Nutritional niche separation in coexisting bog species demonstrated by ¹⁵N-enriched simulated rainfall

Empodisma minus and Sporadanthus ferrugineus (both Restionaceae) coexist in New Zealand raised bogs, yet Sporadanthus have significantly more depleted ¹⁵N natural abundance signatures than coexisting Empodisma. Their root systems are spatially separated with Empodisma having a thick surface layer of about 50 mm of cluster roots overlying the deeper Sporadanthus roots. We hypothesized this root displacement allows Empodisma to preferentially access the primary N input from rainfall, thus establishing niche separation, and tested this using tracer stable isotopes. We aerially applied 1.6 mmol m⁻² of ¹⁵N as (NH₄)₂SO₄ chased by deionized water to simulate a rainfall event of 34 L m⁻². Root/peat matrix cores were harvested after 5 h and analysed for ¹⁵N uptake. Approximately 80% of the tracer applied was recovered in the cores, with 90% of this recovered in the upper Empodisma cluster root layer. Seven weeks after application, young shoots of Empodisma were significantly enriched (mean δ15N = +7.21‰; reference = −0.42‰), whereas those of coexisting Sporadanthus were not (mean δ15N = −2.76‰; reference = −4.24‰). However, we were unable to quantify the ¹⁵N uptake because of the dilution effect of the large biomass. We calculated the contribution of biological nitrogen fixation as a possible alternative source of N in achieving niche separation. The acetylene reduction assay showed minor amounts of nitrogenase activity are associated with Empodisma and Sporadanthus roots (equivalent to 0.045 ± 0.019 and 0.104 ± 0.017 kg N ha⁻¹ year⁻¹respectively). Our results suggest that the species acquire nutrients from different rooting zones, with Empodisma accessing nutrients at the surface from rainfall and Sporadanthus accessing nutrients from mineralization in deeper peat layers. Such niche differentiation probably facilitates species coexistence and may provide a mechanism for slowing the rate of competitive displacement during long-term succession