Michael A. Anderson

A direct link between forest vegetation type and soil organic matter composition

Total carbon storage and turnover in soils can be simulated as a series of pools with different turnover rates, ranging from seasonal to millennial. This approach has emphasized the importance of climatic controls on soil organic matter (SOM) dynamics, but implicitly assumes that SOM is minimally influenced by the nature of the plant material from which it is derived. Here we test this assumption by contrasting the influence of climate and vegetation (oak, manzanita, and conifers) on SOM composition in granitic-derived soils from California. Soils developed under the same climate in the San Gabriel Mountains were compared to soils with varying climate along an elevational transect in the Sierra Nevada range. Solid state TOSS CPMAS 13C NMR was used to semiquantitatively characterize the chemical structure of organic matter in litter layers, and low-density and fine silt fractions isolated from sampled A horizons. For all soils, there was a progressive decrease in O-alkyl C, and an increase in alkyl and carbonyl C from the litter to the low-density and fine silt fractions. The NMR spectra of the low-density fractions, and even more so of the fine silt fractions exhibited clear differences in SOM composition associated with different plant genera, regardless of climate. The carbonyl C dominated under oak, O-alkyl C prevailed under manzanita, and alkyl C was prominent under coniferous vegetation. These results indicate that vegetation, not climate, was the factor controlling SOM composition in these soils, and should be taken as a caution against a simplistic climatic interpretation of storage and turnover rate of carbon in soils.

Soil organic matter processes: characterization by 13C NMR and 14C measurements

Abstract Soil organic matter (SOM) is a central contributor to soil quality as it mediates many of the chemical, physical, and biological processes controlling the capacity of a soil to perform successfully. SOM properties (e.g. C/N ratio, macro-organic matter) have been proposed as diagnostic criteria of overall soil fitness, but their use is hampered by a poor understanding of the basic biochemical principles underlying SOM processes. The objective of this project was to determine the influence of scrub oak (Quercus dumosa Nutt.) and Coulter pine (Pinus coulteri B. Don) vegetation on decomposition and SOM formation processes in a lysimeter installation constructed in 1936 in the San Gabriel mountains of southern California. Soil samples archived during construction of the installation, and A horizons sampled in 1987, were fractionated according to density and mineral particle size to isolate the water floatable (macro-organic matter), fine silt and clay fractions. Carbon turnover rates were determined on all fractions from AMS 14C measurements. Solid state CPMAS TOSS 13C NMR was used to semiquantitatively characterize the chemical structure of organic matter on fresh litter and soil fractions. For the two soils, there was a progressive decrease in O-alkyl C, and an increase in alkyl and carbonyl C from the litter to the floatable, fine silt and clay fractions. These compositional differences were due to the oxidative degradation of the litter material, with preferential decomposition of the cellulose and hemicellulose entities and selective preservation of recalcitrant waxes and resins. In all soil fractions, turnover rates of carbon were longer for the pine than for the oak lysimeter (up to 10 times longer). Also under pine, there was a gradual increase in turnover rate progressing from the floatable to the clay fraction, and differences in turnover rates among fractions may be explained based on differences in carbon chemistry. In contrast, under oak, rapid carbon turnover for all fractions suggested intense biological activity in this soil.

Vegetation control on soil organic matter dynamics

Soil organic matter (SOM) formation is one of the least understood steps of the global carbon cycle. An example is uncertainty around the role of plant communities in regulating SOM formation and turnover. Here we took advantage of the highly controlled conditions at the San Dimas lysimeter installation to quantify the influence of oak and pine vegetation on SOM dynamics. SOM turnover rates, estimated using total C and 14C content of litter and physically separable soil fractions, were faster under oak than under pine. In contrast to the rapid turnover for the oak litter (<2 years), the delay in litter incorporation into the mineral soil under pine was a controlling factor of SOM fluxes.

Enhanced removal of trapped non-aqueous phase liquids from saturated soil using surfactant solutions

Abstract Column experiments were performed under water-saturated conditions to evaluate the effectiveness of low-concentration surfactant solution at removing a trapped light non-aqueous phase liquid (LNAPL) o -xylene and a dense non-aqueous phase liquid (DNAPL) o -dichlorobenzene from soil. Sorption of the ethoxylated alcohol (Witconol SN90) surfactant on the porous material was evaluated using both batch and column flow experiments. The batch sorption results showed that the sorption of the surfactant on sand reached a minimum value near the surfactant concentration selected for the experiments. Negligible sorption of the surfactant was observed in the column sorption experiment at the 90 cm day −1 flow rate used in our study, implying that the residence time was too short for equilibrium sorption to be reached. The NAPLs were removed from the columns by solubilization, emulsification and mobilization. Macroemulsions were formed in both batch and column experiments, suggesting that the NAPL was mostly removed by immiscible displacement. The direction of flow was an important parameter in LNAPL removal, influencing both the location of the chemical in the column and the removal efficiency. The percentage of DNAPL removed from the column was successfully described using a dissolving sphere model with increased apparent solubility. However, the model failed to describe the NAPL distribution in the column adequately, suggesting that a more complex process than the solubility enhancement accounted for by the model is occurring.

