Effect of drainage intensity on lateral carbon fluxes in forested peatlands

Planning of peatland management under contrasting economic and environmental targets requires application of ecosystem models that can account for complex interactions and feedbacks between biogeochemical, hydrological and forest-related processes. Here we present a further developed version of Peatland simulator SUSI (Lauren et al. 2021) that calculates formation, transport and biodegradation of dissolved organic carbon (DOC). We tested the new model structure against laboratory incubation results containing peat columns from Finland, Estonia, Sweden and Ireland. The drainage strategy (distance between ditches and ditch depth) is considerably different in all these countries: In Finland the ditch network forms a dense fish-bone pattern with an average ditch distance of 40 m, in Estonia the ditch distance is longer but the ditches are arranged to square blocks, in Sweden the network is often irregular, and in Ireland shallow ditches are spaced densely. Then we applied SUSI and assessed the impacts of changing ditch depth and distance on stand and soil C balance, lateral C flux and forest growth. We found that DOC export originated mainly from the close proximity of the ditches because the rate of decomposition is highest and the transport time is shortest. Further from the ditches, water residence time can be several months or even longer enabling biodegradation of a large part of DOC during the transport. The results revealed that even small slopes had a remarkable impact on WT, residence time and DOC export; and that the effect of the slope could exceed that of drainage dimensions per se. Overall results indicated that drainage strategy in Finland was particularly sensitive to changes in ditch depth and distance. In contrast, changes in irregular, sparse ditch network in Sweden caused only minor effects on forest growth, C balance and lateral C fluxes.