Kyle Stratechuk | ALES Graduate Seminar

A graduate exam seminar is a presentation of the student’s final research project for their degree.
This is an ALES MSc Final Exam Seminar by Kyle Stratechuk. This seminar is open to the general public to attend.
Thesis Topic: Effects of Stockpiling on Soil Physical Properties and Soil Carbon

MSc with Drs. Sylvie Quideau and Miles Dyck.

Seminar Abstract:

The boreal forest is a major ecosystem in Alberta, subject to a number of anthropogenic disturbances such as oil and gas extraction. Prior to disturbance, top soils are salvaged for use in future reclamation projects; however, a large portion of this soil ends up in stockpiles, inevitably undergoing a number of changes to soil physical properties, aggregate size distribution, carbon distributions, and carbon dynamics. This study sought to assess changes in these parameters, compared against natural sites, to determine implications of stockpiling on the suitability of salvaged soil for future reclamation. Sampling sites were established at an open pit mine and a SAGD development in the boreal forest. Eight stockpiles and six natural sites were selected, with three soil pits being dug per site and three depths sampled per pit.

Soil physical quality changes were assessed using basic soil properties (clay content, bulk density, and total soil carbon), capacity-based measurements (field capacity, plant available water content, air capacity, macroporosity, and the S-index), and energy parameters (air and water retention energies). All of these variables relate to S, defined as the slope at the inflection point of a moisture retention curve, which provides insight on the pore-size distribution of a soil. Larger values denote higher soil physical quality. Clay content and bulk density were higher in natural soils, whereas total soil carbon, S, field capacity, plant available water content, air capacity, macroporosity, air energy, and water retention energy values were higher in stockpiled soils. Significant differences between natural and stockpiled soils were present only in the two subsurface depths. Higher levels of soil carbon, lower bulk densities, and lower clay contents in stockpiled soils were associated with the highest S values for sampled soils, supporting previous observations published in scientific literature. S values were positively correlated with most capacity-based measurements, supporting additional theories that link these parameters together. Air and water retention energies were also positively correlated with S values, though lower values for these parameters are associated with higher soil physical quality. Together, natural soils had lower soil physical quality than stockpiles using basic soil properties, capacity-based measurements, and S, while the outcome using air and water retention energies was inconclusive.

Changes to aggregate size class distributions (4 mm and 250 μm), carbon distributions (total carbon and light fraction carbon), carbon dynamics (basal respiration rates), and soil organic matter quality criteria (C:N ratios and ¹³C) were assessed at both the whole soil and individual aggregate scale. No significant differences in relative proportions of aggregate size classes were found between natural and stockpiled soils for any of the sampling depths, although natural soils did have higher proportions of both aggregate sizes. Higher basal respiration rates and d ¹³C values were found in natural soils, whereas stockpiled soils had greater total and light fraction carbon quantities, along with elevated C:N ratios. Significant differences between natural and stockpiled soils were largely confined to the two subsurface sampling depths. Basal respiration rates negatively correlated with total carbon, light fraction carbon proportions, and whole soil C:N ratios, and positively correlated with whole soil light fraction C:N ratios and d ¹³C values. Similar trends were for each aggregate size class.

The consistent differences between natural and stockpiled soils in subsurface depths, and a lack of differences at the soil surface, demonstrate a lack of pedogenic processes in stockpiled material. As such, stockpiled soils remained more uniform with depth, having characteristics similar to the surface layers of natural forest soils throughout the whole profile. While this may suggest an increased likelihood of reclamation success due to more favorable soil physical properties, the consistent lack of microbial function and changes to in situ organic matter pools may negate these physical benefits. Therefore, it may be advantageous to limit the use of stockpiled materials to the soil surface, where favorable physical properties are more likely to be associated with improved soil function, due to the influence of re-establishing vegetation on microbial community development and subsequent biogeochemical cycles.