The response of soils to extreme weather events will become increasingly important in the future as more frequent and severe floods and droughts are expected to subject soils to drying and rewetting cycles as a result of climate change. These extreme events will be experienced against a backdrop of overall warming. Farmers are adopting cover cropping as a sustainable management practice to increase soil organic matter and benefit soil health. Cover crops may also increase the resilience of soils to help mitigate the impacts of climate change. We examined the legacy of warming and cover crops on the response of soil microbial function to repeated drying and rewetting cycles. We introduced open-top chambers to warm the soil surface of a field plot experiment in which cover crops (single-species monocultures and 4-species polycultures) were grown over the summer after harvest and before planting autumn sown cash crops in a cereal rotation. Soil samples were collected from warmed and ambient areas of the experimental plots in spring, before harvesting the cereal crop. Warming significantly increased, and cover crops significantly decreased, the abundance of genes encoding fungal beta-glucosidase. We quantified respiration (a measure of soil microbial function) with high-frequency CO2 flux measurements after 0, 1, 2, 4 or 8 wet/dry cycles imposed in the laboratory and the addition of barley grass powder substrate at a rate of 10 mg g-1 soil. We observed lower cumulative substrate-induced respiration in soils previously planted with cover crop mixtures than expected from the average of the same species grown in monoculture. Repeated drying and rewetting cycles increased the cumulative substrate-induced respiration rate observed, suggesting that repeated perturbations selected for a community adapted to processing the barley shoot powder more quickly. When we calculated the cumulative respiration after 8 wet/dry cycles, relative to cumulative respiration after 0 wet/dry cycles (which we infer represents the extent to which microbial communities adapted to repeated drying and rewetting cycles), our data revealed that the legacy of warming significantly reduced soil microbial community adaptation, but the legacy of cover crops significantly increased, soil microbial community adaptation. This adaptation of the soil microbial community was positively correlated with the concentration of water-extractable organic carbon in the soils before imposing the drying and rewetting cycles and/or adding the substrate. We conclude that cover crops may enhance the ability of the soil microbial community to adapt to drought events and mitigate the impact of warming, possibly due to the provision of labile organic carbon for the synthesis of osmolytes which then prime the decomposition of labile plant material upon rewetting.

Forests, though very critical for life on Earth, are threatened by various factors and the frequently occurring forest fires are one of the significant causes. Forest fires drastically contribute to climate change on both regional and global scales. Forest fires-of both natural and anthropogenic origins-induce aerosols in the atmosphere and have a significant impact on the health and climate of the region. In this study, we simulate the Uttarakhand (29-31 degrees N, 78-80 degrees E) fire event in India, which occurred in April 2016, using the Weather Research and Forecasting with Chemistry (WRF-Chem) model to estimate the radiative impact of the aerosols emitted due to this fire event and probe into the extent of their transport into the atmosphere. Multiple data from ground-based and satellite observations are used to access the model performance. Our analysis showed that the high values of aerosol optical depths (AODs) during the fire event simulated by WRF-Chem compared very well with MODIS AODs over the Uttarakhand region. The model simulations of the vertical profile of BC corroborate with elevated smoke aerosols derived from CALIPSO. An enhancement of smoke aerosols is observed up to 5-km altitude during the fire event both in the model simulations and observations. The fire has increased the near-surface air temperatures by 1-3 degrees C and decreased the relative humidity by similar to 10% over the affected areas. The NET (shortwave + longwave) atmospheric radiative forcing due to fire varied between similar to 10 and similar to 40 Wm(-2) in the entire affected areas, with the highest values over the source region. The fire-induced atmospheric heating rate varied between 0.5 and 1.4 K/day over the Uttarakhand region.

We describe and evaluate atmospheric chemistry in the newly developed Geophysical Fluid Dynamics Laboratory chemistry-climate model (GFDL AM3) and apply it to investigate the net impact of preindustrial (PI) to present (PD) changes in short-lived pollutant emissions (ozone precursors, sulfur dioxide, and carbonaceous aerosols) and methane concentration on atmospheric composition and climate forcing. The inclusion of online troposphere-stratosphere interactions, gas-aerosol chemistry, and aerosol-cloud interactions (including direct and indirect aerosol radiative effects) in AM3 enables a more complete representation of interactions among short-lived species, and thus their net climate impact, than was considered in previous climate assessments. The base AM3 simulation, driven with observed sea surface temperature (SST) and sea ice cover (SIC) over the period 1981-2007, generally reproduces the observed mean magnitude, spatial distribution, and seasonal cycle of tropospheric ozone and carbon monoxide. The global mean aerosol optical depth in our base simulation is within 5% of satellite measurements over the 1982-2006 time period. We conduct a pair of simulations in which only the short-lived pollutant emissions and methane concentrations are changed from PI (1860) to PD (2000) levels (i.e., SST, SIC, greenhouse gases, and ozone-depleting substances are held at PD levels). From the PI to PD, we find that changes in short-lived pollutant emissions and methane have caused the tropospheric ozone burden to increase by 39% and the global burdens of sulfate, black carbon, and organic carbon to increase by factors of 3, 2.4, and 1.4, respectively. Tropospheric hydroxyl concentration decreases by 7%, showing that increases in OH sinks (methane, carbon monoxide, nonmethane volatile organic compounds, and sulfur dioxide) dominate over sources (ozone and nitrogen oxides) in the model. Combined changes in tropospheric ozone and aerosols cause a net negative top-of-the-atmosphere radiative forcing perturbation (-1.05Wm(-2)) indicating that the negative forcing (direct plus indirect) from aerosol changes dominates over the positive forcing due to ozone increases, thus masking nearly half of the PI to PD positive forcing from long-lived greenhouse gases globally, consistent with other current generation chemistry-climate models.