David M. Hendricks

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.

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.

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.

Changing sources of base cations during ecosystem development, Hawaiian Islands

Sr/ 86 Sr evidence from a soil chronosequence in the Hawaiian Islands demonstrates that the atmosphere supplies >85% of putatively rock-derived Sr in older sites. Initially, bedrock is the dominant source for Sr and other lithophile elements such as Ca, but high rates of weathering and leaching of the substrate by 20 ka lead to a shift to atmospheric sources. The loss of weathering inputs coincides with other physio-chemical changes in the soil and results in a steep decline of base cations in the soil pool. While these patterns imply the potential for limitation of biological productivity by low base cation supply, the atmosphere provides a supply of base cations in excess of nutritional needs, even after nearly all rock-derived base cations have been leached from the soil. This raises the possibility that P limitation in terrestrial ecosystems may develop at least as much because of low rates of atmospheric deposition of P (relative to Ca, K, and other rock- derived elements) as because of its chemical interaction in soil.

Changes in the content and composition of pedogenic iron oxyhydroxides in a chronosequence of soils in southern California

Abstract Studies of the pedogenic iron oxyhydroxides in suites of latest Holocene to middle Pleistocene soils formed on fluvial deposits of the transverse ranges, southern California, indicate that the content and composition of iron oxyhydroxide change in a systematic manner. Analysis of total secondary free iron oxides (dithionite extractable, Fe 2 O 3 d) and ferrihydrite (oxalate extractable, Fe 2 O 3 o) shows that (1) a single-logarithmic model ( Y = a + b log X ) or double logarithmic model (log Y = a + b log X ), where Y is the total mass of pedogenic Fe oxides (g/cm 2 -soil column) and X is soil age, describes the rate of increase in Fe 2 O 3 d with time; (2) the Fe 2 O 3 d content correlates linearly with soil reddening and clay content; (3) the Fe 2 O 3 o Fe 2 O 3 d ratio, which indicates the degree of Fe oxide crystallinity, is moderately high to very high (0.22–0.58) in middle Holocene to latest Pleistocene soils and progressively decreases to less than 0.10 in older soils; (4) the value of the Fe 2 O 3 o Fe 2 O 3 d ratio also appears to be infuenced by climate; and (5) temporal changes in Fe oxide content and mineralogy are accompanied by related, systematic changes in clay mineralogy and organic matter content. These relationships are attributed to a soil environment that must initially favor ferrihydrite precipitation and/or organic matter-Fe complexation. Subsequent transformation to hematite causes increasingly intense reddening and a concomitant decrease in the Fe 2 O 3 o Fe 2 O 3 d ratio. The results demonstrate that iron oxide analysis is useful for numerical age studies of noncalcic soils and shows potential as an indicator of paleoclimates.

Rates and processes of soil evolution on uplifted marine terraces, northern California

