Effects of different types of N deposition on the fungal decomposition activities of temperate forest soils

Nitrogen (N) deposition significantly affects soil microbial activities and litter decomposition processes in forest ecosystems. However, the changes in soil fungi during litter decomposition remain unclear. In this study, ammonium nitrate was selected as inorganic N (IN), whereas urea and glycine were selected as organic N (ON). N fertilizer with different IN-to-ON ratios (1:4, 2:3, 3:2, 4:1, and 5:0) was mixed in equal amounts and then added to temperate forest soils. Half of each treatment was simultaneously added with streptomycin to inhibit soil bacteria. The activities of enzymes involved in litter decomposition (invertase, β-glucosidase, cellulase, polyphenol oxidase, and phosphatase) were assayed after a three-year field experiment. The results showed that enzymatic activities were inhibited by IN addition but accelerated by ON addition in the non-antibiotic addition treatments. An increase in ON in the mixed N fertilizer also shifted enzymatic activities from N inhibition to N stimulation. Similarly, in the antibiotic addition treatments, fungal activities revealed the same trends, but they were seriously inhibited by IN and significantly accelerated by ON. These results indicated that soil fungi were more sensitive to N deposition, particularly to ON. A large amount of ON may convert soil microbial communities into a fungi-dominated system. However, excessive ON deposition (20% IN + 80% ON) caused N saturation and repressed fungal activities. These results suggested that soil fungi were sensitive to N type and that different IN-to-ON ratios may induce diverse ecological effects on soil fungi.