matrix lithology whereby it is not always representative of actual pore fluid. By contrast, a lithology-independent porosity calculation from nuclear magnetic resonance (NMR) and/or core measurements provides reliable evaluations of reservoir capacity. In practice, not all wells include core or NMR measurements. We discovered an interpretation workflow wherein formation porosity and hydrocarbon constituents can be estimated from density and neutron logs using an interactive, variable matrix scale specifically suited for the precalculated matrix density. First, we estimated matrix components from combinations of nuclear logs (photoelectric factor, spontaneous gamma ray, neutron, and density) using Schlumberger’s nuclear parameter calculator (SNUPAR) as a matrix compositional solver while assuming freshwater-filled formations. The combined effects of grain density, volumetric concentration of shale, matrix hydrogen, and neutron lithology units define an interactive matrix scale for correction of neutron porosity. Under updated matrix conditions, the resulting neutron-density crossover can only be attributed to pore volume and saturating fluid effects. Second, porosity, connate-water saturation, and hydrocarbon density are calculated from the discrepancy between corrected neutron and density logs using SNUPAR and Archie’s water saturation equation, thereby eliminating the assumption of freshwater saturation. With matrix effects eliminated from the neutron-density overlay, gas- or light-oil-saturated formations exhibiting the characteristic gas neutron-density crossover become representative of saturating hydrocarbons. This behavior gives a clear qualitative distinction between hydrocarbon-saturated and nonviable depth zones.

Applications of Conventional Logs in Low Resistivity Contrast Tight Gas Reservoirs Identification

It’s a great challenge in identifying gas bearing formation from conventional logs in tight gas sandstones due to the low resistivity contrast caused by high irreducible water saturation. Based on the difference of the principles of three kinds of porosity logs (density, neutron and acoustic logs), three porosities difference method, three porosities ratio method, correlation of neutron and density logs and the overlap method of water-filled porosity and total porosity are introduced to identify tight gas bearing reservoirs. In gas bearing formations, the difference of three porosities is higher than 0.0, the ratio of three porosities is higher than 1.0, the correlation between density and neutron logs is negative, and the water filled porosities are lower than total porosities. On the contrary, in water saturated formations, the difference of three porosities is lower than 0.0, the ratio of three porosities is lower than 1.0, the correlation between density and neutron logs is positive, and the water filled porosities are overlapped with total porosities. Considering the complexity of in-suit formation, when the proposed identification criterion are mainly meet, the pore fluid should be determined, field examples show that the proposed techniques are applicable in tight gas formation identification.

Using Selected Structural Indices to Pinpoint the Field Moisture Capacity of Some Coarse-Textured Agricultural Soils in Southeastern Nigeria

Over- or underestimation of field capacity (FC) of agricultural soils could misguide soil and water management and this might have negative agronomic and environmental impacts. The study sought to identify the moisture tension for reliably estimating in the laboratory the FC of some sandy soils with low-activity clay minerals and at different levels of structure development in Nsukka agroecological zone in southeastern Nigeria. Fifty-four samples of topand subsoils under contrasting vegetation cover at three locations in the zone were analyzed for texture, organicmatter contents, bulk density and total porosity. Saturated hydraulic conductivities (Ksat) of the samples were equallydetermined. Water-conducting and water-filled porosities at each of 0.06-, 0.10- and 0.33-bar tensions were implied from water retention data at the respective tensions. The soils were categorized based on their levels of structure development using a structural stability index [(organic matter: silt+clay) %] as follows: very low (&lt; 4%), low (4-7.5%) and moderate to high (&gt; 7.5%) stability soils. Series of simple correlation tests were run among the waterconducting porosities at the various tensions and the Ksat of the soils. In each case, the soil was assumed to have attained FC at that moisture tension which the associated water-conducting porosity showed significant positive correlation with the Ksat. Our results revealed that the 0.06-bar tension overestimated the FC of the soils. The 0.10-bar tension, the commonly used moisture tension for the purpose in the study area, proved suitable only for soils within the moderate to high structural stability category. From all indications, the 0.33-bar tension best corresponded to the FC of the less structurally developed soils in the other two categories. The level of soil structure development should therefore be considered before deciding the suitable moisture tension for the determination of FC of these and similar soils in other tropical locations.Keywords: Coarse mineral soils, field moisture capacity, saturated hydraulic conductivity, structural stability,water-conducting pores

Low Resistivity Contrast Gas Bearing Formation Identification from Conventional Logs in Tight Gas Sandstones

