Search

Biological soil crusts (biocrusts) are crucial elements of desert ecosystems, exhibiting patchy distribution patterns across the soil surface and significantly impacting surface soil nutrient dynamics. However, the influence of biocrust patch units, serving as fundamental functional entities, on microbial nutrient restriction remains underexplored. This study conducted measurements on moss crust patches of varying sizes and subcrust soils. Stoichiometric analysis of extracellular enzyme activities (EEAs) related to carbon, nitrogen, and phosphorus was performed, along with vector and redundancy analyses to evaluate microbial nutrient limitation and key influencing factors. The findings reveal that both patch size and soil layer of biocrusts collectively influence soil nutrients and enzyme dynamics, with heightened enzyme activity observed in the crust layer. Vector analysis based on EEA stoichiometry indicates that moss crust patch size insignificantly impacts microbial nutrient restriction within the crust layer. However, microbial nitrogen restriction in the subcrust layer demonstrates a “single-peak” trend, initially increasing before gradually declining. This suggests that microorganisms in medium-sized crust patches exhibit peak activity, intensifying nutrient competition. This research underscores the pivotal role of biocrust patch units as fundamental functional entities, offering comprehensive insights into microbial metabolic constraints under crust cover. The findings underscore the significant implications of enzyme stoichiometric characteristics for desert land management and conservation. [ABSTRACT FROM AUTHOR]

Soil structure dynamics during a season depend on management practices and environmental factors. A lightweight autonomous robot (total mass: 3300–4100 kg, wheel load: 700–1200 kg, contact areas: 0.125 m², inflation pressures: 60–280 kPa) was used for sowing (October 2021) and weeding (May 2022) operations on an annually plowed sandy loam field. We took 579 cm³ soil cores at 10- to 18-cm depth in the crop area and wheel tracks before and after the operations to assess the impact from traffic and the potential recovery of topsoil structural properties. We measured air permeability and effective air-filled porosity in the laboratory, and X-ray CT scanned the samples to evaluate soil pore functionality. The first operation (conducted on a moist seedbed) had the largest impact, significantly compacting and reducing the air-filled porosity by 42% (from 0.21 to 0.12 m³ m−3) and decreasing air permeability by 75.8% (from 130 to 31.5 μm²). After 7 months, the crop area and wheel track showed signs of soil consolidation due to environmental factors but not decompaction. The second operation occurred on drier (water content 0.06 g g−1), stronger soil conditions (degree of compactness 100.8%), and recompaction of the wheel track was not observed. Traffic in weak soils can result in seasonal topsoil compaction despite the lighter wheel loads. However, due to the milder impacts, recovery rates might be faster for lightweight machinery than for heavy tractors. Multi-season studies are needed to assess the real potential of lightweight robots to minimize soil compaction risk. [ABSTRACT FROM AUTHOR]

Phosphorus (P) dynamics in the soil subsurface are relatively unstudied, especially in forest soils, and may be important in areas with groundwater‐fed surface water bodies. Here, we use X‐ray absorption near‐edge P spectroscopy (P‐XANES) and X‐ray fluorescence microprobe mapping (µ‐XRF) to examine P speciation and co‐localization with Fe and Al as a function of depth in soils from coniferous and deciduous forests. P‐XANES revealed similar trends in P speciation with depth for both forest types, with Al‐P and Fe‐P being dominant, whereas Ca‐P and organic P were minor constituents. Correlation plots derived from µ‐XRF mapping revealed a strong correlation between P and both Fe and Al at all depths. At depths of up to 378 cm, a range that includes A, E, and B horizons, the relationships between P and Al/Fe were remarkably consistent, consisting of a population of P diffusely associated with Al/Fe, consistent with Fe‐ or Al‐adsorbed P, as well as P linearly correlated with Al/Fe in several discreet groupings, indicative of the presence of specific P compounds. At depths below 378 cm, the relationships between P and Al/Fe changed dramatically, with correlation plotting indicating the disappearance of any diffuse P:Al/Fe relationships. This change occurred concurrently with a decrease in oxalate extractable P, suggesting that the P present in this deep soil horizon may be present in a more stable form that is less available for transport, as well as potentially indicating that P adsorbed to Fe/Al did not readily pass through the overlaying Bw soil horizon. Core Ideas: Organic P and Ca‐P minor constituents of overall P pool, which was dominated by Al‐P and Fe‐P.Similar trends in deciduous/coniferous soils, suggesting litter composition unimportant at these study sites.Correlation plotting revealed consistent P:Al/Fe relationships from surface down to 378 cm (Bw horizon). [ABSTRACT FROM AUTHOR]

Evaluating the spatiotemporal dynamics of soil organic carbon (SOC) is essential for better understanding the role of SOC in global C cycling. In this study, we report on the application of a two‐compartment model for mapping and examining SOC changes in China. We develop a SOC change prediction model combined with an environmental stratification approach and map SOC changes for the top 20‐cm soil depth in China at a grid resolution of 1 km. We perform a model comparison to quantify the effects of environmental stratification on model performance. A mean SOC increase of 0.63 kg m−2 was observed over the past 40 yr. Compared to the model without environmental stratification (the concordance correlation coefficient [CCC]: 0.34–0.40; RMSE: 2.16–2.33 kg m−2), we found that the models incorporating spatial or temporal stratification provide better performance. Notably, the model with spatiotemporal stratification achieved the better performance compared with results without stratification, with the prediction accuracy improved up to 57% in terms of CCC. Our results suggest that both the spatial‐ and temporal‐scale effects of environmental heterogeneity on SOC processes need to be accounted for in the prediction of SOC changes. Furthermore, the results indicate that the developed approach can be used to map SOC changes over large regions. This study also emphasizes that to achieve accurate predictions, it is necessary to choose the proper environmental stratification method to refine the study unit. Core Ideas: A study was conducted to quantify spatiotemporal changes of soil organic C in China.Models were developed based on a two‐compartments model with environmental stratifications.Identification of environmental heterogeneity is critical for accurate soil organic C change estimation. [ABSTRACT FROM AUTHOR]

Soil P is present in multiple forms through the profile with plants potentially accessing different P pools at different depths through distinct pathways. Understanding P pool distribution and dynamics is necessary for sustainable forest management. Our research investigates soil P dynamics in clay (at 60–100 cm depth) and saprolite (at 450–500 cm depth) of the Calhoun Critical Zone Observatory (CCZO) in South Carolina through a 1‐yr greenhouse planting of loblolly pine (Pinus taeda L.) and red maple (Acer rubrum L.). We compared total plant P uptake to changes in soil P pools defined by Mehlich 3 (MIII) P, Hedley P, or total P fractions and to indices of potential iron (Fe) reduction and phosphatase activity as potential pathways of P release. We hypothesized that (a) the soil content of MIII P would be insufficient to meet plant demand; (b) ectomycorrhizal (EM) pine to differ from arbuscular mycorrhizal (AM) maple in accessing organic P pools; and (c) Fe reduction to correlate with declines in NaOH P (associated with Fe‐bound P) with greater decreases in the Fe‐oxide‐rich clay layer. Our results indicated MIII extractable P is plant available but present in insufficient quantities to meet total demand. In Hedley P pools, resin Pi and NaOH Pi supplied the most P for plant growth, and stable P pools (concentrated HCl P and residual P) buffered losses from these P pools. Plant‐available P in these soils seems sufficient to meet long‐term plant demand particularly if deeply rooted trees grow into the saprolite that had higher available soil P supply. Core Ideas: Saprolite was richer than clay in MIII P and total P.Resin Pi and NaOH Pi were dynamic pools that contributed to plant growth.Residual P and con HCl Pi decline indicated stable P pools replenish the loss of labile P.No clear evidence that plants facilitate soil iron reduction.Ectomycorrhizal plants didn't produce more phosphatase than arbuscular plants in the short‐term. [ABSTRACT FROM AUTHOR]

