Eugene F. Kelly

The effect of plants on mineral weathering

This paper is centered on the specific effects of plants on the soil weathering environment; we attempt to address how to quantify this component of the ecosystem and assess feedbacks between plants and weathering processes that influence the degree and rates of mineral weathering. The basic processes whereby plants directly influence the soil chemical environment is through the generation of weathering agents, biocycling of cations, and the production of biogenic minerals. Plants may indirectly influence soil processes through the alteration of regional hydrology and local soil hydrologic regime which determines the residence time of water available for weathering. We provide a brief review of the current state of knowledge regarding the effects of plants on mineral weathering and critical knowledge gaps are highlighted. We summarize approaches that may be used to help quantify the effects of plants on soil weathering such as state factor analyses, mass balance approaches, laboratory batch experiments and isotopic techniques. We assess the changes in the soil chemical environment along a tropical bioclimatic gradient and identify the possible effects of plant production on the soil mineralogical composition. We demonstrate that plants are important in the transfer of atmospheric carbon dioxide into the mineral weathering cycle and speculate how this may be related to ecosystem properties such as NPP. In the soils of Hawaiian rainforests subjected to deforestation, pasture grasses appear to change the proportion of non crystalline to crystalline minerals by altering the soil hydrologic regime or partitioning silica into more stable biogenic forms. A better understanding of the relationship between soil weathering processes and ecosystem productivity will assist in the construction predictive models capable of evaluating the sensitivity of biogeochemical cycles to perturbations.

Oxygen isotopic composition of soil water : Quantifying evaporation and transpiration

Abstract The oxygen isotopic composition of soil water provides an extra quantitative dimension in water balance analysis which allows separation of evaporation from transpiration. Spatial and temporal variations in water content and oxygen isotopic composition in soils along an arid to humid transect in Hawaii reflect the processes of recharge by rain, mixing with antecedent moisture, and evapotranspiration. Rainwater is always more depleted in 18O than is the soil water with which it mixes. Input of 18O-depleted rain increases volumetric water content while lowering the soil-water δ18O value. Evapotranspiration occurs continuously, leading to a decrease in the volumetric water content and an increase in the soil-water δ18O value. These effects are most pronounced at the soil surface and decrease in a downward direction. The frequency of recharge determines temporal variability of these values within a given depth interval, while differences along the transect are due to climatic parameters. Results of a material balance model indicate that evaporation decreases, transpiration increases, and the ratio of evaporation to transpiration decreases with increasing annual rainfall and decreasing temperature.

Carbon isotope systematics of monoaromatic hydrocarbons: vaporization and adsorption experiments

Abstract This study investigates the carbon isotope systematics of benzene, toluene, ethylbenzene, and xylene monoaromatic hydrocarbons (BTEX) with regard to an improved understanding of the behavior of these compounds in the subsurface, particularly during remediation processes. We found that fractionation effects due to vaporization are small and positive for all compounds studied (Δ 13 C vapor–liquid ≌+0.2‰), and that fractionation effects due to soil adsorption are also likely to be small ( 13 C-labeled compounds cannot be resolved from unlabeled compounds by HPLC). We also evaluated use of the isotopic composition of contaminants as tracers of source and migration in the subsurface by performing a survey of the bulk isotopic composition of commercially available sources of BTEX. The results indicate that a wide range of δ 13 C values exist (e.g., benzene, −23.87‰ to −29.40‰). Our work suggests that stable C isotope analysis has great potential for qualifying, and possibly quantifying, the subsurface processes affecting contaminant concentrations. In particular, stable isotope analysis may be especially beneficial for monitoring the efficacy of abiological and biological remediation efforts.

