Robert Horton

Assessing potential of biochar for increasing water-holding capacity of sandy soils

Increasing the water‐holding capacity of sandy soils will help improve efficiency of water use in agricultural production, and may be critical for providing enough energy and food for an increasing global population. We hypothesized that addition of biochar will increase the water‐holding capacity of a sandy loam soil, and that the depth of biochar incorporation will influence the rate of biochar surface oxidation in the amended soils. Hardwood fast pyrolysis biochar was mixed with soil (0%, 3%, and 6% w/w) and placed into columns in either the bottom 11.4 cm or the top 11.4 cm to simulate deep banding in rows (DBR) and uniform topsoil mixing (UTM) applications, respectively. Four sets of 18 columns were incubated at 30 °C and 80% RH. Every 7 days, 150 mL of 0.001 M calcium chloride solution was added to the columns to produce leaching. Sets of columns were harvested after 1, 15, 29, and 91 days. Addition of biochar increased the gravity‐drained water content 23% relative to the control. Bulk density of the control soils increased with incubation time (from 1.41 to 1.45 g cm−3), whereas bulk density of biochar‐treated soils was up to 9% less than the control and remained constant throughout the incubation period. Biochar did not affect the CEC of the soil. The results suggest that biochar added to sandy loam soil increases water‐holding capacity and might increase water available for crop use.

Spatial analysis of hydraulic conductivity measured using disc infiltrometers

Spatial variability of surface hydraulic properties and the extrinsic (e.g., traffic, cropping, etc.) and intrinsic (e.g., soil type, pore size distribution, etc.) factors associated with these properties are important for infiltration and runoff processes in agricultural fields. Disc infiltrometers measured infiltration at 296 sites arranged on two parallel transects. To examine and differentiate the factors contributing to spatial structure under different field conditions these measurements were made in the corn rows, no-track interrows, and wheel track interrows of the field using four different soil water tensions * (0, 30, 60, and 150 mm). Unsaturated hydraulic conductivity (K) and saturated hydraulic conductivity (K,) were maximum in the corn rows and minimum in wheel track inter-rows, with no-track interrows intermediate. Exponents (CZ parameters) of K, and K relationships (K = K, exp -aV) for corn rows and no-track interrows were not significantly different from each other but were significantly different from Q for the wheel track interrows a t P = 0.01 level. Spatial variability o f K and K, values showed some pseudoproportional effect in nugget variance for all three field conditions. No-track interrows clearly showed an inverse trend for semivariogram o f K with changing tension (q) values, whereas differences were found for corn rows and wheel traffic interrows. The spatial structure of (Y for all three field conditions were mostly white noise. Under corn rows, in addition to random variation, a small five-row periodic variation at th e P = 0.20 level, matching the five-row traffic configuration, was discovered. The spatial structure of a was influenced by soil type for the no-track interrows. Spatial structure was absent in wheel track interrows, indicating the destruction of pore structure due to compaction.

A model for regional optimal allocation of irrigation water resources under deficit irrigation and its applications

Abstract It is important to promote efficient use of water through better management of water resources, for social and economical sustainability in arid and semi-arid areas, under the conditions of severe water shortage. Based on the developments in deficit irrigation research, a recurrence control model for regional optimal allocation of irrigation water resources, aiming at overall maximum efficiency, is presented, with decomposition-harmonization principles of large systems. The model consists of three levels (layers). The first level involves dynamic programming (DP) for optimization of crop irrigation scheduling. The second level deals with optimal allocation of water resources among various crops. The last level concerns optimal allocation of water resources among different sub-regions. As a test, this model was applied to the combined optimal allocation of multiple water resources (surface, ground and in-take from the Weihe river) of Yangling, a semi-arid region on the Loess Plateau, China. Exemplary computation showed that not only are the results rational, but the method can also effectively overcome possible “dimensional obstacles” in dynamic programming of multiple dimensions. Furthermore, each sub-model is relatively independent by using various optimization methods. The model represents a new approach for improving irrigation efficiency, implementing water-saving irrigation, and solving the problem of water shortage in the region studied. The model can be extended in arid and semi-arid areas for better water management.

Crop residue effects on surface radiation and energy balance — review

SummaryCrop residues alter the surface properties of soils. Both shortwave albedo and longwave emissivity are affected. These are linked to an effect of residue on surface evaporation and water content. Water content influences soil physical properties and surface energy partitioning. In summary, crop residue acts to soil as clothing acts to skin. Compared to bare soil, crop residues can reduce extremes of heat and mass fluxes at the soil surface. Managing crop residues can result in more favorable agronomic soil conditions. This paper reviews research results of the quantity, quality, architecture, and surface distribution of crop residues on soil surface radiation and energy balances, soil water content, and soil temperature.

