bluuebunny/crossref_metadata_2025 huggingface dataset:
Rhizosphere Effects on Decomposition
Authors: Cheng Weixin , Johnson Dale W. , Fu Shenglei
Published: July 2010

Plant species and soil fertility presumably control rhizosphere effects on soil organic matter (SOM) decomposition, but qualitative and quantitative descriptions of such controls are still sparse. In this study, rhizosphere effects of soybean [Glycine max (L.) Merr.] and spring wheat (Triticum aestivum L.) on SOM decomposition were investigated at four phenological stages under three levels of fertilization in a greenhouse experiment using natural 13C tracers. The magnitude of the rhizosphere effect ranged from 0% to as high as 383% above the decomposition rate in the no‐plant control, indicating that the rhizosphere priming can substantially intensify decomposition. The rhizosphere priming effect was responsible for a major portion of the total soil C efflux. Cumulative soil C loss caused by rhizosphere effects during the whole growing season equated to the amount of root biomass C for the soybean treatment, and 71% of root biomass C for the wheat treatment. Different plant species produced significantly different rhizosphere priming effects. The overall rhizosphere priming effect of soybean plants was significantly higher than for wheat plants. Plant phenology significantly influenced the rhizosphere priming effect. Little rhizosphere effect occurred in both wheat and soybean treatments initially. The priming effect of the wheat rhizosphere reached 287% above the no‐plant control at the flowering stage and declined significantly afterward. The priming effect of the soybean rhizosphere peaked at 383% above the no‐plant control during the late vegetative stage and remained at high levels onward. Contrary to many published reports, NPK fertilization did not significantly modify the rhizosphere priming effect.
Influence of Spatial Structure on Accuracy of Interpolation Methods
Authors: Kravchenko A. N.
Published: July 2010

Effectiveness of precision agriculture depends on accurate and efficient mapping of soil properties. Among the factors that most affect soil property mapping are the number of soil samples, the distance between sampling locations, and the choice of interpolation procedures. The objective of this study is to evaluate the effect of data variability and the strength of spatial correlation in the data on the performance of (i) grid soil sampling of different sampling density and (ii) two interpolation procedures, ordinary point kriging and optimal inverse distance weighting (IDW). Soil properties with coefficients of variation (CV) ranging from 12 to 67% were sampled in a 20‐ha field using a regular grid with a 30‐m distance between grid points. Data sets with different spatial structures were simulated based on the soil sample data using a simulated annealing procedure. The strength of simulated spatial structures ranged from weak with nugget to sill (N/S) ratio of 0.6 to strong (N/S ratio of 0.1). The results indicated that regardless of CV values, soil properties with a strong spatial structure were mapped more accurately than those that had weak spatial structure. Kriging with known variogram parameters performed significantly better than the IDW for most of the studied cases (P< 0.01). However, when variogram parameters were determined from sample variograms, kriging was as accurate as the IDW only for sufficiently large data sets, but was less precise when a reliable sample variogram could not be obtained from the data.
Structure and Function of Peatland‐Forest Ecotones in Southeastern Alaska
Authors: Hartshorn Anthony S. , Southard Randal J. , Bledsoe Caroline S.
Published: July 2010

High‐latitude warming could cause northern peatlands to become C sources. Where peatlands border boreal forests, strong differences in ecosystem C balances reflect drainage differences. Because local drainage conditions could be influenced by alterations in temperature and precipitation regimes, peatland‐forest ecotones represent useful locations for monitoring potential impacts of global warming. We characterized the soils, hydrology, and forest structure along transects bracketing a peatland‐forest ecotone in southeastern Alaska. We expected to find soil properties and processes at the peatland‐forest edge that were intermediate between those from peatland and forest. Instead, we found that above‐ and belowground features of the ecotone did not coincide. Conifers grew on mineral soils, but also grew on Cryofibrists and Cryohemists, soils with high soil organic C (SOC) contents to 100 cm (57 kg m−2) that are significantly greater than the SOC contents of adjacent forested, non‐Histosol pedons. Soil respiration rates (SRR) at peatland‐forest edges (0.08 g CO2–C m−2 h−1), by contrast, were threefold lower than forest rates and did not differ significantly from peatland rates. Respiration rates were strongly influenced by water table height. Peatland and edge water tables were both significantly shallower than forest water tables. Our conceptual model suggests that if additional forest expansion and warmer summers enhance drainage of these edge soils and stimulate SRR to forest‐like levels, 23 kg C m−2 could ultimately be mineralized from these extensive peatland‐forest boundaries. Afforestation of peatland margins under this scenario could represent a transient positive feedback to rising atmospheric CO2 levels.
Utility of Ground‐Penetrating Radar as a Root Biomass Survey Tool in Forest Systems
Authors: Butnor J. R. , Doolittle J. A. , Johnsen K. H. , Samuelson L. , Stokes T. , Kress L.
Published: July 2010