Late summer water status of soils and weathered bedrock in a giant sequoia grove

The natural occurrence of giant sequoia (Sequoiadendron giganteum) is restricted to a mid-elevation zone on the west slope of the Sierra Nevada, California, where summer rainfall is negligible. This study measured the late summer water status of regolith (soil + weathered bedrock) profiles in the Packsaddle Grove on the Sequoia National Forest to assess the distribution of available water by geomorphic position at the end of the dry season. Soil and weathered granitic bedrock samples were collected by auger during September 7–12, 1993, at 30–cm intervals to a depth of 270 cm on triplicated sideslope, swale, and drainage positions for both giant sequoia and non-sequoia microsites throughout the grove. The samples were stored in air-tight cans and were used for water potential and water content determinations in the laboratory. Soil and weathered bedrock water potentials ranged from 0 to −2.1 MPa, generally increasing (becoming less negative; indicating moister conditions) with depth and in drainage positions. No consistent differences between water potentials under the two vegetation types were apparent for any of the landscape positions. The Cr (weathered bedrock) horizons held a substantial quantity of plant-available water. A representative 270–cm thick profile consisting of 90 cm of soil underlain by 180 cm of weathered bedrock is calculated to hold about 44 cm of plant-available water at field capacity, 27 cm of which is stored within the weathered bedrock. Assuming 35% runoff losses, it is estimated that annual precipitation of <68 cm may fail to fully recharge the regolith profile on upland (non-drainage) sites and result in late season water stress. Such conditions of low precipitation have existed in 3 of the past 10 years. Giant sequoia growing in drainages receive additional water from runoff and throughflow; thus they are less likely to experience water stress.

EFFECT OF SOIL PROPERTIES ON DEGRADATION AND SORPTION OF METHYL BROMIDE IN SOIL

Methyl bromide (CH3Br) is currently the most widely used soil fumigant, and its emission into the atmosphere after application reportedly contributes to ozone depletion in the stratosphere. Irreversible degradation and partially reversible sorption reactions affect the quantity of this furnigant reaching the soil surface and escaping into the atmosphere. Incubation studies in closed headspace vials under controlled conditions showed that degradation of CH3Br was highly dependent on soil organic matter content, and to a lesser extent, on the moisture level in the soil. Methylation of CH3Br on organic matter was suggested to be the major reaction that CH3Br undergoes in the soil environment. Other soil constituents such as clay did not contribute to the degradation under moist or air-dried conditions, though enhanced degradation was observed on oven-dried montmorillonite and kaolinite clays. Within soil profiles, degradation of CH3Br decreased with soil depth mainly due to the reduction of soil organic matter content with depth. In both Greenfield and Wasco sandy loams, the degradation rate of CH3Br in soil layers from 0 to 270 cm could be estimated from soil organic matter content. Sorption of CH3Br on moist soils was generally limited, and varied with soil depth. The degree of sorption could be predicted from soil moisture alone or soil moisture and organic matter content.

A Field Study of Water Flow and Virus Transport in Weathered Granitic Bedrock

Where soils are shallow, effluents from on-site wastewater disposal systems (OSWDS) can be dispensed onto underlying weathered bedrock. Viruses contained in the effluents pose a threat to groundwater quality if the weathered bedrock materials do not possess the properties necessary to treat the effluents before they reach groundwater. The extent and pathways of water flow and virus transport in fractured, weathered granitic bedrock were investigated at a field site in southern California. A suspension containing MS-2 bacteriophage, sodium bromide, and blue dye was ponded at the soil–weathered bedrock interface and allowed to infiltrate for 9 h. A trench was excavated, and bedrock samples were collected and assayed for water, bromide, and MS-2 content. Distributions of dye, bromide, and MS-2 indicate that joint fractures facilitated the channeling of water and of viruses to depths >105 cm. Infiltration data suggest that fracture channeling occurred primarily during the first 50 min, after which time vertical convective flow through the bedrock matrix was the primary infiltration and transport process. The transition from fracture to matrix flow was the result of a decrease in fracture aperture caused by the swelling of pedogenic clay in the weathered bedrock matrix. Bromide and MS-2 concentration profiles suggest that the lower extent of matrix flow was 45 cm, below which water flow and virus transport occurred solely via fracture channeling. These results indicate that caution should be used when operating OSWDS on weathered granitic bedrock in California, and emphasize the need to collect morphologic and hydraulic data prior to their installation.

