Impact of stockpile depth and storage time on soil microbial communities

Abstract

Soil removal and stockpiling prior to mining activities have adverse effects on soil health, which raises concerns about the suitability of stockpiled soils as a reclamation substrate. Due to the fundamental roles microbes play in ecosystem function and services, they can be used as indicators of soil health and quality. We analyzed the prokaryotic microbial communities in a chronosequence of 0.5–28-year-old stockpiles at increasing depths (0–300 cm), in two oil-extraction locations in northern Alberta. Our results indicate that the microbial communities of the seven stockpiles examined were more similar to each other than to the undisturbed soils used as reference, which might indicate that stockpiling shifts the microbial community composition outside the range of natural variability. Furthermore, while microbial communities in younger and older stockpiles were dissimilar to the reference soils, the communities of intermediate-age stockpiles were more similar to those in the reference soils. We interpret these divergences in similarity to indicate that initial disturbance leads to a shift in the microbial community, which then recovers following several years of storage, but eventually, long-term storage leads to a secondary divergence from the range of natural variability. Additionally, the bacterial diversity decreased significantly with increasing stockpile depth, which could be attributed to the harsh conditions of the deeper stockpile layers and the scarcity of nutrients. Our findings provide important insights into the impact of soil stockpiling on microbial communities and complement those of earlier studies highlighting the importance of stockpile depth and storage time as predictors of variability of soil microbial diversity and community composition. Furthermore, due to the crucial functions played by microbes in soils, our conclusions should serve for reclamation agencies to modify the stockpiling protocols in order to mitigate the impact of stockpiling on soil microbial communities.