Abandoned agricultural fields (old fields) can be net carbon sinks, due to increased organic matter input from improved primary production, improved soil aggregation, and increasing nitrogen deposition. Most research on old field carbon sequestration has focused on the surface soil (10 to 30 cm). However, in temperate grassland, 58% of the soil organic carbon in the first meter is located between 20 and 100 cm. In 2001 and 2014, we conducted repeated sampling surveys of soil carbon pools in 21 old fields that range in age from 25 to 88 years after abandonment at Cedar Creek Ecosystem Science Reserve, MN. We collected aboveground biomass, litter, soil and roots down to 1 m depth and determined carbon and nitrogen.
On average the surface soil (0-20 cm) accumulated 27 ± 7 g carbon m-2 yr-1, whereas the subsurface soil (40-100 cm) lost 48 ± 11 g carbon m-2 yr-1. In total these old fields lost 2,700 kg per ha over 13 years. This deeper soil carbon loss can be a result of a disturbance legacy, decreased inputs, or increased losses. Subsoil carbon inputs are either from roots or dissolved organic carbon (DOC). Root carbon in soil profile was not correlated with soil carbon. A chronosequence survey in these old fields indicates that up to 89 % of surface soil carbon was lost during agricultural practice. We hypothesize that large soil carbon loss in the surface soil has strongly decreased DOC formation and hence transport to deeper soil depths. We hypothesize that mineralization in subsurface soil is higher than DOC input, leading to the decrease in soil carbon. Our results highlight that incorporating subsurface soil carbon into ecosystem carbon budgets is essential to gain a complete picture of ecosystem carbon changes. Secondly, agriculture might have a legacy that last hundreds of years and thirdly our current estimates of carbon sequestration associated with land abandonment may greatly overestimate the net carbon accumulation.