Traditional methods of measuring tree root biomass are labor intensive and destructive in nature. We studied the utility of ground‐penetrating radar (GPR) to measure tree root biomass in situ within a replicated, intensive culture forestry experiment planted with loblolly pine (Pinus taeda L.). The study site was located in Decatur County, Georgia, in an area of the Troup and Lucy (loamy, kaolinitic, thermic Grossarenic Kandiudults and Arenic Kandiudults, respectively) soils. With the aid of a digital signal processing GPR, estimates of root biomass to a depth of 30 cm were correlated to harvested root samples using soil cores. Significant effects of fertilizer application on signal attenuation were observed and corrected. The correlation coefficient between actual root biomass in soil cores and GPR estimates with corrections for fertilizer application were highly significant (r = 0.86, n = 60, p < 0.0001). Where site conditions are favorable to radar investigation, GPR can be a powerful cost‐effective tool to measure root biomass. Verification with some destructive harvesting is required since universal calibrations for root biomass are unlikely, even across similar soil types. Use of GPR can drastically reduce the number of soil cores needed to assess tree root biomass and biomass distribution. The quality and quantity of information resulting from a detailed GPR survey, combined with soil cores on a subset of plots, can be used to rapidly estimate root biomass and provide a valuable assessment of lateral root biomass distribution and quantity.
Temporal Stability of Soil Moisture in a Large‐Field Experiment in Spain
Authors: Martínez-Fernández José , Ceballos Antonio
Published: July 2010

Here we analyze the temporal stability of the soil moisture content in an area of 1285 km2 located in the central sector of the Duero basin (Spain). The data from a network consisting of 23 soil moisture‐measuring stations (REMEDHUS) over a period of 36 mo (from June 1999 to May 2002) were used. At each station, soil moisture was measured fortnightly, using time domain reflectometry (TDR), at different depths to the bottom of the profile. From the hydrological point of view, the stations are independent from one another and were distributed in space using physiographic and pedological criteria. The results clearly show which stations are representative of wet and dry conditions and the persistence of the representative stations under even the most extreme situations. The temporal evolution of the stability patterns points to a considerable degree of persistence during the periods analyzed. The stations representative of dry conditions are seen to be much more stable at all the depths analyzed (5, 25, 50, and 100 cm) than those identified within the wet sector. Overall, the soil moisture content showed considerable temporal stability throughout of the investigation. However, a clear correlation was observed between mean soil moisture and variance (r = 0.85) across the range of measurements used. Accordingly, temporal stability is always higher when the soils are dry than when the water content is high. It was also observed that the periods with the lowest temporal stability coincided with situations involving the transition from dry to wet. The critical periods in terms of temporal stability are those in which the recharge of water in the profile occurs.
Experimental and Numerical Evaluation of Analytical Volume Balance Model for Soil Water Dynamics under Drip Irrigation
Authors: Mmolawa Khumoetsile , Or Dani
Published: July 2010

The interpretation and subsequent modeling of soil water dynamics in presence of active plant uptake remains a challenge. Despite many simplifying assumptions in their development, analytical models offer simple and easy means for prediction of soil water dynamics that could be readily generalized for development of design and management guidelines. A local volume balance analytical model for predicting soil water dynamics was recently developed and successfully tested with a few point measurements. However the validity of various simplifying assumptions and rigorous evaluation of the model across an entire root zone with spatially variable water distribution was not tested. The objective of this study was to evaluate the analytical model relative to measurements and to a widely used numerical model (Hydrus‐2D [H2D]). Both models were first fitted to measured data at selected locations to estimate hydraulic parameters for subsequent modeling of the entire flow domain. Water content dynamics by the analytical model was in good agreement with the H2D model simulations in the absence of plants. With active plant uptake, the analytical model overestimates root water uptake at some locations when compared with H2D. This was attributed to the lack of consideration of water stress effects due to changing soil‐water status on uptake intensity in the analytical model. The analytical model was also sensitive to the choice of linearizing hydraulic parameters that in turn are dependent on the range of soil water content in the simulation domain.
Comparison of Network Generation Techniques for Unconsolidated Porous Media
Authors: Al-Raoush Riyadh , Thompson Karsten , Willson Clinton S.
Published: July 2010

While network models of porous materials have traditionally been constructed using regular or disordered lattices, recent developments allow the direct modeling of more realistic structures such as sphere packings, microtomographic images, or computer‐simulated materials. One of the obstacles in these newer approaches is the generation of network structures that are physically representative of the real systems. In this paper, we present and compare two different algorithms to extract pore network parameters from three‐dimensional images of unconsolidated porous media systems. The first approach, which utilizes a pixelized image of the pore space, is an extension to unconsolidated systems of a medial‐axis based approach (MA). The second approach uses a modified Delaunay tessellation (MDT) of the grain locations. The two algorithms are validated using theoretical packings with known properties and then the networks generated from random packing are compared. For the regular packings, both methods are able to provide the correct pore network structure, including the number, size, and location of inscribed pore bodies, the number, size, and location of inscribed pore throats, and the connectivity. Despite the good agreement for the regular packings, there were differences in both the spatial mapping and statistical distributions in network properties for the random packings. The discrepancies are attributed to the pixelization at low resolution, non‐uniqueness of the inscribed pore‐body locations, and differences in merging processes used in the algorithms, and serve to highlight the difficulty in creating a unique network from a complex, continuum pore space.
Carbon Storage in Coarse and Fine Clay Fractions of Illitic Soils
Authors: Kahle M. , Kleber M. , Torn M. S. , Jahn R.
Published: July 2010

