Showing papers in "Soil Science Society of America Journal in 2000"

Soil structure and organic matter: I. Distribution of aggregate-size classes and aggregate-associated carbon.

Abstract: Cultivation reduces soil C content and changes the distribution and stability of soil aggregates. We investigated the effect of cultivation intensity on aggregate distribution and aggregate C in three soils dominated by 2:1 clay mineralogy and one soil characterized by a mixed (2:1 and 1:1) mineralogy. Each site had native vegetation (NV), no-tillage (NT), and conventional tillage (CT) treatments. Slaked (i.e., air-dried and fast-rewetted) and capillary rewetted soils were separated into four aggregate-size classes ( 2000 μm) by wet sieving. In rewetted soils, the proportion of macroaggregates accounted for 85% of the dry soil weight and was similar across management treatments. In contrast, aggregate distribution from slaked soils increasingly shifted toward more microaggregates and fewer macroaggregates with increasing cultivation intensity. In soils dominated by 2:1 clay mineralogy, the C content of macroaggregates was 1.65 times greater compared to microaggregates. These observations support an aggregate hierarchy in which microaggregates are bound together into macroaggregates by organic binding agents in 2:1 clay-dominated soils. In the soil with mixed mineralogy, aggregate C did not increase with increasing aggregate size. At all sites, rewetted macro- and microaggregate C and slaked microaggregate C differed in the order NV > NT > CT, In contrast, slaked macroaggregate C concentration was similar across management treatments, except in the soil with mixed clay mineralogy. We conclude that increasing cultivation intensity leads to a loss of C-rich macroaggregates and an increase of C-depleted microaggregates in soils that express aggregate hierarchy.

Organic Matter Influence on Clay Wettability and Soil Aggregate Stability

Abstract: Soil organic matter is thought to increase aggregate stability by lowering the wettability and increasing the cohesion of aggregates. In southwest France, thick humic loamy soils (Vermic Haplubrepts) have been intensively cropped for 40 yr, decreasing the soil organic pool and lowering the soil agregate stability. This study assessed (i) the contribution of organic matter to aggregate stability by decreasing aggregate wettability and (ii) the specific role of clay-associated organic matter. Soil samples with a C content of 4 to 53 g kg -1 were sampled and soil aggregate stability was measured. Aggregate wettability was assessed by measuring water drop penetration times on individual 3-to 5-mm aggregates. The <2-μm fractions were extracted without organic matter destruction and their wettability was determined by measuring contact angles of water on clay deposits. Aggregate stability against slaking was correlated to soil C content (r 2 = 0.71 for fast wetting). Water drop penetration time increased with C contents from 1 to 32 s and was very heterogeneous among individual aggregates from a given soil. The contact angle of water on the clay fraction increased linearly with the C content (r 2 = 0.86). This change in clay wettability could partly explain the higher water stability of soils rich in C.

Soil Thermal Conductivity Effects of Density, Moisture, Salt Concentration, and Organic Matter

Abstract: The thermal conductivity of soil under a given set of conditions is most important as it relates to a soil's microclimate. The early growth and development of a crop may be determined to a large extent by microclimate. The effect of bulk density, moisture content, salt concentration, and organic matter on the thermal conductivity of some sieved and repacked Jordanian soils was investigated through laboratory studies. These laboratory experiments used the single probe method to determine thermal conductivity. The soils used were classified as sand, sandy loam, loam, and clay loam. The two salts used were NaCl and CaCl 2 , while addition of peat moss was used to increase the organic matter content, For the soils studied, thermal conductivity increased with increasing soil density and moisture content. Thermal conductivity ranged from 0.58 to 1.94 for sand, from 0.19 to 1.12 for sandy loam, from 0.29 to 0.76 for loam, and from 0.36 to 0.69 W/m K for clay loam at densities from 1.23 to 1.59 g cm 3 and water contents from 1.4 to 21.2%, The results also show that an increase in the amount of added salts at given moisture content (volumetric solution contents θ ranged from 0.03-0.12 m 3 m -3 for the sand and from 0.09-0.30 m 3 m 3 for the clay loam) decreased thermal conductivity. Increasing the percentage of soil organic matter decreased thermal conductivity, Finally, il was found that the sand had higher values of thermal conductivity than the clay loam for the same salt type and concentrations.

Biochemical quality of crop residues and carbon and nitrogen mineralization kinetics under nonlimiting nitrogen conditions

Abstract: Statistical relationships were established between the fate of C and N from 47 types of crop residues and their biochemical characteristics during a soil incubation at 15°C. The incubations were carried out under nonlimiting N in order to differentiate the effects of biochemical characteristics of residues from those of soil N availability. Depending on the residue, the apparent mineralization of residue C after 168 d varied from 330 to 670 g kg -1 of added C. Mineralization kinetics were described using a two-compartment decomposition model that decomposes according to first-order kinetics. Amounts of C mineralized after 7 d and the decomposition rate coefficient of the labile fraction were related mainly to the soluble C forms of the residue. No statistical relationship was established between the N concentration of residues and their decomposition in the soil. The incorporation of crop residues into soil led to various soil mineral N dynamics. Two residues caused net N mineralization from the time of their incorporation, whereas all the others induced net N immobilization (1-33 g N kg -1 of added C). After 168 d, only residues with a C/N ratio <24 induced a surplus of mineral N compared with the control soil. The mineral N dynamics were related mainly to the organic N concentration of the residues and to their C/N ratio. At the start of incubation, these dynamics were also influenced by the presence of polyphenols in the plant tissues. Finally, this study showed the need to include the biochemical quality of crop residues in any C and N transformation models that describe decomposition. In contrast, the N concentration or C/N ratio of the residues are sufficient to predict the net effects of crop residues on soil mineral N dynamics.

