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• 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 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.

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This document was originally published by Wiley-Blackwell in New Phytologist. The definitive version is available at Copyright restrictions may apply. DOI: 10.1111/j.1469-8137.2010.03427.x

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