H43C-1501
The distribution of iron in a soil chronosequence: the result of biological lifting and surficial accumulation
The abundance of iron increases with soil age in a marine terrace chronosequence (5 terraces aged from 65 to 226 Ka) located northwest of Santa Cruz, California. The iron has two distinct morphologies in the soils. At depths less than 1m on all terraces hard nodules are formed by Fe-oxides cementing and replacing sediment grains. At depths greater than 1m in the youngest terrace (T1), disseminated Fe forms coatings on sediment grains. In terraces 2 through 5 (depths greater than 1m) the disseminated iron becomes increasingly concentrated in mottles within the argillic horizon. Iron nodules do not occur at depths greater than 1m in any of the soils. Iron mineralogy of the nodules is generally goethite with a subset of nodules that are maghemite. Mass change calculations, reveal Fe concentration near the surface and Fe depletion at depth that cannot be accounted for by weathering and compaction of the profile or by the Fe content of eolian additions to the soils. The terrace regoliths are generally unsaturated and aerobic; thus lateral movement of large amounts of reduced Fe is unlikely. Iron as a plant nutrient, unlike other mineral nutrients, is relatively insoluble in aerobic soil solutions. We propose that plant roots and symbiotic fungi (mycorrhizae) transport Fe from deeper in the regolith through the process of biolifting. When released through plant decay, the Fe forms immobile oxides at shallow depths. Iron content of the current grassland vegetation was measured and yearly biomass input of Fe was calculated. The above ground cycling of plant iron when multiplied by the age of the terrace can account for the shallow Fe accumulation in these soils.
H43C-1502
Naturally Occurring Cr and Ni in the Sacramento Valley: I. Geologic and Geomorphic Controls on the Spatial Distribution of Ultramafic Source Elements
We present results of a soil geochemical survey conducted along an east-west transect in northern California. The study area includes the Sierra Nevada, Sacramento Valley, and northern Coast Range. The results show that soil geochemistry in the Sacramento Valley is primarily controlled by the transport and weathering of parent material from the Coast Range and Sierra Nevada. Ultramafic (UM) rocks (e.g. serpentinite) outcrop extensively in both ranges. These rocks and the soils forming from them have elevated concentrations of Cr and Ni. Surface soil samples (n=37) derived from UM rocks of the Sierra Nevada and Coast Range contain 1700 to 10,000 ppm Cr (mean=4345) and 1300 to 3900 ppm Ni (mean=2744). These high concentrations spatially correlate with mapped UM rocks. Valley soils west of the Sacramento River (n=47) contain 80 to 1420 ppm Cr (mean=435) and 65 to 224 ppm Ni (mean=137), reflecting a significant contribution from UM sources in the Coast Range. Valley soils on the east side (n=41) contain 30 to 370 ppm Cr (mean=169) and 16 to 110 ppm Ni (mean=55). The lower Cr and Ni concentrations on the east side reflect the dominance of granitic sources in the Sierra Nevada. Chromium naturally occurs as Cr(III), a non-toxic micronutrient, or Cr(VI) which is a highly soluble toxin and carcinogen. X-ray diffraction and scanning electron microscopy of soils from UM parent show Cr primarily occurs as chromite and other mixed-composition spinels (Al, Mg, Fe, Cr). Chromite contains Cr(III) and is highly refractory. We compared results of a 4-acid digestion (HNO3, HCl, HF, HClO4), which only partially dissolves chromite, and lithium metaborate fusion (LiBO3), which gives total Cr content. The ratio of acid digestion:LiBO3 fusion is low (0.2-0.48, mean=0.32, n=13) in Coast Range UM soils relative to valley soils (0.2-0.99, mean=0.50, n=50), which indicates a lower proportion of chromite in valley soils relative to UM soils. Elevated Cr concentrations (up to 50 ug L-1) in some Sacramento Valley groundwaters suggest that a redox mechanism exists in the weathering of Cr-bearing minerals. We hypothesize that regional-scale transport and weathering of UM material has resulted in enrichment of Cr and Ni in the Sacramento Valley and a change in the residence of Cr.
