Differences in the flow of spruce-derived needle leachates and root exudates through a temperate coniferous forest mineral topsoil

Abstract

Coniferous forest ecosystems are important pools of soil organic carbon (SOC) in the Northern temperate zone. Needle leachates and root exudates represent a significant input of C to these soils and can differently affect soil C cycling because of their differences in chemistry and stoichiometry. This is the first study to investigate the differences in the flow of dissolved organic C (DOC) in the form of needle leachates, root exudates, and their combination through a forest mineral topsoil. We conducted a 5-month microcosm experiment with ecologically relevant additions of 13C-labelled spruce-derived substrates. The proportion of DOC lost from or incorporated into the mineral soil as microbial biomass or soil fractions (free, occluded by or adsorbed onto mineral particles) as well as differences in the priming effect (PE) caused by the two substrates were assessed.Needle leachates (higher in phenolics and C:N ratio) were less utilized by the microbial community than root exudates but caused a higher PE probably because they lacked sufficient N to satisfy microbial N demands. The addition of either substrates failed to change microbial community composition or SOC content in soil fractions. Most of the substrate C in soil fractions was stabilized by adsorption onto mineral particles. On average, 69% of the substrate C was lost via mineralization; only 0.23% via leaching. The most important C pool related to substrate C gain was the C stored in soil fractions (29%); only 1.7% was stored in the microbial biomass. A consideration of all C gains and losses indicates that the addition of spruce-derived substrates resulted in an average net substrate C retention of 31%.