Organic soils can be CO2 sinks in both drained and undrained hemiboreal peatland forests

Drainage of organic soils is associated with increasing soil carbon dioxide (CO2) efflux, which is typically linked to losses in soil carbon (C) stock. In previous studies, drained organic forest soils have been reported as both CO2 sinks and CO2 sources depending on, e.g., soil nutrient and moisture regime. However, most of the earlier research was done in the boreal zone, and both the magnitude of CO2 efflux and the impact of soil moisture regime on soil C stock are likely to vary across different climatic conditions and ecosystems, depending further on vegetation. A 2-year study was conducted in hemiboreal forest stands with nutrient-rich organic soil (including current and former peatlands) and a range of dominant tree species (black alder, birch, Norway spruce, Scots pine) in the Baltic states (Estonia (EE), Latvia (LV), Lithuania (LT)). In this study, we analyzed the CO2 balance of organic soil in drained (19) and undrained (7) sites. To assess the CO2 balance, soil respiration was measured along with the evaluation of CO2 influx into the soil through aboveground (aGV) and belowground (bGV) litter. To characterize the sites and factors influencing the CO2 fluxes, we analyzed soil temperature, soil water table level (WTL), and physical and chemical parameters of soil and soil water. Irrespective of drainage status, the soils functioned as both CO2 sinks and CO2 sources. On average, a close-to-neutral soil CO2 balance (+0.45 ± 0.50 tCO2-C ha-1 yr-1) was observed in drained sites dominated by black alder, birch, or Norway spruce, while drained Scots pine sites showed soil CO2 removals with a mean rate of +2.77 ± 0.36 tCO2-C ha-1 yr-1. In undrained birch- and spruce-dominated sites, soil functioned as a mean CO2 sink at +1.33 ± 0.72 tCO2-C ha-1 yr-1, while the undrained black alder stands showed an uncertain CO2 balance of +1.12 ± 2.47 tCO2-C ha-1 yr-1. Variation in the soil CO2 balance was related to soil macronutrient concentrations and pH: forest types characterized by lower nutrient availability showed greater soil CO2 sink. The reported soil CO2 balance values may be used as regional emission factors (EFs).