A sequential extraction to determine the distribution of apatite in granitoid soil mineral pools with application to weathering at the Hubbard Brook Experimental Forest, NH, USA

Mineral weathering in soils is an important source of many nutrients to forest ecosystems. Apatite, a Ca phosphate mineral, occurs in trace amounts in virtually all igneous and metamorphic rocks and is often found as small mineral inclusions in more weathering-resistant silicate minerals. To better understand the distribution of apatite in soils and its exposure to soil solutions, a sequential extraction method was developed to selectively dissolve minerals from soils so that the amount of apatite in contact with soil solutions versus that armored by silicate minerals could be quantified. The use of three molarities of HNO3 (0.01, 0.1 and 1 M) at three temperatures (0, 10, or 20 degrees C) was explored and it was found that apatite congruently dissolved in 1 M HNO3 at all three temperatures, but did not completely dissolve in weaker HNO3 solutions. Soil horizons, glacial till (i.e., soil parent material), and individual minerals separated from till collected from the Hubbard Brook Experimental Forest (HBEF), NH, were subjected to a 4-step sequential extraction. Chemical analyses of the extracts indicate that 1 M NH4Cl (pH 7; 20 degrees C) removes exchangeable ions, 1 M HNO3 at 20 degrees C primarily dissolves apatite in contact with solutions, 1 M HNO3 at 200 degrees C primarily dissolves biotite and chlorite (and apatite armored by them), and a mixture of concentrated HNO3, HCl, and HF at 200 degrees C dissolves the more refractory minerals including muscovite, alkali feldspar, plagioclase feldspar and quartz (and apatite armored by these minerals). This extraction method was applied to soil profiles from HBEF to demonstrate that it could be used to interpret the abundance of apatite and other minerals as a function of depth. Approximately 70% of the apatite in the HBEF soil parent material is exposed to soil solutions, the remaining 30% is armored in more weathering-resistant micas and feldspars. In the upper soil horizons, the only apatite that has not been weathered from the soil occurs as inclusions in micas and feldspars and thus the rate of dissolution of apatite in weathered soil horizons is controlled by silicate mineral dissolution. (C) 2007 Elsevier Ltd. All rights reserved.