Showing papers in "Soil Science Society of America Journal in 2006"

Soil Water Characteristic Estimates by Texture and Organic Matter for Hydrologic Solutions

Abstract: Hydrologic analyses often involve the evaluation of soil water infiltration, conductivity, storage, and plant-water relationships. To define the hydrologic soil water effects requires estimating soil water characteristics for water potential and hydraulic conductivity using soil variables such as texture, organic matter (OM), and structure. Field or laboratory measurements are difficult, costly, and often impractical for many hydrologic analyses. Statistical correlations between soil texture, soil water potential, and hydraulic conductivity can provide estimates sufficiently accurate for many analyses and decisions. This study developed new soil water characteristic equations from the currently available USDA soil database using only the readily available variables of soil texture and OM. These equations are similar to those previously reported by Saxton et al. but include more variables and application range. They were combined with previously reported relationships for tensions and conductivities and the effects of density, gravel, and salinity to form a comprehensive predictive system of soil water characteristics for agricultural water management and hydrologic analyses. Verification was performed using independent data sets for a wide range of soil textures. The predictive system was programmed for a graphical computerized model to provide easy application and rapid solutions and is available at http://hydrolab.arsusda. gov/soilwater/Index.htm.

Black Carbon Increases Cation Exchange Capacity in Soils

Abstract: Black Carbon (BC) may significantly affect nutrient retention and play a key role in a wide range of biogeochemical processes in soils, especially for nutrient cycling. Anthrosols from the Brazilian Amazon (ages between 600 and 8700 yr BP) with high contents of biomassderived BC had greater potential cation exchange capacity (CEC measured at pH 7) per unit organic C than adjacent soils with low BC contents.Synchrotron-based near edge X-ray absorption fine structure (NEXAFS) spectroscopy coupled with scanning transmission X-ray microscopy (STXM) techniques explained the source of the higher surface charge of BC compared with non-BC by mapping crosssectional areas of BC particles with diameters of 10 to 50 mm for C forms. The largest cross-sectional areas consisted of highly aromatic or only slightly oxidized organic C most likely originating from the BC itself with a characteristic peak at 286.1 eV, which could not be found in humic substance extracts, bacteria or fungi. Oxidation significantly increased from the core of BC particles to their surfaces as shown by the ratio of carboxyl-C/aromatic-C. Spotted and non-continuous distribution patterns of highly oxidized C functional groups with distinctly different chemical signatures on BC particle surfaces (peak shift at 286.1 eV to a higher energy of 286.7 eV) indicated that non-BC may be adsorbed on the surfaces of BC particles creating highly oxidized surface. As a consequence of both oxidation of the BC particles themselves and adsorption of organic matter to BC surfaces, the charge density (potential CEC per unit surface area) was greater in BC-rich Anthrosols than adjacent soils. Additionally, a high specific surface area was attributable to the presence of BC, which may contribute to the high CEC found in soils that are rich in BC.

Bacterial and Fungal Contributions to Carbon Sequestration in Agroecosystems

Abstract: This paper reviews the current knowledge of microbial processes affecting C sequestration in agroecosystems. The microbial contribution to soil C storage is directly related to microbial community dynamics and the balance between formation and degradation of microbial byproducts. Soil microbes also indirectly influence C cycling by improving soil aggregation, which physically protects soil organic matter (SOM). Consequently, the microbial contribution to C sequestration is governed by the interactions between the amount of microbial biomass, microbial community structure, microbial byproducts, and soil properties such as texture, clay mineralogy, pore-size distribution, and aggregate dynamics. The capacity of a soil to protect microbial biomass and microbially derived organic matter (MOM) is directly and/or indirectly (i.e., through physical protection by aggregates) related to the reactive properties of clays. However, the stabilization of MOM in the soil is also related to the efficiency with which microorganisms utilize substrate C and the chemical nature of the byproducts they produce. Crop rotations, reduced or no-tillage practices, organic farming, and cover crops increase total microbial biomass and shift the community structure toward a more fungal-dominated community, thereby enhancing the accumulation of MOM. A quantitative and qualitative improvement of SOM is generally observed in agroecosystems favoring a fungal-dominated community, but the mechanisms leading to this improvement are not completely understood. Gaps within our knowledge on MOM-C dynamics and how they are related to soil properties and agricultural practices are identified.

Wildfire-Produced Charcoal Directly Influences Nitrogen Cycling in Ponderosa Pine Forests

Abstract: Fire is the primary form of disturbance in temperate and boreal forest ecosystems. However, our knowledge of the biochemical mechanisms by which fire stimulates forest N cycling is incomplete. Charcoal is a major byproduct of forest fires and is ubiquitous in soils of most forest ecosystems, yet the biological function of charcoal in soils of forest ecosystems has been greatly overlooked. We conducted a suite of laboratory experiments on soils from ponderosa pine (Pinus ponderosa Laws) forests to determine the influence of charcoal on soil N dynamics and in particular, nitrification. The addition of NH4 1 to forest soils had absolutely no effect on nitrification demonstrating that this process is not substrate limited. The amendment of these soils with NH4 1 and field collected charcoal (1% w/w) significantly increased the nitrification potential, net nitrification, gross nitrification, and decreased the solution concentrations of plant secondary compounds (phenolics). Charcoal had no effect on nitrification in soils (from a grassland site) that had naturally high rates of nitrifier activity. The increase in gross nitrification in forest soils and lack of effect on grassland soils suggests that charcoal may alleviate factors that otherwise inhibit the activity of the nitrifying microbial community in forest soils. These results reveal the biological importance of charcoal and advance our mechanistic understanding of how fire drives nutrient cycling in temperate and boreal ecosystems.

