Robert C. Schwartz

Soil hydraulic properties of cropland compared with reestablished and native grassland

Conversion of cropland to perennial grasses will, over time, produce changes in soil hydraulic properties. The objective of this study was to characterize and compare hydraulic properties of fine-textured soils on adjacent native grassland, recently tilled cropland, and reestablished grassland in the Conservation Reserve Program (CRP) at three locations in the Southern Great Plains. A tension infiltrometer was used to measure unconfined, unsaturated infiltration over a range of supply pressure heads (nominally, h=−150, −100, −50, and −5 mm H2O) at the soil surface. Intact soil cores were sampled within the Ap and Bt horizons to determine bulk density and water desorption curves, θ(h), at potentials ranging from −0.15 to −100 kPa. Unsaturated hydraulic conductivity K(h) over the range in supply pressure heads was estimated using Woodings equation for steady-state flow from a disc source. The van Genuchten water retention model was fitted to θ(h) data to estimate parameter values. Soils in CRP had greater surface bulk densities than their grassland and cropland counterparts. The shape of the soil water retention curve for grassland and CRP land were similar, suggesting that converted croplands had fully reconsolidated. Mean near-saturated hydraulic conductivities of cropland at h=−5 mm were not significantly different from grassland. However, at −150 mm supply pressure head, cropped soils had a mean unsaturated conductivity 2.3 and 4.1 times greater than CRP land and grassland, respectively. Sites in CRP had the lowest (P<0.05) near-saturated hydraulic conductivities (h=−5 mm), which suggest that after 10 years, grasses had not fully ameliorated changes in pore structure caused by tillage. Comparison of unsaturated conductivities for grassland and CRP land suggest that long-term structural development on native grasslands was principally confined to effective pore radii greater than 300 μm. Land use practices had a greater effect on water movement than did soil series, indicating that the modifying effects of tillage, reconsolidation, and pore structure evolution on hydraulic properties are important processes governing water movement in these fine-textured soils.

Assessing Indices for Predicting Potential Nitrogen Mineralization in Soils under Different Management Systems

A reliable laboratory index ofN availability would be useful for making N recommendations, but no single approach has received broad acceptance across a wide range of soils. We compared several indices over a range of soil conditions to test the possibility of combining indices for predicting potentially mineralizable N (N 0 ). Soils (0-5 and 5-15 cm) from nine tillage studies across the southern USA were used in the evaluations. Long-term incubation data were fit to a first-order exponential equation to determine N 0 , k (mineralization rate), and N 0 * (N 0 estimated with a fixed k equal to 0.054 wk -1 ). Out of 13 indices, five [total C (TC), total N (TN), N mineralized by hot KCI (Hot_N), anaerobic N (Ana_N), and N mineralized in 24 d (Nmin_24)] were strongly correlated to N 0 (r > 0.85) and had linear regressions with r 2 > 0.60. None of the indices were good predictors ofk. Correlations between indices and N 0 * improved compared with N 0 , ranging from r = 0.90 to 0.95. Total N and Hush of CO 2 determined after 3 d (Fl_CO2) produced the best multiple regression for predicting N 0 (R 2 = 0.85) while the best combination for predicting N 0 * (R 2 = 0.94) included TN, Fl_CO2 Cold_N, and NaOH_N. Combining indices appears promising for predicting potentially mineralizable N, and because TN and Fl_CO2 are rapid and simple, this approach could be easily adopted by soil testing laboratories.

Mineralizable phosphorus, nitrogen, and carbon relationships in dairy manure at various carbon-to-phosphorus ratios

Phosphorus (P) in animal manure can be an important nutrient for crops or an environmental contaminant if in excess. Organic P in dairy manure may add to the environmentally bioactive P pools upon mineralization. A 353d incubation study of manures containing C:P between 83 and 130:1 was conducted to determine linkages between C and P transformations and the effects of C:P on the immobilization-mineralization of manure P. As C:P widened from 83:1 to 130:1, P mineralization increased and phosphate accumulated at rates between 0.013 and 0.021mgkg(-1) d(-1). Water-extractable C was positively correlated with N:P, particularly at narrow C:P (P<0.001). Absence of a negative feedback by phosphate suggested that P mineralization occurred with degradation of organic P-containing C substrates and appeared incidental to microbial P needs. Carbon content in manure may be managed to lower risks of elevated soluble P and C losses under non-limiting N conditions.

