Fluxes of nitrogen in a semi-natural ecosystem

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Abstract

Nitrogen (N) is known to be a limiting factor in peatlands and as such, the vegetation present has adapted to living in low N conditions. This makes such ecosystems particularly vulnerable to increases in the deposition of reactive N (Nr), which may result in significant changes to its biodiversity and biogeochemistry. Since the industrial revolution, the amount of anthropogenic Nr globally has increased from ~15 Tg N y-1 in the 1860’s to ~187 Tg N y-1 in 2005, and is estimated to reach ~267 Tg N yr-1 by 2050 (Galloway et al. 2004, Galloway et al. 2008). Consequences include acidification, loss of biodiversity, changes in vegetation, N-saturation, eutrophication, health impacts and the release of greenhouse gases.Objectives of this thesis were (i) to measure key components of the N-budget at Auchencorth Moss, a Scottish moorland, for a two year period (Jan 2009 – Dec 2010) and (ii) to compare current deposition rates with those measured 14/15 years previously. Annual fluxes of N inputs were estimated from measurements of wet only deposition, estimates of N-fixation deposition and from atmospheric deposition modelled from hourly concentrations of N containing gases and aerosols. Exports were estimated from stream measurements and from atmospheric emissions modelled from hourly concentrations. Organic N is often an underreported part of the N-cycle, but the results presented here suggest it is an important part of the N story. An attempt to identify (dissolved organic nitrogen) DON compounds in both precipitation and stream water was made using GC×GC-NCD. Ten unique compounds were detected, of which only five could be identified: pyrrole, benzonitrile, dodecylamine, N-nitrosodipropylamine and decylamine. Pyrrole, benzonitrile and three unknown compounds were present in both precipitation and stream samples. Ammonia (NH3) fluxes were measured over a 7 month period in 2009 using a wet-chemistry gradient system with online analysis and calculated with the aerodynamic gradient method. The results were used to refine a bi-directional dynamic exchange model. Several parameters were updated, including an increased stomatal emission potential from 180 to 350, a reduction of the minimum cuticular resistance (Rw,min) used to calculate Rw from 20 s m-1 to 15 s m-1 and an increase in the leaching rate (Kr¬) from the leaf surfaces from -0.01 to -0.1 s-1. The exchange parameterisations used to estimate HNO3, HONO and the aerosol compounds were taken from the literature and earlier studies at the site.Overall, Auchencorth was found to accumulate N, with deposition exceeding export by -1.61 kg N ha-1 yr-1. The main N deposition was from NH3, followed by wet deposition of ammonium. DON, which is not routinely included in N budgets, contributed 6.5% of total deposition. The largest loss of N was as DON via the stream with N losses of -5.31 kg N ha-1 yr-1 or 71.8% of total export. Between 1995 and 2009/2010, deposition decreased by 0.81 kg N ha-1 yr-1, with the wet deposition of inorganic nitrogen decreasing by 25.2%, but dry deposition increasing by 12.5%. DON, N2O and N-fixation were not included in the comparison as they were not measured in 1995.This thesis has demonstrated that DON is an important parameter in the overall N budget, and should be routinely measured when assessing the N status of ecosystems.

Keyword(s)

dry deposition, nitrogen budget; nitrogen, dissolved organic nitrogen, ammonia, GCxGC-NCD

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