Vegetation type determines heterotrophic respiration in subalpine Australian ecosystems
Soils are the largest store of carbon in the biosphere and cool-cold climate ecosystems are notable for their carbon-rich soils. Characterizing effects of future climates on soil-stored C is critical to elucidating feedbacks to changes in the atmospheric pool of CO2. Subalpine vegetation in south-eastern Australia is characterized by changes over short distances (scales of tens to hundreds of metres) in community phenotype (woodland, shrubland, grassland) and in species composition. Despite common geology and only slight changes in landscape position, we measured striking differences in a range of soil properties and rates of respiration among three of the most common vegetation communities in subalpine Australian ecosystems. Rates of heterotrophic respiration in bulk soil were fastest in the woodland community with a shrub understorey, slowest in the grassland, and intermediate in woodland with grass understorey. Respiration rates in surface soils were 2.3 times those at depth in soils from woodland with shrub understorey. Surface soil respiration in woodlands with grass understorey and in grasslands was about 3.5 times that at greater depth. Both Arrhenius and simple exponential models fitted the data well. Temperature sensitivity (Q10) varied and depended on the model used as well as community type and soil depth - highlighting difficulties associated with calculating and interpreting Q10. Distributions of communities in these subalpine areas are dynamic and respond over relatively short time-frames (decades) to changes in fire regime and, possibly, to changes in climate. Shifts in boundaries among communities and possible changes in species composition as a result of both direct and indirect (e.g. via fire regime) climatic effects will significantly alter rates of respiration through plant-mediated changes in soil chemistry. Models of future carbon cycles need to take into account changes in soil chemistry and rates of respiration driven by changes in vegetation as well as those that are temperature- and moisture-driven.
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