The storage of organic C in coarse (0.2–2 μm) and fine (<0.2 μm) clay fractions of illitic topsoils from loess was investigated in terms of the effect of particle size and mineral phase properties. We compared (i) C functional groups by 13C nuclear magnetic resonance spectroscopy (NMR), (ii) the stable C isotope ratio (δ13 C ratio) of organic pools, and (iii) residence time of C by 14C analyses. To investigate relationships between C storage and the size of mineral surface area or the amount of hydrous oxides, specific surface areas (SSAs, BET‐N2 method) and the content of dithionite‐extractable Fe (Fed) were analyzed. The chemistry of the organic matter stored in clay subfractions was different. Compared with coarse clay, fine clay contained relatively (i) more ketonic/aldehyde, carboxyl and phenolic C, and (ii) less anomeric, O‐alkyl, and methoxyl/N‐alkyl C, and had (iii) a lower C content and C/N ratio and (iv) a higher δ13C ratio. In 11 out of 14 fractions, C had turnover times of few centuries or less. In fine clay, the increase in SSA resulting from oxidation of organic matter explained 66% of the variation in C content, in coarse clay 97%. We calculated loadings of mineral surface area with C and Fed Carbon loading exceeded Fed loading in coarse clay while it was of the same range in fine clay. The results may be interpreted as an indication that a certain portion of the mineral surface area controls the C content in both clay subfractions. The character of the important surface may differ between the subfractions.
Carbon and Nitrogen Dynamics in Lumbricus terrestris (L.) Burrow Soil
Authors: Amador José A. , Görres Josef H. , Savin Mary C.
Published: July 2010

The formation of macropores and litter translocation into burrows by anecic earthworms [Lumbricus terrestris (L.)] may be important in controlling the extent to which earthworms affect C and N dynamics. We conducted a mesocosm‐scale laboratory experiment to assess the relationship between corn (Zea mays L.) litter incorporation by L. terrestris and C and N dynamics in burrow soil. We also evaluated the relative contribution of macropores and litter incorporation to C and N dynamics in the burrow soil. Four treatments were employed: control (CTRL), artificial burrows (ARTF), artificial burrows containing corn leaves (LEAF), and amended with L. terrestris (WORM). We measured soil C and N, dissolved organic C, C mineralization, and ammonium and nitrate‐N periodically, and litter removal and litter in burrows over 16 wk. A significant, short‐lived enhancement in C mineralization and in soil C and N was observed in WORM and LEAF treatments. Inorganic N increased with incubation time only in the WORM treatment. Nitrate dominated the inorganic N pool in WORM and LEAF treatments, accumulating in both, and to a lesser extent in the ARTF treatment. Strong correlations were observed between litter remaining aboveground and C mineralization, ammonium‐N, and nitrate‐N in the WORM treatments, whereas only nitrate‐N was correlated with litter resources in LEAF treatment. Our results indicate that C and N dynamics in the burrow soil of L. terrestris are coupled strongly to surface litter removal. Macropores and litter incorporation into macropores by themselves do not appear to explain the effects of L. terrestris on C and N dynamics in burrow soil.
Supervised Landform Classification to Enhance and Replace Photo‐Interpretation in Semi‐Detailed Soil Survey
Authors: Hengl Tomislav , Rossiter David G.
Published: July 2010

A method to enhance manual landform delineation using photo‐interpretation to map a larger area is described. Conventional aerial photo‐interpretation (API) maps using a geo‐pedological legend of 21 classes were prepared for six sample areas totaling 111 km2 in the Baranja region, eastern Croatia. Nine terrain parameters extracted from a digital elevation model (DEM) (ground water depth, slope, plan curvature, profile curvature, viewshed, accumulation flow, wetness index, sediment transport index, and the distance to nearest watercourse) were used to extrapolate photo‐interpretation over the entire survey area (1062 km2). The classification accuracy was assessed using the error matrix, calculated by comparing both the whole API maps and point samples, with the results of classification. The first results, using a maximum‐likelihood classifier, were 58.2% (hill land), 39.1% (plain), and 45.3% (entire area) reproducibility of the training set. Six classes in the plain were responsible for a large proportion of the misclassifications, due to an insufficiently detailed DEM and the complex nature of landforms (point bar complexes, levees, active channel banks), which cannot be explained with the terrain parameters only. Reproducibility for a simplified legend of 15 classes over the study area was improved to 65.8% (plain), 58.2% (hill land), and 63.4% (entire area) using the whole‐API training set. After the simplification of legend (15) and with the iterative (3) selection of point‐sample training set, classification was able to reproduce 97.6% (hill land), 86.7% (plain), and 90.2% (entire area) of the training set. The supervised classification showed fine details not achieved by photo‐interpretation. The number of manual photo‐interpretations that had to be prepared was reduced from 84 to 6. The methodology can be applied by soil survey teams to edit and update current maps and to enhance or replace API for new surveys.
Compost Applications Increase Water‐Stable Aggregates in Conventional and No‐Tillage Systems
Authors: Whalen Joann K. , Hu Quancai , Liu Aiguo
Published: July 2010

