Filled Pore Space Research Articles

Effects of forest harvesting on soil methane fluxes in Florida slash pine plantations

We examined the effects of forest harvesting on the net exchange of methane (CH4) between the atmosphere and sandy soils in two mature slash pine (Pinus elliottii Engelm. var. elliottii) plantations in northern Florida. Before each of the stem-only harvests, soils in these plantations were net sinks for CH4 (-0.03 to -2.6 mg CH4·m-2·d-1) on all but one sampling date. On this one pre-harvest sampling date, soils in one plantation were net sources of CH4. This emission coincided with an average soil moisture content of 83% water filled pore space (WFPS), which was significantly greater than the average soil moisture contents (25-66% WFPS) for all pre-harvest sampling dates. After harvesting, soils in both plantations became net sources of CH4. Average emission rates from harvested soils ranged from 3 to 11 mg CH4·m-2·d-1. Harvested soils were net sources of CH4 for at least 1 year after the harvest. However, the duration of the post-harvest CH4 emission period was reduced by bedding the plantation soils, a typical post-harvest site preparation treatment. Bedded soils in harvested stands were either relatively small net CH4 sources (0.2 mg CH4·m-2·d-1) or net CH4 sinks (-0.4 mg CH4·m-2·d-1). Soil CH4 fluxes were highly correlated with soil moisture contents (r2 = 0.66 and 0.71; significant at p < 0.05), which were strongly influenced by climate and forest management practices. For example, soil moisture contents for one of our sites, averaged over the pre-harvest, post-harvest, and post-harvest-plus-bedding periods were 46, 68, and 38% WFPS, respectively. Our results suggest that increased soil moisture in southern pine plantations induced by either precipitation events or forest management changes the direction of the soil CH4 flux from sink to source.

TEM study of the mechanism of transformation of detrital kaolinite and muscovite to illite/smectite in sediments of the Salton Sea Geothermal Field

The reaction by which kaolinite transforms to I-S has been studied by SEM and TEM using core samples from the Salton Sea Scientific Drilling Project. Kaolinite is abundant from the surface to a depth of ∼300 m (∼115°C), decreasing in abundance and becoming undetectable at a depth of ∼500 m (∼160°C), with a concomitant increase in abundance of I-S. In a 256-m deep sample, kaolinite occurs as large (∼100 μm) detrital grains, in the fine-grained (< 1 μm) matrix, and as packets forming stacks in detrital biotite and muscovite. In sharp contrast, no kaolinite was detected in the sample from a depth of 477 m but illite and minor I-S occur in the matrix. Authigenic chlorite occurs as subhedral crystals in the matrix with subparallel illite crystals, and as muscovite-chlorite stacks. I-S occurs in three ways: (1) Interlayered within kaolinite as a few, curved layers with 10- and 20-A periodicities with the characteristics of collapsed, dehydrated I-S. Along-layer transitions with change in (001) d-value from 7 to 10 A occur. (2) As coalescent, randomly oriented packets filling pore space in the matrix. (3) Interlayered with detrital muscovite. These observations show that R1 T-S in Salton Sea-area sediments forms primarily through alteration of kaolinite and muscovite of detrital origin over a temperature interval of ∼100 to < 200°C, with I-S serving as a precursor to illite. Reaction occurs both by direct layer-by-layer replacement of kaolinite and muscovite, and by dissolution, ion transport and crystallization in pore space. Both processes involve dissolution and crystallization, with only the scale of transport varying from that of along-layer interfaces to the dimensions of pore space. The pH value and activity of K+ in the hydrothermal brines play important roles in the alteration process and control the simultaneous transformation of both kaolinite and muscovite to I-S.