Agricultural system models can help quantify soil water dynamics, crop evapotranspiration, and crop growth with a high temporal resolution to enhance soil and crop management. Simplifying the process of model calibration and validation is an important aspect for crop model use. The objective of this study was to explore the capability of using the Root Zone Water Quality Model (RZWQM2) with soil hydraulic property input parameters (SHPIPs) identified once under a given crop for other crops without recalibrating. A field experiment with soybean [Glycine max (L.) Merr.], corn (Zea mays L.), wheat (Triticum aestivum L.), and fallow soil was conducted from 2015 to 2017. The soil hydraulic property parameter calibration process was evaluated. Four model scenarios of calibrating measured SHPIPs for soybean, corn, wheat, and fallow soil were created. The RZWQM2 simulation results showed high sensitivity to the model inputs of the SHPIPs. Model input parameters of bulk density, field capacity, permanent wilting point, and saturated hydraulic conductivity showed a large effect on model simulations. Model inputs of soil water content θ at field capacity and permanent wilting point showed high impact on water holding capacity and crop development, whereas the model input parameters of saturated hydraulic conductivity and bulk density yielded a high effect on soil water dynamics. All model scenarios presented satisfactory results with regard to soil water dynamics, evapotranspiration, crop growth, and mineral nitrogen, indicating that, for a particular field, RZWQM2 calibrated for one season performs well for other seasons without recalibration. Core Ideas: Representative soil hydraulic measurements are necessary for model simulations.A deviation of model simulations could be due to uncertainty of hydraulic inputs.It is not necessary to re‐calibrate soil hydraulic input parameters for each crop. [ABSTRACT FROM AUTHOR]

Tillage practices and water erosion are the most important anthropogenic and natural processes, respectively, driving soil organic C turnover in agricultural land. The aim of this study was to explore the responses of soil organic C (SOC) turnover to tillage and runoff by comparing the variation of soil aggregate‐associated organic C (AOC) and intra‐aggregate particulate organic C (iPOC) under simulated tillage and runoff conditions. Soil samples were collected from a native vegetation land with no cultivation history in the Mollisol region of Northeast China. After a series of simulated tillage (ST) and simulated runoff (SR) treatments, the samples were incubated for 30 d and then separated through 2‐, 1‐, 0.25‐, and 0.053‐mm sieves by wet‐sieving to obtain different aggregate size fractions. Each aggregate fraction was subsequently shaken for 18 h in 0.5% hexametaphosphate to get different intra‐aggregate particle size fractions. The proportion of the fractions and their AOC and iPOC were determined. The ST treatment promoted the reaggregation of macroaggregates (>2 mm) by accelerating the turnover of their coarse iPOC (0.25–2 mm), leading to a lower concentration of AOC. Runoff transformed larger aggregates (>0.25 mm) to smaller particles (<0.25 mm) without catalyzing C turnover. Fine iPOC (0.053–0.25 mm) could serve as an indicator for AOC (>1 mm) dynamics, especially associated with tillage operations. Our findings highlight the different influences of tillage and runoff, and the negative effect of tillage on SOC dynamics. Core Ideas: The responses of SOC turnover to tillage and runoff are different.Tillage accelerates the turnover of >2 mm soil aggregate and its coarse iPOC (0.25–2 mm).Runoff transforms larger aggregates (>0.25 mm) into smaller particles (<0.25 mm) without catalyzing C turnover.Fine iPOC (0.053–0.25 mm) could serve as an indicator for AOC (>1 mm) dynamics. [ABSTRACT FROM AUTHOR]

Anaeromyxobacter spp. are prevalent and can make important contribution to microbial Fe(III) reduction in flooded paddy soils, which could be greatly regulated by exogenous organic C input. In this study, dynamics in the abundance and structure of metabolically active Anaeromyxobacter community based on RNA level were investigated by a 40‐d anaerobic incubation of paddy soils amended with glucose at 0.1 and 0.5 mol C kg−1 soil, respectively. Compared with the control treatment, glucose additions generally showed obvious inhibitory effect on the Anaeromyxobacter growth; glucose addition at low concentration decreased the abundance of Anaeromyxobacter by 3.56 × 106 to 4.06 × 107 copies g–1 soil within the first 5 d of incubation, whereas a remarkable decrease by 4.54 × 106 to 7.71 × 107 copies g–1 soil occurred after paddy soils were amended with high glucose concentration. Anaeromyxobacter dehalogenans‐like OTU_1 (where OTU stands for operational taxonomic unit) was dominant in the metabolically active Anaeromyxobacter community and showed significant and positive relationship with Fe(III) reduction process. The structure of Anaeromyxobacter community was greatly altered by glucose additions, and divergences mainly occurred after 5 d of incubation. Additionally, glucose additions could enhance interactions within the Anaeromyxobacter community, especially for positive correlations. In general, glucose addition could greatly inhibit the abundance and alter the structure of Anaeromyxobacter community by jointly decreasing soil pH; however, it could promote the final Fe(III) reduction by enhancing the utilization of Anaeromyxobacter to crystalline Fe oxide. Core Ideas: The Anaeromyxobacter dehalogenans‐like group was dominant in the Anaeromyxobacter community.The growth of Anaeromyxobacter was generally inhibited by glucose addition.Glucose addition greatly altered the community structure of Anaeromyxobacter.Glucose addition could enhance interactions within the Anaeromyxobacter community. [ABSTRACT FROM AUTHOR]

Previous studies have indicated that soil properties and texture could lead to differences in N mineralization from applied organic materials. We conducted a 100-day (d) incubation study to measure net N mineralized from feather meal (14–0.1–0.1) and pellet mix (10–1–6) at 150 mg N kg soil −1 (approximately 72 kg N ha−1) incorporated into four common southeastern soils: Cecil 1, Cecil 2, Greenville, and Tifton. A second 23-d study was conducted to determine pH dynamics and the potential for ammonia volatilization from the same soils and materials. There were no differences in net N mineralized from pellet mix among soils, but net N mineralized from feather meal was lower in Greenville soil, which had the greatest clay content. Analysis of the first-order dynamics of N mineralization indicated that even in soils that differ in clay content it may be possible to describe N mineralization from feather meal and pellet mix with a single model. Our results also suggest that the effect of pH on the N mineralization dynamics of these fertilizers may be more important than the effect of clay content. No ammonia loss was observed regardless of soil texture, soil pH buffering capacity, or initial soil pH. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal fungi (AMF) are widespread in terrestrial ecosystems. In addition to their contributions to plant nutrient uptake, AMF also provide many ecological functions including regulation of soil C dynamics. However, both stimulating and retarding soil organic decomposition by AMF have been observed. Here we discuss the possible reasons for such a contradiction. Arbuscular mycorrhizal fungi contribute to soil aggregation mainly through hyphal enmeshment, saprotrophic suppression, and production of glomalin-related soil proteins, while AMF can also stimulate organic decomposition through promoting degradative enzymes, modifying root production and activity, and/or through regulating the microbial community in the mycorrhizosphere and hyphosphere. The role of AMF in C decomposition is strongly dependent on the quality and quantity of different soil C pools. Arbuscular mycorrhizal fungi can stimulate fresh residue decomposition initially through stimulating the decomposition of fresh residues (particularly those having high C/N ratio), whereas for older or decomposed soil organic C, AMF tend to suppress decomposition by promoting soil aggregation. Under elevated CO2 (eCO2), AMF show additive effects on residue decomposition, priming effects, and changes in soil aggregation. Despite organic decomposition rates differing in the short term and long term following litter experiments, our discussion highlights the role of AMF in organic C dynamics. We hypothesize that AMF would benefit soil C gain in the long term and thereby predict that disturbances that impacts negatively on AMF, such as tillage, residue burning, fertilization, and fungicide application, would lead to soil C decline particularly under eCO2. [ABSTRACT FROM AUTHOR]