Soil structure and soil organic matter : II. A normalized stability index and the effect of mineralogy

Abstract: Soil aggregate distribution and stability measurements have been proposed as soil quality indicators. However, pretreatment of soil, antecedent water content and differences in sand size distribution among soils can confound interpretation of these measurements. We propose a normalized stability index (NSI) which (i) compares aggregate distribution after slaking and rewetting to characterize whole soil stability and eliminate confounding effects of pretreatment and antecedent water content, (ii) corrects for the confounding effect of differences in sand size distribution among soils, aggregate size classes and pretreatments, and (iii) normalizes the level of disruption imposed by slaking by using a maximum level of disruption. The NSI was tested on three soils dominated by a 2:1 clay mineralogy and one soil characterized by a mixed (2:1 and 1:1) clay mineralogy. Each site had native vegetation (NV), no-tillage (NT), and conventional tillage (CT) treatments. In soils dominated by 2:1 clays, NSI decreased with increasing cultivation intensity (NV > NT > CT). However, NSI was higher in the soil with mixed clays compared to the other soils and was not different along the cultivation gradient. These observations are hypothesized to be related to the presence of Fe- and Al-oxides and kaolinite. In conclusion, NSI appears to be a good indicator for soil structural quality since it is sensitive to changes in agricultural practices and it minimizes confounding effects. A decrease of SOM levels results in a smaller decrease of soil stability in soils dominated by oxides and 1:1 minerals compared to soils dominated by 2:1 minerals.

Cattle Manure Amendments Can Increase the pH of Acid Soils

Abstract: Crop production on acid soils can be improved greatly by adjusting the pH to near neutrality. While soil acidity is commonly corrected by liming, there is evidence that animal manure amendments can increase the pH of acid soils. The effect of fresh cattle manure on soil acidity and nutrient availability was determined in the laboratory for two acid soils from Beaverlodge and Fort Vermillion in the Peace River region of Alberta, Canada. The effect of manure on soil pH was immediate and persisted during an 8-wk incubation. Manure-amended soil had significantly higher pH than unamended soil, and the highest rate (40 g manure kg -1 , dry weight basis) increased the pH of Beaverlodge and Fort Vermillion soils from 4.8 to 6.0 and 5.5 to 6.3, respectively. The higher pH in manure-amended than unamended soils was attributed to buffering from bicarbonates and organic acids in cattle manure. Mineral N (NH 4 -N + NO 3 -N), available P, K, Ca, and Mg increased immediately after manure application, and available P and K remained significantly higher in manure-amended than unamended soil after the 8-wk incubation. Soils amended with 40 g manure kg -1 had three to four times more plant-available P and K than unamended soils after incubation. Available S concentrations did not differ significantly in manure-amended and unamended soils. Extractable Al and Fe declined slightly after manure application, but did not differ in manure-amended or unamended soils after incubation. No change in the cation-exchange capacity (CEC) of manure-amended soils compared to unamended soils was observed in this study, and it appears that appreciable changes in Al, Fe, and CEC from manure application do not occur in the short-term (weeks). Our results indicate that, in the short-term, cattle manure amendments can increase the pH and the quantity of plant-available P and K in acid soils.

Rhizobia inoculation improves nutrient uptake and growth of lowland rice.

Abstract: Growth-promoting diazotrophs can enhance the growth and development of associated crops by transferring fixed N or by improving nutrient uptake through modulation of hormone-linked phenomena in inoculated plants. Six rhizobial diazotrophs isolated from a wide range of legume hosts were investigated to determine their growth-promoting activities in lowland rice (Oryza sativa L.) during 1997. Seeds and seedlings of rice Pankaj were inoculated with different rhizobia and grown in potted soil supplemented with varied amounts of mineral N. Inoculation with Rhizobium leguminosarum bv. trifolii E11, Rhizobium sp. IRBG74, and Bradyrhizobium sp. IRBG271 increased rice grain and straw yields by 8 to 22 and 4 to 19%, respectively, at different N rates. Nitrogen, P, and K uptake were increased by 10 to 28% due to rhizobial inoculation. Nitrogen-15-based studies indicated that the increased N uptake was not due to biological N 2 fixation (BNF). Inoculation also increased Fe uptake in rice by 15 to 64%. Indole-3-acetic acid (IAA) accumulated in the external root environment of rice plants when grown gnotobiotically with rhizobia. The results indicate that certain strains of rhizobia can promote rice growth and yield, most likely through mechanisms that involve changes in growth physiology or root morphology rather than BNF.

Modeling Soil–Landscape and Ecosystem Properties Using Terrain Attributes

Abstract: Soil-landscape patterns result from the integration of short- and long-term pedogeomorphic processes. A 2-ha hillslope catena in California shows short-distance variation in A horizon depth from 8 to 80 cm and in soil depth from 8 to >450 cm in convex to concave positions. Similar variations in net primary productivity (NPP) and soil C represent significant information often not captured by soil survey maps. Strong correlations between these measured soil-landscape variables and explanatory digital terrain attributes are used to develop quantitative soil-landscape models. We were able to account for between 52 and 88% of soil property variance using easily computed terrain variables such as slope and flow accumulation. Spatial implementation of the models suggest lateral redistribution processes resulting in differential accumulation of C and soil mass in convergent and divergent landscape positions. The models are explicit and quantitative, which enables their use for testing hypotheses about the spatial distribution of fine-scale landscape and ecosystem processes and for parameterizing spatially distributed hydrological and ecosystem simulation models.

