Characterization of soil spatial heterogeneity and improvement of capping materials for oil sands mine reclamation
Dietrich, S.T.. 2018.
Abstract
Surface mining in the Athabasca Oil Sands Region (AOSR), Alberta, Canada creates a
large-scale ecosystem disturbance requiring 'land reclamation'. Mining approvals require that
land reclamation returns the sites to an equivalent land capability class, but this goal has proven
challenging to quantify. Restoring 'ecosystem function' might be more realistic and perhaps,
quantifiable by examining spatial patterns of soil functional indices. Cover soil prescriptions that
emulate close-to-nature conditions and recreate similar realized niches for the reestablishment of
native flora and fauna can increase reclamation success. Surface applications of forest floor
mineral-mix (FFM) sourced from upland forest ecosystems and peat mineral-mix (PM) sourced
from lowland ecosystems are termed "cover soil" and used as a proxy for replacing native
surface soils in upland forest reclamation; yet, there is a wide margin between these surface
amendments and native surface soils. The focus of this research first was the spatial
characterization of heterogeneity of key soil functions such as nutrient bioavailability, soil
respiration, and microbial biomass. We compared reclaimed sites and natural benchmark sites in
the AOSR. Secondly, methods for improvement of cover soil performance were tested.
In 2013 a field study was initiated that compared four different sites: 1) FFM reclaimed
site; 2) PM reclaimed site; 3) type b ecosite recovering from fire; 4) type ab ecosite recovering
from timber harvesting. We identified differences in spatial heterogeneity of nutrient profiles and
soil respiration. Key bioavailable nutrients such as P were significantly different on PM and
showed no heterogeneity. Seasonal patterns of respiration showed variability on natural reference
sites and on FFM, indicating that disturbance had not removed belowground function
completely. PM reclaimed sites showed no strong seasonal respiration patterns indicating
homogeneous belowground function. During 2014 we measured soil microbial biomass (SMB) and soil respiration on six different sites. Two natural benchmarks were added including a 5)
mature type a ecosite forest stand and 6) a type a ecosite recovering from fire to further
characterize spatial heterogeneity on type a ecosites. The data indicated that there were
differences in the pattern of soil respiration. Reoccurring patterns of soil respiration on the
Harvested a/b ecosite indicated a linkage of above- and belowground function. The PM
reclaimed site showed large-scale spatial patterns in SMB, similar to the type a ecosite affected
by severe fire. The FFM site showed smaller scale spatial pattern than sites disturbed by clear-
cutting and severe forest fire. We demonstrated that the amount of SMB and its heterogeneity
increased with time since disturbance, potentially indicating stages of site recovery on
benchmark type a ecosites.
Boreal forest soils of by reclamation targeted ecosites are characterized by higher mineral
soil fractions and notably they contain pyrogenic carbon (PyC) as a native soil component that
affects biogeochemistry. Biochar is a humanmade analog for PyC and its amendment to cover
soils used for reclamation might be a suitable method for reestablishing ecosystems that function
more similarly to upland forest ecosystems recovering from fire. Observations indicated that tree
growth on the tested PM reclaimed sites was lower in comparison to FFM reclaimed sites.
Accordingly, methods for cover soil improvement were tested. The effect of admixing subsoil
with peat and amendment of peat biochar on bioavailable nutrients, foliar nutrient concentration
and stoichiometry, aspen (Populus tremuloides Michx.) growth, soil respiration, root exudation,
and soil organic matter (SOM) stability was evaluated in two greenhouse studies. Seedling
growth increased with admixing subsoil with and without biochar, and there was an overall
positive effect of amendment with biochar in the first study and significant positive effect of
biochar amendment on seedling growth and a reduction of soil respiration in the second study. Our findings suggest that seedlings grown on PM and peat-subsoil mixes were potentially
affected by nutrient deficiency as well as toxicity. Biochar improved seedling nutritional status
and soil organic matter stability was positively correlated with tree growth and increased with
biochar amendment.
Overall our studies demonstrate that by increasing heterogeneity in key soil functions, as
well as assimilation of close-to-nature conditions, PM reclaimed sites can be improved. It is
likely that this could be achieved by admixing of mineral subsoil, biochar amendment, and
potentially targeted fertilizer application.
