Showing papers in "Soil Science Society of America Journal in 1996"

Relating Extractable Soil Phosphorus to Phosphorus Losses in Runoff

Abstract: Phosphorus in agricultural runoff can cause accelerated lake and stream eutrophication. Where producers have applied P at rates exceeding crop uptake, soil P has sometimes become the main source of P in runoff. We hypothesized that soil test P (STP) correlation to dissolved reactive P (DRP) and bioavailable P (BAP) in runoff varies, depending on the extraction method. To investigate which STP extraction method would be best for predicting DRP and BAP concentration and load in runoff, soil samples were taken from the 0- to 2-cm depth of 54 grass plots (5% slopes) on Captina silt loam (fine-silty, siliceous, mesic Typic Fragiudult). The STP was extracted by six methods and the ranges of results (mg kg -1 ) were: 54-490 (Mehlich III), 27-592 (Bray-Kurtz P1), 25-169 (Olsen), 14-110 (distilled water), 23-170 (Fe oxide paper), and 105-1131 (acidified ammonium oxalate). The soil P saturation ranged from 16 to 80%. Simulated rain was applied at 100 mm h -1 and runoff was collected for 30 min. The concentration of DRP in total runoff ranged from 0.31 to 1.81 mg L -1 , and BAP from 0.37 to 2.18 mg L -1 The r 2 values for STP by each extraction method correlated with runoff DRP and BAP, respectively, were: 0.72 and 0.72 (Mehlich III), 0.75 and 0.73 (Bray-Kurtz P1), 0.72 and 0.72 (Olsen), 0.82 and 0.82 (distilled water), 0.82 and 0.82 (iron oxide paper), 0.85 and 0.82 (acidified ammonium oxalate), and 0.77 and 0.76 (P-saturation). All correlations were significant (P < 0.001), but the high r 2 values of those obtained from distilled water, iron oxide paper, and acidified ammonium oxalate extractants indicate better precision for predicting DRP and BAP concentrations in runoff. Correlations of STP with DRP load (range: 43.4 to 472.8 g ha -1 ) and BAP load (54.2 to 542.0 g ha -1 ) were not useful (r 2 < 0.18), possibly because runoff volumes were highly variable.

Modeling competitive adsorption of arsenate with phosphate and molybdate on oxide minerals

Abstract: The mobility of As in soils depends on several factors including redox potential, soil mineralogy, pH, and the presence of other oxyanions that compete with As for soil retention sites. We investigated the effects of pH and competing anions on the adsorption of arsenate [As(V)] on a-FeOOH (goethite) and y-AI(OHb (gibbsite). Batch equilibrium As(V) adsorption experiments were conducted with P and MO as competing anions in order to produce single-anion [As(V), P, and MO] and binary-anion [As(V/P and As(V)/Mo] adsorption envelopes (adsorption vs. solution pH). Arsenate and P single-anion adsorption envelopes were similar with substantial adsorption occurring across a wide pH range, including pH values above the points of zero charge of the oxides. Maximum MO adsorption occurred across a narrower pH range (pH 4-6). On both oxides, equimolar P concentrations decreased As(V) adsorption within the pH range 2 to 11, whereas MO decreased As(V) adsorption only below pH 6. The constant capacitance model was used to predict competitive surface complexation behavior between As(V)/P and As(V)/Mo using intrinsic equilibrium constants [Kd, tint)] optimized from single-anion data. In addition, the model was applied using one-site (monodentate) and two-site (monodentate + bidentate) conceptualizations of the oxide surface. The two approaches gave comparable fits to experimental adsorption data and were consistent with competitive adsorption observed in binary adsorption envelopes. and MO has been described in whole soils with a competitive Freundlich-type isotherm equation (Roy et al., 1986), though the applicability o f the model was limited to cases where the As/P and As/Mo equilibrium concentration ratios were >20. Barrow (1974) investigated As(V) and P competitive adsorption in soil and found that, though As(V) desorbed some previously adsorbed P, a substantial portion o f the bound P was not displaced by As(V). Based on competitive adsorption between As(V) and P on goethite, Hingston et al. (1971) postulated that the goethite surface contains adsorption sites common to both As(V) and P anions, as well as sites that adsorb either one anion or the other. The ability to predict As(V) adsorption in complicated systems such as whole soils will require quantitative information on the adsorption of As(V) by individual soil minerals in the presence of competing anions.

