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Controlled‐release fertilizers (CRFs) have the potential to deliver crop production and environmental benefits through better control of applied nitrogen (N) in cropping systems. Whereas N release from CRFs can be impacted by several factors, there has been a widely held view that soil water has little effect on N release from polymer‐coated CRFs. Past research has often studied the soil water effect as a function of soil water content. This limits the transferability of results. In this study we measured N release from a polymer‐coated urea (PCU) and a polymer‐sulfur–coated urea (PSCU) using undisturbed soil cores at set matric potentials. Soil matric potential had a significant effect on N release from PCU. Release at −1,000 kPa was delayed by up to 30 d compared with −10 kPa. Optical stereo microscopy clarified that this was linked to differences in the rate of water absorption. Theoretical considerations demonstrate that these relatively large differences could not be explained by the effect of soil matric potential on vapor flow. It is possible that soil matric potential interacted with the properties of the coating to change its permeability or the involvement of liquid flow. The effect of soil water on N release from PSCU was less clear. The magnitude of the soil water effect is, therefore, product‐specific and dependent on coating properties. The soil matric potential provided a consistent description of release patterns between soils with contrasting soil water retention characteristics. Soil water effects should hence be studied as a function of soil matric potential.

Autoclaved-citrate extractable (ACE) soil protein is included in some soil health assessments as a biological indicator. Furthermore, soil protein contents may be related to the ability of a soil to make nitrogen (N) available for plants by mineralization. The main objective of this study was to evaluate the correlation between ACE protein and potential net N mineralization in undisturbed soil cores from 57 fields in California under annual crops. Total N in the soils ranged from 0.65 to 12.5 g kg⁻¹, and the sites represented eight Soil Taxonomy orders. Soil ACE protein concentrations ranged from 1.0 to 45.2 g kg⁻¹ soil. Although the correlation between ACE protein and potential net N mineralization was positive, ACE protein explained only 21% of the variability in potential net N mineralization across all sites, which was less than total N. Under the assumption that proteins contain 16% N, ACE protein-N accounted for 28% of total N across all sites. However, in some soils with a high total N content, ACE protein accounted for up to 67% of total N. Because autoclaving is expected to denature some proteins, these values seem very high and are likely caused by the interference of coextracted humic substances. Our results do not suggest that ACE protein is a better predictor of potential net N mineralization than total soil N, which may be at least partly due to an apparent interference of coextracted humic substances with the protein assay.

Capacitance probes (CP) based on responses to electromagnetic properties in soil are widely used for monitoring soil volumetric water content (θ v ), but proper calibration is necessary to obtain reliable results. The objectives of this study were to: (i) compare errors in evaluating θ v when using soil-specific equations as opposed to the manufacturer's default equation; (ii) compare the CP calibration equations obtained in the laboratory and in the field; and (iii) evaluate the accuracy of CP under field conditions using soil-specific equations as opposed to the manufacturer's default equation. Laboratory and field calibrations were conducted by fitting statistical relationships between readings from CP and θ v measured from soil cores. The accuracies of the obtained calibration equations were evaluated under field conditions during two irrigation cycles. In both calibration studies, the manufacturer's default equation consistently overcalculated θ v (RMSE > 0.044 m 3 m-3 ) , whereas custom CP calibration equations provided accurate θ v determinations (RMSE < 0.028 m 3 m -3 ). Even when θ v was overestimated, the manufacturer's default equation was able to reproduce θ v dynamics during irrigation. Therefore, if users are interested in relative differences in θ v , they may use the manufacturer's default equation. If absolute values are needed, however, soil-specific calibration is required. The accuracy of CP in monitoring θ v under field conditions was slightly better using the laboratory calibration equation (RMSE = 0.019 m 3 m -3 ) rather than the field calibration equation (RMSE = 0.023 m 3 m -3 ). The use of a laboratory calibration is recommended given that it is easily reproducible, facilitates work planning, and minimizes uncertainties.