Agricultural practices that alter the soil organic matter (SOM) content are expected to cause changes in soil stability and aggregation. The objective of this study was to evaluate short‐term (<2 yr) changes in water‐stable aggregates (WSA) in a silt‐loam soil under different management regimes. The interactive effects of tillage (no‐till and conventional tillage), crop rotations (continuous corn, corn‐soybean rotation) and composted cattle manure applications [0, 15, 30, and 45 Mg (wet weight) ha−1] on WSA were assessed in a factorial (tillage × crop rotation) split plot (compost) experiment. The proportion of WSA >4 mm was greater in compost‐amended than unamended soils within 1 yr, and the mean weight diameter (MWD) of aggregates increased with increasing compost application rates. By the second year of the study, no‐till soils under continuous corn and the soybean phase of the corn‐soybean rotation had more WSA >4 mm and a greater MWD than any crop rotation in conventionally tilled soils. Increasing the C input to soil increased the MWD of aggregates. The MWD of aggregates was related to the C content of soils under no‐till, but not conventional tillage, suggesting more physical stabilization of organic matter (OM) in no‐till than conventional tillage agroecosystems. Our findings indicate rapid improvements in aggregation of a silt‐loam in the first 2 yr after compost application and the adoption of no‐tillage practices.
Modeling Soil Carbon from Forest and Pasture Ecosystems of Amazon, Brazil
Authors: Cerri C. E. P. , Coleman K. , Jenkinson D. S. , Bernoux M. , Victoria R. , Cerri C. C.
Published: July 2010

Conversion of tropical forest to agricultural management has important implications for C storage in soils and global climate change. The Nova Vida Ranch in the Western Brazilian Amazon basin provided a unique opportunity to study the conversion of tropical forests to pastures established in 1989, 1987, 1983, 1979, 1972, 1951, and 1911, in comparison with uncleared forest. Soils were analyzed for organic C, bulk density, total N, pH, clay content, and biomass C. The forest soil contained 34 Mg C ha−1 in the 0‐ to 30‐cm layer: modeling clearance and conversion to pasture caused an initial fall in the C stock, followed by a slow rise. After 88 yr, the pasture soil contained 53% more C than the forest soil. The increase in total N on conversion to pasture was less marked, which led to C/N ratios in the pasture soils being higher than in the forest soil. The Rothamsted C turnover model (RothC‐26.3) was used to simulate changes in the 0‐ to 10‐ and 0‐ to 30‐cm layer of soils when forest was converted to pasture. The model predicted that conversion to pasture would cause a 54% increase in the stock of organic C in the top 30 cm of soil in 100 yr. The modeled input of plant C to the 0‐ to 30‐cm layer of soil under pasture was assumed to be 8.28 Mg C ha−1 yr−1 The model provided a reasonable estimate of the microbial biomass (BIO) C in the 0‐ to 10‐cm soil layer. This was an independent test of model performance, because no adjustments were made to the model to generate output.
Soil Freezing and the Acid‐Base Chemistry of Soil Solutions in a Northern Hardwood Forest
Authors: Fitzhugh Ross D. , Driscoll Charles T. , Groffman Peter M. , Tierney Geraldine L. , Fahey Timothy J. , Hardy Janet P.
Published: July 2010

Reductions in the depth and duration of snow cover under a warmer climate may cause soil freezing events to become more frequent, severe, and spatially extensive in northern temperate forest ecosystems. In this experiment, snow cover was manipulated to simulate the late development of snowpack and to induce soil freezing at sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis) stands at the Hubbard Brook Experimental Forest (HBEF) in the White Mountains of New Hampshire. The objective of this manipulation was to elucidate the effects of soil freezing on the concentrations and fluxes of soil solution H+, Ca2+, Mg2+, K+, and Na+, as well as values of acid neutralizing capacity (ANC). Mild soil freezing events (soil temperatures never decreased below −5°C) resulted in pronounced acidification of soil solutions, driven primarily by nitrification, in the forest floor of sugar maple stands during the growing season. This mobilization of NO−3 from the forest floor of maple stands was accompanied by the leaching of Ca2+ and Mg2+ in Oa horizon solutions. Responses of soil solution acid‐base chemistry to soil freezing were not evident in yellow birch stands or in the Bs horizon of either vegetation type, emphasizing the importance of vegetation type and the mineral soil in determining the effects of climatic disturbance on drainage water chemistry and nutrient loss. These results suggest that models of soil biogeochemistry in temperate forest ecosystems should consider soil‐freezing events when simulating the acid‐base chemistry of soil solutions and the translocation of nutrient base cations between soil horizons.
Mineralogical and Chemical Characterization of Iron‐, Manganese‐, and Copper‐Containing Synthetic Hydroxyapatites
Authors: Sutter B. , Ming D. W. , Clearfield A. , Hossner L. R.
Published: July 2010

