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DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

Separating Soil Respiration into Plant and Soil Components Using Analyses of the Natural Abundance of Carbon-13

^a Agriculture and Agri-Food Canada, Soils and Crops Research and Development Centre, 2560 Hochelaga Blvd., Ste-Foy, QC, Canada G1V 2J3
^b Dep. of Biological Sciences, Univ. of Lethbridge, 4401 University Dr., Lethbridge, AB, Canada, T1K 3M4
^c Agriculture and Agri-Food Canada, Eastern Cereal and Oilseed Research Centre, Neatby Bldg., Central Experimental Farm, Ottawa, ON, Canada, K1A 0C6

rochettep{at}em.agr.ca

In presence of vegetation, the CO₂–C produced by respiration activity in soils originates from plant C (rhizosphere respiration, R_rh) and from soil C (soil respiration, R_s). Quantitative estimates of the CO₂ produced by each source are required in many studies of C dynamics in the soil–plant system. In this study, we (i) used measurements of the ¹³C value of soil CO₂ to separate total soil respiration (R_t) into subcomponents R_rh and R_s in a maize (Zea mays L.) field under undisturbed conditions and (ii) compared these R_rh estimates with values obtained using the root-exclusion approach. The maximum contribution of R_rh to total respiration was 45%, observed in August. Estimates of R_rh increased from zero 30 d after planting to 2 g CO₂–C m^-2 d^-1 70 d after planting, remained relatively constant at that level in August, and then decreased until the end of the growing season. The total C losses as R_rh were 17% of the crop net assimilation. Estimates of R_s gradually declined from 3.3 g CO₂–C m^-2 d^-1 in late June to 1.4 g CO₂–C m^-2 d^-1 at the end of the season. Losses of soil C represented 6% of total soil C. Variable values of ¹³C of the soil CO₂ in the control plot after Day 250 made the technique less reliable late in the season. However, several observations indicated that the approach has potential to provide quantitative estimates of R_rh and R_s. First, the seasonal pattern of the R_rh estimates coincided with that of the plant growth and physiological activity. Second, the cumulated R_rh across the growing season agreed well with published data obtained using ¹⁴C labeling techniques. Third, in the maize plot, variation in the estimated R_s was closely correlated with changes in soil temperature with a Q₁₀ of 1.99 . Finally, the estimates of R_rh obtained using the isotopic approach agreed well with those obtained using the root exclusion technique.

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