Estimating relative cellulolytic and ligninolytic enzyme activities as functions of lignin and cellulose content in decomposing plant litter

Extracellular enzymes catalyze plant litter decomposition, including enzymes that degrade holocellulose (E2) and lignin (E3). To estimate relative enzyme activities associated with observed patterns of hollocellulose (C2) and lignin (C3) decay, we set observed decay rates equal to reverse Michaelis-Menten equations. Results were consistent with empirical studies, showing a negative relationship of E2/(E2+E3) to litter lignin content, C3/(C2+C3), above a minimum threshold at which lignin begins to decay. This threshold was previously reported to be 40% lignin content, but our results demonstrated substantial variability with litter type and environment. To our knowledge, this is the first mechanistic explanation of microbial allocation of cellulolytic and ligninolytic enzymes as a function of the lignin concentration of the lignocellulose complex but raises further questions about factors controlling the threshold for lignin decay, such as nitrogen availability. •Lignocellulose index (LCI) controls microbial allocation of extracellular enzymes.•We provide an analytical solution to the LCI control on enzyme allocation.•The ratio of cellulose-to-lignin degrading enzyme activities declines with increasing LCI.•LCI threshold values at which ligninolytic enzymes are allocated varies among studies.