bluuebunny/crossref_metadata_2025 huggingface dataset:
Carbon Sequestration in an Aggrading Forest Ecosystem in the Southeastern USA
Authors: Huntington Thomas G.
Published: July 2010

An analysis of C pools at the Panola Mountain Research Watershed (PMRW) near Atlanta, GA, indicates that aggrading forests in the U.S. Southeast are an important regional C sink. The forests in this area were cut in the early 1800s and the land was cultivated until the early 1900s, when farming was abandoned and forest regeneration began. Cultivation resulted in extensive erosion, which depleted soil C pools. The rate of soil C sequestration during the 70‐yr period of forest regeneration was estimated to be between 0.34 (standard error [SE] = 0.12) and 0.79 (SE = 0.19) Mg C ha−1 yr−1. There is a large potential for continued C accumulation in the soil at PMRW based on the difference between current measured soil C pools of 82 Mg C ha−1 at PMRW and 122 Mg C ha−1 at the nearby “undisturbed” Fernbank Forest in Atlanta, GA. The rate of C sequestration in biomass at PMRW was 1.47 Mg C ha−1 yr−1 for the regeneration period, bringing the ecosystem total to between 1.81 and 2.26 Mg C ha−1 yr−1. Carbon sequestration in temperate forest ecosystems partially mitigates the effects of increased atmospheric loading of CO2.
Mineralization, Nitrification, and Denitrification in Histosols of Northern Minnesota
Authors: Zanner C. W. , Bloom P. R.
Published: July 2010

Water content and temperature are influences on N mineralization, nitrification, and denitrification rates in drained Histosols. To look at possible differences in these rates and to enable comparisons to mineral soils, we collected soil samples from two Minnesota Histosols planted in cultivated wild rice (Zizania palustris L.), incubated amended subsamples at 4, 7, 10, 16, and 22°C, and monitored changes in NH4‐N with time. Amendments used were plus or minus NH4 and plus or minus nitrapyrin. Net mineralization rates for Soil 1 (plus‐nitrapyrin treatment) increased from 0.30 to 2.0 mg N kg−1 dry soil d−1 from 4 to 22°C. Nitrification and denitrification occurred simultaneously, with lower denitrification than nitrification rates. Nitrification rates (determined from the zero‐order plots of the disappearance of NH4‐N with time in the plus‐NH4 samples) increased from 4 to 22°C (Soil 1, 17–99 mg N kg−1 d−1; Soil 2, 11–40 mg N kg−1 d−1). Soil 1 nitrified faster than most mineral soils; Soil 2 was comparable with many mineralsoils. Denitrification rates for Soil 1, determined from the decrease in total inorganic N (NH4‐N + NO3‐N; plus‐NH4 samples), increased from 9 mg N kg−1 d−1 at 4°C to 53 mg N kg−1 d−1 at 22°C. There were no differences between the two experimental water contents (corresponding to soil water pressure heads of ‐25 and −50 cm) for any of these rates. The high water‐holding capacity of Histosols increases the potential for simultaneous nitrification and denitrification and will result in removal of native and applied NH4 from the system, thus affecting N use efficiency of both wild rice and upland crops grown on Histosols.
Enhanced Transport of Atrazine Under Irrigation with Effluent
Authors: Graber E. R. , Gerstl Z. , Fischer E. , Mingelgrin U.
Published: July 2010

Enhanced transport of atrazine (2‐chloro‐4‐(ethylamino)‐6‐(isopropylamino)‐s‐triazine; C8H14CIN5) applied to a corn (Zea mays L.) field was observed after irrigation with secondary effluent. Cores to 4 m were obtained from 10 effluent‐irrigated and 10 high‐quality water‐irrigated plots after two growing seasons and two winter rainy seasons. In most of the effluent‐irrigated cores, atrazine was widely distributed with depth, with an average center of atrazine mass at 115 ± 39 cm and a mean variance around the center of mass of 142 cm. In the high‐quality water‐irrigated cores, atrazine was concentrated in the upper soil horizons with an average center of mass at 63 ± 64 cm and a mean variance around the center of mass of 58 cm. The mean centers of atrazine mass in the effluent‐irrigated profiles and high‐quality water‐irrigated profiles are significantly different (Student's t‐test, P = 0.06). The equilibrium sorption isotherm for atrazine in the presence of high‐quality water was nonlinear, with a Freundlich n of 0.87 and a distribution coefficient (Kf) of 1.07. The atrazine isotherm in the presence of effluent had a similar n (0.86) but a significantly lower Kf of only 0.93. Atrazine at a rate of 4 mg/kg−1 was added to soil columns filled with the < 1‐mm size fraction of the 0 to 30‐cm horizon of soil from an uncultivated site adjacent to the field site. Three replicate columns were leached with 0.85 pore volumes of either secondary effluent or 0.005 M CaCl2 solution at constant head. Atrazine in the effluent‐treated columns was more strongly leached from the upper columns and peaked at higher concentrations lower in the columns.
Soil Water Bypass and Solute Transport under Irrigated Pasture
Authors: Prendergast J. Bernard
Published: July 2010

