Showing papers on "Soil organic matter published in 2000"

Soil carbon sequestration and land‐use change: processes and potential

Abstract: SUMMARY When agricultural land is no longer used for cultivation and allowed to revert to natural vegetation or replanted to perennial vegetation, soil organic carbon can accumulate by processes that essentially reverse some of the effects responsible for soil organic carbon losses from when the land was converted from perennial vegetation. We discuss the essential elements of what is known about soil organic matter dynamics that may result in enhanced soil carbon sequestration with changes in land-use and soil management. We review literature that reports changes in soil organic carbon after changes in land-use that favor carbon accumulation. This data summary provides a guide to approximate rates of SOC sequestration that are possible with management, and indicates the relative importance of some factors that influence the rates of organic carbon sequestration in soil. There is a large amount of variation in rates and the length of time that carbon may accumulate in soil that are related to the productivity of the recovering vegetation, physical and biological conditions in the soil, and the past history of soil organic carbon inputs and physical disturbance. Maximum rates of C accumulation during the early aggrading stage of perennial vegetation growth, while substantial, are usually much less than 100 g C m y . Average rates of accumulation are similar for forest or grassland establishment: 33.8 g C m y and 33.2 g C m y respectively. These observed rates of soil organic C accumulation, when combined with the small amount of land area involved, are insufficient to account for a significant fraction of the missing C in the global carbon cycle as accumulating in the soils of formerly agricultural land.

Review of mechanisms and quantification of priming effects.

Abstract: Priming effects are strong short-term changes in the turnover of soil organic matter caused by comparatively moderate treatments of the soil. In the course of priming effects large amounts of C, N and other nutrients can be released or immobilized in soil in a very short time. These effects have been measured in many field and laboratory experiments; however, only a few of the studies were aimed at an extended investigation of the mechanisms of such phenomena. The aim of this overview is to reveal possible causes and processes leading to priming actions using the references on agricultural ecosystems and model experiments. Multiple mechanisms and sources of released C and N are presented and summarized in Tables for positive and negative real and apparent priming effects induced after the addition of different organic and mineral substances to the soil. Soil microbial biomass plays the key role in the processes leading to the real priming effects. The most important mechanisms for the real priming effects are the acceleration or retardation of soil organic matter turnover due to increased activity or amount of microbial biomass. Isotopic exchange, pool substitution, and different uncontrolled losses of mineralized N from the soil are responsible for the apparent N priming effects. Other multiple mechanisms (predation, competition for nutrients between roots and microorganisms, preferred uptake, inhibition, etc.) in response to addition of different substances are also discussed. These mechanisms can be distinguished from each other by the simultaneous monitoring of C and N release dynamics; its comparison with the course of microbial activity; and by the labelling of different pools with 14 C or 13 C and 15 N. Quantitative methods for describing priming effects and their dynamics using 14 C and 15 N isotopes, as well as for non-isotopic studies are proposed.

Soil macroaggregate turnover and microaggregate formation: a mechanism for C sequestration under no-tillage agriculture

Abstract: Soil disturbance from tillage is a major cause of organic matter depletion and reduction in the number and stability of soil aggregates when native ecosystems are converted to agriculture. No-till (NT) cropping systems usually exhibit increased aggregation and soil organic matter relative to conventional tillage (CT). However, the extent of soil organic matter changes in response to NT management varies between soils and the mechanisms of organic matter stabilization in NT systems are unclear. We evaluated a conceptual model which links the turnover of aggregates to soil organic matter dynamics in NT and CT systems; we argue that the rate of macroaggregate formation and degradation (i.e. aggregate turnover) is reduced under NT compared to CT and leads to a formation of stable microaggregates in which carbon is stabilized and sequestered in the long term. Therefore, the link between macroaggregate turnover, microaggregate formation, and C stabilization within microaggregates partly determines the observed soil organic matter increases under NT.

Controls on the dynamics of dissolved organic matter in soils: a review.