Improved Prediction of Unsaturated Hydraulic Conductivity with the Mualem-van Genuchten Model

Abstract: In many vadose zone hydrological studies, it is imperative that the soil's unsaturated hydraulic conductivity is known. Frequently, the Mualem-van Genuchten model (MVG) is used for this purpose because it allows prediction of unsaturated hydraulic conductivity from water retention parameters. For this and similar equations, it is often assumed that a measured saturated hydraulic conductivity (K s ) can be used as a matching point (K o ) while a factor S L e is used to account for pore connectivity and tortuosity (where S e is the relative saturation and L = 0.5). We used a data set of 235 soil samples with retention and unsaturated hydraulic conductivity data to test and improve predictions with the MVG equation. The standard practice of using K o = K, and L = 0.5 resulted in a root mean square error for log(K) (RMSE K ) of 1.31. Optimization of the matching point (K o ) and L to the hydraulic conductivity data yielded a RMSE K of 0.41. The fitted K 0 were, on average, about one order of magnitude smaller than measured K s . Furthermore, L was predominantly negative, casting doubt that the MVG can be interpreted in a physical way, Spearman rank correlations showed that both K 0 and L were related to van Genuchten water retention parameters and neural network analyses confirmed that K 0 and L could indeed be predicted in this way. The corresponding RMSE K was 0.84, which was half an order of magnitude better than the traditional MVG model. Bulk density and textural parameters were poor predictors while addition of K s improved the RMSE K only marginally. Bootstrap analysis showed that the uncertainty in predicted unsaturated hydraulic conductivity was about one order of magnitude near saturation and larger at lower water contents.

Identification of regional soil quality factors and indicators : I. Central and Southern High Plains

Abstract: Appropriate indicators for assessing soil quality on a regional scale using the National Resource Inventory (NRI) are unknown. Our objectives were to (i) identify soil quality factors present at a regional scale, (ii) determine which factors vary significantly with land use, and (iii) select soil attributes within these factors that can be used as soil quality indicators for regional-scale assessment. Ascalon (fine-loamy, mixed, superactive, mesic Aridic Argiustoll) and Amarillo (fine-loamy, mixed, thermic Aridic Paleustalf) soils were sampled from a statistically representative subset of NRI sample points within the Central and Southern High Plains Major Land Resource Areas (MLRA) and analyzed for 20 soil attributes. Factor analysis was used to identify soil quality factors, and discriminant analysis was used to identify the factors and indicators most sensitive to land use within each MLRA. In the Central High Plains, five soil quality factors were identified, with the organic matter and color factors varying significantly with land use. Discriminant analysis selected total organic C (TOC) and total N as the most sensitive indicators of soil quality at a regional scale. In the Southern High Plains, six factors were identified, with water stable aggregate (WSA) content, TOC, and soil salinity varying significantly with land use. Discriminant analysis selected TOC and WSA content as the most sensitive indicators of soil quality in the Southern High Plains. Total organic C was the only indicator that consistently showed significant differences between land uses in both regions.

Comparison of Fatty Acid Methyl Ester (FAME) Methods for Characterizing Microbial Communities

Abstract: Fatty acid profiling is a popular method for characterizing microbial communities of natural systems. Direct extraction of microbial fatty acids in situ would be useful compared with methods that extract lipids first and subsequently release fatty acids from lipids. In this study, two methods for the direct extraction of fatty acids from soils were compared for three cultivated silt loams and one forested sandy clay loam. Fresh soils were analyzed for their fatty acid methyl ester (FAME) profiles by an ester-linked (EL) method and the method of MIDI (Microbial ID, Inc., Newark, DE). Soils were stored four different ways (moist at 4°C, moist at -20°C, air-dried at 25°C, and partially dry at 4°C) and analyzed for FAME profile changes after 30 and 90 d of storage. Eleven and 17 FAMEs were unique to the EL and MIDI method, respectively, but unique FAMEs generally were found in only minute amounts. Soils extracted with the MIDI method yielded more hydroxy FAMEs and short-chain saturated and branched FAMEs. Conversely, EL-extracted soils generally produced more long-chain saturated and branched FAMEs, unsaturated FAMEs, and FAMEs with cyclopropane and methyl groups. Both extraction methods were able to differentiate among communities of different soil types, regardless if soils were fresh or stored. Changes in FAME profiles did occur in stored soils, but the effectiveness of each storage protocol for preserving FAME patterns over time was different among the four soils. While community analyses should be conducted on fresh soil, overall effects of storage were slight compared with those of extraction method and soil type.

Factors Controlling Soil Carbon Levels in New Zealand Grasslands Is Clay Content Important

Abstract: Soil organic matter is a major component of biogeochemical cycles and is important in maintaining soil quality. We investigated relationships between soil organic C and various soil and site properties that may influence long-term soil C accumulation across a range of soil orders in New Zealand. We used pedon and climatic data for 167 pedons under permanent grass, and carried out regression analysis between soil C (0-200 mm) contents it ha -1 ) or concentrations (g kg -1 ) and climatic and soil properties, namely, precipitation, temperature, and contents or concentrations of sand, silt, clay, pyrophosphate-extractable Al (Al py ), Fe oxide, and allophane. Soil clay content or concentration explained little of the variation in soil C across all soils (R 2 < 0.05) and within each soil type. Likewise, mean annual precipitation and temperature explained little variation in soil C content or concentration (R 2 < 0.15 for precipitation, R 2 = 0.04 for temperature). Allophane content or concentration was unrelated to soil C in the soils of volcanic origin; Al py , however, correlated strongly with both soil C content and soil C concentration across all soil types (R 2 = 0.55 and 0.60, respectively). When all factors were combined in a multiple regression analysis, the combination of Al py and allophane contents explained the greatest amount of variation in soil C content (R 2 = 0.57), whereas the combination of Al py , Fe oxide, allophane, and clay concentrations explained the greatest amount of variation in soil C concentration (R 2 = 0.67). Out results suggest that in New Zealand soils, chemical stabilization of organic matter is the key process controlling soil C accumulation, and that clay content relates poorly to long-term soil organic C accumulation.

