Heterotrophic and rhizospheric respiration in coniferous forest soils along a latitudinal gradient

Northern forest soils are a major carbon (C) reservoir of global importance. To estimate how the C balance in these soils will change, the roles of tree roots and soil microbes in C balance should first be decoupled. This study determined how the activity of heterotrophs and tree roots together with root-associated microbes in the rhizosphere varies in coniferous forest soils in boreal, hemiboreal, and temperate climates along a latitudinal gradient using a trenching approach. We created experimental plots without living tree roots, measured soil respiration (CO2 efflux) from these and from unmanipulated plots using the chamber technique, and partitioned the efflux into root-rhizosphere (R-R) and heterotrophic (R-H) respiration. The share of R-R in ecosystem gross primary production (GPP) decreased from north to south in the Scots pine (Pinus sylvestris L.) and the Norway spruce (Picea abies (L.) Karst.) forests, with the exception of a mixed site, where the share of R-R in GPP varied strongly between the years. R-R per ground area and per root biomass were mainly independent of climate within the gradient. R-H per ground area increased from north to south with temperature, while R-H per soil C did not change with temperature. Soil moisture did not significantly affect the respiration components in the northernmost site, whereas soil moisture was positively connected with R-H and negatively with R-R in other Scots pine sites and positively connected with R-R in pure Norway spruce stands. The dynamic ecosystem model LPJ-GUESS was able to capture the seasonal dynamics of R-H and R-R at the sites, but overall accuracy varied markedly between the sites, as the model underestimated R-H in the southern site and R-R elsewhere. Our study provides knowledge about the nature of soil respiration components. The valuable insights can be used in more accurate land-ecosystem modelling of forest ecosystems.