Direct and indirect effects of ammonia, ammonium and nitrate on phosphatase activity and carbon fluxes from decomposing litter in peatland

Here we investigate the response of soils and litter to 5 years of experimental additions of ammonium (NH 4), nitrate (NO 3), and ammonia (NH 3) to an ombrotrophic peatland. We test the importance of direct (via soil) and indirect (via litter) effects on phosphatase activity and efflux of CO 2. We also determined how species representing different functional types responded to the nitrogen treatments. Our results demonstrate that additions of NO 3, NH 4 and NH 3 all stimulated phosphatase activity but the effects were dependent on species of litter and mechanism (direct or indirect). Deposition of NH 3 had no effect on efflux of CO 2 from Calluna vulgaris litter, despite it showing signs of stress in the field, whereas both NO 3 and NH 4 reduced CO 2 fluxes. Our results show that the collective impacts on peatlands of the three principal forms of nitrogen in atmospheric deposition are a result of differential effects and mechanisms on individual components. We found that nitrogen deposition affects microbial activity associated with litter through both indirect and direct mechanisms, but these effects were dependent on the chemical form of inorganic nitrogen compounds.