Root-Derived Carbon and the Formation and Stabilization of Aggregates

Abstract: It is hypothesized that particulate organic matter (POM) contributes to aggregate stability. However, little is known about the dynamics of the POM fraction or its role in aggregate formation. A simulated no-till study was conducted to examine changes in free and aggregate-associated POM during the decomposition of in situ 14 C-labeled roots during a 1-yr incubation in a loess-derived silt loam. Two water pretreatments (capillary-wetted and slaked) were applied to soil samples collected during the incubation, and the samples were then wet sieved to obtain five aggregate size fractions. Densiometric separations were used to isolate free and released POM (frPOM) and intraaggregate POM (iPOM). Small macroaggregates (250-2000 μm) were enriched in iPOM- 14 C on Day 0 which suggested that many of these aggregates formed around cores of new, root-derived POM during the growth and senescence of the oat plants. Slaking resulted in the disruption of many of the small macroaggregates (250-2000 μm) and a large increase in frPOM- 14 C on Day 0. The amount of 14 C released into the frPOM pool with slaking declined with time. In contrast, there was a significant linear increase in the amount of new, root-derived iPOM- 14 C in large microaggregates (53-250 μm) that were released when unstable macroaggregates (>250 μm) slaked. These data support the hypothesis that new microaggregates are formed within existing macroaggregates and provide strong evidence that, in no-till, aggregate formation and stabilization processes are directly related to the decomposition of root-residue and the dynamics of POM C in the soil.

Flush of carbon dioxide following rewetting of dried soil relates to active organic pools.

Abstract: Soil quality assessment could become more standardized with the development of a simple, rapid, and reliable method for quantifying potential soil biological activity. We evaluated the flush of CO 2 following rewetting of dried soil under standard laboratory conditions as a method to estimate an active organic matter fraction. The flush of CO 2 following rewetting of dried soil (3 d incubation at 50% water-filled pore space and 25°C) was assessed for 20 soil series containing a wide range of organic C (20 ± 13 g kg -1 ) from Alberta-British Columbia, Maine, Texas, and Georgia. This flush of CO 2 explained 97% of the variability in cumulative C mineralization during 24 d [y = 12 + 3.3(x); n = 471], 86% of the variability in soil microbial biomass C [y = 337 + 2.4(x); n 399], and 67% of the variability in net N mineralization during 24 d [y = 18 + 0.10(x) - 0.00002(x) 2 ; n = 327]. Accounting for geographical differences in mean annual temperature and precipitation, which could affect soil organic matter quality, further improved relationships between the flush of CO 2 and active, passive, and total C and N pools. Measuring the flush of CO 2 following rewetting of dried soil may have value for routine soil testing of biological soil quality because it (i) is an incubation procedure patterned after natural occurrences in most soils, (ii) exhibits strong overall relationships with active organic pools, (iii) shows relatively minor changes in relationships with active organic pools that may lie due to climatic variables, (iv) has a simple setup with minimal equipment requirements, and (v) has rapid analysis time.

Effects of soil organic matter on the kinetics and mechanisms of Pb(II) sorption and desorption in soil

Abstract: To improve predictions of the toxicity and threat from Pb contaminated soil, it is critical that time-dependent sorption and desorption behavior be understood. In this paper, the sorption and desorption behavior of Pb in a Matapeake silt loam soil (Typic Hapludult) were studied by stirred-flow and batch experiments. In addition, the authors studied the effects of soil organic matter (SOM) on sorption and desorption behavior by treating the soil with sodium hypochlorite to remove the SOM fraction, and using a soil with six times as much SOM (St. Johns loamy sand [Typic Haplaquods]) as the Matapeake soil. Lead sorption consisted of a fast initial reaction in which all of the Pb added to the stirred-flow chamber was sorbed. Following this initial fast reaction, sorption continued and appears to be rate limited. The total amount of Pb sorbed was 102, 44, and 27 mmol kg{sup {minus}1} for the St. Johns soil and the untreated and treated Matapeake soils, respectively. Desorption experiments were conducted on the soils with the background electrolyte as the eluent in the stirred-flow chamber. In the St. Johns soil only, 32% of the total sorbed Pb was desorbed, while 47 and 76% of the sorbed Pb was releasedmore » from the untreated and treated Matapeake soil, respectively. The correlation between SOM in the soils, and the percentage Pb desorbed from the soils suggests that SOM plays an important role in slow desorption reactions of Pb from soil materials. Aging experiments in which sorbed Pb was incubated for 1, 10, and 32 d showed that sorption incubation time had no effect on Pb desorption behavior. Analysis of the treated and untreated Matapeake soils by x-ray absorption fine structure (XAFS) spectroscopy revealed that the local atomic structure of sorbed Pb is distinctly different in the two samples. In the soil treated to remove SOM, the data were well represented by theoretical models using O, Si, and Pb backscattering atoms. In the untreated soil, the XAFS data were best described by O and C backscatterers. These XAFS results confirm that the sorption mechanisms in the two systems are different.« less

