Oliver A. Chadwick

Mineral control of soil organic carbon storage and turnover

A large source of uncertainty in present understanding of the global carbon cycle is the distribution and dynamics of the soil organic carbon reservoir. Most of the organic carbon in soils is degraded to inorganic forms slowly, on timescales from centuries to millennia. Soil minerals are known to play a stabilizing role, but how spatial and temporal variation in soil mineralogy controls the quantity and turnover of long-residence-time organic carbon is not well known. Here we use radiocarbon analyses to explore interactions between soil mineralogy and soil organic carbon along two natural gradients—of soil-age and of climate—in volcanic soil environments. During the first ∼150,000 years of soil development, the volcanic parent material weathered to metastable, non-crystalline minerals. Thereafter, the amount of non-crystalline minerals declined, and more stable crystalline minerals accumulated. Soil organic carbon content followed a similar trend, accumulating to a maximum after 150,000 years, and then decreasing by 50% over the next four million years. A positive relationship between non-crystalline minerals and organic carbon was also observed in soils through the climate gradient, indicating that the accumulation and subsequent loss of organic matter were largely driven by changes in the millennial scale cycling of mineral-stabilized carbon, rather than by changes in the amount of fast-cycling organic matter or in net primary productivity. Soil mineralogy is therefore important in determining the quantity of organic carbon stored in soil, its turnover time, and atmosphere–ecosystem carbon fluxes during long-term soil development; this conclusion should be generalizable at least to other humid environments.

Changing sources of nutrients during four million years of ecosystem development

As soils develop in humid environments, rock-derived elements are gradually lost, and under constant conditions it seems that ecosystems should reach a state of profound and irreversible nutrient depletion. We show here that inputs of elements from the atmosphere can sustain the productivity of Hawaiian rainforests on highly weathered soils. Cations are supplied in marine aerosols and phosphorus is deposited in dust from central Asia, which is over 6,000 km away.

Rapid exchange between soil carbon and atmospheric carbon dioxide driven by temperature change

Comparison of 14C (carbon-14) in archived (pre-1963) and contemporary soils taken along an elevation gradient in the Sierra Nevada, California, demonstrates rapid (7 to 65 years) turnover for 50 to 90 percent of carbon in the upper 20 centimeters of soil (A horizon soil carbon). Carbon turnover times increased with elevation (decreasing temperature) along the Sierra transect. This trend was consistent with results from other locations, which indicates that temperature is a dominant control of soil carbon dynamics. When extrapolated to large regions, the observed relation between carbon turnover and temperature suggests that soils should act as significant sources or sinks of atmospheric carbon dioxide in response to global temperature changes.

Strontium isotopes as tracers of ecosystem processes: theory and methods

Abstract The strontium (Sr) isotope method can be a powerful tool in studies of chemical weathering and soil genesis, cation provenance and mobility, and the chronostratigraphic correlation of marine sediments. It is a sensitive geochemical tracer, applicable to large-scale ecosystem studies as well as to centimeter-scaled examination of cation mobility within a soil profile. The 87Sr/86Sr ratios of natural materials reflect the sources of strontium available during their formation. Isotopically distinct inputs from precipitation, dryfall, soil parent material, and surface or groundwater allow determination of the relative proportions of those materials entering or leaving an ecosystem. The isotopic compositions of labile (soil exchange complex and soil solution) strontium and Sr in vegetation reflect the sources of cations available to plants. Strontium isotopes can be used to track the biogeochemical cycling of nutrient cations such as calcium. The extent of cation contributions from in situ weathering and external additions to soil from dust and rain can also be resolved with this method. In this paper, we review the geochemistry and isotopic systematics of strontium, and discuss the use of this method as a tracer of earth surface processes.

Biological control of terrestrial silica cycling and export fluxes to watersheds

Silicon has a crucial role in many biogeochemical processes—for example, as a nutrient for marine and terrestrial biota, in buffering soil acidification and in the regulation of atmospheric carbon dioxide. Traditionally, silica fluxes to soil solutions and stream waters are thought to be controlled by the weathering and subsequent dissolution of silicate minerals. Rates of mineral dissolution can be enhanced by biological processes. But plants also take up considerable quantities of silica from soil solution, which is recycled into the soil from falling litter in a separate soil–plant silica cycle that can be significant in comparison with weathering input and hydrologic output. Here we analyse soil water in basaltic soils across the Hawaiian islands to assess the relative contributions of weathering and biogenic silica cycling by using the distinct signatures of the two processes in germanium/silicon ratios. Our data imply that most of the silica released to Hawaiian stream water has passed through the biogenic silica pool, whereas direct mineral–water reactions account for a smaller fraction of the stream silica flux. We expect that other systems exhibiting strong Si depletion of the mineral soils and/or high Si uptake rates by biomass will also have strong biological control on silica cycling and export.

Predictive soil mapping: a review

Predictive soil mapping (PSM) can be defined as the development of a numerical or statistical model of the relationship among environmental variables and soil properties, which is then applied to a geographic data base to create a predictive map. PSM is made possible by geocomputational technologies developed over the past few decades. For example, advances in geographic information science, digital terrain modeling, remote sensing, fuzzy logic has created a tremendous potential for improvement in the way that soil maps are produced. The State Factor soil-forming model, which was introduced to the western world by one of the early Presidents of the American Association of Geographers (C.F. Marbut), forms the theoretical basis of PSM. PSM research is being driven by a need to understand the role soil plays in the biophysical and biogeochemical functioning of the planet. Much research has been published on the subject in the last 20 years (mostly outside of geographic journals) and methods have varied widely from statistical approaches (including geostatistics) to more complex methods, such as decision tree analysis, and expert systems. A geographic perspective is needed because of the inherently geographic nature of PSM.