Prolonged soil cultivation has been identified as a major cause of land degradation and a threat to soil quality in drought-prone environments. This study evaluated land use effects on soil carbon (C) fractions and organic C (SOC) quality across three semiarid agro-ecosystems (Harrismith, Tweespruit, and Kroonstad) in central South Africa. Soil samples were collected from croplands, primary and secondary grasslands at the 0- to 200-mm layer in each agro-ecosystem and analyzed for various soil C fractions. The SOC structure was characterized with 13C nuclear magnetic resonance (NMR) spectroscopy. All analyses were done on bulk soil samples. Conversion of primary grasslands into croplands decreased C fractions by 27 to 90% across the three agro-ecosystems, with highest losses recorded in Harrismith and Tweespruit and lowest in Kroonstad, suggesting site-specific conditions acted together with cultivation to effect C losses. The 13C NMR spectra revealed a slight change in SOC structural composition when O-alkyl C decreased with concomitant increase in aromatic and alkyl C due to cultivation, with differences in the range of 1 to 11%. O-alkyl C remained almost the same in Harrismith, suggesting that lignin-derived methoxy groups were probably more dominant than easily decomposable carbohydrates as opposed to O-alkyl C in Tweespruit and Kroonstad. Meanwhile, reversion of cultivated soils into perennial pastures restored and even increased some soil C fractions (by 3-129%) to represent primary grasslands, especially in Kroonstad. Organic C decomposition was lower in the cultivated soils relative to virgin and restored soils. This underscores the importance of determining plant biomass composition because these unusual responses were probably related to vegetation differences. Overall, this study demonstrates the potential of secondary grassland management to rehabilitate degraded cultivated soils with implications for restoration of agro-ecosystem functions and services. [ABSTRACT FROM AUTHOR]

Rapid, easy, and frequent prediction of gross or potential nitrogen mineralization rates (GNMR and PNMR, respectively) is desirable for improved understanding and quantification of soil N dynamics and for enabling advanced sustainable N management. Our goal was to extend the use of excitation–emission matrix (EEM) fluorescence spectroscopy to characterize constituents of soluble organic matter (OM) pools in agricultural soils and to couple these characterizations with advanced chemometric techniques to improve prediction of PNMR and GNMR. To achieve this, EEM-based predictions must be valid across diverse soils, climates, and management systems. Accordingly, we analyzed soil water extracts spanning a broad range of OM contents from midwest United States (MUS) and Israeli (ISL) agroecosystems under organic- and mineral-based N management strategies. Parallel factors analysis, a multiway data analysis method, was used to quantify meaningful EEM spectral components, which were used to detect changes in labile soil OM pools and to predict N mineralization rates. N-way partial least squares regression (NPLS) was also applied to EEM data to obtain spectral factors correlated to total organic carbon concentration potential and gross N mineralization rates. This NPLS analysis led to reliable estimation of all three tested properties for ISL and MUS soils. [ABSTRACT FROM AUTHOR]

Many crops in volcanic regions exhibit symptoms of B deficiency but do not respond to B fertilizer rates that solve deficiency problems in non–volcanic-influenced soils. The US recommended rates are routinely adopted in these regions without accounting for differences in soil B dynamics. Because of the paucity of investigations and the resulting lack of understanding of B adsorption/desorption dynamics in volcanic ash–derived soils over the past 40 yr, it would be incorrect to extrapolate the few data available to all volcanic ash–derived soils because they have not been analyzed as a function of andic character. This paper aims to create an understanding of what soil properties govern B availability in volcanic-influenced soils and to establish if there are low-cost testing methods that can help estimate appropriate field B application rates in these soils. Boron adsorption/desorption isotherms were constructed for a group of volcanic ash–derived and non-volcanic ash–derived soils, and the derived parameters were compared with various standard soil tests. Our results show that soil B availability in volcanic-influenced soils is governed mainly by strong and stable B complexation with short-rangeorder (SRO) minerals. Boron adsorption maxima exhibit a wide variability not directly correlated with clay content but rather with reactivity quantified by SRO mineral and organic matter contents. Organic matter is not a significant variable explaining specific surface area for the overall group of soils, but it becomes significant for soils containing <200 g clay kg-1, suggesting that a new approach to analyzing whole-soil reactivity based on mineral surface abundance is needed. The maximum adsorption capacities of volcanic-influenced soils can be as much as 18-fold higher than for non-volcanic soils, with the reversibility of adsorption reactions decreasing as andic character increases. These findings explain why standard B fertilizer rates fail to solve deficiency problems in volcanic-influenced soils. Because pH in NaF is strongly related with acid-oxalate Al (SRO minerals) and B desorption index, this low-cost test can potentially be used as a measure of soil andic character to better reflect B availability and corresponding field application rates. [ABSTRACT FROM AUTHOR]

It is often a challenge to simulate soil moisture dynamics accurately when limited data are available about the soil. A forward simulation based on pedotransfer function (PTF) estimates of hydraulic parameters indeed produced severely biased moisture fluxes. This study therefore evaluated a new model inversion strategy which is able to use limited data by optimizing factors that scale PTF estimates. An important feature of this strategy is that the model inversion is performed for all profiles simultaneously, thus ensuring that the hydraulic parameters among the different profiles are adjusted at the same time by changing the scale factors while also ensuring that sufficient data are available to make inversion mathematically possible. We evaluated three main inversion models of different complexity. The simplest model (GW5), in which there were five scale factors with no horizon distinction was the most parsimonious in terms of the corrected Akaike's Information Criterion (AICc). GW10 (with two sets of five scale factors for topsoil and subsoil) and GW30 (six sets) performed less well in terms of AICc, but were able to simulate the mean observed moisture profile much better than GW5. Additional inversions demonstrated that textural variation was important to achieve the best performance, thus indicating that the PTF estimates are a necessary part of the inversion. The GW5 model parameters appeared to be insensitive to assumptions made about total root water uptake and its distribution within the profile. The inversion method is simple and can be interfaced without much difficulty with existing vadose zone models. [ABSTRACT FROM AUTHOR]

Bulk density is a commonly measured property during field investigations of soils. Accurate and reliable bulk density measurements are critical for assessing soil quality as well as for converting mass-based measurements to volume-based values. Our objective was to determine if there are significant seasonal changes to the measured bulk density of surface soil horizons in a forested ecosystem. The frame method was used to measure bulk density at monthly intervals for 12 mo at 10 locations within a forested catchment. The measured bulk density was variable with time for all plots, with the strongest seasonality shown in Oi horizons. Seasonal trends in Oe and A horizons were less pronounced. Particularly for O horizons, it appears to be critical to consider the time of year when collecting bulk density data. [ABSTRACT FROM AUTHOR]

Aggregate stability is a key property affecting the movement and storage of water, seedling emergence, and soil sensitivity to erosion. Many studies have shown that aggregate stability changes through time. Field monitoring studies performed with a relatively large (monthly) time step showed the seasonal trend of aggregate stability. However, shorter time step monitoring is required to explore dynamics of aggregate stability at short term. For now, biological activity was recognized to be the main factor of aggregate stability dynamics, but previous studies were currently based on the external stimulation of aggregate stability. The objectives of this study were to assess variations in aggregate stability at short time steps in the field and to identify the factors controlling these variations of stability. A 6-mo field monitoring was performed at short time step (2-5 d) on a bare field of Luvisol without organic amendment. Aggregate stability was measured for both on surface and subsurface materials by the ISO 10930 (2012) method. Rain amount and intensity, air temperature and humidity, soil temperature, water content and hydric history, and soil water repellency were measured as explanatory factors. The results showed that aggregate stability varied greatly (up to 40%) over a few days for both surface and subsurface. Short term dynamics of aggregate stability were already shown by laboratory experiments, but such dynamics was never observed in the field for a bare soil without external stimulation of biological activity. For the surface, short time step variations of surface aggregate stability were primarily controlled by soil water content (WC0 and WC1/2), hydric history (ΔWC4 and API), and rain intensity. While large changes in aggregate stability were found for the subsurface, explanatory factors remain to be found. [ABSTRACT FROM AUTHOR]

