Calculating co-metabolic costs of lignin decay and their impacts on carbon use efficiency

A critical feature of leaf litter decay is the interaction between holocellulose (including both cellulose and hemicellulose) and lignin, described by the lignocellulose index (LCI): LCI = lignin/[lignin + holocellulose], which increases during decomposition. We develop a simple model describing the relationships between the first-order decay rate coefficients (k2 and k3, respectively) for holocellulose (C2) and lignin (C3) litter fractions over the full range of LCI values from 0 to an empirical maximum of ∼0.7 (LCImax). We assume that: (1) k2 and k3 are linear functions of LCI; (2) holocellulose degradation (dC2/dt) provides a net energy yield, while lignin decay (dC3/dt) has a net energy cost; and (3) LCImax is the LCI value at which the yield of dC2/dt equals the cost of dC3/dt. This model is consistent with empirical observations of key aspects of litter decay, such as changes in LCI, and can be used to quantify energetic relationships during decomposition, such as the relationship between potential carbon use efficiency (CUE) and litter quality (LCI).