Litter quality controls early decomposition rates and thus ¹⁴C cycling in old peat

Additional ¹⁴C input in ecosystems is often a concern in radiological assessments, and a secondary pathway exists through litter production and decomposition following atmospheric or belowground uptake of ¹⁴C. To examine how litter quality influences this process, a 90-day laboratory incubation was conducted using peat soil amended with labile litter, recalcitrant litter, or an equal mixed-litter treatment. Peat soils contain strongly ¹⁴C-depleted carbon stocks, whereas fresh plant litter introduces modern carbon that enters biologically active pools during decomposition. Litter mass loss was used as an index of early decomposition and microbial processing. Litter treatment significantly affected mass loss (one-way ANOVA, p < 0.001). Labile litter showed the greatest decomposition (35%), whereas recalcitrant litter decomposed least (10%), with the mixed treatment intermediate (22%). Observed mass loss in the mixed treatment did not differ from the value expected from the two single-litter treatments, indicating an additive mixture effect. To interpret these results in a radiocarbon context, a conceptual framework of carbon pools was developed using previously determined percent modern carbon (pMC) values from the same peat system. Bulk peat represented an old carbon endmember, whereas dissolved organic carbon, microbial biomass, and dissolved inorganic carbon formed intermediate pools enriched relative to peat. The results indicate that fresh litter quality can rapidly regulate early decomposition rates and the transfer of modern carbon into active soil pools, even in ¹⁴C-depleted peat. These findings highlight the need to consider fresh organic inputs, alongside peat age, when evaluating carbon cycling and ¹⁴C transfer in peatland systems.