bluuebunny/crossref_metadata_2025 huggingface dataset:
Carbon Storage in Upland Forests of the Lake States
Authors: Grigal D. F. , Ohmann L. F.
Published: July 2010

Carbon storage and dynamics are receiving increasing attention because of the hypothesized role of CO2 in global climate change. This study was carried out to determine total C storage in Lake States' forests, including C in biomass, forest floor, and mineral soil. Overstory trees were measured and samples of both forest floor and mineral soil (to 1 m) were collected from plots in 169 forest stands across Minnesota, Wisconsin, and Michigan. Five forest types were represented: balsam fir, Abies balsamea (L.) Mill.; jack pine, Pinus banksiana Lamb.; red pine, P. resinosa Ait.; aspen, Populus tremuloides Michx.; and northern hardwoods dominated by sugar maple, Acer saccharum Marsh. There were no strong geographic trends in C storage in biomass, forest floor, or mineral soil across the study area. Storage differed significantly among forest types. Each major C pool was related to a different set of descriptors. Total C storage, the sum of all pools, was related to forest type, stand age, available water, actual evapotranspiration, and soil clay content, explaining about 65% of the variation. Use of soil and site descriptors did not completely account for the strong effects of forest type on C storage. Differences in the size of C pools, as related to time since disturbance and forest type, indicate that C storage in forests of the Lake States can be influenced by forest management activities. Patterns of C storage in these moist temperate ecosystems are not as strongly influenced by climatic variables as is C storage in grasslands to the west.
Corn Growth as Affected by Suspension Fertilizers Containing Fluorosilicic Acid
Authors: Sikora F. J. , Copeland J P. , Dillard E. F. , Burnell J. R.
Published: July 2010

Fluorosilicic acid (H2SiF6) is a suggested additive to suspension fertilizers produced from monoammonium phosphate (MAP) to prevent gelling of the suspension. A greenhouse study was conducted to determine if F added to soil via P suspension fertilizers was detrimental to corn (Zea mays L.) growth. The F sources consisted of reagent‐grade NH4F, NH4SiF6, and three P suspension fertilizers. Equilibria constants and measurements with a F‐ ion selective electrode (ISE) indicated SiF62‐ dissociates to H4SiO4 and F‐ at pH > 4 and total F concentration <1 mM. Therefore, only F‐ needed to be considered as a potential toxic component in soils. At an initial soil pH of 5.8, an 11% dry‐matter weight decrease was observed when a suspension fertilizer with 89 g F kg−1 was added to soil to supply a rate of 170 mg F kg−1 soil. An adequate level of F in P suspension fertilizers is between 16 and 23 g kg−1 and this level did not affect corn dry‐matter weight with application rates ≤ 32 mg F kg−1 soil. At high soil F‐application rates from 30 to 170 mg F kg−1 soil, F was not suspected to be the phytotoxic agent since F concentrations in corn tops were all <0.4 mg kg−1. There was a significant correlation between soil Al and F extracted with water, which led to a hypothesis that decreased corn growth was due to Al phytotoxicity.
Soil pH Gradients near Calcite and Dolomite Particles
Authors: Stevens J. J. , Blanchar R. W.
Published: July 2010

The quality of liming material is normally judged by its neutralizing power, particle‐size distribution, and mineralogy. Historically, three methods have been used to evaluate these factors: (i) observing the effect of a liming material on crop yield, (ii) studying changes in soil pH with time after liming, and (iii) measuring unreacted carbonate. This study used a new method, glass pH microelectrodes, to evaluate the reactivity of limestone in soil. The effect of limestone particle size, mineralogy, and water leaching on the amount of unreacted carbonate and the soil pH around limestone particles was investigated. The reaction rate of calcite and dolomite was proportional to the surface area of the particles. Calcite reacted approximately twice as fast as dolomite. Reproduceable pH gradients exist around limestone particles in soil. In static systems, soil pH varied as a linear function of the reciprocal of distance from the particle surface. These gradients were generally independent of limestone particle size; however, in some cases pH was higher near larger particles. The soil pH next to dolomitic limestone particles was lower than near calcitic particles. Soil pH gradients around leached limestone particles were independent of the direction of water flow. The amount of limestone reacted in leached soil systems could be predicted from the number of particles and the shape of pH gradients around them in unleached soil systems.
Nutrient Distribution following Wheat‐Residue Dispersal by Combines
Authors: Douglas C. L. , Rasmussen P. E. , Allmaras R. R.
Published: July 2010

Uneven crop residue distribution by a combine may creae non‐uniform nutrient distribution in the field long after harvest. Uniform distribution of nutrients across a field has significant impacts on plant growth, soil sampling, fertilizer efficiency, and nutrient loss to surface and subsurface water. This study was conducted to evaluate nonuniformity of N, P, S, and K distribution across a field following combine harvest of soft white winter wheat (Triticum aestivum L.). Twelve self‐propelled combines were evaluated in 1983 and 10 in 1984. Each combine passed over an 11‐m strip of canvas, divided into 0.94 by 0.91 m segments, which was inserted in a swath cut into the unharvested wheat. Residues deposited on any segment of the canvas were collected and residue distribution and N, P, S, and K content of the residue was determined. Distribution of these four nutrients followed the residue distribution and created cyclic nonuniformity across the landscape. Maximum nonuniformity of nutrient distribution, where 1 = perfect uniformity, ranged from 2.7 to 9.0 for N, 1.9 to 7.7 for S, 2.3 to 9.4 for P, and 1.5 to 6.8 for K. Nutrient distribution was more extreme when wheat was cut closer to the ground. Combine modification to improve chaff distribution significantly improved nutrient distribution.
Secondary Manganese/Iron Ratios as Pedochemical Indicators of Field‐Scale Throughflow Water Movement
Authors: McDaniel P. A. , Bathke G. R. , Buol S. W. , Cassel D. K. , Falen A. L.
Published: July 2010