Conjunctive Use of Tension Infiltrometry and Time-Domain Reflectometry for Inverse Estimation of Soil Hydraulic Properties

Moreover, the presence of a wetting front can influence the spatial sensitivity of TDR probes and bias the esti- Infiltration from a tension disc infiltrometer can be applied conjointly mates of optimized parameters (Ferreet al., 2002). with time-domain reflectometry (TDR) measurements of soil water Several studies have measured transient soil water content to improve estimates of field hydraulic parameters. However, interpretation of TDR-measured water contents for use in inverse contents using TDR during three-dimensional axial- optimizations may be problematic when rods are partially within the symmetric flow. Kachanoski et al. (1990) used both wetted zone. The objective of this study was to assess if TDR-mea- curved- and straight-wave guides to monitor the wetting sured soil water contents in addition to cumulative infiltration could front progression and estimate cumulative infiltration improve parameter estimability for the inverse optimization problem. from a point source. Wang et al. (1998) used horizontally Infiltration experiments were conducted with a 0.58-m-diam. cylinder buried probes to measure soil water contents during packed with a loamy sand. Three trifilar TDR probes were inserted infiltration and estimate soil hydraulic parameters based diagonally into the soil to measure transient water contents during on quasianalytical solutions. Because probes were bur- infiltration. Inverse optimizations utilized cumulative infiltration, wa- ied, this precludes the use of these methodologies in ter contents from diagonally placed TDR probes, and a branch of the field applications where an undisturbed soil condition wetting water characteristic (h ) from extracted soil cores. Measured is desired. Schwartz and Evett (2002) used TDR in con- (h ) at one or more pressure heads was required in optimizations to junction with cumulative infiltration to inversely opti- provide a satisfactory description of the water characteristic in the mize hydraulic parameters for a fine-textured soil in the dry region. Optimizations for three infiltration experiments yielded similar parameter estimates with overlapping 95% confidence inter- field. Probes were inserted diagonally at 45 from the vals. The use of diagonal TDR-measured water contents improved soil surface to minimize soil disturbance. They obtained the predicted redistribution of soil water and decreased covariances good agreement between measured and predicted water between parameter pairs that led to better parameter estimability. contents at late times when the wetting front had ex- Optimized simulations predicted water contents in a three-dimen- tended deeper into the profile. However, measured TDR sional region within 0.03 m 3 m 3 of values measured by buried hori- water contents were significantly underestimated by the zontal TDR probes. Parameter estimates were relatively insensitive optimized solution at early times. They attributed the to changes in the assumed averaging depth transverse to TDR rods. poor estimation of TDR water contents at early times For the diagonally placed probes, the dominant gradients in water to physical nonequilibrium processes during transient content were in directions that minimized errors associated with flow near the margins of the wetting front. The assump- assuming a uniform weighting of water content within the TDR sam- tion of uniform weighting of water content (and the pling volume. dielectric constant) across the three-dimensional sam- pling volume of the trifilar probes in the presence of the wetting front (Knight et al., 1994; Ferreet al., 1998)

Effects of Manure Management on Phosphorus Biotransformations and Losses During Animal Production

