Does conversion of forest to agricultural land change soil carbon and nitrogen? a review of the literature

Soil carbon is a large component of the global carbon cycle and its management can significantly affect the atmospheric CO2 concentration. An important management issue is the extent of soil carbon (C) release when forest is converted to agricultural land. We reviewed the literature to assess changes in soil C upon conversion of forests to agricultural land. Analyses are confounded by changes in soil bulk density upon land-use change, with agricultural soils on average having 13% higher bulk density. Consistent with earlier reviews, we found that conversion of forest to cultivated land led to an average loss of approximately 30% of soil C. When we restricted our analysis to studies that had used appropriate corrections for changes in bulk density, soil C loss was 22%. When, from all the studies compiled, we considered only studies reporting both soil C and nitrogen (N), average losses of C and N were 24% and 15%, respectively, hence showing a decrease in the average C: N ratio. The magnitude of these changes in the C: N ratio did not correlate with either C or N changes. When considering the transition from forest to pasture, there was no significant change in either soil C or N, even though reported changes in soil C ranged from -50% to +160%. Among studies that reported changes in soil N as well as soil C, C: N ratios both increased and decreased, with trends depending on changes in system N. Systems with increasing soil N generally had decreased C: N ratios, whereas systems with decreasing soil N had increased C: N ratios. Our survey confirmed earlier findings that conversion of forest to cropland generally leads to a loss of soil carbon, although the magnitude of change might have been inflated in many studies by the confounding influence of bulk-density changes. In contrast, conversion of forest to uncultivated grazing land did not, on average, lead to loss of soil carbon, although individual sites may lose or gain soil C, depending on specific circumstances, such as application of fertiliser or retention or removal of plant residues.