C. Kent Keller

In situ measurement of microbial activity and controls on microbial CO2 production in the unsaturated zone

Carbon dioxide COncentrations were measured at various depths and times in the unsaturated zones of two hydraulically and geochemically contrasting field sites, one in southeastern Washington state, and the other in south central Saskatchewan. In situ CO, production rates were calculated from a mass balance that accounted for diffusive fluxes and partitioning of CO[sub 2] into an advecting aqueous phase. Production rates were compared with (1) microbial abundance and (2) subsurface temperature to determine whether subsurface CO[sub 2] production rates could be expressed as a simple function of these two variables. At the Washington site, subsurface production was successfully expressed as a function of microbial abundance and temperature for a large portion of the year, but not near the end of the growing season. Although subsurface microbes and organic carbon were more abundant at the Saskatchewan site, subsurface CO[sub 2], production rates were generally several orders of magnitude lower than at the Washington site, and no correlation could be established between microbial numbers, temperature, and production rate. The cases where production rates could not be expressed as a function of microbial numbers and temperature suggested conditions in which some other factor, such as nutrient limitations, was controlling.

Macropore transport of bromide as influenced by soil structure differences

Macropore transport of chemicals in soil often causes unexpected contamination of groundwater. The effect of soil structure on the functions of various sized macropores was assessed, investigating transport of nonreactive bromide (Br) under matric heads of 0, � 2, � 5 and � 10 cm using undisturbed soil columns from A, Bw and E horizons of a Thatuna silt loam soil (fine-silty, mixed, mesic Xeric Argialbolls). The experimental breakthrough curves (BTC) for Br were described with a two-region physical nonequilibrium model. Greatest macroporosity occurred in the A horizon and lowest in the E horizon. The measured pore water velocity m under saturated conditions ranged from 18.92 cm day � 1 in the E horizon to 64.28 cm day � 1 in the A horizon. While the greatest dispersivity k occurred in the Bw horizon due to medium subangular blocky and prizmatic aggregates, the lowest dispersivity occurred in the E horizon due to its low macroporosity and massive structure. The fitted mobile water partitioning coefficient b ranged from 0.30 in the A horizon under 0 cm matric head to 0.93 in the E horizon under 0 cm matric head. The calculated values of rate of diffusive mass exchange a decreased with decreasing matric head in A and Bw horizons, and slightly increased and then decreased in the E horizon. The difference among each of the values of the parameters m, b, a and k for the A, Bw and E horizons was greatest under saturated conditions. However, gradually decreasing matric head until about � 3 cm decreased the difference among the values for a particular parameter for different horizons, sharply. The difference remained fairly unchanged with further decreases in the matric head, suggesting that most of the variability in macropore transport of bromide for these horizons caused by pores with radii larger than about 0.5 mm. In A and Bw horizons, there was a sudden change in soil solution movement between � 2 and � 5 cm matric head, indicating that macropore flow generally occurred at matric heads greater than

Hydrogeochemistry of a Clayey Till: 1. Spatial Variability

The spatial variability of hydrogeochemical processes was studied within an 18-m-thick unit of clayey till beneath a flat prairie landscape. Models of major element mass transfer, constrained by chemical, mineralogical, and isotopic measurements on waters and solids, were developed for two end-member water quality types occurring within the unit. The observed pattern of chemical variation, which is predominantly lateral on a scale of tens of meters, is caused by microtopographically controlled differences in the ratio of vertical water flux to oxidation rate associated with depression-focused recharge. Such variation is to be expected where dynamic flow regimes develop in thin surficial tills containing chemically reduced constituents. Observed depletions of major elements from oxidized zones of the till support the mass transfer models and demonstrate that the time scales of persistence of hydrogeochemical variability in the till and of water quality evolution in the aquifer beneath range from 104 to 105 year. Na and S concentrations in such aquifers will depend primarily on ratios of influxes of end-member water quality types from the till above, while Ca, Mg, and inorganic C concentrations will remain high as long as partial pressures of CO2 in the till remain high. CO2 generation in the till is discussed in the companion paper (Keller, this issue).

