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 soil) to soils amended with and without ¹³ C-labeled plant residue. We measured CO ₂ 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.