Changes in soil microbial biomass and microbial activities in response to 7 years simulated pollutant nitrogen deposition on a heathland and two grasslands

The effects of 7 years simulated pollutant nitrogen (N) deposition on soil microbial biomass C (Cmic), soil phosphomonoesterase (PME) activity and utilisation of carbon (C) and organic N and phosphorus (P) sources by soil bacteria are reported for a heathland, an acidic grassland and a calcareous grassland. N additions increased Cmic in the heathland, decreased it in the acid grassland, and had no effect in the calcareous grassland. These effects mirrored the changes in plant cover, indicating close-coupling of plant and microbial responses to chronic N additions. In the heathland, PME activity generally rose with each increment of N addition while in the acid grassland, PME activity significantly increased only in the 14 g N m−2 y−1 (ammonium nitrate) treatment. In the calcareous grassland, PME activity was highly correlated with KCl extractable N (R2 = 0.71), indicating increased PME activity in response to increasing N saturation. At all three sites, PME activity per mg Cmic was greater in plots receiving N inputs, reflecting greater P limitation. In the heathland, N treatments caused a 3-fold increase in the utilisation rate of C and organic N substrates in BIOLOG plates, whereas utilisation of organic P substrates rose 10-fold in response to 8 g N m−2 y−1 and 18-fold in response to 12 g N m−2 y−1. In the acid grassland, utilisation of the C sources decreased in response to the N treatments. The results demonstrate that long-term chronic inputs of pollutant N can significantly increase microbial biomass and activity in N-limited heathland ecosystems, but may reduce microbial biomass and microbial activity in P-limited grasslands.