Thermodynamic models predict that Mn is a more mobile component of soil systems than is Fe and therefore subject to more extensive redistribution within soils and landscapes. This study was initiated to examine the accumulation of secondary Mn (Mnd) relative to that of secondary Fe (Fed) as influenced by field‐scale water movement. Secondary Mn and Fe were measured using a citrate‐bicarbonate‐dithionite (CBD) extract of genetic horizons of 60 soils sampled in an Ultisol‐dominated landscape of the North Carolina Piedmont. Soil water pressure was measured using tensiometers installed at each soil sampling site to infer water flow patterns. For all well‐drained soil borizons, Fed contents correlate well with clay content (r = 0.878***), suggesting that relatively little redistribution of Fe released through weathering has occurred within the landscape. In contrast, Mnd contents are extremely variable among genetically similar horizons and are not well correlated with clay content (r = 0.327***). These differences serve as the basis for using secondary Mn/Fe (Mnd/Fed) ratios as relative indicators of Mn accumulation. Total soil water potential contours for Ap and Bt horizons closely parallel elevation contours, indicating the strong influence of slope gradient and configuration on water movement. Secondary Mn/Fe ratios are highest in Ap horizons of soils occupying lower slope positions and concave areas of the landscape. Total soil water potential data indicate convergent throughflow occurs in these wetter areas of the landscape. In contrast, lower Mnd/Fed ratios are found in soils occupying more convex landscape positions where soil water potential data show that divergent throughflow occurs. Results indicate that the difference in mobility between Mn and Fe in these acid soil systems allows secondary Mn/Fe ratios to be used as pedochemical indicators of field‐scale water movement.
Cropping System and Consolidation Effects on Rill Erosion in the Georgia Piedmont
Authors: West L. T. , Miller W. P. , Langdale G. W. , Bruce R. R. , Laflen J.M. , Thomas A. W.
Published: July 2010

Consolidation and changes in surface horizon properties induced by no‐tillage may reduce soil loss to levels beyond those attributable to surface residue alone. To evaluate effects of consolidation and surface horizon modification induced by long‐term no‐tillage on rill erodibility (Kr) and critical hydraulic shear (τc), simulated rainfall and inflow were applied to plots managed under conventionally tilled monocropped soybean [Glycine max (L.) Merr.], conventionally tilled monocropped grain sorghum [Sorghum bicolor (L.) Moench], and no till double cropped crimson clover (Trifolium incarnatum L.) and grain sorghum at three sites with differing surface horizon clay contents. All surface residue was removed prior to rainfall simulation, and conventionally tilled cropping systems were evaluated in both consolidated and freshly tilled conditions. Increases in organic C from 9.3 to 12.9 g kg−1 and water‐stable aggregates from 50 to 76% induced by no‐tillage resulted in a 60 to 70% decrease in Kr for the no‐till system, compared with conventionally tilled systems. No differences in Kr were observed for the two conventionally tilled systems. No differences in τc were observed among the tillage systems. Consolidation within the conventionally tilled treatments reduced Kr by ≈60% from that observed for freshly tilled soil. The reduction was attributed to greater resistance to detachment by flow associated with soil strength increases due to consolidation. Differences in Kr and τc among the sites were small and were not readily explained by any of the soil properties evaluated. These results indicate that consolidation and modification of surface soil properties under no‐till cropping systems are important factors that contribute to reduced rill soil loss observed under no‐tillage.
Assessment of Functions and Parameter Estimation Methods in Root Water Uptake Simulation
Authors: Rasiah V. , Carlson G. C. , Kohl R. A.
Published: July 2010

Limited information is available on the influence of estimates of parameters of the root water uptake (RWU) function on RWU simulation in macroscopic RWU modeling. This study was conducted to assess the influence of (i) parameter estimation methods, linear vs. nonlinear, and (ii) the type of RWU functions, continuous vs. discontinuous, on the estimates of RWU parameters and RWU simulation. The model calibration and validation were carried out using the data for soybean [Glycine max (L.) Merr.] grown in large soil columns. The estimates of the RWU parameters varied with the estimation method used or the type of function used. The simulations in soil water content (θv) and RWU distribution across space and time varied with the estimates of the parameters used in the flow model. The simulations in θv produced with the estimates from the nonlinear estimation method agreed more closely with the corresponding measured values than did those produced with the estimates from the linear method. A similar trend was observed for the discontinuous vs. continuous functions. The differences in residual sum of squares between the linear vs. nonlinear estimation methods, and that between the continuous vs. discontinuous functions, were significant. The accuracy of the simulations in θv and RWU increased significantly using the nonlinear parameter estimation method and a discontinuous RWU function.
Leaching Effect on the Dissolution of Two Phosphate Rocks in Acid Soils
Authors: Hanafi M. M. , Syers J. K. , Bolan N. S.
Published: July 2010