Soil water content is often determined using various permittivity sensors. The Hydra Probe reports both the real (e') and imaginary (e") components of complex permittivity (e*) and estimates electrical conductivity (σ'). Previously, a 12-wire probe was used with a vector network analyzer to determine e' and e" and σ' and σ" in undisturbed soil cores. The purpose of this study was to compare results from the Hydra Probe and the 12-wire probe for fluids as well as for soil samples taken from six locations, to better understand the interrelations of e', e", and σ' from the Hydra Probe. Undisturbed soil cores were collected from an Iowa prairie and adjacent soybean [Glycine max (L.) Merr.] field, an Iowa forest and adjacent soybean field, Idaho burned and unburned sites, Colorado grassland and dryland wheat (Triticum aestivum L.) sites, Ohio grassed areas, and Texas rangeland and an irrigated field. Because of their density, some of the Texas samples were sieved and repacked. The reported σ', e', and e" values of fluids were compared between the two measurement systems and with theoretical values. Because the theoretical values for soils are not known, the measured values were compared between systems. Both probes showed σ', e', and e" close to the theoretical values for fluids. In soils, we showed that an adjustment was needed in the calculated σ' from the Hydra Probe to account for dielectric relaxation. The soil σ' of the Hydra Probe matched σ' from the 12-wire probe better after the adjustment was made.

Prior research has shown a log-log correlation between ex situ air permeability (k a ) measured at a matric potential of -50 and -100 cm H 2 O and saturated hydraulic conductivity (K sat ) in agricultural soils. We examined the applicability of a previously established log k a vs. log K sat predictive relation in unburned and burned woodland-chaparral and coniferous soils. Specifically, we measured in situ k a in the field using the soil corer air permeameter (SCAP) and measured laboratory K sat on soil cores. To determine postfire soil physical changes, K sat was transformed into water permeability, k w. Then k a and k w were compared in unburned and burned soils. The median k a was lower and median k w was higher in burned than unburned woodland-chaparral soils. The median k a and k w were both lower in burned than unburned coniferous soils. Despite these effects, the 95% prediction interval of the predictive relation includes 80% of the burned soils and 83% of all desert soils. The RMSE values of the predicted log K sat and measured log K sat were 0.371, 0.552, 0.588, and 0.511 m d -1 for unburned and burned woodland-chaparral and unburned and burned coniferous soils, respectively. In comparison, the RMSE for the predicted and measured log K sat was 0.654 m d -1 for an agricultural data set. The overall RMSE for unburned and burned soils including the agricultural soils was 0.551 m d -1 . This suggests that the predictive relation is applicable for unburned and burned desert soils for in situ k a measurements.

Disk infiltrometers are widely used to determine saturated and near-saturated soil hydraulic conductivity. Previous studies have recommended applying a high-permeability material to the undisturbed soil surface to establish a complete hydraulic bond between the disk and the infiltration surface. Other studies have shown that the use of the contact material affects the infiltration, and hence also the determination of saturated and near-saturated conductivity. In this study, we tested a new type of infiltrometer (that we call a hood infiltrometer), which might overcome these problems. Instead of requiring a disk and contact material, it places a water-filled hood, open side down, onto the soil surface. In this study, repeated hood and disk infiltrometer field tests in conjunction with time-domain reflectometry (TDR) measurements were performed at the same location to compare the performance of the two types of infiltrometer. Furthermore, we extracted undisturbed soil cores to measure the hydraulic functions in the lab. The measured hood and disk data were analyzed using Wooding's solution and by numerical parameter optimization technique using uni- and bimodal hydraulic functions. Running the disk infiltrometer with a contact layer provided saturated hydraulic conductivities that were 10 times smaller than corresponding values measured by the hood infiltrometer. We attributed these differences to smearing, sealing, and clogging of pores, which led to additional flow impedances in the soil surface layer. We were able to show, however, that the combined use of hood and disk infiltrometers in conjunction with TDR enabled hydraulic characterization of the soil from saturation to dry conditions.

Soil organic carbon (SOC) sequestration is an important process to mitigate CO 2 emissions. Our objectives were to determine the rates of C sequestration and to determine if the SOC pool was at or approaching equilibrium in plots under long-term (24-yr) conservation (CT) and disk tillage (DT) management. The plots were Norfolk loamy sand (fine-loamy, kaolinitic, thermic, Typic Kandiudult) and were under a row crop rotation. All plots received annual subsoiling, while only plots under DT were surface disked. Soil cores were collected to 90 cm deep. After 24 yr, the only significant increase in SOC occurred in CT plots at a 0- to 5-cm depth. The SOC pool in plots under DT was at a near-steady state, while the SOC pool under CT was not at equilibrium. This supports the conclusion that CT is an effective countermeasure to offset atmospheric CO 2 emissions.

