Carbon Balance in Drained Hemiboreal Peatland Forests

Understanding the carbon (C) balance of forestry-drained peatlands is crucial for addressing climate change. Natural peatlands are significant C sinks; however, soil carbon is released back into the atmosphere after the drainage of these ecosystems. Existing studies often face spatial and temporal variability, as many studies have focused on specific management practices, localized conditions, or short time frames. This narrow scope hinders the generalization of findings across diverse regions and peatland ecosystems. 

This study examines the C balance of hemiboreal drained peatland forests by analyzing C pools and fluxes across various ecosystem components, including soil, vegetation, litter and leaching. Four sites in Estonia were selected, representing two distinct forest types: a drained bog forest (DBF), dominated by Scots pine (Pinus sylvestris), and three transitional fen forest (DTFF) sites, dominated by downy birch (Betula pubescens), Norway spruce (Picea abies), and Scots pine (Pinus Sylvestris), respectively.

The field measurements were conducted over two years (July 2022 to June 2024). Soil heterotrophic respiration (Rhet) during the vegetation period was measured biweekly in trenched plots using an opaque dynamic chamber connected to a portable CO₂ gas analyzer EGM-5. Gas samples of non-vegetation period Rhet and year-round soil methane (CH₄) were collected biweekly using manual static chambers and analyzed with gas chromatography. Soil physical and environmental were continuously measured and recorded at 30-minute intervals using CR1000 data loggers. Soil chemistry was evaluated once during the study period. The leaching of dissolved organic carbon (DOC) was estimated using plate lysimeters installed in the soil at a depth of 40 cm in all the studied stands. Furthermore, above- and belowground biomass and annual production were estimated through field-based measurements and empirical modelling approaches to calculate each site’s C balance.

Preliminary results indicate that Rhet was, on average, significantly higher in DTFF sites than in DBF sites, with levels approximately twice as high and reaching their highest emissions in spruce- and birch-dominated stands. The soil of the DBF site was a net CH₄ source, while the DTFF sites were net CH₄ sinks. Rhet and CH₄ fluxes were primarily influenced by water table depth and soil temperature, with the highest fluxes observed during the peak of snow-free seasons. Carbon accumulation in aboveground vegetation (trees and understory) and inputs through litter were highest in spruce- and pine-dominated DTFF sites and lowest in the DBF site. Carbon losses as DOC in water were highest in DTFF-spruce and DBF sites. Belowground biomass contributed to ecosystem productivity through C inputs from root exudates and production.

In this study, annual net ecosystem production (NEP) and soil carbon balance were estimated using the biometric method. Further investigations of soil C fluxes and their relationships with soil and environmental parameters will be investigated to identify variability across different ecosystem pools and determine the overall C sink or source strength of hemiboreal drained peatland forests.