J. N. Shaw

SOIL CARBON RELATIONSHIPS WITH TERRAIN ATTRIBUTES, ELECTRICAL CONDUCTIVITY, AND A SOIL SURVEY IN A COASTAL PLAIN LANDSCAPE

Soil organic carbon (SOC) estimation at the landscape level is critical for assessing impacts of management practices on C sequestration and soil quality. We determined relationships between SOC, terrain attributes, field scale soil electrical conductivity (EC), soil texture and soil survey map units in a 9 ha coastal plain field (Aquic and Typic Paleudults) historically managed by conventional means. The site was composite sampled for SOC (0-30 cm) within 18.3 × 8.5-m grids (n = 496), and two data sets were created from the original data. Ordinary kriging, co-kriging, regression kriging and multiple regression were used to develop SOC surfaces that were validated with an independent data set (n = 24) using the mean square error (MSE). The SOC was relatively low (26.13 Mg ha−1) and only moderately variable (CV = 21%), and showed high spatial dependence. Interpolation techniques produced similar SOC maps but the best predictor was ordinary kriging (MSE = 9.11 Mg2 ha−2) while regression was the worst (MSE = 20.65 Mg2 ha−2). Factor analysis indicated that the first three factors explained 57% of field variability; compound topographic index (CTI), slope, EC and soil textural fractions dominated these components. Elevation, slope, CTI, silt content and EC explained up to 50% of the SOC variability (P ≤ 0.01) suggesting that topography and historical erosion played a significant role in SOC distribution. Field subdivision into soil map units or k-mean clusters similarly decreased SOC variance (about 30%). The study suggests that terrain attributes and EC surveys can be used to differentiate zones of variable SOC content, which may be used as bench marks to evaluate field-level impact of management practices on C sequestration.

EVALUATING THE SENSITIVITY OF AN UNMANNED THERMAL INFRARED AERIAL SYSTEM TO DETECT WATER STRESS IN A COTTON CANOPY

Airborne thermal infrared (TIR) imagery is a promising and innovative tool for assessing canopy response to a range of stressors. However, the expense associated with acquiring imagery for agricultural management is often cost-prohibitive. The objective of this study was to evaluate a less expensive system, an unmanned airvehicle (UAV) equipped with a TIR sensor, for detecting cotton (Gossypium hirsutum L.) response to irrigation and crop residue management. The experimental site was located on a 6.1 ha field in the Tennessee Valley Research and Extension Center located in Belle Mina, Alabama, where landscapes are gently rolling and soils are highly weathered Rhodic Paleudults. Treatments consisted of irrigation (dryland or subsurface drip irrigation) and crop residue cover (no cover or winter wheat (Triticum aestivum L.)). TIR (7 to 14 µm) imagery was acquired on 18 July 2006 at an altitude of 90 m and spatial resolution of 0.5 m. Coincident with image acquisition, ground truth data consisting of soil water content (0-25 cm), stomatal conductance, and canopy cover were measured within a 1 m radius of each sample location. All sample locations were georeferenced using a real-time kinematic (RTK) GPS survey unit. Analysis of sample locations acquired in multiple flight lines was used to assess the stability and repeatability of the UAV system during an acquisition. Compared to field measurements of stomatal conductance with CVs ranging from 2% to 75%, variability in TIR emittance (CV < 40%) was within the observed tolerance of ground truth measurements of stomatal conductance. Significant differences in canopy cover and stomatal conductance across irrigation treatments allowed testing of the sensitivity of the UAV system. A negative correlation was observed between TIR emittance and stomatal conductance (r = -0.48) and canopy closure (r = -0.44), indicating increasing canopy stress as stomatal conductance and canopy closure decreased. TIR emittance exhibited greater sensitivity to canopy response compared to ground truth measurements, differentiating between irrigation and crop residue cover treatments. TIR imagery acquired with a low-altitude UAV can be used as a tool to manage within-season canopy stress.