The National Aeronautics and Space Administration's (NASA) Advanced Life Support (ALS) Program is evaluating the use of Fe‐, Mn‐, and Cu‐containing synthetic hydroxyapatite (SHA) as a slow release fertilizer for crops that might be grown on the International Space Station or at Lunar and Martian outposts. Separate Fe‐, Mn‐, and Cu‐containing SHA materials along with a transition‐metal free SHA (pure‐SHA) were synthesized using a precipitation method. Chemical and mineralogical analyses determined if and how Fe, Mn, and Cu were incorporated into the SHA structure. X‐ray diffraction (XRD), Rietveld refinement, and transmission electron microscopy (TEM) confirmed that SHA materials with the apatite structure were produced. Chemical analyses indicated that the metal containing SHA materials were deficient in Ca relative to pure‐SHA. The shift in the infrared PO4–ν3vibrations, smaller unit cell parameters, smaller particle size, and greater structural strain for Fe‐, Mn‐, and Cu‐containing SHA compared with pure‐SHA suggested that Fe, Mn, and Cu were incorporated into SHA structure. Rietveld analyses revealed that Fe, Mn, and Cu substituted into the Ca(2) site of SHA. An Fe‐rich phase was detected by TEM analyses and backscattered electron microscopy in the Fe‐containing SHA material with the greatest Fe content. The substitution of metals into SHA suggests that metal‐SHA materials are potential slow‐release sources of micronutrients for plant uptake in addition to Ca and P.
Particle‐Size and Element Distributions of Soil Colloids
Authors: Ranville James F. , Chittleborough David J. , Beckett Ronald
Published: July 2010

Transport of colloids (diameter < 1 μm) through soil has implications for both horizon development and contaminant mobility. Colloid physical properties (size, shape), mineralogy, and surface chemistry are known to influence transport and deposition. Sedimentation field‐flow fractionation‐inductively coupled plasma–mass spectrometry (Sd FFF‐ICP–MS) was used to examine size and element composition distributions of colloids collected from the profile of a texture‐contrast soil located in South Australia. The morphology of the colloids was also examined by transmission electron microscopy (TEM). Colloids collected from the soil matrix, the source of mobile colloids, had significant differences in the size distributions among the three horizons sampled. These differences were consistent with long‐term soil formation processes. Colloids mobilized by rainfall, which were collected from overland flow and infiltration through the soil profile, all showed very similar size distributions. This is consistent with the presence of preferential flow paths and suggests that colloids, and colloid‐associated contaminants, can be transported rapidly through the vadose zone with minimal interaction with the soil matrix. The Sd FFF‐ICP–MS analysis showed variation in element ratios (Fe/Al, Mg/Al), which were used to detect changes in surface coatings and mineralogy over the colloid size range. The study demonstrated the utility of Sd FFF‐ICP–MS for examining the influence of colloid size on element composition and on elucidating colloid transport processes in soils.
Effect of Ammonium, Potassium, and Sodium Cations and Phosphate, Nitrate, and Chloride Anions on Zinc Sorption and Lability in Selected Acid and Calcareous Soils
Authors: Wang Jim Jian , Harrell Dustin L.
Published: July 2010

Zinc availability and mobility in soils is controlled by its interaction with the soil matrix and amendments. Contradicting evidence has been reported for factors influencing Zn behavior in soils. This study was conducted to investigate the effect of common cations and anions on Zn sorption and lability characteristics. Zinc sorption isotherms were conducted on three acid and four calcareous soils in NH4+, K+, and Na+and H2PO4−, NO3−and Cl−backgrounds. Lability of the sorbed Zn was evaluated by DTPA following sorption. Calcareous soils exhibited greater Zn sorption than did acid soils. Predicted Langmuir maxima for Zn sorption differed among the various ionic backgrounds. A majority of the total sorbed Zn (60–96%) was recoverable in the labile fraction. Both NH4+and K+equally decreased Zn sorption, as opposed to Na+in acid and calcareous soils; however NH4+yielded 4 to 12% more of sorbed Zn into the labile pool than did K+in acid soils. Zinc sorption was enhanced by H2PO4−as opposed to Cl−or NO3−in acid soils, but it was decreased in three out of four calcareous soils. The effect of H2PO4−on the lability of the sorbed Zn in acid soils was similar to that of Cl−or NO3−, but in calcareous soils the phosphate held 10‐25% more of the sorbed Zn in the nonlabile pool. It was concluded that even in calcareous soils, total Zn sorption could be impacted by phosphate–Fe oxide interactions. Furthermore, the effect of background ions on the lability of sorbed Zn varied between acid and calcareous soils. These results have important implications on Zn management in relation to other nutrients.
Reducing Analysis Variability of the Illinois Soil Nitrogen Test with Enclosed Griddles
Authors: Klapwyk Jonathan H. , Ketterings Quirine M.
Published: July 2010

The Illinois soil N test (ISNT) is a simple laboratory analysis that shows promise as a tool to predict if additional N application for corn (Zea mays L.) will result in a yield increase. Initial testing of the method at Cornell University showed a high degree of day‐to‐day and within‐griddle soil test N variability. At the University of Illinois, similar findings led to modification of the method to include jar rotations 1.5 and 3 h after initiation of the 5‐h incubation period used in the test. To determine if variability could be reduced without additional labor by enclosing the griddles using plywood boxes, experiments were designed to compare soil test variability using both the open‐bench and enclosed‐griddle methods. Enclosing the griddles decreased the coefficient of variance (CV) (p = 0.013) to 2.5% from 3.8% when analyses were done using the open‐bench method (using 10 exterior positions of the griddle for each method). Enclosing griddles did not impact recovery of added glucosamine‐N but resulted in lower N test values when set at the same temperature as the open‐bench griddles. A comparison study of 14 soils analyzed at the University of Illinois using the standard open‐bench procedure with jar rotation and at Cornell University using the enclosed‐griddle method without rotation showed sample means to be comparable when the enclosed griddles were set at 50.6°C. These results imply that the modified method could be adopted for use beyond well‐controlled laboratory environments but that griddle temperature setting is extremely important.
Soil‐Test Phosphorus and Crop Grain Yield Responses to Long‐Term Phosphorus Fertilization for Corn‐Soybean Rotations
Authors: Dodd Jason R. , Mallarino Antonio P.
Published: July 2010