Downward water flow through soil transports solutes and suspended matter. The velocity of flow can be critical in the process of groundwater pollution, and can determine how effectively this water leaches the root zone of crops. This study quantifies fast soil water fluxes (bypass), and provides some insight into the effectiveness of this bypass in leaching of salt. Three water salinity treatments were imposed on replicated 75‐m2 plots growing irrigated pasture. A tritium‐labeled irrigation was applied to the plots to quantify bypass flow. Penetration of tritium‐labeled water decreased with increased salinity treatment (P < 0.01) for up to 50 d after irrigation application, despite a greater leaching fraction under saline conditions. Bypass decreased with increased salinity because there was higher antecedent water content (P < 0.02) due to lower crop water use, resulting in less soil cracking under saline conditions. Bypass flow was also calculated from a model that used the profile of Cl− concentration below the root zone. However, calculations from the model failed to agree with the experimental results, including the direct measurement of bypass made from recovery of the applied radiotracer. The model failed because it assumed that bypass was of low salinity, whereas the experiment indicated that bypass contributed to leaching. Therefore it was concluded that bypass is best defined in hydraulic terms, rather than in terms of salt leaching. It was also concluded that the Cl− concentration in bypass flow reflected the concentration in the soil matrix. Therefore total water loss below the root zone was adequately quantified by a leaching model of the Cl− mass balance, incorporating the volume‐averaged soil water Cl− concentration.
Determining Manganese Oxidation State in Soils Using X‐ray Absorption Near‐Edge Structure (XANES) Spectroscopy
Authors: Schulze D. G. , Sutton S. R. , Bajt S.
Published: July 2010

The redox chemistry of soil Mn is important in Mn uptake by plants, the movement of trace elements absorbed on or occluded in Mn‐oxides, and the etiology of some soil‐borne plant fungal diseases. The objectives of this study were to develop calibration curves for determining Mn oxidation states in moist soil samples using micro‐x‐ray absorption near‐edge structure spectroscopy and to test the method on selected soils. Oxidation‐state standards were prepared by mixing dry powders of MnSO4·H2O (Mn2+) and a synthetic Na‐birnessite, Na4Mn14O27·9H2O (Mn4+) to obtain various Mn2+ mole fractions, f(Mn2+), where f(Mn2+) = Mn2+/(Mn2+ + Mn4+). Corundum (α‐Al2O3) was used as a diluent to prepare mixtures containing 50, 1000, and 100 000 mg Mn kg−1. Quantitative analysis of Mn oxidation state is possible using either the energy of the pre‐edge peak at 6540 eV or a ratio based on the intensity of the Mn2+ white line peak at 6552.6 eV divided by the intensity of the Mn4+ white line peak at 6560.9 eV. The white line intensity ratio is more sensitive, with an estimated lower detection limit of 20 mg Mn kg−1 using a 300 by 300 µm spot, and predicts f(Mn2+) with an accuracy of ±0.1 mole fraction (95% confidence interval) through most of its range. The f(Mn2+) for four air‐dried Indiana surface soils ranged from 0.26 to 0.44 ± 0.1. Saturation and reduction for 5 d resulted in complete reduction of Mn in the soil matrix.
Pyrite Enhancement of Phosphorus Availability from African Phosphate Rocks: A Laboratory Study
Authors: Lowell K. , Weil R. R.
Published: July 2010

Phosphate rock (PR) and pyrite deposits, indigenous to many African countries, may be useful to resource‐poor farmers for increasing the P and S fertility of highly weathered tropical soils. Combination of PR and pyrite was examined as a means to increase the availability of P from five PR of African origin. Pyrite and PR were combined in several ratios (P/S) and incubated. Mixtures were periodically leached on a vacuum extractor with 0.01 M KCl for a 6‐mo period. Soluble P and pH of the leachate were measured. The pH of the leachates decreased with increasing levels of pyrite. In all cases, soluble P measured in the leachate increased with increasing levels of pyrite. Soluble P measured in the leachate was greatest from Togo and Uganda PR mixtures, much less from Zimbabwe PR, and virtually nil in all but the highest pyrite treatments for both Tanzania and Malawi PR mixtures. Citrate‐soluble P was a less reliable predictor of P release than total P and the percentage of CO2, Al, and Fe in the PR and associated minerals. High pyrite levels with low‐quality rocks generated P release comparable with that from untreated high‐quality rocks. The addition of Fe from the pyrite apparently did not lead to precipitation of substantial amounts of P as it was released from PR. The rocks responded very differently to the pyrite treatment. Although the method is promising for some rocks (Togo and Uganda), it is obviously not useful for other rocks (Malawi).
Influence of Soil Properties on the Biogeochemistry of a Beaver Meadow Hydrosequence
Authors: Johnston C. A. , Pinay G. , Arens C. , Naiman R. J.
Published: July 2010

Previous research about the biogeochemistry of beaver (Castor canadensis Kuhl) impoundments has generally overlooked the influence of soil type on solutes. In this study, soils derived from glacio‐lacustrine and glacio‐fluvial parent materials in northern Minnesota beaver meadows (i.e., former beaver ponds that have drained and revegetated) were studied to: (i) describe soil morphology, (ii) analyze soil and solute chemistry, and (iii) statistically evaluate the relative influence of hydrology and soil type on solute chemistry. With decreasing depth to groundwater, soils in both hydrosequences were increasingly reducing; redox potentials were consistently negative in the Borosaprist. Differences in soil morphology associated with increasing wetness included: (i) increasing thickness of O and A horizons, (ii) decreasing thickness of the Bt horizon (glacio‐lacustrine hydrosequence), (iii) disappearance of E and Bk horizons (glacio‐lacustrine hydrosequence), and (iv) decreasing depth to redoximorphic features. In the glacio‐lacustrine hydrosequence, depth to subsurface concentrations of oxalateextractable Fe (≥800 g m−3) decreased with increasing wetness. Two‐way ANOVA indicated that differences in water chemistry among soils were due to moisture, parent material, and their interaction. Increasing moisture was associated with increased concentrations of Fe2+, Ca2+, and Mg2+ and decreased concentrations of SO4‐S. Glaciolacustrine soils contained higher concentrations of base cations (K+, Ca2+, and Mg2+) than did glacio‐fluvial soils, and so did their soil water. There were pronounced seasonal variations in cation concentrations and significant interannual differences for NH4‐N, total N, and K+. Nitrate concentrations were consistently low and did not differ significantly among any of the groupings.
The Soil Freezing Characteristic: Its Measurement and Similarity to the Soil Moisture Characteristic
Authors: Spaans Egbert J. A. , Baker John M.
Published: July 2010

