Keystone perennial grassland species control soil nitrogen flows

Restoration of many grassland ecosystems is dependent on restoring native N-cycling regimes, through methods such as nutrient stripping, C addition and/or re-establishment of keystone species. We studied topsoil N pools and fluxes under five different understorey vegetation communities of different degradation states of grassy eucalypt woodlands across 11 sites in New South Wales, Australia in order to identify whether high N flux but low standing N concentrations were responsible for the success of reference Themeda triandra communities. The presence of NO3- was the main differentiator between the annual exotic (AE) and reference states, which both demonstrated 2-3 fold increases N flux relative to the depleted states. Reference states were characterised by high proteolysis and dissolved organic N (DON) mineralisation rates, but also low δ15N and a high C:N ratio, which taken together indicate fresh organic matter inputs that turn over quickly. In quantifying fast rates of N cycling yet low NO3- pools in the reference states, our data highlight a productive ecosystem dominated by rapidly cycling DON rather than NO3- in its reference state. The higher NO3- concentrations coupled with fast N cycling in the degraded AE state indicate that although available N production is high in both systems, uptake is lower in the AE state, perpetuating a larger standing NO3- pool that may prevent re-establishment of native species. Steps that promote rapid DON cycling while inhibiting nitrification may lead to a soil biogeochemical status more suitable for the restoration of highly productive native grasslands.