Dissolution of phosphate rock (PR) in soil is a necessary prerequisite for the P in the PR to become plant available. The dissolution of two PR materials, Gafsa (GPR) and Christmas Island A (CIPR), in six acid, Malaysian soils (pH in water 3.0–5.4) was examined in closed‐incubation and open‐leaching systems. Dissolution was determined from the increase in either 0.5 M NaOH‐extractable P (ΔP) or 1 M BaCl2‐triethanolamine (TEA)‐extractable Ca (ΔCa) in soil amended with PR, compared with the control soil. In the closed‐incubation system, dissolution of the reactive GPR was higher (22–71%) than that of the less reactive CIPR (26–42%). Despite an adequate supply of protons, the extent of dissolution of both PR materials was very small at 50 d, after which there was a negligible increase in the amount of P released. Calcium released from the PR materials occupied most of the negatively charged exchange sites and there was an insufficiently large Ca sink for dissolution to continue. Although the extent of PR dissolution increased with increasing P‐retention capacity of the soil, in the closed‐incubation system the size of the sink for Ca determines the extent of PR dissolution in these high P‐sorbing soils. With one exception, the dissolution of both PR materials was larger (56–94%) in the open‐leaching system and there was less difference in the extent of dissolution between the two PR materials and among the various soils. Continuous leaching resulted in the removal of cations (between 4.2 and 29.9 mmolc kg−1 soil), thus providing new sites for the Ca released from PR. The renewal of the Ca sink during leaching was the main reason for the continued dissolution of PR under leaching conditions. Studies of PR dissolution in soils using a closed‐incubation system provide information that is less relevant to the field situation than do related studies involving an open‐leaching system.
Estimating In Situ Unsaturated Hydraulic Properties of Vertically Heterogeneous Soils
Authors: Shouse P. J. , Jobes J. A. , Sisson J. B. , Ellsworth T. R.
Published: July 2010

The majority of procedures for in situ measurement of the unsaturated hydraulic conductivity are variations of the instantaneous profile method. A vertically nonuniform soil requires the unsaturated hydraulic functions to be estimated at each horizon. Scaling systems have evolved in an attempt to reduce the number of hydraulic functions needed to characterize water flow through heterogeneous soils. In this study, we extended the concept of water content (θ) scaling to nonuniform soil profiles, tested the effectiveness of θ scaling for reducing apparent spatial variability, and estimated the unsaturated hydraulic functions for a naturally occurring loamy sand field site. Two instantaneous profile experiments conducted at Etiwanda, CA, provided soil water content and pressure head (h) data vs. depth (z) and time (t). Water retention, θ(h), and hydraulic conductivity, K(θ), functions fitted to data from the 15‐cm depth at Plot 1 were arbitrarily chosen as the reference hydraulic properties to which the other depths and plots were scaled. Based on a unit‐gradient analysis of the drainage data, the slope of the hydraulic conductivity function, dK/dθ, was estimated as z/t. Scaling other depths and plots to the reference location was done using an iterative procedure that provided least‐squares estimates of the two θ scaling parameters (δ and µ) and a corresponding transformed depth variable (z*). Scaled water content, θ*, plotted vs. z*/t, using data from all depths and plots, coalesced to a single curve. Scaling θ successfully coalesced heterogeneous soil hydraulic properties into unique functions for both θ(h) and K(θ).
Dispersivity Values Determined from Effluent and Nonintrusive Resident Concentration Measurements
Authors: Oostrom M. , Dane J. H. , Güven O.
Published: July 2010

As indicated in the subsurface contaminant transport literature, the longitudinal dispersivity values needed to predict salt concentrations become smaller if more details of the flow field are included in the calculations. One way of determining these small‐scale or local longitudinal dispersivities is from breakthrough curves using laboratory columns. A gamma radiation technique is presented that allows non‐intrusive, in situ (resident) salt concentration measurements in saturated sand columns. The technique is demonstrated by means of a series of vertical displacement experiments with deionized water and a NaI solution in a column containing a medium sand. Longitudinal dispersivity values derived from the resident concentration breakthrough curves determined with this technique were consistently smaller than those derived from conventional effluent concentration breakthrough curves. The density configuration of the liquids may affect the apparent dispersivities.
The Analysis of Soil Organic Matter by Pyrolysis‐Field Ionization Mass Spectrometry
Authors: Schnitzer M. , Schulten H. R.
Published: July 2010

At the present time, the analysis of organic matter (OM) in soils requires extraction of the OM, fractionation of the extract into humic acid (HA), fulvic acid (FA), and humin, and purification of each fraction prior to more detailed characterization by chemical, spectroscopic, and other methods. To obviate the need for these laborious and possibly damaging procedures, we examined the possibility of applying pyrolysis‐field ionization mass spectrometry (Py‐FIMS), a method that provides information at the molecular level on complex organic materials, to whole soils and HAs, FAs, and humins separated from these soils. In all materials, carbohydrates, phenols, lignin monomers and dimers, and, to a lesser extent, n‐fatty acids were the major components. Our data indicate that whole soils are the most suitable materials for the analysis of OM in soils. Thus, Py‐FIMS has an advantage compared with existing methods in that it permits soil chemists and other interested scientists to do comprehensive OM analyses on air‐dry soils, even ones with low OM contents, without any pretreatment.
Contrasting Selenate‐Sulfate Interactions in Selenium‐Accumulating and Nonaccumulating Plant Species
Authors: Parker D. R. , Page A. L. , Bell Paul F.
Published: July 2010