Predicting the Gas Diffusion Coefficient in Repacked Soil Water-Induced Linear Reduction Model

Abstract: Investigations of gas transport and fate processes in packed soil systems require knowledge of the gas diffusion coefficient, D p , as a function of air-filled porosity, e. On the basis of the literature, data from six studies over the porosity range of 0.1 to nearly 1.0, it is reconfirmed that the Marshall (1959) model better predicts D p (e) in completely dry, repacked porous media than do the Penman (1940) and Millington (1959) models. The smaller D p value in wet soil, as compared with dry soil at the same air-filled porosity, is accounted for by introducing a water-induced linear reduction (WLR) term, equal to the ratio of air-filled porosity to total porosity, in the D p (e) model. By adding the WLR term in each of the three D p (e) models for dry porous media, the so-called WLR(Marshall), WLR(Penman), and WLR(Millington) D p (e) models for wet soil are developed. To test the three WLR models, D p was measured at different soil-water contents in six differently textured (6-38% clay) repacked soils. The WLR (Marshall) model accurately and best described D p (e) for all six soils and additional soils from the literature. All three WLR models performed better than previous D p (e) models. This study implies that the smaller D p in a wet soil, which is due to water-induced changes in air-filled pore shape and pore connectivity, can be described by a simple, linear function of relative air-filled porosity. The WLR(Marshall) model represents a conceptual and accurate model to predict D p (e) in sieved, repacked soil.

Release of Intracellular Solutes by Four Soil Bacteria Exposed to Dilution Stress

Abstract: The physiological mechanisms utilized by soil bacteria for acclimation to sudden increases in soil water potential are poorly understood. In this study, we examined the physiological responses of soil isolates of Pseudomonas chlororaphis, P. fluorescens, Bacillus pumulis, and Streptomyces griseus to a sudden increase in solution water potential (dilution). Bacterial isolates were cultured at a low solute water potential (-3.0 MPa) and subjected to rapid water potential increases of 0.5 to 2.0 MPa. The small amount of protein and DNA released by a 2.0 MPa dilution suggests that water potential increases up to 2.0 MPa did not cause significant cell lysis. In response to dilution, intracellular solutes were released into the extracellular environment rather than polymerized into osmotically less-active compounds or catabolized to CO 2 . In general, the Gram-positive isolates B. pumulis and S. griseus were more tolerant to dilution than the Pseudomonas spp., since dilution had no effect on culturability, and the amount of solutes released was small (<10% of the intracellular solute pool). The Pseudomonas spp. released a maximum of 22 to 26% of their amino acid pool and 54 to 60% of their low molecular weight neutral sugar pool. The amounts of amino acids and low molecular weight carbohydrates released and the reduction in culturability was, in general, proportional to the magnitude of dilution. Pseudomonas fluorescens tolerated a 0.5 MPa water potential increase, but water potential shocks of greater magnitude resulted in a large reduction in culturability and an increase in the amount of solutes released. These results suggest that a potential source of mineralizable C following the wetting of dry soils is the release of organic compatible solutes from the microbial community.

Relationships Between Soil Test Phosphorus, Soluble Phosphorus, and Phosphorus Saturation in Delaware Soils

Abstract: Methods to identify agricultural soils that contribute to nonpoint-source pollution of surface waters by P are of increasing importance, particularly in areas with high animal densities (animal units per hectare of cropland). Our objective was to determine the relationship between agronomic soil test P (STP = Mehlich 1) and other soil P tests proposed to measure the potential for P loss by erosion, runoff, and leaching. We compared STP with soluble P, P in the fast desorbing pool (strip P), and soil P saturation for 127 soils (122 from Delaware and five from the Netherlands). Soil test P was significantly correlated with total P (r = 0.57 *** , significant at the 0.001 level), soluble P (r = 0.71 *** ), strip P (r = 0.84 *** ), and oxalate-extractable P (P ox ; r = 0.84 *** ), Strip P was a better predictor of soluble P than STP (r 2 0.76 *** ). The ratio of strip P/P ox (the percentage of reversibly sorbed P in the fast desorbing pool) increased as P sorption capacity, estimated from oxalate-extractable Al and Fe (Al ox + Fe ox ), decreased. We also determined the degree of P saturation (DPS) using three methods: Langmuir P sorption isotherms; oxalate extractions of P, Al, and Fe; and STP plus a single-point P sorption index (PSI). Soluble P, STP, and desorbable P increased for DPS values >30%, similar to upper DPS limits in the Netherlands and Belgium. Soils rated agronomically excessive in STP (>50 mg kg -1 ) had higher ratios of soluble P, strip P, and P ox to total P than those in agronomically optimum or lower categories.