Chemical and biological characteristics of physically uncomplexed organic matter

Abstract: Physical fractionation methods are based on the premise that soil organic matter (SOM) can be divided into pools of functional relevance. Physically uncomplexed organic matter (OM) is isolated on the basis of particle size and/or density. Our objective here is to review research on the biological and chemical characteristics of physically uncomplexed OM that demonstrates its value (or otherwise) as a meaningful pool of SOM. Chemical characterization indicates that fractions isolated by size are not identical to those separated by density; even materials separated using variations of a particular fractionation method (i.e., different sizes or different densities) have different chemical and biological properties. Physically uncomplexed OM often contains a substantial portion of whole soil carbon (C) and nitrogen (N) and, compared with the whole soil or heavy fraction, has a wide C/N ratio and high O-alkyl (i.e., carbohydrates) and low carbonyl (i.e., proteins) C contents. The response of physically uncomplexed OM to changes in land use and management practices is greater than that of other labile OM fractions or the whole soil C and N. Studies to elucidate the nutrient availability of physically uncomplexed OM suggest that it is not an immediate source of nutrients. That the quantity of physically uncomplexed OM is not always related to the amount of plant residue inputs suggests that other factors may control its accumulation in soil. Thus the quantity and the biological and chemical properties of physically uncomplexed OM are affected by the amount, composition, and accessibility of plant residues entering the soil; environmental conditions that may enhance or constrain decomposition; and the fractionation technique used.

Beerkan Estimation of Soil Transfer Parameters through Infiltration Experiments—BEST

Abstract: tudying soil hydrological processes requires the determination of soil hydraulic parameters whose assessment using traditional methods is expensive and time-consuming. A specific method, Beerkan estimation of soil transfer parameters referred to as BEST was developed to facilitate the determination of both the water retention curve, {theta}(h), and the hydraulic conductivity curve, K({theta}), defined by their shape and scale parameters. BEST estimates shape parameters from particle-size distribution analysis and scale parameters from infiltration experiments at null pressure head. Saturated water content is measured directly at the end of infiltration. Hydraulic conductivity and water pressure scale parameters are calculated from the steady-state infiltration rate and prior estimation of sorptivity (S) This is provided by fitting transient infiltration data on the classical two-term equations with values from zero to a maximum corresponding to null hydraulic conductivity and using a data subset for which the two-term infiltration equations are verified as valid. BEST was compared with other fitting methods to estimate sorptivity and hydraulic conductivity from infiltration modeling data on the basis of the same infiltration equations for three contrasting soils: agricultural soil, sandy soil, and a coarser fluvioglacial deposit. The other methods failed sometimes to model accurately experimental data and to provide values in agreement with physical principles of water infiltration (negative values for hydraulic conductivity, too high steady-state infiltration rate). None of these anomalies was encountered when modeling cumulative infiltration with BEST. BEST appears to be a promising, easy, robust, and inexpensive way of characterizing the hydraulic behavior of soil.

No-Till Corn/Soybean Systems Including Winter Cover Crops

Abstract: The use of winter cover crops (WCC) such as hairy vetch (Vicia villosa Roth) and cereal rye (Secale cereale L.), in a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation provides long-term benefits that are generally overlooked. There is a particular lack of information regarding the effects of WCC on soil physical and chemical properties. The objective of this study was to assess the effects of four crop sequences (C/S, corn-fallow/soybean-fallow; C-R/S-R, cornrye/soybean-rye; C-R/S-V, corn-rye/soybean-vetch; and C-R/S-VR, corn-rye/soybean-vetch and rye) under no-till on several soil physical and chemical properties. Soil chemical properties included soil organic matter (SOM), pH, total nitrogen (TN), nitrates (NO3–N), and available phosphorus (P). The analyzed soil physical properties analyzed were: water-aggregate stability (WAS), bulk density (Db), penetration resistance(PR),totalporosity(TP),pore-sizedistribution,waterretention properties, and saturated hydraulic conductivity (Ksat). The experimental design was a split-split-plot where whole-plot treatments (sampling period) had a Latin square design and subplot treatments (crop sequences) were arranged in a randomized complete block design with four replications. Compared with winter fallow, crop sequences that included WCC provided substantial benefits from the soil productivity standpoint. Specifically, the use of the C-R/S-Vor C-R/SVR increased SOM down to 30 cm. All WCC sequences improved WAS with increases of 9, 13, and 17% for C-R/S-R, C-R/S-V, and C-R/ S-VR, respectively. Winter cover crop sequences reduced Db and PR of the soil surface and increased total and storage porosity along with plant available water. While the C-R/S-V sequence was the most effective in reducing soil NO3–N, the C-R/S-R sequence was the most effective in fixing soil P.

