Some characteristics of soil respiration in hybrid poplar plantations in northern Alberta

Fast-growing hybrid poplars are being planted in the Canadian prairies to meet the increasing demand for fibre and environmental services of trees and forests; however, the impact of hybrid poplars on C dynamics and storage on previously farmed land is largely unknown for the boreal region. We measured soil CO2 efflux along a chronosequence (3-, 9-, and 11-yr-old stands) of hybrid poplar (Populus deltoides x Populus x petrowskyana var. Walker) plantations and a control agricultural field from June to August 2004. Measurements were made between 0800 and 1800 with a portable Li-Cor 6400-09 system and were based on 4-5 min averaging. We also measured the response to simulated rainfall and the diurnal fluctuation of soil CO2 efflux. Soil CO2 efflux ranged from 1.30 mu mol CO2 m(-2) s(-1) in the 3-yr-old plantation to 5.41 mu mol CO2 m(-2) s(-1) in the agricultural control field, or from 0.17 mu mol CO2 m(-2) s(-1) kg(-1) C (based on soil organic C content to a 0.4 m depth) in the 3-yr-old plantation to 1.09 mu mol CO2 m(-2) s(-1) kg(-1) C in the 11-yr-old plantation. Simulated rainfall applied in the 3-yr-old plantation and a newly planted site resulted in an immediate pulse of CO2 efflux, 2.90 and 2.54 mu mol CO2 m(-2) s(-1), respectively, followed by an efflux rate sustained slightly above pre-irrigation levels. No secondary pulse of soil respiration was observed in the 2-h period following water application. Diurnal variation of soil respiration was found to be small between 0600 and 1900 in the agricultural control field, with values that varied from 2.66 to 3.17 mu mol CO2 m(-2) s(-1). Continued monitoring of soil respiration and other C cycling processes in the chronosequence will improve our understanding of the potential for C sequestration in hybrid poplar plantations in northern Alberta.