Tree species and soil depth affect microbial necromass contribution to soil organic matter in temperate forest soils, surpassing the impact of microbial and faunal community composition

Forest management and climate change inevitably lead to shifts in the composition of temperate forest stands from coniferous to deciduous species, potentially affecting the crucial role of forest soil organic carbon (SOC) stocks in mitigating climate change. Here, we provide the first comprehensive field study focusing on mineral topsoils (0–10 cm) and subsoils (50–60 cm) in temperate deciduous (beech), coniferous (spruce) and mixed forests. Moreover, we investigate the contribution of microbial-derived C to free particulate organic matter (fPOM), occluded POM (oPOM) and mineral-associated OM (MAOM) fractions as affected by differences in the composition of the microbial and micro- and mesofaunal communities. Our results show a relatively high microbial necromass contribution not only to MAOM (61%) and oPOM (36%), but also to fPOM (36%). The contribution of microbial-derived C in MAOM was higher in deciduous (72%) than in coniferous (48%) forest stands and the contribution of microbial-derived C was higher in the topsoil (51%) than in the subsoil (37%). Our data also showed that fungi dominated microbial necromass contribution to POM. In contrast, bacteria dominated microbial necromass contribution to MAOM, and the importance of certain microbial (bacterial) groups in governing microbial-derived soil organic matter (SOM) varies with the forest stand, soil horizon and the SOM fraction. Finally, neither micro- nor mesofaunal community composition as affected by forest stand or soil horizon was associated with microbial necromass contribution to SOM. Our study highlights the need to include the microbial necromass contribution to all SOM fractions in temperate forest soils within SOC models and predictions. Our study also indicates that expected shifts in forest stand composition will lead to higher and more effective SOM stabilization, with the topsoil strongly influenced by a more active soil biota community. Read the free Plain Language Summary for this article on the Journal blog.