High-frequency fire alters C : N : P stoichiometry in forest litter

Fire is a major driver of ecosystem change and can disproportionately affect the cycling of different nutrients. Thus, a stoichiometric approach to investigate the relationships between nutrient availability and microbial resource use during decomposition is likely to provide insight into the effects of fire on ecosystem functioning. We conducted a field litter bag experiment to investigate the long-term impact of repeated fire on the stoichiometry of leaf litter C, N and P pools, and nutrient-acquiring enzyme activities during decomposition in a wet sclerophyll eucalypt forest in Queensland, Australia. Fire frequency treatments have been maintained since 1972, including burning every 2 years (2yrB), burning every 4 years (4 yrB) and no burning (NB). C : N ratios in freshly fallen litter were 29-42% higher and C : P ratios were 6-25% lower for 2 yrB than NB during decomposition, with correspondingly lower 2yrB N : P ratios (27-32) than for NB (34-49). Trends in litter soluble and microbial N : P ratios were similar to the overall litter N : P ratios across fire treatments. Consistent with these, the ratio of activities for N-acquiring to P-acquiring enzymes in litter was higher for 2 yrB than NB, whereas 4 yrB was generally intermediate between 2 yrB and NB. Decomposition rates of freshly fallen litter were significantly lower for 2 yrB (72 ± 2% mass remaining at the end of experiment) than for 4 yrB (59 ± 3%) and NB (62 ± 3%), a difference that may be related to effects of N limitation, lower moisture content, and/or litter C quality. Results for older mixed-age litter were similar to those for freshly fallen litter although treatment differences were less pronounced. Overall, these findings show that frequent fire (2 yrB) decoupled N and P cycling, as manifested in litter C : N : P stoichiometry and in microbial biomass N : P ratio and enzymatic activities. Furthermore, these data indicate that fire induced a transient shift to N-limited ecosystem conditions during the postfire recovery phase.