bluuebunny/crossref_metadata_2025 huggingface dataset:
Simulation of a Potential Error in Determining Soil Salinity from Measured Apparent Electrical Conductivity
Authors: Friedman Shmulik P.
Published: July 2010

A common method for estimating soil salinity is by in situ measurements of the apparent electrical conductivity, ECa, usually by a four‐electrode probe. It is assumed that the ECa can be regarded as taking place in parallel mode by two conductors on a bulk scale: the dissolved (ECb) and the adsorbed (ECs) ions. Therefore, the contribution to the ECa by the soil solution electrolytes, ECb, which serves to assess its salinity, can be deduced by subtracting the estimated ECs from the measured ECa. This assumption is wrong and leads to an evaluation of ECb higher than its real value. This study was conducted to characterize the error in estimating soil solution electrical conductivity, ECw. A simplified model of a randomly diluted and pore‐size‐distribution‐decorated simple cubic lattice serves to describe the pore network of a saturated soil. It is assumed that only within each pore can the electrical conductance be represented by a sum of two conductors: the dissolved and the adsorbed ions, acting in parallel. Using Monte Carlo lattice simulations, it was shown that the error due to the assumption of parallel mode on a bulk scale increases with increasing broadness of the pore‐size distribution, decreasing connectivities, and increasing cation‐exchange capacity. An illustrative example of real soils, typical of irrigated soils, indicated errors of up to 25% for electrolyte concentrations.
Kinetics of Ammonium Adsorption and Desorption by the Natural Zeolite Clinoptilolite
Authors: Kithome M. , Paul J. W. , Lavkulich L. M. , Bomke A. A.
Published: July 2010

The kinetics of NH+4 adsorption and desorption were investigated on the natural zeolite clinoptilolite to ascertain its ability to adsorb and release the important plant nutrient N in its NH+4 form at various pH values and initial NH+4 concentrations. Kinetics of NH+4 adsorption were evaluated on the samples using solutions containing 140.1, 280.2, 560.4, and 840.6 mg L−1 of NH+4‐N at pH 4, 5, 6, and 7, equilibrated for 5, 10, 15, 20, 30, 45, 60, 75, 90, and 120 min. Samples for NH+4 desorption were equilibrated with 70.1, 280.2, 560.4, and 1401 mg L−1 NH+4‐N solution at pH 4, 5, 6, and 7 for 2.5 h, and adsorbed NH+4 extracted with 2 M KCl for 5, 10, 20, 30, 45, 60, 90, 120, 150, 180, and 300 min. Equilibrium time for NH+4 adsorption ranged from 60 min for 140.1 mg L−1 initial NH+4‐N concentration at pH 4 to 120 min for 840.6 mg L−1 initial NH+4‐N concentration at pH 7. Desorption was nearly complete in 150 min for low initial NH+4 concentrations and 200 min for high initial NH+4 concentrations. Amounts of NH+4 sorbed increased with increasing pH and initial NH+4 concentrations. Models evaluated included the first‐order kinetics, modified Freundlich, parabolic diffusion, Elovich, and heterogeneous diffusion. All the models adequately described the NH+4 adsorption process, with r2 values ranging from 0.955 to 0.999. With the exception of first‐order kinetics, they also described the desorption process well, with r2 values ranging from 0.897 to 0.999, for all pH and initial NH+4 concentrations. Reaction rate coefficients (k) were calculated from the modified Freundlich model and ranged from 0.134 to 0.193 min−1 for the adsorption process, and 0.129 to 0.226 min−1 for the desorption process. The models indicated that NH+4 adsorption and desorption by the zeolite was diffusion controlled. Data from this study indicated the potential use of the tested natural zeolite as an NH+4 adsorbent and a controlled‐release NH+4 fertilizer.
Soil Nitrogen, Microbial Biomass, and Respiration along an Arctic Toposequence
Authors: Cheng Weixin , Virginia R. A. , Oberbauer S. F. , Gillespie C. T. , Reynolds J. F. , Tenhunen J. D.
Published: July 2010

To investigate the interactions among mineral N, C availability, microbial biomass, and respiration in arctic soils, we sampled soils five times during a growing season from a toposequence on a slope in northern Alaska. The toposequence consisted of six vegetative types from the ridge top to the stream bank: lichen heath, dry cassiope, moist carex (Carex spp.), water track, tussock tundra (intertussock), and riparian. The spatial distribution and temporal variation of soil mineral N, microbial biomass, soil C availability, and C turnover were soil type dependent. During the growing season, the concentration of soil NH+4‐N decreased in tussock tundra soils but increased in lichen heath soils. Soil C availability at all locations was the highest at the beginning of the growing season and declined thereafter. The C availability index (CAI) and the potential C turnover rate increased as soils became wetter. Tussock‐forming tundra soil was generally colder than other sites and had high C/N ratios, low amounts of mineral N, and a low potential C turnover index, and therefore, was the least biologically active type. In contrast, water track was the most biologically active site in the sequence and had the highest C and N availability, the highest potential C turnover index, and the highest microbial biomass C and N. The mosaic of diverse plant communities and soil types that comprise arctic landscapes necessitates that accurate estimates of large‐scale C or N budget can only be made by integration of all types of plant communities and soils.
Microvariability in Soil Test, Plant Nutrient, and Yield Parameters in Bermudagrass
Authors: Raun W. R. , Johnson G. V. , Lees H. L. , Sembiring H. , Phillips S. B. , Solie J. B. , Stone M. L. , Whitney R. W.
Published: July 2010