Biogeographic patterns in below-ground diversity in New York City's Central Park are similar to those observed globally

Soil biota play key roles in the functioning of terrestrial ecosystems, however, compared to our knowledge of above-ground plant and animal diversity, the biodiversity found in soils remains largely uncharacterized. Here, we present an assessment of soil biodiversity and biogeographic patterns across Central Park in New York City that spanned all three domains of life, demonstrating that even an urban, managed system harbours large amounts of undescribed soil biodiversity. Despite high variability across the Park, below-ground diversity patterns were predictable based on soil characteristics, with prokaryotic and eukaryotic communities exhibiting overlapping biogeographic patterns. Further, Central Park soils harboured nearly as many distinct soil microbial phylotypes and types of soil communities as we found in biomes across the globe (including arctic, tropical and desert soils). This integrated cross-domain investigation highlights that the amount and patterning of novel and uncharacterized diversity at a single urban location matches that observed across natural ecosystems spanning multiple biomes and continents.

Effects of slash-and-burn management on soil aggregate organic C and N in a tropical deciduous forest

Our study examined the effect of slash-and-burn management on the distribution of soil organic carbon (SOC) across water-stable aggregate size fractions. Macroaggregates (>250 μm) are an important source of SOC and soil organic nitrogen in forest soil: they account for approximately 80% of the total C and N content. Slashing and burning did not destroy macroaggregates, but the SOC associated with macroaggregates decreased by 32% due to combustion during burning. Fire also disrupted soil aggregate stabilization by changing the chemical nature of SOC. The largest changes were noted after the first growing season following fire: macroaggregates and associated C decreased 50% while microaggregates increased by the same proportion. The changes in organic C observed after the first growing season can be attributed to macroaggregate instability.

Carbon dioxide consumption during soil development

Carbon is sequestered in soils by accumulation of recalcitrant organic matter and by bicarbonate weathering of silicate minerals. Carbon fixation by ecosystems helps drive weathering processes in soils and that in turn diverts carbon from annual photosynthesis-soil respiration cycling into the long-term geological carbon cycle. To quantify rates of carbon transfer during soil development in moist temperate grassland and desert scrubland ecosystems, we measured organic and inorganic residues derived from the interaction of soil biota and silicate mineral weathering for twenty-two soil profiles in arkosic sediments of differing ages. In moist temperate grasslands, net annual removal of carbon from the atmosphere by organic carbon accumulation and silicate weathering ranges from about 8.5 g m−2 yr−1 for young soils to 0.7 g M−2 yr−1 for old soils. In desert scrublands, net annual carbon removal is about 0.2 g m−2 yr−1 for young soils and 0.01 g m−2 yr−1 for old soils. In soils of both ecosystems, organic carbon accumulation exceeds CO2 removal by weathering, however, as soils age, rates of CO2 consumption by weathering accounts for greater amounts of carbon sequestration, increasing from 2% to 8% in the grassland soils and from 2% to 40% in the scrubland soils. In soils of desert scrublands, carbonate accumulation far outstrips organic carbon accumulation, but about 90% of this mass is derived from aerosolic sources that do not contribute to long-term sequestration of atmospheric carbon dioxide.

Stable isotope composition of soil organic matter and phytoliths as paleoenvironmental indicators

Abstract The stable C isotope composition of organic matter and opal phytoliths in diverse ecosystems demonstrate that soils carry a C isotopic signature that reflects long-term inputs of above- and below-ground C 3 or C 4 biomass. The utility of these isotopic characterization data for paleoenvironmental study is based on a knowledge of dominant soil forming processes as well as geomorphic and climatic conditions. This paper reports on both the theory and applicability of isotopic characterization of soil organic C and opal phytoliths. The theoretical perspectives are discussed in light of broad environmental applications. Details of the sample collection and preparation are provided with insights into quantities and pretreatment required for stable C isotopic characterization. Two case studies from the central Great Plains region are presented. Paleoclimatic interpretations are discussed for a portion of the central Great Plains. These interpretations are based on the stable C isotope data recovered from Holocene paleosols. These data indicate higher proportions of C 3 vegetation persisted during the early Holocene. The concordance in the C isotopic signatures of soil organic matter and phytoliths provide strong biological evidence of regionally cooler conditions. This C isotopic concordance also appears during the mid-Holocene; C isotope values indicate an increase in the proportion of C 4 vegetation, which reflects regionally warmer climatic conditions than present. Isotopic discordance in soil organic C and phytoliths can indicate the degree of diagenesis resulting from pedological alteration. Geomorphic and pedologic evidence indicate that the C isotope discordance between soil organic matter and opal phytoliths is the result of local topographic variations and spatial heterogeneity associated with plant distribution.

