Soil–litter nitrogen transfer and changes in δ13C and δ15N values in decomposing leaf litter during laboratory incubation

AbtractLeaf litter is the main source of nutrient and energy input into the soil. Therefore, detailed knowledge on the short-term variations in the isotopic composition of plant litter is needed for correctly estimating the “isotopic baseline” in stable isotope-based studies on detrital foodwebs. In a laboratory experiment, standardized fragments of freshly fallen leaves of Quercus robur, Ulmus glabra, and Populus tremula were incubated on the surface of natural or 15N-labeled soil during 260 days. At the end of the experiment, the remaining mass represented 62, 53 and 50% of the initial mass for oak, elm and aspen litter, respectively. There was a small decrease in the mean δ13C values during the initial stages of decomposition, although it was inconsistent among the three litter species tested and did not exceed 1.0‰ during the decomposition period. Calculations based on the total N content, as well as the isotope mixing model suggested that up to 50% of the total litter N was incorporated from the underlying soil. The rates of N transfer from the soil to litter were not affected by the disruption of mycelial connections between the soil and litter. As indicated by δ15N values of filter paper placed on the soil surface, labile soil N was depleted by 6–8‰ in 15N relative to bulk soil organic matter. However, in the given experimental settings the input of 15N-depleted labile N from mineral soil was likely counterbalanced by an increase in the 15N-enriched microbial biomass and bulk litter δ15N values changed little.