This study examined spatial and temporal relationships between measured soil moisture and terrain-based proxies for soil moisture dynamics. Two catenas were intensively sampled reflecting a mosaic of differences in degree of soil development in California's Sierra Foothill Region. A catena containing weakly developed soils (Haploxerepts + Haploxeralfs) formed from granitic parent materials was compared to a catena of well-developed soils (Haploxeralfs and Palexeralfs) formed from metavolcanic parent materials. Soil moisture was monitored at 10-, 30-, and 50-cm depths in 15 profiles in the granitic catena and 100 profiles in the metavolcanic catena. Seven postrainfall periods during the 2008-2009 water year were selected to compare terrain shape indices and measured soil moisture. No single terrain index (slope, tangential curvature, profile curvature, mean curvature, topographic prominence, terrain characterization index, and compound topographic index [CTI]) consistently described variability in mean water content or drydown rates, across depth or space. However, within the granitic catena, a combination of CTI and modeled beam radiance consistently accounted for 30 to 70% of the total variance in mean water content at 10 cm, and 10 to 40% at 30- and 50-cm depths. The predictive capacity of digital elevation model (DEM)-derived terrain shape indices for soil moisture dynamics varied widely in time and space, and was influenced by spatial patterns in the degree of soil development. Efforts to describe soil moisture variability are an important attribute of digital soil mapping (DSM). Moreover, soil variability influences soil moisture dynamics, thus synergistic activities are needed to integrate landscape scale variability with digital soil mapping. [ABSTRACT FROM AUTHOR]

Information is needed to mitigate dryland soil greenhouse gas (GHG) emissions by using novel management practices. We evaluated the effects of cropping sequence and N fertilization on dryland soil temperature and water content at the 0- to 15-cm depth and surface C02, N20, and CH4 fluxes in a Williams loam (fine-loamy, mixed, superactive, frigid, Typic Argiustolls) in eastern Montana. Treatments were no-tilled continuous malt barley (Hordeum vulgaris L.) (NTCB), no-tilled malt barley-pea (Pisum sativum L.) (NTB-P), and conventional-tilled malt barley-fallow (CTB-F) (control), each with 0 and 80 kg N ha-1. Gas fluxes were measured at 3 to 14 d intervals using static, vented chambers from March to November 2008 to 2011. Soil temperature varied but water content was greater in CTB-F than in other treatments. The GHG fluxes varied with date of sampling, peaking immediately after substantial precipitation (>15 mm) and N fertilization during increased soil temperature. Total C02 flux from March to November was greater in NTCB and NTB-P with 80 kg N ha-1 than in other treatments from to 2010. Total N20 flux was greater in NTCB with 0 kg N ha-1 and in NTB-P with 80 kg N ha-1 than in other treatments in 2008 and 2011. Total CH4 uptake was greater with 80 than with 0 kg N ha-1 in NTCB in 2009 and 2011. Because of intermediate level of C02 equivalent of GHG emissions and known favorable effect on malt barley yield, NTB-P with 0 kg N ha-1 might mitigate GHG emissions and sustain crop yields compared to other treatments in eastern Montana. For accounting global warming potential of management practices, however, additional information on soil C dynamics and C02 associated with production inputs and machinery use are needed. [ABSTRACT FROM AUTHOR]

Continuous winter wheat (Triticum aestivum L.) represents ~75% of the total grain cropland acreage in Oklahoma. Wheat is mainly sown using conventional tillage (CT), which is linked with low precipitation storage efficiency (PSE), and contributes to water and wind erosion. The objectives of this study were to compare PSE of fallow periods, and water content dynamics of the soil profile during the growing season of winter wheat monoculture under both CT and no-till (NT) systems. Soil moisture was measured weekly from April to November, and every 20 d during the winter period using a neutron probe moisture meter in field experiments at two locations from 2009 to 2011. Significant differences at p < 0.05 level in plant available water (PAW) between CT and NT were observed late in the growing season for one location and year, with CT having PAW values averaging 68 mm greater than NT. Average values of fallow PSE were low, ranging from --10 to 45%. No significant differences at p < 0.05 level were found in fallow PSE and wheat yield with either tillage system. The fallow PSE was 11% in three out of four site-years, which means that often about 90% of summer rainfall is lost. We conclude that soil water dynamics of CT and NT have similar trends during the wheat growing season and that summer fallow PSE in wheat monoculture is inefficient regardless of the tillage system employed. [ABSTRACT FROM AUTHOR]

Correct simulation of overwinter condition is important for the growth of winter crops and for initial growth of spring crops. The objective of this study was to investigate overwinter soil water and temperature dynamics with the simultaneous heat and water (SHAW) model and with its linkage to the root zone water quality model (RZWQM), a hybrid model of RZWQM and SHAW (RZ-SHAW) in a Siberian wildrye grassland under two irrigation treatments (non-irrigation and pre-winter irrigation) in two seasons (2005-2006 and 2006-2007). Experimental results showed that pre-winter irrigation considerably increased soil water content for the top 60-cm soil profile in the following spring, but had little effect on soil temperature. Both SHAW and RZ-SHAW simulated these irrigation effects equally well, which demonstrated a correct linkage between RZWQM and SHAW. Across the treatments and years, the average root mean square deviation (RMSD) for simulated total soil water content (liquid plus frozen) was 0.031 m³ m-3 for both RZ-SHAW and SHAW models, and that for liquid water content alone was 0.028 m³ m-3 for both models. Both models provided better simulation of total and liquid soil water contents under non-irrigation condition than under pre-winter irrigation conditions. On average, RZ-SHAW simulated soil temperature slightly better with an average RMSD of 1.4°C compared to that of 1.8°C by SHAW. Both RZ-SHAW and SHAW simulated the soil freezing process well, but were less accurate in simulating the soil thawing processes, where further improvements are desirable. These simulation results show that the SHAW model is correctly implemented in RZWQM (RZ-SHAW), which adds the capability of RZWQM in simulating overwinter soil conditions that are critical for winter crops. [ABSTRACT FROM AUTHOR]

Soil organic C (SOC) content is one of the most dynamic of soil properties. In this study, we examined the effects of land use change on SOC pools for a single soil series and developed a mapping approach to relate SOC dynamics to land use change. Six paired sites, consisting of adjacent agricultural field and forest within a single delineation, were sampled and the SOC pools determined. The average forest SOC pool (157 Mg ha-1) was significantly higher (P < 0.95) than the field pool (103 Mg ha-1), supporting the importance of land use on SOC pools. We propose the development of a SOC phase based on land use to map such differences. Master 0 and A horizon data should be used to establish SOC phases. Data can be obtained from existing soil surveys, updates, or C accounting activities. Land use classes can be identified with digital imagery and SOC phases can be assigned to all mapping units. Mapping units sampled for C accounting can be resampled to detect patterns and rates of change. This approach provides a robust data set to effectively map and model SOC pools and change across the landscape. [ABSTRACT FROM AUTHOR]

We previously developed a new, coupled-source-sink modeling approach for evaluating an upper bound to the relative water uptake rate (water use efficiency) for two- and three-dimensional geometries. Because the formulated water-flow problem is linear, the relative water uptake rate for any given collection of point (or line) sources and point (or line) sinks is determined by superposing the appropriate solutions while assuming maximum suction at the soil-sink interfaces. In this study, analytical expressions for the maximum possible water uptake rates were determined for various two- and three-dimensional configurations of surface water sources (emitters) and subsurface point (or line) sinks that represent plant roots. Relative water uptake rates were computed for cases of interacting source-sink couples and also for cases of unequal numbers of sources (emitters) and sinks (plants). The water uptake rate can be determined by using as few as three system parameters: the depth and radius of the conceived rooting zone, the soil soeptive number, and design parameters that represent the distances between sources and sinks within rows or between drip lines. Sample computations demonstrate and elucidate the effects of these parameters on the relative water uptake rate of plant roots beneath various configurations of on-surface water sources. For all scenarios, the water uptake rates increased monotonically with increasing radius of the conceived rooting zone or with decreasing separation between emitters or drip lines. We suggest that the calculated relative water uptake rate be used as a design criterion for trickle irrigation systems. [ABSTRACT FROM AUTHOR]

