Weathering Reactions

In addition to the chemical and mineralogical composition of the rock, the nutrient supplying capacity of the geosphere is strongly influenced by weathering rates. The first stage of the weathering process generally involves physical weathering processes that reduce the particle size of the material with no change in the chemical and mineralogical composition of the primary minerals. Physical weathering is very important since it creates greater surface area upon which chemical weathering can act. Chemical weathering involves processes that result in changes in the chemical and mineralogical structure of a primary or secondary mineral. Chemical weathering rates are regulated primarily by mineral surface area, temperature, leaching intensity (availability of water), mineral composition, proton flux, solution pH, and chelate concentration. Chemical weathering often includes the formation of new secondary minerals (i.e., clay minerals) that impart many important characteristics to the soil, such as ion exchange and sorption capacity.

Mineral weathering rates are directly proportional to the mineral surface area exposed to weathering. Physical weathering processes are very important in breaking down the particle size of rocks resulting in an increase in surface area. It is also believed that surface coatings of organic matter or iron oxides may block reactive mineral surfaces and may impede weathering rates because diffusion of reaction products and reactants is hindered by the surface coating.

Climate is the primary determinant of weathering rates. Comparative studies indicate that weathering rates are a linear function of effective precipitation (i.e., precipitation minus evapotranspiration) and an exponential function of temperature (White and Blum, 1995). Thus, weathering rates are substantially greater in tropical rain forests compared with boreal forests.

Primary minerals have widely varying stabilities to chemical weathering in the soil environment. For silicate minerals, the type of linkage between silica tetrahedral constituents is the primary factor affecting its stability. The greater the degree of Si-Si linkages, the more stable the mineral is toward chemical weathering. The stability series of common soil minerals is as shown. Minerals at the top of the diagram are dominant in basic igneous rocks (e.g., basalt) and are relatively unstable due to few Si-Si linkages in their mineral structure. The minerals toward the bottom of the diagram are common in acid igneous rocks (e.g., granite) and are relatively stable due to multiple Si-Si linkages in their structure. As a result, minerals such as olivine quickly disappear with increased weathering while stable minerals such as quartz tend to accumulate. The higher weathering rates in basic igneous rocks result in greater nutrient availability compared to acid igneous rocks, everything else being equal.

The solution pH and proton flux are important factors regulating reaction rates. For most silicate minerals, dissolution rates show a minimum between pH 5 and 8 and increased rates above and below this range. This is due to the effect of H⁺ and OH^- in forming surface active complexes with cations at the mineral surface that destabilizes the internal cation bonds resulting in release of the cation. Since forest soils are generally acidic, processes that lower the pH below 5 will result in increased weathering rates. Any process that contributes protons to the soil solution will enhance weathering rates. Important sources of protons in forest soils include atmospheric deposition, excess cation uptake relative to anion uptake by biota, and biological production of organic acids and carbon dioxide. When plants take up more cations than anions, H⁺ is released (or HCO₃^- taken up) from the root to maintain charge balance within the plant and in the soil solution. This is responsible in part for acidification of the rhizosphere. Root exudation and microbial decomposition of organic matter lead to production of organic acids with an average pK_a due to carboxylic acid groups of approximately 4.5. Organic acids also contribute to enhanced weathering rates due to their chelating abilities. Carbon dioxide is produced by root and microbial respiration in the soil which lead to elevated concentrations of CO₂, often 10 to 50 times atmospheric levels. Carbon dioxide forms carbonic acid (pK_a=6.35) which is the dominant proton donor in many forest soils with pH values greater than 5.

Rates of chemical weathering are very slow relative to ion exchange, sorption/desorption, and organic matter mineralization, occurring on the scale of geologic time. Typical weathering rates of common soil minerals in laboratory dissolution experiments are on the order of 10^-12 to 10^-15 mol/cm²/s at common forest soil pH values (4.5-7). Laboratory weathering rates are difficult to extrapolate to field weathering rates. Field weathering rates estimated from cation denudation rates of watershed mass balance studies indicate weathering rates for entire catchments on the order of 0.1 to 1.5 kmol_c/ha/yr. At the Hubbard Brook Ecological Forest in New Hampshire, weathering is estimated to release 8.0, 4.6, 0.1 and 1.8 kg/ha/yr of Ca, Na, K, and Mg, respectively (Johnson et al., 1968). The actions of biota are believed to increase weathering rates from 3 to 10 times compared to abiotic systems (Drever, 1994).