The scale or resolution where distinct differences in soil test and yield parameters can be detected has not been thoroughly evaluated in crop production systems. This study was conducted to determine if large differences in soil test and forage yield parameters were present within small areas (<1 m2). A 2.13 by 21.33 m area was selected for intensive forage and soil sampling from two bermudagrass [Cynodon dactylon (L.) Pers.] pasture sites (Burneyville and Efaw, OK). Each 2.13 by 21.33 m area was partitioned into 490, 0.30 by 0.30 m (1 by 1 ft) subplots. Bermudagrass forage was hand harvested at ground level from each 0.30 by 0.30 m subplot. Prior to forage harvest, spectral radiance readings (red, 671 ± 6 nm; green, 550 ± 6 nm; and near infrared, 780 ± 6 nm) were recorded from each subplot. Composite samples composed of eight soil cores, 0 to 15 cm deep and 1.9 cm in diameter, were collected from each subplot. At both locations, bermudagrass forage yield harvested from 0.30 by 0.30 m subplots ranged from <1300 to >10 000 kg ha−1. Soil pH ranged from 4.37 to 6.29 within the 2.12 by 21.33 m area at Burneyville and 5.37 to 6.34 at Efaw. No P or K fertilizer would have been recommended at Efaw using mean soil test P and K. The range in recommended fertilizer rates would have been 0 to 31 and 0 to 17 kg P ha−1 and 0 to 107 and 0 to 108 kg K ha−1 at Burneyville and Efaw, respectively, if recommendations were based on individual 0.30 by 0.30 m grid data. Significant differences in surface soil test analyses were found when samples were <1 m apart for both mobile and immobile nutrients.
Modeling Rain Erosivity Using Disdrometric Techniques
Authors: Cerro Carlos , Bech Joan , Codina Bernat , Lorente Jeroni
Published: July 2010

The erosive effect produced by direct raindrop impact on the soil surface may be quantified in terms of the raindrop kinetic energy (KE). This variable is usually calculated from the rainfall rate even though a precise description of the raindrop size distribution is required. In this study, raindrop size and fall velocities were measured for 2 yr in Barcelona, Spain, with an optical disdrometer. Fitted drop‐size distributions were used to obtain relationships between raindrop KE and rainfall rate. A comparison between KE calculated from experimental distributions indicated that the lognormal distribution model was most suitable to provide a KE‐rainfall rate relation. Predicted KE values from this relationship had better agreement with observations (differences of 7%) than predicted values from other models based on different rainfall conditions.
Persistence of Soil Structural Modifications Along a Historic Wagon Trail
Authors: Sharratt B. , Voorhees W. , McIntosh G. , Lemme G.
Published: July 2010

Wagon wheel ruts are still visible along pioneer trails in the USA, which suggests that vehicular traffic can modify soil properties for a century. We compared physical properties of a Barnes loam (fine‐loamy, mixed Udic Haploboroll) across three transects of the 1864 to 1871 Wadsworth Trail. Compaction was evident within wheel ruts of the trail, as penetration resistance and bulk density were 10% greater and water infiltration and air permeability were 50% lower within the wheel ruts than outside the trail. Erosion was also apparent within the wheel ruts, as the greater density could not fully account for the thinner A horizon (60 mm). Our investigation suggests that degradation of soil properties caused by compaction or erosion from wagon wheel or animal traffic may persist for >100 yr. This information underscores the importance that agricultural practices must minimize soil loss or compaction.
Natural Abundance of Nitrogen‐15 in a Forest Soil
Authors: Koba Keisuke , Tokuchi Naoko , Yoshioka Takahito , Hobbie Erik Alan , Iwatsubo Goro
Published: July 2010

Because of measurement difficulties, only a few studies on natural 15N abundance (δ15N) of inorganic N in forest soil have been pursued despite its importance for interpretations of plant δ15N signatures. To investigate stable N isotope ratios in inorganic N, the δ15N values and concentrations of total N, NH+4‐N, and NO−3‐N of forest mineral soils in four profiles were measured along a slope (altitude of 765–870 m) in a coniferous (Japanese red cedar, Cryptomeria japonica D. Don) forest in Japan. Generally, the δ15N values of total N, NH+4‐N, and NO−3‐N increased with increasing soil depth. The values of δ15N ranged from 1.0 to 6.8‰, 2.5 to 15.6‰, and −14.8 to 5.6‰ for total N, NH+4‐N, and NO−3‐N, respectively. Additionally, the δ15N values were different between NH+4‐N and NO−3‐N for each soil depth. Thus, it was concluded that the assumptions about inorganic N used in interpretation of plant δ15N values were valid. Moreover, on upper slope sites where soil inorganic N was predominantly NH+4‐N, the order of δ15N was generally total N > NH+4‐N > NO−3‐N for each depth, whereas the order of δ15N was NH+4‐N > total N > NO−3‐N on lower slope sites where NH+4‐N was less dominant as soil inorganic N and relatively high net nitrification rates were measured. Our results suggested that nitrification plays an important role in regulating δ15N in forest‐soil N.
Spatial Variability of Methane, Nitrous Oxide, and Carbon Dioxide Emissions from Drained Grasslands
Authors: van den Pol‐van Dasselaar A. , Corré W. J. , Priemé A. , Klemedtsson Å. K. , Weslien P. , Klemedtsson L. , Stein A. , Oenema O.
Published: July 2010

