Soil microbial biomass and activity and plant nutrition in semi-natural ecosystems subjected to pollutant nitrogen deposition.

The response of UK semi-natural soil and plant communities to long-term inputs of
pollutant nitrogen (N) is presently poorly understood. This study aims to investigate the
effects of up to 8 years simulated pollutant N additions on plant nutrition and key below
ground nutrient cycling processes in an upland heath, an acid grassland and a calcareous
grassland. An additional series of plots was established in the grasslands that received
short-term N and phosphorus (P) inputs which enabled investigation of NIP interactions and
the determination of plant biomass.
Soil inorganic N concentrations indicated that all of the sites had become 'N saturated', and
that P limitation may be of increasing importance, particularly in the heathland and
calcareous grassland. This was confirmed in the calcareous grassland where significant
increases in above ground biomass were seen in only P treated plots, while in the acid
grassland, there were no growth responses to nutrient additions. A field-based bioassay
was developed which demonstrated that both short and long-term inputs of N significantly
increased root surface phosphomonoesterase (PME) activity of Plantago lanceolata
seedlings. A parallel microcosm-based bioassay revealed a significant relationship between
root surface PME activity of Agrostis capillaris and extractable NfLt concentrations (r =
0.86) in the long-term acid grassland plots, indicating that this site may have reached or be
approaching N saturation.
In response to long-term N additions, soil microbial biomass carbon increased in the
heathland, decreased in the acid grassland and remained constant in the calcareous
grassland. Chloroform fumigation indicated that microbial biomass P decreased in the acid
and calcareous grasslands, while microbial biomass N increased in the heathland and acid
grassland in the N treated plots. Soil PME activity was highly sensitive to the N treatments.
It increased at all sites and was significantly correlated with extractable inorganic N
concentrations (r = 0.71) in the calcareous grassland, indicating close coupling between N
saturation and P limitation. This relationship was also seen in the heathland where there
was increased utilisation of monoester P sources in standard and customised BIOLOG
plates and increased respiration rates in soils amended with organic P compounds.
The implications of these results are discussed, with reference to the critical loads concept
and to recent research by other workers in similar and contrasting environments.