The effects of nitrogen deposition on nitrous oxide fluxes from temperate and tropical forest soils

Abstract

The amount of anthropogenically-created reactive nitrogen (Nᵣ) that enters ecosystems has more than tripled over the past century. A major consequence is the increase in the potent greenhouse gas nitrous oxide (N₂O), with a global warming potential 273 times higher than carbon dioxide on a 100-year time horizon. Emissions of N₂O from natural, as opposed to agricultural, soils are substantial at approximately 6.4 (3.9 – 8.6) Tg N yr⁻¹, which represents around 35% of the total global N₂O budget. It is generally assumed that increases in N availability will lead to a rise in N₂O fluxes. Atmospheric N deposition is a major source of Nᵣ in natural ecosystems such as forests. However, the impacts of increasing N dry deposition on forest ecosystems have been widely ignored. This thesis investigates the impacts of increasing N deposition on N₂O fluxes using temperate and tropical forest soils in Scotland and Sri Lanka, respectively. This research employed a combination of an in-situ field experiment in a temperate birch forest (Glencorse, Scotland) and laboratory incubation experiments using both temperate soils and tropical soils from two different land uses – a secondary tropical forest and a tropical tea plantation. Atmospheric ammonia (NH₃) concentrations, and consequently deposition, were experimentally increased using a unique purpose-built automated NH₃ release system coupled to meteorological conditions in order to more realistically mimic atmospheric N deposition to natural ecosystems. Under laboratory conditions, N availability was increased using ammonium (NH₄₊) aqueous solution as opposed to the more-commonly utilised ammonium nitrate (NH₄NO₃) N enhancement approach, again to more closely mimic the processes in natural rather than agricultural ecosystems. Static and dynamic flux chambers were used to measure the change in N₂O concentrations under field and laboratory conditions, respectively. Fluxes of N₂O were then calculated using linear regression. A wide range of environmental parameters were also measured in order to better understand N dynamics in the study ecosystems. These included total N and total carbon (C) in both soil and vegetation tissue (moss), soil inorganic N availability (in the form of NO₃- and NH₄₊), atmospheric NH₃ concentrations, soil pH, and meteorological conditions (soil and air temperature and moisture, wind speed and direction). In contrast to existing theory, fluxes of N₂O did not increase in response to increased Nᵣ, neither in situ nor during the laboratory incubation experiments for temperate and tropical soils. Fluxes of N₂O remained low (<1.2 nmol m⁻² s⁻¹) for the most part of the experiments. A potential mechanism identified by which to explain these results was carbon limitation of the soil microbial community due to the relatively low C:N ratio (approximately 13.1) at the Glencorse study site. This theory was supported by the observation that adding a source of labile carbon (glucose) increased N₂O fluxes significantly during the laboratory incubation experiments. This corresponded to cumulative fluxes of 2.5 ± 0.7 N₂O-N ng g⁻¹ and 3091 ± 874 N₂O-N ng g⁻¹ following the application of N only and N + C, respectively. The findings of this thesis have practical implications for calculating emission factors, compiling national emission inventories and consequently climate change mitigation policies. Future research should focus on investigating C and N dynamics in response to changing Nᵣ inputs, both in the short and in the long term, and especially the potential trade-offs between increasing C stocks (such as using expansion of semi-natural forest area as a climate change mitigation measure) and the potential for increased N₂O fluxes arising from Nᵣ deposition to these systems