Soil organic matter fractions in experimental forested watersheds

Recent concerns about climate change and atmospheric greenhouse gas concentrations have demonstrated the importance of understanding ecosystem C source/sink relationships. Soil organic matter fractionation was carried out in three paired, forested watershed sites where one of each watershed pair represented a different ecosystem perturbation. The perturbations were 8 years of experimental N amendments at the Bear Brook Watershed in Maine (BBWM), a 50 year old intense wildfire and subsequent regeneration at Acadia National Park (ANP), and a 17 year old whole-tree harvest at the Weymouth Point Watershed (WPW). At each site, mineral soils were sampled by uniform depth increments. Mineral soil (< 2 mm) was separated into light, occluded light, and heavy density fractions by floatation in NaI solution (1.7 g cm(-3)). Mineral soil (< 2 mm) was also separated into particle-size fractions of sand (2.0 to 0.05 mm), silt (0.05 to 0.002 mm), and clay (< 0.002 mm) by wet sieving and centrifugation. Whole soils, and density and particle-size fractions were analyzed for total C and N. Both fractionation schemes showed that all soil organic matter fractions had lower C/N ratios as a result of N enrichment at BBWM. At ANP, soil organic matter fractions generally had lower C/N associated with the wildfire and subsequent shift from softwood to hardwood regeneration. Few significant whole soil and soil organic matter fraction differences were associated with the whole-tree harvest. Within watershed pairs, both density and particle-size fractionation techniques usually indicated similar responses. Soil organic matter fractionation results indicated that there were no consistent shifts in fraction distributions in response to perturbation that were consistent across all paired watershed study sites.