A soil freezing characteristic (SFC) represents the relationship between the quantity and the energy status of liquid water in frozen soil. The SFC is the analogue to the soil moisture characteristic (SMC) and is essential to modeling the transport of water, heat, and solutes in frozen soil. This paper presents a new, automated technique to measure an SFC in situ, for which there has previously been no method. Liquid water content in frozen soil was measured with time domain reflectometry. The corresponding energy status was inferred from accurate soil temperature measurements with a generalized form of the Clapeyron equation. Since both SFC and SMC describe water retention properties in soil, their similarity was investigated. The SMC and SFC agreed to within 1% moisture content across a wide range of matric potentials. Determination of the SMC is reliable at high matric potentials but becomes increasingly inaccurate and time consuming as soil dries. By contrast, the SFC determination becomes more accurate and rapid at lower matric potentials. We thus propose that water retention properties at high matric potentials are best obtained from draining and at low matric potentials from freezing.
Errors in Converting Time Domain Reflectometry Measurements of Propagation Velocity to Estimates of Soil Water Content
Authors: Hook W. R. , Livingston N. J.
Published: July 2010

The determination of soil water content by time domain reflectometry (TDR) involves the measurement of the propagation velocity of a radio frequency (RF) pulse in soil and the conversion of this measurement to an estimate of soil water content. The objective of this study was to quantify errors in this conversion. A comprehensive error analysis can only be carried out if a well‐defined measurement procedure is followed. We define and recommend such a procedure and identify the physical origin and the magnitude of each of the error sources in the procedure. Total measurement errors are calculated as the root mean square sum of each of the individual errors. We express propagation velocity measurements in terms of time intervals, that is, the travel time of an RF pulse in soil (T) relative to that in air (Tair). Our measurement and error calculation procedure is based on an analysis of our own and published data for nonclay soils, which reveals that in all cases, there is a linear relation between T/Tair and volumetric water content (θv) with a mutually independent slope and intercept. A theoretical explanation for this observation is presented. We show that the dominant measurement error source is the transition time of the TDR reflection, which in turn is a function of θv. Absolute measurement errors range from 0.015 to 0.028 m3 m‐3 if the T/Tair vs. θv intercept is known and increase to 0.023 to 0.034 m3 m‐3 if a nominal value (1.55) is used. We recommend that a T/Tair vs. θv slope value of 0.1193 (5% less than theory) be used for measurements in nonclay agricultural soils. This leads to little loss in accuracy. Measurement resolution is primarily a function of the time base error of the TDR instrument and can be as good as 0.0018 m3 m‐3.
Self‐Similitude and Fractal Dimension of Sand Grains
Authors: Barak Phillip , McSweeney Kevin , Seybold Cathy A.
Published: July 2010

Fractal theory postulates that geological materials possess a noninteger, fractal dimension describing length‐surface‐volume relations and particle‐size distribution. We examined sand grains, composed largely of quartz, from several horizons of Wisconsin soils derived from glacial outwash for fractal dimension using image analysis of reflected light micrographs with NIH Image software. No evidence was found to support other than a Euclidean, i.e., regular dimension, relating the geometric properties of: (i) measured particle perimeter as a function of magnification or (ii) particle area and perimeter. In addition, particle‐size distributions were better fit by lognormal distribution than by a fractal fragmentation dimension. Fractal theory would not appear to provide a useful description for these soil materials.
Metal and pH Effects on Adsorption of Poly(vinyl alcohol) by Silicon Oxide
Authors: Schulthess C. P. , Tokunaga S.
Published: July 2010

The adsorption of pentanol onto a smectite shows an adsorption edge at pH 10. Because adsorption similarities between smectites and Si oxides have been noted in the literature, we anticipated that poly(vinyl alcohol) (PVA) would also display an adsorption edge near pH 10 on a Si oxide (SiO2). Adsorption edges between pH 9 and 10 on Si oxide were confirmed when using two PVA polymers of 98 and 88% hydrolysis (MW = 11 000 and 10 000 g mol‐1, respectively). The decrease in adsorption at high pH is attributed to the interference of Na+ adsorption on the Si oxide. The addition of aqueous metals (Al3+, Pb2+, Cu2+, Mg2+, and Ba2+) significantly modified the adsorption envelope of PVA on the Si oxide in various ways. Three mechanisms are proposed: metals may modify the Si surface, interact with the PVA while in the aqueous phase, or form an additional independent surface that can adsorb the PVA polymer. New PVA adsorption envelopes were observed at high pH in the presence of Mg, Cu, and (adsorbed) Pb. The presence of metals also increased the amount of PVA (98% hydrolysis) adsorbed at low pH, but caused no change in, or decreased, the amount of PVA (88% hydrolysis) adsorbed on a mass per area basis. Because of this, we suggest that the metals can modify the effective thickness of the polymers on the surface. The adsorption of Pb and Al on the Si oxide modified the solubility of Si, suggesting a chemisorption reaction with the surface.
Analysis and Field‐Evaluation of the CERES Models' Soil Components: Nitrogen Transfer and Transformations
Authors: Gabrielle Benoît , Kengni Lucas
Published: July 2010