Field experiments are essential for elucidating the processes involved in N 2 O production during soil freezing/thawing (FT) cycles, but laboratory simulations can provide the advantage of controlled conditions. Past studies have used disturbed or undisturbed soil cores placed in a controlled temperature environment, so that FT occurred from the external surface to the inside of the core (omni-directional method, OD). A new method of soil FT for simulating field conditions more dosely in the laboratory was developed and its effect on N 2 O fluxes evaluated. Three methods were examined: OD, uni-directional (UD) with the soil column surrounded by filled-in soil, and a variant of UD with water accessible to the base of the core (UDW). The rate of soil cooling with time was significantly faster and similar at all depths (-0.29°C h -1 ) for OD, than for UD or UDW methods (-0.16°C h -1 at 1 cm, -0.10°C h -1 at 20 cm). This differential cooling resulted in a significant change in soil temperature with depth for UD and UDW methods 0.125°C cm -1 during freezing and -0.35°C cm -1 during thawing, but of only 0.04 and -0.02°C cm -1 for the OD method. Comparison with field temperature data indicated that the UD and UDW method more closely resembled the gradual top-to-bottom freezing of soil layers that occurs in field conditions (changes of -0.20°C h -1 at 1 cm; -0.05°C h -1 at 20 cm). Gravimetric water content in the frozen layer 0 to 20 mm in the UDW (60.6 g kg -1 ) was significantly higher than in UD (44.4 g kg -1 ) and OD (37.6 g kg -1 ) soil columns. Fluxes of N 2 O during thawing were significantly affected by the incubation method used, probably due to the intensity and duration of freezing, and the water content prevalent under each method with OD (13 ng N 2 O-N m -2 s -1 ) > UDW (4.8 ng N 2 O-N m -2 s -1 ) > UD (2.1 ng N 2 O-N m -2 s -1 ). We conclude that the UD (and UDW) method allows for manipulation of FT in soil columns gradually layer-by-layer providing the conditions needed to link the site of N 2 O production in the soil profile with surface N 2 O fluxes in laboratory studies.

Forest soils are often deep andlor coarse-textured, which does not always lend itself to easy unbiased sampling. Two important Pacific Northwest (PNW) forest soil series that are deep and coarse-textured were studied to evaluate methods of estimating soil C: (i) a loamy sand glacial outwash soil (Indianola series, mixed, mesic Dystric Xeropsamments) and (ii) a very gravelly sandy loam glacial outwash soil (Everett series, sandy-skeletal, isotic, mesic Vitrandic Dystroxerepts). Four methods were compared for estimating soil C, including: (i) large pit (0.5 m 2 ) excavation, (ii) dug pit with 54-mm hammer-core bulk-density sampling, (iii) 31-mm soil push sampler, and (iv) clod method. Coarse (>2 mm) fragments were also collected, processed, and analyzed for soil C. Extending soil sampling deeper than 15 cm increased soil C estimates by as much as 120%. The pit excavation method with sand-displacement volume measurements, which is by far the most labor-intensive and time-consuming, was considered the standard by which other methods were compared, as it didn't contain any obvious biases. Soil core methods overestimated the 50%) of the Everett soil. Including C analysis of the >2-mm soil fraction increased soil C estimates by 170% for the Everett series soil (due to organic C contained in the rocks; there were no carbonates) but did not substantially increase the estimate in the Indianola series soil.