Abstract: Dissolved organic matter (DOM) in soils plays an important role in the biogeochemistry of carbon, nitrogen, and phosphorus, in pedogenesis, and in the transport of pollutants in soils. The aim of this review is to summarize the recent literature about controls on DOM concentrations and fluxes in soi

Short communication Soil macroaggregate turnover and microaggregate formation: a mechanism for C sequestration under no-tillage agriculture

Abstract: Soil disturbance from tillage is a major cause of organic matter depletion and reduction in the number and stability of soil aggregates when native ecosystems are converted to agriculture. No-till (NT) cropping systems usually exhibit increased aggregation and soil organic matter relative to conventional tillage (CT). However, the extent of soil organic matter changes in response to NT management varies between soils and the mechanisms of organic matter stabilization in NT systems are unclear. We evaluated a conceptual model which links the turnover of aggregates to soil organic matter dynamics in NT and CT systems; we argue that the rate of macroaggregate formation and degradation (i.e. aggregate turnover) is reduced under NT compared to CT and leads to a formation of stable microaggregates in which carbon is stabilized and sequestered in the long term. Therefore, the link between macroaggregate turnover, microaggregate formation, and C stabilization within microaggregates partly determines the observed soil organic matter increases under NT. q 2000 Elsevier Science Ltd. All rights reserved.

Soil respiration and the global carbon cycle

Abstract: Soil respiration is the primary path by which CO2fixed by land plants returns to the atmosphere. Estimated at approximately 75 × 1015gC/yr, this large natural flux is likely to increase due changes in the Earth's condition. The objective of this paper is to provide a brief scientific review for policymakers who are concerned that changes in soil respiration may contribute to the rise in CO2in Earth's atmosphere. Rising concentrations of CO2in the atmosphere will increase the flux of CO2from soils, while simultaneously leaving a greater store of carbon in the soil. Traditional tillage cultivation and rising temperature increase the flux of CO2from soils without increasing the stock of soil organic matter. Increasing deposition of nitrogen from the atmosphere may lead to the sequestration of carbon in vegetation and soils. The response of the land biosphere to simultaneous changes in all of these factors is unknown, but a large increase in the soil carbon pool seems unlikely to moderate the rise in atmospheric CO2during the next century.

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

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

Solid-Solution Partitioning of Metals in Contaminated Soils: Dependence on pH, Total Metal Burden, and Organic Matter

Abstract: Environmental risk assessment of metals depends to a great extent on modeling the fate and the mobility of metals based on soil−liquid partitioning coefficients. A large variability is observed among the reported values that could be used to predict metal mobility and bioavailability. To evaluate this, soil−liquid partitioning coefficients (Kd) for many elements but especially for the metals cadmium, copper, lead, nickel, and zinc were compiled from over 70 studies of various origins collected from the literature. The relationships between the reported values are explored relative to variations in soil solution pH, soil organic matter (SOM), and concentrations of total soil metal. The results of multiple linear regressions show that Kd values are best predicted using empirical linear regressions with pH (with R 2 values of 0.29−0.58) or with pH and either the log of SOM or the log of total metal and with resulting R 2 values of 0.42−0.76. A semi-mechanistic model based on the competitive adsorption of met...

The role of polyphenols in terrestrial ecosystem nutrient cycling

Abstract: Interspecific variation in polyphenol production by plants has been interpreted in terms of defense against herbivores. Several recent lines of evidence suggest that polyphenols also influence the pools and fluxes of inorganic and organic soil nutrients. Such effects could have far-ranging consequences for nutrient competition among and between plants and microbes, and for ecosystem nutrient cycling and retention. The significance of polyphenols for nutrient cycling and plant productivity is still uncertain, but it could provide an alternative or complementary explanation for the variability in polyphenol production by plants.

Relationship of soil organic matter dynamics to physical protection and tillage

Abstract: Tillage has been reported to reduce organic matter concentrations and increase organic matter turnover rates to a variable extent. The change of soil climate and the incorporation of aboveground C inputs within the soil lead to no unique effect on biodegradation rates, because of their strong interaction with the regional climate and the soil physical properties. The periodical perturbation of soil structure by tools and the subsequent drying‐rewetting cycles may be the major factor increasing organic matter decomposition rates by exposing the organic matter that is physically protected in microaggregates to biodegradation. This paper reviews the assessed effects of tillage on organic matter, the scale, extent and mechanisms of physical protection of organic matter in soils. # 2000 Elsevier Science B.V. All rights reserved.

