Extending the SUSI peatland simulator to include dissolved organic carbon formation, transport and biodegradation - Proper water management reduces lateral carbon fluxes and improves carbon balance.

Drainage intensity and forest management in peatlands affect carbon dioxide (CO2) emissions to the atmosphere and export of dissolved organic carbon (DOC) to water courses. The peatland carbon (C) balance results from a complex network of ecosystem processes from where lateral C fluxes have typically been ignored. Here, we present a new version of the SUSI Peatland simulator, the first advanced process-based ecosystem model that compiles a full C balance in drained forested peatland including DOC formation, transport and biodegradation. SUSI considers site, stand and terrain characteristics as well as the interactions and feedbacks between ecosystem processes and offers novel ways to evaluate and mitigate adverse environmental impacts with thorough management planning. Here, we extended SUSI by designing and parameterizing a mass-balance based decomposition module (ESOM) based on literature findings and tested the ESOM performance against an independent dataset measured in the laboratory using peat columns collected from Finland, Estonia, Sweden and Ireland. ESOM predicted the CO2 emissions and changes in DOC concentrations with a reasonable accuracy for the peat columns. We applied the new SUSI for drained peatland sites and found that reducing the depth to which ditches are cleaned by 0.3 m decreased the annual DOC export by 34 (17 %), 29 (19 %) and 7 (5 %) kg ha-1 in Finland, Estonia and Sweden, respectively, using typical ditch spacing for these countries. Correspondingly, site annual C sink increased by 305, 409 and 32 kg ha-1 in Finland, Estonia and Sweden, respectively. Our results also indicated that terrain slope can markedly alter the water residence time and consequently DOC biodegradation and export to ditches. We conclude that DOC export can be decreased and site C sink increased by reducing the depth to which ditches are cleaned or by increasing the ditch spacing.

Variation in carbon and nitrogen concentrations among peatland categories at the global scale.

Peatlands account for 15 to 30% of the world's soil carbon (C) stock and are important controls over global nitrogen (N) cycles. However, C and N concentrations are known to vary among peatlands contributing to the uncertainty of global C inventories, but there are few global studies that relate peatland classification to peat chemistry. We analyzed 436 peat cores sampled in 24 countries across six continents and measured C, N, and organic matter (OM) content at three depths down to 70 cm. Sites were distinguished between northern (387) and tropical (49) peatlands and assigned to one of six distinct broadly recognized peatland categories that vary primarily along a pH gradient. Peat C and N concentrations, OM content, and C:N ratios differed significantly among peatland categories, but few differences in chemistry with depth were found within each category. Across all peatlands C and N concentrations in the 10-20 cm layer, were 440 ± 85.1 g kg-1 and 13.9 ± 7.4 g kg-1, with an average C:N ratio of 30.1 ± 20.8. Among peatland categories, median C concentrations were highest in bogs, poor fens and tropical swamps (446-532 g kg-1) and lowest in intermediate and extremely rich fens (375-414 g kg-1). The C:OM ratio in peat was similar across most peatland categories, except in deeper samples from ombrotrophic tropical peat swamps that were higher than other peatlands categories. Peat N concentrations and C:N ratios varied approximately two-fold among peatland categories and N concentrations tended to be higher (and C:N lower) in intermediate fens compared with other peatland types. This study reports on a unique data set and demonstrates that differences in peat C and OM concentrations among broadly classified peatland categories are predictable, which can aid future studies that use land cover assessments to refine global peatland C and N stocks.