Labile soil carbon inputs mediate the soil microbial community composition and plant residue decomposition rates

Root carbon (C) inputs may regulate decomposition rates in soil, and in this study we ask: how do labile C inputs regulate decomposition of plant residues, and soil microbial communities? In a 14 d laboratory incubation, we added C compounds often found in root exudates in seven different concentrations (0, 0.7, 1.4, 3.6, 7.2, 14.4 and 21.7 mg C g⁻¹ soil) to soils amended with and without ¹³C-labeled plant residue. We measured CO₂ respiration and shifts in relative fungal and bacterial rRNA gene copy numbers using quantitative polymerase chain reaction (qPCR). Increased labile C input enhanced total C respiration, but only addition of C at low concentrations (0.7 mg C g⁻¹) stimulated plant residue decomposition (+2%). Intermediate concentrations (1.4, 3.6 mg C g⁻¹) had no impact on plant residue decomposition, while greater concentrations of C (> 7.2 mg C g⁻¹) reduced decomposition (-50%). Concurrently, high exudate concentrations (> 3.6 mg C g⁻¹) increased fungal and bacterial gene copy numbers, whereas low exudate concentrations (< 3.6 mg C g⁻¹) increased metabolic activity rather than gene copy numbers. These results underscore that labile soil C inputs can regulate decomposition of more recalcitrant soil C by controlling the activity and relative abundance of fungi and bacteria.