1. Nitrogen and phosphorus supply influences the rate of litter decomposition and nutrient dynamics during decomposition. Besides the total amount of N and P available to decomposers, their relative supply (N : P ratio) might be important, e.g. through an influence on the composition and activity of microbial communities.
2. We carried out two experiments using laboratory microcosms to test that (i) N : P ratios (in either litter or the environment) determine whether N or P limits the rate of decomposition, (ii) the 'critical' N : P ratio between N and P limitation depends on overall nutrient availability, (iii) litter colonization by fungi and bacteria depends on the N : P ratio, and (iv) N : P ratios determine if proportionately more N or P is retained or immobilized by the litter.
3. In one experiment, cellulose as a nutrient-free litter analogue was incubated on sand fertilized with varying N : P supply ratios, whereas in a second experiment, Carex leaf litter with varying N : P ratios was incubated on nutrient-free sand.
4. Results essentially support our hypotheses. Cellulose decomposition was N- or P-limited depending on the N : P supply ratio. The shift from N to P limitation occurred at N : P supply ratios of 1·7–45, depending on overall nutrient supply. Bacteria were most abundant on cellulose at low N : P supply ratios, when decomposition was N-limited, while fungi were relatively more important at high N : P ratios, when decomposition was P-limited. The amounts of mineral N and P immobilized on cellulose and those released from litter, both in absolute terms and relative to supply, depended on the type of nutrient limitation (N vs. P).
5. The N : P ratio of nutrients available to decomposers appears to be an important determinant of plant litter decomposition, the relative importance of fungi and bacteria in litter-associated microbial communities, and litter nutrient dynamics.