The dynamics of soil microorganisms have important implications for the response of subsurface soil ecosystems to perturbations. Traditional indicators such as functional, community, activity, and carbon and nutrient pathway methods have been used to characterize soil microbial processes and ecological function; however, many of these indicators lack the ability to measure changes over large (e.g., landscape) spatiotemporal scales in soil environments. This research introduces the analysis of soil volatile organic compound (VOC) metabolites as nondestructive indicators of subsurface microbial activity and community composition as a function of varying environmental factors. Results of method validation using laboratory microcosms are presented, where VOC metabolites as characterized by gas chromatography and mass spectrometry (GC-MS) were related to CO2 evolution as a measure of microbial activity and to community-level physiological profiles (CLPPs) and fatty acid methyl ester (FAME) community structure techniques. Results included the identification of 72 VOC metabolites produced from the soils, where significant (ρ < 0.05) differences in the estimated amounts and types of compounds produced were observed between treatments. Temporal measurements indicated similarity between VOC production and CO2 evolution, where increased amounts over time were detected. Principal component analysis (PCA) and hierarchical cluster analysis showed chat the VOC results clustered similarly to FAME and CLPP results. Our results suggest there is promise for the use of naturally produced VOCs as potential indicators of soil microbial ecosystems over large spatiotemporal dynamics and environmental perturbations. [ABSTRACT FROM AUTHOR]

Our ability to predict the effects of various organic amendments on soil aggregate stability is limited due to the complexity of the biological, chemical, and physical mechanisms involved. Based on previous experimental results, this study developed a model (Pouloud) to predict the dynamics of aggregate stability following the incorporation of various organic residues under field conditions. Following Monnier's conceptual model and previously published data, a lognormal function is first used to describe changes in aggregate stability after organic inputs under laboratory conditions. Using principal component analysis, the parameters of the lognormal function are associated with the biochemical characteristics of the organic products such as water-extractable polysaccharide, cellulose and hemicellulose, and lignin contents. To simulate aggregate stability dynamics under field conditions, the effects of soil moisture, soil temperature, and N availability are taken into account by specific functions obtained from the literature. When model simulations were compared with experimental results under field conditions, variations in aggregate stability were generally well reproduced. The sensitivity of the model to climate variations and organic residue characteristics was tested. Soil N availability and the substrate lignin content are major factors that influence the prediction of aggregate stability dynamics. Our results suggest that prediction of aggregate stability dynamics under field conditions using organic substrate characteristics and simple climatic data is possible. More work is required to test the model and broaden its applicability to other soil and climatic conditions. [ABSTRACT FROM AUTHOR]

The understanding, prediction, and modeling efficacy of soil organic carbon (SOC) distribution across fields and larger regions requires a large number of samples that are costly to analyze. The objective of this study was to evaluate soil color measurements to predict SOC for agriculture and prairie land uses. Munsell soil color book (B) and chroma meter (C) color readings were taken at the midpoint depth of each horizon (HB and HC) and predetermined depth increments (IB and IC) on 125 cores. Horizon matrix (HD) colors were determined by standard description. A chroma meter was used to determine the color of prepared samples, ground to <2 mm. Both color data sets were used in a regression analysis to predict SOC content by weight and volume. The best predictors for each technique are the models that incorporate Munsell value and chroma along with the depth from which the measurement was taken. Separating samples by land use improved the prediction of SOC. Transforming SOC content by log10 improved the coefficient of determination for nearly all models. The best predictors of SOC were HD for SOC by weight (agricultural field r2 = 0.79, prairie r2 = 0.53), HB for SOC by volume (agricultural field r2 = 0.76, prairie r2 = 0.57), and IC and IB for log-transformed SOC by weight and volume (agricultural field r2 = 0.84, prairie r2 = 0.62). This study indicates that while SOC content predictions can be made with field measurements, there are limitations to their predictive usefulness. [ABSTRACT FROM AUTHOR]

Estuaries have the highest primary productivity of all ecosystems, yet the soils in estuarine environments have been largely overlooked in C sequestration studies. This research quantified organic C stored in the top 1 m of soils from the Taunton Bay estuary in Hancock County, Maine. Taunton Bay is centrally located within Maine's Island-Bay Coastal Complex, a physiographic region that encompasses 47% of the state's coastline. Riverine inputs of sediments and sediment-associated organic matter to this and most Maine estuaries are quite low. These estuary soils form by incremental additions of reworked glaciofluvial marine sediments and estuarine organic matter to the soil surface. The organics are protected from decomposition by anaerobic conditions. As sea level rises, the estuary area increases by inundating and submerging the soils at its edges. The average concentration of soil organic C in the top 1 m of these estuary soils is 2.4% and the average soil bulk density is 0.67 g cm-3. The organic C content is 136 Mg C ha-1, which is greater than the C content in the top 1 m of Maine's upland soils. These results suggest that systematically quantifying and dating the C in estuarine soils will provide valuable data for use in regional and global C budgets and climate models. [ABSTRACT FROM AUTHOR]

Conservation tillage (CT) and cover cropping (CC) are agricultural practices that may provide solutions to address water and air quality issues arising from intensive agricultural practices. This study investigated how CT and CC affect soil organic matter dynamics in a cotton(Gossypium hirsutum L)-tomato (Lycopersicon esculentum Mill.) rotation in California's San Joaquin Valley. There were four treatments: conservation tillage, no cover crop (CTNO); conservation tillage with cover crop (CTCC); standard tillage, no cover crop (STNO); and standard tillage with cover crop (STCC). After 5 yr, the top 30 cm of soil in CTCC had an increase of 4500 kg C ha-1, compared with an increase of 3800 kg C ha-1 in STCC from initial soil C content in 1999. To enhance our understanding of C dynamics in CT systems, we pulse-labeled cotton with 13CO2 in the field and followed the decomposition of both the roots and the shoots through three physical fractions: light fraction (LF), which tends to turnover quickly, and two relatively stable C pools-intraaggregate LF (iLF) and mineral-associated carbon (mC). Soil under CT treatments retained more of the cotton residue-derived C in LF and iLF than ST 3 mo after placement in the field. These differences disappeared after 1 yr, however, with no discernable differences between CT and ST regardless of CC. In California's Mediterranean climate, CT alone does not accumulate or stabilize more C than ST in tomato-cotton rotations, and the addition of cover crop biomass is more important than tillage reduction for total soil C accumulation. [ABSTRACT FROM AUTHOR]

The performance of the PASTIS model was evaluated to simulate soil C and N fluxes under real field conditions with conventional moldboard plowing (CT) and no-tillage (NT) systems differentiated for 33 yr for a loamy soil in northern France. Afterward, the influences on the C and N fluxes by soil temperature, soil water content, and quantity and localization of soil organic matter (SOM) and crop residues in the soil profile were determined. The model PASTIS was able to provide good simulations for the dynamics of soil water content and temperature, CO2 emissions, residue decomposition, and N mineralization. Simulation showed that the presence of the mulch layer in NT reduced cumulative total water evaporation and increased water drainage at the bottom of the 25-cm depth. Furthermore, simulation showed that the larger cumulative total CO2 flux in NT resulted from larger CO2 emissions as a product of crop residue decomposition and not as product of SOM decomposition. The larger amount of accumulated residues of previous crops in NT more than compensated for the slower residue decomposition rate of surface compared with incorporated residues. It was the water content of the surface crop residues that largely controlled the magnitude of this difference in decomposition rate of the crop residues between CT and NT. This means that, besides the amount of crop residues in both tillage systems, the distribution of rainfall and potential evaporation have a large influence on the differences in C and N fluxes between the two tillage systems. [ABSTRACT FROM AUTHOR]

