Movement of nitrogen through a riparian forest in a tropical, agricultural landscape

Connor, Sarah (2012) Movement of nitrogen through a riparian forest in a tropical, agricultural landscape. PhD thesis, James Cook University.

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Riparian zones have been widely reported to function as effective buffers, removing nitrate (NO₃⁻) from groundwater before it is discharged into adjacent streams. This is particularly important in agricultural catchments where additional nitrogen (N) from fertilisers may be leached into groundwater. On coastal plains, NO₃⁻ in groundwater discharged into streams can potentially enrich coastal waters. The permanent removal of NO₃⁻ through denitrification can improve water quality, however incomplete denitrification produces nitrous oxide (N₂O), a greenhouse gas.

Despite copious research in temperate regions, little study has been conducted on the capacity of riparian zones to remove NO₃⁻ from groundwater in the tropics. In agricultural areas of the Australian humid tropics, annual rainfall is high, around 3000 mm, and wet and dry seasons are clearly defined. Wet seasons are characterised by rainfall of high intensity and duration, followed by a dry season producing sporadic small amounts of rainfall.

The overarching questions of this thesis are: in an agricultural landscape in the humid tropics, is NO₃⁻ in groundwater removed as it enters a forested riparian zone and is transported towards the stream? And, are there temporal and spatial differences in patterns of N₂O emissions produced from the riparian forest?

This research is focused on a forested riparian zone 150 m wide, located amongst sugarcane fields, on the coastal plain adjoining the World-Heritage listed Great Barrier Reef lagoon, in the Australian humid tropics.

To gain an understanding of the movement of groundwater through the riparian site, the hydrology of the riparian zone was characterized using measurements of soil water content and water table depth (13 piezometers). In the wet season the system was highly dynamic with large fluctuations in water table levels and long-term inundation of low lying areas. Rapid water table rises were attributed to high in-situ recharge, low air-filled pore space (unsaturated zone), air entrapment and occasional recharge from the creek, and the rapid falls to the steep local hydraulic gradients. The dry season was characterised by a slow moving system with depth to watertable up to 4 m at high locations.

Groundwater entering the riparian zone was found to have low concentrations of NO₃⁻ (mean <0.03 mg NO₃⁻N L⁻¹ over both seasons), however, concentrations increased (by up to 50 fold) as groundwater progressed through the riparian zone, suggesting the riparian zone was a potential source of NO₃⁻ to the adjacent creek. The addition of NO₃⁻ was attributed to nitrification in riparian surface soils, driven by large net primary productivity, including large amounts of litterfall (12.19 Mg ha⁻¹y⁻¹). Nitrate generated in riparian soil was subsequently leached into groundwater in the wet season during rainfall events. Nitrate was also derived from nitrification in groundwater and, potentially, from the mixing of deeper groundwater of higher NO₃⁻ concentrations.

The tropical riparian forest is a large emitter of N₂O, with soil emissions ranging from -24 to 657 μg N₂O-N m⁻2 h⁻¹. Emissions were highest in the wet season but not significantly different between sites of high and low elevation. Similarly, there was no effect on emissions from differing amounts of leaf litter accumulated on the soil surface. Based on differences in soil water content, N₂O emissions were attributed to both denitrification and nitrification. Nitrous oxide was also produced at depth, within the unsaturated zone, with concentrations in soil air ranging from. 0.7 to 157 μg N₂O-N L⁻¹. Dissolved N₂O concentrations in groundwater were greatest at 1.5 m depth and correlated well with NO₃⁻ concentrations in groundwater. Laboratory experiments determined that riparian surface soils were clearly a source of N₂O, and that soils had limited capacity to further reduce N₂O to dinitrogen (N₂). Denitrification potential was up to 20 times greater in surface soils than in soils at 0.5 m depth.

This study has demonstrated that groundwater leaving cultivated land does not always have high concentrations of NO₃⁻ and that NO₃⁻ concentrations are not necessarily reduced during the passage of groundwater through riparian forest. Nitrate generated within the riparian zone, on the other hand, was leached into groundwater during heavy rain events and became a potential source of NO₃⁻ to the creek at our site. Similarly, nitrification and denitrification processes within the forested riparian zone contributed substantial N₂O emissions to the atmosphere, similar in range to emissions reported for other tropical forests. Although there are many environmental benefits to having healthy riparian forest, this study in a humid tropical environment did not show any water quality benefits.

Item ID: 28286
Item Type: Thesis (PhD)
Keywords: agricultural catchments, agricultural pollution, denitrification, groundwater, nitrates, nitrogen, nitrogen cycle, nitrous oxides, riparian forests, runoff, tropical forests, tropics
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Connor, S., Nelson, P.N., Armour, J.D., and Hénault, C. Hydrology of a forested riparian zone in an agricultural landscape of the humid tropics. Agriculture, Ecosystems and Environment. (In Press)

Date Deposited: 08 Aug 2013 02:52
FoR Codes: 04 EARTH SCIENCES > 0406 Physical Geography and Environmental Geoscience > 040608 Surfacewater Hydrology @ 50%
05 ENVIRONMENTAL SCIENCES > 0501 Ecological Applications > 050104 Landscape Ecology @ 50%
SEO Codes: 96 ENVIRONMENT > 9611 Physical and Chemical Conditions of Water > 961102 Physical and Chemical Conditions of Water in Coastal and Estuarine Environments @ 50%
96 ENVIRONMENT > 9611 Physical and Chemical Conditions of Water > 961103 Physical and Chemical Conditions of Water in Fresh, Ground and Surface Water Environments (excl. Urban and @ 50%
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