Soil Microbial Responses to Elevated CO2 and O3 in a Nitrogen-Aggrading Agroecosystem

Climate change factors such as elevated atmospheric carbon dioxide (CO2) and ozone (O3) can exert significant impacts on soil microbes and the ecosystem level processes they mediate. However, the underlying mechanisms by which soil microbes respond to these environmental changes remain poorly understood. The prevailing hypothesis, which states that CO2- or O3-induced changes in carbon (C) availability dominate microbial responses, is primarily based on results from nitrogen (N)-limiting forests and grasslands. It remains largely unexplored how soil microbes respond to elevated CO2 and O3 in N-rich or N-aggrading systems, which severely hinders our ability to predict the long-term soil C dynamics in agroecosystems. Using a long-term field study conducted in a no-till wheat-soybean rotation system with open-top chambers, we showed that elevated CO2 but not O3 had a potent influence on soil microbes. Elevated CO2 (1.5×ambient) significantly increased, while O3 (1.4×ambient) reduced, aboveground (and presumably belowground) plant residue C and N inputs to soil. However, only elevated CO2 significantly affected soil microbial biomass, activities (namely heterotrophic respiration) and community composition. The enhancement of microbial biomass and activities by elevated CO2 largely occurred in the third and fourth years of the experiment and coincided with increased soil N availability, likely due to CO2-stimulation of symbiotic N2 fixation in soybean. Fungal biomass and the fungi∶bacteria ratio decreased under both ambient and elevated CO2 by the third year and also coincided with increased soil N availability; but they were significantly higher under elevated than ambient CO2. These results suggest that more attention should be directed towards assessing the impact of N availability on microbial activities and decomposition in projections of soil organic C balance in N-rich systems under future CO2 scenarios

Air quality policy and fire management responses addressing smoke from wildland fires in the United States and Australia

Wildland fire emissions degrade air quality and visibility, having adverse economic, health and visibility impacts at large spatial scales globally. Air quality regulations can constrain the goals of landscape resilience and management of fire-dependent ecosystems. Here, we review the air quality regulatory framework in the United States, comparing this framework with that of Australia. In the United States, wildland fire management and air quality policies have evolved independently, yet interact to meet diverse public needs. Australian policy development is more recent and decentralised. We find that (1) for maxiumum effectiveness, smoke and fire regulatory frameworks must keep pace with scientific evidence, environmental and social change, and be accompanied by clear regulatory guidance; (2) episodic, non-stationary qualities of fire, and its role in ecosystems, pose specific challenges to regulators and policy-makers; and (3) the complexity of industry-focused air quality policies often leads to unintended consequences for fire management. More research is needed to create and implement more effective fire and air policies and better prepare social-ecological systems to address the challenges of climate change mitigation. These insights may be helpful for countries initially developing complementary fire and air policies, especially as the role of fire becomes more important geopolitically and globally

Satellite based estimates underestimate the effect of CO2 fertilization on net primary productivity

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