Total and Labile Soil Organic Matter in Organic and Conventional Farming Systems

Abstract: Even though organic management practices are intended to enhance soil performance by altering the quantity or quality of soil organic matter (SOM), there is no consensus on how to measure or manage SOM status. We investigated the veracity of common perceptions about SOM quantity in organically and conventionally managed soils by evaluating the relative responsiveness to organic management of particulate organic matter (POM) and the Illinois Soil N Test (IL-N), which has been proposed as a direct measure of labile N. Soil samples were obtained from nine farming systems trials in the USA. Soil organic C (SOC), total N (TN), POM-C, POM-N, and IL-N were compared among manure 1 legume-based organic, legume-based organic, and conventional farming systems. The organic systems had higher SOC and TN concentrations than conventional systems whether or not manure was applied. The POM-C, POM-N, and IL-N concentrations did not differ between manure 1 legumeand legume-based organic systems. The amount of N recovered in POM and IL-N was similar. Organic management enriched soil POMC and -N by 30 to 40% relative to the conventional control and this level of enrichment was two to four times greater than that in any other fraction. The IL-N fraction was not a good measure of labile N as it was less enriched than POM and included recalcitrant components. This is evidenced by the strong correlation between IL-N and SOC, TN, climate and textural characteristics. Particulate organic matter provided clearer evidence of SOM and labile N accrual under organic management. Direct links between POM status and soil N supply and physical condition are being pursued to help farmers manage biologically based fertility.

Spectral Reflectance Methodology in Comparison to Traditional Soil Analysis

Abstract: Traditional soil analyses are expensive, time-consuming, and may also result in environmental pollutants. The objective of this study was to develop and evaluate a methodology to measure soil attributes using spectral reflectance (SR) as an alternative to traditional methods. Tropical Brazilian soils were sampled over a 196-ha area divided into grids. Samples (n 5 184) were obtained from the 0- to 20- and 80- to 100-cm depths and georeferenced. The laboratory SR data were obtained using a Spectroradiometer (400–2 500 nm). Satellite reflectance values were sampled from corrected Landsat Thematic Mapper (TM) images. Particle-size distribution and chemical analysis (organic matter [OM], cation-exchange capacity [CEC], total SiO2 ,F e 2O3, TiO2, sum of cations, cation, and Al saturation) were performed in the laboratory. Statistical analysis and multiple regression equations for soilattribute predictionsusingradiometricdataweredeveloped.Laboratory data used 22 bands and 13 ‘‘Reflectance Inflexion Differences, RID’’ from different wavelength intervals of the optical spectrum. However, the satellite data used only the reflectance of the 1, 2, 3, 4, 5, and 7 TM-Landsat bands. Multiple regression equations were derived from surface and subsurface soil layers. Estimations of some tropical soil attributes were possible using laboratory spectral analysis. Laboratory SR yielded high correlations with traditional laboratory analyses (R 2 . 0.79) for the soil attributes such as clay, sand, TiO2, and Fe2O3. Satellite spectral data correlated well with most of the soil attributes such as clay, Fe2O3, and TiO2 (reaching R 2 5 0.72). The use of soil analysis methodology by satellite and/or ground remote sensing constitutes an alternative to traditional routine laboratory analysis.

Impact of soil texture on the distribution of soil organic matter in physical and chemical fractions

Abstract: Previous research on the protection of soil organic C from decomposition suggests that soil texture affects soil C stocks. However, different pools of soil organic matter (SOM) might be differently related to soil texture. Our objective was to examine how soil texture differentially alters the distribution of organic C within physically and chemically defined pools of unprotected and protected SOM. We collected samples from two soil texture gradients where other variables influencing soil organic C content were held constant. One texture gradient (16–60% clay) was located near Stewart Valley, Saskatchewan, Canada and the other (25–50% clay) near Cygnet, OH. Soils were physically fractionated into coarse- and fine-particulate organic matter (POM), silt- and clay-sized particles within microaggregates, and easily dispersed silt- and clay-sized particles outside of microaggregates. Whole-soil organic C concentration was positively related to silt plus clay content at both sites. We found no relationship between soil texture and unprotected C (coarse- and fine-POM C). Biochemically protected C (nonhydrolyzable C) increased with increasing clay content in whole-soil samples, but the proportion of nonhydrolyzable C within silt- and clay-sized fractions was unchanged. As the amount of silt or clay increased, the amount of C stabilized within easily dispersed and microaggregate-associated silt or clay fractions decreased. Our results suggest that for a given level of C inputs, the relationship between mineral surface area and soil organic matter varies with soil texture for physically and biochemically protected C fractions. Because soil texture acts directly and indirectly on various protection mechanisms, it may not be a universal predictor of whole-soil C content.

Dissolved Organic Matter Characterization Using Multiway Spectral Decomposition of Fluorescence Landscapes

Abstract: Dissolved organic matter (DOM) plays an important role in many soil ecosystem functions. Multidimensional fluorescence spectroscopy of DOM with parallel factor analysis (PARAFAC) of the resulting spectral landscape has been successful in characterizing DOM from a variety of aquatic sources. This study was conducted to assess the multiway PARAFAC approach for quantitatively characterizing the fluorescent landscapes of DOM from aqueous extracts of soils and soil amendments. The DOM was extracted from plant biomass representative of crop, wetlands, and tree species; animal manures; and soils from controlled studies of cropping systems with known histories of organic amendments. The fluorescence landscape spectra were collected in the excitation range from 240 to 400 nm and emission range from 300 to 500 nm in 3-nm increments. The excitation and emission spectra modeled from the PARAFAC analysis showed that the plant biomass, animal manure, and soil DOM contained five fluoresdng components: tryptophan-like (peak location at excitation 270 nm, emission 354 nm), tyrosine-like (273/309 nm), and three humic-substance-like components (>240/465 nm, 306/405 nm, and 315/447 nm). Principal component analysis of the concentration loading showed that the soil-derived DOM was very similar despite the different types and quantities of organic amendments incorporated in the different cropping systems. This study shows that PARAFAC analysis of multidimensional fluorescence spectra can model the chemical profile of terrestrial DOM in a chemically meaningful way. This represents a significant advance over current approaches to interpreting the complex DOM fluorescence spectra.