On the role of microbes in the alteration of submarine basaltic glass: a TEM study

The alteration of submarine basaltic glass that has been attributed to microbial activity was investigated via transmission electron microscopy in order to document glass alteration and to understand the chemical reactions that bacteria could utilize. The features that were examined occur within fresh glass at the alteration front and consist of 0.1–2.3 μm patches or tubes of phyllosilicates rimmed by 0.1–0.3 μm wide zones of altered glass. The altered glass has lost significant Mg, Fe, Ca and Na, slight amounts of Al and Mn, and exhibits an order of magnitude increase of K. This incongruent dissolution occurred at neutral to acid pH and temperatures <90°C. Phyllosilicates consist of montmorillonite–saponite with berthierine interlayers, plus a mica component. The phyllosilicates directly replace altered glass along the perimeters of the phyllosilicate areas, but the centers of many of these areas may be filled pore space formerly occupied by microbes that facilitated glass alteration. If so, the sizes of the bacteria ranged to less than 0.1 μm and into the range of nano-organisms. Iron in secondary products is essentially all ferrous, and is most likely mobilized from glass as the ferrous ion, possibly with the reduction of minor primary ferric iron in the glass. Small amounts of sulfide in secondary pyrite associated with altered glass may be derived from microbial reactions or from the glass itself. Net bulk chemical changes for altered glass plus phyllosilicates include losses of Si, Al, Fe, Mn, Ca and Na, plus possible slight losses of Ti. This material is redistributed into fracture-filling smectite along with significant K and Mg from seawater. The uptake of K requires the circulation of large amounts of seawater through fractures in the glass (>100 fracture volumes), which may be important for the supply of nutrients for microbial activity.

Ion exchange resin–soil mixtures as a tool in net nitrogen mineralisation studies

Mixed-bed ion exchange resins (IER) were mixed with intact soil aggregates and incubated at 60% water filled pore space in closed polyethylene bags for 12 weeks. To test IER effects on N losses, nitrification and net N mineralisation, an arable soil and a grassland soil, differing in organic matter content, were chosen and two crop residues (wheat straw, sugar-beet leaves) with different C-to-N ratios were added to the arable soil. It was proposed that IER might exert an influence on N cycling similar to that of plant roots. Nitrification was inhibited by adsorption of NH 4 in the +IER treatments. Net N mineralisation was greater in the grassland soil than in the arable soil which had less soil organic matter. Without incorporation of additional organic substrates, net N mineralisation was not affected by IER in both soils. Straw addition to the arable soil caused immediate N immobilisation in the −IER treatment, whereas N mineralisation continued in the +IER treatment. Incorporation of sugar-beet leaves into the arable soil highly increased net N mineralisation and microbial biomass N in the −IER treatment. In the +IER treatment, the enhancement of both N mineralisation and microbial biomass N was less pronounced. Thus, IER mixed into soil samples can exert either a stimulating (wheat straw) or dampening (sugar-beet leaves) effect on N mineralisation. Soil–IER mixtures can prevent losses and re-immobilisation of mineralised N and mimic nutrient exchange properties of plant roots. It is concluded that in incubation experiments they can better reflect conditions in the vicinity of roots than incubations without IER or with incorporation of IER in confined resin bags as long as water and aeration conditions are not largely changed. Soil–IER mixtures may also be a useful tool for studying root-induced changes in net N mineralisation.

Effects of urea form and soil moisture on N2O and NO emissions from Japanese Andosols

Nitrous oxide (N2O) and nitric oxide (NO) emissions from soil are affected by many factors. Soil nitrogen source, especially N fertilizer input, and soil moisture might be the most important factors to control these two gases emission rate. In this study, laboratory incubation experiments were conducted to determine the effect of the urea form and the soil moisture on N2O and NO emissions in Japanese Andosols. Results showed that there were no significant differences in the total amount of N2O and NO emissions over 77 d between non-coated and coated urea (CU) treatments, except for NO emission at 40% wfps (water filled pore space) where it was reduced by 23% when CU was applied. As compared to easily decomposable urea (U), however, CU did reduce N2O and NO emissions in the earlier period shortly after fertilization. The results also indicated that soil moisture had a much more significant effect on N2O and NO emissions than the form of urea. From 40% to 100% wfps, there was a positive relationship between N2O emission and soil water content and a negative relationship for NO. The flux ratio of NO/N2O was governed by soil moisture with a greatest value at the lowest wfps treatments for each fertilizer treatment. Soil moisture could be the most important factor controlling N2O and NO emissions when a rich N supply exist in soil.