Diffusion Technique for Preparing Salt Solutions, Kjeldahl Digests, and Persulfate Digests for Nitrogen-15 Analysis

Abstract: When diffusion techniques are used to prepare sample for 15 N analyses, low or variable N recovery is sometimes observed. The effect of low recovery on estimates of 15 N enrichment is unknown. Also, the suitability of diffusion techniques for use with variety of salt solutions and digests containing very low N concentrations ( 92% recovery Of 50 μg NH 4 + -N from 75 mL of 2 M NaCl, total Kjeldahl digests, or 2 M KCl preserved with acid. Leaving sample containers open for 5 d after diffusion for NH 4 + completely eliminated contamination of NO 3 - -N by residual NH 4 + ; however, recovery of NO 3 - was inversely related to the length of the open period. Recovery of NO 3 - -N from 0.5 M K 2 SO 4 and alkaline persulfate digests after a 3-d open period was 90 and 77%, respectively. Complete recovery of NH 4 + or NO 3 - was not required to obtain accurate intimates of 15 N enrichment; however, when recovery was < 100%, the method of blank correction was critical. When the mass of N contamination measured in diffusion blanks was used to blank-correct 15 N enrichments, diffused standards often had 15 N enrichments that were significantly different from nondiffused standards; however, when the mass of N in blanks was intimated using an isotope dilution equation, there was excellent agreement between diffused and nondiffused standards, regardless of the degree of recovery.

Carbon Dynamics of Aggregate-Associated Organic Matter Estimated by Carbon-13 Natural Abundance

Abstract: A major factor controlling soil organic matter dynamics is believed to be the differing degrees of protection from decomposition afforded by the spatially hierarchical organization of soil aggregate structure Changes in the natural 13 C content and in the concentration of soil organic C resulting from the growth of C3 pasture grasses (low δ 13 C PDB ) on former C4 cropland (high δ 13 C PDB ) were used to investigate the turnover and inputs of organic C in water-stable aggregates of different sizes After removal of free and released particulate organic matter (POM) in aggregate size separates (POM with a density ≤185 g cm -3 that was either exterior to aggregates in situ or released from unstable aggregates by slaking), organic C concentrations were greater in macroaggregates (>212 μm) than in microaggregates (53-212 μm) The turnover time (1/k) for C4-derived C was 412 yr for microaggregates, compared with an average turnover of 140 yr for macroaggregates, indicating that old C associated with microaggregates may be both biochemically recalcitrant and physically protected Net input rates of C3-derived C increased with aggregate size (073-113 g kg -1 yr -1 ), supporting the concept of an aggregate hierarchy created by the binding of microaggregates into increasingly larger macroaggregates The net input rate for microaggregates, however, was equal to the rates for small macroaggregates, suggesting that the formation and degradation of microaggregates may be more dynamic than is predicted by their stability in cultivated soils or by the observed turnover times for old C

Sediment and Chemical Load Reduction by Grass and Riparian Filters

Abstract: Vegetated filter strips help reduce non-point source pollution from agricultural areas. Even though they are an accepted and highly promoted practice, little quantitative data exist on their effectiveness under field conditions. The objective of this research was to determine the amount of nutrients and sediment removed by natural and planted filters. This was achieved by collecting and analyzing runoff at field edges and at various locations in vegetated buffers. Total weight of sediment and nutrients in runoff from North Carolina agricultural fields showed that the grass and riparian filter strips studied reduced runoff load by 50 to 80%. Total sediment decrease through the filters was about 80% for both grass and riparian vegetation. The reduction in the chemical load depended on the nutrient and its form. Filters reduced total P load by 50%, but 80% of the soluble PO 4 -P arriving at the field edge frequently passed through the filters. The filters retained 20 to 50% of the NH 4 and approximately 50% of the total Kjeldahl N and NO 3 . High-volume flows commonly overwhelmed both grass and riparian filters next to cultivated fields. Forested ephemeral channels had little vegetation and were effective sediment sinks during the dry season but were ineffective during large storm events because there was little resistance to flow. When possible, drainageways should be designed to hold sediment and to disperse the discharge into a riparian area.

