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Carinata (Brassica carinata A. Braun) is a potential crop for biofuel production, but the risk of injury resulting from carryover of soil herbicides used in rotational crops is of concern. The present study evaluated the carryover risk of imazapic and flumioxazin for carinata. Label rates of imazapic (70 g ai ha−1) and flumioxazin (107 g ai ha−1) were applied 24, 18, 12, 6, and 3 mo before carinata planting (MBP). The same herbicides were applied preemergence right after carinata planting at 1X, 0.5X, 0.25X, 0.125X, 0.063X, and 0X the label rate. When either herbicide was applied earlier than 3 MBP, there was no difference in plant density compared with the nontreated control. Carinata damage was 25%. Quantification of herbicide residues in both soils showed that imazapic moved deeper in the soil profile than flumioxazin. This was more evident in Jackson Springs, where 0.68, 3.52, and 7.77 ng of imazapic g−1 soil were detected (15- to 20-cm depth) when the herbicide was applied at 12, 6 and 3 MBP, respectively, while no flumioxazin residues were detected at the same soil depths and times. When residues were 7.78 and 6.90 ng herbicide g−1 soil in the top 10 cm of soil for imazapic and flumioxazin, respectively, carinata exhibited at least 25% damage. Rotational intervals to avoid imazapic and flumioxazin damage to carinata should be between 6 and 12 MBP depending on soil type and environmental conditions, with longer intervals for the former than the latter.

Seed germination is one of the most important processes in plant biology and ecology because it determines the timing and magnitude of seedling emergence events every growing season influencing community dynamics. Our aim was to determine whether polyethylene glycol (PEG) solutions simulate soil water potential accurately and recreate germination responses to soil water availability. In this study, we compared seed germination of four plant species in PEG and four soils with different textures under six water potentials under controlled laboratory conditions. Total seed germination for all species significantly differed between soil and PEG under the same water potentials, as well as among soil water potentials for each of PEG and soil materials. Due to the inconsistent total germination associated with soil water potential, we evaluated unsaturated soil hydraulic conductivity (Kh) as a predictor of germination. The germination of all species followed the same response to Kh. Germination rate (GR50) was more directly related to water potential than total germination, but Kh provided a more robust description of GR50 across species and soils than PEG-osmotic potentials. Our findings showed that Kh is a more informative variable to predict both total seed germination and germination rate in soil, and caution must be used when considering results obtained using PEG solutions to infer germination behavior under field conditions.

Amending soils with compost has become increasingly common in stormwater management practices. Compost can be a source and sink for nutrients and heavy metals, and it is important to understand the effect of compost on pollutant leaching under different hydrologic conditions. The objectives of this study were (a) to quantify the distribution coefficient (K

Soil bulk density (ρb) may be purposely reduced in agricultural fields using tillage to improve hydraulic properties. However, tillage alters the soil structure, resulting in unstable soils. As the soil stabilizes, ρb increases over time. While this is known, studies on soil hydraulic properties in tilled soils, including comparisons between tilled and non-tilled soils, commonly assume a rigid soil structure. This study presents changes in soil water retention and saturated hydraulic conductivity (Ksat) as ρb increased dynamically with time following tillage at a loam-textured field site. Over the summer of 2015, soil cores were collected at several depths below the surface following precipitation events. Soil water retention curves and Ksat were determined using pressure cells and the constant head method, respectively. Tillage reduced ρb to 0.94 g cm−3. Changes in ρb increased with depth, reaching a ρb of 1.11 g cm−3 in the 0–5 cm layer, and a ρb of 1.42 g cm−3 at the deepest tilled layer. Soil water retention curves were markedly steeper for samples with higher ρb, indicating an overall increase in water retained at a soil matric potential (Ψ) of −33 kPa. Evaluation of two modeling approaches for water retention as a function ρb indicated that changes in water retention with increases in ρb could be reasonably estimated if a matching point was used. No clear relationship between Ksat and ρb was obvious for ρb 1.06 cm3 cm−3, Ksat decreased markedly (order of magnitude) as ρb increased. Hydraulic properties varied strongly depending on time since tillage and soil depth, and results have implications for models of tilled soils, as well as for studies comparing tilled and non-tilled soils.