Emissions of CH4, N2O, and CO2 from soils are the result of a number of biological and physical processes, each influenced by several environmental and management factors exhibiting spatial variability. This study aimed to assess the spatial variability and spatial dependence of CH4, N2O, and CO2 emissions and their underlying soil processes and properties from grasslands on drained peat soil (Terric Histosol). Emissions and possible controlling factors were measured at a field location in Sweden. Measurements were done on two adjacent sites on peat soil on two successive days for each site. Spatial variability was analyzed with trend analysis and variograms. Both sites consumed small amounts of atmospheric CH4, i.e., 0.03 and 0.05 mg CH4 m−2 d−1, and emitted N2O and CO2, i.e., 5 to 19 mg N2O m−2 d−1 and 4 to 6 g CO2 m−2 d−1. Spatial variability of emissions was high, with coefficients of variation of 50 to 1400%. Emissions either showed a spatial trend or were spatially dependent with ranges of spatial dependence of 50 to >200 m. However, spatial dependence of emissions showed differences between sites and short‐term temporal variability. Variograms of emissions and soil processes, which are partly biological in nature and have a high degree of inherent variability, should be interpreted with care.
Loblolly Pine Root Growth and Distribution under Water Stress
Authors: Torreano Scott J. , Morris Lawrence A.
Published: July 2010

A rhizotron study was conducted to assess the relationship between soil water availability and shoot and root growth of loblolly pine (Pinus taeda L.) under conditions where O2 availability and mechanical resistance were considered not limiting. Relationships between soil water availability and (i) root growth rate, (ii) root distribution, (iii) growth periodicity, and (iv) above‐ and belowground allocation were evaluated under four watering regimes ranging from maintaining profile near field capacity (WK 1) to withholding moisture for 4 wk (WK 4). The final distribution of cumulative root growth was similar among watering treatments, even though root growth shifted downward with depth during the growing period due to water depletion in surface soils. Relative root elongation rates were found to be linearly related to soil water potential. The strength of this relationship was improved by considering only periods of active root growth. Reductions in above‐ and belowground growth were proportional among water‐restricting treatments. Stem volume and root growth in the driest treatments were reduced 46 and 41%, respectively, compared with the WK 1 cells, indicating that patterns of biomass allocation above and below ground were similar among treatments. In the absence of other limiting soil factors, flushes of root growth in this study were a function of relatively short‐term changes in soil water potential. Our results indicate that when maximum tree biomass is desired, management efforts in establishing pine should be aimed at reducing any restrictions to root growth during establishment.
Water Content Response of a Profiling Time Domain Reflectometry Probe
Authors: Ferré P. A. , Rudolph D. L. , Kachanoski R. G.
Published: July 2010

We designed a profiling time domain reflectometry (TDR) probe. The probe includes two short rods that measure through two parallel access tubes. Small diameter wires lead to the top of the measurement interval, limiting the sensitivity of the probe to the medium above the region of interest. Larger diameter target rods span the sample interval, leading to greater sensitivity of the probe to the medium surrounding the access tubes within the measurement interval. The probe sensitivity is further increased by sealing the access tubes and filling them with water. Six prototype probes built using readily available components showed a consistent sensitivity to the soil, receiving ≈25% of the total probe response from the medium around the access tubes in the sample interval. We tested three methods of field calibration of the probe. The calibrated profiling‐probe responses produce water‐content profiles comparable to those determined with a neutron probe during the advance of a wetting front.
Sorption of Imidacloprid on Soil Clay Mineral and Organic Components
Authors: Cox L. , Koskinen W. C. , Celis R. , Hermosin M. C. , Cornejo J. , Yen P. Y.
Published: July 2010

Soil clay mineral and organic components have been shown to be responsible for sorption of many pesticides. This study assessed the influence of the different components of the clay fraction of five soils and model sorbents (montmorillonite, Fe oxide, and humic acid) and their binary and ternary associations on sorption of the insecticide imidacloprid (1‐[(6‐chloro‐3‐pyridinyl)‐methyl]‐N‐nitro‐2‐imidazoli‐dinimine). Sorption was very high on a clay fraction containing a high content of low‐laminar‐charge montmorillonite and relatively high organic C content. When organic matter of the soil clays was reduced by H2O2, sorption decreased; however, the sorption coefficient based on organic C (Koc) increased. The increase in Koc was greater on clays whose surface areas substantially increased after H2O2 treatment. This indicates a contribution of the surfaces of mineral components of the soil clay to sorption of imidacloprid. Imidacloprid sorbed on both montmorillonite and humic acid, whereas sorption on Fe oxide was very low. The sorption distribution coefficient, Kd, on montmorillonite increased from 15 to 180 when pH was decreased from 3.5 to 1.0, suggesting protonation of the ‐NH group of the imidazol ring. Sorption on binary and ternary associations of these model sorbents indicated that organic matter is the single most important component affecting sorption of imidacloprid, although there is some contribution of the mineral surfaces.
Sorption and Redox Activity of Cobalt Corrinoids on Hectorite
Authors: Kliewer K. E. , Morra Matthew J.
Published: July 2010

