Mineral stabilization slows losses of peatland carbon following long-term drainage for agriculture

Cultivated peatlands are a major CO2 emission source, but the processes that regulate the decomposition of drained peat are debated, especially as drained peat becomes increasingly shallow. Many cultivated peatlands are underlain by a mineral soil layer. Surface subsidence, oxidation and tillage reduce the peat thickness, which intermixes peat with minerals as peat is lost. A key question is whether this mixing can reduce emissions by making soil organic carbon (SOC) more stable by transforming particulate organic carbon (POC) - which dominates the carbon stock in deep drained peatlands and is readily accessible to microbial decomposers - into mineral associated organic carbon (MAOC), which is less accessible to decomposers. To explore this question, we surveyed ten sites across the East Anglian Fens in England, a once extensive (~3,900 km2) peatland landscape that has been drained for cultivation since the mid-seventeenth century. We used variation in cultivation to establish a peat loss gradient to evaluate how topsoil SOC (0-40 cm) and its forms change as peat is lost. As the peat was lost, the soils became rich in minerals (rising from 46% to 88% silt+clay content), resulting in an initial 11-fold rise in newly-formed MAOC, but a monotonic decline and near-total loss of POC. POC turnover times were 3,158 ± 62 yrs, indicative of peat, and POC was always older than MAOC; consequently, microbially-processed peat along with gradual contributions of recently fixed carbon were sources of MAOC. A four-month laboratory incubation showed that the MAOC:POC ratio was negatively correlated with respiration. We conclude that long-term carbon retention via MAOC formation has the potential to reduce carbon loss from degraded, mineral-mixed peatlands. However, because this MAOC pool is itself vulnerable to loss under continued agricultural drainage, this mechanism is expected to slow rather than halt long-term soil carbon loss from drained peatlands.