A method was developed to obtain large undisturbed soil cores and instrument those cores to collect vadose zone leachate data under agricultural field conditions. Twenty undisturbed soil core lysimeters were constructed at an irrigated-field site in southeastern North Dakota. Construction of the undisturbed lysimeters consisted of utilizing a steel cutting bit to collect large (0.61-m diam. by 1.68-m deep) soil cores in sections of polyvinyl chloride (PVC) pipe, installing a leachate collection system, installing time domain reflectometry (TDR) soil moisture probes, and then finally placing them below grade so normal farming practices could continue. The described method of core collection is an improvement over other methods in that the cutting bit is reusable, the resulting core is larger and deeper than other methods, no heavy static weights are required, and it is not necessary to expose a free-standing soil column. Leachate quality and quantity data is reported for 1990 through 1995 and compared favorably with larger, reconstructed profile lysimeters on the same site. Overall, the lysimeters functioned as intended for the first 6 yr of operation; however, two developed leaks and rodents damaged another. Possible sidewall flow was observed and may be avoidable in future designs.

Forest management practices may alter soil properties, but experimental evaluation of treatment effects is often difficult because large soil variability yields poor statistical sensitivity. This study was conducted to determine if pretreatment soil sampling enhances our ability to detect treatment differences in Pacific Northwest forests. We used statistical simulation and data from 271 pretreatment 30-cm-deep soil cores taken from 70- to 100-yr-old post-fire Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] stands at the Siskiyou National Forest in southwestern Oregon. At this site, an unmanipulated control and six treatments of overstory species and woody debris are being established on 6-ha plots and replicated in three blocks. If only post-treatment measurements were made, the minimum detectable difference (MDD) between extreme treatments would be 26 to 57% of current soil C and N concentrations (g kg -1 ) and masses (kg m -2 ), with C generally having smaller relative MDDs than N. In contrast, if the differences between post- and pretreatment were evaluated, relative MDDs for C properties would remain the same, but those for N would decrease by as much as one-half because N has greater pretreatment variability among plots than C. Masses of C and N, which incorporate information about rock volume and either soil mass or bulk density, have relative MDDs as small as one-half of those associated with concentrations. Evaluation of other sites is required to determine if these observations are consistent for forests in general and, thus, applicable to the design of sampling strategies in large-scale experiments.

Accurate soil mapping at a fine scale is needed for site-specific farming, research on solute transport, and many other applications requiring detailed analysis of both the depth and thickness of soil horizons. Soil mapping techniques currently used are too costly to address the spatial variability of soil mapping units. We propose the use of a profile cone penetrometer (PCP) to assist in mapping soil properties at a landscape scale. Data collected with the PCP show clear changes in soil properties with depth, and were confirmed using profile descriptions from soil pits as well as 4.3-cm-diam soil cores. Combining three-dimensional PCP data with soil attribute information will provide a rapid and effective means to digitally update soil surveys and improve the efficiency and cost effectiveness of sampling techniques.

exploitable by plant roots (Scott et al., 1988a). Similarly, the more continuous the macropores are, the more The role of macropores in soil and water processes has motivated freely gases can interchange with the atmosphere. Conmany researchers to describe their sizes and shapes. Several approaches have been developed to characterize macroporosity, such tinuous macropores also have a direct effect on water as the use of tension infiltrometers, breakthrough curve techniques, infiltration and solute transport in soil. image-analysis of sections of soils, and CAT scanning. Until now, According to Sutton (1991), the size of pore openings efforts to describe macropores in quantitative terms have been concen- is more important for plant growth than is the overall trated on their two-dimensional (2-D) geometry. The objective of soil porosity. Although existing pores constrain the penthis study is to nondestructively quantify the three-dimensional (3etration of roots, they provide favorable conditions for D) properties of soil macropores in four large undisturbed soil colroot growth. Several studies have shown that the presumns. The geometry and topology of macropore networks were determined using CAT scanning and 3-D reconstruction techniques. Our ence of continuous macropores in soil significantly beneresults suggest that the numerical density of macropores varies be- fits root growth (Bennie, 1991). One of the most importween 13 421 to 23 562 networks/m 3