Farmers and nutrient management regulatory agencies are requesting better knowledge of P fertilization impacts on soil‐test P (STP) and crop yield. This study evaluated STP and grain yield of corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] as affected by long‐term P fertilization in three trials evaluated from the 1970s until 2002 near Boone, Kanawha, and Nashua in central, northern, and northeast Iowa. Soils were Aquic Hapludolls and Typic Endoaquolls at Boone, Typic Endoaquolls at Kanawha, and Typic Hapludolls at Nashua. At Boone and Kanawha, treatments were the combinations of three initial STP levels (17–96 mg P kg−1, Bray‐P1) and four annual rates (0–33 kg P ha−1). At Nashua, initial STP was 28 mg P kg−1 and treatments were 0, 22, and 44 kg P ha−1 Ten to twenty years of cropping were needed on soils testing 43 to 96 mg P kg−1 to observe yield response to P. Annual P rates that maintained near optimum STP (16–20 mg P kg−1) were 17, 14, and 13 kg P ha−1 yr−1 at Boone, Kanawha, and Nashua, respectively. Phosphorus required to increase STP 1 mg P kg−1 yr−1 were 23, 28, and 17 kg P ha−1 yr−1 at Boone, Kanawha, and Nashua, respectively. Critical STP concentrations (CC) identified across sites and years were 15 to 21 mg P kg−1 for corn and 12 to 18 mg P kg−1 for soybean. Observed grain yield and STP responses are useful to develop effective P management plans for corn–soybean rotations under approximately similar conditions to those in this study.
Soil Carbon Stabilization in Converted Tropical Pastures and Forests Depends on Soil Type
Authors: López-Ulloa M. , Veldkamp E. , de Koning G. H. J.
Published: July 2010

The influence of soil C stabilization mechanisms is normally not considered in studies on the effects of land use changes. Instead, observed changes are typically explained by differences in litter input. As a result, it is not well known if and how quickly newly incorporated C is stabilized in soils. Our goals were to find out how much soil C was stabilized in two different soil orders (Andisols and Inceptisols) and which are the responsible mechanisms of C stabilization. Furthermore, we looked for evidence that newly incorporated soil C was stabilized in these contrasting soil orders. We selected 25 sites in northwestern Ecuador with two paired plots per site: one plot where pasture was converted to secondary forest and one plot where forest was converted to pasture. In all the plots, soil C content, stocks, and stable isotope (δ13C) signal were measured in the surface soil. The δ13C values were used to estimate the stocks of soil C derived from forest (Cdf) and from pasture (Cdp) in all plots. We calculated correlations between these stocks and soil and environmental characteristics to identify mechanisms of soil C stabilization. Our results show that long‐term stabilization in Andisols was through formation of metal–humus complexes and allophane, while in Inceptisols long‐term stabilization was through sorption to clay minerals. We found evidence that recently incorporated C was not stabilized in Andisols, while in Inceptisols, poorly crystalline (hydr‐) oxides seemed to have stabilized part of this soil C. We conclude that unless soil C stabilizing mechanisms are explicitly considered, we will not be able to predict the direction and magnitude of changes in soil C stocks following land use changes in the tropics.
Saturation, Reduction, and the Formation of Iron–Manganese Concretions in the Jackson‐Frazier Wetland, Oregon
Authors: D'Amore David V. , Stewart Scott R. , Huddleston J. Herbert
Published: July 2010

The Jackson‐Frazier wetland is a rare, relatively undisturbed wetland in the Willamette Valley of Oregon that experiences repetitive annual flooding and drying cycles. Redoximorphic features formed by these wetting and drying cycles have not been calibrated with saturation and reduction. Long‐term measurements of soil saturation and soil redox potential (EH) were combined with measurements of Fe and Mn concentrations to understand the formation of these redoximorphic features in the wetland soils. The Jackson‐Frazier wetland is underlain by three stratigraphic units that control its hydrology: Holocene alluvium (A and Bss horizons), Malpass clay (2Bt horizon), and Irish Bend silts (3BC horizon). All three units are saturated for up to 9 mo each year, leading to EH values that fall below the Fe reduction threshold. Extractable Fe and Mn in nodules, concretions, and soft masses document substantial depletions from the matrix soil in all three units. In the 3BC, diffuse halos of Fe accumulation and matrix chromas from two to four indicate a source of Fe that can be reduced, translocated, and concentrated in nodular form. In the Bss and 3BC horizons, soft masses surrounding concentrations and higher ratios of oxalate to dithionite‐citrate‐bicarbonate Fe (FeOx/FeDCB) in the larger concentrations suggest that nodules and concretions are currently forming by accretion of Fe. In the 2Bt, virtually all of the Fe and Mn is gone from the matrix, and concretion boundaries are sharp. These are not relict features, as the current conditions would favor continued formation had the supply of Fe and Mn not been exhausted.
Tillage Effects on Subsurface Drainage
Authors: Moroizumi Toshitsugu , Horino Haruhiko
Published: July 2010

