Separating rhizosphere respiration from total soil respiration in two larch plantations in northeastern China

The potential capacity of soil to sequester carbon in response to global warming is strongly regulated by the ratio of rhizosphere respiration to respiration by soil microbial decomposers, because of their different temperature sensitivities. To quantify relative contributions of rhizosphere respiration to total soil respiration as influenced by forest stand development, we conducted a trenching study in two larch (Larix gmelini (Rupr.) Rupr.) plantations, aged 17 and 31 years, in northeastern China. Four plots in each plantation were randomly selected and trenched in early May 2001. Soil surface CO₂ effluxes both inside and outside the plots were measured from May 2001 to August 2002. Soil respiration (i.e., the CO₂ effluxes outside the trenched plots) varied similarly in the two plantations from 0.8 μmol m^-2 s^-1 in winter to 6.0 μmol m^-2 s^-1 in summer. Rhizosphere respiration (i.e., CO₂ efflux outside the trenched plots minus that inside the plots) varied from 0.2 to 2.0 μmol m^-2 s^-1 in the old forest and from 0.3 to 4.0 μmol m^-2 s^-1 in the young forest over the seasons. Rhizosphere respiration, on average, accounted for 25% of soil respiration in the old forest and 65% in the young forest. Rhizosphere and soil respiration were significantly correlated with soil temperature but not with soil water content. We conclude that the role forests play in regulating climate change may depend on their age.