Weathering and Soils

Erosional and climatic effects on long-term chemical weathering rates in granitic landscapes spanning diverse climate regimes

C. Riebe, J.W. Kirchner, R. Finkel, Erosional and climatic
effects on long-term chemical weathering rates in granitic C. Riebe, J.W. Kirchner, R. Finkel, Erosional and climatic
effects on long-term chemical weathering rates in granitic

We used cosmogenic nuclide and geochemical mass balance methods to measure long-term rates of chemical weathering and total denudation in granitic landscapes in diverse climatic regimes. Our 42 study sites encompass widely varying climatic and erosional regimes, with mean annual temperatures ranging from 2 to 25 jC, average precipitation ranging from 22 to 420 cmyear 1, and denudation rates ranging from 23 to 755 tkm 2year 1. Long-term chemical weathering rates range from 0 to 173 tkm 2 year 1, in several cases exceeding the highest granitic weathering rates on record from previous work. Chemical weathering rates are highest at the sites with rapid denudation rates, consistent with strong coupling between rates of chemical weathering and mineral supply from breakdown of rock. A simple empirical relationship based on temperature, precipitation and long-term denudation rates explains 89–95% of the variation in long-term weathering rates across our network of sites. Our analysis shows that, for a given precipitation and temperature, chemical weathering rates increase proportionally with freshmaterial supply rates. We refer to this as ‘‘supply-limited’’ weathering, in which fresh material is chemically depleted to roughly the same degree, regardless of its rate of supply from breakdown of rock. The temperature sensitivity of chemical weathering rates is two to four times smaller than what one would expect from laboratory measurements of activation energies for feldspar weathering and previous inter-comparisons of catchment mass-balance data from the field. Our results suggest that climate change feedbacks between temperature and silicate weathering rates may be weaker than previously thought, at least in actively eroding, unglaciated terrain similar to our study sites. To the extent that chemical weathering rates are supply-limited in mountainous landscapes, factors that regulate rates of mineral supply from erosion, such as tectonic uplift, may lead to significant fluctuations in global climate over the long term.

A spheroidal weathering model coupling porewater chemistry to soil thicknesses during steady-state denudation

Fletcher, R.C., Buss, H.L., Brantley, S.L., 2006.Aspheroidal weathering
model coupling porewater chemistry to soil thicknesses during
steady-state denudation. Earth Planet. Sci. Lett. 244, 444–457.

Spheroidal weathering, a common mechanism that initiates the transformation of bedrock to saprolite, creates concentric fractures demarcating relatively unaltered corestones and progressively more altered rindlets. In the spheroidally weathering Rio Blanco quartz diorite (Puerto Rico), diffusion of oxygen into corestones initiates oxidation of ferrous minerals and precipitation of ferric oxides. A positive ΔV of reaction results in the build-up of elastic strain energy in the rock. Formation of each fracture is postulated to occur when the strain energy in a layer equals the fracture surface energy. The rate of spheroidal weathering is thus a function of the concentration of reactants, the reaction rate, the rate of transport, and the mechanical properties of the rock. Substitution of reasonable values for the parameters involved in the model produces results consistent with the observed thickness of rindlets in the Rio Icacos bedrock (≈2–3cm) and a time interval between fractures (≈200–300 a) based on an assumption of steady-state denudation at the measured rate of 0.01cm/a. Averaged over times longer than this interval, the rate of advance of the bedrock–saprolite interface during spheroidal weathering (the weathering advance rate) is constant with time. Assuming that the oxygen concentration at the bedrock–saprolite interface varies with the thickness of soil/saprolite yields predictive equations for how weathering advance rate and steady-state saprolite/soil thickness depend upon atmospheric oxygen levels and upon denudation rate. The denudation and weathering advance rates at steady state are therefore related through a condition on the concentration of porewater oxygen at the base of the saprolite. In our model for spheroidal weathering of the Rio Blanco quartz diorite, fractures occur every ∼250yr, ferric oxide is fully depleted over a four rindlet set in ∼1000yr, and saprolitization is completed in ∼5000yr in the zone containing ∼20 rindlets. Spheroidal weathering thus allows weathering to keep up with the high rate of denudation by enhancing access of bedrock to reactants by fracturing. Coupling of denudation and weathering advance rates can also occur for the case that weathering occurs without spheroidal fractures, but for the same kinetics and transport parameters, the maximum rate of saprolitization achieved would be far smaller than the rate of denudation for the Rio Blanco system. The spheroidal weathering model provides a quantitative picture of how physical and chemical processes can be coupled explicitly during bedrock alteration to soil to explain weathering advance rates that are constant in time.

Topographic control of soil microbial activity: a case study of denitrifiers

Florinsky, 1. V., S. McMahon, and D. L. Burton. 2004.
Topographic control of soil microbial activity: a case study of
denitrifiers. Geoderma 119:33-53.