Time-dependent changes in soil development have been evaluated for two flights of uplifted marine terraces with similar site characteristics and a range in terrace ages (∼ 3.9 ka to 330 ka) in northern California. Six pedogenic properties of soils developed in unconsolidated deposits overlying bedrock marine platforms have systematic, time-dependent trends, but each property differs in rate of change and in the time period over which its changes are useful as relative age dating or correlation tools. Changes in accumulated mass of organic carbon and pH are useful properties only for the youngest soils, and probably for soils less than 10–20 ka in this climatic region. Soils on terraces older than 29 ka have nearly similar amounts of mass of organic carbon, with values of about 3–6% by weight for the upper meter of the soil profile. Soil reaction (pH) in the top 20 cm of the soil profile decreases at a rapidly declining rate, reaching and maintaining a value of ∼ 5 by 39–40 ka. Total mass of accumulated clay is an excellent indicator of relative soil age during all stages of soil development. The rate of accumulation of clay is about 0.4 g/cm2 soil column per thousand years, so that pedons on terraces with inferred ages of about 124 ka have accumulated at least 50 g/cm2 soil column of clay. Systematic and linear increases in accumulated mass of several forms of pedogenic iron make them very useful for chronological purposed for at least 240 ky of soil development. Crystalline forms of iron enable greater distinction among young soils than amorphous forms, which have negligible accumulations until at least 40 ka. The rate of accumulation of crystalline forms of iron (Fed−Feo) is about 0.02 g/cm2 soil column per thousand years, so that pedons on terraces with inferred ages of about 124 ka have accumulated about 3 g/cm2 soil column of crystalline forms of pedogenic iron. Changes with time in accumulated mass of amorphous forms of pedogenic aluminum, Alo, are negligible within the first 29 ka, then reflect a rapid increase in rate of accumulation until at least 124 ka. Two pedons from the same terrace (inferred age 124 ka) contain strikingly different amounts of pedogenic Alo, however, and suggest that Alo may be an unreliable indicator of the degree of soil development. In addition to mass values of different soil substances, maximum percent values were calculated. Soils developed on marine terraces older than 29 ka have at least 10% maximum clay, older than 100 ka at least 30% maximum clay, and older than 240 ka at least 40–50% maximum clay. Soils on terraces younger than 29 ka have less than 1% maximum Fed, and soils older than 240 ka at have at least 4% maximum Fed. These easily obtained values provide textural and chemical boundaries for separating marine terraces younger or older than about 29, 100, and 240 ka in this region, but are useful only as very general tools for correlation. Maximum percent clay is a more reliable indicator of soil age than maximum percent Fed, as the latter is greatly influenced by local groundwater flux, and percent values of Fed have a high degree of scatter.

The mass balance of soil evolution on late Quaternary marine terraces, northern California

Mass-balance interpretation of a soil chronosequence provides a means of quantifying elemental addition, removal, and transformation that occur in soils from a flight of marine terraces in northern California. Six soil profiles that range in age from several to 240,000 yr are developed in unconsolidated, sandy- marine, and eolian parent material deposited on bedrock marine platforms. Soil evolution is dominated by (1) open-system depletion of Si, Ca, Mg, K, and Na; (2) open-system enrichment of P in surface soil horizons; (3) relative immobility of Fe and Al; and (4) transformation of Fe, Si, and Al in the parent material to secondary clay minerals and sesquioxides. Net mass losses of bases and Si are generally uniform with depth and substantial—in some cases approaching 100%; however, the rate of loss of each element differs markedly, causing the ranking of each by relative abundance to shift with time. Loss of Si from the sand fraction by dissolution and particle-size diminution, from ∼100% to 80% to 90% to <10% concurrently with an increase of AI in the organic sesquioxide and clay phases, to 10% and 50%, respectively, while only minor increases occur in the nonorganic sesquioxide phases.

The influence of slope aspect on soil weathering processes in the Springerville volcanic field, Arizona

Abstract A comparison was made between soils on north- and south-facing slopes of six cinder cones in the Springerville volcanic field (SVF), Arizona, in order to determine the influence of slope aspect on soil weathering processes. Twenty-four soil pedons were sampled on different aspects of six cinder cones. To control for the influence of slope on pedogenesis, all sample sites possessed slopes of 17±2°. Soil weathering processes were characterized by solum depth, texture, and Ca:Zr chemical weathering indices. Quartz and mica were used to identify eolian additions to the volcanic soils. Accelerated rates of weathering and soil development were found to occur in soils on south-facing slopes while no trend with aspect was found for eolian additions. Accelerated rates of weathering and soil development may influence cinder cone degradation and cone morphology.

Potassium fertility status of several Sonoran desert soils

There have been several recent reports of cotton lint yield response to potassium (K) fertilization from areas east and west of Arizona in the Cotton Belt. However, there is no documentation of the K status in southern Arizona within these Sonoran Desert soils. The physical, chemical, and mineralogical properties affecting the K status of 10 soils (six Entisols, three Aridisols, and one Mollisol) common to cultivated soils of the Sonoran Desert were studied to determine which soils might respond to K fertilization. The dominant clay minerals included mica, vermiculite, smectite, and palygorskite. All the soils contained some vermiculite but, none contained more than 5% vermiculite in the clay fraction