For estimating N losses in soil‐crop systems with the simple and functional CERES models, we evaluated their N modules and compared them with the more complex SLIM (for solute transport) and NCSOIL (for N mineralization) models. SLIM is based on the concept of immobile and mobile water regions in soil, and CERES on a piston‐flow hypothesis. In NCSOIL, the soil organic matter (SOM) mineralizes through two active pools, whereas in CERES the SOM decomposes as a whole with a gross decay rate. We used data on mineral N dynamics under bare soils (silt loam, loam, and sandy loam) from 1‐yr‐long experiments at three locations in France, including measurements of weekly NO3 leaching fluxes. The original CERES mineralization submodel did not correctly simulate N supply from potentially degradable SOM. When using NCSOIL instead, the simulations improved and CERES predicted NO3 leaching reasonably well, with a root mean square error of 6 to 21 kg N ha‐1, representing 5% of the yearly flux. The SLIM model performed as well, and better simulated the intense NO3 percolation regime that occurred in wintertime. Its immobile water fraction parameter had to be calibrated, however, or the yearly leaching flux was underestimated. When linked to NCSOIL, the CERES model showed a good potential for estimating N dynamics in soil, even if its piston‐flow type of NO3 transfer was not always relevant. In such case, SLIM was a more appropriate approach, although it required a site‐specific calibration.
Comparison of Three Methods to Obtain the Apparent Dielectric Constant from Time Domain Reflectometry Wave Traces
Authors: Timlin D. J. , Pachepsky Ya. A.
Published: July 2010

A calibration of time domain reflectometry (TDR) probes to measure soil water in a particular soil is desirable since no consistent relationship between water content and apparent dielectric constant (Ka) has been found. We compared three procedures to compute Ka from TDR traces: a manual method that uses a plot of the wave trace, a derivative‐based computer algorithm, and fitting a simulated TDR trace to a measured one with a simple multiple reflection model. We added a model of inertia to the multiple reflection model to approximate the rise time of the cable tester. The TDR traces were measured in situ with a Tektronix 1502B cable tester (Tektronix, Inc., Wilsonville, OR) on samples from two soils with contrasting textures, Beltsville silt loam (fine‐loamy, mixed, mesic Typic Fragiudult) and Rumford loamy sand (coarse‐loamy, siliceous, thermic Typic Normudult). Ten‐centimeter probes with three rods were used. The water contents were measured gravimetrically with soil cores. Calibration with apparent dielectric constants obtained from the derivative‐based algorithm had the smallest standard error for both soils, and the manual method was better than the wave simulation method. The intercepts and slopes of the calibration equations for the manual and derivative methods were not significantly different from each other for both soils. The wave simulation method, which gives physically meaningful values for Ka, and characteristic impedances can be a useful tool for theoretical studies.
The Characterization of Water in Peat
Authors: McBrierty V. J. , Wardell G. E. , Keely C. M. , O'Neill E. P. , Prasad M.
Published: July 2010

The functionality and ultimate use of peat are intimately bound up with the way in which absorbed water behaves in the host matrix. Control of the amount of absorbed water, the dynamics of water loss in drying, and the gain of water in rewatering are important determinants in its commercial exploitation. We examined the nature and behavior of water in a range of different Irish peat samples by exploiting the combined capabilities of differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and proton nuclear magnetic resonance (NMR). Up to four different forms of water were detected in heavily hydrated peat: absorbed or tightly bound water, which is non‐freezable and behaves like a glass at low temperatures; two forms of loosely bound water; and bulk water. Increasing temperature across the subambient regime revealed the sequential onset of mobility of the different forms of water. The transitions are well‐defined in terms of both water concentration and the temperature at which motion sets in. Unlike hydrated polymers, NMR estimates of the maximum amounts of bound water at low temperatures depend on overall water content, reflecting the extent to which the peat matrix can restructure and swell when hydrated. The TGA, which maps out the loss of water under controlled conditions of temperature and atmosphere, offers additional evidence on the mechanism of dehydration, indicating that drying on a small scale under laboratory conditions is not grossly impeded by internal barriers. The gratifying consistency between the data from the three experimental techniques used, along with pertinent data in the literature, give additional insights into the role of water in peat.
Two‐Dimensional Model for Water, Heat, and Solute Transport in Furrow‐Irrigated Soil: I. Theory
Authors: Noborio K. , McInnes K. J. , Heilman J. L.
Published: July 2010

Because of salinity, many areas with irrigated farmland have suffered from reduced food and fiber production, especially where irrigation water or soil contains large amounts of soluble salts. In those areas, ridge‐furrow tillage is a common practice and furrow irrigation is a popular irrigation method. Effects of water content, temperature, and solute concentration on transport phenomena in soil are hard to predict without complex models. The geometry of a ridgefurrow surface contributes to complexities in heat and vapor transport, and hence, in the distribution of water, temperature, and salt in soil. To improve understanding of water, heat, and solute transport in furrow‐irrigated and salt‐affected soil, we developed a two‐dimensional mechanistic model using the Galerkin finite element method (FEM). The model was designed to consider interactive effects of water content, temperature, and solute concentration on water, heat, and solute transport. To simulate field conditions, an energy balance equation was applied to the ridge‐furrow surface to provide boundary conditions for water and heat transport. To utilize a single FEM solver, equations governing transport of water, heat, and salt in soil were generalized as a diffusive‐convective type equation. Numerical schemes for the model were tested by comparing simulated results with analytical or semianalytical solutions. Good agreements between FEM and analytically calculated soil water content, temperature, and solute concentration were obtained.
Role of Redox Potential in Chemical Transformations of Selenium in Soils
Authors: Jayaweera Gamani R. , Biggar James W.
Published: July 2010

