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Limitations in available techniques to separate autotrophic (root) and soil heterotrophic respiration have hampered the understanding of forest C cycling. The former is here defined as respiration by roots, their associated mycorrhizal fungi and other micro-organisms in the rhizosphere directly dependent on labile C compounds leaked from roots. In order to separate the autotrophic and heterotrophic com- ponents of soil respiration, all Scots pine trees in 900 m 2 plots were girdled to instantaneously terminate the supply of current photosynthates from the tree canopy to roots. Hogberg et al . ( Nature 411, 789-792, 2001) reported that autotrophic activity contributed up to 56% of total soil respiration during the first summer of this experiment. They also found that mobilization of stored starch (and likely also sugars) in roots after girdling caused an increased apparent heterotrophic respiration on girdled plots. Herein a transient increase in the d 13 C of soil CO 2 efflux after girdling, thought to be due to decomposition of 13 C- enriched ectomycorrhizal mycelium and root starch and sugar reserves, is reported. In the second year after girdling, when starch reserves of girdled tree roots were exhausted, calculated root respiration increased up to 65% of total soil CO 2 efflux. It is suggested that this estimate of its contribu- tion to soil respiration is more precise than the previous based on one year of observation. Heterotrophic respira- tion declined in response to a 20-day-long 6 ∞ C decline in soil temperature during the second summer, whereas root respiration did not decline. This did not support the idea that root respiration should be more sensitive to variations in soil temperature. It is suggested that above-ground pho- tosynthetic activity and allocation patterns of recent pho- tosynthates to roots should be considered in models of responses of forest C balances to global climate change. |
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