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Increasing atmospheric CO2concentration impacts the terrestrial carbon (C) cycle by affecting plant photosynthesis, the flow of photosynthetically fixed C belowground, and soil C pool turnover. For managed agroecosystems, how and to what extent the interactions between elevated CO2and N fertilization levels influence the accumulation of photosynthesized C in crops and the incorporation of photosynthesized C into arable soil are in urgent need of exploration. We conducted an experiment simulating elevated CO2with spring wheat (Triticum aestivumL.) planted in growth chambers. 13C-enriched CO2with an identical 13C abundance was continuously supplied at ambient and elevated CO2concentrations (350 and 600 μmol mol–1, respectively) until wheat harvest. Three levels of N fertilizer application (equivalent to 80, 120, and 180 kg N ha-1soil) were supplied for wheat growth at both CO2concentrations. During the continuous 62-d 13CO2labeling period, elevated CO2and increased N fertilizer application increased photosynthesized C accumulation in wheat by 14%–24% and 11%–20%, respectively, as indicated by increased biomass production, whereas the C/N ratio in the roots increased under elevated CO2but declined with increasing N fertilizer application levels. Wheat root deposition induced 1%–2.5% renewal of soil C after 62 d of 13CO2labeling. Compared to ambient CO2, elevated CO2increased the amount of photosynthesized C incorporated into soil by 20%–44%. However, higher application rates of N fertilizer reduced the net input of root-derived C in soil by approximately 8% under elevated CO2. For the wheat-soil system, elevated CO2and increased N fertilizer application levels synergistically increased the amount of photosynthesized C. The pivotal role of plants in photosynthesized C accumulation under elevated CO2was thereby enhanced in the short term by the increased N application. Therefore, robust N management could mediate C cycling and sequestration by influencing the interactions between plants and soil in agroecosystems under elevated CO2.

Increasing human demands for Earth's resources are hastening many environmental changes and creating a need to incorporate routine monitoring of ecosystem functions into forest management. Soil carbon (C) cycling is undergoing substantial changes in terrestrial ecosystems under global change and anthropic disturbances, resulting in the transformation between soil C sources and sinks. Therefore, 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 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 forest. The effects of climate warming and N deposition on soil C cycling are complex, especially temporal scale of short-term versus long-term. Climate warming can decay soil organic matter to emit substantial CO2, whereas the duration of warming shows the different effects on soil C loss ranging from ~1 to 15 Mg C ha-1. Short-term soil warming mainly reduces labile soil C pool, and increases the decomposition of recalcitrant soil C compounds (e.g., lignin) with lasting soil warming, whereas longer-term warming may restrain soil C loss due to impoverished soil C substrate and microbial community. Moderate N addition is conducive to enhancing soil C storage (approximately 2–22 Mg C ha-1) through increasing plant productivity including above- and below-ground biomass; however, chronic N deposition or excess N addition can result in soil acidity, lowering N use efficiency and plant growth, further resulting in no change and decline in soil C pool. Fire and logging lead to a large quantity of soil C loss viadamaged plant productivity and increased organic matter degradation, exacerbating global warming. Especially, severe fire can cause large amount of soil C loss of ~16–34 Mg C ha-1in our reviewed data. Meanwhile, black C input induced by fire and plant residual C input rooted in logging can increase soil recalcitrant C proportion 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, soil microorganisms are crucial in driving soil C cycling through C capture and N recycling of plant and soil combined with microbial technologies and approaches of metagenomics and metabolomics. We therefore suggest that clarifying the relationship among plant-SOM-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-term soil C feedback from high-latitude forests to environmental changes.