Robert C. Graham

Wood-ash composition and soil pH following intense burning

Thousands of hectares of wildlands are burned annually in the western United States. The composition and mineralogy of wood-ash produced by severe burning, and the changes in pH of soils underlying the ash, were examined at five sites in Califor-nia. Soil pH increased by as much as 3 pH units (to pH 10.5) immediately after burn-ing compared with unburned soil. Approx-imately 1 to 2% of each burn area was affected to a maximum observed depth of 20 cm. The major component of fresh, white wood-ash is calcite, while K and Na carbonates are present in minor amounts. The initial very high pH values of wood-ash and surface soil are caused by K and Na oxides, hydroxides, and carbonates. These compounds are very soluble and do not persist through the wet season. The calcite is much less soluble and was present in soils 3 years after burning, maintaining moderately alkaline pH in surface soils that are normally neutral to strongly acid.

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.

Rock to regolith conversion: Producing hospitable substrates for terrestrial ecosystems

Weathering processes transform hard fresh rock into friable weathered rock, which is then physically disrupted to become soil. These regolith materials mantle the land masses and support terrestrial life but their formation involves some of the least understood of Earth’s surficial processes. The conversion of biologically inert hard rock to a hospitable substrate for organisms begins with the production of porosity by weathering. Porosity allows water to flow through weathered rock, but it also imparts a water-holding capacity so that water can be stored for prolonged use by organisms. Organisms themselves, in the form of microbes and plant roots, invade the rock as porosity forms. Production of porosity is the fundamental process responsible for converting rock into a medium capable of supporting terrestrial ecosystems. Consequently, the rate of porosity formation during rock weathering is the ultimate measure of the production and sustainability of ecosystemfunctional substrates. INTRODUCTION Fresh bedrock exposed at the land surface is an inhospitable substrate for most life. Exposed bedrock has very low porosity and hydraulic conductivity (Zhao, 1998; Schild et al., 2001); consequently, rain and snowmelt run off from it immediately. Water is not stored, so plants do not have a reservoir from which they can extract moisture as needed during dry periods. Furthermore, although hard bedrock contains elements such as P, Ca, Mg, and K that are essential for life, they are not readily accessible to organisms because they are bound within crystalline mineral structures. Once hard rock is weathered, it develops abundant porosity, first as friable bedrock, and later, when this weathered bedrock is physically disrupted, as soil. The development of extensive porosity is the key process in converting rock from a biologically inert material to a medium from which biota can gain nutrients, stored water, and a vast underground habitat. Here we describe the mechanisms and implications of transforming nonporous hard rock into porous regolith. We focus on granitic rock because it is a major component of Earth’s crust (15% of the land area) and because it is relatively consistent in its weathering behavior (Twidale and Vidal Romaní, 2005). GSA Today, v. 20, no. 2, doi: 10.1130/GSAT57A.1 E-mails: robert.graham@ucr.edu, arossi1@umd.edu, khubbert@fs.fed.us POROSITY FORMATION AND GRANITIC ROCK WEATHERING Unweathered granitic plutons are commonly jointed. The joints are the result of stresses on the rock mass, including those associated with thermal, tectonic, and erosional unloading processes. Joint spacings range from several decimeters to several meters, can be orthogonally oriented, and depend on the geologic history of the rock. In unweathered bedrock, the joint fractures are empty planar voids that range in width from a fraction of a millimeter to more than a centimeter (Bergbauer and Martel, 1999). Fractures are the main source of hydraulic connectivity in unweathered bedrock (Paillet, 1993). The rock mass between the joints contains minor porosity, usually 1% or less (Twidale and Vidal Romaní, 2005), in the form of microfractures <1 μm wide and microporosity within mineral grains (Sardini et al., 2006). The microfractures are generated by stresses incurred during cooling, hydrothermal activity, or tectonism (Schild et al., 2001). Micropores within mineral grains form during crystallization and cooling. Meteoric water flowing down joint fractures initially enters the bedrock mass through inherent microfractures, thereby beginning the chemical weathering process (Meunier et al., 2007). In biotite-bearing granites, ion exchange weathering is an important first step in generating bulk rock porosity. The replacement of interlayer K by hydrated Mg cations results in expansion of the biotite structure as the mineral is transformed to vermiculite (Wahrhaftig, 1965; Nettleton et al., 1970; Isherwood and Street, 1976). This expansion, which involves a 30%–40% increase in volume, exploits the weakness imparted by the lithogenic microfractures and shatters the rock. A smaller expansion of biotite has been noted to occur upon oxidation of the Fe within its structure (Buss et al., 2008). In either case, the rock matrix loses much of its mechanical strength (Arel and Önalp, 2004) and is transformed into a regolith material referred to as saprock (Anand and Paine, 2002) (Fig. 1A). The rock mass is now permeated by a continuous network of mesofractures (Fig. 1B). It maintains the original rock texture (Fig. 1C) but is friable and can be crumbled by hand into its individual grain sizes (Fig. 1D). Individual mineral grains in saprock are not extensively chemically altered (Wahrhaftig, 1965; Girty et al., 2003). The mesofracture network in saprock opens up the rock mass to extensive percolation of water and vastly increases the surface area for weathering. At this point, hydrolysis becomes an effective weathering process, attacking feldspars and other weatherable minerals. Feldspars are weathered preferentially along twin planes (Fig. 1B), and are eventually

