Mineral weathering in soil as a function of landscape position, vegetation and fungi at the Hubbard Brook Experimental Forest, New Hampshire.

Abstract

Mineral weathering provides nutrients to vegetation, neutralizes acid deposition, and controls soil water composition. In the northeastern USA, acid deposition has leached exchangeable calcium, an essential plant nutrient, from soil. This dissertation investigates factors that affect the replenishment of exchangeable calcium by mineral weathering, especially landscape position, vegetation type and fungi, at the Hubbard Brook Experimental Forest (HBEF), New Hampshire. Long-term mineral weathering rates, calculated from changes in soil chemical composition compared to parent material, are twice as rapid at higher elevations where conifers are abundant than at lower elevations where deciduous trees are dominant. The average weathering rate is 35 meq (Ca+Na+Mg+K) m<super> -2</super> yr<super>-1</super> - one-third the current rate of base cation loss from the watershed via stream flow. This confirms that the pool of exchangeable base cations in HBEF soil is being diminished. The relative availability of different Ca-bearing minerals to the plant-available pool was investigated. A sequential extraction method was developed to identify the following soil pools of Ca: exchangeable pool, easily-weathered minerals and weathering-resistant minerals. The weathering of apatite [Ca₅(PO₄)₃(F,Cl,OH)], a trace mineral in HBEF soil, was found to account for &sim;20% of Ca loss from HBEF during the last 14,000 years. To explore the distribution of apatite in soils across the northeastern USA, the sequential extraction procedure was performed on several types of soil parent material collected from 18 locations. Apatite, identified by a Ca:P ratio of 5:3 in a 1M HNO₃ extract, is present in soils developed on crystalline-silicate rocks. Soils developed on sedimentary rocks do not contain appreciable amounts of apatite. Fungal response to Ca- and P-bearing minerals in soil was also examined. Mesh bags containing apatite or wollastonite (CaSiO₃) were buried in HBEF soils for two growing seasons. Fungal biomass, quantified as phospholipid fatty acid content, increased in bags containing apatite beneath beech stands. Fungi in a mixed spruce-fir stand showed no response to either Ca-bearing mineral. Fungal biomass in sugar maple decreased in wollastonite-amended bags relative to a control.