Effect of Soil Conditions on the Degradation of Cloransulam‐Methyl

AbstractHerbicide efficacy and environmental fate are often controlled by soil conditions. Aerobic soil laboratory studies were undertaken to determine the degradation of the herbicide cloransulam‐methyl [N‐(2‐carbomethoxy‐6‐chloro‐phenyl)‐5‐ethoxy‐7‐fluoro[1,2,4]triazolo [1,5‐c]pyrimidine‐2‐sulfonamide] for a range of soil factors. Treatments included soil temperature (5, 15, 25, 40, and 50°C), moisture (20, 40, and 60% water filled pore space), and soil type. The soils under study were a Drummer silty clay loam (fine‐silty, mixed, super‐active, mesic Typic Endoaquolls) and a Cisne silt loam (fine, smectitic, mesic Vertic Albaqualfs). Variability in molecular degradation was investigated using two radiolabeled forms ([Phenyl‐UL‐14C] and [Pyrimidine‐7,9‐14C]). Dissipation of parent compound in soil solution and sorbed phases, formation of radiolabeled metabolites, 14C mineralization, total microbial respiration, and bound residue formation were measured for up to 120 d. Dissipation of parent and formation of bound residues in Drummer soil increased with greater temperatures. The influence of temperature on 14C mineralization, however, was dependent on position of radiolabel, suggesting that distinct groups of microorganisms degrade different parts of the molecule at higher temperatures. Only 14C mineralization was influenced by moisture, with response depending on soil type. Increasing moisture resulted in more 14C mineralization in Drummer, but not Cisne soil, which was attributed to increased microbial access to pesticide at greater moisture contents in Drummer soil. Reduced availability, suggested by greater sorption in Drummer soil, may explain persistence of parent in this soil. Bound residues were more extensive and exhibited greater dependence on biological activity in Cisne soil, owing to enhanced dissipation of parent compound in this soil.

Properties of Urbic Anthrosols from an abandoned shunting yard in the Ruhr area, Germany

The soil conditions of an abandoned shunting yard in the Ruhr area were studied to find the most important factors influencing plant growth and nutrient cycling. The chemical and physical conditions of five Urbic Anthrosols with different development histories were compared with those of a nearby Calcic Cambisol under agricultural use. In the second half of the 19th century, the ground level of the shunting yard site was raised about 2 m. The soils are now mainly Regosols, mostly well drained in the topsoil. The coarse material contents (>2 mm), which are up to 100% in base layers of railway tracks, and the quality of the man-made substrates (crushed rock, slags, ashes, coke) are the most important factors influencing plant growth. Poor physical structure and low contents of fine fraction (<2 mm) lead to low water storage. Together with a low available nutrient stock, these properties lead to poor establishment of vegetation. The burning of coal by steam engines produced highly polluted ashes that fill pore spaces in the railway ballast layers. All the investigated fine substrata of the shunting yard layers exhibit a higher magnetic susceptibility than the rural soil. The ashes still contain unburned coal so that, although the total organic C content of the man-made soils reaches 37%, the wide C:N ratio does not indicate the actual quality of humic substances. The acid neutralization potential of the topsoil layers of the shunting yard will be consumed by acidic atmospheric deposition within a few decades, whereas in the surrounding Calcic Cambisols, it will take more than a thousand years. This may cause further problems of groundwater quality, as the heavy metals will become more soluble. The low soil quality makes such sites suitable for vegetation species that cannot compete in the surrounding very eutrophic agricultural landscape.

Using a boundary line approach to analyze N2O flux data from agricultural soils.

Predicting the N2O flux from soils is difficult because of the complex interplay of the various processes involved. In this study a boundary line approach was used to apply results from mechanistic experiments to N2O flux data resulting from measurements on field scale in southern Germany. Boundary lines were fitted to the rim of the data points in scattergrams depicting readily obtainable soil variables against the measured N2O flux. The boundary line approach is based on the hypothesis that this line depicts the functional dependency between the two variables. For determining these boundary lines a novel method was applied. The function best representing the relationship between the N2O flux and soil temperature had a maximum above 23 °C and the one between the N2O flux and the water filled pore space (WFPS, to represent water content) had a maximum at 72% WFPS. In the range of 0–20 mg N kg-1 the relationship between N2O flux and nitrate in the soil was best described by a linear function, whereas in the range of 0–35 mg N kg-1 a Michaelis–Menten function was more appropriate. The boundary lines specified in this study are in agreement with existing theoretical concepts as well as experimental results obtained under controlled and field conditions as reported in the literature. Therefore, the boundary line approach can be used to improve empirical models for predicting the N2O flux in the field.