Ecology and Management of Forest Soils

Abstract: Forest soils have characteristic properties that set them apart from the usual types of soils used for growing crops. Often rocky, of poor quality, and in mountainous terrain, forest soils are nevertheless the foundation of the entire forest ecosystem, supporting a diverse assemblage of plants and animals. They are often fragile soils, easily subject to erosion by road building or logging operation. A proper understanding of forest soils is an essential component of understanding forest ecology and maintaining the diversity and productivity of forested land. This new edition emphasizes the ecological aspects of forest soils. It is global in its scope, discussing soil types ranging from the tropical rainforest soils of Latin America to the boreal forest soils of Siberia. Separate chapters discuss the physical, chemical, and biological aspects of forest soils, with additional chapters on soil organic matter, roots, and biogeochemistry. It highlights the site specific factors that are important in each case and discusses practical management aspects including: nutrition management, thinning, site preparation techniques, soils for nursery and seed orchard operation, problem soils, atmospheric deposition of nutrients, soil acidity, and techniques for sustaining and improving long term soil productivity.

Handbook of soil science

Abstract: Soil Physics, AW Warrick Physical Properties of Primary Particles Dynamic Properties of Soils Soil Water Content and Water Potential Relationships Soil Water Movement Energy and Water Balances at Soil-Plant-Atmosphere Interfaces Solute Transport Soil Structure Soil Gas Movement in Unsaturated Systems Soil Spatial Variability Soil Chemistry, PM Huang The Chemical Composition of Soils Soil Organic Matter The Soil Solution Kinetics and Mechanisms of Soil Chemical Reactions Redox Phenomena Soil Colloidal Behavior Ion Exchange Pehnomena Chemisorption and Precipitation Reactions Abiotic Catalysis Soil pH and pH Buffering Soil Biology and Biochemistry, EA Paul Microbiota Soil Fauna Microbially Mediated Processes Nitrogen Transformations Soil Fertility and Plant Nutrition, EJ Kamprath Bioavailability of Major Essential Nutrients Bioavailability of Micronutrients Nutrient Interactions in Soil and Plant Nutrition Soil Fertility Evaluation Fundamentals of Fertilizer Application Nutrient and Water Use Efficiency Pedology, LP Wilding Geomorphology of Soil Landscapes Pedogenic Processes Pedological Modeling Soil Taxonomy Other Systems of Soil Classification Classification of Soils Land Evaluation for Landscape Units Soil Mineralogy, JW Stucki The Alteration and Formation of Soil Minerals by Weathering Phyllosilicates Oxide Minerals Poorly Crystalline Aluminosilicate Clays Interdiscipinary Aspects of Soil Science, I Shainberg Salinity Sodicity Hardsetting Soils Biogeochemistry of Wetlands Acid Sulfate Soils Soils and Environmental Quality Water Erosion Wind Erosion Land Application of Wastes Conservation Tillage Soil Quality Soil Databases, M F Baumgardner From the Soil Map of the World to the Digital Global Soil and Terrain Database: 1960-2002 SOTER: The World Soils and Terrain Database Development of a 05 Degrees by 05 Degrees Resolution Global Database The Canadian Soil Database United States Soil Survey Database The Use of Soil Databases in Resource Assessments

Carbon input by plants into the soil. Review.

Abstract: The methods used for estimating below-ground carbon (C) translocation by plants, and the results obtained for different plant species are reviewed. Three tracer techniques using C isotopes to quantify root-derived C are discussed: pulse labeling, continuous labeling, and a method based on the difference in 13 C natural abundance in C3 and C4 plants. It is shown, that only the tracer methods provided adequate results for the whole below-ground C translocation. This included roots, exudates and other organic substances, quickly decomposable by soil microorganisms, and CO 2 produced by root respiration. Advantages due to coupling of two different tracer techniques are shown. The differences in the below-ground C translocation pattern between plant species (cereals, grasses, and trees) are discussed. Cereals (wheat and barley) transfer 20%-30% of total assimilated C into the soil. Half of this amount is subsequently found in the roots and about one-third in CO2 evolved from the soil by root respiration and microbial utilization of rootborne organic substances. The remaining part of below-ground translocated C is incorporated into the soil microorganisms and soil organic matter. The portion of assimilated C allocated below the ground by cereals decreases during growth and by increasing N fertilization. Pasture plants translocated about 30%-50% of assimilates below-ground, and their translocation patterns were similar to those of crop plants. On average, the total C amounts translocated into the soil by cereals and pasture plants are approximately the same (1500 kg C ha -1 ), when the same growth period is considered. However, during one vegetation period the cereals and grasses allocated beneath the ground about 1500 and 2200kg C ha -1 , respectively. Finally, a simple approach is suggested for a rough calculation of C input into the soil and for root-derived CO 2 efflux from the soil.

