Organic soil carbon balance in drained and undrained hemiboreal forests

Abstract. Drainage of organic soils is associated with increasing soil carbon (C) efflux, which is typically linked to losses in soil C stock. In previous studies, drained organic forest soils have been reported as both C sinks and sources depending on, e.g., soil nutrient and moisture regime. However, most of the earlier research was done in boreal region, and both the magnitude of C 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 two-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, Latvia, Lithuania). In this study, we analysed the C balance of organic soil in drained (19) and undrained (7) sites. To assess the C balance, soil respiration was measured along with evaluation of C influx into the soil through aboveground and belowground litter. To characterize the sites and factors influencing the C fluxes, we analysed soil temperature, water table level, physical and chemical parameters of soil and soil water. On average, no changes in soil C stocks (0.45±0.50 t C ha⁻¹ year⁻¹) were observed in drained sites dominated by black alder, birch, or Norway spruce, while drained Scots pine sites showed soil C removals with a mean rate of 2.77±0.36 t C ha⁻¹ year⁻¹. In undrained birch- and spruce-dominated sites, soil functioned as mean C sink at 1.33±0.72 t C ha⁻¹ year⁻¹, while the undrained black alder stands showed an uncertain C balance of 1.12±2.47 t C ha⁻¹ year⁻¹. The variability in C balances were influenced by the nutrient-rich soil exhibiting a wide range of nutrient conditions and organic matter quality. Thus, indicating that soil macronutrient concentrations and pH can determine whether the soil functions as a C source or sink.