Methane production and ebullition in a shallow, artificially aerated, eutrophic temperate lake (Lake Elsinore, CA).

Methane is an important component of the gases released from lakes. Understanding the factors influencing the release is important for mitigating this greenhouse gas. The volume of methane (CH4) and other gases in sediments, and the rate of CH4 ebullition, were determined for an artificially aerated, shallow, eutrophic freshwater lake in Southern California. Gas volume was measured at 28 sites in July 2010, followed by monthly sampling at 7 sites through December 2011. Gas volumes measured in July 2010 at the 28 sites exhibited a complex dependence on sediment properties; the volume of CH4 and other gases was negligible in very coarse-textured sediment with low water and organic carbon contents. Gas volumes increased strongly with increased silt content, and were highest in sediments with intermediate water contents (60 to 70%), organic carbon contents (2 to 3%) and depths (approximately 4m). Methane was the dominant gas collected from sediment (80 to 90%), while carbon dioxide comprised roughly 2 to 3% of sediment gas in the lake. Gas sampling during cool winter months revealed very low or undetectable volumes of gas present, while sediment gas volumes increased markedly during the spring and early summer months, and then declined in late summer and fall. The rate of CH4 ebullition, quantified with an echosounder, also varied markedly across the lake and seasonally. High rates of ebullition were measured at all 7 sites in July 2011 (up to 96mmolCH4m(-2)d(-1)), while the rates were >50% lower in September and negligible in December 2010. Ebullition rates were inversely correlated with depth and most other sediment properties, but strongly positively correlated with sand content. No simple relationship between ebullition rate and sediment gas volume across the set of sites was found, although ebullition rates at individual sites were strongly related to gas volume.

PHYSICAL AND HYDRAULIC PROPERTIES OF WEATHERED GRANITIC ROCK IN SOUTHERN CALIFORNIA

Weathered granitic rock is a material with properties intermediate between soil and hard rock. It retains structural features of the hard rock, including joint fractures, but also has porosity generated as a result of weathering of primary minerals, clay formation, and root invasion. This study evaluated the physical and hydraulic properties of moderately and highly weathered granitic rock (class 5 and 6, respectively, of the classification scheme of Clayton and Arnold (1972)) in the San Jacinto Mountains of southern California. Class 5 and 6 samples exhibited comparable bulk densities (approximately 1.94 g cm−3), porosities (27%), and particle size distributions. Saturated hydraulic conductivities were also similar for both weathering classes (approximately 5.5 cm h−1). Water retention data indicate that about 50% of the water held at saturation is drained at −100 cm head. Effective pore size distributions calculated from water retention data indicate that 25% of the total porosity was associated with pores >100 μm in diameter.

Modeling the impact of body-contact recreation on pathogen concentrations in a source drinking water reservoir

Abstract A modeling study was conducted to evaluate the impact of body-contact recreation (e.g., water skiing, jet skiing, swimming) on pathogen concentrations in a source drinking water reservoir under construction in eastern Riverside County in Southern California. A hybridized Monte Carlo-finite segment model was used to predict pathogen concentrations in the reservoir resulting from pathogen inputs associated with shed fecal material and accidental fecal releases (AFRs). Monte Carlo techniques were incorporated into the finite segment model to define characteristics about individual recreators which affect pathogen loading to the reservoir (e.g., infection, pathogen shedding rate, location). Results of simulations are provided in the form of cumulative distribution and probability density functions derived from uncertainty analyses. The model predicted considerable spatial and temporal variability in pathogen concentrations within the reservoir, with elevated levels of Cryptosporidium , rotavirus, and poliovirus in the epilimnion during periods of high recreational use. Predicted Giardia concentrations were lower than the other pathogens. Hypolimnetic concentrations of all pathogens were generally 1–3 orders of magnitude lower than the overlying epilimnetic concentrations. Model results also suggest that field sampling will underestimate the mean, range and variance of pathogen concentrations in the reservoir. The model was further modified to include a particle tracking scheme to allow for transport of aggregated fecal material. Results from simulations using this approach demonstrate a potential for high pathogen loads due to body-contact recreation periodically reaching treatment plants.