Age of soil organic matter and soil respiration: radiocarbon constraints on belowground c dynamics

Abstract: Radiocarbon data from soil organic matter and soil respiration provide pow- erful constraints for determining carbon dynamics and thereby the magnitude and timing of soil carbon response to global change. In this paper, data from three sites representing well-drained soils in boreal, temperate, and tropical forests are used to illustrate the methods for using radiocarbon to determine the turnover times of soil organic matter and to partition soil respiration. For these sites, the average age of bulk carbon in detrital and Oh/A-horizon organic carbon ranges from 200 to 1200 yr. In each case, this mass-weighted average includes components such as relatively undecomposed leaf, root, and moss litter with much shorter turnover times, and humified or mineral-associated organic matter with much longer turnover times. The average age of carbon in organic matter is greater than the average age predicted for CO2 produced by its decomposition (30, 8, and 3 yr for boreal, temperate, and tropical soil), or measured in total soil respiration (16, 3, and 1 yr). Most of the CO 2 produced during decomposition is derived from relatively short-lived soil organic matter (SOM) components that do not represent a large component of the standing stock of soil organic matter. Estimates of soil carbon turnover obtained by dividing C stocks by hetero- trophic respiration fluxes, or from radiocarbon measurements of bulk SOM, are biased to longer time scales of C cycling. Failure to account for the heterogeneity of soil organic matter will result in underestimation of the short-term response and overestimation of the long-term response of soil C storage to future changes in inputs or decomposition. Comparison of the 14 C in soil respiration with soil organic matter in temperate and boreal forest sites indicates a significant contribution from decomposition of organic matter fixed.2 yr but ,30 yr ago. Tropical soil respiration is dominated by C fixed ,1 yr ago. Monitoring the 14 C signature of CO2 emitted from soils give clues as to the causes of

Management options for reducing CO2 emissions from agricultural soils

Abstract: Crop-based agriculture occupies 1.7 billion hectares, globally, with a soil C stock of about 170 Pg. Of the past anthropogenic CO2 additions to the atmosphere, about 50 Pg C came from the loss of soil organic matter (SOM) in cultivated soils. Improved management practices, however, can rebuild C stocks in agricultural soils and help mitigate CO2 emissions.

Will changes in soil organic carbon act as a positive or negative feedback on global warming

Abstract: The world's soils contain about 1500 Gt of organic carbon to a depth of 1m and a further 900 Gt from 1-2m. A change of total soil organic carbon by just 10% would thus be equivalent to all the anthropogenic CO2 emitted over 30 years. Warming is likely to increase both the rate of decomposition and net primary production (NPP), with a fraction of NPP forming new organic carbon. Evidence from various sources can be used to assess whether NPP or the rate of decomposition has the greater temperature sensitivity, and, hence, whether warming is likely to lead to an increase or decrease in soil organic carbon. Evidence is reviewed from laboratory-based incubations, field measurements of organic carbon storage, carbon isotope ratios and soil respiration with either naturally varying tempera- tures or after experimentally increasing soil temperatures. Estimates of terrestrial carbon stored at the Last Glacial Maximum are also reviewed. The review concludes that the temper- ature dependence of organic matter decomposition can be best described as: d(T) = exp(3.36 (T 40)/(T + 31.79)) where d(T) is the normalised decomposition rate at temperature T (in C). In this equation, decomposition rate is normalised to '1' at 40 C. The review concludes by simulating the likely changes in soil organic carbon with warm- ing. In summary, it appears likely that warming will have the effect of reducing soil organic carbon by stimulating decomposition rates more than NPP. However, increasing CO2 is likely to simultaneously have the effect of increasing soil organic carbon through increases in NPP. Any changes are also likely to be very slow. The net effect of changes in soil organic carbon on atmospheric CO2 loading over the next decades to centuries is, therefore, likely to be small.

Carbon isotope ratios in belowground carbon cycle processes

Abstract: Analyses of carbon isotope ratios (δ13C) in soil organic matter (SOM) and soil respired CO2 provide insights into dynamics of the carbon cycle. δ13C analyses do not provide direct measures of soil CO2 efflux rates but are useful as a constraint in carbon cycle models. In many cases, δ13C analyses allow the identification of components of soil CO2 efflux as well as the relative contribution of soil to overall ecosystem CO2 fluxes. δ13C values provide a unique tool for quantifying historical shifts between C3 and C4 ecosystems over decadal to millennial time scales, which are relevant to climate change and land-use change issues. We identify the need to distinguish between δ13C analyses of SOM and those of soil CO2 efflux in carbon cycle studies, because time lags in the turnover rates of different soil carbon components can result in fluxes and stocks that differ in isotopic composition (disequilibrium effect). We suggest that the frequently observed progressive δ13C enrichment of SOM may be related to a g...