The shoots of primary Se‐accumulating plant species can accrue Se to several thousand micrograms per gram of dry weight, even when growing in gypsiferous soils, yet no detailed studies of the mutual antagonism between SeO4 and SO4 during plant uptake have been conducted with these species. In a comparative study, we grew a nonaccumulator (alfalfa, [Medicago sativa L.]) and a primary Se accumulator [Astragalus bisulcatus (Hook.) A. Gray] in identical nutrient solutions with varied SeO4 (2–80 µM) and SO4 (0.5–15.5 mM) concentrations for 21 (alfalfa) or 32 to 35 d (A. bisulcatus). Shoot S concentrations in alfalfa were increased by increases in solution SeO4, but only when shoot Se was above ≈20 µg g−1, suggesting that SeO4‐induced stimulation of S uptake may be a result of incipient Se toxicity. Similar stimulations of S uptake were less apparent in A. bisulcatus. Alfalfa shoot Se concentrations ranged from 4 to 154 µg g−1, while the same treatments resulted in shoot levels of 175 to 1200 µg g−1 in A. bisulcatus. Uptake of SeO4 by alfalfa was profoundly inhibited by increases in solution SO4, while Se uptake by A. bisulcatus was much less, although still significantly, affected. Comparison of molar Se/S ratios in plants and nutrient solutions indicated discrimination against Se by alfalfa, but preferential accumulation of Se by A. bisulcatus. A reevaluation of previously published results was in general agreement with our findings, and suggested that primary Se accumulators have a unique ability to accumulate SeO4 in the face of competition from SO4. These species could thus prove useful in efforts to remediate Se‐contaminated soils or sediments that are also enriched in SO4. Overall plant discrimination between SeO4 and SO4 may be related to (i) discrimination between the two analogues during initial absorption, or (ii) differential retranslocation of these elements to the root with subsequent afflux to the external solution.
Soil Carbon, Nitrogen, and Fine Root Biomass Sampling in a Pine Stand
Authors: Ruark Gregory A. , Zarnoch Stanley J.
Published: July 2010

Spatial variability of soil properties can be great in forested settings. A study was conducted to determine the most efficient methodology for sampling surface mineral soils in a mature loblolly pine (Pinus taeda L.) forest for total C, total N, and fine root biomass to obtain stand level estimates. Specifically, we evaluated the variation within and among plots, the performance of small‐ and large‐diameter corers, the effect of stem proximity, the influence of fine roots on soil C and N estimates, and the effect of compositing samples. We concluded that (i) C and N levels always increased with proximity to stems; (ii) proximity to a stem did not affect mean fine root biomass estimates; (iii) variability of the estimates of C and N consistently decreased with increased distance from the stem, but no trend occurred for fine root biomass; (iv) for total stand estimates of N and fine root biomass, stratification with respect to tree stems does not yield a gain in precision, for C precision increased only slightly; (v) C associated with fine roots represented ≈5% of the total C in a core sample, while N in roots was < 2% of its total; (vi) for C and N estimates, the variation with a 1.8‐cm‐diam. core was similar to that obtained with a 6.1‐cm core; and (vii) chemical analyses could be reduced and precision maintained by compositing the soil core samples.
Soil‐Vegetation Relationships in Franciscan Terrain of Northwestern California
Authors: Popenoe J. H. , Bevis K. A. , Gordon B. R. , Sturhan N. K. , Hauxwell D. L.
Published: July 2010

This study sought to discover how soil and vegetation patterns are related in the mountains of northwestern California. Soils were compared by vegetation‐epipedon type and drainage class, with elevation as a covariate. Vegetation types were (i) open prairie, (ii) coniferinvaded prairie, (iii) oak woodland, (iv) conifer forest with umbric or mollic epipedon, and (v) conifer forest with ochric epipedon. The level of organic carbon (OC) was similar in the upper 10 cm of mineral soil under all vegetation types, but soils under forest and oak woodland had less cumulative OC and N, and a greater decrease with depth than soils under prairie. Ochric epipedons under forest had wider C/N ratios than umbric and mollic epipedons under other vegetation types. Exchangeable Ca2+ and base saturation decreased with increasing elevation. Exchangeable Ca2+ decreased with depth under oak woodland and forest, and pH decreased with depth under oak woodland. Under prairie, pH increased with depth, and Ca2+ exhibited little change. Soils under prairie and invaded prairie had lower exchangeable K+ in upper horizons than soils under forest or oak woodland. Drainage‐impaired subsoils had higher bulk density, pH, and exchangeable Ca2+ than well‐drained subsoils. Conifer invasion was restricted to well‐drained soils. Except for conifer‐invaded prairie, closely associated soil and vegetation patterns indicated that the vegetation mosaic has persisted for a long time.
Tillage Studies with a Corn‐Soybean Rotation: Hydrology and Sediment Loss
Authors: Edwards W. M. , Triplett G. B. , Van Doren D. M. , Owens L. B. , Redmond C. E. , Dick W. A.
Published: July 2010

