Response of enzyme activities to nitrogen addition in forest floors of different C-to-N ratios

The effect of different inputs of mineral N on several enzyme activities involved in the C and N cycles was investigated using Oa material of forest floors from four Norway spruce [Picea abies (L.) Karst.] sites with different C-to-N ratios. The samples from each site were treated with five different concentrations of mineral N (as liquid NH4NO3). All samples were incubated aerobically for 15-20 weeks at 15degreesC and at field capacity. Respiration was measured weekly. At the end of the incubation period, four enzyme activities (endoglucanase, beta-glucosidase, polyphenol oxidase and beta-glucosaminidase) and microbial biomass were determined. Endoglucanase activity was increased and beta-glucosidase activity was decreased by N additions only in Oa material having a wide C-to-N ratio. In N-supplemented samples of low C-to-N ratio, increased polyphenol oxidase activities were often detected as a consequence of N addition. beta-Glucosaminidase activity responded positively to mineral N additions, particularly in Oa samples with low internal N concentration. The results of the present study indicate that the effects of N additions on enzymatic activities of organic matter in late stages of decomposition are related to the C-to-N ratio. Increasing inputs of mineral N to spruce ecosystems may especially affect C-hydrolyzing enzyme activities in soils with wide C-to-N ratio leading to an incomplete degradation of cellulose and thus reduced C availability to micro-organisms.