Nutrient limitation and enzyme activities during litter decomposition of nine wetland species in relation to litter N:P ratios

1. Litter decomposition can be limited by nitrogen or phosphorus, but conditions under which either nutrient is limiting remain uncertain. We investigated whether this depends on nutrient limitation during plant growth, on N : P ratios of the litter, or on activities of C-, N- and P-mineralizing enzymes. 2. Nine herbaceous species were grown at N : P supply ratios (N : P-s) of 5-160 (mass-based). Fresh leaf litter was incubated in the laboratory for 5-10 weeks, partly with N or P fertilization, to determine mass loss and activities of extracellular enzymes. 3. Both plant growth and litter decomposition were N-limited for plants grown at N : P-s < 20, and P-limited for those grown at N : P-s >= 40. Litter N : P ratios varied widely among species and did not predict which nutrient limited decomposition, except that litter with N : P > 22 always had P-limited decomposition. 4. The activities of beta-glucosidase, chitobiase and phosphatase differed widely among species but were poorly related to litter nutrient concentrations and mass loss. Within some species, phosphatase activity increased towards high litter N : P ratios, suggesting P limitation of decomposers. 5. We conclude that there is no unique critical N : P ratio discriminating between N- and P-limited decomposition because this critical N : P ratio is species-dependent and may also depend on the physical conditions under which plants were grown.