Land managers and policymakers need information about soil change caused by anthropogenic and non-anthropogenic factors to predict the effects of management on soil function, compare alternatives, and make decisions. Current knowledge of how soils change is not well synthesized and existing soil surveys include only limited information on the dynamic nature of soils. Providing information about causes and attributes of soil change and the effects of soil change on soil function over the human time scale (centuries, decades, or less) should be a primary objective of 21st century soil survey. Soil change is temporal variation in soil across various time scales at a specific location. Attributes of change include state variables (dynamic soil properties), reversibility, drivers, trends, rates, and pathways and functional interpretations include resistance, resilience, and early warning indicators. Iterative elements of the blueprint for action described in this article are: (i) identify user needs; (ii) conduct interdisciplinary research and long-term studies; (iii) develop an organizing framework that relates data, processes, and soil function; (iv) select and prioritize soil change data and information requirements; (v) develop procedures for data collection and interpretation; and (vi) design au integrated soilecosystem-management information system. Selection of dynamic soil properties, soil change attributes, and functional interpretations to be included in future soil surveys should be based on analyses comparing the benefits of meeting user needs to the costs of data acquisition and delivery. Implementation of the blueprint requires increased collaboration among National Cooperative Soil Survey partners and other research disciplines. [ABSTRACT FROM AUTHOR]

Current management and harvesting practices can compact forest soils and degrade soil health. However, effects of soil compaction on microbial processes and composition are poorly understood. We measured microbial community responses to compaction in a sandy loam and a clay loam soil under laboratory and field conditions. Treatments of no, moderate (approximately 20% increase in bulk density), and severe (approximately 40% increase) compaction were manually applied to intact soil cores. A 67-d laboratory experiment, punctuated by four sampling dates, was used to evaluate microbial indices (biomass, respiration, total and culturable bacteria and fungi, N mineralization, surface CO2 efflux, C use (Biolog), and phospholipid fatty acids [PLFA]) and their relationship to soil physical properties (bulk density, pore-size distribution, water-holding capacity [WHC], gas diffusion). Macropores (>30 μm diam.) were reduced 50 to 90% in compacted samples. In contrast, habitable-sized pores for micro- organisms (0.2–30 μm diam.) increased at least 40% in both soils with compaction. Despite these changes, microbial measures were either unaffected by compaction or showed inconsistent increases (e.g., fun- gal hyphae, C use, total PLFA) across sampling periods and soil types. Surface CO2 efflux was reduced 34 to 51% in severe compaction samples. Minimal changes in microbial respiration indicate that reduced efflux was due to restricted gas diffusion. Microbial indifference to compaction also was verified at two mixed-conifer plantations in northern California. Soil strength values, ranging from 75 to 3800 kPa (no to severe compaction), were unrelated to either microbial respiration or biomass. The results show broad tolerance of microbial communities from contrasting soil textures to compaction, and indicate a poor link between physical and biological indices of soil health. [ABSTRACT FROM AUTHOR]

Identification of diagnostic soil organic matter (SOM) fractions and the mechanisms controlling their formation and turnover is critical for better understanding of C dynamics in soils. Enhanced microaggregate formation and stabilization of C due to reduced macroaggregate turnover has been proposed as a mechanism promoting C sequestration in no-tillage (NT) compared with conventional tillage (CT) systems in temperate soils dominated by 2:1 clay mineralogy. We evaluated the contribution of macroaggregate-protected microaggregates to total soil organic carbon (SOC) sequestration in NT relative to CT in three soils differing in clay mineralogy: a 2:1 claydominated soil (2:1), a soil with mixed clay mineralogy [2:1 and 1:1] and oxides (mixed), and a soil dominated by (1:1) clay minerals and oxides (1:1). Microaggregates (mM) were isolated from macroaggregates from 0- to 5- and 5- to 20-cm soil layers. Particulate organic matter (POM) located within the microaggregates (intra-mM-POM) was separated from POM outside of the microaggregates (inter-mMPOM) and the mineral fraction of the microaggregates (mineral-mM). In all three soils, total SOC as well as microaggregate-associated C (mM-C) was greater with NT compared with CT. Although less than half of the total SOC under NT was associated with the microaggregate fraction, more than 90% of the total difference in SOC between NT and CT was explained by the difference in mM-C in all three soils. Thus, we identified and isolated a fraction that explains almost the entire difference in total SOC between NT and CT across soils characterized by drastically different clay mineralogy. [ABSTRACT FROM AUTHOR]

Isotopic labeling techniques have the potential to elucidate soil P dynamics and the fate of P sources added to the soil, but they have rarely been applied to highly weathered tropical soils. We collected soils from two crop rotations [continuous maize (COM; Zea mays L.) and maize-crotaiaria (MCF; Crotalaria grahamiana Wight & Arn.) fallow rotation] in a field experiment in Kenya and incubated them for 9 wk after addition of a plant residue or inorganic phosphorus (Pi), both labeled with 33P and added at 6 mg P kg-1 soil, or after carrier-free labeling of isotopically exchangeable soil phosphorus (soil IEP). The amount of P and recovery of 33P were determined in resin-extractable Pi (Presin), microbial P (Phex), and in a 0.1 M NaOH extract of samples from which Presin and Phex had been removed. The Presin increased after addition of Pi, while Phex increased after plant residue amendment, involving considerable microbial uptake of soil P. The recovery of 33P in Presin followed the order added Pi > soil IEP > plant residue, and decreased steadily from 7 to 22% after 1 d to 3 to 5% after 9 wk. The recovery of 33P in Phex remained constant throughout the incubation, being greater after plant residue amendment (15%) than in the other two treatments (4–7%). An additional 66 to 76% of 33P was recovered in the NaOH extract, as much as 27% of which was in organic phosphorus (Po) after plant residue amendment and 2 to 8% in the other two treatments. Similar to P dynamics after plant residue amendment, the comparison of the two rotations indicated a shift toward Phex and Po with increasing microbial activity due to previous fallow biomass incorporation. [ABSTRACT FROM AUTHOR]

Soil P can be fracfionated by methods such as the Hedley extraction method to help identify and quantify soil P. Measuring the increases in soil P fractions from P fertilizer does not indicate whether external P can control a P fraction because of transfers among P fractions. Similarly, correlations between soil P fractions and plant P uptake do not directly indicate whether a P fraction affects plant P uptake. The objectives of this research were to describe the transformation among fractions, the effect of P fertilizer on P fractions, and the relationship between plant P uptake and P fractions by a state-space modeling approach, and to minimize the dimension of the state-space model by introducing concepts of controllability and observability and by implementing the General Decomposition Theorem. A P fraction was said to be controllable if its size can be controlled directly by the amount of external P fertilizer, or indirectly through transformations among P fractions. A P fraction was considered observable if it contributed to plant P uptake directly, or indirectly through transformations among P fractions. The jointly controllable and observable P fractions identified by the General Decomposition Theorem were minimum to describe the measured soil P dynamics in selected soil-plant systems, while other fractions were redundant in the cases described. Results showed that only strip-P, inorganic NaHCO3-Pi, organic NaHCO3-Po, and inorganic NaOH-Pi of the seven fractions were necessary and sufficient to describe the P dynamics in example Mollisol, Vertisol, Ultisol, and Oxisol. [ABSTRACT FROM AUTHOR]

A better understanding of C turnover, with estimates of root-derived C, is needed to manage soil C sequestration. The objective was to evaluate the long-term treatment and environmental effects on unharvestable soil C components. Two N fertilizer treatments and a control were imposed during 29 yr of continuous corn (Zea mays L.) with stover removal as silage vs. stover return during grain harvest with moldboard plow (MB) tillage. Soil organic carbon (SOC) declined and natural 13C abundance (δ13C) increased during the 29-yr period. Field averages of SOC and δ13C (0-30 cm) were 96.4 Mg ha-1 and -17.3%o in 1965; respective values in 1995 were 78.9 Mg ha-1 and -16.6%o. Loss of SOC was greater with stover removed or no fertilization, but δ13C increased for all treatments. Stover yield (SY), SOC, and δ13C data were applied to a model to estimate unharvestable C and predict total source C (SC) input from corn. The SC for 29 yr totaled 172 to 189 Mg ha-1 when stover was harvested and 268 to 284 Mg ha-1 when stover was returned. The SC input from unharvestable sources was 1.8 times more than SC from aboveground stover when N was added and 1.7 when N was not added. The root-to-shoot ratio was 1.1 when N was added and 1.2 with no N. Only 5.3% of the SC was retained as SOC. Unharvestable C contributions to rhizodeposition are much larger than suggested from controlled studies including C-enriched COz followed by soil respiration or CO2 efflux measurements. [ABSTRACT FROM AUTHOR]