Maize Root Biomass and Net Rhizodeposited Carbon

Abstract: Assessment of net primary productivity of maize (Zea mays L.)-based agroecosystems is dependent on both above and belowground dry matter production that is ultimately returned to the soil as residue and decaying roots. Root to shoot ratio (R/S) is a parameter often used to estimate root biomass (RB) when shoot biomass is measured or estimated. The labor intensive nature of root sampling and wide variety of sampling techniques has lead to a paucity of maize RB data in the literature, and few researchers have endeavored to characterize R/S throughout an entire growing season. In this paper, the results of 45 maize root studies published in 41 journal articles are summarized and the data used to generate estimates of maize RB and R/S versus days after emergence (DAE). The data from these studies indicate that on average, RB was maximized just after anthesis at approximately 31 g plant -1 (13.6 g C plant -1 ) and that average R/S varied from a high of 0.68 at emergence to a low of 0.16 at physiological maturity. Net rhizodeposited C as a percentage of total net root-derived belowground C at time of sampling (%NRC) was reported for 12 maize studies and varied between 5 and 62%. The wide variation in the %NRC was shown to be highly correlated with an index combining irradiance level, photoperiod, and ambient temperature, suggesting a strong dependence of net rhizodeposited C on rate of photosynthesis and soil respiration. The net belowground C deposition at maize physiological maturity is estimated as 29 ± 13% of shoot biomass C for maize that has not experienced stress.

Does the acid hydrolysis-incubation method measure meaningful soil organic carbon pools?

Abstract: The literature was reviewed and analyzed to determine the feasibility of using a combination of add hydrolysis and CO 2 -C release during long-term incubation to determine soil organic carbon (SOC) pool sizes and mean residence times (MRTs). Analysis of 1100 data points showed the SOC remaining after hydrolysis with 6 M HCI ranged from 30 to 80% of the total SOC depending on soil type, depth, texture, and management. Nonhydrolyzable carbon (NHC) in conventional till soils represented 48% of SOC; no-till averaged 56%, forest 55%, and grassland 56%. Carbon dates showed an average of 1200 yr greater MRT for the NHC fraction than total SOC. Long-term incubation, involving measurement of CO 2 evolution and curve fitting, measured active and slow pools. Active-pool C comprised 2 to 8% of the SOC with MRTs of days to months; the slow pool comprised 45 to 65% of the SOC and had MRTs of 10 to 80 yr. Comparison of field 14 C and 13 C data with hydrolysis-incubation data showed a high correlation between independent techniques across soil types and experiments. There were large differences in MRTs depending on the length of the experiment. Insertion of hydrolysis-incubation derived estimates of active (C a ), slow (C s ), and resistant pools (C r ) into the DAYCENT model provided estimates of daily field CO 2 evolution rates. These were well correlated with field CO 2 measurements. Although not without some interpretation problems, acid hydrolysis-laboratory incubation is useful for determining SOC pools and fluxes especially when used in combination with associated measurements.

Emissions of Nitrous Oxide and Carbon Dioxide

Abstract: Innovative management practices are required to increase the efficiency of N fertilizer usage and to reduce nitrous oxide (N 2 O) and carbon dioxide (CO 2 ) emissions from agricultural soils. The objectives of this study were to evaluate the feasibility of using conservation tillage and N fertilizer placement depth to reduce N 2 O and CO 2 emissions associated with corn (Zea mays L.) production on clay loam soils in Eastern Canada. A 3-yr field study was established on a wheat (Triticum aestivum L.)-corn-soybean [Glycine max (L.) Merr.] rotation with each phase of the rotation present every year. Investigations were focused on the corn phase of the rotation. The tillage treatments following winter wheat included fall moldboard plow tillage (15 cm depth), fall zone-tillage (21 cm width, 15 cm depth), and no-tillage. The N placement treatments were "shallow" placement of sidedress N (2-cm depth) and "deep" placement of sidedress N (10-cm depth). Nitrous oxide emissions were measured 53 times and CO 2 emissions were measured 43 times over three growing seasons using field-based sampling chambers. There was a significant tillage and N placement interaction on N 2 O emissions. Averaged over all three tillage systems and site-years, N 2 O emissions from shallow N placement (2.83 kg N ha -1 yr -1 ) were 26% lower than deep N placement (3.83 kg N ha -1 yr -1 ). The N 2 O emissions were similar among the tillage treatments when N was placed in the soil at a shallow depth. However, when N was placed deeper in the soil (10 cm), the 3-yr average N 2 O emissions from zone-tillage (2.98 kg N ha -1 yr -1 ) were 20% lower than from no-tillage (3.71 kg N ha -1 yr -1 ) and 38% lower than those from moldboard plow tillage (4.81 kg N ha -1 yr -1 ). Tillage type and N placement depth did not affect CO 2 emissions (overall average = 5.80 Mg C ha -1 yr -1 ). Hence, zone-tillage and shallow N placement depth reduced N 2 O emissions without affecting CO 2 emissions.