Halogenated synthetic organic compounds are widespread contaminants of the environment; however, little is known concerning their potential dehalogenation by extracellular corrinoids. Our primary objective was to determine if corrinoids sorbed to soil colloids are redox active and thus viable electron transfer mediators of contaminant dehalogenation in reduced soils and sediments. Dicyanocobínamide, cyanocobalamin (vitamin B12), and aquocobalamin were sorbed onto Ca2+‐, K+‐, and Na+‐hectorite and sorption isotherms determined. Additional assessment of sorption reactions was performed using x‐ray diffraction (XRD) and infrared (IR) spectroscopy. Redox states of the sorbed corrinoids in hectorite suspensions were monitored using ultraviolet and visible (UV‐VIS) spectrophotometry. Corrinoid molecular size and charge, as well as the saturating cation, controlled corrinoid sorption. The XRD results indicated that Co corrinoids intercalated hectorite, expanding the basal spacing from 1.45 to 2.21 nm for Ca2+‐, 1.17 to 2.70 nm for K+‐, and 1.31 to 2.81 nm for Na+‐hectorite. The IR spectra of bound corrinoids yielded little information on binding mechanisms, but indicated that axial ligands remained intact. The central metal of bound corrinoids was reduced to Co(I) by Ti(III) and reoxidized to Co(III) by introduction of O2, thus demonstrating that redox activity of sorbed corrinoids was maintained and reversible. Microbially produced extracellular corrinoids may act as potential electron transfer mediators when in association with clays, possibly participating in the reductive dehalogenation of organic contaminants.
In Situ Decomposition of Grass Roots as Affected by Elevated Atmospheric Carbon Dioxide
Authors: van Ginkel J. H. , Gorissen A.
Published: July 2010

The effects of elevated CO2 on belowground C input, on decomposition of roots in situ vs. decomposition of disturbed roots, and on soil microbial biomass were investigated in a perennial grass species. Forty ryegrass (Lolium perenne L.) plants were homogeneously 14C‐labeled in two controlled environments for 115 d in a continuous 14CO2 atmosphere at 350 and 700 µL CO2 L‐1 and two soil N levels (low, LN, and high, HN). Thereafter, some of the plants were destructively harvested. Undisturbed root systems of the remaining plants were incubated in situ (IRS) for comparison with a disturbed incubation of the dried and ground roots (DRS) in their original soils. At the start of the incubation, elevated CO2 had increased total 14C‐labeled soil C input by 44 and 27% at LN and HN, respectively, compared with input at ambient CO2. After incubation for 230 d, 40% of 14C soil content was mineralized to 14CO2 in the disturbed system and 52% in the intact system. Native soil organic matter (SOM) decomposition of the DRS was lower than the SOM decomposition of the IRS. The formation of 14C‐labeled soil microbial biomass (14C‐SMB) in the soil with DRS was 130% higher than in the soil with the IRS. Elevated CO2 decreased the decomposition of roots and root‐derived products by 10% and increased the size of the 14C‐SMB by 28% for both IRS and DRS, whereas the decomposition of SOM was not affected by CO2 at either LN or HN. After plant growth and in situ incubation, the 14C‐labeled C in the soil solution showed a highly positive correlation with the amount of 14C‐SMB. The ratio between 14C‐labeled microorganisms and total 14CO2 evolved was not affected by elevated CO2. It seems that microorganisms adapt to changing soil C input under elevated CO2 and there is no effect on their turnover and behavior.
Water and Nitrogen Management of Sugarcane Grown on Sandy, High‐Water‐Table Soil
Authors: Obreza T. A. , Pitts D. J. , Anderson D. L.
Published: July 2010

Little information exists regarding water and N fertilizer management for sugarcane (Saccharum spp.) production on Florida's high‐water‐table sandy soils. We hypothesized that sugar yield and N‐use efficiency would be affected by water table depth and N fertilizer application timing. Sugarcane (cv. CP 72‐1210) was grown in >1‐ha plots for three seasons on Basinger sand (siliceous, hyperthermic Spodic Psammaquent) to determine the effects of water table depth (0.46 vs. 0.57 m), N fertilization frequency (13 vs. 7 split applications for 3 yr, at 224 kg N ha‐1 yr‐1), and Mg fertilizer rate (0 vs. 60 kg Mg ha‐1 yr‐1) on cane and sugar yields. Annual mean high‐ and low‐water‐table differences were 0.13, 0.11, and 0.10 m, resulting in a 0.2 to 1.4 J kg‐1 difference in soil water matric potential at middle of the root zone, and a 0.02 to 0.11 m3 m‐3 difference in soil water concentration in the top 0.30 m. Three‐year mean yields for low vs. high water table were 73.7 vs. 67.9 t sugarcane ha‐1 and 9.23 vs. 8.51 t sugar ha‐1. High vs. low N fertilization frequency yielded 75.0 vs. 66.5 t sugarcane ha‐1 and 9.41 vs. 8.33 t sugar ha‐1. There were no water level × N fertilization frequency interactions. Where mean Mehlich 1 extractable Mg was 25 mg kg‐1, Mg fertilization did not affect yield, suggesting that this Mg level should be classified in the unresponsive (“high”) range. Although increasing N fertilization frequency increases the fertilization program cost, its use is justified by increased sugar yield.
Predicting Long‐Term Soil Carbon Storage from Short‐Term Information
Authors: Hyvönen Riitta , Ågren Göran I. , Bosatta Ernesto
Published: July 2010