H43C-1503
Naturally Occurring Cr and Ni in the Sacramento Valley: II. Mn Oxides and the Mobility of Cr(VI) and Ni
Soil manganese oxides can strongly affect the mobility and redox state of several toxic trace metals. We are studying the biogeochemical origin of Mn oxides and their association with Cr and Ni in soils of the Sacramento Valley, California. Both Cr and Ni are likely derived from ultramafic rocks that underlie Coast Range drainages to the west of the study area. The impact of weathering and erosion of these rocks is evident in the high levels of total Cr (80 to 1420 μg g-1) and nickel (65 to 224 μg g-1) that occur broadly in western Sacramento Valley soils. Although much of the Cr is bound in refractory spinels as Cr(III), some mobilization of Cr is apparent in the coincidence of enriched soils with high contents of Cr(VI) in ground water. Data from the National Water Information System (NWIS) shows 7 of 12 sampled wells within a 600 km2 area in the Sacramento Valley having Cr(VI) concentrations between 60 and 100% of the CA maximum contaminant level for drinking water (50 μg l-1). A 3-meter depth soil profile collected within the lower Putah Creek watershed was examined to investigate processes contributing to the oxidation and mobilization of natural Cr(III). Hydroxylamine hydrochloride-reducible Mn was determined for 8 depth intervals as a measure of manganese oxide occurrence. Concentrations of reducible Mn varied between 360 and 690 μg g-1 with depth and peaked at 2.7 m below the surface. Concentrations of anion exchangeable Cr(VI) were as high as 6 ng g-1 and were positively correlated (r2=0.59; p=0.07) with reducible Mn. Scanning electron microscopy of soil minerals from the 2.9 to 3.0 m interval showed Cr-bearing spinel grains enclosed within Mn oxide micro concretions suggesting a potential mechanism for the oxidation of natural Cr(III) to mobile Cr(VI). Consistent with the known tendency of Ni to sorb on Mn oxides, substantial Ni (13 to 45 μg g-1) was released in the reducible Mn fraction and it strongly correlates (r2=0.76; p=0.005) with reducible Mn concentration. Further work is exploring potential biogeochemical origins of the Mn oxides within these soils using phospholipid fatty acid and rDNA analyses and assessing the effects of seasonal redox cycling events on Mn oxides and the availability of Cr(VI) and Ni within the vadose zone of valley soils.
H43C-1504
Controls on Arsenic Retention in Surface and Subsurface Environments: Resolving the Impact of Iron Reduction
A transition from oxidizing to reducing conditions has long been implicated in increasing aqueous As concentrations. Confounding processes controlling the release of As, reductive transformation of ferrihydrite, a common Fe(III) (hydr)oxide, has recently been shown to promote As retention rather than release. Elucidating the processes controlling As desorption and subsequent migration in surface and subsurface environments and how environmental factors (for example, availability of labile carbon and duration/extent of flooding) affect these processes will allow predictions to be made regarding long-term stability of As in soil and sediment. In turn, this can aid in evaluating the likelihood of having measurable As in groundwater. To better resolve these processes, here we examine As desorption from ferrihydrite-coated sands pre-sorbed with As(III) at circumneutral pH under Fe-reducing conditions with the dissimilatory iron reducing bacterium (DIRB) Shewanella putrefaciens strain CN- 32. We reveal that upon iron reduction, transformation of As-bearing ferrihydrite results in As(III) retention. However, over time there is a shift from reductive transformation to reductive dissolution of the As-bearing Fe phase(s) coupled with prolonged release of As to the aqueous phase. Our results suggest that arsenic retention may increase or decrease depending on the type of iron oxide, secondary iron transformations, and duration of reducing conditions. Immediately following a transition to anaerobic conditions there is potential for As retention on newly formed ferric/ferrous (hydr)oxide phases; however prolonged reduction will result in both the dissolution of ferric/ferrous (hydr)oxides and release of aqueous arsenic.
