Changes in soil chemistry following a watershed-scale application of wollastonite (CaSiO3) at Hubbard Brook, New Hampshire, USA. Chris E. Johnson, Charles T. Driscoll, and Young-Il Cho Syracuse University, Dept. of Civil & Environmental Engineering, Syracuse, NY 13244, USA Whole-Watershed Treatment With Wollastonite In October, 1999, Watershed 1 at the Hubbard Brook Experimental Forest in New Hampshire, USA, was amended with 850 kg/ha of Ca in the form of wollastonite (CaSiO3). The aim of this long-term experiment is to replace calcium believed to have been leached from the soil in the 20th century by acid rain. Wollastonite was used instead of lime or other more common Ca amendments because the natural geochemical source of Ca in the region is principally silicate minerals. Hubbard Brook Experimental Forest Hubbard Brook Experimental Forest, New Hampshire, USA: Annual Precipitation: 1295 mm; Temperature: -9 °C (Jan), 19 °C (July) Predominantly hardwoods (beech, maple, birch) Some conifers (red spruce, balsam fir) at higher elevations Spodosols (Typic Haplorthods) > Inceptisols (Typic Dystrochrepts) Acidic soils: pHw ~ 3.5 - 4.0 in O horizon Coarse-grained soils: loamy sands Recovery of Forest Ecosystems from Chronic Acidification Acidic deposition, primarily in the form of sulfuric and nitric acid, has resulted in the acidification of soils and surface waters of the northeastern United States, and many other regions of the world. There is a growing consensus that this acidification has resulted in the depletion of available calcium from many base-poor soils. Soil calcium depletion may partly explain the sluggish response of surface waters in the northeastern United States to recent decreases in acidic deposition, and may also be related to declining forest health in the region. This study extends results reported in Cho et al. (In Press) and is part of an ecosystem-level investigation of the response of vegetation, soils, fauna, and drainage waters to Ca amendment using wollastonite. Hypotheses: Wollastonite addition increases the exchangeable Ca in forest floor and upper mineral soil horizons. Increasing exchangeable Ca is balanced by a compensatory decline in exchangeable Al. Timing of soil chemical changes reflect a downward migration of Ca released from the added wollastonite. Soil Sampling: Samples collected from 75-100 sites in July, 1998 (pre-treatment), 2000, 2002, and 2006. The Oi and Oe horizons (L and F) were collected together as a single sample. After sampling the Oa (H) horizon, soil cores were collected from the upper 10-cm of mineral soil. Samples were air-dried and sieved (5-mm for O horizons, 2-mm for mineral soils). Oi+Oe samples were ground in a Wiley mill. Analyses and Response Variables: Exchangeable cations (Al, Ca, Mg, K, Na) were measured in 1 M NH4Cl extracts. Exchangeable acidity (assumed to be Al+H) was measured in 1 M KCl extracts. Exchangeable H computed by difference. Effective cation exchange capacity: CECe = Exch. Acidity + Ca + Mg + K + Na Effective base saturation: BSe = 100∙(Ca + Mg + K + Na)/CECe Soil pH measured in deionized water (pHw) and 0.01 M CaCl2 (pHs). “Total” concentrations of Al, Ca, Mg, K, Na were measured by ashing overnight (500 °C), then digesting the residue in concentrated HNO3. Soil Chemical Changes The goal of this experiment was to replace exchangeable Ca believed to have been depleted from the soil by decades of acid deposition. Our data suggest that the experiment was successful in this regard. Major findings include: Exchangeable Ca increased in all horizons after wollastonite application (Figure at right). Exchangeable acidity decreased in the Oi+Oe and Oa horizons after wollastonite application (Figure at right). Patterns in the increases in exchangeable Ca and decreases in exchangeable acidity reflect the progressive downward migration of Ca in the soil. Chemical changes were detected in the Oi+Oe horizon in 2000, the year after treatment, while changes did not occur until 2002 in the Oa horizon, and 2006 in the mineral soil (Figure at right). The increases in exchangeable Ca were not fully compensated by decreases in exchangeable Al and/or H (Figure below). As a result, CECe increased in the Oi+Oe and Oa horizons. Our data support the interpretations of Nezat et al. (2010), who proposed three stages to explain increases in stream water Ca concentration (Figure below right). The penetration of Ca into mineral soils that we observed after 2002 (Year 3) coincides with their “infiltration” stage, in which wollastonite-derived Ca is believed to reach the stream from soil sources. Mass Balance for Added Wollastonite The wollastonite loading for the watershed was 1015 kg Ca/ha. Measurements from collectors indicated that the wollastonite was fairly evenly distributed (figure below left). Measured “total” Ca pools in the O horizons and the upper mineral soil indicate: The increase in Ca due to wollastonite addition could be largely accounted for in the Oi+Oe horizons in the year after application (2000). (Figure below right) There has been a progressive downward migration of Ca in the soil, with the Oa horizon pool increasing in 2002 and the mineral soil pool increasing in 2006. (Figure below right) Of the 850 kg/ha of Ca added as wollastonite, approximately 250 kg/ha are no longer in the forest floor or the upper mineral soil (Table below). Possible fates include: uptake in forest vegetation, migration to mineral soil, and export as stream water. CECe Peters et al. (2004) Horizon “Total” 1998 Calcium (kg/ha) 2000 ± Std. Error 2002 2006 Oi + Oe 144 ± 10 989 ± 62 825 ± 52 620 ± 38 Oa 106 ± 14 119 ± 10 212 ± 19 156 ± 15 Upper Mineral Soil 91 ± 7 84 ± 11 97 ± 9 166 ± 15 Total 340 ± 18 1191 ± 65 1133 ± 65 942 ± 50 Change from Previous Sampling Year +851 -58 -191 Nezat et al. (2010) References Cho, Y., Driscoll, C.T., Johnson, C.E., and Siccama, T.G. In Press. Chemical changes in soil and soil solution after calcium silicate addition to a northern hardwood forest. Biogeochemistry. Nezat, C.A., Blum, J.D., and Driscoll, C.T. 2010. Patterns of Ca/Sr and 87Sr/86Sr variation before and after a whole watershed CaSiO3 addition at the Hubbard Brook Experimental Forest, USA. Geochim. Cosmochim. Acta 74:3129-3142. Peters, S.C., Blum, J.D., Driscoll, C.T., and Likens, G.E. 2004. Dissolution of wollastonite during the experimental manipulation of Hubbard Brook Watershed 1. Biogeochemistry 67: 309-329. Thanks! Funding: National Science Foundation: LTER and Ecosystems Programs. Infrastructure: U.S. Forest Service, Northern Research Station Sampling and Design: Tom Siccama, Ellen Denny. Sample Prep & Analysis: Mary Margaret Koppers.