Peat decomposability in managed organic soils in relation to land use, organic matter composition and temperature

Abstract. Organic soils comprise a large yet fragile carbon (C) store in the global C cycle. Drainage, necessary for agriculture and forestry, triggers rapid decomposition of soil organic matter (SOM), typically increasing in the order forest < grassland < cropland. However, there is also large variation in decomposition due to differences in hydrological conditions, climate and specific management. Here we studied the role of SOM composition on peat decomposability in a variety of differently managed drained organic soils. We collected a total of 560 samples from 21 organic cropland, grassland and forest soils in Switzerland, monitored their CO₂ emission rates in lab incubation experiments over 6 months at two temperatures (10 and 20 °C) and related them to various soil characteristics, including bulk density, pH, soil organic carbon (SOC) content and elemental ratios (C / N, H / C and O / C). CO₂ release ranged from 6 to 195 mg CO₂-C g⁻¹ SOC at 10 °C and from 12 to 423 mg g⁻¹ at 20 °C. This variation occurring under controlled conditions suggests that besides soil water regime, weather and management, SOM composition may be an underestimated factor that determines CO₂ fluxes measured in field experiments. However, correlations between the investigated chemical SOM characteristics and CO₂ emissions were weak. The latter also did not show a dependence on land-use type, although peat under forest was decomposed the least. High CO₂ emissions in some topsoils were probably related to the accrual of labile crop residues. A comparison with published CO₂ rates from incubated mineral soils indicated no difference in SOM decomposability between these soil classes, suggesting that accumulation of recent, labile plant materials that presumably account for most of the evolved CO₂ is not systematically different between mineral and organic soils. In our data set, temperature sensitivity of decomposition (Q₁₀ on average 2.57 ± 0.05) was the same for all land uses but lowest below 60 cm in croplands and grasslands. This, in turn, indicates a relative accumulation of recalcitrant peat in topsoils.