Soil thermal properties play important roles in dynamic heat and mass transfer processes, and they vary with soil water content (θ) and bulk density (ρ b). Both θ and ρ b change with time, particularly in recently tilled soil. However, few studies have addressed the full extent of soil thermal property changes with θ and ρ b. The objective of this study is to examine how changes in ρ b with time after tillage impact soil thermal properties (volumetric heat capacity, C ᵥ, thermal diffusivity, k, and thermal conductivity, λ). The study provides thermal property values as functions of θ and ρ b and of air content (n ₐᵢᵣ) on undisturbed soil cores obtained at selected times following tillage. Heat pulse probe measurements of thermal properties were obtained on each soil core at saturated, partially saturated (θ at pressure head of −50 kPa) and oven‐dry conditions. Generally, k and λ increased with increasing ρ b at the three water conditions. The C ᵥ increased as ρ b increased in the oven‐dry and unsaturated conditions and decreased as ρ b increased in the saturated condition. For a given θ, a larger ρ b was associated with larger thermal property values, especially for λ. The figures of C ᵥ, k and λ versus θ and ρ b, as well as C ᵥ, k and λ versus n ₐᵢᵣ, represented the range of soil conditions following tillage. Trends in the relationships of thermal property values with θ and ρ b were described by 3‐D surfaces, whereas each thermal property had a linear relationship with n ₐᵢᵣ. Clearly, recently tilled soil thermal property values were quite dynamic temporally due to varying θ and ρ b. The dynamic soil thermal property values should be considered in soil heat and mass transfer models either as 3‐D functions of θ and ρ b or as linear functions of n ₐᵢᵣ. HIGHLIGHTS: Thermal property values for a range of θ and ρ b were measured on undisturbed soil cores. Freshly tilled soil thermal property values were quite dynamic temporally. The thermal property values of a tilled soil were described as 3‐D surfaces with θ and ρ b. The thermal property values of a tilled soil varied linearly with n ₐᵢᵣ.

The Mualem-van Genuchten model has been widely used for estimating unsaturated soil hydraulic conductivity (Ku) from measured saturated hydraulic conductivity (Ks) and fitted water retention curve (WRC) parameters. Soil bulk density (ρb) variations affect the accuracy of Ku estimates. In this study, we extend the Mualem-van Genuchten model to account for the ρb effect with ρb-related WRC and Ks models. We apply two functions (A and B) that relate the van Genuchten WRC model to ρb and two models (1 and 2) that estimate Ks with various ρb. By combining the ρb-related WRC functions and Ks models, we develop four integrated approaches (i.e., A1, A2, B1, and B2) for estimating Ku at various ρb. Ku measurements made on five soils with various textures and ρb are used to evaluate the accuracy of the four approaches. The results show that all approaches produce reasonable Ku estimates, with average root mean square errors (RMSEs) less than 0.35 (expressed in dimensionless unit because logarithmic Ku values are used). Approach A2, with an average RMSE of 0.25, agrees better with Ku measurements than does Approach A1 that has an average RMSE of 0.28. This is because Model 2 accounts for the WRC shape effect near saturation. Approaches A1 and A2 give more accurate Ku estimates than do Approaches B1 and B2 which both have average RMSEs of 0.35, because Function A performs better in estimating WRCs than does Function B. The proposed approaches could be incorporated into simulation models for improved prediction of water, solute, and gas transport in soils.

Current research on bioenergy crops shows that perennial grasses can yield substantial amounts of dry biomass with relatively low inputs of water and fertilizer. In order to minimize competition with food production, it has been suggested that bioenergy crops could be directed to land areas less suitable for commodity crops, commonly referred to as “marginal” lands. These are land units with inherent limitations to vegetative growth and production, which may be due to several factors (soil physical and chemical properties, climatic conditions, etc.). However the term “marginal” is an adjective with imprecise meaning, and objective criteria for determining “marginal” lands for siting bioenergy crops are necessary. Here we propose that such criteria may be based on soil survey classifications and realistic yield estimates, and we show an example of its use to justify site selection for bioenergy crops in different regions of North Carolina.