Morphologic and hydraulic properties of soils with water restrictive horizons in the Georgia Coastal Plain

Hydraulic and morphological properties of soils from a 0.36-ha site in the Georgia Coastal Plain were evaluated. Objectives included characterizing the morphological and hydraulic properties ofmajor horizons in soils with plinthite, determining the extent of preferential flow, and relating flow/transport parameters derived from breakthrough curve analyses to morphological properties. These soils have developed from Miocene aged sediments and are classified in fine-loamy, siliceous, thermic families of Plinthaquic, Aquic, Arenic Plinthic, Plinthic, and Typic Kandiudults. Morphological evidence indicates that BC horizons are restrictive to vertical percolation of drainage water. Methylene blue dye staining, K sat and breakthrough curves (analyzed by two region/MIM using CXTFIT) were measured on 15-cm-diameter undisturbed cores to determine the effects ofargillic horizons, argillic horizons with plinthite, and subjacent BC horizons on hydraulic properties of the soils. K sat for seven sampled pedons averaged 1.6 X 10 -2 , 1.1. X 10 -2 , and 3.8 X 10 -3 mm s -l for Bt, Btv, and BC horizons, respectively. Dye staining and model output (MIM) indicate a greater degree of preferential flow with depth and subsequent less mobile area contributing to water flow. Analyses of the flowpaths indicate water is translocated in regions with relatively higher porosity that also contain a higher proportion of coarser pores. Micrographs indicate that flowpaths are associated with bio-pores and areas ofbetter aggregation in the Bt horizons and structural voids in the BC horizons. BC horizons in these soils are less permeable because ofincreased clay content, differences in pore characteristics, and less cross-sectional area contributing to flow.

IRON AND ALUMINUM OXIDE CHARACTERIZATION FOR HIGHLY-WEATHERED ALABAMA ULTISOLS

Characterization of oxide and oxyhydroxide iron (Fe) and aluminum (Al) forms in highly-weathered Ultisols of the southeastern United States is necessary to develop further understanding of colloidal-facilitated transport of pollutants, sorption of contaminants, erosion, and soil genesis. The objective of this study was to examine Fe and Al oxides from several highly-weathered Ultisols and evaluate their relationships with particle size fractions and other soil chemical and physical properties. Samples contained in the argillic horizon of 13 highly-weathered pedons were examined. These pedons either contained kandic horizons, were in a kaolinitic mineralogical family, or were in a siliceous mineralogical family with a subactive cation exchange capacity activity (CEC) class. Standard characterization analyses were performed on all pedons. Samples were fractionated into a coarse (2 to 2000 μm) and fine (< 2 μm) fraction, and ammonium oxalate (Feo and Alo) and dithionite-citrate-bicarbonate (Fed and Ald) extractable Fe and Al were quantified in each. Selective dissolution treatments were conducted on < 2 μm fractions prior to mineralogical analyses. A modified differential XRD (DXRD) was used for further Fe oxide characterization. Pretreatments worked well for concentrating crystalline oxide forms. All Fe and Al extracted forms were higher in the fine than the coarse fraction. The Fe oxides were highly crystalline, but a significantly higher ratio of Feo/Fed was found in the fine as compared to the coarse fraction (0.041 to 0.020, respectively). Both goethite and gibbsite quantities were negatively correlated with effective cation exchange capacity (ECEC) (r = −0.54 and −0.80, respectively). The Feo/ Fed ratio in the < 2 μm fraction was positively correlated (r = 0.49) with hematite content, while it was negatively correlated with goethite quantities (r = −0.61), suggesting a ferrihydrite precursor is associated with hematite formation.

Soils on Quaternary terraces of the Tallapoosa River, Central Alabama

Fluvial terrace chronosequences provide excellent opportunities for evaluating pedogenesis. We evaluated soil development on Quaternary terraces of the Tallapoosa River of central Alabama. Our objective was to ascertain pedogenic markers for these relatively low terraces. Past studies and other evidence suggest landscape ages ranging from contemporary floodplains to mid- to late Pleistocene terraces. The six pedons evaluated ranged from Typic Udifluvents and Fluventic Dystrudepts on the floodplain to fine-loamy and coarse-loamy Typic Paleudults on relatively higher terraces. Soil physical properties (particle size, bulk density, 15 bar H2O content), soil chemical properties (total and extractable elements, cation exchange capacity, Fe and Al extractions, heavy mineral content), morphological properties, and micromorphological properties were evaluated. Many soil physical and chemical properties (averaged for B horizons) are related to the soil chronosequence, including properties established in earlier studies as being indicative of soil development. The effective cation exchange capacity and cation exchange capacity 100 g−1 clay (P ≤ 0.05 and 0.01, respectively), oxalate-extractable Fe (P ≤ 0.05), oxalate-extractable Fe/dithionite extractable Fe (P ≤ 0.10), and total K/total Ti (P ≤ 0.05) all decreased at higher levels. In addition, Mehlich extractable Cu (P ≤ 0.10), Fe (P ≤ 0.05), and Zn (P ≤ 0.05), along with total Mg (P ≤ 0.01), Mn (P ≤ 0.10), and Ca (P ≤ 0.10) decreased with higher terrace levels. Principal component analysis and cluster analysis indicated that soils on terrace levels < 5500 years B.P. are similar, and soils on levels > 5500 years B.P. are similar. The first principal factor, which accounted for 65% of the variance and is composed of both chemical and physical properties, separated terrace levels satisfactorily. The results are generally consistent with decreasing inherent fertility with increasing soil development.

