Nitrogen dynamics in alpine soils of south-eastern Australia

Abstract

Context: The Australian Alps are recognised by UNESCO as a globally significant mountain range. Soils underpin all of these ecosystem services. However, sparse data exists on alpine soils. Aims and methods: We explored nitrogen dynamics of soils from four high mountain sites, using a combination of new and established field and laboratory techniques. Key results: Organic and inorganic N were of the same order of magnitude, with around twice as much inorganic N as organic N. Forty three small (<250 Da) organic N compounds were detected, with concentrations 30 times greater in microbial and salt-extractable pools than free in the soil solution. The net N mineralisation rate decreased four-fold over the growing season. The organic matter decomposition rate was close to the global mean (k = 0.017), while the stabilisation factor was high (0.28) in comparison with other ecosystems globally. Conclusions: These results begin to illuminate the complexity of the belowground processes that have formed the high C soils of the Australian Alps. The combination of moderate turnover times and high stabilization of organic matter support Costin's theory that these mountain soils formed in place as a result of biological activity, rather than reflecting their geological substrata. The pools of organic N adsorbed to mineral soil surfaces and bound up within microbes lend support to a theory of tight N cycling, with little organic or inorganic N free in the soil solution. Implications: This new knowledge of soil N dynamics can support land managers to design successful restoration works to preserve alpine soil ecosystem services impacted by climate change, feral animal disturbance, weed invasion and the increase in summer tourism infrastructure.