Soil respiration and georespiration distinguished by transport analyses of vadose CO2, 13CO2, and 14CO2

Georespiration and soil respiration operate on organic carbon pools of vastly different sizes and mean residence times (MRT). Both processes occur in the shallow subsurface at the Dalmeny site in southern Saskatchewan, Canada. Steady and transient, heterogeneous simulations of vadose CO2,13CO2, and 14CO2 show that at least 98% of all subsurface respiration occurs in the solum where the MRT of labile soil carbon is about 10 years. Root respiration dominates the total during the growing season. Remaining CO2 generation occurs near the capillary surface at 6.5–7.5 m depth, where δ14C of respired CO2 indicates an MRT of about 22,000 years. This value is consistent with a respiration substrate dominated by Cretaceous-age kerogen in the till. The simulated oxidation/georespiration rate at this depth is also consistent with observed depletion of kerogen C from the vadose zone during the Holocene. Field relations in this setting indicate that georespiration is controlled hydrogeologically by the development of aerobic vadose zones; we speculate that this may be more generally true on a global basis. Where soil parent materials contain ancient carbon, georespiration should be considered as a possible factor complicating studies of soil carbon turnover.

Carbon dioxide respiration in the deep vadose zone: Implications for groundwater age dating

In the deep vadose zone at the Dalmeny site, subsurface gas samples were collected and analyzed for CO2 and the 13C and 14C ratios of that CO2. High concentrations of CO2 depleted in 14C near the water table necessitate the use of an open-system model of calcite dissolution to match observed dissolved inorganic carbon 14C ratios just below the water table. Groundwater age-dating models assuming closed-system calcite dissolution predict incorrect groundwater age dates at the Dalmeny site. These results and our field observations suggest that such errors may generally occur where deep-vadose generation of nonmodern CO2 is not accounted for in groundwater age-dating exercises.

Hydrogeochemistry of a Clayey Till: 2. Sources of CO2

The spatial distributions of inorganic and organic carbon in gas, aqueous, and solid phases were studied in an 18-m-thick surficial deposit of fractured clayey calcareous till. Large PCO2 increases were observed with increasing depth at the bottoms of vadose zones from which solid organic carbon had been depleted. Hydrochemical reaction simulations indentified a reaction path consistent with PCO2, O2, major element chemistry, 13C, and 14C isotope observations; this path is “open” to a biogenic CO2 reservoir with PCO2 higher than, and 14C composition considerably different from, that of the soil zone. Such CO2 reservoirs, occurring in deep-vadose/shallow saturated zones and thus isolated by overlying sediments from temperature fluctuations which limit soil-zone respiration, may be common in the Western Glaciated Plains as evidenced by prevalence of hard, high-PCO2 waters in the regions intertill aquifers. Carbon 14 data suggest that in the till, both dissolved organic C and dissolved inorganic C have two sources: soil organic matter and kerogen in the till matrix. Regardless of source(s), initial 14C activities of both aqueous C pools at the water table are “diluted” substantially relative to the actual residence time of the water; unappreciated, this could cause large relative errors in the calculation of radiocarbon-based groundwater age dates.

Seasonal Variability of Adsorption and Exchange Equilibria in Soil Waters

Chemical analyses for major ions have been conducted on waters,collected on an approximately weekly basis over the period April, 1993 toNovember, 1996, that drain three small experimental ecosystems(“sandboxes”) at Hubbard Brook, New Hampshire. One sandbox is planted withpine trees, another with grass, and the third is left “bare” (actually itis covered sporadically by bryophytes and lichens). Results show linearcorrelations, independent of discharge, between the concentrations ofdissolved Na+ and K+ on the one hand andCa++ and Mg++ on the other for all threesandboxes. No correlations between singly charged and doubly chargedcations were found. These correlations are interpreted to represent cationexchange equilibria between soil waters and clay minerals plus soil organicmatter. The correlation slope, representing the exchange constant, for Na vsK is different for the pine-covered sandbox than for the other two whereasfor Ca vs Mg the correlation is independent of the presence or absence oftrees. We interpret this as representing a shift of cation exchangeequilibria in the pine sandbox by the activities of growing trees.Concentrations of Na, K, Ca, Mg, and H4SiO4from the barren and grass-lined sandboxes were found to vary seasonally witha marked sinusoidal pattern which was independent of the discharge from eachsandbox. (The discernment of a similar pattern in the tree lined sandbox wasdifficult due to a lack of discharge over much of the year.) Concentrationmaxima occurred in August and minima in February, and there is a closeparallelism with soil temperature. We interpret this as representingtemperature induced variations in cation exchange equilibria and silicaadsorption. Independence from highly varying water discharge, e.g.,. thataccompanying severe rainstorms, indicates rapidly re-attained equilibrium.Variations in the concentrations of cations are likely due to exchange withunmeasured cations, probably H+ or dissolved Al species, as aresult of possible seasonal changes in internal acid production and externalinput of acid rain to the sandboxes. Internal production may represent aresponse to seasonal changes in respiration rate as it responds toseasonally varying temperature. Added to this is the effect of temperatureon exchange equilibrium. Seasonal variations in dissolved silica are mostlikely due to the dependence of adsorption/desorption equilibria ontemperature. The temperature dependence of a number of silica-consumingreactions are consistent with the measured values.