H43C-1505
Diffusion-limited iron transformations in artificial soil aggregates: The impact of small-scale heterogeneity on cycling of redox-sensitive elements
Structured soils are typically heterogeneous composites of chemical and biological constituents within an intricate physical framework, which has variable geometry, composition and stability expressed over spatial scales of several orders of magnitude. In such settings, solutes move preferentially (by advection) through macropores and slowly (by diffusion) into intra-aggregate micropores, which promotes the establishment of redox gradients at the aggregate scale. Consequently, in such structured environments characterized by mass transfer limitation and redox gradients within soil aggregates, metals distribution can be strongly localized and the interrelated transport and biogeochemical processes control the fate of redox-sensitive contaminants and metals. Iron (hydr)oxides are particularly ubiquitous in soils and sediments and hence exert a pronounced effect on the fate and transport of nutrients and contaminants. As they are subject to both biotic and abiotic redox transformations, iron cycling depends on a tight interplay between hydrodynamic transport, and (bio)geochemical reactions depending on substrate distribution and microbial activity patterns. In this study, we present an experimental/modelling approach aimed at a qualitative and quantitative understanding of bioreductive processes at the microscale, and between advective and diffusive domains. Artificial soil aggregates, representing systems of intermediate complexity, were used to study the coupling of physical, chemical, and biological processes affecting iron oxides transformations, under environmentally relevant geometries. We used novel aggregate-based reaction flow cell experiments and reactive transport modeling to determine mass transfer and biogeochemical redox controls on the cycling of iron ranging from micropore- to aggregate-scales. Aggregates were made of ferrihydrite coated-sand and inoculated with Shewanella putrefaciens. Lactate was added in the input solution. Chemical gradients, spatial distribution of bacteria, and solid phase constituents were determined, to quantify magnitude, as well as temporal and spatial heterogeneity in biotransformation rates of iron. After 9 days of reaction, a slight and uniform transformation of ferrihydrite results in approximately 10% (mol Fe) of goethite and 10% (mol Fe) magnetite. While this distribution remains steady within the outer portion of the aggregate, toward the aggregate center, goethite becomes the dominant product (60% (mol Fe)) after 36 days of reaction. Due to the localized buildup of both Fe(II) and bicarbonate, up to 15% (mol Fe) siderite also results within aggregate centers while no magnetite was detected. Our results demonstrate the large variation in biotransformation of iron within soil aggregates characterized by a transition between advective and diffusive transport domains. They illustrate the importance of small-scale chemical conditions, dynamics of bioreductive processes at the microscale, and the microbial dynamics in situ for assessing bulk elemental cycling.
H43C-1506
How can residence time control on weathering rates be distinguished from hydrologic control?
Based on laboratory studies, chemical weathering rates in the critical zone (CZ) have been considered to be a function of temperature and the chemistry of CZ pore waters. Because the rate of soil water flow through the critical zone determines, in part, the chemistry of the pore water, this control on chemical weathering rates could be considered hydrologic. More recently, however, several workers have discovered that the age (or residence time) of CZ materials also exerts a strong control on weathering rates. Here we demonstrate how the age of CZ materials varies in space on the catena scale (where erosion rates are changing in time, linking weathering to geomorphic processes) and on the soil profile scale (where weathering fronts propagate downward through CZ materials). We also show how hydrologic controls may be expected to lead to different spatial variations in chemical weathering rates than CZ material age controls, and how these differences can be harnessed to determine which of the two hypothesized controls dominate in eroding landscapes.