Effects of forest harvesting on soil methane fluxes in Florida slash pine plantations

We examined the effects of forest harvesting on the net exchange of methane (CH4) between the atmosphere and sandy soils in two mature slash pine (Pinus elliottii Engelm. var. elliottii) plantations in northern Florida. Before each of the stem-only harvests, soils in these plantations were net sinks for CH 4 (-0.03 to -2.6 mg CH4·m -2 ·d -1 )o n all but one sampling date. On this one pre-harvest sampling date, soils in one plantation were net sources of CH4. This emission coincided with an average soil moisture content of 83% water filled pore space (WFPS), which was signifi- cantly greater than the average soil moisture contents (25-66% WFPS) for all pre-harvest sampling dates. After har- vesting, soils in both plantations became net sources of CH4. Average emission rates from harvested soils ranged from 3t o 11 mg CH 4·m -2 ·d -1 . Harvested soils were net sources of CH4 for at least 1 year after the harvest. However, the duration of the post-harvest CH4 emission period was reduced by bedding the plantation soils, a typical post-harvest site preparation treatment. Bedded soils in harvested stands were either relatively small net CH 4 sources (0.2 mg CH4·m -2 ·d -1 ) or net CH4 sinks (-0.4 mg CH4·m -2 ·d -1 ). Soil CH4 fluxes were highly correlated with soil moisture con- tents (r 2 = 0.66 and 0.71; significant at p < 0.05), which were strongly influenced by climate and forest management practices. For example, soil moisture contents for one of our sites, averaged over the pre-harvest, post-harvest, and post-harvest-plus-bedding periods were 46, 68, and 38% WFPS, respectively. Our results suggest that increased soil moisture in southern pine plantations induced by either precipitation events or forest management changes the direction of the soil CH4 flux from sink to source.

Denitrification and N mineralization from hairy vetch ( Vicia villosa Roth) and rye (Secale cereale L.) cover crop monocultures and bicultures

N mineralization, N immobilization and denitrification were determined for vetch, rye and rye-vetch cover crops using large packed soil cores. Plants were grown to maturity from seed in cores. Cores were periodically leached, allowing for quantification of NO3 − and NH4 + production, and denitrification incubations were conducted before and after cover crop kill. Gas permeable tubing was buried at two depths in cores allowing for quantification of N2O in the soil profile. Cover crops assimilated most soil N prior to kill. After kill, relative rates of N mineralization were vetch > rye-vetch mixture > fallow > rye. After correcting for N mineralization from fallow cores, net N mineralization was observed in vetch and rye-vetch cores, while net N immobilization was observed in rye cores. Denitrification incubations were conducted 5, 15 and 55 days after kill, with adjustment of cores to 75% water filled pore space (WFPS). The highest denitrification was observed in vetch cores 5 days after kill, when soil NO3 − and respiration rates were high. Substantially lower denitrification was observed on subsequent measurement dates and in other treatments probably due to either limited NO3 − or organic carbon in the soil. On day 5, 3%, 23%, 31% and 31% of the N2O was recovered in the headspace of fallow, vetch, rye and rye-vetch cores, respectively. The rest was stored in the soil profile. In a field study using intact soil cores, denitrification rates also peaked 1 week after cover crop kill and decreased significantly thereafter. Results suggest greater potential N losses from vetch than rye or rye-vetch cover crops due to rapid N-mineralization in conjunction with denitrification and potential leaching, prior to significant crop N-assimilation.

Nitrous oxide emissions and methane oxidation by soil following cultivation of two different leguminous pastures

Nitrous oxide (N2O) emissions and methane (CH4) consumption were quantified following cultivation of two contrasting 4-year-old pastures. A clover sward was ploughed (to 150–200 mm depth) while a mixed herb ley sward was either ploughed (to 150–200 mm depth) or rotovated (to 50 mm depth). Cumulative N2O emissions were significantly greater following ploughing of the clover sward, with 4.01 kg N2O-N ha–1 being emitted in a 48-day period. Emissions following ploughing and rotovating of the ley sward were much less and were not statistically different from each other, with 0.26 and 0.17 kg N2O-N ha–1 being measured, respectively, over a 55-day period. The large difference in cumulative N2O between the clover and ley sites is presumably due to the initially higher soil NO3 – content, greater water filled pore space and lower soil pH at the clover site. Results from a denitrification enzyme assay conducted on soils from both sites showed a strong negative relationship (r=–0.82) between soil pH and the N2O:(N2O+N2) ratio. It is suggested that further research is required to determine if control of soil pH may provide a relatively cheap mitigation option for N2O emissions from these soils. There were no significant differences in CH4 oxidation rates due to sward type or form of cultivation.