USING SITE-SPECIFIC SUBSOILING TO MINIMIZE DRAFT AND OPTIMIZE CORN YIELDS

Subsoiling is often required to alleviate soil compaction; however, deep tillage can be expensive and time-consuming. If this tillage operation is conducted deeper than the compacted soil layer, energy is wasted. However, if this tillage operation is conducted shallower than the compacted soil layer, energy is again wasted, and plant roots may be prevented from penetrating the compacted layer. Technologies are now available that allow subsoiling to be conducted at the specific depth of the compacted layer, which would conserve natural resources without sacrificing crop yields. An experiment was conducted over four years in a field located in southern Alabama to evaluate whether the concept of site-specific subsoiling (tilling just deep enough to eliminate the hardpan layer) would reduce tillage draft and energy requirements and/or reduce crop yields. Average corn (Zea mays L.) yields over this four-year period showed that site-specific subsoiling produced yields equivalent to those produced by the uniform deep subsoiling treatment while reducing draft forces, drawbar power, and fuel use.

Mineralogy of eroded sediments derived from highly weathered Ultisols of central Alabama

Coarse-textured surface horizons are common in highly weathered southeastern US coastal plain soils. These soils have historically been managed under conventional tillage practices, but conservation tillage management practices are increasing. Although clay (<2 �m) contents are low in these surface horizons (typically < 1 0 0gk g −1 ), the reactive nature of this fraction tends to play a dominant role in colloidal facilitated pollutant transport. Studies have suggested that enrichment of certain minerals occurs in transported sediments, thus, we evaluated sediment size and the mineralogical partitioning of clay minerals of soil versus runoff sediment under simulated rainfall. In addition, because water dispersible clay (WDC) has been shown to be correlated with soil erodibility, we evaluated WDC differences as a function of tillage practices. Plots were established at a site in the upper coastal plain of central Alabama, where soils classified as coarse-loamy, siliceous, subactive, thermic Plinthic Paleudults and Typic Hapludults (FAO-Acrisols). Surface tillage treatments were established in 1988, and included conventional tillage (CT) versus no surface tillage (NT) treatments, with crop residue remaining or removed, and with or without paratilling (non-inversion subsoiling). Within these plots, simulated rainfall (target intensity = 50 mm h −1 for 2 h) was applied to replicated 1 m × 1 m areas, and runoff and sediment were collected. Mineralogical analyses of soils and sediment were conducted using thermogravimetric analysis (TGA) and X-ray diffraction (XRD) techniques. WDC quantities were highly correlated with percent soil organic carbon (SOC) (r 2 = 0.76), which was a function of tillage treatment. Although no differences in the mineralogy of the <2 �m sediment were observed between tillage treatments, runoff sediments (<2 �m) were enriched in quartz (qtz) and relatively depleted with kaolinite compared to in situ soils. These findings will facilitate development of mechanistic models that predict sediment attached losses of nutrients and pesticides. Published by Elsevier Science B.V.

Using Remote Sensing Data to Evaluate Surface Soil Properties in Alabama Ultisols

Evaluation of surface soil properties via remote sensing could facilitate soil survey mapping, erosion prediction, and allocation of agrochemicals for precision management. The objective of this study was to evaluate the relationship between soil spectral signature and surface soil properties in conventionally managed row crop systems. High-resolution remote sensing data were acquired over bare fields in the Coastal Plain, Appalachian Plateau, and Ridge and Valley provinces of Alabama using the Airborne Terrestrial Applications Sensor multispectral scanner. Soils ranged from sandy Kandiudults to fine textured Rhodudults. Surface soil samples (0 to 1 cm) were collected from 161 sampling points for gravimetric soil water content, soil organic carbon, particle size distribution, and citrate dithionite extractable iron content. Surface roughness and crusting were also measured during sampling. Two methods of analysis were evaluated: (1)multiple linear regression using common spectral band ratios and (2)partial least-squares regression. Our data show that thermal infrared spectra are highly, linearly related to soil organic carbon, sand and clay content. Soil organic carbon content was the most difficult to quantify in these highly weathered systems, where soil organic carbon was generally <1.2%. Estimates of sand and clay content were best using partial least-squares regression at the Valley site, explaining 42 to 59% of the variability. In the Coastal Plain, sandy surfaces prone to crusting limited estimates of sand and clay content via partial least-squares and regression with common band ratios. Estimates of iron oxide content were a function of mineralogy and best accomplished using specific band ratios, with regression explaining 36 to 65% of the variability at the Valley and Coastal Plain sites, respectively.

