Do Root Exudates Enhance Peat Decomposition?

Despite the importance of understanding controls on microbial carbon (C) mineralization in peat soils, the role of vascular plant root exudates is still unclear. To determine whether root exudates could stimulate enhanced decomposition of peat, we utilized an in-vitro method involving the addition of a solution similar to root exudates (6 glucose-C: 2 citrate-C: 2 amino acid-C, at 3 addition levels) to peat, incubating the mixture and measuring CO2 produced over 20 d and microbial biomass and dissolved organic carbon (DOC) at the end of the incubation. We defined priming as inorganic C (IC) production (CO2 + calculated dissolved inorganic C) during the incubation being greater than that attributed to the control peat plus the added C. An addition level of 0.083 mg C g(-1) dry peat, estimated to represent root exudation over one 12-h sunny day in a bog, caused an enhancement in IC production that exceeded that produced in the controls and the amount of added C after 8 d, with rates levelling to control values after 15 d. At the end of the incubation nearly 3 times the amount added C had been mineralized, relative to the control, however this represented only 4% of total microbial respiration in the controls. Although the priming effect pattern appeared to be real throughout repeated measurements in our experiments, the statistical probabilities were not always large due to high variability in background CO2 production levels. Given the observed long lag-times and overall small magnitude and large variability in observed effects, we conclude that although priming of decomposition appears to occur in peatlands, it likely has only a minor overall impact on net C loss to the atmosphere.