Amount and stability of recent and aged plant residues in degrading peatland soils

- Miscanthus cropping in organic soils leads to an accumulation of C4-Carbon.
- The accumulation rate of Miscanthus-C is similar to those found in mineral soils.
- Accumulating C4-C does not compensate C loss from peat decomposition.
- Decomposing SOC of drained organic soils comprises young and old carbon.
- Young carbon decomposes at higher rates than old peat derived carbon.

Peatlands store large amounts of soil organic carbon (SOC). Drainage, required for agriculture, aerates these organic soils and triggers rapid peat decomposition. In turn, cultivation of organic soils is also accompanied by input of young organic carbon (YOC) from plant residues. The extent to which YOC inputs compensate for oxidative peat loss is unknown. Furthermore, the lability of YOC in organic soils introduced by cultivation has never been examined. Here we studied the amount and lability of YOC in two adjacent drained organic soils by a combined 13C and 14C approach. Soils have been under intensive arable use for several decades and were both cultivated, inter alia, with corn, a C4 plant. In 1995, one soil was converted from annual cropping to permanent cultivation with Miscanthus x giganteus, another C4 plant, while the other was converted to permanent C3 grassland in 2009. Using δ13C signatures, we analysed the fractions of C4 derived carbon in the soil and in CO2, during one month of soil incubation. This enabled us i) to estimate C4-C accumulation in both soils, and ii) to assess the lability of C4-C carbon that accumulated either at least five years prior to sampling (current grassland soil) or until sampling (current Miscanthus soil). The fraction of C4-C derived SOC in the Miscanthus soil was 0.19 ± 0.024 in the top 30 cm, corresponding to an accumulation rate of 1.6 ± 0.2 t C4-C ha−1 yr−1. This accumulation rate is in the range of rates found for fertile mineral soils cultivated with Miscanthus. Yet, this C4-C accumulation rate is below average C-losses of agriculturally used organic soils. The grassland soil contained a smaller fraction of 0.08 ± 0.002 C4-C in SOC. The rates of total CO2 emitted from the two soils did not differ, but the fraction of CO2 derived from C4-C was significantly higher in the Miscanthus soil (0.53 ± 0.05) than in the grassland (0.29 ± 0.04) soil. Hence, in both soils YOC was more labile than bulk SOC. The ratio between the fraction of decomposing C4-C and C4-C in SOC was the same for both soils indicating a similar lability of currently accumulated and aged C4-C. In both soils, the 14C age of emitted CO2 was younger than that of SOC, confirming an increased lability of YOC over old SOC.