Changes in land use and in climate may induce changes in soil organic matter storage in different ecosystems. Predictions of future soil organic matter storage are based on extrapolations from short‐term observations. Certain errors in the extrapolation procedure will have larger consequences than others. We analyzed this error propagation using the continuous‐quality theory. We show that parameter values that predict litter‐decomposition rates within 5% for 2 yr can give rise to errors in steady‐state soil organic matter stores of a factor of three or more, whereas comparable errors in parameter estimates derived from decomposition studies of whole soil organic matter keep the deviations in soil organic matter within 10%. The processes that have to be quantified most accurately are those regulating the interaction between soil organic matter and soil texture and those regulating the rate of change in the quality of the soil organic matter during decomposition. The error propagation from errors in decomposer growth rate, and hence climatic factors, and litter quality is limited, but that from decomposer efficiency is somewhat higher. We also compared the continuous‐quality approach with some other models.
Preferential Flow in Water‐Repellent Sands
Authors: Bauters Tim W. J. , Steenhuis Tammo S. , Parlange Jean‐Yves , DiCarlo David A.
Published: July 2010

Water‐repellent soils occur all over the world and affect both groundwater pollution and crop yield. The finger‐like wetting patterns in these soils have many similarities with unstable wetting fronts in coarse‐grained sandy soils. Our objectives were to study the water movement in water‐repellent sand and to examine how the theory for unstable wetting fronts applies to water‐repellent sands. Infiltration experiments, in which moisture content and matric potentials were measured, were carried out in slab chambers with identical sands but with different levels of water repellence. Soil water characteristics were determined in separate experiments. Infiltration in the hydrophilic soil resulted in a uniform and horizontal front. All water‐repellent sands showed a fingered flow pattern. For negative water‐entry values, water infiltrated without delay. For positive values, water entered the soil only after the depth of the ponded water equaled or exceeded the water‐entry pressure, which increased with increasing repellency. The finger widths predicted with the unstable flow theory agreed rather well with the observed values. In general, the research showed that the wetting patterns of water‐repellent sands depended directly on the soil water characteristic curve. This implies that the type of wetting front and risk to groundwater pollution can be predicted based on laboratory‐measured soil hydraulic properties.
A Field Apparatus for Measuring Unconfined Compressive Strength
Authors: Cook B. J. , Nimlos T. J.
Published: July 2010

Laboratory techniques for measuring unconfined compressive strength are standardized by testing groups (e.g., American Association of State Highway and Transportation Officials), often requiring technologically complex and expensive equipment. However, much of this technology is not available in developing countries. We describe the construction, operation, calibration, and application of a modified hydraulic jack for determining unconfined compressive strength. Three calibration curves are presented to provide options for laboratories with varying technological capabilities. Application shows that variation in measurements comes from natural variation within materials rather than from the modified jack. Our modified jack is simple, inexpensive, and precise. Testing shows it to have a high correlation to standard laboratory techniques (R2 = 0.89) and establishes it as a reliable measure of unconfined compressive strength.
Reaction Sequence of Nickel(II) with Kaolinite: Mineral Dissolution and Surface Complexation and Precipitation
Authors: Eick Matthew J. , Fendorf Scott E.
Published: July 2010

The dissolution kinetics of Ni(II) sorbed to kaolinite at pH 7 were examined as a function of initial aqueous Ni(II) concentrations; the local molecular structure of the Ni surface complexes were determined using extended x‐ray absorption fine structure (EXAFS) spectroscopy. The dissolution of kaolinite was nonstoichiometric with a preferential release of Si over Al. The dissolution rate, RSi, increased with an increase in the aqueous concentration of Ni. A rate law was developed representing a fractional‐order dissolution reaction with respect to the surface concentration of Ni. Absorption spectra were obtained from kaolinite samples washed with 0.10 M NaNO3 adjusted to pH 3.4 to remove amorphous Al‐hydroxide surface deposits or adsorbed Al and from kaolinite that was untreated. For all samples, EXAFS results revealed the presence of multinuclear Ni surface complexes with a similar, but not identical, local structural environment to pure crystalline Ni(OH)2. The Ni‐Ni bond distances were shorter (3.06–3.11 Å) than in Ni(OH)2(s) (3.13 Å) and increased with an increase in surface coverage. Additionally, Al was present in all but the highest surface coverage as a second neighbor backscatterer. At the lower surface coverages (Γ ≤ 0.406 µmol m‐2), Al is present at 2.69 Å, indicating the formation of a bidentate Ni surface complex. With increased surface coverage, the d(Ni‐Al) increases to 2.96 Å, which is believed to result from a Ni,Al‐hydroxide precipitate. No significant structural differences were found between kaolinite pretreatments. Based on these results, a hypothesized mechanism for Ni sorption on kaolinite is proposed.
Flow and Reactivity Effects on Dissolved Organic Matter Transport in Soil Columns
Authors: Weigand Harald , Totsche Kai U.
Published: July 2010

Dissolved organic matter (DOM) plays a prominent role in the transport of contaminants in porous media. As DOM has to be considered as a reactive component, flow regime and sorbent reactivity should affect overall DOM transport in an important way. We focused on DOM transport in unsaturated column experiments using quartz sand (QS) and goethite‐coated quartz sand (GS). Rate constrictions to DOM sorption were investigated by varying the volumetric flow rate, while extent and reversibility of sorption were studied in consecutive adsorption and desorption steps. In the QS, DOM retention was low and unaffected by changes in flow rate. Desorption‐step breakthrough curves (BTCs) and mass balances show full reversibility of the sorption process. However, DOM retention in GS was significant and sensitive to flow variation, indicative of nonequilibrium sorption. At lower flow rates, DOM breakthrough exhibited a change in curvature (shoulder) due to the superimposition of two BTCs representing reactive and nonreactive DOM fractions. Transport was successfully modeled assuming these two fractions governed overall DOM mobility. At higher flow rates, the BTC shoulder vanished due to reduced contact time between the DOM and the solid phase (rate‐limited sorption). Sorption of DOM on GS is accompanied by a marked rise in effluent pH, indicative of a ligand‐exchange mechanism. Recovery of DOM during desorption was incomplete due to either partially irreversible sorption or strongly rate‐limited desorption. Increased DOM mobility in the consecutive adsorption step resulted from partial blocking of sorption sites by the initial pulse of DOM.
Estimating Calcium‐Magnesium Selectivity in Smectitic Soils from Organic Matter and Texture
Authors: Curtin D. , Selles F. , Steppuhn H.
Published: July 2010

