2008 Joint Annual Meeting (5-9 Oct. 2008): The Relationship of Soil 15N to N Status in New Zealand Pasture and Intensive landuse systems.
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698-24 The Relationship of Soil 15N to N Status in New Zealand Pasture and Intensive landuse systems.



Tuesday, 7 October 2008
George R. Brown Convention Center, Exhibit Hall E
Bryan Stevenson, Landcare Research, Private Bag 3127, Hamilton, New Zealand, Roger Parfitt, Landcare Research, Palmerston North, New Zealand, Troy Baisden, GNS Science, Lower Hutt, New Zealand and Louis Schipper, University of Waikato, Private Bag 3105, Hamilton, New Zealand
The concept of N saturation has largely been developed in forested ecosystems of the northern hemisphere where chronic atmospheric N deposition is a large input into these N poor systems. Soil 15N values have been suggested as an indication of N saturation and/or N loss in systems since nitrification and denitrification fractionate the N isotopes in soil. Additionally, past studies have indicated that soil 15N values generally correlate with C/N ratios. Agriculture (particularly pasture) and other intensively managed landuse systems receive large inputs of N and may also be approaching N saturation, with consequences for nitrate leaching to surface and groundwater.

We hypothesised that fractionating N losses (as opposed to non-fractionating losses or simply rate of N cycling) from intensively managed systems would lead to enriched (more positive) soil 15N values. In initial work to verify this hypothesis, soil 15N, C/N ratio, and 56-day N mineralisation were measured in a variety of New Zealand’s agronomic systems. Preliminary data indicates that soil 15N values become progressively more enriched with increasing management intensity, from a mean δ15N value of  approximately 1.0 ‰  in (unmanaged) indigenous forest (n=20) to approximately 5.3 ‰ (n=30) in dairy pasture. Furthermore, δ15N values correlate more strongly with soil C/N ratio than N mineralization rate. We discuss the relationships between 15N, C/N ratio, and N mineralization rate across these systems.

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