Role of Organic Acids in Phosphate Mobilization from Iron Oxide

Abstract: Phosphate deficiency often limits crop production in acid tropical soils because of the strong bonding of phosphate by Fe and Al oxides. Organic-acid exudation from roots is one reported plant adaptation to P deficiency. The objective of this study was to predict the efficacy of this P-deficiency stress response in different soil types by investigating the mechanism of organic-acid-induced P mobilization from different oxide minerals. Greater proportions of Fe and initially adsorbed P were released from ferrihydrite when compared with goethite. More P was released and Fe dissolved at pH 4.0 than pH 5.5 or 7.0 from both oxides. For ferrihydrite, the order of effectiveness of the organic ligands for P release at pH 4 was citrate (19% of the total initially adsorbed P) . malate (14%) . tartrate (5%).. oxalate 5 malonate 5 succinate (0.3–1.2%). For Fe release at pH 4, the order was oxalate (18% of total oxide suspension Fe dissolved) < citrate (17%) . malonate(13%).malate(8%).tartrate(5%)..succinate(0.02%). Faster phosphate readsorption in the case of oxalate than citrate probablyaccountedforthelowapparentreleaseofPbyoxalateinspite of its greater Fe dissolution. At the smaller adsorbed-P concentration (1/4 of the adsorption maximum), the predominant mechanism of organic-acid induced P release was ligand-enhanced dissolution of the Fe oxide rather than ligand exchange. At 3/4 of the adsorption

Percolation Theory for Flow in Porous Media

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Management Effects on Soil Physical Properties in Long-Term Tillage Studies in Kansas

Abstract: Five long-term tillage studies in Kansas were evaluated for changes in soil properties including soil organic carbon (SOC), water holding capacity (WHC), bulk density, and aggregate stability. The average length of time these studies have been conducted was 23 yr. Soil properties were characterized in three depth increments to 30 cm, yet changes due to tillage, N fertility, or crop rotation were found primarily in the upper 0- to 5-cm depth. Decreased tillage intensity, increased N fertilization, and crop rotations that included cereal crops had greater SOC in the 0- to 5-cm soil depth. Only one of five sites had greater WHC, which occurred in the 0- to 5-cm depth. Aggregate stability was highly correlated with SOC at all sites. No-tillage (NT) had greater bulk density, but values remained below that considered root limiting. Soil organic C levels can be modified by management that can improve aggregate stability, but greater SOC did not result in greater WHC for the majority of soils evaluated in this study.

Aggregation and organic matter protection following tillage of a previously uncultivated soil

Abstract: Understanding the effects of tillage on soils following years or decades of no-till is critical for developing C conservation strategies. To date, short-term responses to tillage in previously uncultivated or other long-term no-till soils have primarily focused on total C changes, which are difficult to detect. Tillage effects on soil conservation and C permanence may be better predicted by changes in more readily detected factors known to affect C storage such as aggregation and physically protected C. We annually plowed replicated plots in a previously uncultivated midsuccessional field between 2002 and 2004 and investigated changes in the distribution of aggregates, physically protected C, and light fraction (LF) organic matter. Within 60 d of initial cultivation, soil aggregates in the 2000- to 8000-μm size class declined from 0.47 to 0.15 g g -1 at 0- to 7-cm soil depth and from 0.32 to 0.23 g g -1 at 7 to 20 cm. Lower levels of aggregation persisted through the winter and spring of the following year. Inter-aggregate, unprotected light fraction (LF) increased following cultivation, as did particulate C in soil fractions with densities < 1.9 g cm -3 . Changes in the mass of total soil C were not detectable after 3 yr but the vertical distribution of all soil C pools was altered by plowing. Our study demonstrates that plowing once immediately and substantially alters aggregation and LF and particulate C dynamics and that these conditions persist. Results suggest that no-till soils need to be continuously maintained to protect aggregation and physically stabilized C pools.

Cross-reference system for translating between genetic soil classification of China and soil taxonomy

Abstract: Soil classification systems are not consistent among countries or organizations thereby hindering the communication and organizational functions they are intended to promote. The development of translations between systems will be critical for overcoming the gap in understanding that has resulted from the lack of a single internationally accepted classification system. This paper describes the application of a process that resulted in the translation of the Genetic Soil Classification of China (GSCC) to Soil Taxonomy (ST). A brief history of soil classification in China is also provided to familiarize readers with GSCC and its origins. Genetic Soil Classification of China is the attribute base for the recently assembled digital form of the 1:1 000 000 soil map of The People's Republic of China. The translation between GSCC and ST was based on profile, chemical, and physical descriptions of 2540 soil series. First, the 2540 soil series were classified to their equivalent soil order, suborder, great group, and subgroup according to ST and GSCC subgroup descriptors. Order names for both classification systems were then linked to corresponding map units in the 1:1 000 000 digital soil map of China using a geographic information system (GIS). Differences in classification criteria and in the number of orders of the two systems (there are more GSCC orders than ST orders) meant that each GSCC order could possibly be assigned to more than one ST order. To resolve the differences, the percent correspondence in area between orders was determined and used as the criterion for assigning GSCC orders to ST orders. Some percentages of correspondence were low so additional processing was used to improve the assignment process. The GSCC suborders were then matched with ST orders. When the area for each order was summarized, the percentage of correspondence increased except for two subgroups in the Ferrasols order.