It’s a great challenge in identifying gas bearing formation from conventional logs in tight gas sandstones due to the low resistivity contrast caused by high irreducible water saturation. Based on the difference of the principles of three kinds of porosity logs (density, neutron and acoustic logs), three porosities difference method, three porosities ratio method, correlation of neutron and density logs and the overlap method of water-filled porosity and total porosity are introduced to identify tight gas bearing reservoirs. In gas bearing formations, the difference of three porosities is higher than 0.0, the ratio of three porosities is higher than 1.0, the correlation between density and neutron logs is negative, and the water filled porosities are lower than total porosities. On the contrary, in water saturated formations, the difference of three porosities is lower than 0.0, the ratio of three porosities is lower than 1.0, the correlation between density and neutron logs is positive, and the water filled porosities are overlapped with total porosities. Considering the complexity of in-suit formation, when the proposed identification criterion are mainly meet, the pore fluid should be determined, field examples show that the proposed techniques are applicable in tight gas formation identification. Key words : Low resistivity contrast gas bearing formation; Tight gas sandstones; Identification; Difference of three porosities; Ratio of three porosities; Correlation of neutron and density logs

Herbivore trampling as an alternative pathway for explaining differences in nitrogen mineralization in moist grasslands

Studies addressing the role of large herbivores on nitrogen cycling in grasslands have suggested that the direction of effects depends on soil fertility. Via selection for high quality plant species and input of dung and urine, large herbivores have been shown to speed up nitrogen cycling in fertile grassland soils while slowing down nitrogen cycling in unfertile soils. However, recent studies show that large herbivores can reduce nitrogen mineralization in some temperate fertile soils, but not in others. To explain this, we hypothesize that large herbivores can reduce nitrogen mineralization in loamy or clay soils through soil compaction, but not in sandy soils. Especially under wet conditions, strong compaction in clay soils can lead to periods of soil anoxia, which reduces decomposition of soil organic matter and, hence, N mineralization. In this study, we use a long-term (37-year) field experiment on a salt marsh to investigate the hypothesis that the effect of large herbivores on nitrogen mineralization depends on soil texture. Our results confirm that the presence of large herbivores decreased nitrogen mineralization rate in a clay soil, but not in a sandy soil. By comparing a hand-mown treatment with a herbivore-grazed treatment, we show that these differences can be attributed to herbivore-induced changes in soil physical properties rather than to above-ground biomass removal. On clay soil, we find that large herbivores increase the soil water-filled porosity, induce more negative soil redox potentials, reduce soil macrofauna abundance, and reduce decomposition activity. On sandy soil, we observe no changes in these variables in response to grazing. We conclude that effects of large herbivores on nitrogen mineralization cannot be understood without taking soil texture, soil moisture, and feedbacks through soil macrofauna into account.

Physicochemical properties of mixtures of inorganic supporting materials affect growth of potato (Solanum tuberosum L.) plantlets cultured photoautotrophically in a nutrient-circulated micropropagation system

The objective of this study was to determine the effect of physicochemical properties of inorganic supporting materials on the growth of potato plantlets in a nutrient-circulated photoautotrophic micropropagation (NCM) system. Nine mixtures consisting of combinations of perlite and/or vermiculite with three particle sizes (PL, PM, and PS = perlite large, medium, and small; VL, VM, and VS = vermiculite large, medium, and small, respectively) were used to cultivate potato plantlets in the NCM system. The ratios of vermiculite to perlite in the nine mixtures were determined by cation exchange capacity (CEC) and three distinct levels of water content. Among the nine mixtures with different values of porosity, bulk density, pH, and EC, the mixtures with high ratios of PS or VS had higher levels of water content, total porosity, and water-filled porosity than those with high ratios of PL or VL. Water holding capacity increased in mixtures with high levels of vermiculite. The EC and pH ranges of all mixtures were 1.2 to 2.5 mS·cm−1 and 6.3 to 7.2, respectively. Two mixtures containing 40 and 50% VS had exceptionally the highest EC and pH values. The optimum perlite and vermiculite content for potato plantlet growth was found in two treatments (PL:VL = 30:70 and PM:PS:VL:VM = 20:10:40:0) with a water content of 37% to 47% and CEC of approximately 17 cmol·kg−1. Regardless of the mixtures of supporting materials, conventional polycarbonate culture boxes retarded the growth of potato plantlet as compared with the NCM system. In conclusion, this study showed that the combinations of vermiculite and perlite with different particle sizes made different physical and chemical properties, and some of these mixtures improved growth of potato plantlets in the NCM system.

