A Synthesis of Sphagnum Litterbag Experiments: Initial Leaching Losses Bias Decomposition Rate Estimates

Abstract. Decomposition is one of the major controls of long-term sequestration of carbon in northern peatlands. Our knowledge of the magnitude and controls of decomposition rates is derived to a large extent from litterbag experiments. Similar to other litter types, initial leaching losses may bias decomposition rates of Sphagnum, but their magnitudes and variability are not well known.

We present a meta-analysis of 15 Sphagnum litterbag studies to estimate initial leaching losses (l₀), to test whether initial leaching losses can bias Sphagnum decomposition rate estimates (k₀), and to analyze how initial leaching losses increase errors in k₀ estimates.

Average l₀ estimates range between 3 to 18 mass-%, average k₀ estimates 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 estimated large l₀ (>5 mass-%) also 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 be possible only if initial leaching losses are explicitly considered.