Comparison of kriging and inverse-distance methods for mapping soil parameters

Abstract: Variable-rate technology may provide a means of increasing fertilizer use efficiency by matching applications to specific conditions at a given field location. Effective implementation of this technology depends on accurately characterizing the spatial variability of soil parameters used to define the application rate. Kriging and inversedistance-squared are two commonly used techniques for characterizing this spatial variability and interpolating between sampled points. To assess the accuracy of these techniques, data sets obtained from grid sampling two field research sites were used in a prediction-validation comparison of ordinary kriging and inverse-distance methods using powers p = 1, 2, and 4. The accuracy of the inverse-distance methods tended to increase with the power of distance for data sets with a coefficient of variation less than about 25% (typical of soil organic matter). However, for data sets with greater variation (such as soil NO 3 - ), inverse-distance prediction methods using high distance powers (2 or 4) can give very inaccurate predictions. The accuracy of predictions from kriging was generally unaffected by the coefficient of variation, and was relatively high for all of the sampling configurations considered in this study. These tendencies were also observed using 48- and 72-m subsamples, although the use of wider sampling spacings greatly reduced the information in the maps constructed by each method. Careful thought should be given to the choice of sample spacing and interpolation method to be used before data are collected. Summary statistics, and the coefficient of variation in particular, are simple measures that can give an indication of the relative accuracy of the inverse-distance and kriging mapping approaches.

Artificial Neural Networks to Estimate Soil Water Retention from Easily Measurable Data

Abstract: Indirect estimation of soil water retention from easily measurable data of soil surveys is needed to extend the applicability of hydrological models. Artificial neural networks (ANN) are becoming a common tool for modeling complex input-output dependencies. The objective of this work was to compare the accuracy of ANN and statistical regressions for water retention estimation from texture and bulk density. We used data on water contents at eight matric potentials for 130 Haplustoll and 100 Aquic Ustoll soil samples. Although the differences were not always statistically significant, ANN predicted water contents at selected matric potentials better than regression. The performances of ANN and regressions were comparable when van Genuchten's equation was fitted to data for each sample, and parameters of this equation were estimated from texture and bulk density. The precision of parameter estimations was lower than the precision of estimating water contents at a given soil water potential with both ANN and regressions. Grouping samples by horizons improved the precision of the estimates, especially in subsoil. Because they can mimic natural many inputs-many outputs relationships, ANN may be useful in the estimation of soil hydraulic properties from easily measurable soil data.

Molybdenum Adsorption on Oxides, Clay Minerals, and Soils

Abstract: Molybdenum adsorption behavior was investigated on various crystalline and x-ray amorphous Al and Fe oxide minerals, clay minerals, CaC@, and arid-zone calcareous and noncalcareou s ssoils. Molybdenum adsorption on botb Al and Fe oxides exhibited a maximum at low pH extending to about pH 4 to 5. Above pH 5 adsorption decreased rapidly, with little adsorptio noccurring above pH 8. Molybdenum adsorption was higher for the oxide minerals having higher specific surface area and lower crystallinity. Molybdenum adsorption on the clay minerals exhibited a peak near pH 3 and then decreased rapidly with increasing pH until adsorption was virtually zero near pH 7. The magnitude of MO adsorption on clays increased in the order: kaolinite < illite < montmorillonite. Shifts in point of zero charge were observed on Al and Fe oxides and kaolinite following MO adsorption, indicating an inner-sphere adsorption mechanism for MO on these surfaces. Molybdenum adsorption behavior on three arid-zone noncalcareous soils resembled that on clays, exhibiting a peak near pH 3 to 4 and decreasing with increasing pH up to pH 7. This behavior is expected since the oxide content of these soils is low. Molybdenum adsorption on calcite and tw ocalcareous arid-zone soils was low, indicating that CaCOJ is not a significant sink for MO in soils.

The Soil Freezing Characteristic: Its Measurement and Similarity to the Soil Moisture Characteristic

Abstract: 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.

Characterization of Miller‐Similar Silica Sands for Laboratory Hydrologic Studies

Abstract: 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.

Numerical Evaluation of Static-Chamber Measurements of Soil—Atmosphere Gas Exchange: Identification of Physical Processes

Abstract: The exchange of gases between soil and atmosphere is an important process that affects atmospheric chemistry and therefore climate. The static-chamber method is the most commonly used technique for estimating the rate of that exchange. We examined the method under hypothetical field conditions where diffusion was the only mechanism for gas transport and the atmosphere outside the chamber was maintained at a fixed concentration. Analytical and numerical solutions to the soil gas diffusion equation in one and three dimensions demonstrated that gas flux density to a static chamber deployed on the soil surface was less in magnitude than the ambient exchange rate in the absence of the chamber. This discrepancy, which increased with chamber deployment time and air-filled porosity of soil, is attributed to two physical factors: distortion of the soil gas concentration gradient (the magnitude was decreased in the vertical component and increased in the radial component) and the slow transport rate of diffusion relative to mixing within the chamber. Instantaneous flux density to a chamber decreased continuously with time; steepest decreases occurred so quickly following deployment and in response to such slight changes in mean chamber headspace concentration that they would likely go undetected by most field procedures. Adverse influences of these factors were reduced by mixing the chamber headspace, minimizing deployment time, maximizing the height and radius of the chamber, and pushing the rim of the chamber into the soil. Nonlinear models were superior to a linear regression model for estimating flux densities from mean headspace concentrations, suggesting that linearity of headspace concentration with time was not necessarily a good indicator of measurement accuracy.

