Disentangling organic matter inputs and outputs during litter decomposition using C4-plant teabags

Decomposition rates are commonly assessed from mass loss in litterbags or teabags, but underestimation may occur if organic matter (OM) enters the bags during decomposition, e.g. through colonization by fungal hyphae. We used C4-plant (lemongrass) teabags and an isotopic mass-balance approach to quantify external OM inputs to decomposing litter during a 24-month field study in two C3-plant ecosystems, a temperate forest and a heath tundra, with contrasting climates and vegetation. After 24 months, OM inputs represented 4% of recovered mass in the tundra and 22% in the forest, corresponding to an underestimation of decomposition by 3% to 6%, with stronger biases in the forest due to accelerated litter processing and belowground activity. The use of litter- or teabags containing plant material with isotopic signatures distinct from the studied system enables the quantification of OM inputs entering the bags during litter decomposition, thereby improving the comparability of decomposition rates across ecosystems.