EFFECT OF INCREASED NITROGEN AVAILABILITY ON SOIL ENZYME PERFORMANCE IN WETLANDS OF NORTHEAST CHINA

Due to the enhanced organic matter decomposition induced by climate warming as well as increased external nitrogen (N) inputs, increased soil N availability in wetlands can alter soil enzyme activities and hence ecosystem C dynamics. An incubation experiment was conducted to assess the effects of increased N availability on soil enzyme activities in freshwater marsh and peatland of Northeast China. N was added as ammonium nitrate solution at four levels (0, 0.1, 0.2, and 0.5 mg N/g soil for N0, N1, N2, and N3, respectively). Results showed that the effect of N availability increasing on soil enzyme activities varied with soil types, fertilization rates, and incubation times. Increased N availability inhibited urease activity in marsh soils throughout the incubation, whereas N2 and N3 treatments suppressed urease activity in peat soils only at 6 days. Marsh soil invertase activity increased after 15 days of incubation because of N addition. N availability increasing stimulated the peat soil protease activity initially, but negative effect was observed at the end of incubation. In both soils, increased N availability had no effect on acid phosphatase except for N3 treatment at 27 days. With increased N addition levels, dissolved organic carbon (DOC) decreased in marsh soils, but did not change in peat soils. These findings suggest that, in marsh ecosystems, increased N availability alters soil urease and invertase activities and DOC pool, and thus changes the soil organic C dynamics. However, soil urease and DOC are not sensitive to N availability increasing in peat soils.