Air-drying depresses rates of leaf litter decomposition

Air-dried leaf litter is universally used in decomposition experiments, but limnological research indicates that air-drying considerably increases the susceptibility of litter to leaching. In an aspen forest in the Rocky Mountains of Alberta, Canada, I compared first-year decomposition of air-dried leaf litter (7–8% moisture) of trembling aspen (Populus tremuloides) and green alder (Alnus crispa) against decomposition of freshly fallen leaf litter that was still moist (57–62% moisture) when placed in the litterbags. Contrary to expectations, mass loss was substantially faster from fresh litter than from air-dried litter of both species, even in the first month in the field. Over the 334-d experiment, fresh alder leaves decayed nearly twice as fast as air-dried leaves; fresh aspen leaves decayed 62% faster than air-dried leaves. Air-drying was estimated to add at least another year to the time required to reach 50% mass loss. Moisture content of fresh leaves was higher than that of air-dried leaves at every sampling time. Fresh aspen litter rapidly accumulated external N during only the first month, while air-dried aspen litter accumulated external N throughout the experiment. N-rich alder leaves released N in the first week in the field, but the loss was greater for fresh leaves than for air-dried leaves and the content difference persisted for the remainder of the experiment. Both mass loss rates and rates of N uptake and release may be substantially underestimated by using air-dried leaves in decomposition experiments.