Opposite priming responses to labile carbon versus oxygen pulses in anoxic peat

Abstract

Vegetation shifts in peatlands might change the stability of soil organic carbon (SOC) stocks via rhizosphere priming effects. However, mechanisms and magnitude of priming effects in peat soils are poorly understood. Beyond supplying C-rich root exudates - a central driver of priming in upland soils - wetland vascular plants supply oxygen to reducing soil systems.

We evaluated priming effects in anoxic peat soils driven by labile C-exudate inputs (glucose), oxygen inputs and their interaction. Using incubation experiments, we mimicked oxygen loss and exudation rates of wetland plants and separated peat SOC- and glucose-derived respiration rates using a C stable isotope approach.

Oxygen pulses and oxygen + glucose pulses stimulated SOC mineralization through positive priming of > + 350% and > + 200%, respectively. By contrast, glucose pulses alone caused negative priming, with the most negative effect (< - 70%) at maximum glucose input. However, even glucose-C inputs smaller than the estimated microbial biomass C led to negative or no priming. Both positive and negative priming effects continued for several weeks after inputs stopped and increased in magnitude.

We demonstrate that labile C inputs into an anoxic soil can strongly suppress SOC mineralization, in contrast to positive priming effects often observed in oxic upland soils. We hypothesize that negative priming driven by preferential substrate usage is amplified in anoxic soils due to electron-acceptor exhaustion through exudate-fueled respiration. Our results imply that expansion of vascular plants into peatlands could stimulate SOC mineralization through root oxygen loss, while labile C inputs might stabilize SOC.