Topography may affect soil microbial processes, however, the use of topographic data to model and predict the spatial distribution of soil microbial properties has not been widely reported. We studied the effect of topography on the activity of denitrifiers under different hydrologic conditions in a typical agroecosystem of the northern grasslands of North America using digital terrain modelling (DTM). Three data sets were used: (1) digital models of nine topographic attributes, such as elevation, slope gradient and aspect, horizontal, vertical, and mean land surface curvatures, specific catchment area, topographic, and stream power indices; (2) two soil environmental attributes (soil gravimetric moisture and soil bulk density); and (3) six attributes of soil microbial activity (most probable number of denitrifiers, microbial biomass carbon content, denitrifier enzyme activity, nitrous oxide flux, denitrification rate, and microbial respiration rate). Linear multiple correlation, rank correlation, circular–linear correlation, circular rank correlation, and multiple regression were used as statistical analyses. In wetter soil conditions, topographically controlled and gravity-driven supply of nutritive materials to microbiota increased the denitrification rate. Spatial differentiation of the denitrification rate and amount of denitrifying enzyme in the soil was mostly effected by redistribution and accumulation of soil moisture and soil organic matter down the slope according to the relative position of a point in the landscape. The N2O emission was effected by differentiation and gain of soil moisture and organic matter due to the local geometry of a slope. The microbial biomass, number of denitrifiers, and microbial respiration depended on both the local geometry of a slope and relative position of a point in the landscape. In drier soil conditions, although denitrification persisted, it was reduced and did not depend on the spatial distribution of soil moisture and thus land surface morphology. This may result from a reduction in soil moisture content below a critical level sufficient for transient induction of denitrification but not sufficient to preserve spatial patterns of the denitrification according to relief. Digital terrain models can be used to predict the spatial distribution of the microbial biomass and amount of denitrifying enzyme in the soil. The study demonstrated a feasibility of applying digital terrain modelling to investigate relations of other groups of soil microbiota with topography and the system ‘topography–soil microbiota’ as a whole.

Mudslide-caused ecosystem degradation following Wenchuan earthquake 2008

Ren, D., et al., 2009. Mudslide-caused ecosystem degradation following Wenchuan earthquake 2008. Geophysical Research Letters 36, L05401.

We have applied a scalable and extensible geo-fluid model that considers soil mechanics, vegetation transpiration and root mechanical reinforcement, and hydrological processes to simulate two dimensional maps of the landslides occurrence following the 2008 Wenchuan earthquake. Modeled locations and areas generally agree with observations. The model suggests that the potential energy of earth was lowered by 1.52×1015 J by these landslides. With this, the vegetation destroyed transfer ~235 Tg C to the dead respiring pool and transforms 5.54×10-2 Tg N into unavailable sediments pools and the atmosphere. The cumulative CO2 release to the atmosphere over the coming decades is comparable to that caused by hurricane Katrina 2005 (~105 Tg) and equivalent to ~2% of current annual carbon emissions from global fossil fuel combustion. The nitrogen loss is twice as much as that released by the 2007 California Fire (~2.5×10-2 Tg). A significant proportion of the nitrogen loss (14%) is in the form of nitrous oxide, which can affect the atmospheric ozone layer.

Influence of landscape position and vegetation on long-term weathering rates at the Hubbard Brook Experimental Forest, New Hampshire, USA

Nezat, C. A., J. D. Blum, A. Klaue, C. E. Johnson, and T. G. Siccama
(2004), Influence of landscape positions and vegetation on long-term
weathering rates at the Hubbard Brook Experimental Forest, New
Hampshire, USA, Geochem. Cosmochim. Acta, 68(14), 3065– 3078.

The spatial variability of long-term chemical weathering in a small watershed was examined to determine the effect of landscape position and vegetation. We sampled soils from forty-five soil pits within an 11.8-hectare watershed at the Hubbard Brook Experimental Forest, New Hampshire. The soil parent material is a relatively homogeneous glacial till deposited 14,000 years ago and is derived predominantly from granodiorite and pelitic schist. Conifers are abundant in the upper third of the watershed while the remaining portion is dominated by hardwoods. The average long-term chemical weathering rate in the watershed, calculated by the loss of base cations integrated over the soil profile, is 35 meq m2 yr1—similar to rates in other 10 to 15 ka old soils developed on granitic till in temperate climates. The present-day loss of base cations from the watershed, calculated by watershed mass balance, exceeds the long-term weathering rate, suggesting that the pool of exchangeable base cations in the soil is being diminished. Despite the homogeneity of the soil parent material in the watershed, long-term weathering rates decrease by a factor of two over a 260 m decrease in elevation. Estimated weathering rates of plagioclase, potassium feldspar and apatite are greater in the upper part of the watershed where conifers are abundant and glacial till is thin. The intra-watershed variability across this small area demonstrates the need for extensive sampling to obtain accurate watershed-wide estimates of long-term weathering rates.


Emmanual J Gabet, O J Reichman, and Eric W Seabloom (2003)
The Effects of Bioturbation on Soil Processes and Sediment Transport
Annual Review Earth Planet Science:249-73.

Plants and animals exploit the soil for food and shelter and, in the process, affect it in many different ways. For example, uprooted trees may break up bedrock, transport soil downslope, increase the heterogeneity of soil respiration rates, and inhibit soil horizonation. In this contribution, we review previously published papers that provide insights into the process of bioturbation. We focus particularly on studies that allowus to place bioturbation within a quantitative framework that links the form of hillslopes with the processes of sediment transport and soil production. Using geometrical relationships and data from others’ work, we derive simple sediment flux equations for tree throw and root growth and decay.