Nitrous oxide emissions from intensive agricultural systems: Variations between crops and seasons, key driving variables, and mean emission factors

Emissions of nitrous oxide from intensively managed agricultural fields were measured over 3 years. Exponential increases in flux occurred with increasing soil water‐ filled pore space (WFPS) and temperature; increases in soil mineral N content due to fertilizer application also stimulated emissions. Fluxes were low when any of these variables was below a critical value. The largest fluxes occurred when WFPS values were very high (70–90%), indicating that denitrification was the major process responsible. The relationships with the driving variables showed strong similarities to those reported for very different environments: irrigated sugar cane crops, pastures, and forest in the tropics. Annual emissions varied widely (0.3–18.4 kg N2O‐N ha−1). These variations were principally due to the degree of coincidence of fertilizer application and major rainfall events. It is concluded therefore that several years' data are required from any agricultural ecosystem in a variable climate to obtain a robust estimate of mean N2O fluxes. The emissions from small‐grain cereals (winter wheat and spring barley) were consistently lower (0.2–0.7 kg N2O‐N per 100 kg N applied) than from cut grassland (0.3–5.8 kg N2O‐ N per 100 kg N). Crops such as broccoli and potatoes gave emissions of the same order as those from the grassland. Although these differences between crop types are not apparent in general data comparisons, there may well be distinct regional differences in the relative and absolute emissions from different crops, due to local factors relating to soil type, weather patterns, and agricultural management practices. This will only be determined by more detailed comparative studies.

Glass-ionomer cement: evaluation as an orbital implant.

To assess the potential of a porous glass-ionomer cement (GIC) as an alternative material for spherical orbital implants, the handling, side effects and rates of fibrovascular ingrowth of this material were compared with those of a synthetic hydroxyapatite (HA) implant. Twenty-one GIC and 8 HA uncovered 14-mm spheres were implanted into the orbits of New Zealand albino rabbits. Postoperative reactions, animal's behaviour, weight increase and socket conditions were monitored. Light and electron microscopy of the exenterated orbits were performed 2, 3 and 6 months after primary insertion. Implanting of GIC was easier than HA. Postoperatively all animals did well. Three HA and 1 GIC implant caused conjunctival dehiscences, but no implant extrusion was observed. Histologically, both materials caused mild inflammation in the surrounding connective tissue capsule, decreasing with time. GIC implants proved to be not truly porous, with only peripheral pores partly occupied by relatively acellular collagenous connective tissue. Free glass particles were observed in both the connective tissue and giant cells, occupying the partly filled pore spaces. HA implants showed extensive ingrowth of vital host tissue from the beginning. Considering the clinical findings and the mild inflammation in the connective tissue capsule surrounding both materials, they would appear to be equally well tolerated at the implant site. The significantly different microstructure and the histological results make GIC, despite better handling, less suitable as an orbital implant.

Laboratory study of NO flux from agricultural soil: Effects of soil moisture, pH, and temperature

Unique laboratory test chambers with attendant procedures are described, and the results of a comprehensive test protocol are discussed in terms of the mechanical, chemical and biological factors contributing to NO flux from agricultural soil to the lower levels of the troposphere. Soil moisture content, pH, and temperature are investigated to determine the effects of these important variables on NO flux. The flux is seen to increase with temperature and is greatest at pH &lt;5 and pH &gt;8 for the ranges studied. Further, NO flux is seen to decrease as soil moisture content is &lt;1% or &gt;45% water filled pore space. Mechanical, chemical, and biological factors in the soil which contribute to these observed fluxes are addressed.