Fertilization effects on soil organic matter turnover and corn residue C storage

Abstract: Soil organic matter turnover is influenced by N ; thus long-term fertilization of corn (Zea mays L.) may significantly affect soil organic matter levels. Effects of fertilization on soil organic matter turnover and storage of residue C under continuous corn were evaluated using soils from a long-term field experiment in Ontario. Total organic C and natural 13 C abundance measurements indicated that fertilized soils had more organic C than unfertilized soils, the difference accounted for by more C 4 -derived C in the fertilized soils. About 22 to 30% of the soil C in the plow layer had turned over and was derived from corn in the fertilized soils ; in unfertilized soils only 15 to 20% was derived from corn. Assuming that organic matter turnover follows first-order kinetics, the half-life of C 3 -derived C in the surface 10 cm of both soils was the same, about 19 yr. Natural 13 C abundance measurements and estimates from a soil organic matter model indicate that 10 to 20% of the added residue C was retained in the soil. Fertilized soils had more light fraction (LF) C than unfertilized soils. More than 70% of the C in the LF of fertilized soils was derived from corn ; in unfertilized soils only 41% was derived from corn. The half-life of C 3 -derived C in the LF was shorter than 10 yr. These results indicate that adequate fertilization increases crop yields, in turn leading to greater C storage, and that fertilization does not significantly alter the rate of turnover of native soil organic matter.

Isotope Discrimination during Decomposition of Organic Matter: A Theoretical Analysis

Abstract: 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 13 C during the course of decomposition. Isotope effects on decomposer growth rate, efficiency, and dispersion in quality increase, on the other hand, 13 C 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.

Uranium(VI) Adsorption on Goethite and Soil in Carbonate Solutions

Abstract: 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 CO 2 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 [UO 2 (CO 3 ) 2 2- and UO 2 (CO 3 ) 3 4- ] 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.

Characterizing the Dependence of Gas Diffusion Coefficient on Soil Properties

Abstract: Knowledge of the gaseous diffusion coefficient is necessary to properly model gas movement in porous media. In this study, a nonreactive tracer (freon-12) and a reactive tracer (hexafluorobenzene) were used to evaluate the relationship between the ratio of the gas diffusion coefficient in soil (D g s ) to that in free air (D g a ) and the volumetric air content (a) with a two-chamber diffusion system. The measured D g s /D g a values in our experiments (i.e., the relative gas diffusion coefficients) showed a similar relationship to air content as data obtained from various studies reported in the literature. Our data and values taken from other studies were used to test the validity of various models. When compared with data obtained from experiments with disturbed soils, the Penman model D g s /D g a = 0.66 a was found to overestimate the measured relative diffusion coefficient, whereas the Millington-Quirk model D g s /D g a = a 10/3 /φ 2 , where φ is the soil porosity, underestimated it. Another Millington-Quirk relationship D g s /D g a = a 2 /φ 2/3 , which has been largely overlooked in the literature, was found to provide significantly better agreement with measured relative diffusion coefficients in various disturbed soils of different texture. On the contrary, no universal relationship was found when data from both disturbed and undisturbed soil experiments were evaluated. The Troeh model D g s /D g a = [(a - u)/(1 - u)] v had the flexibility to fit all of the experimental data when both of the model parameters were varied simultaneously ; however, no obvious correlation was found between soil properties and the parameters. The limitations of any universal form for gas tortuosity model in natural soils were analyzed with percolation theory.

Chloride increases cadmium uptake in Swiss chard in a resin-buffered nutrient solution