H43C-1507
Soil and Solution Based Assessments of Weathering along a Hillslope Transect in Coastal California
Understanding the genesis of hillslope soils is challenging. They are the products of geomorphic, hydrologic, and geochemical processes that are interacting among themselves and are affected by the soils they shape. Our goal is to mechanically and quantitatively integrate the soil production and transport, chemical weathering of minerals, and solute fluxes with the observed topographic variation of soil elemental compositions. We studied a grass covered hillslope in coastal California where geomorphic processes of soil production and transport are well characterized. The parent material is clay-rich sandstone and bioturbation drives the slope- dependent soil transport. 10 sites were excavated to the depth of 10-20 cm beyond the soil-saprolite boundary, and soil and saprolite samples were collected for total chemical analyses of major elements. Soil solution was sampled throughout the year at multiple depths at 6 hillslope locations, along with outflow stream samples. We report that long term weathering rates, determined by the enrichment of weathering resistant element (Zr), are approximately proportional to soil production rates, whereas solute measurements indicate fastest chemical weathering rate near the toeslope where the soil production rate is lowest due to the thick soils. In saprolite, the Zr enrichment increased with increasing soil thickness, a trend consistent with the fact that soil production rate declines with soil thickness. If we assume mineral residence time in the saprolite increases with a decreasing conversion rate of saprolite to soil, the saprolite residence time may explain the greater degrees of saprolite chemical alteration in the depositional slope. Comparisons of elemental compositions of the soils and saprolite suggest that less than 10 % of the original saprolite mass has been lost via dissolution and leaching during the soil formation. Despite the mass losses of most elements, phosphorous and calcium appear to be selectively retained within the soils. Along the hillslope, the fractional mass loss only slightly increases in the downslope direction, indicating the balance between the rates of dissolution mass loss and soil production as the soil material moves downslope. This finding contrasts with the solute flux measurements. Greater mineral-water contact times and water flux in the lower slope positions combined to result in short-term chemical weathering loss rates (mass per volume of soil per time) that are greater than at the upper slope positions. This discrepancy may occur because colluvial flux increases in the downslope direction; the soils in the downslope direction quickly pass through the zone of highest chemical weathering rate such that the fractional mass losses of the soils increase only slightly. Additionally, at any given slope position, short-term weathering rates from solute fluxes increased with both soil temperature and soil CO2 concentrations. Because the colluvial flux increases in the downslope direction, and data for both long term and annual to storm timescale weathering fluxes are available, this study site allows us to test whether chemical weathering rates are controlled primarily by hydrologic processes or mineral residence time (see abstract by Mudd, Yoo, and Sanderman, this session).
H43C-1508
Chemical Weathering of Shales in the Shale Hills Catchment (Central Pennsylvania, USA)
Shales have been documented to be important in determining global fluxes of C, P and Pt-group elements worldwide as large amounts of shales are exposed at the Earth's surfaces. However, few studies have been performed to understand the chemical weathering rates of shales and their contribution to the overall silicate weathering fluxes on global scales. The Shale Hills catchment is situated in central Pennsylvania, on Rose Shale formation of Silurian age, and it is one of several sites where shale weathering rates will be studied and compared along a latitude and thus a climate gradient. The Shale Hills is a 7.9 ha, V-shaped catchment, with slopes around 16-18%. Previous hydrologic experimentations have included monitoring the soil moisture contents and modeling the water flow dynamics in the soil zones. In this study, we characterized soils and soil waters in three locations along an elevation transect in the Shale Hills catchment (ridge top, middle slope and valley floor), as well as the first-order streams. The parent materials are comprised of primarily illite, quartz and K-feldspar. The dominant chemical reactions in the soil profiles are dissolution of K-feldspar to form illite and further dissolution of illite to more stable kaolinite. The depths to bedrock are controlled by elevation, where the soils are the shallowest at the ridge top (only 30 cm), and the soils in the valley floor are deepest, over 60 cm deep. The extent of elemental loss due to shale weathering also shows dependence on topography, from high in valley floor to low at the ridge top. In all three sites, Ca is leached more quickly than other cations and more than 50% has been weathered during soil development. The soil water composition is very consistent with soil chemistry. Soil waters from the ridge top are dilute, while those from the valley floor are much higher in ionic strength. In all soil waters, Ca is the dominant cation, followed by Mg, Na and K. The variation of cation concentrations with depth is distinctively different among three sites, which is related to soil texture and thus the water flowpaths. The stream reflects mixing among soil waters of different chemistry (shallow versus deep), as ground waters contribute less than 5% of riverine discharge. Temporal variations of stream chemistry are related not only to the temperature-dependent mineral weathering rates, but also the different proportions of these sources during high-flow and low-flow seasons. This work focuses on a hillslope transect, and well-characterized hydrology of the catchment makes it possible to investigate chemical weathering processes in multi-dimensions.
