The dynamics of soil OC and N after conversion of forest to cropland

In this study, we investigated the dynamics of soil organic carbon (OC) and nitrogen (N) following the conversion of forest to cropland. The cropland had been converted from adjacent forest areas 4, 50, or 100 years previously. Our specific objectives were 1) to determine the dynamics of OC and N in density fractions and of OC and N derived from forest and cropland and 2) to examine the contributions of these changes to changes in total OC and N. Conversion to cropland led to rapid losses of soil OC and N. The decreases in total soil OC and N in the 0-10 cm depth were mainly due to decreases in light-fraction OC and N. In the 10-20 cm depth, the decreases in total soil OC and N were determined by the loss of OC and N from both the light- and heavy-fractions. The losses in forest-derived OC and N were larger than the gains in crop-derived OC and N. The predicted losses of forest-derived OC and N in both depths were almost 2 times the losses of total soil OC and N. The mean residence times of forest-derived OC and N were shorter than those of total soil OC and N. Our findings indicated that the dynamics of soil OC and N after the conversion of forest to cropland were dominated by the losses of forest-derived and light-fraction OC and N.