Regional intensification of animal production has drastically changed the natural landscape and the ecological balance on millions of hectares of agricultural lands. Animal manure, which is a substantial source of environmental phosphorus (P), has accumulated and now often exceeds the nutrient utilization capacity of the plant production sector within watersheds with a high density of livestock feeding operations. How manure is handled, collected, stored, and disposed of determines the dominant types of transformations and bioavailability of the various P forms excreted in the manure (given the animal species and its diet). Livestock and poultry utilize feed P inefficiently, and unavoidable P excretion occurs even at recommended feeding levels. P excretion increases over and above that level, in proportion to the rise in dietary P intake. Although the use of dietary phosphohydrolases improves feed digestibility and recovery of feed organic P forms, the practice also increases the proportion of water-extractable P excreted in feces. The latter phenomenon may appear advantageous during the reuse of manure as a biofertilizer in crop production, but high levels of water-soluble P also present undue risk of dispersal and contamination of the environment. Linkages between production system characteristics and animal manures were examined to gain an improved understanding of management-induced transformations of manure P forms on the farm. Co-transformations of manure C and N influence the turnover of P and accumulation of water-extractable P in stored manures and amended soils. Mineralization of organic P forms occurs in manure in spite of elevated initial water-extractable P levels. A wide C:P molar ratio enhances manure organic P mineralization, given the need of manure-borne organisms to assimilate C substrates and obtain metabolic energy. P losses from animal production systems occur primarily via P transport in rainwater or snowmelt, and in water percolating through soil. The identification of likely loss pathways on grazinglands, animal production facilities, manure storage structures, and treated fields contributes to the development of comprehensive mitigation strategies to ultimately reduce the environmental footprint of animal agriculture.

Design of Access-Tube TDR Sensor for Soil Water Content: Testing

Soil water measurement is important in water management for irrigation and hydrologic sciences. The purpose of this paper is to describe the design of a cylindrical access-tube mounted waveguide for use in time-domain reflectometry (TDR) for in-situ soil water content sensing. In order to optimize the design with respect to sampling volume and losses, we derived the electromagnetic fields produced by a TDR sensor with this geometry. Using this analytical derivation, the effects on sampling area, waveform shape, and losses while varying design and soil water content were examined. It was found that when the soil and tube substrate have identical dielectrics, then sampling area has a local extremum. Tube radius has the largest impact of any geometrical parameter on sampling area with increases in radius causing increases in sampling area. Increasing electrode separation angle increases the sampling area slightly. The effects on TDR waveform are greatest for soil water content, tube dielectric, and tube radius: where increasing any of these increase delay and dispersion.Soil water measurement is important in water management for irrigation and in hydrologic sciences. The purpose of this paper is to develop and test the design of a cylindrical access-tube mounted waveguide for use in time-domain reflectometry (TDR) for in situ soil water content sensing. Several prototypes with varying geometrical parameters were constructed. The sensors were compared by evaluating the characteristics of reflected waveforms from a (200-ps) step pulse in different media, including air, triethylene glycol, deionized water, and over a range of water contents in sand and a clay loam soil. Sensors with greater separation between electrodes, achieved by means different tube diameters or the separation angles, tend to have greater field penetration in both sand and clay. In addition, sensors with the shortest electrode separation show greater sensitivity to soil electrical conductivity. Together, these trends demonstrate that the propagating electromagnetic fields above 0 Hz do not take the transverse electromagnetic form commonly assumed in the analysis of TDR probes.

Design and Field Tests of an Access-Tube Soil Water Sensor

Accurate soil profile water content monitoring at multiple depths has heretofore been possible only using the neutron probe (NP) but with great effort and at infrequent time intervals. Despite the existence of several frequency domain electromagnetic (EM) sensor systems for profile water content measurements, accuracy and spatial representativeness has been precluded by fundamental problems related to soil conductivity and structure effects on the volume explored by the EM field of these sensors, which causes nonrealistic spatial variation in reported profile water contents. Time domain reflectometry (TDR) methods have the distinct advantage of employing a moving EM field that must pass through and be affected by both the drier and wetter soil structures in which the TDR electrodes are embedded. This article describes a profiling water content system based on TDR. The design, laboratory calibration, and field testing is detailed. The sensor system provides unattended, real-time, data acquisition. And, it can be installed without disturbing the soil around the access tube on the outside of which the TDR electrodes are embedded. The correlation coefficient between neutron probe and TDR measured soil water content was 0.94 with a slope of 1.40 and an intercept of -0.08 m3m-3. Bias between TDR and NP readings (TDR-NP) was positive at all depths below 10 cm, ranging from 0.021 and 0.096 m3 m-3. Uncertainty in data of ~0.012 m3 m-3 for soil water, and uncertainty in bulk electrical conductivity of 0.030 S m-1 (both partly due to unreliable mechanical electrical connections) shows that improvements must be made before such a system is acceptable for widespread use.

Fringe capacitance correction for a coaxial soil cell.

