Carbon and Nitrogen Accumulation in Post-Agricultural Forest Soils of Western New England

The agricultural history of New England is reflected in the species composition, biomass, and productivity of the second-growth forests of the region. There are also effects of agricultural use on soil morphology, soil organic C (SOC) and N content, erosion status, and C stability, which can be specific to site-level characteristics or prior agricultural use. We conducted a survey of SOC amounts in chronosequences comprising 25 post-agricultural second-growth forests. Time since abandonment was estimated from tree ages, and the chronosequences were bounded by sites currently in agricultural use (n = 6) and minimally disturbed old forest (n = 4). At sites currently used for pasture, hay, or row crops, the median amount of SOC (to a depth of 55 cm) was 6.5 kg C m(-2) compared with 10.3 kg C m(-2) in the soils of the oldest forest stands. The chronosequences indicated that nearly all of the SOC accumulation occurred in the first century of afforestation. There were significant trends of SOC accumulation in the forest floor, upper mineral horizons (0-10 or 10-20 cm), and B horizons (20-55 cm) of soils that had been used for pasture, hay, or row crops but not for former woodlots. Multivariate analysis showed that years since agricultural abandonment and growing season degree days were the most important variables related to SOC content (68% sum of squares explained). There was a significant trend of N accumulation (1.3 g N m(-2) yr(-1)) in soils (to a depth of 55 cm) formerly used for cultivated crops but not for other former uses. Farms in this region were an important C sink during the last 150 yr; however, the capacity of former agricultural soils to gain SOC in the future is probably limited.