The porosities of ordinary chondrites: Models and interpretation

Abstract— Densities and porosities for 285 ordinary chondrites have been assembled and analyzed. Measured chondrite porosities are bimodal; finds have an average porosity of &lt;3%, whereas fall porosities average 7% but range from zero to &gt;30%. We conclude that mild degrees of weathering fill pore spaces, lowering grain densities and porosities without significantly changing the bulk size or mass of the sample. By assuming an original pristine grain density (as a function of the meteorite's mineralogy—determined by its class), we can derive model pristine porosities. These model porosities cluster around an average value of 10% for all classes of ordinary chondrites.Ordinary chondrites do not show any correlation of porosity (model or measured) with petrographic grade or sample size (over a range from 0.2 g to 2 kg). However, we do see a correlation between shock state and porosity. Shock‐blackened meteorites are less porous than other meteorites. Furthermore, less severely shocked meteorites show a much broader range of porosities, with the maximum porosity seen among meteorites of a given shock class falling linearly as a function of that shock class. This is consistent with the idea that shock compresses and closes pore space.Analysis of meteorite porosity provides a lower bound to the fine‐scale porosity of asteroids. Our densities, even with 10% primordial porosity, are significantly higher than inferred densities of possible asteroid parent bodies. These asteroids are probably loose piles of rubble.

Pore level imaging of fluid transport using synchrotron X-ray microtomography

Recently developed high resolution computed microtomography (CMT) using synchrotron X-ray sources is analogous to conventional medical CT scanning and provides the ability to obtain three-dimensional images of specimens with a spatial resolution on the order of micrometers. Application of this technique to the study of core samples has previously been shown to provide excellent two- and three- dimensional high resolution descriptions of pore structure and mineral distributions of core material. Recently, computed microtomographic endpoint saturation images of a fluid filled sandstone core sample were obtained using a microtomographic apparatus and a high energy X-ray beam produced by a superconducting wiggler at the National Synchrotron Light Source at Brookhaven National Laboratory. Images of a 6 mm subsection of the one inch diameter core sample were obtained prior and subsequent to flooding to residual oil. Both oil and brine phases were observable within the imaged rock matrix. The rock matrix image data was used as input to a fluid transport simulator and the results compared with the end point saturation images and data. These high resolution images of the fluid filled pore space have not been previously available to researchers and will provide valuable insight to fluid flow, and provide data as input into and validation of high resolution porous media flow simulators, such as percolation-network and lattice Boltzmann models.

A new membrane tube technique (METT) for continuous gas measurements in soils

We present a novel technique for monitoring trace gas concentrations in the air filled pore space of the soil with a high temporal resolution. The method is based on gas diffusing from the air filled pore space into air flowing through an air permeable, hydrophobic, polypropylene tube (Accurel® PP V8/2). Gas permeation efficiencies of the membrane tube were determined for NO, N2O, CO2 and 222Rn in the laboratory. For a length of 1.5 m and flows smaller than 0.8 L min-1, the permeation efficiency was larger than 96%. The effective diffusion coefficients of NO, N2O, CO2 and 222Rn in the membrane are 6.2, 6.6, 5.6 and 6.6 times smaller than the corresponding diffusion coefficients in air, respectively. For tubes shorter than 1.5 m the contribution of pressure gradient induced transport into the membrane tube is below 0.9% of concentration gradient induced transport.

Three‐dimensional tomographic velocity structure of upper crust, Coaxial segment, Juan de Fuca Ridge: Implications for on‐axis evolution and hydrothermal circulation