In 2012, only 2.7% of North Carolina’s cotton (Gossypium hirsutum L.) was irrigated compared to the national average of 39%. The small size and nonuniform shape of most North Carolina fields are not conducive for a center pivot system. However, benefits to yield due to irrigation in North Carolina have been reported, specifically in years receiving below average or sporadic rainfall. The objective of this research was to investigate the impact of subsurface drip irrigation (SDI) on growth and yield of early- and late-maturing cotton cultivars at varying planting dates in eastern North Carolina. In 2014, the site received more than 750 mm of rainfall and no differences were observed for any parameters between irrigated and non-irrigated plots. Total rainfall in 2015 and 2016 was lower with several extended periods without rain events. There was a greater plant height increase and dry weight accumulation throughout the growing season in response to SDI. Cotton yields were increased by SDI in 2015 and 2016. Cultivar only influenced lint yield in 2016 with the earlier-maturing ‘PHY 333 WRF’ having greater lint yield than ‘PHY 499 WRF’. Planting date did not influence yield under irrigated conditions, and the timing of rainfall played a role similar to previous reports in North Carolina. Irrigation applied via SDI will increase cotton plant stature, fruit retention, and yield in response to deficit moisture conditions, independent of planting date or cultivar.

Tillage practices, cover crops, and planting dates can influence soil moisture and temperature conditions at planting and cotton (Gossypium hirsutum L.) stand establishment and yield. A study was conducted in North Carolina at two locations from 2014 through 2016. Treatments included six tillage systems of fall and spring conventional raised beds and flat strip tillage planted in early and late May, with and without a wheat (Triticum aestivum L.) cover crop. Tillage treatments include conventional spring raised beds, fall strip till, stale seedbed, at-plant strip till, pre-plant strip till, and stale seedbed with at-plant strip till. Except for fall strip tillage, no tillage systems reduced plant populations compared to conventionally tilled cotton in any environment. Crop growth rates were similar in conventional and spring strip-till systems and were lower in four planting date environments with stale seedbeds. In 2016, in-row soil resistance was measured from 0- to 30-cm depth using a conical penetrometer both at planting and post-harvest. Plots without any spring tillage had the greatest soil resistance for all measurements and depths. All plots with spring tillage had similar soil resistance to at least the 15-cm depth from which point the conventional spring beds had the least soil resistance through the 30-cm profile. Late planted cotton in 2014 showed inconsistent yield differences among tillage systems between locations. When pooled over location and year for 2015 and 2016, however, tillage system did not influence cotton yield. These data indicate that cotton yields in reduced-till systems are comparable to cotton grown in conventional systems in North Carolina soils.

Soils adjacent to urban surfaces are often impaired by construction activities that degrade the natural structure and function of the soil, resulting in altered physical, hydraulic, and vegetative properties that limit the infiltration, storage, and filtration of stormwater runoff. A management approach to enhance the efficacy of vegetated roadside soils for runoff control is the use of compost in conjunction with tillage to improve soil conditions and facilitate improved hydrological function, the establishment of vegetative biomass, and increased nutrient and pollutant attenuation. The purpose of this study was to determine the efficacy of soil improvement measures to reduce runoff volumes and improve water quality along roadsides over time. The effects of tillage with and without compost on 1) bulk density and infiltration rates, 2) runoff volumes, and 3) runoff water quality were evaluated during multiple storm events along two long-established interstate roadsides in North Carolina during 2015 and 2017. Experimental plots were established in the grassed areas adjacent to roads and consisted of an untreated control, tillage only, and tillage amended with compost. Tillage alone did not reduce runoff in roadside soils, however, tillage with compost did improve runoff capture. The patterns in hydrologic performance within and among sites suggests that the incorporation of compost in tilled soils may influence storage potential through different effects on soil properties, such as decreasing bulk density or improving vegetation establishment, thereby increasing evapotranspirative withdrawals, depending on soil texture. Tillage increased sediment concentrations in runoff, however, net export of sediments was reduced with the inclusion of compost due to the reduction of runoff quantities compared to undisturbed areas and tillage alone. Control and treatment plots were equally effective in reducing dissolved nutrient and metal concentrations, however, the improved hydrologic performance in plots with compost decreased net nutrient and metal export in most storms. The results of this study suggest that the incorporation of compost in compacted urban soils may provide significant improvements for biological and physical soil properties that affect stormwater interception and infiltration.