Root Influence on Nitrogen Mineralization and Nitrification in Avena barbata Rhizosphere Soil

Abstract: Micro– 15 N pool dilution was used to quantify rates of gross N mineralization, consumption, and nitrification in bulk soil and in soil within 2 mm of root sections of Avena barbata (slender wild oats), an annual grass common to California oak woodland-savannas. Rates of gross N mineralization in rhizosphere soil (9.2 mg N kg 21 d 21 ) were about ten times higher than in bulk soil (1.0 mg N kg 21 d 21 ). Total bacterial numbers in soil adjacent to roots were slightly higher than in bulk soil; protozoa biomass was not measurably different. Changes in bacterial numbers or standing stocks of bacterial N could not account for rates of N mineralization. Nitrification potential values were similar in bulk and rhizosphere soil, yet gross rates of nitrification were highly dependent on location along the root. Gross nitrification rates in soil near the root tip were the same as those in bulk soil, while rapid uptake of NH4 by older sections of root (8–16 cm from the tip), appeared to limit nitrification rates. Only small differences in microbial community structure between bulk and rhizosphere soil were detected by terminal restriction fragment length polymorphism (TRFLP) analysis. While the small increases in bacterial numbers and changes in community composition may in-part explain the increased rates of N mineralization, other microbial-root interactions are likely involved in accelerating the flux of N from organic sources to the plant-available NH4 pool. The high rates of N mineralization observed in soil immediately adjacent to roots should facilitate plant access to N. Most of the stocks and fluxes determined in these studies exhibited distinct spatial patterns along the plant root that may have significantly impacted N-availability to the plant.

Soil Properties and Carbon Sequestration of Afforested Pastures in Reclaimed Minesoils of Ohio

Abstract: Land-use change affects many soil properties, including soil organic carbon (SOC) pool, and the transfer of atmospheric CO2 to terrestrial landscapes. The objective of this study was to evaluate the effects of converting pastureland to Australian pine (Casuarina spp) and Black locust (Robinia pseudoacacia L) forest on selected soil physical and chemical properties and SOC sequestration in reclaimed minesoils (RMS) of southeastern Ohio. The study sites were surface mined for coal, reclaimed and managed as pasture, and then converted into woodland 10 yr before the present study. Soil pH and electrical conductivity (EC) were higher in the RMS than in a nearby undisturbed hardwood forest. Conversion to Australian pine decreased soil pH and EC in the top 20 cm. Bulk densities of the RMS ranged from 1.24 to 1.82 Mg m 23 , and only minor changes were observed in soil bulk density after land-use conversion. Mean weight diameter (MWD) and root biomass increased significantly (P , 0.05) with conversion of pasture to Australian pine or Black locust. In addition, aggregate stability was greater in RMS under hardwood forest than under pasture. Conversion to the Australian pine forest increased the SOC pool in the top 50 cm by 6 Mg ha 21 (11%) in 10 yr. However, the N pool in the top 50 cm was not affected by the land-use conversion from pasture to Australian pine. Conversion to Black locust increased the SOC pool in the top 50 cm by 24 Mg ha 21 (42%), while the N pool increased by 10% under Black locust in 10 yr. The increase in the SOC pool was accompanied by an increase in the C/N ratios and root biomass in both Australian pine and Black locust sites in the 20- to 50-cm depth. Establishment of tree plantation has a greater potential for SOC sequestration than pastures in the RMS.

Hydration Energy Determines Isovalent Cation Exchange Selectivity by Clay Minerals

Abstract: Cation exchange is one of the most venerable concepts in soil science, yet it needs rethinking. This paper presents an extremely simple conceptual framework for interpreting many observed trends in cation exchange. Takingthe example of Cs-K exchange, the methods of computational molecular mechanics found that Cs-montmorillonite is considerably higher in energy than K-montmorillonite at constant water content, in agreement with inferences from a new thermodynamic cycle representation of cation exchange. Since montmorillonite selects Cs + over K + in real experiments, these results mean that alkali cation-exchange selectivity is controlled by selectivity of the solution phase for the more strongly hydrated cation. Thus the clay does not "select" for Cs + over K + in any positive sense and it may be more useful to consider cation exchange as a partitioning reaction: Given two cations of equal valence, the more weakly hydrated will tend to partition into the "subaqueous" smectite interlayer phase. This concept seems not only parsimonious, but also more accurate than other hypotheses for cation exchange selectivity that impute more favorable interactions between smectite surfaces and the selected cations; such theories err by ignoring energy changes in the solution phase. This simple partitioning concept rationally explains the alkali and alkaline earth selectivity sequences as well as the selectivities of smectites for organic cations over inorganic, for larger organic cations over smaller, and for organometallic complexes over the uncomplexed metal.

Overview of the symposium proceedings, meaningful pools in determining soil carbon and nitrogen dynamics

Abstract: Extraction of soil organic matter (SOM) fractions has been a longstanding approach to elucidating the pivotal roles of SOM in soil processes. Several types of extraction procedures are commonly used, and all provide partial information on SOM function. This report and accompanying papers summarize the information regarding SOM functions in real-world issues that has been gained through physical or chemical fractionations. Each procedure has its strengths and weaknesses; each is capable to some degree of distinguishing labile SOM fractions from nonlabile fractions for studying soil processes, such as the cycling of a specific soil nutrient or anthropogenic compound, and each is based on an agent for SOM stabilization. Physical fractionations capture the effects on SOM dynamics of the spatial arrangement of primary and secondary organomineral particles in soil, but they do notconsiderchemicalagentsforSOMstabilization.Theyappearbetter suited for C cycling than N cycling. Chemical fractionations cannot consider the spatial arrangement, but their purely organic fractions are suitable for advanced chemical characterization and can be used to elucidate molecular-level interactions between SOM and nutrients or other organic compounds. During all fractionations, the potential exists for sample alteration or mixing of material among fractions. We call for better coordination of research efforts by (i) developing integrated fractionation procedures that include physical, chemical, and/or biological components, and (ii) categorizing fractionations by their most suitable applications, defined by the nutrient, compound, or soil process in question, land use or crop type, crop management strategies, soil type, and possibly other factors. Selecting the most suitable fractionation procedure for a given research application would enable more precise approximation of the functional SOM pool.

