Soil carbon fluxes in sub-alpine ecosystems

Background/Question/Methods

Many ecological processes in alpine areas of Australia are highly geared to expected changes in climate over coming decades. Changes in climate will likely include lesser duration and depth of snow cover, reductions in amount and greater seasonal variation in rainfall, and increased temperature extremes, as well as an overall increase in mean temperatures. Plant species and communities in alpine and sub alpine areas are seemingly often at the edge of their ecological range, making them sensitive to such changes. In Australia generally, including alpine and sub-alpine areas, fire plays a major role in determining the species composition of plant communities. High country landscapes are usually described as shifting mosaics of grassland, shrubland and woodland, where boundaries demarcating each community type are often sharp and due to combinations of climatic and past fire regimes.  In much of this high country, all three community types replace one another over scales of tens of metres.

Community type can affect soil respiration through its influence on the quality and quantity of organic material supplied to the soil through leaf litter, root decomposition and exudates.  The size and quality of inputs of potential substrates, are crucial ‘drivers’ of rates of respiration. Shifts in vegetation type due to fire and climate change are thus strong influences on stocks of soil carbon.

Results/Conclusions

We adopted an Arrhenius approach to modelling relationships between temperature and soil respiration in long-term laboratory incubations. We focused on woodland, shrubland and grassland in sub-alpine Australia. Rates of respiration were faster for shrubland soils than for other community types.  Initial rates of CO₂ efflux from shrubland (dominated by N-fixing genera) were almost twice that from woodland and five times that of grassland soils. Respiration rates declined during long-term incubations (>100 days) by > 85% for grassland, >66% for woodland and >60% for shrubland soils.  However, the temperature sensitivity of the labile and resistant carbon pools remained similar for all community types.

Climatic changes leading to shifts in vegetation type (possibly via shifts in fire regime), increased length of growing seasons, and changes in snow depth and duration, are likely to have marked influences on soil respiration through changes in carbon cycling.