From a black to a gray box — a mass balance interpretation of pedogenesis

We utilize chemical elements as tracers in a mass balance analysis that provides functional relationships among soil chemical composition, bulk density, and volume change in relation to parent material. These analytical functions are based on the principle of conservation of mass and include a term quantifying mass flux into/out of the soil and between horizons. We apply the technique to the oldest member of a chronosequence developed on marine terraces in northern California. It is an Alfisol that evolved from beach sand to its present state — nearly one-third of its weight is composed of secondary clay minerals — in about 240 ky. Aside from large increases in organic carbon, desilication is the dominant factor in soil evolution; 29% (50.83 g cm−2) Si was leached from the beach sand during pedogenesis. The rate of desilication is roughly 2.1 t km−2 yr−1 (0.21 g cm−2 ky−1), an order of magnitude slower than that implied by the Si denudation rate calculated for the Mattole River watershed. Weathering of primary minerals and synthesis of secondary minerals is relatively well-advanced suggesting that the rate of desilication may be declining. The local beach is composed of quartz and sodic plagioclase with smaller amounts of chlorite, mica, and kaolinite. The soil has substantially different mineralogy: sand is dominated by quartz, and clay is dominated by kaolinite/halloysite, chloritic intergrades, and gibbsite. Bases are also leached, though the total mass was much less; 57% (2.55 g cm−2) of Na in the beach sand was lost as plagioclase weathered. By focusing on elemental and mineralogical gains and losses, we emphasize the essential connection between the pedologic environment and the external hydrochemical environment.

The chemistry of pedogenic thresholds

Abstract Pedogenesis can be slow or fast depending on the internal chemical response to environmental forcing factors. When a shift in the external environment does not produce any pedogenic change even though one is expected, the soil is said to be in a state of pedogenic inertia. In contrast, soil properties sometimes change suddenly and irreversibly in a threshold response to external stimuli or internal change in soil processes. Significant progress has been made in understanding the thermodynamics and kinetics of soil-property change. Even in the open soil system, the direction of change can be determined from measures of disequilibrium. Favorable reactions may proceed in parallel, but the most prevalent and rapid ones have the greatest impact on product formation. Simultaneous acid–base, ion exchange, redox and mineral-transformation reactions interact to determine the direction and rate of change. The nature of the governing reactions is such that soils are well buffered to pH change in the alkaline and strongly acid regions but far less so in the neutral to slightly acid zones. Organic matter inputs may drive oxidation–reduction processes through a stepwise consumption of electron acceptors (thereby producing thresholds) but disequilibrium among redox couples and regeneration of redox buffer capacity may attenuate this response. Synthesis of secondary minerals, ranging from carbonates and smectites to kaolinite and oxides, forms a basis for many of the reported cases of pedogenic inertia and thresholds. Mineralogical change tends to occur in a serial, irreversible fashion that, under favorable environmental conditions, can lead to large accumulations of specific minerals whose crystallinity evolves over time. These accumulations and associated “ripening” processes can channel soil processes along existing pathways or they can force thresholds by causing changes in water flux and kinetic pathways.

DEFORMATIONAL MASS TRANSPORT AND INVASIVE PROCESSES IN SOIL EVOLUTION

Soils are differentiated vertically by coupled chemical, mechanical, and biological transport processes. Soil properties vary with depth, depending on the subsurface stresses, the extent of mixing, and the balance between mass removal in solution or suspension and mass accumulation near the surface. Channels left by decayed roots and burrowing animals allow organic and inorganic detritus and precipitates to move through the soil from above. Accumulation occurs at depths where small pores restrict further passage. Consecutive phases of translocation and root growth stir the soil; these processes constitute an invasive dilatational process that leads to positive cumulative strains. In contrast, below the depth of root penetration and mass additions, mineral dissolution by descending organic acids leads to internal collapse under overburden load. This softened and condensed precursor horizon is transformed into soil by biological activity, which stirs and expands the evolving residuum by invasion by roots and macropore networks that allows mixing of materials from above.

Refractory element mobility in volcanic soils

Refractory trace element concentrations in strongly weathered Hawaiian soils ranging in age from 20 to 4100 ka are highly elevated over parent-rock values due to extensive mass loss of more soluble major elements during pedogenesis. Nb and Ta exhibit virtually no mobility. Soil Nb/Ta ratios are within the range of fresh bedrock even when soil Nb concentrations are residually enriched by a factor of 10. In contrast, Al, Zr, and Hf are depleted relative to Nb in surface soil horizons but are enriched at depth, clearly indicating mobility of these elements. Variations in Th/Nb ratios in soil profiles indicate significant Th mobility within the soil column. However, mass-balance calculations require that accretion of Th-enriched Asian dust has resulted in a net increase in Th in some soils. Soils developed on a 150 ka rainfall gradient show that the mobility and loss of Zr increase with mean annual precipitation.