Three‐dimensional models of compressional velocity and azimuthal anisotropy from tomographic inversions using 23,564 ocean bottom seismometer P wave arrivals define systematic lateral variations in seismic structure of the CoAxial segment of the Juan de Fuca Ridge (JdFR). Over much of the segment the across‐axis structure is roughly axisymmetric, characterized by a progressive increase in dike velocities moving away from the ridge axis. This trend is most apparent in the basal dikes, where on‐axis velocities are about 800 m/s slower than those measured elsewhere within the rift valley. The on‐axis sheeted dikes also exhibit ridge‐oriented azimuthal anisotropy, with a peak‐to‐peak amplitude of about 600 m/s. Outboard of the rift valley, beneath ridge flanks with fault scarps, velocities in the upper 1500 m of crust are reduced. The maximum amplitude of this anomaly is about 700 m/s, located near the top of the sheeted dikes. Variations in the three‐dimensional velocity model are believed to reflect changes in crustal porosity, from which we infer an axisymmetric porosity model for seismic layer 2 of the CoAxial segment. As the crust ages, the evolution of layer 2 porosity could occur in the following way: (1) the porosity of zero‐age, on‐axis dikes is set at formation by the contraction of molten material, (2) hydrothermal alteration fills pore spaces as the dikes move away from the center of the axial valley, and (3) normal faulting on the ridge flank scarps opens fractures and increases porosity of the upper dikes as they move off‐axis. At the north end of the segment, dike velocities are several hundred meters per second slower, on average, and the across‐axis structure is lost. The transition from a coherent, aligned seismic structure to a less distinct pattern with reduced velocities may represent a shift from magmatic to amagmatic extension moving away from the Cobb hotspot on the ridge axis. The porosity structure we have derived for the CoAxial segment suggests an alternative to the usual hydrothermal circulation model of cross‐axis convection cells. A circulation model with along‐axis convection cells located entirely within the axial valley appears to be more compatible with our data.

Simulation modeling of nitrogen trace gas emissions along an age gradient of tropical forest soils

Applications of a process simulation model for ecosystem production and soil nutrient cycling were carried out to test hypotheses concerning the controls on nitrogen trace gas fluxes in tropical forests. Assuming that emissions of trace gases (N 2O and NO) can help characterize patterns in N cycling among forests of different age and nutrient status, we applied a new daily time step version of the CASA (Carnegie-Ames-Stanford approach) model in an attempt to reproduce soil biophysical conditions previously measured in three forest stands that comprise a soil age gradient (ranging from 200 to 185 000 years old) on the island of Hawaii. We compared model-predicted soil moisture, temperature, N mineralization, and N trace gas emission rates to measurements at forest sites made during 1990 and 1991. Results showed that predicted water filled pore space (WFPS) in the soil is consistently lower than measured WFPS, possibly due to incomplete understanding of moisture holding capacity, drainage properties, and small scale variability of these volcanic soils. Simulations correctly predicted the observed difference of an order of magnitude between N 2O emission fluxes in the youngest and the oldest forest soils. Nevertheless, specific day-by-day comparison of predicted and measured N 2O fluxes reveal only occasional agreement within an order of magnitude tolerance level. Although soil moisture conditions appear favorable for emission on NO at these sites, as predicted by the generalized model design, lack of observed NO emissions from the previous field and laboratory studies suggest that: (1) our hypothesized NO emission levels as a function of WFPS should be truncated at lower levels of soil moisture availability; and/or (2) nitrification potentials of natural forest soils should be considered in revised versions of the model.

Nitrogen Cycling in Piedmont Vegetated Filter Zones: I. Surface Soil Processes

AbstractSurface runoff is a major transport mechanism for particulate‐bound and dissolved N species from agricultural fields. One means of reducing nutrient loading in surface waters is the use of vegetative filter zones. The objective of this study was to evaluate the effectiveness of two forested filter zones (FFZ) for removing N from runoff in the Piedmont region of North Carolina. We used a spreading device to ensure dispersed flow in the FFZ. In addition to measuring inputs and outputs from each FFZ, we characterized the N cycle in the surface 30 cm of the soil profile to determine the fate of different N species retained in the FFZ. N loading increased as water passed through FFZ1: NO3‐N increased by 1.6 kg ha−1 yr−1, organic‐N increased by 13.4 kg ha−1 yr−1 and NH4‐N decreased by 0.2 kg ha−1 yr−1. The second FFZ was more effective with net retention of 0.2 kg ha−1 yr−1 for NO3‐N, 0.5 kg ha−1 yr−1 for organic‐N and 0.2 kg ha−1 yr−1 for NH4‐N. The FFZ were ineffective during the winter and spring when water filled pore space exceeded 35% in FFZ1 and 25% in FFZ2, and infiltration was low. Infiltration was the key factor controlling N pollutant removal from surface runoff. Therefore, the clayey soils of the Piedmont may not be as effective as the sandy coastal plain soils studied by other authors. Results from the analysis of the N cycle suggest that both uptake by the vegetation and leaching to deeper soil layers were the dominant fates of inorganic‐N.