Laboratory determination of soil hydraulic characteristics (HC) is performed on cylindrical confined soil cores, while resin-coated small natural aggregates are prevalent for determining the Soil Shrinkage Characteristic Curve (SSCC). Because of the different geometry and volume of clods and cores, incorporation of shrinkage in HC of clay soils could be affected by the use of SSCC determined on natural aggregates or on confined cores. The objectives of this study were (i) to investigate differences between the SSCC obtained on resin-coated natural aggregates (volume V = 20-30 cm 3 ) and on cylindrical confined cores having a volume (650 cm 3 ) close to that of cores used for HC laboratory determination, (ii) to test the performance of different models proposed for analytical interpretation of the SSCC, and (iii) to derive regression equations predicting the SSCC from routinely measured soil physical properties. Using twenty-one Sicilian soils of variable shrink-swell behavior, we found significantly larger specific volume (n ), indicating less shrinkage, in the cylindrical confined cores. The investigation also proved the good fitting of a two-line model to the measured SSCC and the possibility to predict the basic shrinkage line from the clay content. These results suggest that incorporation of shrinkage in HC of clay soils should be based on the SSCC measured or predicted on cores with geometry and dimensions as those used for routine laboratory measurement of HC.

To predict the effects of water or leachate infiltration into unsaturated field soils requires measurements of both the field-saturated hydraulic conductivity (K fs ) and the matric flux potential (Φ m ). Measurement of these soil parameters commonly requires steady-state infiltration rates, which, in the slowly permeable clay liners that are found underlying municipal landfills, can require several weeks or months to be attained. The transient, capillary-dominated, early-time method of determining K fs and Φ m has been found to provide a much faster, yet still reliable estimate of these flow parameters. Here, an analytical solution was utilized for early-time, one-dimensional infiltration under the conditions of an initial constant head followed by a falling-head phase. Six laboratory trials were conducted using precisely compacted clay cores to determine K fs and Φ m during both early-time and steady-state flow domains. The results of these trials showed a favorable comparison between these two techniques, with the early-time constant-head and falling-head methods producing K fs estimates that were 60 and 75% of their respective steady-state values. This suggests that the early-time determination of K fs and Φ m on slowly permeable media has a definite practical advantage over lengthy steady-state analyses. A new, expeditious procedure for determining steady-state K fs values of highly compacted soil cores, involving use of a Mariotte-equipped pressure plate chamber, was also developed and validated.

Soil probes are hammered into the soil to sample uniform, undisturbed soil cores, and can also be used to dig uniform holes for permanent installation of devices to monitor soil moisture and soil solution composition. Extracting soil probes becomes increasingly difficult at increasing depths, particularly when the soil is dry, frozen, or has a cohesive consistency. The construction and operation of a device for extracting soil probes is described. The device can be constructed from inexpensive and universally available galvanized pipe and pipe joints, and a 25 mm width x 2 m length nylon strap. The device can be used to extract other soil sampling equipment that requires upward vertical force. A single person can generate over 1000 kg of upward force on the probe without excessive effort. Use of the probe does not strain the back, which is an important consideration as back injuries are a serious concern with manual soil probe extraction.

In this study, the effects of storage and sieving of a mineral soil (Typic Fragiortbod) on the P chemistry of the soil solution have been quantified. Intact and sieved soil cores were stored for 0, 1, 3, and 8 d at 4°C prior to obtaining solutions, which were analyzed for total dissolved phosphorus (TDP), molybdate-reactive phosphorus (MRP), dissolved organic phosphorus (DOP), and dissolved condensed phosphorus (DCP). Storage of both sieved and intact soil cores influenced the amount and chemical composition of P in solution. Concentrations of TDP, MRP, and DOP displayed a rapid decline with increase in storage time, whereas DCP concentrations, which were initially very low, increased. Concentrations of TDP were significantly (P < 0.05) different between solutions extracted from intact and sieved soils on the same day as sampling and this was accounted for by DOP and MRP, which were significantly (P < 0.05) larger in sieved soil. The proportions of MRP, DOP, and DCP were similar in solutions extracted from intact and sieved soils on the same day, even for Day 0 where a large difference in TDP was apparent. The MRP fraction was consistently larger for intact cores (44%) than sieved (38%) on all days and varied little with length of storage. The DOP component, which dominated initially, decreased with increase in storage time, while DCP increased to become the major fraction after 8 d of storage. This study has highlighted the potential effects of soil storage and sieving on the P concentration and composition of soil solution. The results also have important implications for soil solution sampling and subsequent data interpretation.

