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Carbon and Nitrogen Mineralization and Microbial Succession in Oil Sands Reclamation Soils Amended with Pyrogenic Carbon Open Access


Author or creator
Mackenzie, M.D.
Hofstetter, S.
Hatam, I.
Lanoil, B.
Additional contributors
Tar Sands
Nitrogen Mineralisation
Carbon Mineralisation
Oil Sands
Peat Mineral Mix
Type of item
Canada, Alberta, Fort McMurray
Land reclamation of oil sands disturbed boreal forests in Alberta is a challenging task facing companies with surface mine leases. The government requires reclamation to equivalent land capability, which is a vague statement at best. Agronomic theories and methodologies have been applied in the past with mixed success. We believe that ecological theory and new methods, designed to tease apart ecosystem function, should be applied to reclaiming ecosystems similar to native forests. This new reclamation ecology should start with disturbance theory and in boreal Alberta that means recovery from wildfire. All organisms in boreal forests, from the biggest trees to the smallest bacterium, are adapted to regular pulses of fire. Fire causes plant mortality and therefore changes in competition and resource availability. It also results in the partial combustion of organic matter, which creates pyrogenic carbon (PyC) and changes the soil chemical environment. Pyrogenic C is the substrate legacy of fire. It is resistant to decomposition and remains in the soil for hundreds of years. It has high surface area and adsorbs organic and inorganic compounds readily, which affects the availability of nutrients. It can be manufactured by an industrial process called pyrolysis, where is referred to as biochar. We believe that rebuilding native forest soils in the reclamation environment will require the use of biochar to stimulate functional similarity to native ecosystems, in terms of nutrient availability and microbial community succession. This is because peat is being used as a reclamation surface soil, but does not follow the typical first-order decomposition kinetics of native forest soils, due to distinct differences in organic matter quality. We believe that PyC will help to align peat decomposition kinetics and retain nutrients in surface soils. We also believe that it will create microbial community diversity and structure to be similar to the NFS. A 90 day laboratory incubation was conducted to examine the effect of PyC additions on carbon (C) and nitrogen (N) mineralization in two common oil sands reclamation surface soils, peat mineral mix (PMM) and forest floor mineral mix (FFM), and one native forest soil (NFS) recovering from wildfire. Three different kinds of PyC were used in the incubation, including charcoal collected from a local wildfire event, biochar pyrolyzed from willow chips, and petroleum coke, a by-product of oil sands upgrading. Micro-lysimeter chambers were used to build small soil columns of each soil type, to which PyC was added in replicate. These micro-lysimeters allowed for gas sampling from a soil head space for analysis of microbial respiration and therefore activity, and soil solution sampling for analysis of inorganic N. Samples were collected and analyzed on days 0, 1, 3, 7, 10, 14 and then every week after that for the duration of the incubation. After incubation, soil samples were extracted for microbial sequencing by paired end Illumina sequencing of the 16S rRNA gene for bacteria and ITS 1-2 gene for fungi to examine microbial community diversity and structure. Results indicated that the different PyC types increased C mineralization compared to the control, which suggests that it stimulates microbial activity and therefore respiration similar to the NFS. Literature also suggests that it undergoes some surface modifications at the molecular level upon addition to soils, which we also feel is reflected in the increased respiration. In contrast, PyC caused a decrease in N mineralization, which we believe is the result of N retention on PyC. This stems mostly from the fact that it is counter-intuitive that microbial activity would be increased (respiration), but the product of that activity (inorganic N) decreased. In reality, it is incredibly difficult to measure nutrient retention on PyC, but some literature provides evidence for this theory. PyC did not align decomposition kinetics for PMM as we believed it would, except in the case of N mineralization with biochar perhaps making it a soil amendment worth more attention. Pet-coke consistently performed the same or worse than the control in terms of C and N mineralization. Molecular sequencing of bacterial DNA showed that PyC, except for pet-coke, increased diversity in both the FFM and NFS, but not PMM which was higher to begin with. In ordination space, there is clear microbial succession from the control to the biochar, which indicates that biochar has a strong effect on the community structure. Fungal sequencing indicated that FFM had the highest diversity which was lowered by biochar to the level of NFS. However, no clear effect of PyC could be established for fungal community structure. These results clearly indicate that the addition of PyC has an effect on C and N mineralization, and microbial community diversity and structure in oil sands reclamation surface soils. The direction and magnitude of the effects has some similarity to the effect of PyC in NFS, and therefore should be considered as a soil amendment. However, the full interpretation of these results requires more work in terms of prescribing surface soil mixtures that will lead to a high degree of similarity between reclaimed ecosystems and native ecosystems. This work provides some preliminary evidence to support a paradigm shift towards reclamation ecology.
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