Abstract: One of the possible hypotheses to explain the high Cd availability at elevated Cl concentration in soil is that chloro-complexes of Cd are available for root uptake. In order to test this hypothesis, Swiss chord [Beta vulgaris ssp. cicla (L.) Koch, cv. Fordhook Giant) was grown in nutrient solution continuously recirculated over a chelating resin (Chelex-100) partially loaded with Cd. Treatments were increasing concentrations of Cl (0.01, 40, 80, and 120 mM) in the nutrient solution with fourfold replication. Solution Na concentrations and ionic strengths were equalized in all treatments by compensating with NaNO3. Increasing Cl concentrations in solution did not affect dry weights of either roots or shoots. Activity of Cd 2+ in solution was well buffered during plant growth using the resin system. Complexation of Cd 2+ by Cl increased soluble Cd in culture solutions but the calculated activity of Cd 2+ was not significantly affected by increasing concentrations of Cl in solution. As solution Cl concentration increased, Cd concentrations in plant shoots increased from 6.5 to 17.3 mg kg -1 and in roots from 47 to 106 mg kg -1 . We concluded that enhancement of Cd uptake by Cl in soils need not be related only to enhanced diffusion of Cd 2+ through soil to the root but that (i) CdCl n 2-n (in addition to Cd 2+ ) species in solution are phytoavailable and/or (ii) Cl enhances diffusion of Cd 2+ through the unstirred liquid layer adjacent to the root surface or through the apoplast to sites of Cd uptake within the root itself.

Soil textural control on decomposition and soil organic matter dynamics

Abstract: Soil texture affects litter decomposition by altering soil water availability, pore size distribution, nutrient availability, and surface area. We examined the direct and indirect effects of soil texture on litter decomposition and soil organic matter mineralization during 91-d laboratory incubations. Treatments included texture (73, 55, and 40% sand), soil water pressure (−0.012, −0.033, and −0.30 MPa), and nutrient availability (with or without additional N and P). Wheat litter (Triticum aestivum L.) was placed on the surface or incorporated into the soil. Soil texture had no effect on litter decomposition (P >- 0.23). Litter decomposition was fastest for the −0.012 MPa treatment across all soil types (P < 0.01), and the difference between water pressure treatments was greatest in the loam (40% sand) soil. The effects of texture and soil water pressure could be combined into one variable (percentage water-filled pore space), which accounted for more of the variability in litter decomposition and native soil C mineralization than either texture or soil water pressure alone. Surface-applied litter decomposed significantly faster than incorporated litter, but the effect was not consistent across different soils (P = 0.04). Litter addition stimulated the mineralization of native soil C, the greatest effect occurring when litter was incorporated into the fine-textured soil. Contribution of the Rocky Mtn. For. Range Exp. Stn., Fort Collins, CO.

Determination of Microbial Biomass and Nitrogen Mineralization following Rewetting of Dried Soil

Abstract: 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. Contribution from the Texas Agric. Exp. Stn.

Sustained Productivity of Forests Is a Continuing Challenge to Soil Science

Abstract: Scientific uncertainties about ecosystem processes and greater awareness of the need for environmental care are sources of public anxiety over forest management. Partly because of this, the negative impacts of poor forestry practices are often emphasized, overlooking the achievements in sustainable forestry. The forest estates should be viewed and managed as a continuum so that the overall need for production of wood and the protection of environmental values can be met. We need practical goals of forest management. One goal should be to ensure that the trend in forest productivity is nondeclining or is positive through successive rotations and harvests, while maintaining and enhancing the quality of the soil resource base in perpetuity. The conflicts about the use of native forests for wood harvesting, while maintaining all conservation values, can be lessened if the value of each forest is ranked within a scale ranging from wood production to conservation. The growing demand for wood and concerns for land care can be met in part by expanding plantation forestry. Questions concerning management strategies for sustainable forestry are global in scope, but the genesis and application of practices for achieving this are local and are based fundamentally on the soil. The expectation of developing soil-based sustainability indicators can be realized only if the expectation is backed by substantial research linking changes in soil properties, ecosystem processes, and productivity at the landscape level. Challenges are many and include interdisciplinary approaches to research and forest management, application to ensure economic prosperity, and positive approaches to communication.

Recently deposited organic matter in soil water-stable aggregates

Abstract: Some suggest that young labile soil organic matter accumulates preferentially in water-stable macroaggregates (>250 um) where it acts as a transient binding agent We determined the proportions of recently deposited C ( 2 mm) than for smaller ones Assuming first-order kinetics, the estimated half-life of the Cj-derived C of stable aggregates >2 mm was 13 yr, which corresponds to that reported for macro organic matter in similar systems Analysis of water-stable macroaggregates under corn showed that they were enriched in recently deposited C relative to microaggregat es and to the whole soil, which partly compensated for their loss in Cj-C On average, 20% of the C in water-stable aggregates >1 mm was derived from corn whereas this value was down to 9% in the whole soil and 1% in the microaggregates The results of this study provide further quantitative evidence that slaking-resistant macroaggregates are enriched in, and probably stabilized by, recently deposited organic matter

