PLANT SUCCESSION AND SOIL CHEMICAL WEATHERING ON POST-GLACIAL LANDSCAPES, SE ALASKA

Southeast Alaska exhibits one of the fastest rates of glacier retreat, plant succession, and soil development in the world. Previous soil chronosequence studies have shown a predictable pattern of soil formation from Entisols to Spodosols and plant succession from pioneer to climax species 270 years after moraine deposition. However, little is known about plant successional controls on soil chemical weathering on post-glacial landscapes. This study aims to elucidate the connection between plant succession and soil chemical weathering in response to climate change in Southeast Alaska. To accomplish this goal a soil chronosequence was established on the moraines of the Herbert Glacier valley, an extension of the Juneau Icefield northwest of the state capital. Seven pedons were sampled and morphologically described; the proportion of different plant species was estimated and recorded at each site. Moraine age was constrained using tree cores. Soil samples were analyzed for elemental composition (XRF), isotopic and C/N composition (IRMS), and Fe and Al enrichment (DCB, ammonium oxalate, and sodium pyrophosphate extractions). Preliminary results show accumulation of carbon and decrease in pH over time consistent with increasing soil development. XRF data will be used in the CIA and WISP chemical weathering indices to establish degree of cation depletion in soils over time. These data will be compared to the plant successional sequence to determine correlation. We expect to see greater rates of weathering early in the chronosequence as Sitka alders are N-fixing and in the process produce nitric acid. Later in the chronosequence we expect to see rates of chemical weathering taper off as alders are replaced by Sitka spruce and Western hemlock. Drawing connections between soil processes and plant succession will provide a greater understanding of the ecosystem and geochemical dynamics as climate change leads to glacier retreat and exposure of new landscapes.