Toluene biodegradation rates in unsaturated soil systems versus liquid batches and their relevance to field conditions

Contaminant biodegradation in unsaturated soils may reduce the risks of vapor intrusion. However, the reported rates show large variability and are often derived from slurry experiments that are not representative of unsaturated conditions. Here, different laboratory setups are used to derive the biodegradation capacity of an unsaturated soil layer through which gaseous toluene migrates from the water table upwards. Experiments in static unsaturated soil microcosms at 6-30% water-filled porosity (WFP) and unsaturated soil columns at 9, 14, and 27% WFP were compared with liquid batches containing the same culture of Alicycliphilus denitrificans. The biodegradation rates for the liquid batches were orders of magnitude lower than for the other setups. Hence, liquid batches do not necessarily reflect optimal conditions for bacteria; either oxygen or toluene mass transfer at the cell scale or the absence of soil-water-air interfaces seemed to be limiting bacterial activity. For the column setup, the rates were limited by mass supply. The microcosm results could be described by apparent first-order biodegradation constants that increased with WFP or through a numerical model that included biodegradation as a first-order process taking place in the liquid phase only. The model liquid phase first-order rates varied between 6.25 and 20h(-1) and were not related to the water content. Substrate availability was the primary factor limiting bioactivity, with evidence for physiological stress at the lowest water-filled porosity. The presented approach is useful to derive liquid phase biodegradation rates from experimental data and to include biodegradation in vapor intrusion models.

Hysteresis of Unsaturated Hydromechanical Properties of a Silty Soil

Laboratory tests to examine hysteresis in the hydrologic and mechanical properties of partially saturated soils were conducted on six intact specimens collected from a landslide-prone area of Alameda County, California. The results reveal that the pore-size distribution parameter remains statistically unchanged between the wetting and drying paths; however, the wetting or drying state has a pronounced influence on the water-entry pressure, the water-filled porosity at zero suction, and the saturated hydraulic conductivity. The suction stress values obtained from the shear-strength tests under both natural moisture and resaturated conditions were mostly bounded by the suction stress characteristic curves (SSCCs) obtained from the hydrologic tests. This finding experimentally confirms that the soil-water retention curve, hydraulic conductivity function, and SSCC are intrinsically related.

The primary sources of carbon loss during the crop-establishment period in a subtropical Oxisol under contrasting tillage systems

The physical protection of mineralizable carbon (C) in aggregates has been identified as the primary mechanism of soil C stabilization. Therefore, it is possible to hypothesize that the disruption of aggregate by soil tillage is a key process driving C losses during the crop-establishment period. However, these findings are based on studies performed in temperate soils. Limited information is available for studies performed in subtropical and tropical soils, especially in Oxisols, which are rich in oxides that provides chemical C stabilization. This study was performed in southern Brazil in a long-term soil-management experiment carried out in a clay Typic Haplorthox in Cruz Alta (RS). During the 22nd year of the experiment, carbon dioxide (CO 2–C) emissions, temperature, and soil moisture were intensively evaluated over a 21-day summer crop-establishment period using a closed infrared CO 2-flux chamber. The cropping system investigated was an intensive crop rotation following the soil input of winter-cover crops (black oat ( Avena strigosa Schreb) + common vetch ( Vicia sativa L.) under two contrasting tillage systems, conventional tillage (CT) and no-till (NT). The apparent contributions to CO 2–C losses by resident soil C associated with aggregate disruption and recent crop-residue C input were assessed in treatments with crop-residue input (+R) and with crop-residue removed (−R). An exponential-decay model was used to fit the differences in CO 2–C flux between CT − R and NT − R (apparent aggregate-disruption effect) and between CT + R and CT − R (apparent recent crop-residue C input effect). As expected, the CT + R showed an increase of 72% in CO 2–C losses relative to NT + R. During the three-week crop-establishment period, crop-residue C input was the primary source of CO 2–C emissions under CT. The CO 2–C losses under CT were equivalent to 65% of the aboveground C input by winter cover crops, whereas this value decreased to 35% in NT. Exponential-decay modeling of the data for the first week showed that approximately 20% of the CO 2–C losses under CT were related to the exposure of mineralizable resident soil C due by tillage operations. The analysis showed that this value decreased to only 2% for the three-week period. The CO 2–C emissions exhibited a positive linear relationship with soil temperature and soil water-filled porosity under NT, but a similar relationship was found only with soil temperature under CT. For this Oxisol during the crop-establishment period, the physical aggregate disruption induced by long-term CT played a secondary role in CO 2–C losses relative to the recent crop-residue C input from tillage operations.