Summary Tree stems from wetland, floodplain and upland forests can produce and emit methane (CH4). Tree CH4 stem emissions have high spatial and temporal variability, but there is no consensus on the biophysical mechanisms that drive stem CH4 production and emissions. Here, we summarize up to 30 opportunities and challenges for stem CH4 emissions research, which, when addressed, will improve estimates of the magnitudes, patterns and drivers of CH4 emissions and trace their potential origin. We identified the need: (1) for both long‐term, high‐frequency measurements of stem CH4 emissions to understand the fine‐scale processes, alongside rapid large‐scale measurements designed to understand the variability across individuals, species and ecosystems; (2) to identify microorganisms and biogeochemical pathways associated with CH4 production; and (3) to develop a mechanistic model including passive and active transport of CH4 from the soil–tree–atmosphere continuum. Addressing these challenges will help to constrain the magnitudes and patterns of CH4 emissions, and allow for the integration of pathways and mechanisms of CH4 production and emissions into process‐based models. These advances will facilitate the upscaling of stem CH4 emissions to the ecosystem level and quantify the role of stem CH4 emissions for the local to global CH4 budget.

Managing leaks in urban natural gas (NG) distribution systems is important for reducing methane emissions and costly waste. Mobile surveying technologies have emerged as a new tool for monitoring system integrity, but this new technology has not yet been widely adopted. Here, we establish the efficacy of mobile methane surveys for managing local NG distribution systems by evaluating their ability to detect and locate NG leaks and quantify their emissions. In two cities, three-quarters of leak indications from mobile surveys corresponded to NG leaks, but local distribution companies' field crews did not find most of these leaks, indicating that the national CH 4 activity factor for leaks in local NG distribution pipelines is underestimated by a factor of 2.4. We found the median distance between mobile-estimated leak locations and actual leak locations was 19 m. A comparison of emission quantification methods (mobile-based, surface enclosure, and tracer ratio) found that the mobile method overestimated leak magnitude for the smallest leaks but accurately estimated size for the largest leaks that are responsible for the majority of total emissions. Across leak sizes, mobile methods adequately rank relative emission rates for repair prioritization, and they are easily deployed and offer efficient spatial coverage.

Precipitation pulses in arid ecosystems can lead to temporal asynchrony in microbial and plant processing of nitrogen (N) during drying/wetting cycles causing increased N loss. In contrast, more consistent availability of soil moisture in mesic ecosystems can synchronize microbial and plant processes during the growing season, thus minimizing N loss. We tested whether microbial N cycling is asynchronous with plant N uptake in a semiarid grassland. Using (15)N tracers, we compared rates of N cycling by microbes and N uptake by plants after water pulses of 1 and 2 cm to rates in control plots without a water pulse. Microbial N immobilization, gross N mineralization, and nitrification dramatically increased 1-3 days after the water pulses, with greatest responses after the 2-cm pulse. In contrast, plant N uptake increased more after the 1-cm than after the 2-cm pulse. Both microbial and plant responses reverted to control levels within 10 days, indicating that both microbial and plant responses were short lived. Thus, microbial and plant processes were temporally synchronous following a water pulse in this semiarid grassland, but the magnitude of the pulse substantially influenced whether plants or microbes were more effective in acquiring N. Furthermore, N loss increased after both small and large water pulses (as shown by a decrease in total (15)N recovery), indicating that changes in precipitation event sizes with future climate change could exacerbate N losses from semiarid ecosystems.

Soil–atmosphere exchange significantly influences the global atmospheric abundances of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). These greenhouse gases (GHGs) have been extensively studied at the soil profile level and extrapolated to coarser scales (regional and global). However, finer scale studies of soil aggregation have not received much attention, even though elucidating the GHG activities at the full spectrum of scales rather than just coarse levels is essential for reducing the large uncertainties in the current atmospheric budgets of these gases. Through synthesizing relevant studies, we propose that aggregates, as relatively separate micro‐environments embedded in a complex soil matrix, can be viewed as biogeochemical reactors of GHGs. Aggregate reactivity is determined by both aggregate size (which determines the reactor size) and the bulk soil environment including both biotic and abiotic factors (which further influence the reaction conditions). With a systematic, dynamic view of the soil system, implications of aggregate reactors for soil–atmosphere GHG exchange are determined by both an individual reactor's reactivity and dynamics in aggregate size distributions. Emerging evidence supports the contention that aggregate reactors significantly influence soil–atmosphere GHG exchange and may have global implications for carbon and nitrogen cycling. In the context of increasingly frequent and severe disturbances, we advocate more analyses of GHG activities at the aggregate scale. To complement data on aggregate reactors, we suggest developing bottom‐up aggregate‐based models (ABMs) that apply a trait‐based approach and incorporate soil system heterogeneity.

Trees are sources, sinks, and conduits for gas exchange between the atmosphere and soil, and effectively link these terrestrial realms in a soil-plant-atmosphere continuum.We demonstrated that naturally produced radon-222 ( 222 Rn) gas has the potential to disentangle the biotic and physical processes that regulate gas transfer between soils or plants and the atmosphere in field settings where exogenous tracer applications are challenging.Patterns in stem radon emissions across tree species, seasons, and diurnal periods suggest that plant transport of soil gases is controlled by plant hydraulics, whether by diffusion or mass flow via transpiration.We establish for the first time that trees emit soil gases during the night when transpiration rates are negligible, suggesting that axial diffusion is an important and understudied mechanism of plant and soil gas transmission.

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