Experimental Warming Increases Seasonal Methane Uptake in an Alpine Meadow on the Tibetan Plateau

Effects of whole-soil warming on CH4 and N2 O fluxes in an alpine grassland

Global climate warming could affect the methane (CH4 ) and nitrous oxide (N2 O) fluxes between soils and the atmosphere, but how CH4 and N2 O fluxes respond to whole-soil warming is unclear. Here, we for the first time investigated the effects of whole-soil warming on CH4 and N2 O fluxes in an alpine grassland ecosystem on the Tibetan Plateau, and also studied the effects of experimental warming on CH4 and N2 O fluxes across terrestrial ecosystems through a global-scale meta-analysis. The whole-soil warming (0-100 cm, +4°C) significantly elevated soil N2 O emission by 101%, but had a minor effect on soil CH4 uptake. However, the meta-analysis revealed that experimental warming did not significantly alter CH4 and N2 O fluxes, and it may be that most field warming experiments could only heat the surface soils. Moreover, the warming-induced higher plant litter and available N in soils may be the main reason for the higher N2 O emission under whole-soil warming in the alpine grassland. We need to pay more attention to the long-term response of greenhouse gases (including CH4 and N2 O fluxes) from different soil depths to whole-soil warming over year-round, which could help us more accurately assess and predict the ecosystem-climate feedback under realistic warming scenarios in the future.

Variation in methane uptake by grassland soils in the context of climate change - A review of effects and mechanisms

Grassland soils are climate-dependent ecosystems that have a significant greenhouse gas mitigating function through their ability to store large amounts of carbon (C). However, what is often not recognized is that they can also exhibit a high methane (CH₄) uptake capacity that could be influenced by future increases in atmospheric carbon dioxide (CO₂) concentration and variations in temperature and water availability. While there is a wealth of information on C sequestration in grasslands there is less consensus on how climate change impacts on CH₄ uptake or the underlying mechanisms involved. To address this, we assessed existing knowledge on the impact of climate change components on CH₄ uptake by grassland soils. Increases in precipitation associated with soils with a high background soil moisture content generally resulted in a reduction in CH₄ uptake or even net emissions, while the effect was opposite in soils with a relatively low background moisture content. Initially wet grasslands subject to the combined effects of warming and water deficits may absorb more CH₄, mainly due to increased gas diffusivity. However, in the longer-term heat and drought stress may reduce the activity of methanotrophs when the mean soil moisture content is below the optimum for their survival. Enhanced plant productivity and growth under elevated CO₂, increased soil moisture and changed nutrient concentrations, can differentially affect methanotrophic activity, which is often reduced by increasing N deposition. Our estimations showed that CH₄ uptake in grassland soils can change from -57.7 % to +6.1 % by increased precipitation, from -37.3 % to +85.3 % by elevated temperatures, from +0.87 % to +92.4 % by decreased precipitation, and from -66.7 % to +27.3 % by elevated CO₂. In conclusion, the analysis suggests that grasslands under the influence of warming and drought may absorb even more CH₄, mainly because of reduced soil water contents and increased gas diffusivity.

Ignoring the Effects of Photovoltaic Array Deployment on Greenhouse Gas Emissions May Lead to Overestimation of the Contribution of Photovoltaic Power Generation to Greenhouse Gas Reduction

Photovoltaic (PV) power generation is one of the world's most promising options for carbon emission reduction. However, whether the operation period of solar parks can increase greenhouse gas (GHG) emissions in hosting natural ecosystems has not been fully considered. Here, we conducted a field experiment to compensate for the lack of evaluation of the effects of PV array deployment on GHG emissions. Our results show that the PV arrays caused significant differences in air microclimate, soil properties, and vegetation characteristics. Simultaneously, PV arrays had more significant effects on CO₂ and N₂O emissions but a minor impact on CH₄ uptake in the growing season. Of all the environmental variables included, soil temperature and moisture were the main drivers of GHG flux variation. The sustained flux global warming potential from the PV arrays significantly increased by 8.14% compared to the ambient grassland. Our evaluation models identified that the GHG footprint of PV arrays during the operation period on grasslands was 20.62 g CO₂-eq kW h^-1. Compared with our model estimates, GHG footprint estimates reported in previous studies were generally less by 25.46-50.76%. The contribution of PV power generation to GHG reduction may be overestimated without considering the impact of PV arrays on hosting ecosystems.

