Using 15N to determine a budget for effluent-derived nitrogen applied to forest

Area of Science:

• Environmental Science
• Ecology
• Biogeochemistry

Background:

• Land-based municipal effluent irrigation schemes are used for wastewater management.
• Understanding nitrogen (N) fate in these systems is crucial for environmental sustainability.
• Stable isotope analysis offers a method to trace N within complex ecosystems.

Purpose of the Study:

• To determine the fate of effluent-derived nitrogen (N) in a land-based irrigation scheme.
• To quantify N storage and loss pathways within a production conifer forest and associated wetland.
• To assess the long-term sustainability of effluent irrigation based on N dynamics.

Main Methods:

• Stable nitrogen (N) isotope analysis was used to trace effluent-derived N.
• Over 900 metric tonnes of effluent N were applied to 192 ha of forest over 11 years.
• N storage in forest components (wood, soil) and wetland, and N export via streamflow were quantified.

Main Results:

• Forest storage accounted for 50% of applied N, with significant immobilization in wood and soil.
• The wetland retained 13% (115 t) of applied N, and denitrification accounted for 3% (23 t).
• Stream export of effluent N was estimated at 29% (263 t), with 88% of stream nitrate-N being effluent-derived.

Conclusions:

• The forest and wetland ecosystem intercepted or denitrified 65% of applied N.
• The forest ecosystem is currently over-supplied with N, suggesting current application rates are unsustainable.
• Long-term sustainability of the current effluent irrigation scheme is questionable due to N accumulation and export.