Irrigated agricultural areas with high levels of soil Se face the dilemma of elevated Se levels in drainage and groundwater systems, which will adversely affect the environment and the wildlife. The release of Se from soils to water systems is dependent on the speciation of Se, which is primarily governed by the redox potential. This study describes the dynamics of redox transformations of naturally occurring Se in a soil matrix as a continuous function of time during a sequence of oxic‐anoxic‐oxic transition. An experimental setup where a solution stream is continuously passed through a soil column in a closed loop to reach an equilibrium between the soil and the solution was used. The pH and redox potential (Eh) were recorded and the solution was analyzed over time for various Se forms and Mn2+. During soil reduction, the total soluble Se and SeO2−4 decreased, while the SeO2−3 and other Se forms (organic Se, elemental Se, and selenide) increased initially, then decreased. Both soluble Se and SeO2−4 decreased during the anoxic phase, slowly at first, then more rapidly, and were characterized by first‐order rate constants. During reoxidation, the total soluble Se and SeO2−4 increased, SeO2−3 initially increased followed by a decrease, and other Se forms decreased. Decrease in SeO2−3 during reduction may be partly due to the precipitation of MnSeO3. This study showed that the soil Eh plays an important role in mobilizing Se into water systems.
Organic Matter Fractions from Organically and Conventionally Managed Soils: II. Characterization of Composition
Authors: Wander M. M. , Traina S. J.
Published: July 2010

Diffuse reflectance Fourier transform infrared (DR‐FTIR) spectroscopy was used to investigate the effects of organic and conventional management practices on soil organic matter (SOM) chemistry. We characterized physically and chemically isolated SOM fractions collected from Rodale's Farming Systems Trial (FST) experiment and used ratios of reactive (O‐containing) and recalcitrant (C, H and/or N) functional group heights to characterize SOM fraction composition. Based on peak ratio comparisons, we determined that humic acid (HA) in the manure‐amended organic rotation was more reactive than HA isolated from crop‐residue‐amended soils. Reactive/recalcitrant (O/R) peak ratios indicated that fulvic acid (FA) and particulate organic matter or light fraction (LF) isolated in April were most reactive in the organic cash‐grain‐based rotation. By June LF reactivity had changed; O/R ratios of LF and litter (LT) fractions isolated from the conventional and organic cash‐grain soils were greater than the manure‐amended soil's ratios. We did not draw any conclusions about management impacts on SOM composition from spectra of fine‐clay‐(FC) and sand‐ (S) sized fractions. Using DR‐FTIR we were able rank SOM fraction reactivity; average O/R ratios were FA > LT > HA > LF > FC. No consistent link between SOM fraction O/R ratios and reported C/N ratios or biological availability was made. Humic acid O/R ratios were most consistent with other estimates of overall SOM biological activity. More research is needed to clarify the relationship between the composition of specific SOM fractions and their functional significance in mineral soils.
Soil Organic Carbon Pools of Two Tropical Soils Inferred by Carbon Signatures
Authors: Hsieh Yuch‐Ping
Published: July 2010

Labile and stable soil organic carbon (SOC) pools of two tropical soils of Belize, Central America, as influenced by land use management, were investigated using a combination of 14C and 13C techniques. The sizes and mean residence times (MRT) of the labile pools were found to be in the range of 1.5 to 59.8 g/kg and 5 to 8 yr, respectively, and those of the stable SOC pools, 25.1 to 65.2 g/kg and 250 to 388 yr, respectively. Both the labile and stable SOC pools have significantly shorter MRT than those of the temperate soils. The labile SOC pools of sugarcane (Saccharum officinarum L.) in continuous management averaged 12.9 and 19.8 g/kg, respectively, for Xaibe (Lithic Ustropept) and Louisville (Typic Pellustert) soils. Continuous sugarcane management reduced the labile pool to about one‐third the size of the corresponding secondary forests. The labile SOC pools is sensitive to changes in management practices and its size represents the intrinsic sustainable soil productivity. A radiocarbon single‐plot approach to study the dynamics of SOC pools was developed using the 14C and 13C signatures of the soil and those of the CO2 evolved. The results of the single‐plot method and that of the paired‐plot method agree well within the range of the experimental errors.
Isotope Discrimination during Decomposition of Organic Matter: A Theoretical Analysis
Authors: Ågren Göran I. , Bosatta Ernesto , Balesdent Jérôme
Published: July 2010

Stable isotope composition is a powerful characteristic of the development of soil organic matter, but there is a need to understand the causes of and to predict the changes in isotopic composition during organic matter decomposition. We used the continuous quality theory to derive a set of equations to analyze how properties of the litter interact with properties of the decomposers to produce different patterns of isotopic composition. The distribution of isotopes between different chemical fractions in the litter is in itself such that it should lead to increasing depletion of 13C during the course of decomposition. Isotope effects on decomposer growth rate, efficiency, and dispersion in quality increase, on the other hand, 13C concentration. The magnitudes of these effects have to be, in relative terms, considerably greater than that of initial litter chemical composition. The equations derived were also successfully tested on experimentally observed patterns of changes in C isotopic composition in an agricultural soil and some forest soils.
Determination of Microbial Biomass and Nitrogen Mineralization following Rewetting of Dried Soil
Authors: Franzluebbers A. J. , Haney R. L. , Hons F. M. , Zuberer D. A.
Published: July 2010

