The Relationship of Soil 15N to N Status in New Zealand Pasture and Intensive landuse systems.

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.