Tillage near the soil surface may greatly influence drainage discharge and pressure head values in the subsurface zone. In this study, a controlled comparative experiment was conducted in the field under natural weather conditions, using a tilled and an untilled column, to evaluate the effects of tillage on subsurface drainage discharge and pressure head values. There were no significant differences between the two columns in subsurface drainage before tillage treatment before the study, as checked by a preliminary experiment. After tilling one column of the two columns, cumulative subsurface drainage discharge was larger and occurred earlier for the tilled column than for the untilled column. The measured drainage discharge and pressure head values were evaluated using the water movement model, HYDRUS version 6.0, which numerically solves the Richards equation. The HYDRUS model reproduced measured values well for subsurface drainage discharge and pressure head values after the tillage, as determined by root mean squared error (RMSE), mean bias error (MBE), and R2 Therefore, it is concluded that the model, which includes the effects of tillage on the hydraulic properties, can explain the reasons for differences in observed drainage in the two columns. Moreover, the effects of tillage depth on subsurface drainage discharge were simulated. The results implied that the effect of tillage on subsurface drainage were induced by the conditions of pressure head just before the first rainfall, and this effect was equal to, or greater than, the effects of changes in hydraulic properties due to tillage.
Relationship between Clay Content, Clay Type, and Shrinkage Properties of Soil Samples
Authors: Boivin Pascal , Garnier Patricia , Tessier Daniel
Published: July 2010

The availability of methods for quasi‐continuous measurements of soil shrinkage curves allowed the development of new models. The exponential (XP) model allows the calculation of the volume of two pore phases in the soil, namely macro and micropore volumes. The micropore volume is identified with the pore volume of the soil clay matrix according to the assumption that the maximum swelling of the clay matrix (MS) is the point of minimum water content of the structural shrinkage (Point D). This is discussed using undisturbed and repacked soil samples with various clay contents and clay types. The slope of the shrinkage curves as a function of equivalent saturated‐pore radius show a transition in pore type around a 10‐μm pore radius, where smaller and more deformable pores start to desaturate. This corresponds to the fitted D point and is close to the size of the largest pores in the clay matrix or clay–silt phase reported in the literature. The calculated micropore volume and micropore swelling properties are close to clay paste properties reported in the literature. At low water content, the specific micropore volume is independent from clay content. At Point D, the shrinking capacity of the specific micropore volume decreases with increasing clay content for clay contents below 40%. Our results show that Point D can be identified with the MS of the clay matrix, and that the XP model can be used to calculate the swelling properties of clays in the soil, without extraction.
An Assessment of Cadmium Availability in Cadmium‐Contaminated Soils using Isotope Exchange Kinetics
Authors: Gray C. W. , McLaren R. G. , Günther D. , Sinaj S.
Published: July 2010

There has been much research conducted to find suitable methods for the determination of available Cd in soils. In recent years, an isotope exchange kinetics (IEK) technique has been tested to describe the kinetic transfer of ions from the soil solution to the solid phase. Although the IEK technique has been successful in describing nutrient availability in soils, it has not been widely applied to study contaminant availability. In this study, experimental conditions to determine exchangeable Cd in soils using IEK were determined along with a measurement of Cd availability using the IEK technique for 20 topsoils. The results indicated that isotopically exchangeable Cd [E(t) values] predicted from short‐term isotopic kinetics (≤60 min) were only successful in predicting exchangeable Cd up to 24 h of exchange. After 24 h, E(t) values were significantly overestimated when compared with measured E(t) values. A compartmental analysis revealed that there were differences in the distribution of Cd in exchange pools between soils contaminated with Cd from different sources. The percentage of Cd located in the E(1 min) pool, was on average 21% of total Cd for the P‐fertilizer soils compared with 13% for the biosolids‐amended soils. In contrast, the biosolids‐amended soils had on average 42% of total Cd located in the E(1 min–24 h) pool compared with 25% for the P‐fertilizer soil. The E(>24 h) pool averaged 57% for the P‐fertilizer soil compared with 46% for the biosolids‐amended soil. The IEK technique may be a useful tool to provide information on Cd availability in soils.
Predicting Soil Nitrogen Mineralization Dynamics with a Modified Double Exponential Model
Authors: Wang W. J. , Smith C. J. , Chen D.
Published: July 2010

The double exponential model that separates mineralizable soil organic N into active (Na) and slow (Ns) pools has been widely used to describe net N mineralization dynamics. However, the biological meanings of the model parameters and their relationships to soil properties and environmental conditions remain to be elucidated. In the present study, 18 soils were incubated at 35°C and 55% water‐holding capacity (WHC) for 41 wk and two soils at 16 factorial combinations of temperature (5, 15, 25, or 35°C) and moisture (8, 11, 15, or 19%) for 29 wk. Although the model closely fitted the net N mineralization data, the model parameters often appeared to lack biological meanings and could vary with incubation time, temperature, and soil moisture in unpredictable manners. Thus, the conventional double exponential model was modified by (i) using defined mineralization rate constants for Na and Ns under standard temperature (35°C) and moisture (approximately 55% WHC); and (ii) using soil‐specific and fixed Na and Ns values to estimate temperature‐ and moisture‐dependent rate constants under non‐standard conditions. This technique basically eliminated the time effect on the estimates of pool sizes and resulted in the rate constants with a consistent Q10 response to temperature and linear response to moisture changes. Predictions of soil net N mineralization dynamics under field conditions using the parameters estimated in laboratory agreed closely with the measured data. Multiple regression analysis indicated that the size of Na is correlated with the initial water‐soluble organic N and microbial biomass in soil, whereas Ns represents the combined effects of all the factors regulating long‐term net N mineralization.
Nitrogen Mineralization from Humic Acid Fractions in Rice Soils Depends on Degree of Humification
Authors: Ve Nguyen Bao , Olk D. C. , Cassman K. G.
Published: July 2010