Routine soil testing procedures that are rapid and precise are needed to evaluate agricultural surface soils for their potential to mineralize C and N. Our objectives were to determine the optimum preincubation time after rewetting of dried soil for estimating soil microbial biomass (SMB) and to identify a quick, reliable biochemical predictor of soil N mineralization potential. Biochemical determinations of SMB were screened on a Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) having five levels of soil organic C (SOC) as a result of long‐term management. Determinations used (i) field‐moist soil and (ii) soil that was air dried, rewetted, and preincubated for 0.2, 1, 3, 6, 10, and 15 d. Biochemical determinations included arginine ammonification, substrate‐induced respiration (SIR), cumulative C and net N mineralization, and SMBC using the chloroform fumigation‐incubation (CFI) method. Preincubation periods of 1 and 15 d prior to fumigation gave estimates of SMBC using CFI most similar to those determined on field‐moist soil. Arginine ammonification and SIR determinations on dried soil were highly variable, making longer preincubation periods necessary. Carbon mineralization during all preincubation periods was highly correlated to (i) SMBC using CFI determined on field‐moist and dried soil with all preincubation periods and (ii) net N mineralization during 21 d for the Weswood soil, as well as for seven additional soil series each having five to eight levels of SOC. The CO2‐C evolved during the first day after rewetting of dried soil is recommended for rapid estimation of SMBC and potential N mineralization because of its simplicity and precision.
Pore Size Distribution and Hydraulic Conductivity Affected by Tillage in Northwestern Canada
Authors: Azooz R. H. , Arshad M. A. , Franzluebbers A. J.
Published: July 2010

Tillage management can affect crop growth, in part by altering the pore structure and hydraulic properties of soil. We hypothesized that water retention, pore size distribution, and unsaturated hydraulic conductivity (k) differed under conventional tillage (CT) and no tillage (NT). We evaluated this hypothesis on a Donnelly silt loam (fine‐loamy, mixed, frigid Typic Cryoboralf) and a Donnelly sandy loam (coarseloamy, mixed, frigid Typic Cryoboralf) in northwestern Canada. Soil cores were collected from the 0‐ to 300‐mm depth in 75‐mm increments. Water retention was measured at 10 pressure levels from ‐2 to −400 kPa to calculate pore size distribution and k. Both soils retained 0.04 to 0.09 m3 m−3 more water under NT than under CT. The volume fraction of total porosity with pores <7.5 µm in diameter (effective pores for retaining plant‐available water) in the silt loam averaged 0.49 and 0.58 m3 m−3 under CT and NT, respectively, and in the sandy loam averaged 0.39 and 0.51 m3 m−3 under CT and NT, respectively. In contrast, the volume fraction of total porosity with pores >150 µm in diameter (pores draining freely with gravity) in the silt loam averaged 0.29 and 0.23 m3 m−3 under CT and NT, respectively, and in the sandy loam averaged 0.35 and 0.24 m3 m−3 under CT and NT, respectively. Conventional tillage appeared more likely to interrupt capillaries than NT, since large differences in k between tillage regimes were observed below a depth of 75 mm with increasing moisture deficit. Continuous NT management increased water storage of both silt loam and sandy loam soils in this cold, semiarid region.
Organic Carbon Characteristics in a Spruce Swamp Five Years after Harvesting
Authors: McLaughlin James W. , Liu Guo , Jurgensen Martin F. , Gale Margaret R.
Published: July 2010

Forest harvesting and regeneration may cause changes in soil and solution chemistry that adversely affect environmental quality. Organic C content in the forest floor and mineral soil, and dissolved organic carbon (DOC) fractions in the soil solution and groundwater were investigated in a black spruce [Picea mariana (Miller) BSP] swamp 5 yr after the application of two silvicultural prescriptions: whole‐tree harvest only and whole‐tree harvest followed by site preparation bedding. Soil organic carbon (SOC) content in the forest floor of both treatments were significantly lower than that of an uncut control stand. However, SOC in the upper 50 cm of mineral soil of the bedded treatment was similar to that of the control, while both were about 20% higher than SOC in the harvest‐only treatment. Total DOC concentrations ranged from 12 to 87 mg/L in the soil solution and 6 to 46 mg/L in the groundwater and did not differ among treatments. Hydrophobic acids were the dominant DOC fraction across all treatments, but the proportion of hydrophobic strong acids increased at the expense of hydrophilic acids in the harvest‐only treatment. Carboxyl content of hydrophilic acids in the soil water was 7.0 mmol/g hydrophilic compound in the control stand and 4.4 mmol/g in the treatments. Hydrophobic acid carboxyl content across treatments was 4.6 mmol/g hydrophobic substance. Relative to C loss and changes in water chemistry, these results indicate that 5 yr after whole‐tree harvesting, this forest site has not yet fully equilibrated.
Characterization of Miller‐Similar Silica Sands for Laboratory Hydrologic Studies
Authors: Schroth M. H. , Istok J. D. , Ahearn S. J. , Selker J. S.
Published: July 2010

