Effects of substrate quality, temperature, and water content on the decomposition of Sphagnum peat

• Fresh peat decomposition at high temperatures is limited by water content. • In degraded peat, substrate availability limits decomposition at all temperatures. • Decomposition of both fresh and degraded peat at 7 °C is higher than at 14 °C. • Degraded peat can decompose even under very low water contents. • Decomposition of low water content peat can increase under high temperatures. Peatlands, with their high water tables and anoxic conditions, have inherently low organic matter decomposition rates, making them vital carbon reservoirs. We designed a laboratory incubation experiment to investigate the interactive effects of substrate quality, temperature, and water content on the decomposition rate of Sphagnum peat. Fresh and degraded peat was collected from three different depths (0–5, 5–15, and 15–30 cm) of an Australian alpine peatland. The water contents of the peat samples were adjusted to four levels (field-moist or 50 %, 400 %, or 1500 %) and incubated at four temperatures (7, 14, 21, and 28 °C) for 70 days. Overall, fresh peat had ∼ 50 % higher decomposition rate than degraded peat. Both fresh and degraded peat incubated at 7 °C had higher or similar decomposition rates to peat at 14 °C, regardless of water content, likely due to the Sphagnum peat being dominated by psychrophilic microbes that have optimal metabolism at low temperatures. Further, the duration for which peat at 7 °C had a higher decomposition rate than at 14 °C declined as water content declined in the fresh peat and as peat depth increased in degraded peat. These findings indicate that the decomposition rate of fresh peat with a high percentage of labile carbon content is determined by the availability of liquid water required for microbial metabolism, while in degraded peat, substrate availability controls the decomposition rate. Our study provides critical insights into carbon release dynamics in southern hemisphere Sphagnum peatlands, which can inform strategies for managing and conserving these critical carbon reservoirs.