Slope Length Effects on Soil Loss for Steep Slopes

Abstract: Empirical soil erosion models continue to play an important role in soil conservation planning and environmental evaluations around the world. The effect of hillslope length on soil loss, often termed the slope length factor, is one of the main and most variable components of any empirical model. In the most widely used model, the Universal Soil Loss Equation (USLE), normalized soil loss, L, is expressed as a power function of slope length, λ, as L = (λ/22.1) m , in which the slope exponent, m, is 0.2, 0.3, 0.4, and 0.5 for different, increasing slope gradients. In the Revised Universal Soil Loss Equation (RUSLE), the exponent, m, is defined as a continuous function of slope gradient and the expected ratio of rill to interrill erosion. When the slope gradient is 60% and the ratio of rill to interrill erosion is classified as moderate, the exponent m has the value of 0.71 in RUSLE, as compared with 0.5 for the USLE. The purpose of this study was to evaluate the relationship between soil loss and slope length for slopes up to 60% in steepness. Soil loss data from natural runoff plots at three locations on the Loess Plateau in China and data from a previous study were used. The results indicated that the exponent, m, for the relationship between soil loss and the slope length for the combined data from the three stations in the Loess Plateau was 0.44 (r 2 = 0.95). For the data as a whole, the exponent did not increase as slope steepness increased from 20 to 60%. We also found that the value of m was greater for intense storms than for less intense storms. These experimental data indicate that the USLE exponent, m = 0.5, is more appropriate for steep slopes than is the RUSLE exponent, and that the slope length exponent varies as a function of rainfall intensity.

Quantitative Characterization of Humic Substances by Solid-State Carbon-13 Nuclear Magnetic Resonance

Abstract: The compositions of humic acids (HAs) from various Histosols in North America and Europe, of similarly treated plant-extracted materials (PEMs), of coal-extracted humic acids, and of International Humic Substances Society (IHSS) Florida peat were quantified by solid-state 13 C nuclear magnetic resonance (NMR). In order to obtain quantitative intensities, the peak areas in direct-polarization 13-kHz magic-angle spinning (DPMAS) 13 C NMR spectra were corrected for incomplete relaxation by factors measured in cross-polarization spin-lattice relaxation time (CP/T 1 ) experiments with total sideband suppression (TOSS). The elemental compositions (%C, %H, %O + N) of a peat sample, 8 HAs and 2 PEMs were estimated from the NMR results and compared with chemical analyses, as well as solution NMR for two of the HAs. The results are in good agreement, which shows that DPMAS corrected by CP/T 1 -TOSS permits quantitative characterization of HAs and PEMs. The compositions of the PEMs deviate significantly from those of the Histosol HAs. The compositions in terms of nine types of chemical groups were computed. The investigated HAs consist of more than 60% of aromatic and CO carbons (including both carbonyl and carboxyl groups). Previous cross-polarization magic-angle spinning (CPMAS) NMR experiments have significantly underestimated the ratio of sp 2 -to sp 3 -carbons; in particular, the true COO carbon fraction is a factor of two larger than estimated by CPMAS NMR. In spite of their wide range of geographical origins, the compositions of the Histosol HAs appear to be relatively uniform, suggesting that the search for a general model of Histosol HA structure is worthwhile. Eight models proposed in the literature do not reproduce the experimentally determined compositions, but a few models show promising partial agreement.

Predicting the Gas Diffusion Coefficient in Undisturbed Soil from Soil Water Characteristics

Abstract: The gas diffusion coefficient in soil (D P ), and its dependency on soil physical characteristics, governs the diffusive transport of oxygen, greenhouse gases, fumigants, and volatile organic pollutants in agricultural, forest, and urban soils. Accurate models for predicting Dp as a function of air-filled porosity (e) in natural, undisturbed soil are needed for realistic gas transport and fate simulations. Using data from 126 undisturbed soil layers, we obtained a high correlation (r 2 = 0.97) for a simple, nonlinear expression describing D P at -100 cm H 2 O of soil water potential (D P,100 ) as a function of the corresponding air-filled porosity (e 100 ), equal to the volume of soil pores with an equivalent pore diameter >30 μm. A new D P (e) model was developed by combining the D P,100 (e 100 ) expression with the Burdine relative hydraulic conductivity model, the latter modified to predict relative gas diffusivity in unsaturated soil. The D P,100 and Burdine terms in the D P (e) model are both related to the soil water characteristic (SWC) curve and, thus, the actual pore-size distribution within the water content range considered. The D P (e) model requires knowledge of the soil's air-filled and total porosities and a minimum of two points on the SWC curve, including a measurement at -100 cm H 2 O. When tested against independent gas diffusivity data for 21 differently textured and undisturbed soils, the SWC-dependent D P (e) model accurately predicted measured data and gave a reduction in root mean square error of prediction between 58 and 83% compared to the classical, soil type-independent Penman and Millington-Quirk models. To further test the new D P (e) model, gas diffusivity and SWC measurements on undisturbed soil cores from three 0.4-m soil horizons (sandy clay loam, sandy loam, and loamy sand) within the 4 to 7 m depth below an industrially polluted soil site were carried out. For these deep subsurface soils the SWC-dependent model best predicted the measured gas diffusivities.

Methane and Nitrous Oxide Emissions from a Rice Field in Relation to Soil Redox and Microbiological Processes

Abstract: Paddy rice fields provide an environment for production of two important greenhouse gases, CH4 and N2O, because of variations in soil characteristics, moisture content, and microbial activity during the cropping season Emissions of CH4 and N2O from a paddy rice field in northern China were measured in situ by static chamber technique during March to December in 1995 and 1996 Factors affecting gas emission, including soil temperature, pH, and redox potential (Eh), were measured as well Emissions of CH4 and N2O were strongly correlated with changes in soil redox potential Significant CH4 emission occurred only at soil redox potential lower than approximately -100 mV, while the emission of N2O was not significant below +200 mV A significant inverse relationship between CH4 and N2O emissions was observed (r = -049, n = 16, 5% confidence level) The results suggest the possibility of using management practices to maintain the redox potential in a range where both N2O and CH4 emissions are low The activities of six related bacteria groups (zymogenic bacteria, acetic acid and hydrogen-producers, methanogens, CH4 oxidizers, and nitrifiers and denitrifiers) in the soil were also measured in an effort to explain the relationship between gas emission and soil microbiological processes Methane emission was significantly related to the logarithm number of zymogenic bacteria (r = 076, n = 12, 1% confidence level), as well as to soil redox potential (r = -072, n = 12, 1% confidence level) Both zymogenic bacteria number and soil redox potential appear to be predicators of CH4 emission potential