To study denitrification, a method was developed for simultaneous measurement of CO2, N2, N2O, and NO production in soil cores. The technique does not interfere with biochemical processes in the soil and is fully automated to enable highly time-resolved measurements for prolonged periods. Soil cores (intact or repacked) are connected to individual gas-tight circuits (up to nine). Gas is continuously circulated through the soil cores to avoid diffusional problems. Each individual circuit can be automatically sampled for chromatographic gas analysis. The N20 production rates can be measured accurately down to 0.04 μg N kg -1 measuring interval -1 (MI). Determination of N 2 , CO 2 , and O 2 production rates is less sensitive: 0.09 mg N kg -1 MI -1 , 0.04 mg C kg -1 MI -1 , and 0.02 mg O kg -1 MI -1 , respectively. Dynamically changing gas production rates are monitored accurately, and total N gas production under anaerobiosis is balanced by NO 3 - disappearance.

An important physical property of agricultural soils is their friability or tendency to fracture into small fragments when tilled. Because friability varies greatly with soil water content, simple tests to determine the point of maximum friability are needed. Previous tests have failed to simulate a set of conditions that are necessary to achieve effective soil fragmentation in tillage. The most important of these conditions is a loading path that imposes a compression/shear ratio conducive to fracture and that also provides continuous energy for crack propagation after fracture initiation. Also, many tests do not directly measure the amount of soil fragmentation. The latter is a primary variable of interest when considering soil reaction to tillage. A new procedure is described that provides the necessary fracture conditions and gives a direct measure of the amount of soil fragmentation under any desired compression/shear ratio. Forces, displacements, and energy expenditures during the soil fragmentation process can be measured. The amount of soil fragmentation determined by the method was found to be very sensitive to compression/shear ratio, particularly in highly plastic soils. However, different soils always ranked the same for all ratios. Friability indices were measured with the technique on a set of soil cores sampled in the field at different soil water matric pressures. The matric pressure at which these indices indicated maximum soil friability agreed well with the point of maximum friability determined by tillage of the same soil

Hydraulic conductivity is the single most important hydraulic parameter for flow and transport-related phenomena in soil, but the results from different measuring methods vary under different field conditions. To evaluate the performance of four in situ saturated hydraulic conductivity (K,) measuring methods, K, measurements were made at four depths (15, 30, 60, and 90 cm) and five locations on a glacial-till soil of Nicollet (fine-loamy, mixed, mesic Aquic Hapludoll)Clarion (fine-loamy, mixed, mesic Typic Hapludoll) association. The four in situ methods were: (i) Guelph permeameter, (ii) velocity permeameter, (iii) disk permeameter, and (iv) double-tube method. The K, was also determined in the laboratory on undisturbed soil cores collected from all the five sites and four depths. The Guelph permeameter method gave the lowest K, values, possibly because of small sample size, whereas the disk permeameter and double-tube methods gave maximum values for KS with minimum variability, possibly because of large sample size. Maximum variability in KS values for soil cores at shallow depths may have occurred because of the presence or absence of open-ended macropores. Estimates of K,, however, are most comparable for the velocity permeameter and the laboratory method using a constant-head permeameter.

Understanding of soil water and solute transport processes requires knowledge of the soil hydraulic properties. A simple evaporation method for the determination of the hydraulic conductivity function and the water retention characteristic was developed and applied to a range of soils with different texture and structure. During evaporation from the top of a 6-cm-high soil core, soil water pressure head at 1.5 and 4.5 cm below the soil surface was measured with tensiometers several times. At the same time, evaporative water loss was determined by weighing the soil column. The procedure for calculation of hydraulic functions was evaluated via numerical simulations. Results from the numerical experiment confirm the underlying theory. A limitation of the evaporation method is the fact that, at water contents near saturation where hydraulic conductivity is high, hydraulic gradients cannot be determined with sufficient accuracy. Other measurement techniques are suggested that can supplement the evaporation method in the wet range.