Vineyards’ canopy architecture and row structure pose unique challenges in modeling the radiation partitioning and energy exchange between the vine canopy and the interrow area. The vines are often pruned and manipulated to be strongly clumped, while mechanical harvesting requires wide rows, often with vine height to vine spacing ratio > 1. This paper estimates the intercepted radiation by the canopy, and the effect of this interception on the below canopy surface energy balance and evapotranspiration (ET). Measurements were conducted in an east–west oriented vineyard in CA during intensive observation periods as part of the grape remote sensing atmospheric profile and evapotrnspiration eXperiment (GRAPEX). Below canopy incoming shortwave radiation was measured at multiple positions across the interrow, and the surface energy balance/ET below the vine rows was measured for only one growing season (in 2015) using three micro-Bowen ratio (MBR) systems. These MBR systems were deployed across the interrow, in the north, center, and south of the interrow. A significant spatial and temporal variability in radiation was observed since the vines were not significantly pruned or manipulated and thus grew randomly into the interrow. However, when averaged across the interrow using the radiation sensor array, the values appeared to give reliable mean radiation extinction conditions that agreed with model estimates. The variation in the surface energy fluxes were dominated by the amount of transmitted radiation, while soil moisture was a second order effect. Daily estimates of ET from the three micro-Bowen ratio systems, weighted by their respective representative sampling area, yielded estimates similar to values computed by the correlation-based flux partitioning method, which utilizes high-frequency eddy covariance data measured above the canopy.

Pressures on water availability for irrigation of turfgrasses continue in many parts of the United States as climate and weather patterns shift and populations increase. It is essential to understand underlying factors controlling water loss to more precisely predict irrigation requirements and develop new strategies for improving effective use of water. In this study, we investigate two key components of potential water loss from a bentgrass (Agrostis stolonifera L.) system that have not previously been examined in detail: 1) water loss in darkness, and 2) water loss through evaporation directly from the soil. The experiments were conducted in controlled environment chambers with intact cores from the field. An automated gravimetric system and soil moisture probes allowed precise measurements of water loss over ranges of vapor pressure deficits (VPD). The gravimetric and soil probe results indicated that substantial evapotranspiration occurred in darkness, at rates 40 to 60% of that in the light across VPDs. Simulations using field weather data from dry and humid environments indicated nighttime water loss rates would be expected to be 30 to 40% of that in the light. Using cores treated with a fast-acting, desiccating herbicide that eliminated transpiration but kept core resistances intact, evaporation directly from the soil surface was estimated to account for 40% of total water loss in the light and 60 to 70% in the dark. The results, collectively, indicated that water loss in darkness must be separately accounted for to accurately estimate daily evapotranspiration totals and irrigation requirements. Furthermore, because of the very high potential for evaporative water loss in the light and dark, efforts to improve water use efficiencies in the turfgrass system should include strategies that regulate both transpiration by the plant and evaporation from the soil surface.

Unsaturated hydraulic conductivity (K) of surface soil changes substantially with space and time, and it is of great importance for many ecological, agricultural, and hydrological applications. In general, K is measured in the laboratory, or more commonly, predicted using soil water retention curve and saturated hydraulic conductivity. In the field, K can be determined through infiltration experiments. However, none of these approaches are capable of continuously monitoring K in-situ at fine depth scales. In this study, we propose and investigate an approach to continuously estimate fine depth-scale K dynamics under field conditions. Evaporation rate and change in water storage in a near-surface soil layer are measured with the heat pulse method. Then, water flux density at the lower boundary of the soil layer is estimated from evaporation rate, change in water storage, and rainfall or irrigation rate using a simple water balance approach. Finally, K values at different soil depths are derived using the Buckingham-Darcy equation from water flux densities and measured water potential gradients. A field experiment is performed to evaluate the performance of the proposed approach. K values at 2-, 4-, 7.5-, and 12.5-cm depths are estimated with the new approach. The results show that in-situ K estimates vary with time following changes in soil water content, and the K-water content relationship changes with depth due to the difference in bulk density. In-situ estimated K-matric potential curves agree well with those measured in the laboratory. In-situ K estimates also show good agreement with the Mualem-van Genuchten model predictions, with an average root mean square error in log10 (K, mm h−1) of 0.54 and an average bias of 0.17. The new approach provides reasonable in-situ K estimates and has potential to reveal the influences of natural soil conditions on hydraulic properties as they change with depth and time.

