Development of carbon and nitrogen stable isotope abundances in afforested soil profiles of abandoned cropland over five decades

Afforestation is used as a climate change mitigation measure and for reduction of nitrate losses to ground- and surface water. Temporal development of soil carbon (C) storage and nitrogen (N) retention since conversion from cropland to forest are not well understood. Stable isotope composition provides footprints of soil processes and can be utilized to give insight on soil C and N processing in a changing soil environment from cropland to forest. With this in mind, the present study investigates vertical and temporal development of soil δ¹³C and δ¹⁵N of the forest floor (FF) and underlying mineral soil profiles in an afforestation chronosequence. The chronosequence is confined to four oak forest stands established in 1970, 1977, 1988, and 2009 that have been sampled in 1998, 2011, and 2022 in the same plots resulting in an age span from forest age 2 to 52 years plus an additional pre-conversion cropland and a 200-year-old forest serving as an old growth reference. The samples used were collected in six soil depths including the FF to a maximum of 50 cm depth and were analyzed for C, N, and their stable isotope composition. Over time, the forest soil develops a natural stable isotope gradient with depth for both C and N. The cropland soil has the highest N content and exhibited a high and homogeneous δ¹⁵N through 40 cm soil depth (8.2 - 9.3 ‰). Despite a continuous C3 plant cover, a small decrease in δ¹³C from input plant residue can be distinguished after afforestation, resulting in a depleted FF compared to the cropland soil. 10 years after afforestation a soil δ¹³C and δ¹⁵N profile has developed with a depleted signal in the FF and the top 5 cm of the mineral soil. The depth gradients develop further over time towards the old growth reference. Both gradients develop as a result of a new isotopically depleted FF and following depletion of the top layers of the mineral soil due to mixing of new organic matter. From these footprints, an accumulation of new C in the mineral soil can be hypothesized despite another study having shown this specific forest’s mineral soil to be a C source to the atmosphere during the initial 40 years after afforestation. Additionally, a degree of C turnover is apparent from the steady state of the C isotope composition at the old growth reference where in most cases there is no significant difference in δ¹³C between sampling campaigns at either depth or between the three topsoil layers of the mineral soil at either campaign. Finally, only minimal amounts of N may have been lost via denitrification, as this process is typically associated with significant fractionations that would have resulted in soil δ¹⁵N enrichment, which is not observed in the top mineral soil.