Selectivity coefficients are essential to describe the distribution of cations between the exchange and solution phases of soil. Because the factors influencing Ca‐Mg exchange have not been adequately defined, models commonly incorporate the dubious assumption that soils do not discriminate between these two cations (i.e., the selectivity coefficient is 1). The purpose of this study was to identify key variables influencing Ca‐Mg selectivity in smectite‐dominated soils of the Canadian prairies. Selectivity coefficients (K) were measured by reacting soils that differed in texture (50–500 g clay kg‐1), organic matter (4–96 g C kg‐1), and pH (4.1–7.3) with solutions containing mixtures of CaCl2 and MgCl2. The value of K for the reaction Ca + Mg‐soil ⇌ Mg + Ca‐soil ranged from 1.1 to 3.4, depending on soil type and the composition of the reacting solution. Preference for Ca increased as the equivalent fraction of Ca in solution decreased from 0.8 (average K = 1.7) to 0.05 (average K = 2.9). We identified organic matter as the major source of Ca‐preferring sites. The ratio of organic matter to clay was the best single indicator of Ca‐Mg selectivity. Measurements on soils whose pH had been altered by laboratory addition of Ca(OH)2 or field application of anhydrous NH3 indicated that selectivity was independent of pH, even though the proportion of organic sites increased as pH increased. A simple two‐site model with K of 3.9 for organic matter and K of 1.3 for clay described our data reasonably well, although selectivity for Ca tended to be underpredicted at low Ca loading.
Increased Soil Enzyme Activities under Two Row Crops
Authors: Bergstrom D. W. , Monreal C. M.
Published: July 2010

This study examined the influence of crop on the spatial pattern of soil enzyme activities in a Rego Humic Gleysol (Aquoll) strip‐cropped to corn (Zea mays L.) and soybean [Glycine max (L.) Merr.]. Soil samples (0–23‐cm depth) were collected at 20‐cm spacings along transects across adjacent plant rows. Activities of dehydrogenase, urease, glutaminase, phosphatase, arylsulfatase, and β‐glucosidase were compared for soil samples collected from within vs. between crop rows. Spectral analysis was used to test for a cyclical pattern of soil enzyme activities along transects. The spatial pattern of all six soil enzyme activities was sometimes influenced by the crop. Greater enzyme activity in the row than in the furrow was observed in approximately one‐third of the cases examined. In approximately two‐thirds of the cases where the class comparison indicated greater enzyme activity in the row than in the furrow, enzyme activity appeared to cycle at a period equal to the row spacing. Periodograms also indicated cycling at other periods, and there was considerable random variation in enzyme activities along the transects. Dehydrogenase and β‐glucosidase activities differed between strips of corn and soybean on some sampling dates. The spatial pattern of soil enzyme activities indicated that they behaved similarly as general indices of microbial activity at the scale of measurement. Comparisons of soil enzyme activities between sites within the growing season should account for influence of the crop.
Arbuscular Mycorrhizae and Water Stress Tolerance of Wyoming Big Sagebrush Seedlings
Authors: Stahl Peter D. , Frost Sandra M. , Williams Stephen E. , Schuman Gerald E.
Published: July 2010

Although Wyoming big sagebrush (Artemisia tridentata Nutt. ssp. wyomingensis Beetle and Young) is widespread in the western USA, reestablishment of this native shrub on disturbed lands by direct seeding is problematic. A number of theories have been proposed to explain this difficulty. Included are the hypotheses that seedlings are unable to obtain adequate moisture and are handicapped by reduced levels of mycorrhizae in perturbed soils. We conducted a greenhouse study to examine the influence of vesicular arbuscular mycorrhizae (VAM) and seedling age on soil moisture stress tolerance of Wyoming big sagebrush seedlings. Results demonstrated greater survival of mycorrhizal seedlings than nonmycorrhizal seedlings as soil dried down past soil water potential values of ‐2.5 MPa to as dry as −3.8 MPa. For all different aged seedlings tested (30, 45, 60, 90, 120, 150 d), the degree of soil dryness resulting in death of mycorrhizal seedlings was significantly greater (P < 0.01) than that causing death of nonmycorrhizal seedlings. Analysis of variance indicated a significant interaction of seedling age and mycorrhizae on moisture stress tolerance. Experimental data suggest that as sagebrush seedlings age, the beneficial influence of arbuscular mycorrhizae on soil water stress tolerance increases.
Ammonia Volatilization from Urea Amended with Triple Superphosphate and Potassium Chloride
Authors: Ouyang D. S. , Mackenzie A. F. , Fan M. X.
Published: July 2010

