Apatite weathering

A more direct approach to study EM weathering is the use of tracer elements as used by Blum et al. (2002). They examined calcium uptake by different tree species in the Hubbert Brook Experimental Forest in northeastern USA. They used the fact that the different calcium sources (atmospheric deposition, calcium silicate weathering and apatite weathering) have different Ca/Sr ratios. In the ecosystem, strontium is believed to behave in a similar fashion as calcium (Aberg et al., 1990). Atmospheric deposition, calcium silicate minerals and soil water have similar Ca/Sr ratios (120-300), but apatite has a much higher Ca/Sr ratio (2200-2920). 

We recognize that the addition of apatite grains to the soil, even in bags, could have an effect on the proximal biological activity (Wallander, 2000). Moreover, despite our SEM observations confirming losses of mineral masses recorded for apatite, it is still difficult to conclusively establish the contribution of rhizosphere processes on apatite weathering as long as this effect is not quantified. 

Essentially all native soil P originates from apatitic minerals. As apatites weather, new minerals are formed, which are relatively soluble, and make up the labile P pool, vdiich replenishes P in soil solution. Many less soluble forms are also formed that comprise a nonlabile pool, which is a large but f rly inert storehouse for P. 

Banfif.ld, J. F. Eggleton, R. A. 1989. Apatite replacement and rare earth mobilization, fractionation, and fixation during weathering. Clays and Clay Minerals, 37, 113-127. 

Tazaki, K., Fyee, W. S. Djssanayake, C. B. 1987, Weathering of apatite under extreme conditions of leaching. Chemical Geology, 60, 151-162. 

The Hostrock and Backfill Material. Most crystalline igneous rocks, including granite and gneiss, are composed of a comparatively small number of rock forming silicate minerals like quartz, feldspars (albite, microcline, anorthite etc.) micas (biotite, muscovite) and sometimes pyroxenes, amphiboles, olivine and others. Besides, there is a rather limited number of common accessory minerals like magnetite, hematite, pyrite, fluorite, apatite, cal cite and others. Moreover, the weathering and alteration products (clay minerals etc.) from these major constituents of the rock would be present, especially on water exposed surfaces in cracks and fissures. 

Blum, J. D., Klaue, A., Nezat, C. A. et al. (2002). Mycorrhizal weathering of apatite as an important calcium source in base-poor forest ecosystems. Nature, 417, 729-31. 

Arsenic in soilds has been fractionated by Jackson s T28) procedure for soil phosphorus (15. 27). In this laboratory, a modification of Jackson s procedure is being used on sediment solids. A series of 1 molar solutions of NH4CI, NH4OH, acid ammonium oxalate (29) and HCl are used in sequence. The chloride fraction, or exchangeable fraction, contains weakly adsorbed, coulombically bound arsenic. The hydroxide fraction, contains iron and aluminum arsenate precipitates and surface precipitates (i.e. adsorbed arsenic species having both chemical and coulombic bonding to oxide surfaces). The oxalate, or reductant soluble fraction, contains arsenic occluded in amorphous weathering products. The acid, or calcium, fraction contains arseno-apatites. 

Blum et al. (2002) and Probst et al. (2000) have found, in areas similarly cation-depleted by acid deposition, that a considerable proportion of calcium released by weathering came from apatite dissolution. This apatite was utilized directly by ectomycorrhizal tree species (spruce and fir) bypassing the soil exchange complex. In the Blum et al. (2002) study, ectomycorrhizal fungi associated with the roots of conifers provided 95% of the calcium found in the foliage of the trees and 35% of the Ca leaving the mixed conifer hardwood watershed in stream water. 

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