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

Chronosequential analysis of soil properties has proven to be a valuable approach for estimating ages of geomorphic surfaces where no independent age control exists. In this study we examined pedogenic silica as an indicator of relative ages of soils and geomorphic surfaces, and assessed potential sources of the silica. Pedogenic opaline silica was quantified by tiron (4,5-dihydroxy-l,3-benzene-disulfonic acid [disodium salt], C6H4Na2O8S2) extraction for pedons in two different chronosequences in southern California, one in the San Timoteo Badlands and one in Cajon Pass. The soils of both of these chronosequences are developed in arkosic sediments and span 11.5 to 500 ka. The amount of pedogenic silica increases with increasing duration of pedogenesis, and the depth of the maximum silica accumulation generally coincides with the maximum expression of the argillic horizon. Pedogenic silica has accumulated in all of the soils, ranging from 1.2% tiron-extractable Si (Sim) in the youngest soil to 4.6% in the oldest. Primary Si decreases with increasing duration of weathering, particularly in the upper horizons, where weathering conditions are most intense. The loss of Si coincides with the loss of Na and K, implicating the weathering of feldspars as the likely source of Si loss. The quantity of Si lost in the upper horizons is adequate to account for the pedogenic silica accumulation in the subsoil. Pedogenic silica was equally effective as pedogenic Fe oxides as an indicator of relative soil age in these soils. [ABSTRACT FROM AUTHOR]

An incubation system was developed to study the dynamics of greenhouse gas emissions from soil cores at different matric potentials. It combines the facility to establish and easily alter matric potential in soil columns, contained in nine parallel incubation chambers, with the feature of highly time resolved measurements of NO, N2O, N2, O2, and CO2. Studies on chamber performance demonstrated that the repeated application of specified suctions or pressures gave highly reproducible water contents and that the water release curves were similar to those obtained with traditional methods. The system was used to examine how soils preincubated at -5 or -75 kPa for 90 d responded to matric potential increases during a subsequent anaerobic incubation. The data showed that as large pores were filled with water rates of both CO2 evolution and total denitrification increased up to -2 kPa. This is in contrast to aerobic studies that show a decline in CO2 evolution at such high matric potentials. The data suggest that C that was not decomposed in large air-filled pores during the aerobic preincubation became colonized only when these pores became water-filled in the subsequent anaerobic incubation and provided substrate for denitrifying organisms. These studies demonstrate the capacity of the incubation system to explore the complex interaction between soil structure and the microbially mediated soil processes responsible for gaseous N and C emissions. This interaction is crucial to understanding factors controlling gaseous emission rates but has historically been difficult to be examined directly. [ABSTRACT FROM AUTHOR]

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

It is well known that soil CO2 flux can exhibit pronounced day-to-day variations; however, measurements of soil CO2 flux with soil chambers typically are done only at discrete points in time. This study evaluated the impact of sampling frequency on the precision of cumulative CO2 flux estimates calculated from field measurements. Automated chambers were deployed at two sites in a no-till corn/ soybean field and operated in open system mode to measure soil CO2 fluxes every hour from 4 March 2000 through 6 June 2000. Sampling frequency effects on cumulative CO2-C flux estimation were assessed with a jackknife technique whereby the populations of measured hourly fluxes were numerically sampled at regular time intervals ranging from I d to 20 d, and the resulting sets of jackknife fluxes were used to calculate estimates of cumulative CO2-C flux. We observed that as sampling interval increased from I d to 12 d, the variance associated with cumulative flux estimates increased. However, at sampling intervals of 12 to 20 d, variances were relatively constant. Sampling once every 3 d, estimates of cumulative C loss were within ±20% of the expected value at both sites. As the time interval between sampling was increased, the potential deviation in estimated cumulative CO2 flux increased such that sampling once every 20 d yielded potential estimates within +60% and -40% of the actual cumulative CO2 flux. A stratified sampling scheme around rainfall events was also evaluated and was found to provide more precise estimates at lower sampling intensities. These results should aid investigators to develop sampling designs to minimize the effects of temporal variability on cumulative CO2-C estimation. [ABSTRACT FROM AUTHOR]

A lack of understanding about the selenite adsorption behavior on hydroxyaluminum (HyA)- and hydroxyaluminosilicate (HAS)interlayered phyllosilicates led us to conduct the present study. The kinetics of selenite adsorption on montmorillonite (Mt), HyA(OH/ Al = 2.0)-Mt, HAS1(OH/A1 = 2.0; Si/Al = 0.24)-Mt, and HAS2(OH/ Al = 2.0; Si/Al = 0.48)-Mt were studied at pH 4.5, with an initial selenite concentration of 0.025 mM, a clay concentration of 0.5 g L-1, temperatures of 288, 298, 308, and 318 K, and background electrolyte concentration of 10-2 M NaNO3. Of the six different kinetic models tested, the second-order rate equation best described the kinetic data obtained for the initial fast reaction (5-30 min) followed by a slow reaction (30-180 min) in the adsorption systems. Elevated temperatures brought about a substantial increase in the rate constants. Compared with Mt, different HyA/HAS-Mts had 2 to 21 times higher rate constants for the fast reaction and up to five times higher rate constants for the slow reaction. Silication of HyA-Mt to form HAS1-Mt and HAS2-Mt substantially lowered the rate constants for both the fast and slow reactions. For the fast reaction, Mt had the highest activation energy and HyA-Mt had the lowest activation energy (around four times lower than Mt); silication increased the activation energy of selenite adsorption on the HAS-Mts. The pre-exponential factor, an index of the frequency of selenite collision with the clay surface, was remarkably lower for the HyA/HAS-Mts in comparison with Mt. The data obtained in the present study are of fundamental significance in understanding the role of Al interlayering and coating and silication of Al polymers on expansible phyllosilicates in influencing the dynamics of Se in soil and related environments. [ABSTRACT FROM AUTHOR]

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

The availability of methods for quasi-continuous measurements of soil shrinkage curves allowed the development of new models. The exponential (XP) model allows the calculation of the volume of two pore phases in the soil, namely macro and micropore volumes. The micropore volume is identified with the pore volume of the soil clay matrix according to the assumption that the maximum swelling of the clay matrix (MS) is the point of minimum water content of the structural shrinkage (Point D). This is discussed using undisturbed and repacked soil samples with various clay contents and clay types. The slope of the shrinkage curves as a function of equivalent saturatedpore radius show a transition in pore type around a 10-µm pore radius, where smaller and more deformable pores start to desaturate. This corresponds to the fitted D point and is close to the size of the largest pores in the clay matrix or clay-silt phase reported in the literature. The calculated micropore volume and micropore swelling properties are close to clay paste properties reported in the literature. At Iow water content, the specific micropore volume is independent from clay content. At Point D, the shrinking capacity of the specific micropore volume decreases with increasing clay content for clay contents below 40%. Our results show that Point D can be identified with the MS of the clay matrix, and that the XP model can be used to calculate the swelling properties of clays in the soil, without extraction. [ABSTRACT FROM AUTHOR]

