Effects of global change and human disturbance on soil carbon cycling in boreal forest: A review

Increasing human demands for Earth's resources are hastening many environmental changes and creating a need to incorporate the routine monitoring of ecosystem functions into forest management. Under global change and anthropogenic disturbances, soil carbon (C) cycling in terrestrial ecosystems is undergoing substantial changes that result in the transformation between soil C sources and sinks. Therefore, the forest C budget requires an understanding of the underlying soil C dynamic under environmental disturbances. The present review focuses on the response and feedback of soil C cycling to global change (climate warming and nitrogen (N) deposition) and human disturbances (fire and logging) and detects the association of soil C cycling with soil C and N efflux and inflow in boreal forests. The effects of climate warming and N deposition on soil C cycling are complex, especially at short-term temporal scales. Climate warming can decay soil organic matter (SOM) to emit substantial amounts of CO2, and differing warming durations result in different effects on soil C loss, ranging from ca. 1 to 15 Mg C ha-1. Short-term soil warming mainly reduces the labile soil C pool and increases the decomposition of recalcitrant soil C compounds (e.g., lignin), whereas longer-term warming may limit soil C loss due to impoverished soil C substrate and microbial communities. Moderate N addition is conducive to enhancing soil C storage (ca. 2-22 Mg C ha-1), by increasing plant productivity including above-and belowground biomass; however, chronic N deposition or excess N addition can result in soil acidity, reducing N use efficiency and plant growth and further resulting in no changes or declines in soil C pool. Fire and logging lead to a large quantity of soil C loss via impaired plant productivity and increased organic matter degradation, exacerbating global warming. In particular, severe fire can cause a large amount of soil C loss, ca. 16-34 Mg C ha-1 in the data we reviewed. Meanwhile, the black C input induced by fire and the plant residual C input from the roots of logged trees can increase the proportion of recalcitrant soil C and enhance the stability of soil C pool. We also highlight the positive feedback of forest restoration to soil C storage after fire and logging disturbances, indicating that effective forest restoration projects (e.g., afforestation and natural forest recovery) are necessary to sequester soil C belowground. Additionally, combined with microbial technologies and metagenomics-and metabolomics-based approaches, soil microorganisms are proved crucial for driving soil C cycling via C capture and the N recycling of plants and soil. We, therefore, suggest that clarifying the relationship among plant, SOM, and microorganisms is essential to better evaluate soil C cycling and to predict how boreal forests respond to global change and human disturbances. Further work is needed to assess long-ter m soil C feedback from high-latitude forests to broader regions.