Determining thermal conductivity (λ) and volumetric heat capacity (C) of partially frozen soils with a dual probe heat pulse (DPHP) sensor is challenging because an applied heat pulse melts ice surrounding the heater probe. Examining DPHP temperature changes with a commonly-used analytical solution that only accounts for heat conduction leads to inaccurate λ and C estimates for partially frozen soils at temperatures between −5 °C and 0 °C. In order to determine λ and C accurately and simultaneously, it is necessary to understand how various properties of partially frozen soil influence the temperature changes produced by DPHP sensors. The objective of this study is to determine the sensitivity of DPHP temperature changes to soil conditions and soil thermal properties. A numerical solution for radial heat conduction with soil freezing and thawing is developed. A series of simulations are performed, in which various errors are imposed onto a selected model parameter while other model parameters are held constant, and sensitivity coefficient values (φ) of the time of maximum probe temperature (tm) and of the maximum probe temperature rise (Tm) for each parameter are calculated. Temperature changes at the measurement probe are quite sensitive to initial soil temperature (φ values for tm and for Tm are −0.99 and 0.99, respectively), λ (φ value for tm is −0.93), and parameters determining the shape of the soil freezing characteristic (FC) curve, i.e., saturated water content θs (φ values for tm and for Tm are 0.59 and −0.73, respectively) and n (φ values for tm and for Tm are −2.7 and 2.4, respectively). Temperature changes are not very sensitive to C (φ values for tm and for Tm are 0.034 and −0.15, respectively). Although previous investigations tried to determine C by inverse analysis, this sensitivity analysis shows that the influence of C on temperature response to a heat pulse is masked by that of the FC. Thus, λ and FC parameters are the best candidate parameters to be determined by inverse analysis of DPHP data. This new result will guide further testing of DPHP sensors in partially frozen soils.

Summary Goals: In regions such as the eastern United States, excess vine vigor can be problematic. In this region, it is common to plant a perennial grass between rows, which can compete with vines for water and nutrients. The purpose of this research was to determine the effect of vegetation-free strip (VFS) width beneath the planted row on vine growth and fruit quality. The current recommendation for VFS width is 90 to 120 cm. However, modification of the VFS width can provide additional competition, limiting vine vigor. Determining the optimal width and effect of the VFS on vine size, berry composition, and yield would allow growers to optimize groundcover management in this region. Key Findings: Reducing VFS width decreased pruning weight/m cordon, shoot number/m cordon, lateral shoot number/cane, and summer fresh hedging weights. Narrowing the VFS width was most effective in the two of four years with the least rainfall. Yield/m cordon was reduced by narrowing VFS width, but not to below normal adjusted crop loads. Cluster weight, number of berries/cluster, and cluster number/m cordon were also reduced by narrowing VFS width. Berry soluble solids and total anthocyanins increased and TA decreased with decreasing VFS width, improving berry quality. Postveraison natural weed population growth in the VFS did not affect vine growth or fruit yield and composition. Impact and Significance: In the eastern United States, high rainfall and humidity promote excessive vine growth and immense pest pressure for Vitis vinifera production. Improved canopy characteristics could increase fruit quality by reducing pest pressure, by increasing soluble solids and total anthocyanin concentrations, and by improving the balance between pH and titratable acidity (TA). Increasing competition for water and nutrients during the growing season (by narrowing the VFS or allowing late-season weed competition) may be an effective way to accomplish these improvements. In this study, narrower VFS width in a tall fescue ( Festuca arundinacea var. ‘Kentucky 31’) groundcover reduced vine vegetative growth and positively influenced berry composition. Vineyard weed populations that established naturally postveraison did not affect vine size, yield, or fruit quality.

