Soil Microstructure and Solution Chemistry of a Compacted Forest Soil in a Sub-Boreal Spruce Zone in Canada

This study was conducted to identify and quantify changes to soil microstructure and solution chemistry in order to understand the long-term effects of machinery-induced compaction on the soil. The study area near Log Lake, northeast of Prince George, BC, is one of three installations in the Sub-boreal Spruce Zone of the Long-Term Productivity Study of BC Ministry of Forests and Range. Selected plots representing major treatment combinations from the foregoing experiments were used for the current study. Soil thin sections from undisturbed (control) and compacted soils were prepared to study soil microstructure. Soil solution was extracted from field-moist samples using the immiscible displacement-centrifugation technique. Results showed that compaction reorganized soil substances and changed soil pore space characteristics. Porous types of soil microfabrics (granic, granoidic, plectic) that are common in control (uncompacted) soils, are replaced by dense types of soil microfabrics (such as banded or porphyric fabrics) in compacted soils. The microscopic porosity in compacted soils appeared lower than in control samples. Compaction decreased soil macroporosity and reduced the connectivity of soilpores as well. Void types that have less connectivity, such as vughs and planar voids, are increased in compacted soils, likely at the expense of voids that have -better -connectivity (compound packing voids). An increase in proportion of isolated vughs relative to overall soil microscopic pore space was also observed. This is probably related to formation of “relict” macropores, resulting from soil compaction. The decrease in soil microscopic pore space and pore connectivity might have restricted nutrient movement or enhanced precipitation of organic salts. Soil solution chemistry investigation revealed a decrease in Ca and K in soil solution to such a degree that might be sufficient to curtail vigorous plant growth.