Conventional batch adsorption-isotherm experiments are conducted under conditions that are generally not representative of field conditions. Our objective was to develop and illustrate a technique that uses continuous, Unsaturated flow through intact soil cores and gives information analogous to standard batch absorption experiments. Soil cores were placed on filter funnels and continuous flow was induced by controlled vacuum. Solution, 10 times the weight of the soil, was applied and recycled until apparent equilibrium concentrations were observed. While the total solution-to-soil ratio was 10:1, the amount in contact with soil at any instant was small. The procedure was replicated on separate cores with different initial solute concentrations to give different final equilibrium concentrations as a function of solute adsorbed per gram of soil. The technique was illustrated by observing adsorption of forest floor dissolved organic C to cores of AB horizon soil, and was compared with conventional batch adsorption data. Isotherms using both methods were linear but the slope of the batch isotherm was somewhat less. This flowthrough technique provides a way to obtain adsorption-isotherm data under more realistic

A lightweight hand-sampler is described for taking uncompacted cores up to 1.5 m long and 49 by 49 mm in section. Cores were successfully taken from tilled gravelly brown earths or had intact unshattered earthworm tunnels even in samples with a high clay conlent. The sampler comprises two halves, two lengths of light, high quality angle-steel (2 m long × 50 mm wide). The two half-lengths are inserted separately into the soil. Tips are acutely pointed and sharpened with the chamfer to the outside. These cutting edges are constricted internally to give relief to the core. Steel caps are welded to the top of each half-length to receive hammer blows. The first half-length to be driven into the soil has a cube of hardwood fixed at the upper end and another is positioned just above the soil surface. These serve as alignment bearings for the second half-length. It is positioned tightly against the first half-length and driven until the steel caps just touch and overlap. The entire sampler and soil core are then withdrawn using a lever, the tip of which keys sequentially into a series of holes bored along the first half-length. A short steel tube attached horizontally across a piece of heavy marine-plywood provides a fulcrum for the lever.

Denitrification may be an important pathway for N loss from desert ecosystems. Few studies, however, have investigated the factors limiting this process in a desert environment. A factorial experiment was conducted to determine the factors controlling denitrification in the northern Chihuahuan Desert. Specifically, we measured the response of denitrification to additions of water, C, N, and C + N. Soil cores were collected from four vegetation zones along an alluvial piedmont. Dry cores were subjected to five treatments: (i) water; (ii) water + NO₃; (iii) water + C; (iv) water + NO₃ + C; and (v) a control (no additions). When denitrification rates were averaged across vegetation zones and patch types (between or under vegetation), the following treatment effects were significantly different: water + NO₃ + C > > > water + NO₃ = water » water + C > control. These results indicate that denitrification at this site is limited by the availability of water. In wet soil cores, C additions immobilized available NO₃ and suppressed denitrification. When water + NO₃ + C was added (C/N = 22), however, denitrification was significantly greater than when water + NO₃ were added. This result indicates that C and N interact to control denitrification in wet desert soils. No evidence for an overall NO₃ limitation in moist cores was found. Surprisingly, denitrification rates in wet cores of nutrient-poor desert soils (≈32.9 ng N cm⁻² h⁻¹) were similar to those measured in the nutrient-rich soils of temperate and tropical forests. When extrapolated to an annual rate, denitrification for this site is 7.22 kg N ha⁻¹ yr⁻¹. Extreme drying-wetting cycles common in desert ecosystems may account for the high rates observed.

The channels created in soil by roots and soil animals (biopores) play an important role in the subsequent movement of water, air, and new roots through irrigated clay soils in southeastern Australia. The maintenance of these biopores is critical for both crop productivity and erosion control. If these biopores are to remain open, they must be able to withstand the vertical stresses associated with vehicle and animal traffic. This study had three aims: (i) to examine the influence on porosity and permeability of naturally occurring channels, (ii) to determine the magnitude of vertical stresses that artificially created channels could withstand, (iii) to test the effect of channel angle to the direction of the stress. Air-filled porosity of soil cores at —10 J/kg was decreased in a linear fashion as the applied stress was increased from 50 to 400 kPa. The reduction in air-filled porosity was relatively independent of the size or the presence of biopores. However, the intrinsic permeability of the soil cores to air was positively correlated with the diameter of the biopores, and the permeability was little affected by applied stresses up to 200 kPa if the initial diameter of the biopores was >3.5 mm. The resistance of the channels to stresses was greatest if the vertical stress was parallel to the axis of the channels. Agricultural practices on these soils should not only encourage formation of biopores, but should also minimize stresses >200 kPa to the subsoil so that these biopores can remain open. View complete article To view this complete article, insert Disc 5 then click button8

An improved method for collecting intact cylindrical soil cores is presented. A soil cylinder was carved in a soil pit using a steel chimney flange to aid in the formation of a straight and uniform core. The cylinder sides were encased in fiberglass-reinforced polyester resin. The ends of the soil core were coated with liquid rubber latex. The latex can be easily removed later with minimal soil disturbance. The technique was used to prepare soil cores of 15-cm diam. And up to 1 m in length. The encasement material is lightweight, strong, and waterproof. Fittings for flow-measuring equipment can be easily attached to the resulting core. Soil cylinders encased by this method are well suited for saturated and unsaturated laboratory investigations.