Sensible Heat Observations Reveal Soil-Water Evaporation Dynamics

Soil-water evaporation is important at scales ranging from microbial ecology to large-scale climate. Yet routine measurements are unable to capture rapidly shifting near-surface soil heat and water processes involved in soil-water evaporation. The objective of this study was to determine the depth and location of the evaporation zone within soil. Three-needle heat-pulse sensors were used to monitor soil heat capacity, thermal conductivity, and temperature below a bare soil surface in central Iowa during natural wetting/ drying cycles. Soil heat flux and changes in heat storage were calculated from these data to obtain a balance of sensible heat components. The residual from this balance, attributed to latent heat from water vaporization, provides an estimate of in situ soil-water evaporation. As the soil dried following rainfall, results show divergence in the soil sensible heat flux with depth. Divergence in the heat flux indicates the location of a heat sink associated with soil-water evaporation. Evaporation estimates from the sensible heat balance provide depth and time patterns consistent with observed soil-water depletion patterns. Immediately after rainfall, evaporation occurred near the soil surface. Within 6 days after rainfall, the evaporation zone proceeded 13 mm into the soil profile. Evaporation rates at the 3-mm depth reached peak values 0.25 mm h 1 . Evaporation occurred simultaneously at multiple measured depth increments, but with time lag between peak evaporation rates for depths deeper below the soil surface. Implementation of finescale measurement techniques for the soil sensible heat balance provides a new opportunity to improve understanding of soil-water evaporation.

Effect of solute application method on preferential transport of solutes in soil.

Abstract In soils containing macropores, both water and solutes can move preferentially, bypassing much of the soil matrix. The objective of this study was to examine the effect of solute application method on preferential solute transport in soil materials containing differing amounts of macroporosity. Transport experiments were conducted with three undisturbed soil columns (18 cm diameter, 33–35 cm long) that contained extensive networks of macropores. The macroporosity of the soil columns was first characterized by measuring the saturated hydraulic conductivity and drained porosity, and by conducting saturated, steady-fluid-flow miscible displacement experiments in which a 0.05 N CaCl 2 solution displaced a 0.01 N CaSO 4 solution. These measurements indicated extensive macroporosity and a range in macroporosity among the three columns. Two transport experiments were then conducted with each of the three columns. The initial moisture condition of the soil in both experiments was a gravity-drained profile. In the first transport experiment, a 200-ml pulse of 0.05 N CaCl 2 solution was ponded on the surface of each of the three columns and allowed to infiltrate. A solution of 0.01 N CaSO 4 was then immediately ponded on the soil surface. In the second transport experiment, after leaching the excess chloride remaining in the columns from the first experiment, 40 ml of a fivefold more concentrated CaCl 2 solution (0.25 N CaCl 2 ) was uniformly dripped on the soil surface of the same three columns (no ponding), allowed to redistribute for 15 min, and then leached with a steady flow of 0.01 N CaSO 4 solution. The resulting breakthrough curves for the two solute application methods were distinctly different, indicating a sensitivity of the transport process to solute application method, or solute boundary condition. In addition, the curves showed that the sensitivity to application method decreased with decreasing soil macroporosity. An analysis of the breakthrough curves is presented to explain the experimental results obtained. Two factors are important in understanding the results. The initial water status of soil macropores, saturated or drained, is important in determining the degree of dispersion that takes place in individual macropores and thus in the soil as a whole. Second, mass transfer of solute into the soil matrix as a consequence of both diffusion and solution absorption from macropores is important in causing dispersion of the solute. The implications of these findings for conducting solute transport experiments are discussed.