Estimating soil labile organic carbon and potential turnover rates using a sequential fumigation–incubation procedure

Zoua, X.M.; Ruanc,H.H.; Fua, Y.; Yanga, X.D.; Sha, L.Q. 2005. Estimating soil labile organic carbon and potential turnover rates using a sequential fumigation–incubation procedure.. Soil Biology & Biochemistry 37 :1923-1928.

Labile carbon is the fraction of soil organic carbon with most rapid turnover times and its oxidation drives the flux of CO2 between soils and atmosphere. Available chemical and physical fractionation methods for estimating soil labile organic carbon are indirect and lack a clear biological definition. We have modified the well-established Jenkinson and Powlson’s fumigation–incubation technique to estimate soil labile organic carbon using a sequential fumigation–incubation procedure. We define soil labile organic carbon as the fraction of soil organic carbon degradable during microbial growth, assuming that labile organic carbon oxidizes according to a simple negative exponential model. We used five mineral soils and a forest Oa horizon to represent a wide range of organic carbon levels. Soil labile organic carbon varied from 0.8 mg/g in an Entisol to 17.3 mg/g in the Oa materials. Potential turnover time ranged from 24 days in an Alfisol to 102 days in an Ultisol. Soil labile organic carbon contributed from 4.8% in the Alfisol to 11.1% in the Ultisol to the total organic carbon. This new procedure is a relatively easy and simple method for obtaining indices for both the pool sizes and potential turnover rates of soil labile organic carbon and provides a new approach to studying soil organic carbon.

Hurricane Effects on Soil Organic Matter Dynamics and Forest Production in the Luquillo Experimental Forest, Puerto Rico: Results of Simulation Modeling

Hurricane Effects on Soil Organic Matter Dynamics and Forest Production in the Luquillo Experimental Forest, Puerto Rico: Results of Simulation Modeling
Robert L. Sanford, Jr., William J. Parton, Dennis S. Ojima and D. Jean Lodge
Vol. 23, No. 4, Part A. Special Issue: Ecosystem, Plant, and Animal Responses to Hurricanes in the Caribbean (Dec., 1991), pp. 364-372

The forests and soils at Luquillo Experimental Forest (LEF), Puerto Rico, are frequently disturbed by hurricanes occurring at various frequencies and intensities. We have derived a forest version of the Century soil organic matter model to examine the impact of hurricanes on soil nutrient availability and pool sizes, and forest productivity in the tabonuco forest at Luquillo. The model adequately predicted aboveground plant production, soil carbon, and soil nitrogen levels for forest conditions existing before Hurricane Hugo. Simulations of Hurricane Hugo and of an historical sequence of hurricanes indicated a complex pattern of recovery, especially for the first 10 yr after the hurricanes. After repeated hurricanes, forest biomass was reduced, while forest productivity was enhanced. Soil organic matter, and phosphorus and nitrogen mineralization stabilized at higher levels for the LEF than for hurricane-free tabonuco forest, and organic soil phosphorus was substantially increased by hurricanes. Results from these simulations should be regarded as hypotheses. At present there is insufficient data to validate the results of hurricane model simulations.

Soil carbon stocks at regional scales

Milne, E., Powlson, D.S., Cerri, C.E.P. (Eds.), Soil carbon stocks at regional scales. Agric. Ecosyst. Environ., 122 (1), 13-25.

The appropriate management of soil organic carbon (SOC) is important at a range of scales. At the local scale, good management of SOC determines ecosystem and agroecosystem function, influencing (amongst other things) soil fertility and soil physical properties such as aggregate stability and water holding capacity. At the global scale, SOC management is important because of its role in the global carbon cycle and therefore, the part it plays in the mitigation or exacerbation of atmospheric levels of greenhouse gases (GHGs). Soil organic C is highly sensitive to changes in land use, with changes from native ecosystems such as forest or grassland to agricultural systems almost always resulting in a loss of SOC. Likewise, the way in which land is managed following land use change has also been shown to affect SOC stocks. We therefore have the opportunity in the future to adopt land use and land management strategies that lead to greater C storage in the soil, thereby improving soil fertility, minimising loss of C from soils to the atmosphere and potentially mitigating the effects of GHGs. Maximising this opportunity will require the formulation of policy at the national and sub-national scale.

Soil Studies in the El Verde Rain Forest

Some Physical and Chemical properties of tropical rain forest soils at El Verde, Puerto Rico, were investigated before and after treatments with gamma radiation from a 137Cs source and after mechanical defoliation of the vegetation. Flushes of soil ions were observed in soils after treatment; subsequently, ioic concentrations were lower than they were at the start. The profiles of cations indicated heterogeneous patterns with both poscolization and laterication active. Infiltration rates for the relatively undisturbed soils studied were unusually high (30 and 60 cm/hr) whenu associated with low bulk density. Lower infiltration rates and higher bulk-density values were associated with level areas rather than with slopes.
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