Testing an Artificial Neural Network for Predicting Soil Hydraulic Conductivity

Abstract: Multilinear regression has been used extensively to predict soil hydraulic properties, both the θ(h) and K(h) relationships, from easily obtainable soil variables. As an alternative, this study investigated the performance of an artificial radial basis neural network in predicting some K(h) values from other variables. This kind of neural network may be seen as a multivariate interpolation technique, which can theoretically fit any nonlinear continuous function. Neural networks are characterized by parameters that must be optimized to solve a given problem. We used fitting procedure requiring only two parameters to ensure a unique solution. These two parameters were determined by data splitting. Hypothetical data bases with uncertainties were simulated to analyze the performance of the neural network in predicting a nonlinear relation derived from a classical model for K(h). A soil data base covering a broad spectrum of soil horizons was used to test the neural network in solving multivariate problems. Numerical simulations showed that the neural network was sensitive to large uncertainties in the data base. It was more efficient than a multilinear regression when the uncertainties were small. Experimental results showed that the neural network was more efficient than the multilinear regression for predicting K(h = -1 m) or K(h = - 2.5 m) from two qualitative and five quantitative soil variables. It was also more efficient than two independent multilinear regressions, one for the sandy samples and the other for the loamy and clayey samples. Provided that a large data base with accurate K values is available, artificial neural networks can be useful to predict θ(h) and K(h) over a broad spectrum of soils.

Landscape and Seasonal Patterns of Nitrous Oxide Emissions in a Semiarid Region

Abstract: As a prerequisite for quantification of annual N 2 O emissions at a regional scale, this study was conducted to determine the landscape-scale patterns and seasonal fluctuations of N 2 O emission and to demonstrate the linking relationships between large-scale controllers and proximal factors of N 2 O emission An area of the Black soil zone of central Saskatchewan, Canada, was stratified into three main textural areas: clay loam, fine sandy loam, and sandy Within each textural area, representative sites were selected based on land use: unfertilized and fertilized cropland, fallow, pasture, and forest sites A consistent landscape-scale pattern of N 2 O emission was observed; footslope positions had higher N 2 O fluxes than shoulder positions The role of topography is attributed to its strong influence on the hydrologic and pedologic processes in the landscape, which, in turn, regulate the soil factors controlling N 2 O emission at the microscale level The seasonal fluctuation of N 2 O emission was influenced by precipitation Pulses of activity were observed during summer, after rainfall events following N fertilizer application, and during spring thaw At the regional scale, the sandy area had lower N 2 O emissions than the fine-textured areas The general order of N 2 O evolved among the land uses was forest < pasture < fallow unfertilized < fertilized cropland Our results showed the importance of developing spatially based, predictive relationships between N 2 O emission and its controlling factors Linking these relationships with large-scale integrative variables, such as soil texture and land use, provides means for extrapolating N 2 O fluxes from landscape to regional scale

Soil organic matter pools during early adoption of conservation tillage in northwestern Canada

Abstract: Changes in soil organic matter (SOM) pools during adoption of reduced (RT) or zero tillage (ZT) can influence soil physical properties, nutrient cycling, and CO 2 flux between soil and atmosphere. We determined soil organic C (SOC), soil microbial biomass C (SMBC), basal soil respiration (BSR), and mineralizable N to a depth of 200 mm at the end of 3, 5, and 6 yr after implementation of tillage management on a Falher clay (fine, montmorillonitic, frigid Typic Natriboralf) near Rycroft, Alberta, in a canola (Brassica campestris L.)-wheat (Triticum aestivum L.)-barley (Hordeum vulgare L.)-fallow cropping system. At the end of 6 yr, SOC was not different among tillage regimes and averaged 8.6 kg m -2 . At the end of 3 and 5 yr, SMBC was not significantly different among tillage regimes, but at the end of6 yr SMBC was 7% greater in RT and 9% greater in ZT than in conventional tillage (CT). Basal soil respiration and mineralizable N at the end of 6 yr were not different among tillage regimes following barley and averaged 2.7 g CO 2 -C m -2 d -l and 5.0 g inorganic N m -2 24 d -1 , respectively. However, BSR following fallow was 2.2, 2.5, and 2.6 g CO 2 -C m -2 d -1 in CT, RT, and ZT, respectively. Mineralizable N following fallow was 5.8 g inorganic N m -2 (24 d) -1 in RT and ZT and 7.3 g inorganic N m -2 (24 d) -1 in CT. At 0 to 50 mm, there was no significant increase in SOC at the end of 6 yr, a 17 to 36% increase in SMBC, and a 12 to 69% increase in BSR with ZT compared with CT, depending on rotation phase. Relatively small changes in SOM pools with adoption of conservation tillage may be attributable to the large amount of SOM initially present and the cold, semiarid climate that limits SOM turnover.