When soybean [Glycine max (L.) Merr.] is produced on sloping fields using conventional tillage and mechanical cultivation for weed control, potential soil losses are high, limiting production to relatively flat lands. With the advent of conservation tillage practices and herbicides for weed control, soybean production with tolerable soil losses on sloping fields has become technically possible. To evaluate soil loss under these new conditions, runoff and erosion were measured on six small watersheds that were farmed for 6 yr in a corn (Zea mays L.)‐soybean rotation with conservation tillage. Two of the watersheds were chiseled each year, two others paraplowed, and the other two received no‐tillage. A rye (Secale cereale L.) cover crop was established each winter following soybean. With average yearly rainfall of 930 mm, yearly runoff was 63.5 mm and varied between 28 and 108 mm. Average yearly soil loss for the 6‐yr period was 526 kg/ha, <10% of the allowable soil loss. Two storms caused most of the erosion, with a single storm accounting for 42% of the total measured soil loss. Earlier, these same watersheds averaged 5750 kg/ha of measured soil loss during the corn years of a corn‐wheat‐meadow‐meadow rotation with conventional tillage.
Tillage Studies with a Corn‐Soybean Rotation: Surface Runoff Chemistry
Authors: Owens L. B. , Edwards W. M.
Published: July 2010

When soybean [Glycine max (L.) Merr.] is grown on sloping soils, there is a high potential for soil and nutrient losses. The purpose of this study was to compare nutrient losses in surface runoff across a range of watershed conditions when tillage practice was a variable. For 6 yr in east‐central Ohio, nutrient concentrations and transport in surface runoff were measured from six small (<1‐ha) watersheds planted to a corn (Zea mays L.)‐soybean rotation. Two watersheds were chisel plowed each year, two were paraplowed, and two received no‐tillage. Rye (Secale cereale L.) provided winter cover following soybean harvest. Nitrate‐N and K concentrations in surface runoff were greater during corn years than soybean years, but there was no significant difference among tillage treatments. There were no consistent differences between crops or among tillage practices for the transport of soluble P, soluble K, and total organic C (TOC). Most of the NO3‐N loss was in the runoff from a small percentage of runoff events. Although NO3‐N concentrations in surface runoff frequently exceeded 10 mg/L during the corn years, the actual amount of N lost was small. But because of year‐to‐year variation in runoff, which masked most of the differences resulting from cropping or tillage practice, there is a need for long‐term research (>6 yr) to assess the environmental risks associated with a particular management practice.
Long‐Term Nutrient Accumulation Rates in the Everglades
Authors: Reddy K. R. , DeBusk W. F. , DeLaune R. D. , Koch M. S.
Published: July 2010

Anthropogenic nutrient inputs to the northern Everglades of Florida during the last three decades have resulted in alteration of vegetation and soil nutrient storage. Due to the nutrient‐limited status of this ecosystem, increased loading may have altered the capacity for long‐term nutrient accumulation. Our study was conducted to determine the potential long‐term nutrient accumulation rates for this ecosystem along a gradient of nutrient loading. Accumulation rates were calculated using the vertical peat accretion rates, as determined by 137Cs dating, and nutrient concentration profiles. Intact soil cores were obtained along a 15‐km transect and evaluated as a function of distance from the inflow structure. Soil cores were sectioned into 1‐cm‐depth increments and analyzed for 137Cs, P, N, C, and selected cations. Vertical accretion rates of peat decreased logarithmically with distance from the inflow, with rates of 1.1 cm yr−1 at 0.3 km from the inflow to about 0.25 cm yr−1 in unimpacted sawgrass (Cladium jamaicense Crantz‐dominated areas. Phosphorus, N, and C accumulation rates in soil and floodwater total P concentrations also showed similar relationships. The P accumulation rates ranged from 0.54 to 1.14 g P m−2 yr−1 in cattail (Typha spp.)‐dominated areas, and 0.11 to 0.25 g P m−2 yr−1 in sawgrass‐dominated areas. The C/P and N/P accumulation ratios increased with distance from the inflow, suggesting that a greater proportion of P accumulated in the system, compared with C and N. Similar P retention coefficients were obtained when calculated using either changes in surface water total P concentration, or the long‐term P accretion rates. These findings suggest that P was either directly adsorbed by soil or precipitated with Ca in the water column and deposited on the soil surface. This hypothesis was further supported by a highly significant correlation between P and Ca accretion rates, suggesting that Ca‐bound P controls equilibrium concentrations in this ecosystem.
Ammonium Oxidation and Nitrate Reduction in Sediments of a Hypereutrophic Lake
Authors: D'Angelo E. M. , Reddy K. R.
Published: July 2010

