Separating soil respiration into plant and soil components using analyses of the natural abundance of carbon-13

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 C-13 value of soil CO2 to separate total soil respiration (R-s):into subcomponents R-rh and R-s in a maize (Zea mays L.) field under undisturbed conditions and (ii) compared these Rrh 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 Closses as R-rh ere 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 approximate to 6% of total soil C. Variable values of delta(13)C 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 Rib 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 C-14 labeling techniques. Third, in the maize plot, variation in the estimated R-s was closely correlated with changes in soil temperature with a Q(10) of 1.99 (r(2) = 0.87). Finally, the estimates of R-rh obtained using the isotopic approach agreed well with those obtained using the root exclusion technique.