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.

Spatial distribution and properties of ash and thermally altered soils after high-severity forest fire, southern California

After a century of fire suppression, dense forests in California have fueled high-severity fires. We surveyed mixed conifer forest with 995-1178 trees ha −1 (stems > 10 cm diameter at breast height), and nearby pine- oak woodland having 175-230 trees ha −1 , 51 days after a severe burn, to contrast the spatial extent and properties of thermally altered soil at sites with different tree densities. Water-repellent soils were more extensive in forest than woodland. Deposits of white ash, composed largely of calicite, covered at most ∼25% of the land surface, in places where large fuel items (e.g. logs, branches, exfoliated oak bark) had thoroughly combusted. At least 1690 kg ha −1 of CaCO3 in ash was deposited over the forest, and at least 700 kg ha −1 was added to the woodland. Combustion of logs and large branches also reddened the underlying yellow-brown soil as deep as 60 mm (average 8 mm), and over ∼1-12% of the land surface. The reddened soils have magnetic susceptibilities that are three to seven times greater than surrounding unreddened soils within the burn, indicating thermal production of maghemite. Such fire-altered conditions persist over spatial and temporal scales that influence soil genesis in Mediterranean-type climate regions.

Decade-scale changes of soil carbon, nitrogen and exchangeable cations under chaparral and pine

Four large lysimeters on the San Dimas Experimental Forest, each filled with similar parent material and planted with monocultures of native species in 1946, provide a unique opportunity to quantify short-term effects of plant species on soil properties. The four species under which soils were investigated are scrub oak (Quercus dumosa Nutt.) , chamise (Adenostoma fasciculatum Hook. and Am.), ceanothus (Ceanothus crassifolia Torr.), and Coulter pine (Pinus coulteri B. Don). A mass-balance approach was used to measure changes in C, N, exchangeable base cations, and exchangeable acidity to a depth of 1 m in the mineral soils over a 41-year period. The C content increased in all of the soils, but the greatest change was in the soil under oak (3.7 kg mm3). more than doubling the original amount. Since the source of C in these soils is the photosynthetic fixation of atmospheric CO,, the mass of C accumulated reflects the magnitude of the CO2 sink provided by chaparral soils in their initial stages-of formation. The calculated rate of soil C accumulation is as much as 0.09 kg mm3 yr-‘ .The increase in N was highest in the soil under ceanothus (0.12 kg mP3), the only Nz-fixing species in this study. Exchangeable Ca increased by 25.7 mol mm3 in the soil under oak, while the maximum increase in exchangeable Mg was 5.5 mol m -3 also under oak. Exchangeable Na was leached from all of the soils (a maximum of 2.4 mol mP3 lost from under chamise and ceanothus) and K was slightly depleted.