Accurate measurement of moisture content is a prime requirement in hydrological, geophysical and biogeochemical research as well as for material characterization and process control. Within these areas, accurate measurements of the surface area and bound water content is becoming increasingly important for providing answers to many fundamental questions ranging from characterization of cotton fiber maturity, to accurate characterization of soil water content in soil water conservation research to bio-plant water utilization to chemical reactions and diffusions of ionic species across membranes in cells as well as in the dense suspensions that occur in surface films. One promising technique to address the increasing demands for higher accuracy water content measurements is utilization of electrical permittivity characterization of materials. This technique has enjoyed a strong following in the soil-science and geological community through measurements of apparent permittivity via time-domain-reflectometry (TDR) as well in many process control applications. Recent research however, is indicating a need to increase the accuracy beyond that available from traditional TDR. The most logical pathway then becomes a transition from TDR based measurements to network analyzer measurements of absolute permittivity that will remove the adverse effects that high surface area soils and conductivity impart onto the measurements of apparent permittivity in traditional TDR applications. This research examines an observed experimental error for the coaxial probe, from which the modern TDR probe originated, which is hypothesized to be due to fringe capacitance. The research provides an experimental and theoretical basis for the cause of the error and provides a technique by which to correct the system to remove this source of error. To test this theory, a Poisson model of a coaxial cell was formulated to calculate the effective theoretical extra length caused by the fringe capacitance which is then used to correct the experimental results such that experimental measurements utilizing differing coaxial cell diameters and probe lengths, upon correction with the Poisson model derived correction factor, all produce the same results thereby lending support and for an augmented measurement technique for measurement of absolute permittivity.

In-Soil and Down-Hole Soil Water Sensors: Characteristics for Irrigation Management

The past use of soil water sensors for irrigation management was variously hampered by high cost, onerous regulations in the case of the neutron probe (NP), difficulty of installation or maintenance, and poor accuracy. Although many sensors are now available, questions of their utility still abound. This study examined down-hole (access tube type) and insertion or burial type sensors for their ability to deliver volumetric water content data accurately enough for effective irrigation scheduling by the management allowed depletion (MAD) method. Down-hole sensors were compared with data from gravimetric sampling and field-calibrated neutron probe measurements. Insertion and burial type sensors were compared with a time domain reflectometry (TDR) system that was calibrated specifically for the soil; and temperature and bulk electrical conductivity measurements were also made to help elucidate sensor problems. The capacitance type down-hole sensors were inaccurate using factory calibrations, and soil-specific calibrations were not useful in a Central Valley California soil and a Great Plains soil. In both soils, these sensors exhibited spatial variability that did not exist at the scale of gravimetric and NP measurements or of irrigation management, resulting in errors too large for the MAD approach. Except for one, the point sensors that could be buried or inserted into the soil gave water contents larger than saturation using factory calibrations. The exception was also the least temperature sensitive, the others exhibiting daily water content variations due to temperature of >= 0.05 m3 m-3 water content. Errors were related to bulk electrical conductivity of this non-saline but clayey soil.

Relationship between LAI and Landsat TM Spectral Vegetation Indices in the Texas Panhandle

Abstract: Mapping and monitoring leaf area index (LAI) is important for spatially distributed modeling of surface energy balance, evapotranspiration and vegetation productivity. Remote sensing can facilitate the rapid collection of LAI information on individual fields over large areas in a time and cost-effective manner. However, there are no LAI models available for the major summer crops in the Texas Panhandle. The main objective of this study was to develop statistical relationship between LAI and Landsat Thematic Mapper (TM) based spectral vegetation indices (SVI) for major crops in the Texas Panhandle. LAI was measured in 48 randomly selected commercial fields in Moore and Ochiltree counties. Data collection was made to coincide with Landsat 5 satellite overpasses on the study area. Numerous derivations of SVIs were examined for estimating LAI using ordinary least square regression models such as linear, quadratic, power and exponential models. The R2 values for the selected models varied from 0.76 to 0.84 with the power function model based on the normalized difference between TM bands 4 and 3 (NDVI) producing the best results. Analysis of the results indicated that the SVI-LAI models based on the simple ratio i.e. the ratio of TM bands 4 and 3, and NDVI are most sensitive to LAI.