Crop residue effects on electrical conductivity of Tennessee Valley soils

Electrical conductivity (ECa) measurements taken with soil-contact sensors are commonly used to evaluate field-scale soil variability. Several studies have evaluated soil property effects on ECa, however, relatively few studies have evaluated the impacts of crop residues on these measurements. Crop residues may impact ECa readings, thus affecting the ability of ECa to depict spatial patterns of static soil properties. Residue effects on ECa measurements were evaluated in the intensively cropped Tennessee Valley region of Alabama where the adoption of conservation tillage practices that utilize cover crops is increasing. Ten transects (3.1 m wide by 73.2 m long) were established within a 0.22 ha (30.5 m×73.2 m) no-till field in a corn (Zea mays L.)-wheat (Triticum aestivum L.)-soybean (Glycine max L.) rotation. Soils classified in fine-loamy, siliceous, semiactive, thermic Typic Paleudults. Geo-referenced ECa measurements (at 0–30 and 30–90 cm depths) collected with a coulter mounted direct-contact sensor were taken with 1) crop residue (3789±955 kg ha−1) in place, 2) residue removed from five randomly selected transects, and 3) residue incorporated through shallow disking (5 to ∼10 cm). Measurements were taken at operating speeds of 2.2 and 4.4 m s−1. Sand and clay content and gravimetric water content (θg) were highly correlated with ECa, and regression equations using measured soil properties were developed that explained between 56% and 86% of the ECa variability. Difference in operating speeds had minimal effects on ECa measurements. Significant differences were observed between ECa values for residue remaining vs residue removed, but differences were small (∼0.5 mS m−1). When compared within two groups stratified by clay content (< or > 200 g kg−1), larger differences in ECa(0–30 cm) values for residue removed vs residue remaining were observed. Minimal differences in geostatistical parameters were observed. Overall, for the residue quantities and soils evaluated in this study, residue had minimal impacts on ECa values.

Lateral flow in loamy to sandy Kandiudults of the Upper Coastal Plain of Georgia (USA)

Abstract Interest in site-specific agronomic management in intensively cropped regions necessitates characterization of subsurface water movement for efficient water management (irrigation timing) and control of off-site agrichemical movement. Soils formed in fluvial sediments in portions of the Upper Coastal Plain of Georgia (USA) are extensively used for peanut, cotton, and corn production. Certain proximate soils in this region possess contrasting subsoil properties, and it was hypothesized that these differences would have major effects on water redistribution across the landscape. This could be important in irrigation management, where soils possessing increased impedance to vertical flow could require decreased irrigation as opposed to soils without vertical flow restrictions. At a site near Plains, GA. (USA), hydraulic properties of soils with differences in overlying sand thickness and contrasting argillic horizon textures (sandy vs. loamy) were evaluated. The soils were predominantly in loamy and sandy families of Typic, Arenic, and Grossarenic Kandiudults. Laboratory measurements, field monitoring of matric potentials under simulated and natural rainfall, and modeling (VS2DT) were utilized to evaluate soil hydraulic properties. Reduction in vertical K s occurred in horizons containing higher clay (argillic horizon). Changes in tension and build ups in hydraulic gradients associated with infiltration and redistribution events existed above and within horizons with low K s . Evidence suggested there was less groundwater recharge occurring in the loamy than in the sandy pedons, suggesting more pronounced lateral flow occurred in the loamier soils. Model simulations of water movement across a slightly sloping (1%) simulated landscape indicated lateral gradients of flow existed within the solum of these soils. Analyses of tracer (Br) movement suggested a very slight lateral redistribution occurred within a relatively short monitoring period within the sandy pedons Bt1 horizon, and the Bt2 and Bt3 horizons of the loamy pedon. Evidence suggested both loamy and sandy argillic horizons slightly, but not overwhelmingly, induced lateral flow on these landscapes.