Ammonia volatilization from urea reduces N fertilizer efficiency of crops. Addition of acidic materials to urea may reduce NH3 volatilization and improve N use efficiency. The objectives of this study were to compare the effects of compacted or blended mixtures of urea and triple superphosphate (TSP), urea and KCl, or urea, TSP, and KCl on NH3 volatilization and N use efficiency by corn (Zea mays L.) under a no‐till system. A Ste. Rosalie clay (Typic Humaquept) and an Ormstown silty clay loam (Typic Humaquept) were used with three rates of TSP (0, 26, and 52 kg P ha‐1) and two rates of KCl (0 and 100 kg K ha‐1). Total NH3 volatilization for 9 d following fertilization ranged from 27 to 31 g kg‐1 of added urea‐N on the Ste. Rosalie soil and from 50 to 82 g kg‐1 on the Ormstown soil with urea alone. Ammonia volatilization decreased when TSP, KCl, or both were applied with urea. Nitrogen recovery of added urea, N use efficiency, and corn yields increased with added TSP and KCl. Compacted mixtures of urea with TSP, KCl, or both had an advantage over blended fertilizers in reducing NH3 volatilization and improving N use efficiency and corn yields.
Impact of Saturated Hydraulic Conductivity on the Prediction of Tile Flow
Authors: Mohanty B. P. , Skaggs T. H. , van Genuchten M. Th.
Published: July 2010

Preferential flow through macropores and other structural voids in field soils most often occurs at or near saturation. Our earlier research revealed significant differences in the value of the saturated hydraulic conductivity (Ks) of a glacial till soil in central Iowa when obtained with five different measurement techniques. The five techniques included one laboratory constant‐head permeameter method and four in situ methods: disc permeameter, Guelph permeameter, velocity permeameter, and double‐tube permeameter. Differences in measured Ks values were attributed to differences in sample size, the existence or absence of open‐ended macropores, and measurement principles. In this study, we used the different Ks estimates in a two‐dimensional numerical model, CHAIN_2D, to predict water flow into a subsurface tile drain in the same field. Comparisons between predicted and observed tile flows were made during four crop growing seasons. Preferential flow observed in the tile drain during large storm events was predicted best by the model when using Ks values measured with the disc permeameter method, which least disturbed the boundary conditions of the flow field and better accounted for the macropore structures of the field soil. Quantitative and qualitative findings suggest that the disc permeameter was best suited for the field site.
Determination of Soluble Phosphorus in the Presence of Organic Ligands or Fluoride
Authors: He Z. L. , Baligar V. C. , Ritchey K. D. , Martens D. C.
Published: July 2010

In the ascorbic acid‐molybdenum blue method for measurement of soluble P, the relationship between color intensity and P concentration in solution and color stability is greatly affected by organic and inorganic ligands such as oxalate, citrate, tartrate, or F. The objective of this study was to evaluate the effect of these ligands and to optimize the conditions for determination of inorganic P in the presence of the interfering ligand. The critical concentrations of the ligand at which P recovery was significantly decreased were 1.5 mM for oxalate, 3.0 mM for citrate or tartrate, and 10 mM for fluoride. The interference of the ligand with P determination was overcome by excess amounts of ammonium molybdate (AM) added before the color developing reagent (CDR). The critical molybdate/ligand molar ratios (excluding the amount of molybdate added in the CDR) required to completely eliminate the interference of the ligand were 1.12 for oxalate, 0.84 for citrate, 0.56 for fluoride, and 0.34 for tartrate. The presence of ligand and excess amount of AM did not affect the linearity of color intensity against P concentration but did alter the slope, suggesting that the modified method is reliable for P determination in the presence of an interfering ligand provided that the standard curve is prepared in the same matrix as the sample.
Quantification of Biodegradable Dissolved Organic Carbon in Soil Solution with Flow‐Through Bioreactors
Authors: Yano Yuriko , McDowell William H. , Kinner Nancy E.
Published: July 2010

A flow‐through bioreactor method for drinking water or stream water was modified for the rapid determination of biodegradable dissolved organic C (BDOC) in small, discrete samples such as soil solutions collected in lysimeters. Biodegradable dissolved organic C was measured as the difference in dissolved organic C (DOC) concentration between the inlet and outlet when a sample was pumped through a column colonized by indigenous soil microbes. Both feed water and sample solutions were adjusted to ≈ 10 mg C L‐1 to maintain a consistent environment in the column, and inorganic nutrients (N, P, S, and K) were added to avoid a nutrient limitation on C degradation. Biodegradable dissolved organic C values obtained by this method were consistent for a year and were insensitive to changes in ambient temperature (13–24°C) and flow rate (0.36–0.42 mL min‐1). Dextrose, humic acid, and dextrose‐humic acid solutions were used to test the bioreactors. Only 5% or less of the humic acid C was lost in the bioreactor, suggesting limited abiotic sorption of C. Nearly 100% loss of dextrose, coupled with large increases in dissolved inorganic C (DIC), suggested biotic degradation of BDOC within the bioreactors.
Long‐Term Effects of Fertilization and Rotation on Denitrification and Soil Carbon
Authors: Drury C. F. , Oloya T. O. , Tan C. S. , van Luyk C. L. , McKenney D. J. , Gregorich E. G.
Published: July 2010