The use of well‐characterized porous media can simplify and improve the efficiency of laboratory subsurface flow and transport experiments. The objective of this study was to present a comprehensive set of hydrologically relevant properties for a unique set of commercially available silica sands. Features of sands selected for characterization included high sphericity, high batch‐to‐batch consistency, Miller‐similarity, and availability in large quantities. Samples of four different sand grades (12/20, 20/30, 30/40, and 40/50 sieve sizes) were characterized for physical properties, chemical composition, water retention, three‐phase air‐non‐aqueous‐phase liquid (NAPL)‐water saturation‐pressure relationships for water and a model NAPL, Soltrol 220, and saturated and unsaturated hydraulic conductivity. Properties common to all sand grades included high chemical purity and low organic matter content. Water retention curves featured well‐defined air entry pressures and the Miller‐similarity of the media was demonstrated for both static and dynamic properties. During water retention measurements, we determined that the common assumption of a uniform vertical water content distribution in retention cells can result in significant errors in uniform porous media. A numerical correction procedure was developed and successfully applied to correct fitted water retention curve parameters, illustrating that potential errors of up to 70% in volumetric water content are made without proper analysis. The characterization data for the four sand grades presented here should facilitate their use in a wide range of laboratory flow and transport studies.
Spatial Structure of Solute Transport Variability in an Unsaturated Field Soil
Authors: Ellsworth T. R. , Boast C. W.
Published: July 2010

Three solutes were applied in series under steady, unsaturated flow to determine the spatial scale dependence of transport. Twenty‐seven days after solute application, a 2 by 2 by 2 m3 field plot was excavated using two measurement scales, 22.2 by 22.2 by 10 cm3 (scale L) and 7.4 by 7.4 by 10 cm3 (scale l), providing two sets of 81 samples for each 0.1‐m layer to a depth of 2.0 m. For each chemical and for each scale, vertical moment analysis was performed on the data for each of the 81 vertical columns. Evidence of enhanced local horizontal mixing with increasing mean travel time was observed in two ways: (i) a decrease in mass recovery variance at scale L for NO−3 (center of mass at 80‐cm depth) vs. Br− (46‐cm depth of center of mass), and (ii) a horizontal correlation range in solute concentrations in individual 0.1‐m‐thick layers, which was lowest between 70 and 130 cm deep. Point variograms of the vertical moments for each solute were estimated by simultaneous deconvolution of the two observed (at scales l and L) area‐averaged variograms. Dispersion variance analysis (based on these point variograms) characterized the variability within the plot of mass recovery and center of mass depth as a function of sample size. Several approaches for characterizing plot‐ or field‐scale transport features from soil‐core scale measurements were evaluated. The horizontal dependence of the vertical variance increased with mean travel time. For each tracer, this dependence was reasonably characterized as the sum of the dispersion variance in the center of mass and the average L‐scale vertical variance.
Uranium(VI) Adsorption on Goethite and Soil in Carbonate Solutions
Authors: Duff M. C. , Amrhein C.
Published: July 2010

Elevated concentrations of U are found in agricultural drainage waters from the San Joaquin Valley, CA, which are often disposed of in evaporation basins that are frequented by waterfowl. To determine the factors that affect aqueous U concentrations in the basins, sorption experiments with U(VI) were performed at various CO2 partial pressures, dissolved Ca, Mg, and P concentrations, and carbonate alkalinities. Synthetic waters, comparable in inorganic constituents to irrigation and drainage waters, were prepared, spiked with 0.1 (soil) and 2 mg U(VI) L−1 (synthetic goethite), and analyzed for U, P (when applicable), and major ions. Total chemical analyses were input into the computer program FITEQL to determine U(VI) speciation and generate U(VI) adsorption constants with the diffuse layer model (also referred to as the two‐layer model). Maximum adsorption occurred in solutions with low carbonate alkalinities (≤3 mmol L−1), ionic strengths (≤0.03 M), Ca concentrations (≤4 mmol L−1), and P concentrations (<0.005 mmol L−1 for soil). Lesser and negligible adsorption was attributed to the predicted formation of highly soluble, negatively charged U(VI) carbonates [UO2(CO3)2−2 and UO2(CO3)4−3] that did not strongly adsorb to soil surfaces. Calcium and, to some degree, Mg competition with positively charged U(VI) species for surface sites was observed at low carbonate alkalinities (<3 mmol L−1 for goethite; <14 mmol L−1 for soil). At high carbonate alkalinities, carbonates competed with anionic U(VI) species for adsorption sites. Study results suggest that elevated U concentrations in the drainage waters are due to the speciation of dissolved U(VI) into negatively charged carbonate complexes.
Nitrification of Ammonium Nitrogen in Soils Treated with XDE‐474
Authors: Kpomblekou‐A K. , Killorn Randy
Published: July 2010

