Soil Carbon Quantities and Quality From Post-Agricultural Forests in Western New England.

Using quantitative pits, we sampled chronosequences of afforested soils in western New England to determine the amount of carbon lost during the period of agricultural use, as well as rates of C accumulation after agricultural abandonment. Chronosequences were developed for three former agricultural land uses: cultivated cropland, pastured or hayed fields, and woodlot. Active farms served as our theoretical zero time points, while old forests (>250 years), served as our likely C-maximum. Changes in C-quality between land uses and forest ages were established using laboratory incubations and particulate organic matter fractionation.
Our data showed a significant direct relationship between time since the abandonment of agricultural and carbon amount for the organic horizons (Oe and Oa) of formerly cultivated and pastured / hayed plots, but not for sites formerly used as woodlot. Likewise, there was a significant relationship between C content and time for the plowed horizons (0-20 cm) of previously cultivated ground but not for other former uses. Our results suggest that cultivation reduced the C-content of plowed soils by 35% to a depth of 20 cm, and that recovery of the C-pool requires ≈120 years. Modern cultivated soils respired C at a greater rate than pastured land, however differences in respiration between afforested plots were negligible. Cumulative respiration and respiration rate were greatest in soils from the oldest forests. At regional scales, other factors (topographic and climatic) may be better determinants of afforested soil C than former land use and forest age.