Soil Water Evaporation Suppression By Sand Mulches

This paper reports experiments performed to study sand as a soil mulch. The objective was to determine the comparative effectiveness of 0-, 2-, and 6-cm-thick covering sand layers in suppressing evaporation from columns of soil. Measurements were made by using potential evaporation rates of 1.1 and 0.55 cm/d. In addition to evaporation, soil water distribution with depth was measured for the different sand-cover treatments. Five treatments were studied: check (no sand mulch), 6 cm of coarse sand (C6), 6 cm of fine sand (F6), 2 cm of coarse sand (C2), and 2 cm of fine sand (F2). After 35 d of experiment, the cumulative evaporations for the check, C6, F6, C2, and F2 treatments were measured as 6.79, 1.50, 1.55, 3.76, and 4.62 cm of water, respectively, at a potential evaporation of 1.1 cm/d and, for the potential evaporation of 0.55 cm/d, was correspondingly 6.68, 0.95, 1.21, 2.71, and 4.28 cm. These sets of numbers show that there was marked evaporation reduction for the sand mulches with respect to bare soil (check). The 6-cm sand mulches were the most effective evaporation suppressors. For equal mulch thickness, coarse sand was only slightly more effective than fine sand. Results from soil water distributions with depth for the various treatments also indicated that the sand mulches were effective in conserving soil water against evaporation losses. The mulches were effective in this order: C6 > F6 > C2 > F2.

Infiltration and macroporosity under a row crop agricultural field in a glacial till soil

Previous field-scale infiltration studies showed difference in the magnitude and the trend of spatial variation of infiltration rates under different soil water tensions. In different studies the differences in infiltration rates are caused by management practices, relative field positions, and soil and topographic setups, hence warranting further site-specific infiltration studies. In this study, variability in infiltration rate (I ψ ) at four soil water pressure heads, ψ, were investigated in a no-tillage agricultural field under corn rows, nontrafficked interrows, and trafficked interrows in a central Iowan glacial till soil. Automated disc infiltrometers were used to measure infiltration at 0-, 30-, 60-, and 150-mm tensions at 296 sites arranged on two parallel transects perpendicular to corn rows. Mean infiltration rates at different soil water tensions were found maximum under corn row, minimum for trafficked interrow, and intermediate for nontrafficked interrow positions. Maximum variability was found for larger pores (those conducing water at 0-mm tension) under all three surface positions (corn row, CV = 85% ; trafficked interrow, CV = 95% ; nontrafficked interrow, CV = 124%). Infiltration at saturation (0-mm tension) showed a different scale of heterogeneity than infiltration at other (30-, 60-, and 150-mm) tensions, and approximately 90% of the saturated flux moves through macropores (>1-mm diameter) that constitute less than 3% of the total surface area at three field positions. Spatial analysis of I ψ indicated a larger proportion of random variations under all three field positions in the glacial till soil. In addition to the large random noise, a small spatial structure of 7.6 to 11.4-m range was found for I ψ (at all four tensions) under corn row position, and only for I 150 under (nontrafficked and trafficked) interrow positions.

Comparison of techniques for extracting soil thermal properties from dual-probe heat-pulse data

Temperature-by-time data obtained using a dual-probe heat-pulse device can be analyzed using two different approaches to determine the soil thermal diffusivity, volumetric heat capacity and thermal conductivity. One approach, referred to as the single-point method, is based on accurate identification of the peak in the temperature-by-time measurements. The second approach involves a nonlinear model fit of the appropriate temperature model to the temperature-by-time data. In this paper, we analyze dual-probe heat-pulse data and show how the soil thermal properties determined using these two approaches compare. The single-point method is easy to apply, but results are sensitive to choice of the peak value, which can be difficult to identify if the data are sparse and contain noise. The nonlinear model fit (Marquardt method) copes better with broad, flat peaks and sparse, noisy data. Soil thermal properties obtained using either approach should be checked by comparing the fitted model with the measured temperature-by-time data. By doing so, one can quickly determine the validity of the results.

A Robust-Resistant Approach to Interpret Spatial Behavior of Saturated Hydraulic Conductivity of a Glacial Till Soil Under No-Tillage System

A central Iowa glacial till soil under no-tillage condition was studied for its spatial behavior of saturated hydraulic conductivity (K) at the surface soil layers. Hydraulic conductivity measurements both in situ and in the laboratory were made at two depths of 15 and 30 cm at regular intervals of 4.6 m on two perpendicular transects crossing each other at the center of the field. Simplified split-window median polishing in conjunction with a robust semivariogram estimator were used to examine the spatial structure of the glacial till material. Results of this study indicated a nested structure of K at 30 cm depth. Soil clustering at the experimental site at intervals of 20 m, in addition to the soil microheterogeneity, contributed to variation in K, with an overall range of spatial dependence of K up to 60 m. Medians of split windows of 23 m width were found to be the “solo representatives” or “summary points” of the soil clusters contributing to spatial structure. In situ and laboratory measurements for K showed consistency in their trends even though some parametric variations were observed. K values observed near the soil surface at a depth of 15 cm were dominated by white noise and directional trends.