Soil Nitrate and Water Dynamics in Sesbania Fallows, Weed Fallows, and Maize

Abstract: We hypothesized that the integration of trees into agricultural land-use systems can reduce NO3 leaching and increase subsoil N utilization. A field study was conducted on a Kandiudalfic Eutrudox (Ochinga site) and a Kandic Paleustalf (Muange site) in the subhumid highlands of Kenya to measure changes in soil NO3 and water to 200-cm depth for one rainy season in four land-use systems (LUS): (i) planted tree fallow using Sesbania sesban (L.) Merr., (ii) unfertilized maize (Zea mays L.), (iii) weed fallow, and (iv) bare fallow. Subsoil (50-200 cm) NO3-N at the start of the season ranged from 58 to 87 kg ha-1 for the four LUS and two sites. In maize, subsoil NO3-N differed by <5 kg ha-1 between planting and harvest at both sites. In sesbania, subsoil NO3-N decreased by 22 kg ha-1 at both sites, whereas in weed fallow subsoil NO3-N decreased by 26 and 38 kg ha-1 at Ochinga and Muange, respectively. At both sites, subsoil water contents at the start of the season were similar in the four LUS; but at the end of the season, soil water at 100 to 200 cm was significantly lower for sesbania than for maize. Adsorption of NO3 increased with soil depth. Sorbed NO3 at 100 to 200 cm was about 60% in the Kandiudalfic Eutrudox and about 15% in the Kandic Paleustalf. Rotation of maize with either a sesbania fallow or a weed fallow can result in more effective subsoil NO3 and water utilization than maize monoculture.

Leaching of Nitrogen from Slow-Release Urea Sources in Sandy Soils

Abstract: Application of readily soluble forms of N fertilizers to sandy soils may cause leaching of NO 3 -N resulting in contamination of groundwater. The leaching loss of N may be reduced to some extent by using slow-release forms of N. An intermittent leaching and incubation technique, to mimic natural occurrence of rainfall and dry conditions, was used to examine the leaching of N from readily soluble (NH 4 NO 3 ) and slow-release fertilizers [isobutylidene diurea (IBDU) and a polyolefin resin-coated urea, Meister] in Wabasso (sandy, siliceous, hyperthermic Alfic Haplaquod) and Candler (hyperthermic, uncoated Typic Quartzipsamment) soils. After 29 d, the cumulative recovery of the applied fertilizer N in the leachate for the treatments decreased in the following order: NH 4 NO 3 (88-100%) > IBDU (27-32%) > Meister (11.5-11.7%). A significant portion (19.5-35.5%) of the total N leached from IBDU and Meister was in the urea form in the initial leaching; however, after 9 d, N03 and NH4 forms represented the major portion of leachate total N. Although N from NH 4 NO 3 was leached completely from the Candler sand, 12% of the applied N (as NH 4 NO 3 ) was not recovered in the leachate from the Wabasso sand. Cumulative NH4-N leached from the Wabasso sand was only 58% of that from the Candler sand. The results demonstrate that the amounts and forms of fertilizer N leached from the sandy soils depend on the solubility of the fertilizer, the soil type, and the duration of intermittent leaching.

Availability of Residual Phosphorus in Manured Soils

Abstract: In many areas with confined animal operations, continual manure application has increased soil P above amounts sufficient for optimum crop yields. In these areas, it is of economic and environmental importance to determine how long high-P soils will remain above crop sufficiency and identify soils where P contents would decrease most rapidly under similar management conditions. Thus, the surface 5 cm of 23 high-P soils (85-419 mg kg -1 Mehlich-3 P) in Oklahoma and Texas, which had received beef feedlot, poultry, or swine manure (90-1880 kg P ha -1 yr -1 for up to 35 yr) were successively extracted with Fe-oxide-impregnated paper strips to investigate residual soil P availability. A decrease in strip P with successive extractions followed the equation: Strip P = a(extraction number) -b (r 2 of 0.88-0.98). The rate of P release to strips (exponent b) decreased more rapidly as soil P sorption saturation increased (R 2 of 0.79). Phosphorus saturation also accounted for 85% of the variation in the total amount of P released to strips from manured soils in 15 successive extractions (51-572 mg kg - 1). Fractionation of soil P before and after strip extraction showed bicarbonate inorganic P contributed most of the P released to strips (46%). The above equation also described soil P release in several published field studies (r 2 of 0.77-0.98). Thus, successive strip extraction of soil has the potential to describe soil factors controlling the availability of residual P and identify soils where high P contents may be less buffered and, thus, decrease more rapidly than others under similar management conditions.

