Significant methane undersaturation during austral summer in the Ross Sea (Southern Ocean)

Dissolved methane (CH4) was measured at 9 stations along a transect at 75° S in the Ross Sea during austral summer in January 2020. CH4 undersaturation (mean: 82 ± 20 %) was found in the water column, with a mean air-sea CH4 flux density of −0.58 ± 0.48 μmol m−2 day−1, which suggests that the Ross Sea was a net sink for atmospheric CH4 during the austral summer. Simple box-model calculations revealed that the CH4 depletion should occur in the surface mixed layer because of CH4 oxidation and advection of CH4-poor waters. We propose that freshwater injection caused by sea-ice melting in summer dilutes CH4 concentrations within the surface layer and thus increases its potential for atmospheric CH4 uptake in the Ross Sea. Thus, we argue that both CH4 consumption and sea-ice melting are important drivers of CH4 undersaturation, which implies that the high-latitude area of the Southern Ocean is a sink for atmospheric CH4. We estimated that the Southern Ocean (> 65° S) takes up about 0.02 % of the global CH4 emissions and thus represents a minor sink for atmospheric CH4.

Changes in GHG Emissions Based on Irrigation Water Quality in Short-Term Incubated Agricultural Soil of the North China Plain

A worsening water shortage is threatening the sustainable development of agriculture in the North China Plain (NCP). How to make effective use of inferior water resources and alleviate the impact of insufficient water resources on agricultural environments is one of the urgent problems in agricultural production. Although agriculture plays an important role in greenhouse gas (GHG) emissions, the effects of irrigation water quality on such emissions in the NCP are not clear. In this study, we used a short-term incubation experiment to test the effects of the irrigation water quality (underground water (UW), saline water (SW), and reclaimed water (RW)) and frequency (high (H) and low (L)) on regulating the soil GHG emissions of the NCP. The results indicated that RW treatment increased the CO2 and N2O emissions by 15.00% and 20.81%, respectively, and reduced the CH4 uptake by 12.50% compared with the UW treatment. In addition, SW treatment decreased the CO2 and N2O emissions and CH4 uptake by 35.18%, 40.27%, and 20.09% against UW treatment, respectively. The high-frequency water added to the soil significantly increased the GHG emissions for all water qualities applied. Compared with UW, the global warming potential was significantly increased by RW_H and RW_L with 26.48% and 14.5% and decreased by SW_H and SW_L with 32.13% and 43.9%, respectively. Compared with the increase brought by reclaimed water, changing irrigation water sources from conventional groundwater to saline water (4 g L−1) will moderately reduce GHG emissions under the worsening water shortage conditions occurring in the NCP.

The Effect of Multi-Years Reclaimed Water Irrigation on Dryland Carbon Sequestration in the North China Plain

Reclaimed water is an alternative water source which could alleviate the shortage of water resources in agricultural systems. Many researchers have studied the effect of reclaimed water on soil environment, crop yield, etc. However, carbon sequestration in reclaimed water irrigated agricultural systems is less studied. This study investigates methane uptake and photosynthesis in reclaimed water irrigation systems contributing to carbon sequestration estimation and analyzes the important factors impacting them. The results show that CH4 uptake is related to soil water-filled pore space (WFPS) with a quadratic and it has the highest uptake when WFPS is between 40 and 50%. Long-term reclaimed water irrigation could significantly decrease (p < 0.05) CH4 uptake and macroaggregate stability in the topsoil. However, reclaimed water had no significant impact on photosynthesis in comparison. The type of fertilizer is an important factor which impacts CH4 emission from soil; urea had a lower CH4 uptake and a higher CO2 emission than slow-released fertilizer. Overall, reclaimed water irrigation could effectively decrease soil carbon sequestration. A soil wetted proportion level of 40–50% was recommended in this study for favorable methane oxidation. Slow-released fertilizer in reclaimed water irrigated agriculture could better control soil carbon emission and soil carbon absorption.

Accelerated Discovery of CH4 Uptake Capacity MOFs using Bayesian Optimization

High-throughput computational studies for discovery of metal-organic frameworks (MOFs) for separations and storage applications are often limited by the costs of computing thermodynamic quantities, with recent studies reliant ab initio results for a narrow selection of MOFs and empirical force-field methods for larger selections. Here, we conduct a proof-of-concept study using Bayesian optimization on CH4 uptake capacity of hypothetical MOFs for an existing dataset (Wilmer et al, Nature Chem. 2012, 4, 83). We show that less than 0.1% of the database needs to be screened with our Bayesian optimization approach to recover the top candidate MOFs. This opens the possibility of efficient screening of MOF databases using accurate ab-initio calculations for future adsorption studies on a minimal subset of MOFs. Furthermore, Bayesian optimization and the surrogate model presented here can offer interpretable material design insights and our framework will be applicable in the context of other target properties.