Measurement of soil-water δ18O values by direct equilibration with CO2

Abstract Isotopic analysis of soil water requires complete extraction of water from the soil matrix to ensure against isotopic fractionation. A variant of the standard CO 2 –water equilibration technique avoids distillation of soil water that can cause isotopic fractionation. Reproducibility of direct equilibration is between 0.3‰ and 0.4‰. Longer equilibration times are necessary for soil with a fine particle size distribution and/or low water content than for standard CO 2 –water equilibrations. Differences in the δ 18 O value between unsterilized and sterilized (by γ-radiation) sample pairs suggests that respiration can contribute to changing the δ 18 O value of soil water. Measured δ 18 O values of samples prepared by adding water with a known δ 18 O value to dry soil increase with increasing water content and are always more negative than the δ 18 O value of the added water, especially at low water contents. The combination of contamination by residual water, heterogeneities due to difficulty with physical mixing of the sample, possible kinetic fractionations when small amounts of water are added to dry soil, contribute to these trends. These tests suggest that direct equilibration is a quick and relatively easy approach for measuring the δ 18 O value of soil water.

Effect of burning of tropical deciduous forest soil in Mexico on the microbial degradation of organic matter

Slash and burn conversion of tropical deciduous forest can result in significant disruption of soil nutrient cycling, particularly in terms of the dynamics of microbial populations. This study deals with the effect of fire and ash input on microbial respiration and on distribution of C within water-stable aggregate in soils during a long-term incubation experiment (164 days). In 0–2 cm samples, the forest soil with ash had the lowest total CO2-C evolved during incubation. In the top 2 cm soil burned samples, grass amendment did not increase respiration; it did increase respiration, however, in the undisturbed forest soil. Our results suggest that the fire affected microbial activity through both soil heating and chemical changes. As indicated by the results of the grass amendment to burned sample treatment the high temperature killed some soil microorganisms, mainly those associated with the use of newly added C. In addition, ash input appears to have constrained microbial activity through changes in soil chemistry.Soil heating and ash input also affected the distribution of C across different size fractions of soil aggregates. Labile C associated with macroaggregates (>250 μm) was destroyed during fire and did not represent an important source of available labile C for microbial activity. We concluded that the combination of organic C redistribution among size-aggregate fractions and microbial communities alteration by fire are critical for soil C dynamic under pasture condition.

Paleopedologic and geomorphic evidence for Holocene climate variation, Shortgrass Steppe, Colorado, USA

Abstract Radiocarbon dates of paleosols in northeastern Colorado indicate distinct periods of stability and soil formation with intervening periods of instability resulting in soil truncation or burial. A combination of pedologic and geomorphic indicators were used to resolve the duration of, and prevailing climate during, these periods. Five sites, each having a paleosol, were examined using both traditional soil analyses and grain-size statistics, the latter to decipher the mode of parent material deposition. Twenty local stream, dune and bedrock deposits were analyzed using grain-size statistics to establish benchmarks for comparison with soils. Field investigation supported by grain-size frequency statistics indicate early Holocene, middle Holocene and contemporary soils all formed in alluvium. Organic C and phytolith data suggest the early and middle Holocene climatic conditions were more favorable for plant productivity than the present climate. Soil development in early and middle Holocene paleosols suggests wetter soil moisture regimes than present. Low parent material carbonate contents suggest an eolian source for the carbonate in the Bk and Btk horizons. The presence of paleosol Btk horizons suggests a decrease in precipitation at the end of soil-forming intervals followed by drought and subsequent soil burial.