Backscattered electron scanning images of soil porosity for analyzing soil compaction around roots

Abstract: Thin sections of soil surrounding maize roots were studied in scanning electron microscopy using the backscattered electron mode. The soil is a fine-loamy, mixed, mesic, Typic Eutrochrept. Backscattered electron scanning images (BESI) of the porosity surrounding a selected maize (Zea mays L.) root were studied by image analysis at the scale of micro- and mesopores. Image analysis of BESI revealed that the porosity was 22 to 24% less within the soil surrounding the root than in the bulk soil. The bulk density increased up to 1.80 Mg m -3 close by the root-soil interface, although it was 1.54 Mg m -3 in the bulk soil. The porosity reduction consisted of a removal of the mesopores and a decrease in micropores, which resulted from the packing of skeleton grains with the porous clayey phase. The micropores were affected, although they are usually thought to be altered only with difficulty under natural conditions. A model which was developed earlier for soil compression around roots of plants growing on remolded soils was fitted to the experimental data by nonlinear regression analysis.

Preservation of Plant Residues in Soils Differing in Unsaturated Protective Capacity

Abstract: This study tested the hypothesis that the decomposition of applied residue C in soil is not determined by soil texture per se but by the degree of saturation of the protective capacity of a soil. Soil protective capacity is defined as the maximum amount of C associated with clay and silt (<20 μm) in grassland and uncultivated soils. To test this hypothesis, 14 C-labeled ryegrass (grown in a phytotron and continuously labeled with 14 CO 2 ; specific activity 546 Bq mg -1 C) was mixed with 11 soil samples differing in texture and saturation deficit, the latter being the difference between the actual and the maximum amount of C associated with the <20-μm fraction. After 3 d of incubation, the percentage of applied 14 C that had respired showed a significant correlation (r = -0.85) with the saturation deficit. After 53 d of incubation, the amount of 14 C respired showed a significant correlation with the saturation deficit of the fine-textured soils (r = - 0.88), but not with those of coarse-textured soils. The correlation between 14 CO 2 production and soil texture was weak. The results confirm the hypothesis that the degree of saturation of the protective capacity of a soil predicts the decomposition rate of residue C better than does soil texture alone.

Errors in Converting Time Domain Reflectometry Measurements of Propagation Velocity to Estimates of Soil Water Content

Abstract: 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 (T air ). 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/T air and volumetric water content (θ v ) 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 θ v . Absolute measurement errors range from 0.015 to 0.028 m 3 m -3 if the T/T air vs. θ v intercept is known and increase to 0.023 to 0.034 m 3 m -3 if a nominal value (1.55) is used. We recommend that a T/T air vs. θ v 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 m 3 m -3 .

Changes in ponderosa pine site productivity following removal of understory vegetation

Abstract: Competition from understory vegetation for water and nutrients can limit productivity of young forest stands. Less is known of the effect of understory vegetation on long-term stand growth or soil organic properties. The effect of understory vegetation on periodic annual increments (PAIs) of basal area, height, and volume for ponderosa pine (Pinus ponderosa Doug].) in central Oregon at 4 or 5-yr intervals was examined for a 35-yr period. Soil C, N, and microbial biomass C (MBC) were also quantified after 32 and 35 yr with and without understory vegetation on a sandy loam pumice soil (Xeric Vitricryand). Five tree spacings, ranging from 2.0 to 8.0 m (1542469 trees ha -1 ), in combination with two understory treatments (understory vegetation present or continuously absent) were installed in 1959. Total understory vegetation cover averaged 3570 between 1959 and 1994 for treatments with understory vegetation present, and was dominated by three shrub species: antelope bitterbrush [Purshia tridentata (Pursh) DC.], greenleaf manzanita (Arctostaphylos patula Green), and snowbrush (Ceanothus velutinus Dougl. ex Hook.). Covariance analyses of PAIs for each successive interval were performed using appropriate stand parameters at the start of each interval as covariates. Tree growth was reduced by competing understory vegetation during the first 12 to 20 yr only; understory vegetation did not reduce the adjusted PAIs during the last 15 yr. Soil C and N were measured incrementally to a depth of 24 cm. Presence of understory vegetation resulted in greater C and N in the O horizon and upper 4 to 12 cm of mineral soil. Seasonal MBC, measured at 14-d intervals from May to November, was greater when understory vegetation was present. The results suggest that understory vegetation plays an important role in maintaining soil quality.