Excess soil moisture and fresh carbon input are prerequisites for methane production in podzolic soil

Boreal upland forests are generally considered methane (CH4) sinks due to the predominance of CH4 oxidising 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 18200 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 resulting net efflux, that should be incorporated in process models describing global CH4 cycling.

Impacts of Irrigation Managements on Soil CO2 Emission and Soil CH4 Uptake of Winter Wheat Field in the North China Plain

The North China Plain is an important irrigated agricultural area in China. However, the effects of irrigation management on carbon emission are not well documented in this region. Due to the uneven seasonal distribution of rainfall, irrigation is mainly concentrated in the winter wheat growing season in the North China Plain. In this study, we estimated CO2 emission and soil CH4 uptake from winter wheat fields with different irrigation methods and scheduling treatments using the static chamber-gas chromatography method from April to May 2017 and 2018. Treatments included three irrigation methods (surface drip, sprinkler, and border) and three irrigation scheduling levels that initiated as soon as the soil moisture drained to 50%, 60%, and 70% of the field capacity for a 0–100 cm soil profile were tested. The results showed that both the irrigation methods and scheduling significantly influenced (p < 0.05) the cumulative CO2 and CH4 emission, grain yield, global warming potential (GWP), GWP Intensity (GWPI), GWPI per unit irrigation applied, and water use efficiency (WUE). Compared to 60% and 70% FC, 50% FC irrigation scheduling de-creased accumulated CH4 uptake 26.8–30.3% and 17.8–25.4%, and reduced accumulated CO2 emissions 7.0–15.3% and 12.6–19.4%, respectively. Conversely, 50% FC reduced GWP 6.5–13.3% and 12.5–19.4% and lower grain yield 10.4–19.7% and 8.5–16.6% compared to 60% and 70% FC irrigation scheduling in 2017 and 2018, respectively. Compared to sprinkler irrigation and border irrigation, drip irrigation at 60% FC increased the accumulated CH4 uptake 11.3–12.1% and 1.9–5.5%, while reduced the accumulated CO2 emissions from 7.5–8.8% and 10.1–12.1% in 2017 and 2018, respectively. Moreover, drip irrigation at 60% FC increased grain yield 5.2–7.5% and 6.3–6.8%, WUE 0.9–5.4% and 5.7–7.4%, and lowered GWP 8.0–9.8% and 10.1–12.0% compared to sprinkler and border irrigation in 2017 and 2018, respectively. The interaction of irrigation scheduling and irrigation methods significantly impacted accumulated CH4 uptake, cumulative CO2 amount, and GWP in 2018 only while grain yield and WUE in the entire study. Overall, drip irrigation at 60% FC is the optimal choice in terms of higher grain yield, WUE, and mitigating GWP and GWPI from winter wheat fields in North China Plain.

Different responses of ecosystem CO₂ and N₂O emissions and CH₄ uptake to seasonally asymmetric warming in an alpine grassland of the Tianshan

An experiment was conducted to investigate the effect of seasonally asymmetric warming on ecosystem respiration (Re), CH4 uptake, and N2O emissions in alpine grassland of the Tianshan of central Asia, from October 2016 to September 2019. The annual means of Re, CH4, and N2O fluxes in growing season were 42.83 mg C m−2 h−1, −41.57 µg C m−2 h−1, and 4.98 µg N m−2 h−1, respectively. Furthermore, warming during the non-growing season increased Re and CH4 uptake by 7.9 % and 10.6 % in the growing season and 10.5 % and 9.2 % in the non-growing season, respectively. However, the increase in N2O emission in the growing season was mainly caused by the warming during the growing season (by 29.7 %). The warming throughout the year and warming during the non-growing season increased N2O emissions by 101.9 % and 192.3 % in the non-growing season, respectively. The Re, CH4 uptake, and N2O emissions were positively correlated with soil temperature. Our results suggested that Re, CH4 uptake, and N2O emissions were regulated by soil temperature, rather than soil moisture, in the case of seasonally asymmetric warming. In addition, the response rate was defined by the changes in greenhouse gas fluxes driven by warming. In our field experiment, we observed the stimulatory effect of warming during the non-growing season on Re and CH4 uptake. In contrast, the response rates of Re and N2O emissions were gradually attenuated by long-term annual warming, and the response rate of Re was also weakened by warming over the growing season. These findings highlight the importance of warming in the non-growing season in regulating greenhouse gas fluxes, a finding which is crucial for improving our understanding of C and N cycles under the scenarios of global warming.