Effects of warming on nutrient dynamics in the Australian Alps

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the Department of Agricultural Science, School of Life Sciences, Faculty of Science, Technology and Engineering, La Trobe University, Bundoora.

In the Australian Alps, the predicted rise in temperatures, decrease in precipitation and shifts in plant communities could have important consequences on carbon and nutrient dynamics. In order to determine the future impacts of a warmer climate on the nutrient dynamics in alpine soils, this thesis aims to (1) investigate the effects of warming on the decomposition and nutrient dynamics of litters of a range of common alpine species, (2) determine how the warming-associated changes in nutrient dynamics from litter decomposition affect the nutrient dynamics in the soil, and (3) investigate the effects of long-term warming on the soil microbial potential to mineralize C and N with litter addition and also under dry-rewet conditions. In the first field experiment, warming effects on the decomposition and nutrient releases from litters of eight alpine species were investigated over 633 days. Warming, using open-top chambers, induced soil drying which reduced decomposition and nutrient releases. The second field experiment then aimed to determine how these nutrient releases affect soil nutrients under different climatic conditions. To that effect, litters of two dominant alpine species were submitted to three elevations over 321 days. Total mass and carbon losses and nutrient releases of both litters were lowest at the warmest elevation. Soil nutrients were unaffected by elevation and were immobilized under decomposing litters. Thus, short-term (0-2 years) warming and drying decreased litter decomposition and nutrient releases, with little direct impact on soil nutrients. The next experiment investigated the effects of 10-years warming on soil microbial activity and biomass, and its ability to decompose and mineralize carbon and nitrogen from two contrasting litter types. Previously warmed soils had reduced N mineralization, microbial biomass and activity, but greater relative increases in C and N mineralization when amended with litters. Lastly, the effects of long-term warming and dry-rewet frequency, on the decomposition of soil organic matter and nitrogen mineralization were investigated. Dry-rewet and increasing the number of dry days reduced microbial biomass and activity, while increasing inorganic nitrogen. However, after 16 days at optimum moisture, dry-rewetted soils had similar total carbon losses through respiration to constantly moist soils. In conclusion, warming-induced drying limited decomposition in the short-term. A consistent reduction in available substrate from litter decomposition under warming could explain the decline in microbial biomass and activity in the long-term and could potentially lead to decreased carbon and nutrient cycling in this ecosystem.