Organic Matter Influence on Clay Wettability and Soil Aggregate Stability

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

Controls over carbon storage and turnover in high-latitude soils

Abstract: Despite the importance of Arctic and boreal regions in the present carbon cycle, estimates of annual high-latitude carbon fluxes vary in sign and magnitude. Without accurate estimates of current carbon fluxes from Arctic and boreal ecosystems, predicting the response of these systems to global change is daunting. A number of factors control carbon turnover in high-latitude soils, but because they are unique to northern systems, they are mostly ignored by biogeochemical models used to predict the response of these systems to global change. Here, we review those factors. First, many northern systems are dominated by mosses, whose extremely slow decomposition is not predicted by commonly used indices of litter quality. Second, cold temperature, permafrost, waterlogging, and substrate quality interact to stabilize soil organic matter, but the relative importance of these factors, and how they respond to climate change, is unknown. Third, recent evidence suggests that biological activity occuring over winter can contribute significantly to annual soil carbon fluxes. However, the controls over this winter activity remain poorly understood. Finally, processes at the landscape scale, such as fire, permafrost dynamics, and drainage, control regional carbon fluxes, complicating the extrapolation of site-level measurements to regional scales.

Soil carbon cycling in a temperate forest: radiocarbon-based estimates of residence times, sequestration rates and partitioning of fluxes

Abstract: Temperate forests of North America are thought to be significant sinks of atmospheric CO2. We developed a below-ground carbon (C) budget for well-drained soils in Harvard Forest Massachusetts, an ecosystem that is storing C. Measurements of carbon and radiocarbon ( 14 C) inventory were used to determine the turnover time and maximum rate of CO2 production from heterotrophic respiration of three fractions of soil organic matter (SOM): recognizable litter fragments (L), humified low density material (H), and high density or mineral-associated organic matter (M). Turnover times in all fractions increased with soil depth and were 2-5 years for recognizable leaf litter, 5-10 years for root litter, 40-100+ years for low density humified material and >100 years for carbon associated with minerals. These turnover times represent the time carbon resides in the plant + soil system, and may underestimate actual decomposition rates if carbon resides for several years in living root, plant or woody material. Soil respiration was partitioned into two components using 14 C: recent photosynthate which is metabolized by roots and microorganisms within a year of initial fixation (Recent- C), and C that is respired during microbial decomposition of SOM that resides in the soil for several years or longer (Reservoir-C). For the whole soil, we calculate that decomposition of Reservoir-C contributes approximately 41% of the total annual soil respiration. Of this 41%, recognizable leaf or root detritus accounts for 80% of the flux, and 20% is from the more humified fractions that dominate the soil carbon stocks. Measurements of CO 2 and 14 CO2 in the soil atmosphere and in total soil respiration were combined with surface CO2 fluxes and a soil gas diffusion model to determine the flux and isotopic signature of C produced as a function of soil depth. 63% of soil respiration takes place in the top 15 cm of the soil (O + A + Ap horizons). The average residence time of Reservoir-C in the plant + soil system is 81 years and the average age of carbon in total soil respiration (Recent-C + Reservoir-C) is 41 years. The O and A horizons have accumulated 4.4 kgC m -- 2 above the plow layer since abandon- ment by settlers in the late-1800's. C pools contributing the most to soil respiration have short enough turnover times that they are likely in steady state. However, most C is stored as humified organic matter within both the O and A horizons and has turnover times from 40 to