Internal N cycling processes in sediments and the overlying water column may contribute to the eutrophication of lake systems. One of the major mechanisms for N loss in these systems is through biological oxidation and reduction of N species in the aerobic and anaerobic sediment zones, coupled with exchange processes between these zones. These mechanisms were measured using flooded, intact sediment columns and batch incubations with bulk sediments collected from a hypereutrophic lake. In continuously stirred batch incubations with aerated sediment, NH+4 oxidation to NO−3 (nitrification) showed two‐phase, zero‐order kinetics. The rapid first phase of nitrification (0.36 mg N L−1 h−1) was due to the oxidation of NH+4 initially present in the sediment, while the slower second phase (0.15 mg N L−1 h−1) was limited by the rate of production of NH+4 during ammonification of organic N. Denitrification as determined by the C2H2‐blockage technique was found to be limited by NO−3 availability. Under NO−3 nonlimiting conditions, the denitrification rate was 0.11 mg N L−1 h−1, but a fivefold decrease was measured at low NO−3 concentration (1 mg NO3 L−1). Denitrification was the major NO−3 reductive process in the surface 27‐cm sediment depth. Assimilatory 15NO−3 reduction into the organic 15N fraction was also a significant NO−3 loss mechanism at the sediment surface. Dissimilatory 15NO−3 reduction to 15NH+4 became the dominant NO−3 reductive pathway at sediment depths >27 cm. Losses of up to 90% of the floodwater 15NH+4 or 15NO−3 was largely attributed to sequential nitrification‐denitrification reactions.
Simulating Nutrient Uptake by Single or Competing and Contrasting Root Systems
Authors: Smethurst P. J. , Comerford N. B.
Published: July 2010

Simulation of nutrient uptake by competing root systems is necessary to assist in the quantitative understanding of these processes, to predict the consequences of competition for nutrients, and to prioritize future research on the mechanisms of nutrient competition. Our objective was to apply previously published concepts to a computer model based on solute transport theory that would have the ability to simulate nutrient uptake by root systems of one or two competing plant species. Analytical solutions were used for calculating the volume of soil allocated to each root system and the concentrations of solute at the root surfaces. We included in the model (i) a depletion zone that increased with time until it reached the no‐transfer boundary, (ii) an adjustment of the average concentration in the depletion zone to account for newly encountered solute as the zone increased in radius, (iii) a variable root‐absorbing power (α), and (iv) a routine to correctly account for all soil in simulations with two contrasting root systems. Predicted uptake for single‐species scenarios in soils of contrasting nutrient supply characteristics was verified against that predicted by the commonly used Barber‐Cushman model, NUTRIENT UPTAKE (maximum discrepancy was 23%), which utilizes a numerical solution. The ratio of uptakes predicted by both models was a function of α, root radius, effective‐diffusion coefficient, and buffer power in a more complex way than previously suggested in the literature. Sensitivity analysis indicated that, of all parameters, K uptake by slash pine (Pinus elliottii Engelm. var. elliottii) relative to competing grass would be most sensitive to variations in root‐length density of the grass, or in soil water content, if only soil parameters were examined.
Ferrihydrite, Lepidocrocite, and Goethite in Coatings from East Texas Vertic Soils
Authors: Wang H. D. , White G. N. , Turner F. T. , Dixon J. B.
Published: July 2010

Iron oxide coatings from ped surfaces and pores of three rice paddy soils and one non‐paddy soil near Beaumont, TX, provided samples for studying Fe oxide mineralogy and the relationship of extractable Fe to extractable Al, Si, and P. Analyses of Fe oxide coatings, concentrated by a combination of scraping, sonication, centrifugation, and magnetic separation, showed that these Fe oxide coatings cement clay minerals and quartz particles together. Iron oxide coatings in all soils contained lepidocrocite and smaller amounts of goethite. An improved differential x‐ray diffraction (DXRD) method, in situ DXRD on a nonreflecting quartz plate, allowed the identification of ferrihydrite, which is otherwise difficult to identify because of its poor crystallinity and low concentration. Most of the ferrihydrite was dissolved by 10 min in pH 3 ammonium oxalate in the dark (AOD). Sequential 10‐, 50‐, and 180‐min AOD treatments extracted lepidocrocite of progressively increasing particle size. The 180‐min AOD treatment extracted minimal amounts of goethite. The DXRD and chemical data indicate that the paddy soils have more ferrihydrite in relation to total Fe oxides than the non‐paddy soil. The P/Fe ratio decreased with each AOD treatment step for all soils. This study demonstrates that physical separation and in situ AOD treatment can be used with the DXRD method to identify ferrihydrite, lepidocrocite, and goethite in soil clays containing extractable Fe concentrations as low as 10 to 44 g/kg. We concluded that a short incremental AOD treatment (≈10–30 min) is a better approach to DXRD of ferrihydrite in <2‐µm fractions of seasonally reduced soils than the much longer treatment recommended in the past for soils.
Winter‐Associated Changes in Dry‐Soil Aggregation as Influenced by Management
Authors: Layton J. B. , Skidmore E. L. , Thompson C. A.
Published: July 2010

Changes in surface soil structure during winter can increase soil erodibility. This study was conducted to examine winter‐associated changes in dry‐soil aggregation as influenced by crop, residue, and tillage system. These changes were studied at Hays, KS, on a Harney silt loam (fine, montmorillonitic, mesic Typic Argiustoll) in a winter wheat (Triticum aestivum L.)‐grain sorghum [Sorghum bicolor (L.) Moench]‐fallow rotation. Three tillage systems were used — clean tillage with residue buried, stubble‐mulch tillage, and zero tillage. Residue and crop cover present during the winter was wheat residue, sorghum residue, and winter wheat. Measurements of dry aggregate stability, aggregate‐size distribution as geometric mean diameter, and aggregate density were made before and after the winters of 1988–1989 and 1989–1990. Little difference occurred between clean and stubble‐mulch tillage systems, which were usually different from the zero‐tillage system. Greater changes in aggregation occurred during the 1989–1990 winter when precipitation was greater. Residue maintained higher surface water contents, decreased freeze‐thaw cycling and drying by sublimination, and decreased fluctuations in water content. Aggregates from plots with low residue cover decreased in stability more than aggregates from high residue treatments. Generally, differences in aggregation between tillage systems were maintained during the drier winter and minimized during the wetter winter. Soil aggregates were smaller, less dense, and less stable on the zero‐tillage plots in March 1989. Therefore, insufficient residue production for wind erosion control in a zero‐tillage system could lead to more erodible conditions than in a conventional tillage system.
Streamlines for Diffusive Flow in Vertical and Surface Tillage: A Model Study
Authors: Kirkham M. B.
Published: July 2010