Soil Community Analysis Using DGGE of 16S rDNA Polymerase Chain Reaction Products

Abstract: Separation of polymerase chain reaction (PCR)-amplified 16S rDNA products using denaturing gradient gel electrophoresis (DGGE) was tested as a means to study microbial community composition in hulk soil samples. DNA was extracted from six soils from agroecosystems in Norway and the USA under different agronomic treatments (crop, rotation, and tillage); one soil is contaminated with polyaromatic hydrocarbons (PAH, 700 mg kg 1 ), Two sets of primers specific for Bacteria (V3 and the V6/V9 regions of 16S rRNA) and another for Archaea (V3 region of 16S rRNA) were used to determine the contribution of each domain to the microbial community. Reproducible, characteristic profiles of the communities were obtained by DGGE separation of the PCR amplification products. The number of fragments resolved by DGGE indicated bacterial diversity was far greater than that of the Archaea in the agricultural soils examined. Only the soil contaminated with PAHs had reduced bacterial diversity, evidenced by a distinet DGGE profile. The results showed that the method is useful as an initial step to discriminate among communities because it is rapid and multiple samples can be easily screened. There are some limitations, but under highly selective conditions it is possible to distinguish communities from different soils and to indicate the presence of numerically dominant populations.

Adsorption and Transport of Uranium(VI) in Subsurface Media

Abstract: Uranium(VI) adsorption and transport in three natural, heterogeneous subsurface media were investigated in batch and column experiments. The rate of U(VI) adsorption to the natural samples was rapid over the first few hours of the experiments, and then slowed appreciably after 24 to 48 h. The adsorption of U(VI) to the samples was also nonlinear, suggesting a decreasing attraction for the surface with increased surface loading. The extent of adsorption on each of the media was strongly pH-dependent, increasing sharply as the pH increased from 4.5 to 5.5 and then decreasing sharply over the pH range 7.5 to 8.5 as the concentration of dissolved carbonate and U(VI)-carbonate complexes increased. The similarities in the pH-dependent behavior between the three materials despite differences in bulk mineralogy was likely due to the similar Fe contents of the materials. The transport of U(VI) through packed columns of the soils and sediments was significantly retarded but reversible. The local equilibrium assumption and the batch-measured adsorption isotherms dramatically underestimated the degree of retardation observed in the columns. The U(VI) displacement experiments were modeled with the one-dimensional advective-dispersive equation and several different model formulations describing the interactions of U(VI) with the solid phase. These models were able to fit the observed breakthrough curves within 0.1 root mean square error of the initial concentration.

Organic farming: challenge of timing nitrogen availability to crop nitrogen requirements.

Abstract: Groundwater has become increasingly degraded by NO 3 , and this degradation has been partially attributed to the use of commercial inorganic N fertilizers. Conversion from conventional fertilizer management to organic farming has been proposed as a means to reduce groundwater degradation. Matching soil inorganic N supply with crop N requirement on a temporal basis is important to achieve high yield and low water degradation. Dynamics of N mineralization from two manures and N-uptake dynamics for two crops were derived from published data, and multi year simulations were done using the ENVIRON-GRO computer model, which accounts for N and irrigation management effects on crop yield and N leaching. The temporal N-mineralization and N-uptake curves did not match well. The potential N uptake for corn (Zea mays L.) exceeded the cumulative mineralized N during a significant period that would cause reduced yield. Wheat (Triticum aestivum L.) has a low and flat N-uptake peak, so that the cumulative mineralized N met N demand by wheat during the growing season. A crop with a very high maximum N-uptake rate, such as corn, would be difficult to fertilize with only organic N to meet peak demands without excessive N in the soil before and after crop growth. In order to satisfy crop N demand, a large amount of manure, which would leave much N or subsequent leaching, must be applied. It took two or more years after conversion to organic sources of N to reach maximum yield because of carryover of unmineralized manure and accumulation of mineralized N after crop uptake which was not completely leached during the winter. High initial applications to build up the organic pool followed by reduced inputs in subsequent years would be appropriate.

Predicting depressional storage from soil surface roughness.

Abstract: Runoff may be reduced by temporal water storage in depressions at the soil surface. Depressional storage is difficult to measure and is usually estimated from some roughness index. The objective of this study was to compare the ability of selected roughness indices to describe maximum depressional storage (MDS). Height measurements were taken on 221 tilled soil surfaces across a range of roughnesses. Maximum depressional storage was determined from digital elevation models (DEMs). The MDS values ranged from 0 to 13 mm. Five roughness indices were calculated from transects across these DEMs: random roughness (RR), tortuosity (T), limiting elevation difference (LD) and slope (LS), and mean upslope depression (MUD). Regression analysis of MDS on each of five roughness indices showed that RR best described depressional storage (r 2 = 0.80). Prediction of MDS in the field based on RR has an uncertainty of ± 3 mm (95% confidence interval). Variation was due to RR and its nonspatial nature. To improve predictions of MDS, the spatial configuration of the surface has to be taken into account.

