A synthesis of Sphagnum litterbag experiments: initial leaching losses bias decomposition rate estimates

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Abstract. Our knowledge of the magnitude and controls of Sphagnum decomposition rates is derived to a large extent from litterbag experiments that do not explicitly consider initial leaching losses. Previous research on vascular plants suggests that decomposition rate (k0) estimates from litterbag experiments are biased when initial leaching losses (l0) are ignored. In contrast, the magnitude and variability of l0 for Sphagnum litterbag experiments are not well known, and it is therefore also not known how much Sphagnum k0 estimates are biased. As Sphagnum is the main peat-forming species in many northern peatlands, and biases in k0 estimates can propagate and amplify in long-term peatland models, minimizing such bias is necessary for accurate predictions of peat accumulation. We present a meta-analysis of 15 Sphagnum litterbag studies to estimate initial leaching losses (l0), to analyze how much Sphagnum k0 estimates are biased when the decomposition model ignores initial leaching losses and to analyze how much the variance in k0 estimates increases due to initial leaching losses even when they are estimated by the decomposition model. Average l0 estimates range between 3 mass-% to 18 mass-%, and average k0 estimates range between 0.01 to 1.16 yr−1. Simulations and models fitted to empirical data indicate that ignoring initial leaching losses leads to an overestimation of k0. An error analysis suggests that k0 and l0 can be estimated only with relatively large errors because of limitations in the design of most available litterbag experiments. Sampling the first litterbags shortly after the start of the experiments allows more accurate estimation of l0 and k0. We also estimated large l0 (>5 mass-%) for only air-dried samples, which could imply that Sphagnum litterbag experiments with dried litter are unrepresentative for natural decomposition processes in which l0 may be smaller according to leaching experiments with fresh litter. We conclude that comparing results of litterbag experiments between experimental treatments and between studies and accurately estimating decomposition rates may only be possible if initial leaching losses are explicitly considered.