Methods of tillage must permit adequate flow of O2 to plant roots. The objective of this work is to calculate streamlines of O2 flow from chisel holes oriented vertically and horizontally (surface tillage) in the soil to plant roots. Streamlines are perpendicular to equipotential lines. They also have the property that the difference in value between two streamlines gives the quantity of fluid (in this case, O2) flowing between them. Therefore, streamlines are often more important than equipotentials. Streamlines for flow of O2 from the chisel holes in the soil to plant roots were calculated by use of the relaxation method, in which both a coarse grid (2.54 cm on a side) and a fine grid (1.27 cm on a side) were used. Transit times were calculated from the flownet resulting from the streamlines and equipotential lines. Calculations with the coarse grid showed that, for the vertical chisel case, 75% of the flow goes out of the side of the chisel opening closest to the sheet of plant roots and 25% of the flow goes out of the side of the chisel opening farthest away from the sheet of plant roots; for the fine grid, the values are 63 and 37% for the two sides of the chisel hole, respectively. For both the vertically and horizontally tilled cases, stagnant areas were identified in the soil. Transit times were short for both tillage cases (on the order of minutes), confirming the general assumption that diffusion is the major mechanism of gas transport in soil.
Phosphorus Uptake by Bahiagrass from Spodosols: Modeling of Uptake from Different Horizons
Authors: Ibrikci H. , Comerford N. B. , Hanlon E. A. , Rechcigl J. E.
Published: July 2010

Bahiagrass (Paspalum notatum Flugge), when grown on a Myakka fine sand in Florida (sandy, siliceous, hyperthermic Aeric Alaquod), did not respond to P fertilization even though Mehlich‐1 extractable P was low in the surface soil horizon. Previous research suggested that the Bh horizon was a potential source of P. The objective of this study was to evaluate the plant availability of P in the Bh horizon and to determine to what degree existing nutrient uptake theories explain the uptake of P by bahiagrass under greenhouse conditions. Sod and soil samples were collected from unfertilized bahiagrass growing on a Spodosol. Sod was replanted into soil samples. Plant and root samples were collected from unfertilized bahiagrass growing on a Spodosol. Sod was replanted into soil samples. Plant and root samples were collected at 18, 36, 61, and 90 d. Selected soil and plant model parameters were estimated at each harvest period. Bahiagrass was able to extract more P from the Bh horizon material than from Ap or E horizon materials. The COMP8 computer model was used to simulate uptake of P by bahiagrass. Observed and predicted P uptake by bahiagrass followed the same relative pattern (Bh > Ap > E). There was statistically good agreement between observed and predicted P uptake for 18‐d‐old plants, but less agreement at 90 d (within 20%) from the Bh horizon. Predicted P uptake values from the Ap and E were statistically lower than observed values at both harvest dates. Selected extraction procedures indicated that there was no P release from the mineral forms in the Ap horizon. Data suggest the plant‐induced mineralization was an important P supplying mechanism in this pot study.
Incorporating Variability into Soil Map Unit Delineations
Authors: Rogowski A. S. , Wolf J. K.
Published: July 2010

Published soil surveys comprise a nationwide soils data base available to users. Although adequate for farm planning and agricultural production, the surveys lack information on spatial and temporal variability of soil attributes. Such information is increasingly needed for modeling of water flow and contaminant transport, in geographical information systems (GIS) applications, and for environmental impact assessment. The proposed approach combines spatially interpolated distributions of measured values with soil map unit delineations within a GIS framework. Data analysis provides insights into the potential continuity and variability of attributes within and between the map units. The result is a map that preserves the map unit boundaries but incorporates spatial variability of attribute data into map unit delineations. The approach is illustrated using bulk density and hydraulic conductivity data for surface soil horizons at a farm scale. Semivariograms based on measured values are constructed and cross‐validated. Subsequently, GIS overlays of soil survey attributes and kriged overlays of measured data can be prepared. These GIS overlays can be combined pixel by pixel according to a simple rule. Results indicate that spatial distributions of bulk density and hydraulic conductivity are similar to original map unit delineations, but include aspects of the attribute continuity and variability from geostatistical analysis. Such combined representations of bulk density and hydraulic conductivity are superior, because they contain more information than either the soil survey map, or the kriged interpolation of discrete data.
Aluminum Influence on Iron Oxides: XVII. Unit‐Cell Parameters and Aluminum Substitution of Natural Goethites
Authors: Schwertmann Udo , Carlson Liisa
Published: July 2010

