We evaluated controls of N cycling on a precipitation gradient from <300 mm/yr to >3300 mm/yr on Kohala Volcano, Hawaii. The natural abundance of 15N is used widely as an integrated indicator of N cycling processes, but there are multiple steps in the N cycle at which N isotopes are fractionated, making it challenging to map N isotope abundance to its controls. We measured natural abundance 15N in multiple ecosystem pools along the gradient (soils, N fixing and non-fixing plants, and nitrate), and developed a simple model of N cycling to integrate its controls.
Results/Conclusions
The natural abundance of 15N varied substantially along the gradient. In soil it ranged from +3 per mil in the wettest sites to ~+14 per mil in sites receiving ~400 mm/yr of precipitation, before declining to ~+10 per mil in the driest sites. In non-N-fixing plants, it ranged from 0 per mil to +11 per mil, being parallel to soil but 3 to 4 per mil less enriched in 15N. Nitrogen-fixing plants ranged from -1 per mil to +4 per mil; 15N in fixers was most enriched (suggesting that N fixation contributed least to plant N) in the sites with the highest rates of net N mineralization and in the driest sites. Nitrate-15N ranged from -15 per mil to +14 per mil, it was close to soil organic 15N in the drier sites but substantially (and variably) depleted in the wetter sites. A simple model of N cycling, N fixation, and 15N fractionation as they vary with precipitation suggests that rainfall variation is an important underlying driver of N inputs, outputs, and availability (and so of 15N natural abundance) along the precipitation gradient, especially in the drier sites.