Response of phosphomonoesterase activity in the lichen Cladonia portentosa to nitrogen and phosphorus enrichment in a field manipulation experiment

• Effects of nitrogen (N) enrichment on the heathland lichen Cladonia portentosa were quantified to test the hypothesis that modified N : phosphorus (P) relationships observed in this species in N-polluted natural environments are a direct effect of increased N deposition, and to evaluate potential confounding effects of N form and P availability. • Cladonia portentosa was harvested from experimental plots in lichen-rich peatland vegetation (background total N deposition of 8 kg N ha)1 yr)1) treated for 4 yr with additional wet N deposition at 0, 8, 24 and 56 kg N ha)1 yr)1 as either NH4 + or NO3 ), and with or without P added at either 0.6 or 4 kg P ha)1 yr)1. • Nitrogen enrichment increased thallus N concentration, N : P mass ratio and phosphomonoesterase (PME) activity by factors of up to 1.3, 1.4 and 1.7, respectively, effects being independent of N form. Phosphomonoesterase activity was tightly related to thallus N : P ratio with additions of P at 4 kg ha)1 yr)1 depressing PME activity by a factor of 0.4. • Nitrogen enrichment induces P-limitation in C. portentosa with attendant changes in chemical and physiological characteristics that could be used as sensitive biomarkers with which to detect low levels of N pollution.