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Pyrogenic carbon (PyC) derives from incomplete combustion of organic matter and is ubiquitous in terrestrial and aquatic systems. Most PyC is inherently more stable against decomposition than plant residues, and PyC therefore forms an important component of the global carbon (C) cycle. During the Holocene, about 436 Pg organic C accumulated in northern peatlands, and we hypothesize that PyC may contribute substantially to that C stock. We studied 70 samples from 19 intact and degraded European peatland sites and analyzed their PyC content by 13C nuclear magnetic resonance spectroscopy and molecular modeling and peat age and accumulation by radiocarbon dating. Classification of a peatland as either intact or degraded was based on the comparison between apparent and expected long-term C accumulation rates. On average, PyC amounted for 13·5% of soil C across sites, and accounted for up to 50% at single sites. The amount of PyC increased significantly with peat age. Degraded peatlands had lost approximately 56 kg C m�2, half of their former C stock. However, degraded peat had higher PyC contents than intact one. Selective enrichment of PyC during both peat build-up and decomposition seems to be an important factor fostering PyC accumulation. Assignment of our results to peatlands of the northern hemisphere, stratified by age, revealed an estimated PyC stock of 62 (±22) Pg. Our estimate indicates a substantial and hitherto unquantified contribution of northern peatlands to global PyC storage. © 2017 The Authors. Land Degradation & Development Published by John Wiley & Sons Ltd.

Major common production activities on organic soils require drainage, which results in the loss of important ecosystem services such as carbon storage. This paper provides an overview of the farming situation on organic soils in a western region of Switzerland characterized by intensive vegetable farming and therefore by high opportunity costs of adopting sustainable practices on these soils. Based on a review of the academic and grey literature and interviews with regional experts, we find that the main challenges to a change in management practices include the profitability of the current land use, the difficult economic environment of farmers, the cultural background associated with the region, and the absence of systematic data on soil properties. We provide a comparative overview of policy options that could promote sustainable use of organic soils, and present the diverse economic, environmental, and social implications of potential future scenarios of the development on these soils, namely pursuing the current land use versus adopting peat-preserving land use. Based on a basic economic analysis, we find that considering the carbon benefits of preserving the soils, current offsetting-carbon policies cannot compensate the opportunity cost of switching land use on organic soils used for intensive vegetable farming. With the price of carbon offsetting options, only up to the half of the opportunity cost would be covered. We therefore stress the need for a long-term vision of the management decision on these soils and for eliciting society’s willingness to invest in preserving organic soils.

Drainage and land-use are key factors that trigger CO 2 emissions from cultivated organic soils. Drainage enhances aeration, and land-use might affect peat decomposition due to the input of fresh organic matter (FOM). The effect of FOM addition on peat decomposition of agriculturally used organic soils has seldom been quantified experimentally. In this study, we i) incubated soil samples taken from three adjacent former peatland sites that were drained and managed as cropland, perennial grassland and forest, respectively, and measured CO 2 release over 16 months at 20 °C. In a second experiment, ii) we incubated soil samples from the same three sites with and without adding corn straw as FOM (2% w/w) at 20 °C over three weeks. The 13 C and 14 C signatures of soil organic carbon (SOC) and emitted CO 2 enabled us to apportion the amount of decomposed corn, as well as to estimate relative effects of corn addition on the decomposition of SOC from old peat (SOC old ) and from young soil organic carbon (SOC young ). In the first experiment, samples lost between 0.03 and 0.09 mg CO 2 -C mg SOC − 1 in the order forest > grassland > cropland. The higher 14 C values of the emitted CO 2 vs. that of SOC indicated that SOC young was more easily decomposable. FOM addition induced negative (− 14.8 ± 5.2%), positive (+ 14.9 ± 4.6%) and neutral priming (− 14.2 ± 30.0%) of SOC decomposition in the forest, grassland and cropland samples, respectively. Most importantly, we find that the relative contribution of SOC old to the overall CO 2 release consistently decreased after FOM application, whereas decomposition of SOC young was rather stimulated. The latter finding is in line with previous studies on intact peat. Our results show that young and old C pools in managed organic soils respond differently to the addition of fresh plant residues, and FOM addition can effectively reduce the decomposition of old peat. Hitherto negative priming was never reported for agriculturally used organic soils and it might be caused by the overall poor decomposability of old peat.

Drained peatlands are CO2 hotspots and lose important soil functions over time. In contrast to mineral soils, their high carbon density induces long lasting and high emissions. These emissions can be estimated using various approaches which cover different system boundaries in time and space. Here we compare 5 years flux measurements from manual chambers with a soil profile based method to estimate carbon losses from two temperate fens under different management intensity drained at the end of the 19th century. According to the flux measurements, both grassland sites currently lose significant amounts of carbon as CO2 in the order of 7.1 and 9.1 t CO2-C ha−1a−1 when managed non-intensively or intensively, respectively. Profile based estimates, which make use of the difference in ash concentration along the soil profile, reveal a total of 284 and 619 t C ha−1 since the onset of drainage. These substantial losses are accompanied by a sharp decrease in peat quality as measured by NMR spectroscopy, confirming that a large part of former topsoil material is already mineralized. On average, the profile based estimate converts to smaller annual loss rates of 2.2 (non-intensive) and 4.8 t CO2−C ha−1a−1 (intensive) management. Our data, together with historical flux measurements at this site, provide evidence that peat decomposition rates increased over time, despite declining organic matter quality. We suggest that higher management intensities (i.e., higher fertilization and changes in carbon export from the field), including drainage, and increased mean annual temperature may be important factors for higher emissions today. These two methods are complementary in terms of time horizon and system boundary and, in conjunction, confirm the long-term emission potential of temperate drained organic grassland soils.

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 13 C and 14 C 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 δ 13 C signatures, we analysed the fractions of C4 derived carbon in the soil and in CO 2 , 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 CO 2 emitted from the two soils did not differ, but the fraction of CO 2 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 14 C age of emitted CO 2 was younger than that of SOC, confirming an increased lability of YOC over old SOC.