NITROGEN SUPPLY EFFECTS ON PRODUCTIVITY AND POTENTIAL LEAF-LITTER DECAY OF CAREX SPECIES FROM PEATLANDS DIFFERING IN NUTRIENT LIMITATION

We investigated the effect of increased N-supply on productivity and p otential litter decay rates of Carex species, which are the dominant v ascular plant species in peatlands in the Netherlands. We hypothesized that: (1) under conditions of N-limited plant growth, increased N-sup ply will lead to increased productivity but will not affect C:N ratios of plant litter and potential decay rates of that litter; and (2) und er conditions of P-limited plant growth, increased N-supply will not a ffect productivity but it will lead to lower C:N ratios in plant litte r and thereby to a higher potential decay rate of that litter. These h ypotheses were tested by fertilization experiments (addition of 10 g N m(-2) year(-1)) in peatlands in which plant growth was N-limited and P-limited, respectively. We investigated the effects of fertilization on net C-fixation by plant biomass, N uptake, leaf litter chemistry an d potential leaf litter decay. In a P-limited peatland, dominated by C arex lasiocarpa, there was no significant increase of net C-fixation b y plant biomass upon enhanced N-supply, although N-uptake had increase d significantly compared with the unfertilized control. Due to the N-f ertilization the C:N ratio in the plant biomass decreased significantl y. Similarly, the C:N ratio of leaf litter produced at the end of the experiment showed a significant decrease upon enhanced N-supply. The p otential decay rate of that litter, measured as CO2-evolution from the litter under aerobic conditions, was significantly increased upon enh anced N-supply. In a N-limited peatland, dominated by C. acutiformis, the net C-fixation by plant biomass increased with increasing N-supply , whereas the increase in N-uptake was not significant. The C:N ratio of both living plant material and of dead leaves did not change in res ponse to N-fertilization. The potential decay rate of the leaf litter was not affected by N-supply. The results agree with our hypotheses. T his implies that atmospheric N-deposition may affect the CO2-sink func tion of peatlands, but the effect is dependent on the nature of nutrie nt limitation. In peatlands where plant growth is N-limited, increased N-supply leads to an increase in the net accumulation of C. Under con ditions of P-limited plant growth, however, the net C-accumulation wil l decrease, because productivity is not further increased, whereas the amount of C lost through decomposition of dead organic matter is incr eased. As plant growth in most terrestrial ecosystems is N-limited, in creased N-supply will in most peatlands lead to an increase of net C-a ccumulation.