H43C-1509
Weathering of Glacial Moraines in the French Alps
Samples of recent glacial sediments and 9 soil profiles ranging in age from 360 years to ~300Ky were collected from near Chamonix toward Lyon, France. The mineralogy of the <2 mm size fraction of each soil as a function of depth was analyzed by quantitative XRD. Soil profiles 17 Kyr and younger were derived almost exclusively from crystalline rocks. Older moraines also contain limestone and dolomite. The 360 yr moraine has a well developed organic horizon, but no clear indication of silicate weathering. Within the ~10 Ky till, chlorite (formed in the source rock by retrograde alteration of biotite) was weathered to smectite in the upper ~20 cm, but there was no major change in plagioclase or other primary silicate concentrations or kaolinite formation. The ~17 Ky till showed plagioclase weathered in the top 15 cm, and an incipient A horizon. The older tills (>100 Ky, and closer to Lyon) all had well developed A and B horizons, with the B horizon depth increasing with the till age from ~65 to ~150 cm. Carbonates in the A and B horizons were severely depleted, but abruptly reached concentrations near that of the unaltered till at the base of the B horizon, the same depth at which silicate weathering appeared to cease. There appeared to be a correspondence between carbonate depletion and silicate weathering, although the nature of the relationship remains unclear. The amount of plagioclase and K-feldspar weathering in the soil horizons increased with age, as did the development of an argillic kaolinite horizon. Estimates of weathering rates of plagioclase and K-feldspar in the older tills fell in the mid-range of observed weathering rates from unglaciated granitic watersheds, with dissolution rate between 6.8 and 40 times 10-20 mol/cm2/sec. This suggests that the high surface area of the primary silicates in the clay-size fraction of these tills played a subordinate role in controlling the rate of chemical weathering, and that weathering models with a large dependence on mineral surface area may not describe the weathering behavior of this chronosequence. Rather, other factors such as the evolution of solution chemistry may have a major control on weathering rates, leading to a weathering regime with 1st-order kinetics dependent primarily on exposure age.
H43C-1510
Seasonal Variations of Biogeochemical Characteristics in Predominantly Anaerobic Groundwater From a Riverine Alluvial Aquifer
Hydrogeochemical parameters were investigated for groundwater from six multi-level wells (up to 30 m deep) in a riverine alluvial aquifer with intense agricultural activities of rice, barley and vegetable cultivation during two sampling campaigns in rainy summer (July) and dry spring (March) season to identify seasonal variation in biogeochemical processes in the aquifer. The alluvial aquifer is located in flooldplains of Mangyeong River, western part of South Korea near the city of Jeonju. pH, concentrations of Na, Cl, Ca, F have little difference between the two sampling periods. Electrical conductivity (EC) and concentrations of HCO3, Mg, SO4 slightly increased as a whole from rainy season to dry season. Dissolved concentrations of major ions in river water increased by more than three fold during the seasons. These feature indicates that the groundwater system is relatively stable and less affected by the river in hydrogeochemical aspects. Dissolve oxygen (DO) concentrations were less than 1 mg/L for most of the wells whereas two wells turned to aerobic conditions in dry season which can be attributed to removal of stagnant water in the paddy fields. NO3 concentrations decreased significantly in dry season at most well points near the paddy fields. This indicates that denitrification is dominant over nitrate supply by infiltrating water from the land surface which is likely to be significantly decreased after harvesting of rice during the dry season. However, NO3 increased in upper zones (<10 m) in two wells near the barley and vegetable fields, which suggest continued nitrate supply from the crop fields to the upper part of the aquifer during the season. Fe concentrations have relatively small variations in most of wells for two sampling periods. However, from rainy season to dry season, Fe concentrations significantly increased more than two fold in two wells whereas NO3 concentration decreased below detection limit from 2 to 8 mg/L. The increase of Fe resulted in the increase of HCO3 concentrations by two fold and significant increase of EC, which even transformed hydrogeochemical pattern of major ions in those intervals, which can be attributed to the much higher proton consumption in iron reduction than in denitrification. This is indicative of the seasonal change in electron-accepting processes from nitrate reduction to iron reduction along with decrease in nitrate supply from the land surface. Seasonal variations in concentrations of DO, NO3, and Fe show that the temporal change in agricultural activities on land surface significantly affects biogeochemical processes in an alluvial aquifer. Denitrification of agriculturally derived NO3 and iron reduction in the alluvial aquifer can significanlty affect water chemistry of the river.
