Rethinking the sub-Antarctic terrestrial N-cycle: evidence for organic N acquisition by Marion Island grasses

Organic N (oN, e.g., amino acids) is an important N-resource for plants in soils replete with oN but not inorganic N (iN; i.e., NH₄⁺ and NO₃⁻), such as cold ecosystems with temperature-limited soil decomposition rates. However, sub-Antarctic literature assumes that plants only acquire iN, potentially underestimating plant-available N. We hypothesised that Marion Island (− 46.90°, 37.75°) grasses (Polypogon magellanicus, Poa cookii, Agrostis stolonifera and Poa annua) acquire oN and that oN relative to iN provision affects plant growth. We investigated oN and iN uptake and growth responses in two hydroponics experiments. In situ N (¹⁵N-glycine, ¹⁵NO₃⁻ and ¹⁵NH₄⁺) acquisition was investigated at three field sites with decreasing faunal influence, thus iN input and microbial activity. When plants grown in mire water were supplied with ¹⁵N-glycine or ¹⁵NO₃⁻, root δ¹⁵N enrichment was highest for glycine-supplied plants. In the second hydroponics experiment, plant N-uptake rates (nmol g biomass⁻¹ s⁻¹) were significantly higher for glycine than NO₃⁻, but relative growth rates (g g⁻¹ d⁻¹) lower on glycine. There were species-specific biomass allocation responses to N concentration (4 mM and 0.4 mM) and N-form (glycine and NO₃⁻). Glycine-supplied grasses at the low iN concentration field sites had significantly higher δ¹⁵N enrichment relative to those at sites with high iN, suggesting higher oN uptake when iN is limiting. We demonstrate the importance of accounting for oN acquisition in the sub-Antarctic. As a system with high soil oN relative to iN, plants may predominantly meet N-demands through oN rather than iN acquisition.