Litter N:P ratios indicate whether N or P limits the decomposability of graminoid leaf litter

The N: P ratio of leaf litter may determine if decomposability is N- limited ( litter with low N: P ratio) or P- limited ( litter with high N: P ratio). To test this hypothesis and to determine the threshold between N and P limitation, we studied relationships between litter N and P concentrations, litter mass loss and effects of fertilisation on litter mass loss in laboratory experiments. Leaf litter of 11 graminoid species was collected in Swiss and Dutch wetlands, yielding 84 litter samples with a broad range of N and P concentrations ( 3.2 - 15.1 mg N g(-1), 0.04 1.93 mg P g(-1)) and with N: P mass ratios ranging from 5 to 100. On nutrient- free sand, dry mass loss after five or ten weeks ( 5.5 - 53% of initial mass) correlated positively with the N and P concentrations of the litter. Within species, mass loss correlated mainly with N for litter with low N: P ratio, and with P for litter with high N: P ratio, in agreement with our hypothesis. Among species, however, these relationships did not exist, and decomposition rather correlated with the specific leaf area. When the litter was incubated on fertilised sand, 35 out of 50 litter samples decomposed faster than on nutrient- free sand. Decomposition was generally accelerated by P fertilisation ( i. e. P- limited) when the N: P ratio of the litter was above 25 and the P concentration below 0.22 mg g(-1), supporting our hypothesis. N- limited decomposition was not significantly related to the litter N: P ratio but occurred rarely for litter with N: P ratio greater than 25, and only for litter with N concentration below 11.3 mg g(-1). We conclude that the N: P ratio of leaf litter indicates whether its decomposability is more likely to be N- or P- limited. The critical N: P ratio ( threshold between N and P limitation) appeared to be 25 for graminoid leaf litter.