H43C-1511
Autochthonous dissolved organic matter in karst waters: Evidence from fluorescence spectroscopy
Fluorescence spectroscopy provides a rapid, sensitive tool for probing the nature of dissolved organic matter (DOM) without the need for isolation or concentration of the material. This makes it particularly useful for examining waters from environments where collection of large volumes of water is impractical and for large-scale surveying of waters from many sites within a single hydrogeological system. In this study, excitation-emission matrices were collected on waters collected from the Edwards Aquifer (central Texas), Frasassi Caves (Italy), Lower Kane Cave (Wyoming), and other karst sites. The spectral characteristics from the subsurface systems were unlike those observed for surface waters or soil-derived humic substances. Analysis of the data using several different methods suggests that the DOM from these sites is derived primarily from microbial sources with little, if any, terrestrial material present. Because few studies have been conducted with significant microbial contributions to DOM, especially in the absence of allochthonous input, our results provide a unique collection of DOM signatures constrained and controlled by microbial influences. Moreover, our work provides important information about carbon sources and cycling in karst ecosystems.
H43C-1512
Two modeling approaches for quantifying hydrologic and biologic controls on large-scale nitrogen cycling, Upper Rio Grande, NM
Variations in nutrient concentrations can be caused by both abiotic changes in hydrology and biotic processes. Most process-level studies of nutrient cycling are conducted in small catchment systems and at points on large river systems. Relatively less understanding has been developed on how biotic and abiotic processes influence large-scale nutrient concentrations and variability in large river systems. To address this issue, we performed biannual synoptic chemical sampling along a 640 km reach of the Upper Rio Grande for five years to determine the large-scale patterns in dissolved carbon and nitrogen concentrations and then used two different and simple models to evaluate the abiotic and biotic processes that generate the observed large-scale patterns. First, we used a Cl mixing model, validated with Br to quantify the effects of evapoconcentration, tributaries, and point sources on dissolved nitrogen and carbon concentrations. Ratios of observed to predicted concentrations close to 1 suggest that abiotic hydrologic processes are the dominant controls on concentrations while ratios departing from 1 indicate that biological processes are important controls. Our conservative mixing model generally captured patterns in DOC concentrations, suggesting minimal, net biological processing. In contrast, both nitrate and TDN concentrations were altered biogeochemically in all reaches. In areas where observed and predicted values differed, the spatial variability of river characteristics was more strongly correlated to relative nutrient retention than seasonal or inter-annual discharge variability. Second, we used an integrated surface water – groundwater dynamic simulation model to evaluate the agricultural conveyance and riparian systems as potential nitrogen removal locations. Under conservative behavior, modeled nitrate concentrations were higher than observed in the groundwater, river, and conveyance channels. We calibrated the model using denitrification in the groundwater and uptake by riparian vegetation and crops to match modeled to observed concentrations. Neither uptake of nitrate by riparian vegetation and crops or denitrification alone reduced nitrate sufficiently. However, a combination of 10% denitrification in the groundwater, riparian uptake equal to 90% of ET and crop uptake equal to 50% of ET resulted in nitrate concentrations that generally matched the magnitude and seasonal variations of observed nitrate concentrations.
