Soil Drainage and Forest Type Influences On Soil Organic Carbon Fractions In a New England Forested Watershed.

Soil organic carbon (SOC) fractions vary across forested watersheds among distinct soil drainage classes and vegetative communities.  Defining the continuum between labile and recalcitrant SOC in these systems is important for our understanding of biogeochemical cycling processes influencing carbon (C) and ecosystem health.  We conducted a plot scale field study across three different mineral soil drainage classes (moderately well drained – MWD; somewhat poorly drained – SWPD; poorly drained - PD) and in two dominant forest types (coniferous, broad-leaved deciduous) in a temperate forested watershed in eastern Maine.  Soil respiration (SR), soil temperature (Ts), and gravimetric soil moisture (SM) were measured as a monthly time series from June through November 2010.  Samples were simultaneously collected from the O and uppermost 5 cm of the mineral soil and analyzed for hot water extractable carbon (HWEC) to track temporal patterns of labile solid phase soil C.  In addition, SOC fractionation was conducted to determine operationally defined labile, stable and recalcitrant SOC fractions from various depths in adjacent quantitative pits in each plot.  Seasonal trends were evident in SR, Ts, SM, and HWEC.  Maximum values for SR and Ts were observed during July and August 2010, while maximum values for SM were observed in early June and late fall.  SWPD soils had a 31% and 7% higher mean SR compared to MWD and PD soils, respectively.  The deciduous forest type had higher mean SR compared to the coniferous forest type in MWD and SWPD soils, with the trend reversing in PD soils.  Evidence from this study suggests SWPD soils play an important role in landscape resilience to drought stress during the growing season.