Variation in annual carbon fluxes affecting the SOC pool in hemiboreal coniferous forests in Estonia

Estimation of soil-related carbon (C) fluxes is needed to understand the dynamics of the soil organic carbon pool, to determine changes in the carbon balance and functioning of forest ecosystems, and to support climate change policies. The objective of the study was to analyse the variation in the most dynamic soil C input (tree and understory above- and belowground litter production) and output (soil respiration) fluxes, in addition to the forest floor, understory and fine root biomass stocks, in eight different Scots pine (Pinus sylvestris L) and Norway spruce (Picea abies (L) Karst) sites growing on mineral soils in Estonia. Further, the impact of soil C input and output fluxes on the soil organic carbon (SOC) pool was examined, and the net ecosystem production (NEP) of the stands was estimated. Fine root production (FRP) of the trees constituted 53% and 28% and needle litter constituted 25% and 28% of the total annual C input to the soil in the Norway spruce and Scots pine stands, respectively. The total FRP of the trees and the understory roots and rhizomes ranged from 211 to 1040 g m(-2) yr(-1), of which the understory comprised up to 28%. The mean annual soil respiration (Rs) rate was 5.7 +/- 0.3 and 6.5 +/- 0.3 Mg C ha(-1) yr(-1) in the pine and spruce stands, respectively, and did not differ significantly between the two groups of stands. The SOC pool of the studied stands depended significantly on both the above- and belowground C input fluxes. Tree-derived litter had the strongest effect on the SOC pool, while the Rh as the main soil C output flux showed no significant impact. The NEP ranged from 4.2 to - 1.8 Mg C ha(-1) yr(-1) and demonstrated a strong negative correlation with stand age. The results affirm the importance of belowground as well as aboveground litter production on carbon accumulation in forest soils.