Autotrophic and heterotrophic respiration of a poplar plantation chronosequence in northwest China

The partitioning of soil respiration is helpful for understanding the mechanism of soil CO2 production and for estimating net forest C budget. We investigated heterotrophic (Rh) and autotrophic (Ra) components of soil CO2 efflux (Rs) in a poplar chronosequence (3-, 8-, and 15-yr-old) using a trenching method in 2007 and 2008. Our results showed that Rh and Ra varied with stand age. Mean Rh increased from 1.85 μmol m−2 s−1 in the 3-yr-old stand to 2.59 μmol m−2 s−1 in the 15-yr-old stand, while Ra decreased from 3.41 μmol m−2 s−1 in the 3-yr-old stand to 1.89 μmol m−2 s−1 in the 15-yr-old stand. There was a significant stand-age effect on Ra. Autotrophic respiration in the three stands all peaked in June, while the maximum Rh and soil temperature occurred in July. The seasonal variations of Ra and Rh could be well explained by soil temperature (P < 0.01). The contribution of Ra to Rs during the growing season was 57.1% averaged over the three stands. We found that Rh was positively correlated with dead fine root biomass (P < 0.01) and with forest floor litter mass (P < 0.05), and that Ra was significantly correlated with live fine root biomass. The root respiration significantly decreased with stand age, which was explained by a decrease in live fine root biomass over the age sequence, whereas the increase of heterotrophic respiration could be attributed predominantly to an increase in the labile C input with stand age. Our results highlight the importance of stand-age effects on different components of soil respiration and its significance to the estimation of forest C sink potential over a rotation.