Reduced throughfall decreases autotrophic respiration, but not heterotrophic respiration in a dry temperate broadleaved evergreen forest

Abstract

Climate change may have major implications on soil respiration dynamics and the carbon sink strength of forest soils. To assess the effect of climate change on soil respiration (RS), it is crucial to understand individual responses of autotrophic (RA) and heterotrophic (RH) components. We investigated the effect of continuously (20 months) reduced throughfall (TFR, −40%) and the influence of seasonal changes in soil temperature and moisture on RS, RA and RH, partitioned by root exclusion, in a dry temperate broadleaved evergreen eucalypt forest in south-eastern Australia. TFR decreased mean RS from 4.7±0.1 (Control) to 3.8±0.1 (TFR) μmolCO2m−2s−1 (−19%). TFR indicated a strong decrease in RA from 2.5±0.1 (Control) to 1.5±0.1 (TFR) μmolCO2m−2s−1 (−40%), but had no effect on RH. The mean relative contribution of RH to RS was 47% in the Control and increased to 61% under TFR. RS was the result of distinct seasonal patterns and dependencies of RH and RA on environmental variables. Soil temperature was a good predictor of RH (Control: R2=0.72, TFR: R2=0.75), but not of RA. In contrast, RH was not limited by soil moisture, while RA was partly influenced by soil moisture (Control: R2=0.29, TFR: R2=0.56). The lack of response of RH to changes in soil moisture (seasonal and under TFR) was likely influenced by the high rainfall conditions such that soil moisture did not decrease to a point where it limited soil microbial decomposition processes. Our results show that TFR implied the strongest effect on RA and that changes in soil temperature and moisture alone do not sufficiently explain seasonal changes in RA and RS. This indicates that biotic factors, such as plant internal carbon allocation, may exert a stronger influence on RA and hence, RS. In short-term a reduction in rainfall will lead to a decrease of soil respiration in dry temperate broadleaved evergreen eucalypt forests. The magnitude of this decrease and its persistence under extended drought will be greatly influenced by seasonal and inter-annual climate variability and potential changes in plant carbon allocation.