The effects of simulated nitrogen deposition on extracellular enzyme activities of litter and soil among different-aged stands of larch

Abstract

Aims Nitrogen (N) addition could affect the rate of forest litter and soil organic matter decomposition by regulating extracellular enzyme activity (EEA). The impact of N addition on EEA may differ across different age stands with different organic matter quality. We were interested in whether the impact of N addition on EEA in litter and mineral soil during the growing season was dependent on stand age of a larch plantation in North China. Methods We added three levels of N (0, 20 and 50 kg N ha −1 year −1 ) in three age stands (11, 20 and 45 years old) of Larix principis-rupprech- tii plantation in North China. We measured potential activities of β-1,4-glucosidase (BG), cellobiohydrolase (CB), β-1,4-N-acetyl- glucosaminidase (NAG) and phenol oxidase (PO) in litter (organic horizon) and mineral soil (0-10 cm) during the second growing sea- son after N amendment. We also measured C and N concentrations, microbial biomass C and N, and KCl-extractable ammonium and nitrate in both litter and mineral soil. temperature. Stand age had a strong effect on EEA in both litter and mineral soil, and this effect differed between litter and mineral soil as well as between different enzymes. N addition did not signifi- cantly affect the activities of BG or CB but significantly suppressed the activity of NAG in litter. We also found stand age-specific responses of PO activity to N addition in both litter and mineral soil. N addition suppressed PO activity of the high C:N ratio litters in 20- and 45-year-old stands but had no significant effect on PO activity of the low C:N ratio litter in 11-year-old stand. Moreover, N addition inhibited PO activity of the high C:N ratio soil in 20-year- old stand but had no significant impact on PO activity of the low C:N ratio soils in 11- and 45-year-old stands. Overall, stand age had a greater effect on EEA in litter and mineral soil compared to 2 years of N addition. Moreover, the effect of N addition on PO activity is stand age dependent, which may affect the long-term soil carbon storage in this forest.