Trace Gas Emission in Chambers

Abstract: Non-steady-state (NSS) chambers are widely used to measure trace gas emissions from the Earth’s surface to the atmosphere. Unfortunately, traditional interpretations of time-dependent chamber concentrations often systematically underestimate predeployment exchange rates because they do not accurately represent the fundamental physics of diffusive soil gas transport that follows chamber deployment. To address this issue, we formally derived a time-dependent diffusion model applicable to NSS chamber observations and evaluated its performance using simulated chamber headspace CO2 concentration data generated by an independent, three-dimensional, numerical diffusion model. Using nonlinear regression to estimate the model parameters, we compared the performance of the non-steady-state diffusive flux estimator (NDFE) to that of the linear, quadratic, and steady-state diffusion models that are widely cited in the literature, determined its sensitivity to violation of the primary assumptions on which it is based, and addressed some of the practicalities of its application. In sharp contrast to the other models, NDFE proved an accurate and robust estimator of trace gas emissions across a wide range of soil, chamber design, and deployment scenarios. N

Hot Water-Extractable Nitrogen as an Indicator of Soil Nitrogen Availability

Abstract: There is keen interest among soil scientists in identifying chemical assays that may be used as predictors of soil N mineralization potential. Our objective was to determine if hot water-extractable N (16-h extraction at 80°C) is a useful predictor of mineralizable N and plant N availability. In a group of 30 New Zealand soils, representing different management histories and parent materials, hot water extracted between 2.6 and 8.7% of total N. The extracted N consisted mainly (∼80%) of organic N, with the remainder being NH 4 -N, generated by hydrolysis of heat-labile organic N. The C/N ratio of the extracted organic matter was relatively low (mean 8:1 vs. 11:1 for total organic matter), indicating that it included N-rich substrates (i.e., substrates likely to have high mineralization potential). However, about three-quarters of the extracted organic N was relatively recalcitrant, i.e., it did not hydrolyze to ninhydrin-reactive N (NH 4 -N, amino acid-N, amino sugar N) when treated with 1 M HCl for 6 h at 80°C. The contribution of mineralized N to plant N uptake was measured using a greenhouse-grown oat (Avena saliva L.) crop, which received no added N. Hot water-extractable N accounted for 50% of the variation in plant N derived from mineralization (PNDM), compared with 16% for total soil N, 32% for anaerobically mineralizable N (AMN), and 24% for NH 4 -N released by hot 2 M KCl. The best predictor of PNDM was N mineralized in a 28-d aerobic incubation at 20°C (79% of variability in PNDM explained).

Quantity and spatial variability of soil carbon in the conterminous United States

Abstract: We estimated the soil organic carbon (SOC) and soil inorganic carbon (SIC) inventory for the conterminous USA using the State Soil Geographic Database (STATSGO). The relative contribution of each soil order and Land Resource Region (LRR) to the national SOC and SIC inventory was determined. There are 302 to 1499 310 8 Mg of SOC and 226 to 937 3 10 8 Mg of SIC in the upper 2 m of soil in the conterminous USA. About 30 and 80% of the upper 2-m SOC is in the 0- to 0.20- and 0- to 1.0-m soil layers, respectively. For SIC, only about 8% of the upper 2-m SIC is in the upper 0.2 m, and about 50% is in the top 1.0-m layer. The relative spatial variability of SOC increases dramatically as soil depth increases while the largest relative variability of SIC is in the surface layer. Because of its large area (27% of the soil area in the conterminous USA), Mollisols are the largest contributors both to the SOC stock (about 31 to 39%) and to the SIC stock (about 43 to 44%) in the conterminous USA. The results ofthisstudyprovideaviewofsoilCpartitioningbytaxonomicgroup and land resource area, information that may be useful for assessing the impact of land use and climatic change on SOC and SIC pools.

Detection of carbon stock change in agricultural soils using spectroscopic techniques

Abstract: Soil organic carbon (SOC) represents one of the major pools in the global C cycle. Therefore, even small changes in SOC stocks cause important CO, fluxes between terrestrial ecosystems and the atmosphere. However, SOC stocks are difficult to quantify accurately due to their high spatial variability. The aim of this paper is to evaluate the potential of Imaging Spectroscopy (IS) using the Compact Airborne Spectrographic Imager (CASI; 405-950 nm) and field spectroscopy with an Analytical Spectral Devices spectrometer (ASD; 350-2500 nm) to measure SOC content in heterogeneous agricultural soils. We used both stepwise and partial least square (PLS) regression analysis to relate spectral measurements to SOC contents. Standard Error of Prediction (SEP) for the ASD ranged from 2.4 to 3.3 g C kg(-1) depending on soil moisture content of the surface layer. Imaging spectroscopy performed poorly, mainly due to the narrow spectral range of the CASI. Tests using both the CASI and the Shortwave infrared Airborne Spectrographic Imager (SASI; 900-2500 nm) showed better results. The variation in soil texture and soil moisture content degrades the spectral response to SOC contents. Currently, SEP allows to detect a SOC stock change of 7.2-9.9 Mg C ha(-1) in the upper 30 cm of the soil, and is therefore still somewhat high in comparison with changes in SOC stocks as a result of management or land conversion (0.34.9 Mg C ha(-1) yr(-1)). A detailed SOC maps produced by IS reflected the patterns in SOC contents due to the recent conversion from grassland to cropland.

