Global temperatures are rising from increasing concentrations of greenhouse gases in the atmosphere associated with anthropogenic activities. Global warming includes a warmer shift in mean temperatures as well as increases in the probability of extreme heating events, termed heat waves. Despite the ability of plants to cope with temporal variations in temperature, global warming is increasingly presenting challenges to agroecosystems. The impact of warming on crop species has direct consequences on food security, there-fore understanding impacts and opportunities to adapt crops to global warming necessi-tates experimentation that allows for modification of growth environments to represent global warming scenarios. Published studies addressing crop responses to warming are extensive, however, in-field studies where growth temperature is manipulated to mimic global warming are limited. Here, we provide an overview of in-field heating techniques employed to understand crop responses to warmer growth environments. We then focus on key results associated with season-long warming, as expected with rising global mean temperatures, and with heat waves, as a consequence of increasing temperature variability and rising global mean temperatures. We then discuss the role of rising tem-peratures on atmospheric water vapor pressure deficit and potential implications for crop photosynthesis and productivity. Finally, we review strategies by which crop photosyn-thetic processes might be optimized to adapt crops to the increasing temperatures and frequencies of heat waves. Key findings from this review are that higher temperatures consistently reduce photosynthesis and yields of crops even as atmospheric carbon dioxide increases, yet potential strategies to minimize losses from high-temperature exist.

Subtropical and tropical wetlands play a prominent role in the global carbon (C) cycle; yet factors that influence their C fluxes remain uncertain. We collected measurements from a temporarily flooded subtropical wetland over 3 years to investigate environmental drivers impacting CO 2 and CH 4 fluxes. The wetland was a sink of CO 2 (−469 to −380 g C‐CO 2 · m −2 · year −1 ) and a source of CH 4 (25.1 to 32.1 g C‐CH 4 · m −2 · year −1 ) to the atmosphere. Dry season CH 4 emissions represented 41 to 49% of the annual budget, reflecting the importance of continuous CH 4 flux measurements. Gross primary productivity (GPP) increased with temperature and radiation, and the influence of VPD on GPP varied with soil inundation. Higher water tables decreased R eco and increased GPP, and a higher GPP in turn lead to enhanced R eco likely through enhancements of GPP on autotrophic respiration. This suggests that the impact of the water table on R eco depends on the cancelling effects of hydrology and GPP. Emissions of CH 4 increased with soil temperature, water table, and GPP until soils were inundated at which point temperature and GPP became the main drivers. Water table and temperature influenced GPP and CH 4 fluxes, and increases in GPP directly enhanced CH 4 emissions. In addition to impacting C fluxes directly through water table depth, hydrology also determined the hierarchy of the dominance of factors controlling C fluxes and their response. The positive climate forcing of subtropical wetlands may be dictated by plant‐mediated and climate interactions, with hydrological factors playing a major role in determining the greenhouse gas sink or source strength of subtropical wetlands.

Wetlands are an important source of CH4 globally. However, uncertainty surrounding the impact of anthropogenic activities on CH4 emissions from wetlands limits understanding of how these ecosystems will respond to management changes. Furthermore, by neglecting the potential for management to influence CH4 emissions likely inflates error of CH4 emissions for regional and global CH4 models. This study employed a replicated factorial experimental design to investigate how management of the agricultural landscape, including grazing and/or management intensity, influences net CH4 emissions from embedded, seasonal subtropical wetlands. This research further determined key mechanisms by which management decisions at the landscape scale modulate CH4 emissions from the embedded wetlands. Net CH4 exchange was measured using a closed chamber system over two complete wet/dry seasonal cycles in 16 wetlands embedded in either agronomically improved pastures (improved wetlands) or less intensively grazed unfertilized seminative pastures (seminative wetlands), as well as in grazed and ungrazed wetlands in each treatment. Emissions of CH4 were higher from improved wetlands (2.82 μmol m−2 s−1) than seminative wetlands (0.75 μmol m−2 s−1), particularly during the wet season. Enhanced CH4 emissions in improved wetlands relative to seminative wetlands were caused by increased soil wetness and by higher biomass in improved seminative wetlands. Unlike subtropical flooded pastures, our results showed that grazers do not alter CH4 emissions from subtropical wetlands. Current and future changes in management intensity of pastures may cause shifts in net soil CH4 emissions from embedded subtropical wetlands, which could further enhance this emission source.

The impact of grazing on C fluxes from pastures in subtropical and tropical regions and on the environment is uncertain, although these systems account for a substantial portion of global C storage. We investigated how cattle grazing influences net ecosystem CO2 and CH4 exchange in subtropical pastures using the eddy covariance technique. Measurements were made over several wet-dry seasonal cycles in a grazed pasture, and in an adjacent pasture during the first three years of grazer exclusion. Grazing increased soil wetness but did not affect soil temperature. By removing aboveground biomass, grazing decreased ecosystem respiration (R-eco) and gross primary productivity (GPP). As the decrease in R-eco was larger than the reduction in GPP, grazing consistently increased the net CO2 sink strength of subtropical pastures (55, 219 and 187 more C/m(2) in 2013, 2014, and 2015). Enteric ruminant fermentation and increased soil wetness due to grazers, increased total net ecosystem CH4 emissions in grazed relative to ungrazed pasture (27-80%). Unlike temperate, arid, and semiarid pastures, where differences in CH4 emissions between grazed and ungrazed pastures are mainly driven by enteric ruminant fermentation, our results showed that the effect of grazing on soil CH4 emissions can be greater than CH4 produced by cattle. Thus, our results suggest that the interactions between grazers and soil hydrology affecting soil CH4 emissions play an important role in determining the environmental impacts of this management practice in a subtropical pasture. Although grazing increased total net ecosystem CH4 emissions and removed aboveground biomass, it increased the net storage of C and decreased the global warming potential associated with C fluxes of pasture by increasing its net CO2 sink strength.

The cultivation of energy cane throughout the Southeastern United States may displace grazed pastures on organic soil (Histosols) to meet growing demands for biofuels. We combined results from a field experiment with a biogeochemical model to improve our understanding of how the conversion of pasture to energy cane during early crop establishment affected soil GHG (CO2, CH4, and N2O) exchange with the atmosphere. GHG fluxes were measured under both land uses during wet, hot and cool, dry times of year, and following a fertilization event. We also simulated the impact of changes in precipitation on GHG exchange. Higher fertilization of cane contributed to greater emission of N2O than pasture during warmer and wetter times of the year. The model predicted that energy cane emitted more nitrogen than pasture during simulated wetter than drier years. The modeled emission factor for N2O was 20 to 30-fold higher than the default value from IPCC (1%), suggesting that the default IPCC value could dramatically underestimate the consequences of this land conversion on the climate system. Predicted soil CH4 and CO2 fluxes were higher in pasture than energy cane, and this difference was not affected by increasing precipitation. Model simulations predicted that soils under first year cane emit more GHGs than pasture, particularly during wet years, but this difference disappeared two years after energy cane establishment. Our results suggest that management practices may be important in determining soil GHG emissions from energy cane on organic soils particularly during the first year of cane establishment.