Influence of fir root zone on soil structure in a 23 m forest transect: the fractal approach

Abstract This paper reports the results of a joint colloidal chemistry and fractal approach to the study of aggregate structures in the A 1 (0–10 cm) and A 1 (B) (20–30 cm) horizons of a sandy acid brown soil collected along a 23 m transect under three fir trees (eastern France, declined fir ecosystem). The amounts of both organic carbon and Al oxalate clearly discriminate between the two horizons studied but no significant root zone differences were observed. However, the A 1 horizons collected in the fir root zone contained much more polysaccharides than the corresponding non-root zone horizons, and root zone soil polysaccharides had both a plant and a microbial origin. The impact of a poorly-ordered Al hydrous oxides-organic matter balance (Al ox -C significant negative correlation) on both physico-chemical and structural characteristics of soil millimetric aggregates was demonstrated. The soil zero point of charge (ZPC) was mainly correlated both with positively charged poorly-ordered Al hydrous oxides and with negatively charged organic matter: ZPC = 7.3Al ox (%)−0.14C(%)+3.69 ( r =0.85; P More porous and water-stable aggregates as fractal domains over a larger range of spatial scales were observed in the A 1 , organic horizon rather than in the deeper Al-rich A 1 (B) horizon. Finally, the surface fractal dimension D s (as determined by mercury porosimetry) was found to be a better discriminator than the porosity between the root zone and non-root zone of the soils.