Various long‐term crop management strategies are known to have differing effects on soil organic C. This laboratory study explored the effect of long‐term (35 yr) fertilization and crop rotation on soil organic C and denitrification capacity at different depths of a Brookston clay loam soil (fine‐loamy, mixed, mesic Typic Argiaquoll). We related denitrification capacity to soil biochemical (CO2 production, organic C, microbial biomass C, soluble organic C) and soil structural properties. Denitrification capacity was determined as the increase in N2O that occurred when NO‐3‐amended soils were incubated anaerobically in the presence of acetylene. Treatments included fertilized and nonfertilized plots of continuous corn (Zea mays L.), continuous bluegrass (Poa pratensis L.), and rotation corn (corn‐oat [Avena sativa L.]‐alfalfa [Medicago sativa L.]‐alfalfa). Soils from an adjacent mixed deciduous woodlot were also sampled. Soils from the woodlot had higher denitrification capacities than the continuous or rotation corn treatments. Among the agricultural treatments, the soil under bluegrass had the greatest denitrification capacity followed by the soil under corn rotation, with the continuous corn having the lowest capacity. Long‐term fertilization resulted in 35% higher denitrification capacity and 65% higher CO2 production than nonfertilized soils. Denitrification capacity across all depths in the agricultural soils was correlated with CO2 production (r2 = 0.76), microbial biomass C (r2 = 0.60), and organic C (r2 = 0.54); however, the relationship between denitrification capacity and soil structure was not as strong (r2 = 0.28).
USLE‐M: Empirical Modeling Rainfall Erosion through Runoff and Sediment Concentration
Authors: Kinnell P. I. A. , Risse L. M.
Published: July 2010

Runoff is a factor in rainfall erosion, and models that include explicit consideration of runoff provide better predictions of erosion than models like the Universal Soil Loss Equation (USLE) and the revised USLE (RUSLE) that do not. The USLE‐M is a modified version of the USLE based on the rainfall erosivity factor for an event, which is the product of the 30‐min erosivity index (EI30) and the runoff ratio (QR). As a consequence, USLE‐M contains explicit consideration of the effects of soil, crop, and crop management on runoff and sediment concentration. Also, the explicit consideration of runoff means that values of many of the other factors used in the USLE have to be reevaluated because the effects of variations in factors like soil and crop management on runoff are now being treated separately from the effects on sediment concentration. We present examples of new values for the soil erodibility (K) and crop management (C) factors resulting from an analysis of data from USLE runoff and soil loss plots at a number of locations in the USA in terms of the new modified version of the USLE. The new K values varied from 1.4 to 3.9 times the USLE K values. The new C values varied from 1.1 to 32.3 times the USLE C values. Procedures are also outlined for obtaining first approximations of the new K and C values from existing USLE values. Because of the explicit consideration of runoff, USLE‐M will, as a general rule, predict event soil loss better than the USLE. The USLE and USLE‐M are equally effective in predicting erosion for impervious conditions (QR = 1) but the efficiency of the USLE decreases as the proportion of the rain infiltrating increases, while that of the USLE‐M does not.
Harvesting Effects on Long‐Term Changes in Nutrient Pools of Mixed Oak Forest
Authors: Johnson D. W. , Todd D. E.
Published: July 2010

The effects of sawlog harvesting (SAW) vs. whole‐tree harvesting (WTH) on C and nutrient budgets of a mixed oak forest near Oak Ridge, TN, were assessed by sampling soils and vegetation just prior to and 15 yr after harvesting. Fifteen years after harvest, large woody residues in the SAW treatment lost >80% of their mass and nutrient content. Greater concentrations of Ca, K, and Mg were found in both foliage and soils in the SAW treatment than in the WTH treatment, but there were no signs of deficiency in these nutrients and no differences in growth due to treatment. There were nod eclines in exchangeable Ca2+ in the WTH treatment, and the increases in soil exchangeable Ca2+ pools in the SAW watersheds were approximately equal to the amount of Ca2+ released by decomposing residues. On the other hand, no treatment effects on soil C, vegetation biomass, species composition, vegetation N or P concentration, soil bulk density, or soil N were found 15 yr after treatment. Contrary to what has been speculated on regarding the basis of nutrient budgets, this study showed no detrimental effects of WTH on productivity or soil nutrient status (i.e., reductions from preharvest conditions). The results did show, however, that nutrients (especially Ca) from logging residues are largely retained in soils and reflected in elevated foliar nutrient concentrations.
Wheat Root Growth Responses to Enhanced Ammonium Supply
Authors: Gao S. , Pan W. L. , Koenig R. T.
Published: July 2010

Slowing nitrification with a nitrification inhibitor (NI) provides an enhanced NH4 supply (EAS), which has improved N uptake efficiency and crop yield. A physiological response to EAS has been demonstrated in container experiments with restricted rooting volumes and uniform N distribution, but less is known about root responses to EAS when distributions of N forms are not uniform. Greenhouse and 2‐yr field experiments were conducted to determine how the root system of wheat (Triticum aestivum L.) responds to EAS. Three N sources: (i) calcium nitrate (NO3); (ii) urea with a nitrification inhibitor, nitrapyrin (NH4 + NI); and (iii) 50% NO3 + 50% NH4 + NI were uniformly incorporated in the field into the top 20 cm of a Shano silt loam soil (Coarse‐silty, mixed, superactive, mesic Xeric Haplocambid). Urea with a NI increased NH4 in the top 30 cm of soil and increased root surface area densities in this soil layer at the boot stage in both years. Rooting was either decreased or not affected by NH4 + NI relative to NO3 in the 30‐ to 90‐cm subsoil. In a greenhouse experiment, NO3, NH4 + NI, or NH4 + NI + CaCl2 were uniformly mixed into the top 20 cm of soil (diffused) or banded at the 20‐cm depth in rooting‐boxes (5 by 17 by 70 cm). Nodal root density at boot stage increased in the fertilized zones with NH4 + NI, mostly in the presence of CaCl2, compared with NO3, while seminal roots in the fertilized zones were only enhanced by banded NO3. Wheat root types responded differentially to N forms and the associated differences in vertical distribution.