Organic and Biodynamic Management Effects on Soil Biology

Abstract: Biodynamic agriculture is a unique organic farming system that utilizes, in addition to the common tools of organic agriculture, specific fermented herbal preparations as compost additives and field sprays. The objective of this work was to determine whether biodynamic compost or field spray preparations affect the soil biological community in the short term, beyond the effects of organic management. Four fertilizer options: (i) composted dairy manure and bedding (organic fertilization), (ii) the same material composted with biodynamic compost preparations, (iii) mineral fertilizers, and (iv) no fertilizer were investigated with and without the biodynamic field spray preparations. Both biodynamic and nonbiodynamic composts increased soil microbial biomass, respiration, dehydrogenase activity, soil C mineralized in 10 d (MinC), earthworm (Lumbricus terrestris) population and biomass, and metabolic quotient of respiration per unit biomass (qCO 2 ) by the second year of study. No significant differences were found between soils fertilized with biodynamic vs. nonbiodynamic compost. Use of biodynamic field sprays was associated with more MinC and minor differences in soil microbial fatty acid profiles in the first year of study. There were no other observed effects of the biodynamic preparations. Organically and biodynamically managed soils had similar microbial status and were more biotically active than soils that did not receive organic fertilization. Organic management enhanced soil biological activity, but additional use of the biodynamic preparations did not significantly affect the soil biotic parameters tested.

Nitrogen mineralization from field-applied beef cattle feedlot manure or compost.

Abstract: To apply manure or compost to fulfill N requirements of a crop, the amount of N mineralized in actual field conditions needs to be determined. Nitrogen mineralization from composted and noncomposted beef cattle feedlot manure applied to no-till and conventional tillage systems was determined under field conditions for 3 yr. Manure, composted manure, and inorganic fertilizer were applied to provide for N needs of corn. A no-treatment check was also included. An in situ resin method was used to determine N mineralization from a soil receiving manure, compost, and no treatment during the growing season (June-October). Of the organic N applied the previous autumn, =11% was mineralized from composted manure and 21% from noncomposted manure during the succeeding growing season. Lower N availability from compost reflects the loss of easily convertible N compounds during composting and the presence of stable N compounds. Nitrogen mineralization was similar in the no-till and conventional tillage systems even though manure and compost were surface-applied in the no-till. Nitrogen mineralization was significantly, but not closely (R 2 = 0.21), related to thermal unit (cumulative mean daily temperature >0°C). Mineralization rate constants indicated that availability of residual manure and compost N was less than expected. The in situ mineralization approach seems to be a good method of measuring N mineralization during the growing season or during periods when the soil is not frozen or excessively dry. Nitrogen mineralization needs to be considered when manure and compost are used for an environmentally acceptable crop production system.

Influence of time on soil response to no-till practices.

Abstract: The number of growing seasons required for no-till practices to improve soil properties should be considered before changing management systems. To evaluate this time factor, an 8-yr tillage study was conducted on a Grenada silt loam (fine-silty, mixed, active, thermic Glossic Fragiudalfs) using cotton (Gossypium hirsutum L.), grain sorghum [Sorghum bicolor (L.) Moench]-corn (Zea mays L.), and soybean [Glycine max (L.) Merr.]-wheat (Triticum aestivum L.) as test crops. Soil samples were characterized for soil organic matter (SOM), pH, exchangeable Ca and Mg, extractable P, K, Fe, Mn, Cu, and Zn, aggregate stability (AS), water dispersible clay (WDC), total clay (TC), and modulus of rupture (MR) at time 0, 4, and 8 yr. Within 4 yr, no-till (NT) resulted in statistically significant (P ≤ 0.05) differences compared to conventional tillage (CT). The surface 2.5 cm of the NT treatments had higher levels of SOM, exchangeable (a, and extractable P, Mn, and Zn, but lower extractable K, Fe, and Cu. Tillage had no effect on exchangeable Mg and pH. No-till also resulted in higher AS, and lower MR, WDC, and TC in the lop 2.5 cm, relative to CT. The differences in soil properties between tillage treatments were essentially independent of crop. Instead, the results are controlled by relative amounts of SOM and clay, and the extent to which these properties change with time. Undoubtedly, NT practices can improve several fertility and erodibility-related properties of this soil within 4 yr, and enhance its sustainability.

Identification of Regional Soil Quality Factors and Indicators II. Northern Mississippi Loess Hills and Palouse Prairie

Abstract: Diversity of soil series present in a region may hinder identification of soil quality factors and indicators at a regional scale. Our objectives were (i) to identify soil quality factors for a diverse population of soils at the regional scale, (ii) to determine which factors vary significantly with land use, (iii) to select indicators from these factors that can be used with the National Resource Inventory (NRI) for monitoring soil quality, and (iv) to compare these results to a similar study involving only a single soil series. One hundred eighty-six points representing 75 soil series in the Northern Mississippi Valley Loess Hills and 149 points representing 58 soil series in Palouse and Nez Perce Prairies were sampled from a statistically representative subset of NRI sample points and analyzed for 20 soil attributes. Factor analysis was used to identify soil quality factors and discriminant analysis was used to identify factors and indicators most sensitive to land use within each region. In the Northern Mississippi Valley Loess Hills, five soil quality factors were identified. Discriminant analysis selected potentially mineralizable N (PMN), microbial biomass C (MBC), water stable aggregates (WSA), and total organic C (TOC) as the most discriminating attributes between land uses. In the Palouse and Nez Perce Prairies, six factors were identified. Discriminant analysis selected TOC and total N as the most discriminating attributes between land uses. The soil quality factors were similar among three of the four regions, but TOC was the only indicator common to all regions for distinguishing among land uses.