Contribution of root and rhizosphere respiration to the annual variation of\rcarbon balance of a boreal Scots pine forest

Abstract

A large part of gross primary production (GPP) is consumed in root and rhizosphere respiration (Rr ). To measure Rr , a group of evergreen coniferous Scots pine (Pinus sylvestris) trees were girdled in a 45-year-old even aged forest in Hyyti ¨ al ¨ a, Southern Finland. In the girdling, phloem and bark were removed from breast height around the trees. We measured soil CO2 effluxes with a dynamic chamber at the girdled plot and at a non-girdled control plot in close vicinity. Before the girdling, effluxes were 22% higher at the plot to be girdled compared to the control plot. We scaled the measurements so that before girdling the effluxes representing total soil respiration (Rs) were at the same level. We compared the Rr and Rd to GPP measured with eddy covariance system. Our results show that Rr has higher seasonal variation than Rd , and also spatial variability was higher for Rr . The annual Rr :Rs and Rr:GPP-ratios were 0.36 and 0.21, respectively. Rr :Rd varied seasonally and in late summer and in autumn Rr exceeded Rd . Rr followed GPP with a delay of several weeks. During winter and spring Rr was very low, even when GPP and soil temperature had significantly risen. We conclude that Rr and Rd have different response to the environment and that for Rr the substrate availability is a more important explaining variable than soil temperature.