Nitrogen Retention by Sphagnum fuscum in Laboratory Mesocosms: Responses to Experimentally Added NH4+-N and NO3--N

To evaluate the effects of increasing atmospheric N deposition on net C and N accumulation in Sphagnum-dominated bogs, we conducted a laboratory mesocosm experiment. Following 45days of watering with N-free rainwater, mesocosms were exposed to simulated atmospheric N deposition values of 0, 4.14, 8.18, and 12.42mgNm(-2) da(-1) over a 108day period. We quantified N retention from leachate N concentrations and from changes in N contents of the Sphagnum/peat mesocosms. As N loading of simulated atmospheric N deposition increased, so did the net retention of simulated atmospherically deposited N. Our hypothesis of a decrease in N retention efficiency with increasing N loading was not observed. Further, for each N loading treatment, rates of retention of simulated atmospherically deposited N remained constant over time. We did not observe a threshold N deposition below which N would be retained in mesocosms and above which N passed through the mesocosms. Substantially more N was retained in mesocosms that was added in simulated atmospheric N deposition, suggesting an unaccounted for N source, most likely biological N-2 fixation. In locations where N-2 fixation represents a much greater source of new N to Sphagnum-dominated bogs, as in northern Alberta, Canada, increasing atmospheric N deposition may have a minimal effect on overall net N accumulation in peat.