SYNOPSIS Polycyclic Aromatic Hydrocarbons (PAHs) in Soil — a Review

Abstract: PAHs are mainly produced by combustion processes and consist of a number of toxic compounds. While the concentrations of individual PAHs in soil produced by natural processes (e.g., vegetation fires, volcanic exhalations) are estimated to be around 1—10 μg kg−1, recently measured lowest concentrations are frequently 10 times higher. Organic horizons of forest soils and urban soils may even reach individual PAH concentrations of several 100 μg kg−1. The PAH mixture in temperate soils is often dominated by benzofluoranthenes, chrysene, and fluoranthene. The few existing studies on tropical soils indicate that the PAH concentrations are relatively lower than in temperate soils for most compounds except for naphthalene, phenanthrene, and perylene suggesting the presence of unidentified PAH sources. PAHs accumulate in C-rich topsoils, in the stemfoot area, at aggregate surfaces, and in the fine-textured particle fractions, particularly the silt fraction. PAHs are mainly associated with soil organic matter (SOM) and soot-like C. Although the water-solubility of PAHs is low, they are encountered in the subsoil suggesting that they are transported in association with dissolved organic matter (DOM). The uptake of PAHs by plants is small. Most PAHs detected in plant tissue are from atmospheric deposition. However, earthworms bioaccumulate considerable amounts of PAHs in short periods. The reviewed work illustrates that there is a paucity of data on the global distribution of PAHs, particularly with respect to tropical and southern hemispheric regions. Reliable methods to characterize bioavailable PAH pools in soil still need to be developed.

Dynamics of soil nitrogen and carbon accumulation for 61 years after agricultural abandonment

Abstract: We used two independent methods to determine the dynamics of soil carbon and nitrogen following abandonment of agricultural fields on a Minnesota sand plain. First, we used a chronosequence of 19 fields abandoned from 1927 to 1982 to infer soil carbon and nitrogen dynamics. Second, we directly observed dynamics of carbon and nitrogen over a 12-yr period in 1900 permanent plots in these fields. These observed dynamics were used in a differential equation model to predict soil carbon and nitrogen dynamics. The two methods yielded similar results. Resampling the 1900 plots showed that the rates of accumulation of nitrogen and carbon over 12 yr depended on ambient carbon and nitrogen levels in the soil, with rates of accumulation declining at higher carbon and nitrogen levels. A dynamic model fitted to the observed rates of change predicted logistic dynamics for nitrogen and carbon accumulation. On average, agricultural practices resulted in a 75% loss of soil nitrogen and an 89% loss of soil carbon at the time of abandonment. Recovery to 95% of the preagricultural levels is predicted to require 180 yr for nitrogen and 230 yr for carbon. This model accurately predicted the soil carbon, nitrogen, and carbon : nitrogen ratio patterns observed in the chronosequence of old fields, suggesting that the chronose- quence may be indicative of actual changes in soil carbon and nitrogen. Our results suggest that the rate of carbon accumulation was controlled by the rate of nitrogen accumulation, which in turn depended on atmospheric nitrogen deposition and symbiotic nitrogen fixation by legumes. Our data support the hypothesis that these aban- doned fields initially retain essentially all nitrogen and have a closed nitrogen cycle. Multiple regression suggests that vegetation composition had a significant influence on the rates of accumulation of both nitrogen and carbon; legumes increased these rates, and C 3 grasses and forbs decreased them. C4 grasses increased the C:N ratio of the soil organic matter and thereby increased the rate of carbon accumulation, but not nitrogen accumulation.

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

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

A process-oriented model of N2O and NO emissions from forest soils: 1. Model development

Abstract: To predict emissions of nitrous oxide (N 2 O) and nitric oxide (NO) from forest soils, we have developed a process-oriented model by integrating several new features with three existing models, PnET, Denitrification-Decomposition (DNDC), and a nitrification model. In the new model, two components were established to predict (1) the effects of ecological drivers (e.g., climate, soil, vegetation, and anthropogenic activity) on soil environmental factors (e.g., temperature, moisture, pH, redox potential, and substrates concentrations), and (2) effects of the soil environmental factors on the biochemical or geochemical reactions which govern NO and N 2 O production and consumption. The first component consists of three submodels for predicting soil climate, forest growth, and turnover of soil organic matter. The second component contains two submodels for nitrification and denitrification. A kinetic scheme, a so-called anaerobic balloon,' was developed to calculate the anaerobic status of the soil and divide the soil into aerobic and anaerobic fractions. Nitrification is only allowed to occur in the aerobic fraction, while denitrification occurs only in the anaerobic fraction. The size of the anaerobic balloon is defined by the simulated oxygen partial pressure which is calculated based on oxygen diffusion and consumption rates in the soil. As the balloon swells or shrinks, the model dynamically allocates substrates (e.g., dissolved organic carbon, ammonium, nitrate, etc.) into the aerobic and anaerobic fractions. With this approach, the model is able to predict both nitrification and denitrification in the same soil at the same time. This feature is important for soils where aerobic and anaerobic microsites often exist simultaneously. With the kinetic framework as well as its interacting functions, the PnET-N-DNDC model links ecological drivers to trace gas emissions. Tests for validating the new model are published in a companion paper [Stange et al., this issue].