Nanoscale Biogeocomplexity of the Organomineral Assemblage in Soil: Application of STXM Microscopy and C 1s-NEXAFS Spectroscopy

Abstract: Methodological constraints limit the extent to which existing soil aggregationmodels explaincarbon(C) stabilization insoil. Wehypothesize that the physical infrastructure of microaggregates plays a major role in determining the chemistry of the occluded C and intimate associations between particulate C, chemically stabilized C and the soil mineral matrix. We employed synchrotron-based scanning transmission X-ray microscopy (STXM) coupled with near-edge X-ray absorption fine structure (C 1s-NEXAFS) spectroscopy to investigate the nanoscale physical assemblage and C chemistry of 150-mm microaggregates from a Kenyan Oxisol. Ultra-thin sections were obtained after embedding microaggregates in a sulfur block and sectioning on a cryo-microtome at 255C. Principal component and cluster analyses revealed four spatially distinct features: pore surfaces, mineral matter, organic matter, and their mixtures. The occurrence of these features did not vary between exterior and interior locations; however, the degree of oxidation decreased while the complexity and occurrence of aliphatic C forms increased from exterior to interior regions of the microaggregate. At both locations, compositional mapping rendered a nanoscale distribution of oxidized C clogging pores and coating pore cavities on mineral surface. Hydrophobic organic matter of aromatic and aliphatic nature, representing particulate C forms appeared physically occluded in 2- to 5-mm pore spaces. Our findings demonstrate that organic matter in microaggregates may be found as either oxidized C associated with mineral surfaces or aromatic and aliphatic C in particulate form. Using STXM and C 1s-NEXAFS we are for the first time able to resolve the nanoscale biogeocomplexity of unaltered soil microaggregates.

Probability Distribution and Spatial Dependence of Nitrous Oxide Emission

Abstract: Climate controls soil N 2 O emissions via its effects on soil properties such as water-filled pore space. Changes in climate should produce changes in the probability distribution and spatial dependency of soil N 2 O data. Knowing the extent of the changes in the distribution of this data is important for validating model predictions. The objectives of this study were to describe the probability distributions and estimate the spatial dependency of soil N 2 O emission data. On a hummocky, agricultural landscape in Saskatchewan, N 2 O emission data and related soil variables were taken from a 128-point transect 15 times over 2 yr. Probability distributions were compared using a Chi-square test. The range in spatial correlation was determined using the indicator semivariogram with a nested model fit approach. The mean N 2 O flux ranged from 25.3 to -0.2 ng N 2 O-N m -2 s -1 . Probability distributions ranged in shape from reverse J-shape through log normal to symmetrical. The majority of distributions were statistically different from each other, showing a lack of temporal stability. Mean N 2 O flux and distribution shape followed an event-based/background emission pattern. High flux events had statistically similar, reverse J-shaped distributions. As mean N 2 O flux decreased to a background level distribution, shape changed to log normal and symmetrical forms. A high nugget/sill ratio characterized the majority of sampling dates, although spatial dependency was generally moderate. Flux values in the fourth quartile tended to have a spatial dependency of 15 m, probably reflecting a topographic control at a landform element scale.

Organic Matter Study of Whole Soil Samples Using Laser-Induced Fluorescence Spectroscopy

Abstract: Fluorescence spectroscopy relies on the fluorescence emitted by rigid conjugated systems and thus can be used to assess the soil organic matter (SOM) humification. This technique is generally applied to solution samples of humic substances, and so far no information exists about its applicability to whole untreated soil samples. The laser-induced fluorescence (LIF) spectroscopy is proposed as a novel technique to assess the organic matter humification in whole soil samples. We sampled the 0- to 2.5-, 2.5- to 5-, 5- to 10-, 10- to 15-, and 15- to 20-cm layers of three Oxisols of long-term experiments located in two sites of the Brazilian Cerrado. The humification index based on LIF spectroscopy (H L I F ) of whole soil samples showed a dose correlation with the humification indexes A 4 /A 1 , I 4 6 5 /I 3 9 9 , and A 4 6 5 obtained after fluorescence spectroscopy analysis of the dissolved humic adds. The H L I F in soils under native cerrado or subjected to no-tillage increased from the top to the deepest layer, which is consistent with the deposition of labile organic matter from plant residues on the soil surface. The soils subjected to conventional tillage, however, showed relatively constant H L I F along the profile, possibly because homogenization imparted by disturbance of the arable layer. Accordingly, for the two top layers, the soils under no-tillage showed a lower H L I F than for conventionally tilled soils. Laser-induced fluorescence spectroscopy is a promising technique to assess humification in whole soil samples, particularly in Oxisols, which due to high concentration of Fe 3 + are not feasible to electron spin resonance (ESR) and Carbon-13 nuclear magnetic resonance ( 1 3 C NMR) spectroscopy, unless previous treatment is employed.