Latent soil effects of grazing and ammonium deposition on Deschampsia flexuosa tillers inserted and grown in heather moorland soil

Abstract

Upland heather moorlands support a range of increasingly rare and threatened biota, making them a priority habitat for conservation and restoration. Over-grazing and nitrogen deposition are two of the most important threats to maintaining these heather moorlands, yet there remains a paucity of research into their combined long-term effects. During the summer of 2008, we established an experiment within an existing research site at Pwllpeiran, mid-Wales. The original site, established in 1996, investigated long-term grazing and N deposition treatments (ammonium and nitrate) on heather moorland. Previous findings from the site, following a decade of treatment, suggested no significant, detectable impacts of treatments on soil C:N ratio and the foliar nitrogen of vegetation.The aim of our study was to investigate short- (2008) and long-term (1996–2008) N deposition treatment impacts, in combination with past grazing (1990–2007), on soil nutrient bioavailability. Soil cores were harvested and aboveground vegetation removed. Tillers of the grass Deschampsia flexuosa were planted into these cores which were then reciprocally transplanted back into the field experiment. The N deposition treatment was continued but grazing was excluded. D. flexuosa biomass changes were next assessed and leaf chemistry investigated using the metabolic fingerprinting method Fourier-transform infrared spectroscopy (FTIR) following three months of growth in the field (May–August 2008).Grazing treatment (on its own) had significant negative impacts on aboveground biomass and significant changes in plant chemistry were also revealed through the metabolic fingerprinting method Fourier transform infrared spectroscopy (FTIR). Short-term N deposition treatments during 2008 had no impacts on D. flexuosa growth or chemistry. There were also no detectable latent effects of long-term nitrate treatments on either growth or chemistry of D. flexuosa. However, plants grown in plots that had received long-term treatments of ammonium (NH4+) had significantly lower poly-phenolic contents (revealed by FTIR) than plants grown in either nitrate (NO3−) or control plots, suggesting detectable latent effects of N application in its reduced form. Further work needs to be undertaken to assess the relevance of residual soil nitrogen pools post N deposition and grazing.