H43C-1513
Determination Of Baseflow Inputs In A The San Miguel River Basin, A Riparian Area In The North American Monsoon Region
The North American Monsoon is a critical climatalogical phenomenon in Northwest Mexico and Southwest United States. Little is known however of how important this climate phenomenon is to long term ground water resources in this region and such information would be useful for water management. One of the better-studied systems in this region is the San Pedro, in the United States. Using geochemical and isotopic mixing techniques the waters sources of the San Pedro have been shown to include a significant fraction of summer floodwater. For this study we are looking to extend this understanding to other basins in the area; moving to basins with more and less Monsoonal influence. The more Monsoon dominated basin we are investigating is the Rio San Miguel, Mexico, a Rio Sonora tributary. We chose this river due to its several perennial reaches. At this time we have samples from two years during pre-monsoon (May) and monsoon season (July), and winter (February). The isotopic data gives a clear pattern of seasonal variability of recharge source composition. The chemical data helps us differentiate what processes are occurring along the 80 km river reach of our study. The river has 5 separate perennial reaches; the reaches group in two different groups of water source. In the lower part of the basin the data show more evaporated water than in the upper basin. The isotopic data clearly show a differentiation processes in precipitation, with more depleted water at high altitude. Surface and ground water average isotopic values are close to each other, with surface water closer in composition to summer precipitation than groundwater.
H43C-1514
Biologically-Mediated Weathering of Minerals From Nanometre Scale to Environmental Systems
The Weathering Science Consortium is a multi-disciplinary project that aims to create a step change in understanding how biota control mineral weathering and soil formation (http://www.wun.ac.uk/wsc). Our hypothesis is that rates of biotic weathering are driven by the energy supply from plants to the organisms, controlling their biomass, surface area of contact with minerals and their capacity to interact chemically with minerals. Symbiotic fungal mycorrhiza of 90% of plant species are empowered with an available carbohydrate supply from plants that is unparalleled amongst soil microbes. They develop extensive mycelial networks that intimately contact minerals, which they weather aggressively. We hypothesise that mycorrhiza play a critical role through their focussing of photosynthate energy from plants into sub-surface weathering environments. Our work identifies how these fungal cells, and their secretions, interact with mineral surfaces and affect the rates of nutrient transfer from minerals to the organism. Investigating these living systems allows us to create new concepts and mathematical models that can describe biological weathering and be used in computer simulations of soil weathering dynamics. We are studying these biochemical interactions at 3 levels of observation: 1. At the molecular scale to understand interactions between living cells and minerals and to quantify the chemistry that breaks down the mineral structure; 2. At the soil grain scale to quantify the activity and spatial distribution of the fungi, roots and other organisms (e.g. bacteria) and their effects on the rates at which minerals are dissolved to release nutrients; 3. At soil profile scale to test models for the spatial distribution of active fungi and carbon energy and their seasonal variability and impact on mineral dissolution rates. Here we present early results from molecular and soil grain scale experiments. We have grown pure culture (Suillus bovinus, Paxillus involutus) mycorrhizal mycelial networks associated with pine trees in otherwise sterile (agar) and also non-sterile (peat) microcosms, which include mineral sections and powders of biotite, apatite and quartz. 14C labelling has been used to map C flux through the microcosms and to determine the transfer of photosynthate energy into the weathering arenas. We have used Vertical Scanning Interferometry (VSI) to assess volumetric alteration of mineral substrates in contact with fungi. Focused Ion Beam (FIB)- Transmission Electron Microscope (TEM) work provides evidence for increased mechanical forcing and possible alteration of biotite surfaces with greater fungi contact time. We also present real-time in situ observations of mineral-organic acid and mineral-exudate interactions using Atomic Force Microscopy (AFM). http://www.wun.ac.uk/wsc