Soil compaction around Eucalyptus grandis roots: a micromorphological study

Abstract

This study aimed to evaluate the effects of Eucalyptus grandis root growth on localised soil compaction and fracturing. Undisturbed soil (Kandiustox) samples subjected to root growth pressure were used, employing 2 methods to study the phenomenon: (i) micromorphological analysis of thin sections of soil samples compacted by roots of 0.3, 0.9, 1.3, 2.8, 3.5, 6.4, 8.0, 9.0, and 10.2 cm diameter, carried out in the zone under direct root influence, up to 1 cm from the root–soil surface, compared with a reference area at a distance of 3 cm from the contact surface; (ii) a localised infiltration test to assess the time taken to infiltrate one drop of water into the surface of root-compacted soils, compared with the time taken in a reference sample without root compaction. The soil compaction was greater around root diameters >3.5 cm, and this effect was accompanied by reduced water infiltration in the soil surface at the root contact. Presence of chiseling fractures at an approximate 45° angle to the soil surface suggested helicoidal growth of the E. grandis root, causing both soil compression and shearing. At microscopic level the soil-root contact showed clay-oriented features, microfractures, fungi coatings, and micro-slickensides. The lower infiltration rate in the compacted soil–root surface is associated with both physical (compaction) and chemical (possibly hydrophobicity) mechanisms. The use of micromorphological techniques and image analysis allowed the observation and quantification of soil porosity in the vicinity of roots.