Legacy impacts and recovery of δ15N, δ13C and C/N storage in soils due to historic land use

Physical and chemical modifications within soils have been proposed as a marker of the Anthropocene, as soils can preserve modifications due to past land use for long periods of time. Soils are the primary terrestrial reservoir of C and N and are especially important for sequestration and emission of C related to land use changes. The northeast US has a well-documented sequence of deforestation and reforestation related to land use changes following European settlement, yet the impact of land use and recovery on C and N stocks and isotopes is still poorly constrained. We analyze δ¹⁵N, δ¹³C, and C/N to evaluate changes to soil C and N related to historical land use across an Anthropocene chronosequence comprised of four land use classes that vary in terms of duration of disturbance and recovery time from past agricultural activity. Reforested soils show minimal difference in δ¹⁵N and δ¹³C and display no overall statistical relationship with abandonment length, while modern agricultural soils have higher δ¹⁵N values. Differences in total C and N between land use classes are more distinct, as SOC decreases and total N increases with longer land use duration. Historic agriculture increased C and N storage, and recently abandoned land still has the potential to act as a sink to store more organic C. In total, land use imparts clear changes to SOC and N stocks that persist long after abandonment, providing a distinct marker of anthropogenic activities. However, stable C and N isotopes of soils within reforested classes show only slight differences between land use classes, indicating shorter timescales of isotopic resetting of C and N signatures following abandonment.