Carbon-rich peat soils have been drained and used extensively for agriculture throughout human history, leading to significant losses of their soil carbon. One solution for rewetting degraded peat is wet crop cultivation. Crops such as rice, which can grow in water-saturated conditions, could enable agricultural production to be maintained whilst reducing CO2 and N2O emissions from peat. However, wet rice cultivation can release considerable methane (CH4). Water table and soil management strategies may enhance rice yield and minimize CH4 emissions, but they also influence plant biomass allocation strategies. It remains unclear how water and soil management influences rice allocation strategies and how changing plant allocation and associated traits, particularly belowground, influence CH4-related processes. We examined belowground biomass (BGB), aboveground biomass (AGB), belowground:aboveground ratio (BGB:ABG), and a range of root traits (root length, root diameter, root volume, root area, and specific root length) under different soil and water treatments; and evaluated plant trait linkages to CH4. Rice (Oryza sativa L.) was grown for six months in field mesocosms under high (saturated) or low water table treatments, and in either degraded peat soil or degraded peat covered with mineral soil. We found that BGB and BGB:AGB were lowest in water saturated conditions where mineral soil had been added to the peat, and highest in low-water table peat soils. Furthermore, CH4 and BGB were positively related, with BGB explaining 60% of the variation in CH4 but only under low water table conditions. Our results suggest that a mix of low water table and mineral soil addition could minimize belowground plant allocation in rice, which could further lower CH4 likely because root-derived carbon is a key substrate for methanogenesis. Minimizing root allocation, in conjunction with water and soil management, could be explored as a strategy for lowering CH4 emissions from wet rice cultivation in degraded peatlands.

Wetlands are the largest natural source of methane (CH 4 ) globally. Climate and land use change are expected to alter CH 4 emissions but current and future wetland CH 4 budgets remain uncertain. One important predictor of wetland CH 4 flux, plants, play an important role in providing substrates for CH 4 -producing microbes, increasing CH 4 consumption by oxygenating the rhizosphere, and transporting CH 4 from soils to the atmosphere. Yet, there remain various mechanistic knowledge gaps regarding the extent to which plant root systems and their traits influence wetland CH 4 emissions. Here, we present a novel conceptual framework of the relationships between a range of root traits and CH 4 processes in wetlands. Based on a literature review, we propose four main CH 4 -relevant categories of root function: gas transport, carbon substrate provision, physicochemical influences and root system architecture. Within these categories, we discuss how individual root traits influence CH 4 production, consumption, and transport (PCT). Our findings reveal knowledge gaps concerning trait functions in physicochemical influences, and the role of mycorrhizae and temporal root dynamics in PCT. We also identify priority research needs such as integrating trait measurements from different root function categories, measuring root-CH 4 linkages along environmental gradients, and following standardized root ecology protocols and vocabularies. Thus, our conceptual framework identifies relevant belowground plant traits that will help improve wetland CH 4 predictions and reduce uncertainties in current and future wetland CH 4 budgets.

Methane (CH4) is a strong greenhouse gas that is produced in anoxic soil conditions. Wetlands are the largest natural source of CH4 globally because their anoxic soils provide suitable habitats for CH4-producing Archaea. Both global and regional wetland CH4 budgets remain unconstrained due to large uncertainties in wetland extent and high spatio-temporal variability in CH4 dynamics that are in part driven by wetland spatial heterogeneity. High wetland spatial heterogeneity often results from the variability in microtopography, soil hydrological and chemical properties, and vegetation and microbial composition. Intertwined together, the different abiotic and biotic variables further contribute to the ratio between CH4 production, consumption and transport processes in wetland soil, resulting in either net CH4 emission or uptake. However, the contribution of different abiotic and biotic factors to CH4 flux variability in wetlands remains unclear, increasing uncertainties in up-scaling CH4 emissions from plot to ecosystem and regional scales. Therefore, including well-defined spatial heterogeneity into wetland CH4 bottom-up estimates can help improve the regional and global CH4 budget calculations.This study investigates the effect of spatial heterogeneity on observed CH4 emissions in ten different wetland sites with varying climatic conditions. Our approach will include up-scaling chamber measurements from different land cover classes to the level of the eddy covariance (EC) footprint. The compiled chamber datasets include both manual (n=5) and automatic (n=5) measurements that have been combined with EC measurements (FLUXNET-CH4 database) based on matching timestamps. First, the chamber observations will be compared to the corresponding EC measurements without accounting for spatial heterogeneity. Then, various remotely sensed environmental variables, such as leaf area index (LAI) and topographic wetness index (TWI), in high spatial resolution will be used to create land cover classes and combined with a modeled footprint to include spatial heterogeneity in the up-scaling of point-level measurements in all sites. Preliminary results suggest that the chamber and EC observations differ significantly in magnitude between seasons and sites. In general, chamber observations had a larger range than EC, which we expected, given that chambers capture finer spatial heterogeneity than EC. However, no consistent trends emerged in the difference in magnitude between chamber and EC. We expect that the inclusion of spatial heterogeneity into the footprint model will decrease the differences between up-scaled chamber and EC observations for all sites. Notably, we expect that the inclusion of proxies for soil moisture, plant functional type (PFT) and aerenchyma will improve footprint-level comparisons to chamber-level data. We will present updated comparisons of EC and chamber data with and without inclusion of spatial heterogeneity. Altogether, this study will establish a workflow for combining wetland CH4 data from different measurement types (EC and chamber) and will allow global syntheses to use more of the available data to constrain CH4 budgets.