Contributions of water supply from the weathered bedrock zone to forest soil quality

One measure of forest soil quality is the ability of the soil to support tree growth. In mediterranean-type ecosystems, such as most of Californias forests, there is virtually no rainfall during the summer growing season, so trees must rely on water stored within the substrate. Water is the primary limitation to productivity in these forests. Many forest soils in California are relatively thin, but are underlain by thick zones of weathered bedrock. Weathered granitic bedrock, the most prevalent lithology, has available water capacities (0.12-cm water/cm rock) that approach those of soils (0.2-cm water/cm soil) and, because the weathered rock zone is usually so much thicker (several meters) than the soil (<1 m), it almost always constitutes the larger storage reservoir for plant-available water. The weathered bedrock retains the original rock fabric and is friable and easily excavated, but the primary minerals are not thoroughly altered to clay minerals, so it is not considered saprolite. Roots of ponderosa pine (Pinus ponderosa) seedlings penetrate through the soil and encounter weathered bedrock within the first 2 years on many sites. Thus, the influence of the weathered bedrock zone on plant growth begins early. Root access to the weathered bedrock is restricted to fractures, which are often spaced about 50 cm apart. Water is extracted from the intervening rock matrix through unsaturated flow toward the root mat in the fractures and by mycorrhizal fungal hyphae that penetrate the rock matrix. At one site in the Sierra Nevada, 30-year-old Jeffrey pine (P. jeffreyi) depleted the soil-held water by mid-June and relied on water stored in the weathered bedrock until the rainy season began in October. In this case, the weathered bedrock supplied at least 70% of the water used by the trees during the growing season. In the same area, we found that thin soils are not a detriment to pine productivity when they are underlain by a thick zone of weathered bedrock. In mediterranean-type ecosystems, the weathered bedrock zone is an essential component of the plant water supply system and is thus an important contributor to forest soil quality.

Transformations of 2:1 phyllosilicates in 41-year-old soils under oak and pine

Abstract The large, unconfined lysimeters at the San Dimas Experimental Forest, in southern California, provide a unique setting in which to study decade-scale vegetation effects on mineral weathering. We investigated the 2:1 phyllosilicate mineralogy of lysimeter soils under 41-year-old monocultures of scrub oak (Quercus dumosa Nutt.) and Coulter pine (Pinus coulteri B. Don), and compared the results to archived original fill material. X-ray diffraction showed that mica increased relative to vermiculite in the clay and medium silt fractions of A horizons under both oak and pine, compared to the archived original fill material. The increase, however, was far greater under oak than under pine. No mineralogical differences were observed in the subsurface horizons of either oak or pine, compared to archived material. Nonexchangeable K increased by 23% in the clay fraction of the oak A horizon, and increased by 5% in the clay fraction of the A horizons under pine, relative to archived parent material. Strong evidence supports biocycling as the basis for observed decade-scale mineral transformations. We conclude that the return of K to the soil surface through litter decomposition, and subsequent fixation by vermiculite, resulted in increased mica contents in A horizons. More K may have been fixed by vermiculite in the oak A horizon compared to pine due to greater K concentration in the oak litter pool; earthworm-mediated mineralization of K from organic matter under oak; and presence of fewer roots at the surface under oak, and, consequently, less plant removal of K from the A horizon.

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.

Root distribution and seasonal water status in weathered granitic bedrock under chaparral

Soils in mountainous terrain are often thin and unable to store sufficient water to support existing vegetation through dry seasons. This observation has led to speculation about the role of bedrock in supporting plant growth in natural ecosystems, since weathered bedrocks often have appreciable porosity and, like soil, can store and transmit water. This study, within a chaparral ecosystem in southern California, was designed to determine the extent of rooting within weathered granitic bedrock and to measure the relative contributions of soil and weathered bedrock to water-use by chaparral shrubs (Adenostorna fasciculatum Hook and Am., Arctostaphylos glandulosa Eastw., and Ceanothus greggii A. Gray). The rooting pattern was mapped from the wall of a trench excavated into the weathered bedrock. Water contents of soil-weathered bedrock profiles were measured at one to four week intervals for two years using a neutron probe. Chaparral roots penetrate deeply (≥ 4 m) into the weathered bedrock and are largely confined to joint traces. During summer dry seasons, the shrubs extracted 39.4 cm of water from a 2.9-m-thick zone of weathered bedrock — accessing nearly ten times as much water as from the 0.35-m-thick soil (Typic Xerorthents). Although it is commonly neglected in ecological inventories and analyses, weathered bedrock can be an essential ecosystem component, particularly where soils are thin and seasonal drought occurs.