Forest floor and mineral soil manipulations influence the soil biogeochemical properties important for loblolly pine (Pinus taeda L.) tree growth. The impacts of forest floor manipulations on soil aggregate stability and the presence of ectomycorrhizal fungi (EMF), was assessed to elucidate the relationship between EMF abundance and aggregate stability. The study site consists of a 14-year-old loblolly pine plantation managed by Weyerhaeuser Company in the Lower Coastal Plain, approximately 8 miles east of New Bern, North Carolina, USA. The soil samples were collected from the top 7.62 cm of each soil treatment which includes three levels of forest floor retention: removed, control, and doubled and two levels of forest floor mixing with the mineral soil: mixed and unmixed. Ectomycorrhizal fungi abundance was evaluated by ester-linked fatty acid methyl ester analysis and microbial community functionality was assessed by acid-phosphatase activity measurement. Aggregate stability was assessed using the aggregate mean weight diameter approach. Results indicate that the forest floor manipulations had no significant impact on aggregate stability and EMF abundance. However, a positive relationship between EMF abundance and aggregate stability was identified. Removing the forest floor resulted in a soil bulk density increase of 0.18 g cm−3 compared to doubling the forest floor. Our results demonstrate that some mineral soil properties recover relatively quickly from forest floor manipulations. The study informs forest managers interested in how soil responds to forest floor manipulation and the interaction between EMF and aggregate stability.

Soil pore size and arrangement control the heterogeneous distribution of nutrients, water, and air and, therefore, are likely a superimposed and integrated factor dictating the soil microbial community structure. It is known that soil with more large pores can potentially harbor more diverse microbes under low hydraulic connectivity. Still, there is scant information on how soil pore size distribution (PSD) governs the composition and association of the microbial community. This work examined PSD effects on microbial community compositions and networks via marker gene high-throughput sequencing of both DNA and cDNA. Three soils with a large variation in silt content (~11–73%) and their combinations at different mass ratios were used to enhance the continuity in silt content and thus PSD. Investigations were made under different levels of total pore volume and pore hydraulic connectivity by incubating soils at varied bulk densities and moisture contents for 50 days. The total of six soils was dichotomized into two PSD groups based on soil water retention curves, with PSD-1 of more macro- and mesopores (>30 μm) and PSD-2 of more micropores (

The thermo-TDR (time domain reflectometry) technique has been applied for measuring soil bulk density (ρb) in-situ. However, the accuracy of thermo-TDR measured ρb data, as influenced by plant roots, has not been studied. In this study, we applied the extended de Vries heat capacity model to examine the influences of roots on thermo-TDR sensor performance for measuring ρb dynamics in the root zone. Soil samples were collected at multiple depths and horizontal positions over time during a maize growth period, and ρb values were determined gravimetrically and indirectly from thermo-TDR measurements. Results showed that by using the extended de Vries model, the thermo-TDR measured ρb agreed well with the gravimetric values. Ignoring root contribution to bulk soil heat capacity introduced 6.7%, 13.8% and 13.9% errors in thermo-TDR measured ρb data on the loamy sand, sandy loam, and clay loam soils, respectively. A critical root density of 0.037 g cm−3 was determined beyond which roots may induce ρb errors greater than 0.1 g cm−3 with the thermo-TDR technique.

