Soil Carbon Chronosequences from Post-Agricultural Forests in Western New England.

Using quantitative soil pits, we sampled chronosequences of post-agricultural northern hardwood forest soils in the Hopkins Memorial Forest; Williamstown, MA, to determine the amount of carbon lost during the period of agricultural use, as well as the rates at which C accumulates after abandonment. Chronosequences based on the time of abandonment were developed for the three main agricultural uses: cultivated cropland, pasture or hay, and woodlot. Active farms served as our theoretical zero time points and old growth stands in the region served as our likely maximum for C-accumulation. We then tested this chronosequence model throughout the three main physiographic provinces of the Berkshire-Taconic landscape: carbonate lowlands, Taconic uplands, and Berkshire highlands. Our data show a significant direct relationship between time since abandonment and carbon amount for the organic horizons (Oe and Oa) of cultivated as well as pastured or hayed plots but not for stands formerly used as woodlots. Likewise there was a significant direct relationship between C content and time for plowed horizons (0-20 cm) of cultivated ground, but not for the top 20 cm of mineral soils that were formerly under other agricultural land uses. Our best estimate suggests that cultivation reduced the C-content of plowed soils by 50% to a depth of 20 cm, and that complete recovery of the C-pool requires approximately 120 years. Management practices of post-settlement New England farms differ significantly from those used by modern farms. These methodological differences complicate efforts to quantify the recovery of carbon in the western New England landscape.