Under limited water resources, modern irrigation methods tend to save water and improve water and salt regimes within the root zone. This study deals with the combined effects of water application methods and tillage practices on water and salt distributions, runoff production, and soil loss in a field irrigated with moving irrigation systems (MIS). An experiment was conducted in a cotton (Gossypium hirsutum L. cv. Sivon) field at Hazorea, Israel, where the main soil type is vertisol (Typic Chromoxerets). Sprinkling (SP) and flooding (FL) MIS, and conventional (CT) and microbasin (MB) tillage, were compared in terms of runoff and soil loss from runoff microplots (5 m²), soil water content and salinity distribution with depth, yield, and plant height. Under SP conditions, no runoff and soil loss were obtained for either tillage practice. In the FL/CT treatment, the mean runoff and soil loss were about 25% of the irrigation water and 0.59 kg m[sup -2]. The FLfMB treatment reduced the runoff and soil loss to 5.8% and 0.02 kg m[sup -2], respectively. The soil water contents in the SP treatments were generally lower than in the FL treatments, especially in the 0.1to 0.6-m soil layer. No significant differences in the soil salinity, plant height, and seed-cotton yield were observed between the treatments. Microbasins tillage reduces water losses under flooding MIS to a point where they become practically similar to those obtained under sprinkler MIS. It can potentially lead to lower water losses if the microbasins storage capacity is matched to the water application rate, to avoid runoff. [ABSTRACT FROM AUTHOR]

Repeated N applications to field crops, either as inorganic fertilizers or animal manures, can lead to N buildup in soils with potential longterm environmental hazards. The objective of this 3-yr field study was to monitor total- and mineral-N levels in soil after repeated fertilizer or single or repeated dairy manure applications, and to compute an N balance for the soil-crop system. Unlabeled and [sup 15]N-enriched dairy manure were used. The experiment was conducted on a Piano corn silt loam continuously cropped to corn (Zea mays L.) Manure increased total- and NO[sub 3]-N levels in soil, especially in the 0- to 30-cm depth and in plots receiving frequent and recent manure applications. Manure increased NO[sub 3]-N in the 0- to 30-cm soil layer more than fertilizer N, whereas the opposite was true in the 30- to 60- and 60to 90-cm layers. There was a clear NO[sub 3]-N buildup with repeated manure treatments. Unlabeled N measurements were not accurate enough to track trends in soil total N levels, hampering the calculation of an N balance. [sup 15]Nitrogen-labeled manure allowed for direct measurement and provided more accurate estimates of N recovery in soils and crops. During the 3-yr study period, an average of 18% of applied manure [sup 15]N was recovered in corn silage and 46% remained in the soil. Unaccounted-for [sup 15]N (36%) was assumed to be lost mainly by NH[sub 3] volatilization and denitrification. Most (82%) of the [sup 15]N remaining in soil was present in the top 30 cm, irrespective of frequency of manure application. Although costly and time-consuming, the use of [sup 15]Nlabeled manure provided a much better approach to study the fate of manure N within the soil-crop system, compared with unlabeled manure. [ABSTRACT FROM AUTHOR]

Calculating the dynamics of soil water content (Θ) near the surface and modeling soil water evaporation (E[sub 5]) are critical for many agricultural management strategies. This study was performed to develop a model to simulate soil water redistribution during second-stage evaporation (SSE). In this model, the daily change of Θ was estimated from the difference between the initial Θ (Θ[sub i]) and air-dry Θ (Θ[sub ad]), multiplied by a conductance coefficient (C). The C represents the fraction of the remaining soil water (Θ[sub i] - Θ[sub ad]) that can be removed in I d during SSE and is a power function of soil depth. Testing the dependency of C and α (the slope of cumulative evaporation [E[sub c]] vs. square root of time [t[sup ½]]) on soil characteristics was done using theoretical and laboratory data. Then the whole model was evaluated in laboratory and field conditions by measuring Θ for different soils at different depths during SSE. Linear relationships with zero intercept were found between α and drained upper limit Θ (Θ[sub dul]) with slope and r² = 1.19 and 0.69 and 1.39 and 0.95 for laboratory and theoretical data, respectively. Conductance coefficient and Θ[sub dul] were correlated with r² > 0.9. Root mean square error (RMSE) between measured and estimated Θ in the field was highest (0.014 cm³ cm[sup -3]) at depths of 3 and 6 cm and lowest (0.005 cm³ cm[sup -3]) at the 9-cm depth. The model gave reasonable estimates of both water redistribution and E[sub s] during SSE and is expected to work well for soils for which the diffusivity theory holds. [ABSTRACT FROM AUTHOR]

Volumetric soil-water depth profiles at nine sample locations on a 2-ha hillslope were monitored throughout the 1997-1998 El Niño and the 1998-1999 La Niña cycles. A hydrological model integrating the soil-water measurements with digital terrain analysis and a onedimensional water balance model was developed to map dominant hydrological patterns of soil-water storage and lateral flow redistribution. Statistical correlations between hydrologic behavior and the compound topographic index (CTI) generated from a digital elevation model (DEM) were used to generate spatially distributed input parameters of initial water storage and soil-controlled evapotranspiratiun utilized in the model. The results suggest that differences in water storage and availability are highly modified by climatic conditions and local topography. Nearly three times higher than normal rainfall in the El Niño year caused deeper infiltration of water and led to significant subsurface water redistribution into the concave hillslope positions which remained moist throughout the 1998 summer. Water infiltration and distribution was diminished considerably in the drier than normal La Niña, and led to a complete dry-down in 1999, Actual evapotranspiration was 87% of total precipitation during the El Niño, compared with 100% in the subsequent La Niña. The good correlation between modeled and measured water storage shows that even a simple one-dimensional model combined with the appropriate input parameters is a suitable tool for estimating changes in soil-water content on hillslopes where lateral flow is a significant functional component of the soil hydrology. [ABSTRACT FROM AUTHOR]

Soil suitability assessments for land use planning are commonly based on on-site specific topographic, soil, and climatic characteristics, and are often neglecting the effects of physical landscape processes by water. The spatial and temporal variability of the landscape requires specific input data and modeling procedures. Existing studies aiming at the landscape level often are data-driven and operating at detailed resolutions of seconds and hours for single slopes or catchments. This study aims at the coarser level of multiple catchments over a period of 10 yr. A dynamic modeling approach is applied to a study area fur the effects of soil redistribution within a landscape (run-on, run-off, erosion, and sedimentation) on subsequent soil water availability. Simulation scenarios include factors of water routing, soil depth, and erodibility. Different approaches for surface run-off routing have a major influence on the magnitude and spatial patterns of soil redistribution. Also initial conditions such as soil depth, parent material, and erodibility have spatial impacts upon soil erosion and sedimentation within the landscape. Locally decreasing water storage capacity (onsite) may cause increased run-off and erosion at lower positions in the landscape (off-site). Localized soil redistribution can cause significant changes in actual soil depth and indirectly affect available soil water. The changing patterns of soil redistribution for the different scenarios are both related to modeling techniques as well as to the implemented boundary conditions. This study indicates that at the landscape scale spatial variability in for example soil properties is inherent to both the complexity of the landscape (parent material) and on-site and off-site effects of controlling processes. [ABSTRACT FROM AUTHOR]

Investigates salinization and alkalinization processes as a function of tidal inundation frequency across low-lying transgressive landscapes of the Chesapeake Bay region, in Dorchester County, Maryland. Pedogenic transformation of Ultisols to Alfisols and eventually to Histosols in response to sea-level rise; Description of changes in the submerging soils along the Mid-Atlantic coast using sodium adsorption ratio and exchangeable sodium percentage; Assessment of soil alkalinity using exchangeable sodium percentage.

Studies changes in soil organic carbon (SOC) dynamics during a 51-year period on the Breton Classical Plots at Breton, Alberta in Canada by using historical records of crop yield, soil management and soil carbon measurements. Influence of carbon inputs and manure on SOC content over diverse management; Distribution of SOC content among kinetic soil compartments of increasing stability; Estimation of the SOC parameters.

Investigates sulfate pools and desorption in the weathered substrata of a forested catchment on granite to quantify the importance of these layers to sulfate dynamics. Factors affecting sulfate concentration on groundwater; Variability of sulfate pools and its desorption behavior; Higher sulfate pools in weathered substrata than in mineral soil.