Constructing roads and buildings often involves removal of topsoil, grading, and traffic from heavy machinery. The result is exposed, compacted subsoil with low infiltration rate (IR), which hinders post-construction vegetation establishment and generates significant runoff, similar to impervious surfaces. Our goal was to assess tillage and adding amendments for improving density and maintaining perviousness of subsoils compacted during construction. The effects of tillage with and without amendments on (1) soil compaction, (2) IR, and (3) vegetative growth at five sites in North Carolina, USA were evaluated over a period of up to 32 months. The sites, representing a range of soil conditions, were located at three geographic regions; one in the Sandhills (located in Coastal Plain), one in the mountains, and three in the Piedmont. Amendments varied by site and included: (1) compost, (2) cross-linked polyacrylamide (xPAM), and (3) gypsum. Bulk density (BD) and soil penetration resistance (PR) tests were used to characterize soil physical condition. The IR was measured using a Cornell Sprinkle Infiltrometer. Vegetative growth was evaluated by measuring shoot mass and vegetative cover at all sites and root density at the Piedmont sites. Tillage decreased BD and PR compared to the compacted soil at four out of five sites for observations ranging from 24 to 32 months. Compost was applied to four sites prior to tillage and reduced BD in two of them compared to tillage alone. The IR in the tilled plots was maintained at about 3–10 times that of the compacted soil among the five sites over the monitoring periods. Adding amendments did not increase IR relative to tillage alone except at one Piedmont site, where compost and xPAM increased IR at 12 months and compost at 24 months after site establishment. Vegetative responses to tillage and amendments were inconsistent across sites. Results suggest that tillage is a viable option to reduce bulk density and increase infiltration for areas with compacted soils where vegetation is to be established, and that the effect is maintained for at least several years.

Summary Measuring ice contents (θi) in partially frozen soils is important for both engineering and environmental applications. Thermo-time domain reflectometry (thermo-TDR) probes can be used to determine θi based on the relationship between θi and soil heat capacity (C). This approach, however, is accurate in partially frozen soils only at temperatures below −5 °C, and it performs poorly on clayey soils. In this study, we present and evaluate a soil thermal conductivity (λ)-based approach to determine θi with thermo-TDR probes. Bulk soil λ is described with a simplified de Vries model that relates λ to θi. From this model, θi is estimated using inverse modeling of thermo-TDR measured λ. Soil bulk density (ρb) and thermo-TDR measured liquid water content (θl) are also needed for both C-based and λ-based approaches. A theoretical analysis is performed to quantify the sensitivity of C-based and λ-based θi estimates to errors in these input parameters. The analysis indicates that the λ-based approach is less sensitive to errors in the inputs (C, λ, θl, and ρb) than is the C-based approach when the same or the same percentage errors occur. Further evaluations of the C-based and λ-based approaches are made using experimentally determined θi at different temperatures on eight soils with various textures, total water contents, and ρb. The results show that the λ-based thermo-TDR approach significantly improves the accuracy of θi measurements at temperatures ≤−5 °C. The root mean square errors of λ-based θi estimates are only half those of C-based θi. At temperatures of −1 and −2 °C, the λ-based thermo-TDR approach also provides reasonable θi, while the C-based approach fails. We conclude that the λ-based thermo-TDR method can reliably determine θi even at temperatures near the freezing point of water (0 °C).

Soil structure-dependent properties are subject to changes with time and depth under the influences of agronomic practices and environmental factors. Wetting and drying (W/D) cycles following tillage alter the structure and pore-related functions of field soils. This study investigates the effects of W/D cycles on temporal changes of soil porosity and water retention curves (SWRCs) of a tilled layer (0- to 15-cm) during a post-tillage period. Soil volumetric water content (θ) and matric potential (ψₘ) dynamics at three depths were monitored continuously, and the total porosity (Pₜ) was determined gravimetrically. Nine W/D cycles were identified in the experimental period. Soil Pₜ decreased gradually with time and depth and became relatively stable after four W/D cycles, and the SWRCs shifted toward higher θ values at a specific ψₘ. The magnitude of SWRC change was relatively small during the first to third W/D cycles when the degrees of saturation were relatively low, reached the maximum after the fourth W/D cycle during which the soil was nearly saturated, and became less significant thereafter. Soil water holding capacity was improved during the W/D processes mainly due to the reduction of effective porosity and development of residual porosity. During the earlier four W/D cycles, a higher initial θ also contributed to the increase of the residual porosity, and thus enhanced the soil water holding capacity. The difference, however, tended to disappear in later W/D cycles. The data show clearly that the θ–ψₘ relationships following tillage are dynamic.