Lessons from the Sandbox: Is Unexplained Nitrogen Real?

AbstractIn their review of 24 studies of forest nitrogen (N) budgets, Binkley and others (2000) found that only one of them supported the conclusion that an N accumulation of more than 25 kg N ha−1 y−1 is possible without known symbiotic N2–fixing plants. They contended that, given how well the N cycle is known, new N accumulation pathways are unlikely. They also concluded that the Hubbard Brook sandbox study (Bormann and others 1993) was insufficiently replicated and had low precision in vegetation and soil estimates. Here we reevaluate and extend the sandbox analysis and place the findings in a broader context. Using multiple methods of estimating vegetation N accumulation in pine sandboxes, we arrived at results that differed from the reported rates but still strongly supported large biomass N accumulation. The original studys conclusions about soil N changes were strengthened when new evidence showed that N accumulated in lower horizons and that the sandboxes were successfully homogenized at the beginning of the experiment. Unexplained ecosystem N accumulation ranged from about 40 to 150 kg ha−1 y−1, with 95% confidence intervals that did not include zero. No evidence was found that could balance the sandbox ecosystem N budgets without adding unexplained N. Unreplicated experiments, such as the sandboxes, can explore the possibility that N can accumulate in ways not explainable by mass balance analysis, but they cannot quantify the frequency and extent of the phenomenon. New studies should combine substantive microbiological, mass balance, and process research using multiple direct measures of N2 fixation.

Hydrogeologic parameters affecting vadose‐zone microbial distributions

Abstract The vadose zone and its contaminant‐attenuating processes are physically interposed between surface contamination and groundwater supplies. Given the potential role of microorganisms in mediating vadose‐zone chemical processes, it is vital to understand vadose microbial distributions and factors controlling those distributions. Vadose and shallow saturated zone sediments obtained from cores drilled to approximately 8 m below the surface at two hydrogeologically contrasting sites, named Dalmeny and Washington State University (WSU), were examined for culturable heterotrophic bacteria, total organic carbon (TOC), and sediment texture. Pore‐water elutions were analyzed for dissolved organic carbon, sulfate, and inorganic nitrogen species. Numbers of cultured bacteria (103‐107 g−1) generally decreased with depth at both sites. The TOC decreased uniformly with depth at WSU where soil processes are the sole carbon source; at Dalmeny, where both soil and kerogen carbon are present, TOC was higher and re...

Nitrate in tile drainage of the semiarid Palouse Basin.

Topographically heterogeneous agricultural landscapes can complicate and accelerate agrochemical contamination of streams due to rapid transport of water and chemicals to poorly drained lower-landscape positions and shallow ground water. In the semiarid Palouse region, large parts of these landscapes have been tile drained to enhance crop yield. From 2000-2004 we monitored the discharge of a tile drain (TD) and a nearby profile of soil water for nitrate concentration ([NO(3)]), electrical conductivity level (EC), and water content to develop a conceptual framework describing soil nitrate occurrence and loss via subsurface pathways. Tile-drain baseflow [NO(3)] was consistently 4 mg N L(-1) and baseflow EC was 200 to 300 microS cm(-1). Each year sudden synoptic increases in TD discharge and [NO(3)] occurred in early winter following approximately 150 mm of fall precipitation, which saturated the soil and mobilized high-[NO(3)] soil water throughout the profile. The greatest TD [NO(3)] (20-30 mg N L(-1)) occurred approximately contemporaneous with greatest TD discharges. The EC decrease each year (to approximately 100 microS cm(-1)) during high discharge, a dilution effect, lagged approximately 1 mo behind the first appearance of high [NO(3)] and was consistent with advective transport of low-EC water from the shallow profile under saturated conditions. Water-budget considerations and temporal [NO(3)] patterns suggest that these processes deliver water to the TD from both lower- and upper-slope positions, the latter via lateral flow during the high-flow season. Management practices that reduce the fall reservoir of soil nitrate might be effective in reducing N loading to streams and shallow ground water in this region.