Anoxic wetland soils are the biggest natural source of methane (CH4) globally. Climate-driven changes to soil moisture regimes are expected to alter wetland CH4 production, consumption, and transport in a variety of different ways. At the same time, moisture changes also influence plants and their traits, especially belowground. Wetland plants provide key carbon substrates for methanogenesis, and transport CH4 to the atmosphere but also oxygen into the soils, which can promote CH4 consumption. We tested hypotheses on these complex abiotic and biotic interactions to understand how belowground plant traits influence net CH4 emissions in wetlands. Specifically, we address the following questions 1) which root traits are most important for wetland CH4 processes? 2) how does water table manipulation influence the link between root biomass and CH4? 3) how does soil moisture influence the amount of root-derived carbon emitted as CH4? First, using a literature review, we developed a conceptual framework describing root traits that would be most related to CH4 processes, highlighting trait categories regulating CH4 substrate provision and transport. Second, using a field manipulation in an experimental rice system, we found that root biomass and CH4 emissions are positively linked, but only under low moisture conditions. Lastly, in a lab incubation, we found that the amount of root-derived C-CH4 increases with increased fresh root litter, in water saturated peat soils. Overall, our studies identify root traits and their moisture interactions that should be considered in wetland CH4 measurements and models.

Boreal upland forests are generally considered methane (CH4) sinks due to the predominance of CH4 oxidizing bacteria over the methanogenic archaea. However, boreal upland forests can temporarily act as CH4 sources during wet seasons or years. From a landscape perspective and in annual terms, this source can be significant as weather conditions may cause flooding, which can last a considerable proportion of the active season and because often, the forest coverage within a typical boreal catchment is much higher than that of wetlands. Processes and conditions which change mineral soils from acting as a weak sink to a strong source are not well understood. We measured soil CH4 fluxes from 20 different points from regularly irrigated and control plots during two growing seasons. We also estimated potential CH4 production and oxidation rates in different soil layers and performed a laboratory experiment, where soil microcosms were subjected to different moisture levels and glucose addition simulating the fresh labile carbon (C) source from root exudates. The aim was to find the key controlling factors and conditions for boreal upland soil CH4 production. Probably due to long dry periods in both summers, we did not find occasions of CH4 production following the excess irrigation, with one exception in July 2019 with emission of 18 200 µg CH4 m−2 h−1. Otherwise, the soil was always a CH4 sink (median CH4 uptake rate of 260–290 and 150–170 µg CH4 m−2 h−1, in control and irrigated plots, respectively). The median soil CH4 uptake rates at the irrigated plot were 88 % and 50 % lower than at the control plot in 2018 and 2019, respectively. Potential CH4 production rates were highest in the organic layer (0.2–0.6 nmol CH4 g−1 d−1), but some production was also observed in the leaching layer, whereas in other soil layers, the rates were negligible. Potential CH4 oxidation rates varied mainly within 10–40 nmol CH4 g−1 d−1, except in deep soil and the organic layer in 2019, where potential oxidation rates were almost zero. The laboratory experiment revealed that high soil moisture alone does not turn upland forest soil into a CH4 source. However, a simple C source, e.g., substrates coming from root exudates with high moisture, switched the soil into a CH4 source. Our unique study provides new insights into the processes and controlling factors on CH4 production and oxidation, and the resulting net efflux that should be incorporated in process models describing global CH4 cycling.

Boreal upland forests are generally considered methane (CH4) sinks due to high methane consumption rates. However, some studies have shown a boreal upland forest soil turning from a CH4 sink to a source after long-term abundant precipitation but the factors affecting this change remain largely unknown. In a simulated rainfall experiment, we investigated soil moisture effects on CH4 flux. We also evaluated the influence of simultaneous soil temperature increase, organic litter addition and root exclusion on CH4 flux. The study was conducted in a northern boreal upland forest soil (Kenttärova forest in Kittilä, Finland) in summer 2018. Split-plot design was used in the experiment with soil moisture being the main treatment variable and soil warming (T), organic litter addition (A) and organic litter and root exclusion (E) subtreatment variables. The design included two main plots: irrigation (I) and control (C), within which each subtreatment was replicated three times. In addition to the T, A and E manipulations, plots without additional manipulations (O) were included for the assessment of the effect of only soil moisture increase (n=4). CH4 flux was measured at least once a week using chambers. Soil moisture and temperature were also continuously measured. Contrary to our expectation, the soil remained a CH4 sink after experimental rainfall increase but irrigation and control sites differed significantly in their CH4 fluxes, indicating a strong decreasing effect of soil moisture on soil CH4 sink. All treatments had lowest CH4 uptake rates in August, possibly as a result of high soil moisture. The IA treatment had the lowest uptake rates possibly due to a reduction in gas diffusion. The IE treatment had contrasting results to all other treatments and showed a positive relationship between uptake rate and soil moisture, but the causes remained unsolved and the results were highly uncertain. T treatment had no effect on uptake likely due to a failure to create soil temperature differences and thus the interactions were not reliably analyzed. Our results suggest that the CH4 flux response to soil moisture may have been more related to changes in CH4 consumption than production. Based on these results, future irrigation experiments should especially assess the combined effect of organic litter addition and soil moisture on soil CH4 sink over multiple years. Improvements in the manipulation methods, experimental design and sensors will also be needed.