Although the chemical nature of soil organic matter (SOM) is thought to affect the mineralization rate of N bound in SOM, little direct evidence exists for such effects. To test the hypothesis that the N mineralization rate is affected by the degree of SOM humification, we added equivalent amounts of humic acid (HA) N as either the labile mobile humic acid (MHA) fraction or the more humified calcium humate (CaHA) fraction to two lowland rice (Oryza sativa L.) soils, which were subsequently incubated under anaerobic conditions for 6 wk. The HA fractions had been chemically extracted from seven irrigated lowland rice soils from Vietnam and the Philippines. In both incubation soils, the amount of N mineralized from the added HA fractions decreased exponentially as the optical density of the HA at 465 nm (E4, index of humification) increased (R2 = 0.94–0.98). Mineralization of humic N was also strongly negatively associated with the amount of N or C contained in each HA fraction per kilogram of soil extracted, suggesting a more recalcitrant nature of the fractions in those soils where conditions allowed them to accumulate. Nitrogen mineralization was less for both the MHA (by 26%) and CaHA (by 41%) when incubated in the International Rice Research Institute (IRRI) soil compared with the Tanhoi soil. The most plausible explanation for this decrease is increased stabilization of the added HA by the relatively abundant Ca+2 in the IRRI soil. We conclude that the degree of humification of the MHA and CaHA fractions plays an important role in governing the rate of N mineralization in lowland rice soils because (i) N mineralization from these fractions was affected by their degree of humification, and (ii) the fractions are a small but important component of total soil N.
Spatial Variability in Soil Ion Exchange Chemistry in a Granitic Upland Catchment
Authors: Stutter M. I. , Deeks L. K. , Billett M. F.
Published: July 2010

Advances in quantifying the spatial variability of soil properties made for agricultural soils are not being mirrored for naturally structured upland soils. The objectives of this study were to determine the degree of spatial variability and variance structure of cation exchange chemistry in a granitic, heather moorland site (Northeast Scotland). Two 20 by 20 m soil plots, a Typic Placaquod and a Typic Humaquept, were sampled at O and B horizon depths at 104 locations in a regular grid overlayed with cluster (Placaquod) and transects (Humaquept) patterns. Soils were analyzed for pH and exchangeable cation, physical, and hydraulic properties. Results showed strongly significant vertical differences in exchange chemistry between surface organic and mineral horizons at either site for all chemical properties. Strong lateral variability was also apparent within the plots. Coefficients of variation (CV) were 18 to 52% for O horizon chemical properties, with similar variability between sites. In B horizons CV values were 26 to 119%, the highest associated with Humaquept chemical properties. Compared with the Placaquod the Humaquept had lower mean pH, but higher mean concentrations of exchangeable Ca and Mg in both horizons. Geostatistical analyses highlighted a generally strong degree of spatial dependence to Placaquod properties, particularly in the organic horizon where correlation ranges were greater. By contrast, the majority of properties for the Humaquept showed random, pure nugget variance indicating no spatial correlations at this scale of observation. It is postulated that seasonal water logging of the Humaquept may explain some differences in the exchange chemistry between sites. These differences in chemistry and in the spatial patterns of variability have implications not only for modeling the role of such soils in controlling the hydrochemical environment of the uplands, but also for the design of field soil sampling strategies for accurately quantifying soil properties.
An Automated Soil Line Identification Routine for Remotely Sensed Images
Authors: Fox Garey A. , Sabbagh G. J. , Searcy S. W. , Yang C.
Published: July 2010

The soil line is a linear relationship between the near‐infrared (NIR) and red (R) reflectance of bare soil as characterized by slope and intercept parameters. Vegetation indices use soil line parameters extensively in crop growth analyses. Research indicates that the soil line can be related to site‐specific soil conditions within a field, especially organic C content. This relationship may provide a means for directing soil sampling. However, these soil and crop growth remotely sensed predictions require accurate estimates of soil line parameters. Determining soil line parameters by manually extracting reflectance characteristics of bare soil pixels can be cumbersome. This research proposes an automated soil line identification routine capable of deriving soil line parameters from bare soil or vegetated remotely sensed images. The automated routine estimates soil line parameters by deriving a set of minimum NIR digital numbers across the R band range. Pixels that contradict soil line theory are removed through an iterative process. The routine was evaluated using bare soil images of two fields in the Midwest USA and 15 multispectral digital video images of South Texas grain sorghum fields dominated by vegetated cover. This research compared soil line parameters derived from the automated routine to actual soil line parameters obtained by extracting R and NIR digital numbers from identifiable bare soil pixels within the images and also by manually inspecting plots of R versus NIR digital numbers for all pixels within an image. The routine performed reasonably well in matching the estimated actual soil line parameters with minimal adjustment between images.