One-step outflow procedures represent potentially rapid and convenient procedures to obtain soil-water-diffusivity and unsaturated-hydraulic-conductivity data. This study presents modifications of the existing procedures that improve and simplify the estimation of the soil water diffusivity and associated water content. The derivation is similar to that of Passioura's. An equation is derived that describes the soil water diffusivity as a function of average soil water content rather than the water content at the top end of the soil core. The results may be used over a range of water contents similar to Passioura's original procedure. The modifications presented here provide a simpler analysis that is more readily applied to one-step out-flow experiments. Contribution of the Nebraska Agric. Exp. Stn. Journal no. 9174.

Intact soil cores are difficult to extract from reduced-tillage plots using available equipment due to the tendency of cores to separate at planes of weakness. A split-tube sampler was developed to minimize disruptions of the natural soil fabric that frequently occur as the core is removed from the sampling equipment. Construction details are provided that permit this equipment to be attached to standard hydraulically driven soil-coring equipment. Use of this equipment, in conjunction with sample containers that are resistant to laboratory pretreatment procedures and impregnating resins, greatly reduced sample handling and the potential for creating structural anomalies.

Cross sections of a 203-mm diam. soil core taken from a site under Kentucky bluegrass (Poa pratensis L.) were scanned using x-ray computed tomography (CT) to determine if macropores such as cracks, earthworm holes and root channels could be distinguished and characterized. The core was physically sectioned at the same locations as the CT scans to verify the size and location of pores seen on the CT images. Macropores 1 mm and larger could be easily and quickly distinguished by the method. By manipulation of the scanner images, air-filled pores, roots and stones were identified within the soil matrix. A wet bulk density plus an air-filled macroporosity were calculated for scans. Prior to sectioning, a dye solution was ponded on the surface of the core and allowed to infiltrate to assess the continuity of pores through the core.

Laboratory measurements of saturated hydraulic conductivity (K sat ) of soil cores from nine North Carolina Coastal Plain Ultisols demonstrate that K sat generally decreases with increasing depth, while grade of blocky structure generally becomes stronger with depth. The best single predictor (second-order polynomial, r 2 = 0.81, p 30 μm (LP) as calculated from water retention characteristics, followed by the coefficient of linear extensibility measured on soil pastes (COLE paste, r 2 = 0.50) (...)

Seasonal rates of nonsymbiotic nitrogen fixation in the surface soil of oak-hickory forest plots exposed to 30 years of annual and periodic (4-year) prescribed burning were estimated using an in-situ acetylene reduction procedure. Fixation rates were low, averaging 0.1 kg·ha⁻¹·year⁻¹ for all soil cores assayed in April, August, and October 1980 and April 1981 with approximately 11% of samples displaying activity. Burning treatments had no influence on fixation rates or proportion of samples displaying activity. Highest rates were found in autumn with an average rate for active samples of 2.2 kg·ha⁻¹·year⁻¹. Higher autumn fixation rates may be related to higher quantities of high energy foliar leachates which could induce a significant seasonal peak on sites demonstrating higher rates of fixation.

(…) Our objective was to characterize the distribution of urease activity in the surface of two Oklahoma soils (Grant silt loam, fine, silty, mixed, thermic Udic Argiustoll and Norge clay loam, fine, silty, mixed, thermic Udic Paleustoll) which had been in conventional, reduced, and no-till wheat (Triticum aestivum L.) residue management for four consecutive years. Soil cores 60-mm deep were taken from residue management plots that had been previously fertilized with 168 kg urea-N ha −1 yr −1 and separated into 10-mm increments. (…)