Use of soil cores and minirhizotrons for the long-term study of the response of Norway spruce fine roots (Picea abies (L.) Karst.) to the increase in CO2 concentration in the atmosphere and nutrition. The fine root biomass of Norway spruce (Picea abies (L.) Karst.) grown in open top chambers (OTCs) under elevated CO2 since 1987 was investigated with soil cores and minirhizotrons, at Vielsalm (Ardenne, Belgium). Four levels of CO2 concentration (700, 580, 470 and 350 µmol.mol-1 CO2) were applied

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... in combination with two nutrition regimes: the optimal nutrition and the nutrition corresponding to the local brown forest soil. First soil coring was performed in 1997 to describe the root biomass and root distribution according depth, at the initial stage of this study. Soil coring was repeated in 2003. Two observation and measurement campaigns using minirhizotrons were carried out in 2001 and in 2002. These latter measurements shown a very large proportion of very fine roots within the root classes classically reported in the literature: ]0-1 mm], ]1-2 mm], ]2-5 mm] and > 5 mm. Among the 2252 roots we regularly observed in minirhizotrons, more than 90 % were thinner than 0.5 mm in diameter, less than 2 % were in the range [1-3 mm[ in diameter and their mean was 0.28 mm. Calculations on the 42 Biotechnol. Agron. Soc. Environ. 2004 8 (1), 41-53 Lebègue C., Laita É., Perrin D., Pissart G. roots observed with minirhizotrons shown that root biomass under elevated atmospheric CO2 concentrations (580 and 700 µmol.mol-1 CO2) were two times higher than root biomass under ambient concentration (350 µmol.mol-1 CO2). As these figures are based on specific root length (SRL), they are supposed to be largely underestimated. A larger proportion of these roots were exploring the upper soil horizons under elevated CO2. Norway spruce showed an annual production cycle of fine roots, independent from the atmospheric CO2 concentration. Increased CO2 affected root branching. Under elevated CO2 roots were shorter and four times more branched t [...]