Calcium Mineralogy in Forest Soil Profiles From Three Northeastern Sites Having a Wide Range in Total Calcium.

In the late 1980’s, compilations of watershed mass balance studies suggested that calcium (Ca) was the nutrient element most likely to be depleted from eastern U.S. forests over time. Subsequently, much research was initiated to improve estimates of mineral weathering flux, the weak link in the mass balance conclusions. However, there is still much uncertainty in the weathering estimates, and in the regional extent of Ca depletion. Recent studies have suggested that minerals such as apatite and allanite may contribute to Ca flux in excess of their trace amounts in forest soils. In addition, Ca-oxalate biominerals are known to be formed in forest soils and have been suggested to form an overlooked stock of this nutrient. In this study, a complete mineralogic accounting of the total Ca pool in forest soils was attempted. Three sites were selected to provide a range in total Ca concentrations: Sleepers River Research Watershed (SRRW) with high total Ca, Hubbard Brook Experimental Forest (HBEF) with moderate total Ca, and the Neversink watershed in the Catskills of NY with very low total Ca.  Using synchrotron based micro-xrd and electron probe micro analysis (EPMA), we have confirmed the presence of apatite and whewellite (Ca-oxalate monohydrate) in soils at HBEF and epidote in soils at SRRW.  We have also compared quantitative mineralogic inventory of Ca sources, obtained by EPMA, with total and exchangeable Ca concentrations, obtained via traditional chemical analyses.  Results highlight the importance of identifying the specific mineral phases to understand weathering rates and potential Ca depletion.