Retardation of NH+4 nitrification constitutes one means to conserve fertilizers applied to soils. A new nitrification inhibitor, XDE‐474, has been developed by DowElanco Chemical Company (Indianapolis, IN) and needs to be evaluated. Laboratory studies evaluated the effect of nitrapyrin [2‐chloro‐6‐(trichloromethyl)pyridine] and XDE‐474 on nitrification rates in five Iowa soils varying markedly in organic C (OC), pH, and texture. A 10‐g sample of surface soil treated with 1 mL of solution containing 2 mg N as either NH4OH or (NH4)2SO4 and 4 µg a.i. g−1 soil of nitrapyrin or XDE‐474 was incubated at 20 ± 1°C for 0, 7, 15, 30, 45, and 60 d. First‐order equations were used to calculate the maximum nitrification rate (Kmax), duration of lag period (t′), period of maximum nitrification (Δt), and duration of lag plus maximum nitrification period (ts). Nitrification rates were significantly affected by N sources and inhibitors used. In the Okoboji soil (43.0 g kg−1 OC), nitrification of NH4OH was 19.5 mg kg−1 d−1 NO−3‐N in the control soil but reduced to 15.2 or 4.43 mg kg−1 d−1 NO−3‐N when nitrapyrin or XDE‐474 were added, respectively. Extension of the lag period due to the inhibitors varied from 0.73 d in the Okoboji soil to 18.8 d in the Storden soil (11.0 g kg−1 OC). Multiple regression equations expressing the dependence of Kmax on OC, pH, and clay showed that only OC and pH were significant in accounting for the variation in Kmax. The resulting three‐dimensional response surface indicated that XDE‐474 was less affected than nitrapyrin by changes in OC and pH.
Small‐Scale Heterogeneity of Aluminum and Heavy Metals in Aggregates along a Climatic Transect
Authors: Wilcke Wolfgang , Amelung Wulf
Published: July 2010

Concentrations and forms of AI and heavy metals in aggregate core and surface fractions of forest soils in Central Europe differ significantly. The objectives of this study were to (i) test if there is a similar small‐scale metal heterogeneity in topsoils of the native North American prairie and (ii) investigate the influence of climatic parameters on this heterogeneity. Aggregates of eight A horizons along a climatic transect in the North American prairie were separated into core and 0.5‐mm‐thick surface fractions. Seven forms of Al, Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn were determined in the aggregate fractions using a sequential extraction procedure. The differences of metal concentrations between aggregate surface and core fractions are 1 to 25% of the total concentrations of the core fractions. The percentages of easily extractable metals are up to 120% higher in the aggregate surface than in the core fractions. The percentages of the mainly silicate‐bound metals (extractable only with strong acids) are up to 25% lower than in the core fractions. These results support the hypotheses that (i) preferential weathering of aggregate surfaces causes a shift from strongly to more weakly bound metal forms and (ii) atmospheric metal input is sorbed preferentially at the aggregate surfaces, and (iii) this input is weakly bound only. Lead is significantly enriched in the surface fractions compared with the cores, indicating ubiquitous input of Pb to the soils. The depletion of the mainly silicate‐bound metals in the aggregate surface fractions compared with the core fractions correlates with the mean annual temperature and the mean annual precipitation, which both favor silicate weathering.
Phosphorus Retention as Related to Morphology of Sandy Coastal Plain Soil Materials
Authors: Harris W. G. , Rhue R. D. , Kidder G. , Brown R. B. , Littell R.
Published: July 2010

Phosphorus retention in sandy coastal plain soils can be low enough that surface water quality is jeopardized by agricultural P loadings. Objectives of this study were to: (i) determine if discrete morphological characteristics could effectively differentiate sandy soils with respect to P retention and (ii) test the efficacy of a rapid chemical assessment of relative P adsorption (single‐point isotherm) for sandy materials. Soil samples from 96 surface and subsurface horizons of randomly selected Alaquod, Quartzipsamment, and Paleudult pedons were used. These pedons had previously been described, sampled, and characterized as part of the Florida Cooperative Soil Survey Program. Two groups of uncoated Quartzipsamments (<5% silt plus clay) were distinguished: those having “clean” (coating‐free) and “slightly coated” grains. Eluvial horizons from Alaquods were also designated as clean because of a dominance of coating‐free sand grains. Single‐point P adsorption isotherms were obtained for all samples, and multipoint adsorption and desorption isotherms for 21 of the 96 samples. Single‐point isotherms effectively arrayed sandy material with respect to relative P adsorption. Sand‐grain coatings significantly enhanced P adsorption and resistance to desorption. All clean samples readily desorbed P regardless of origin or amount adsorbed. Thus clean sands pose a greater hazard for P leaching than sands with grain coatings. Clay content was closely related to P adsorption, but silt content was not. The P‐retention distinction between clean and other Quartzipsamments is more marked than the “uncoated vs. coated” USDA family criterion, the latter being confounded by the inclusion of silt content.
Water and Solute Movement under Conventional Corn in Central Spain: I. Water Balance
Authors: Román R. , Caballero R. , Bustos A. , Díez J. A. , Caballero A. , Cartagena M. C. , Vallejo A.
Published: July 2010

Partitioning temporal changes in soil water storage to water balance components (evapotranspiration [ET], drainage) under field conditions remains a vexing task. An in situ protocol using neutron probes and tensiometers was established on a sandy loam (Typic Xerofluvent) under irrigated, conventional corn (Zea mays L.) with the objectives to separate ET from deep drainage and to improve the accuracy of estimating the water balance components. An experimental research site (0.4 ha) at La Poveda Field Station near Madrid (Spain) was divided into 24 plots of 9 by 11.1 m each. The heterogeneity of the research site had been previously determined and the experimental field was instrumented accordingly. Vacuum soil‐water extractors were placed in all plots, but neutron probe and tensiometers were installed in only eight plots based on soil texture. Plot management included nine furrow irrigations of 404 mm and rainfall of 250 mm throughout the growing season. Measurements of water flow and soil water storage for periods of a few days allowed the determination of ET and drainage with the soil water balance equation. Four water flow patterns and six water balance cases were identified. Results showed a cumulative ET of 532 ± 70 mm and total drainage of 75 ± 22 mm below the 1.4‐m soil depth. Peak drainage took place 20 d before maximum ET and represented 19% of total irrigation water.