Functional traits drive the difference in soil respiration between Gilbertiodendron dewevrei monodominant forests patches and Scorodophloeus zenkeri mixed forests patches in the Central Congo basin.

Abstract

In tropical rainforests, soil respiration accounts for the major part of ecosystem respiration, yet a deep understanding of the influence of forest type and species composition is still lacking. We therefore selected patches of the rainforest in the Central Congo basin differing in their species composition, some patches under the Scorodophloeus zenkeri Harms mixed forests (MIF) and others in the Gilbertiodendron dewevrei (De Wild.) J.Leonard monodominant forests (MOF). We measured daily soil respiration over a one-year period. By fitting a simple conceptual model of soil respiration, including fine root biomass, soil organic C stocks and ground climate measurements (soil moisture and temperature), we attempted to distinguish autotrophic and heterotrophic soil respiration, and to better understand the drivers behind total soil respiration. On an annual basis, soil respiration was 10% higher under MOF (22.10 Mg C ha−1 y−1) compared to MIF (20.01 Mg C ha−1 y−1) (p < 10−3). While the estimated autotrophic and heterotrophic soil respiration contributed about equally to soil respiration in MOF, autotrophic soil respiration slightly dominated (59%) in MIF. In both forests, the combined contribution of litterfall inputs and fine roots productivity was lower than the heterotrophic flux, with the largest difference observed under MOF (−6.16 Mg C ha−1 year−1) compared to MIF (−2.62 Mg C ha−1 year−1). The sensitivity analysis of the model showed that the higher heterotrophic soil respiration under MOF was driven by the twofold C accumulation in MOF topsoil compared to MIF. Soil moisture was a major driver of temporal changes in soil respiration, but hardly impacted the differences in annual soil respiration between forests. While the difference in SOC accumulation between forests was driven by the low nutrient to C ratios of Gilbertiodendron dewevrei tissues, additional research is needed to identify the causes behind the unbalanced C budget.