Rhizosphere priming in a warming Arctic: Are peatlands insusceptible?

Rapid warming is currently accelerating Arctic carbon cycling, including increased permafrost thaw and CO₂ production from soil organic matter decomposition, but also CO₂ uptake by plants. Plants can additionally stimulate soil organic matter decomposition near their roots, via the rhizosphere priming effect. In a recent modeling study, we showed that priming can accelerate Arctic soil carbon loss at a globally relevant rate, and spatial analysis pointed to large potential contributions from carbon-rich permafrost peatlands. At the same time, the high carbon content of peatlands might render them insusceptible to input of easily available organic compounds by plant roots, which is considered a key component of priming. We here investigated the susceptibility of permafrost peat soils to priming by plant compounds under aerobic conditions that resemble the dominant rooting zone. To that end, we combined a 30-week laboratory incubation of peat soils from five circum-Arctic locations, with a literature meta-analysis of priming studies of Arctic peat and mineral soils. The combined experimental and literature data showed substantially and significantly weaker priming susceptibility of peat than mineral soils. Organic carbon addition increased CO₂ production from soil organic matter in mineral, but not in peat soils. Organic nitrogen addition had a significant effect on both sample types, but that of peat was much weaker. These observations point at fundamental, mechanistic differences in the response of peat and mineral soil organic matter decomposition to changing carbon and nitrogen availability. In a new model sensitivity analysis, we show that insusceptibility of peatlands to priming would substantially reduce estimates of priming-induced carbon loss from the circum-Arctic. While our study suggests a limited effect of plant-released organic compounds on peat decomposition, it does not preclude a vegetation effect on decomposition under natural conditions. The large carbon stocks of circum-Arctic peatlands and expected changes in vegetation and drainage, call for increased efforts to quantify the combined effect of living plants on soil processes beyond carbon input.