Aluminum‐iron substitution in goethites is a widely used parameter to characterize soil goethites and to gain information on the pedogenic environment. The extent of Al substitution is obtained from calibration curves consisting of linear relationships between the unit‐cell edge length c and the Al content of synthetic goethites. We derived calibration curves from natural goethites obtained from two contrasting genetic environments: 84 samples from tropical soils and 30 lake ore samples from Finland. Significant correlations between unit‐cell parameters and chemically determined Al (dithionite‐citrate‐bicarbonate‐extractable Al minus citrate‐bicarbonate‐extractable Al) were obtained for both groups. The negative slopes of the regressions for the tropical goethites were significantly greater and closer to the slope for 106 synthetic goethites, and to the Vegard line, than those of goethites from lake ores. This is tentatively attributed to the formation environment: the temperature is lower while the rate of formation and the presence of crystallization inhibitors are probably higher in the lake environment. The younger age may also have had some influence on crystal perfection.
Mineralogy of a Holocene Chronosequence on Andesitic Beach Sediments in Costa Rica
Authors: Nieuwenhuyse A. , Jongmans A. G. , van Breemen N.
Published: July 2010

We investigated weathering and neoformation of minerals in a Holocene (<5000 yr) soil chronosequence on sandy, andesitic, ocean beach ridges in humid tropical Costa Rica using micromorphological, mineralogical, and chemical analyses. Troposamments are present on the younger beach ridges and Hapludands are on the older ones. The parent materials of all soils are sands with similar mineralogical composition: andesitic rock fragments, plagioclase, and pyroxene dominate, with minor amounts of opaque minerals. None of the parent materials contained >13% (v/v) volcanic glass. Weathering and neoformation of minerals with increasing soil age is characterized by (i) increasing pellicular and linear alteration of sand grains and (ii) decrease of the sand fraction and concomitant increase of finer material. Andesitic rock fragments weather more rapidly than plagioclase and pyroxene mineral grains. The alteration rates of the latter two are similar. Clay content in the ≈ 2000‐yr‐old soil is several times higher than in soils developed in rhyolitic parent materials of similar age in New Zealand. Formation of allophane with Al/Si ratios ranging from 1.9 to 3.8 takes place mainly in the B horizons. Aluminum‐humus complexes, allophane, and Al oxides and hydroxides are mainly formed in the A horizons. Small amounts of gibbsite were noticed in soils older than 2000 yr. Small amounts of 2:1 and 1:1 clay minerals present in the clay fraction of all soils are thought to be inherited from the parent material, which contains sand‐sized bodies of clay and andesitic rock fragments with clay pseudomorphs, both consisting of 2:1 and 1:1 clay minerals.
The Upper Limit of Solute Dispersion and Soil Hydraulic Properties
Authors: Scotter D. R. , Ross P. J.
Published: July 2010

Current theory inadequately describes the effects on solute movement of local variations in pore‐water velocity. A satisfactory theory for solute flow in structured soil has yet to be developed. As a possible starting point for such a development, we present a simple analytical description of the maximum amount of solute dispersion that can occur during steady water flow in a vertically uniform soil. The distribution of soil water velocities is found from the derivative of the unsaturated hydraulic conductivity‐water content relationship. With just this information, given the water flux and solute boundary conditions, the solute distribution can be described, assuming no mixing between water (and solute) flowing at different velocities in the soil. The analytical solution describes not only the upper limit of the dispersion that can occur, but also the maximum depth to which the leading edge of a surface‐applied pollutant can travel, given a steady water flux for a certain time. It is shown that the steeper the hydraulic conductivity curve, the greater the expected difference between the soil solution flux concentration (measured with a lysimeter) and resident concentration (measured by soil sampling). Transverse solute diffusion between water moving at different velocities reduces the dispersion below the upper limit described here. Incorporating a description of such diffusion into the approach given here may be a fruitful alternative to considering dispersion as enhanced longitudinal diffusion in soil water moving at a uniform velocity.
Unsaturated Hydraulic Conductivities of Fiberglass Wicks and Designing Capillary Wick Pore‐Water Samplers
Authors: Knutson John H. , Selker John S.
Published: July 2010

Passive capillary samplers (PCAPS) use the capillary potential of fiberglass wicks to draw pore water from unsaturated soils. The sampling suction of a wick should be matched to the pressure head, h, in the soil in which the PCAPS will be installed. Currently there is no theoretically based procedure for matching wicks to soils, a problem this study solves. When matched, the top of a wick has approximately the same h as the soil across multiple fluxes. Unsaturated hydraulic conductivities, K(h), were measured in wicks. These data exhibited exponential behavior, and estimates of wick saturated conductivities, Ksat, ranged from 220 to 1380 cm/h. The Ksat values were compared with predictions from a capillary tube model (R2 = 0.85). Moisture contents, θ(h), were measured in wicks. Hysteresis was observed, and the draining curves were used to predict K(h) using the van Genuchten equation coupled with the Mualem conductivity model. The van Genuchten equation fit the draining θ(h) data well (R2 > 0.953), but Mualem's K(h) values did not reproduce the exponential predictions. A solution for unsaturated flow was used to calculate h in wicks given the flux, q, and length. Calculated values of h were within 25% of the measured values for high fluxes and within 5% of the measured values for low fluxes. A least‐squares procedure for matching the h vs. q curve of a wick to a soil was used to design PCAPS for application in two soils.