Reference for different sensitivities of greenhouse gases effluxes to warming climate among types of desert biological soil crust

There is much uncertainty about how climate warming will impact greenhouse gases (GHG) budget in dry environments due to the lack of available data for desert biocrust soil. We implemented a 2.5-year field measurement of CO₂, CH₄ and N₂O effluxes in cyanobacteria-dominated, moss-dominated and mixed (cyanobacteria, moss and lichen) biocrust soils using open-top-chambers to simulate climate warming (1.2 °C on average). Desert biocrust soils generally acted as a weak sink of atmospheric CH₄ and N₂O. Although warming effects on daily CO₂, CH₄, and N₂O effluxes varied depending on sampling date and biocrust soil, there was no significant difference in daily, monthly and seasonal average CO₂, CH₄ and N₂O effluxes between warming and control in most cases for three biocrust soils. However, warming caused a marginal (p = 0.06) decrease (14.2%) in annual accumulative CO₂ efflux in moss-dominated biocrust soil due to the drought effects caused by warming indirectly and OTC sheltering of precipitation directly, while there was no significant difference between warming and control for cyanobacteria-dominated and mixed biocrust soils, implying a neutral response of GHG effluxes to climate warming. These results suggest that the GHG budget in arid desert biocrust soil would not be significantly changed in the warmer future when the direct negative effects of drought on CO₂ effluxes were excluded. Therefore, a marginal decrease of accumulative CO₂ effluxes in response to warming coupled with drought for moss-dominated biocrust soil might offer a weak negative feedback to warming and drier climate change pattern.

Increased annual methane uptake driven by warmer winters in an alpine meadow

Pronounced nongrowing season warming and changes in soil freeze-thaw (F-T) cycles can dramatically alter net methane (CH₄ ) exchange rates between soils and the atmosphere. However, the magnitudes and drivers of warming impacts on CH₄ uptake in different stages of the F-T cycle are poorly understood in cold alpine ecosystems, which have been found to be a net sink of atmospheric CH₄ . Here, we reported a year-round ecosystem daily CH₄ uptake in an alpine meadow on the Qinghai-Tibetan Plateau after a 5-year warming experiment that included a control, a low-level warming treatment (+2.4℃ at 5 cm soil depth), and a high-level warming treatment (+4.5℃ at 5 cm soil depth). We found that warming shortened the F-T cycle under the low-level warming and soils did not freeze under the high-level warming. Although both warming treatments increased the mean CH₄ uptake rate, only the high-level warming significantly increased annual CH₄ uptake compared to the control. The warming-induced stimulation of CH₄ uptake mainly occurred in the cold season, which was mostly during spring thaw under low-level warming and during the frozen winter under high-level warming due to a longer period with thawed soil. We also found that warming significantly stimulated daily CH₄ uptake mainly by reducing near-surface soil water content in the warm season, whereas both soil water content and temperature controlled daily CH₄ uptake in different ways during the autumn freeze, frozen winter, and spring thaw periods of the control. Our study revealed a strong warming effect on CH₄ uptake during the entire F-T cycle in the alpine meadow, especially the unfrozen winter. Our results also suggested the important roles of soil pH, available phosphorus, and methanotroph abundance in regulating annual CH₄ uptake in response to warming, which should be incorporated into biogeochemical models for accurately forecasting CH₄  fluxes under future climate scenarios.

Effects of increased precipitation combined with nitrogen addition and increased temperature on methane fluxes in alpine meadows of the Tibetan Plateau

Climate change and anthropogenic activities have resulted in increased atmospheric methane (CH₄) concentration. Increased nitrogen deposition and precipitation accompanies climate warming and can change soil carbon and nitrogen dynamics and microbial processes and alter CH₄ fluxes. To quantify the sink of the vast alpine meadows of the Tibetan Plateau and to examine how precipitation addition (P), warming (W), and nitrogen addition (N) affect CH₄ fluxes in alpine meadows, we conducted continuous 3-growing season experiments in an alpine meadow using the static chamber and gas chromatograph method. Soil CH₄ samples were collected during the early, peak, and late stages of the growing season from 2015 to 2017. Our results suggested that neither P, W, nor N had an interaction effect on soil CH₄ uptake. P significantly increased and decreased the copies number of particulate methane monooxygenase alpha subunit (pmoA) and methyl-coenzyme M reductase alpha subunit (mcrA), respectively. However, P significantly decreased CH₄ uptake, particularly under the combined treatment of P and N. Compared with the control, CH₄ uptake decreased under P, N, PW, and PN by 50.64%, 6.24%, 39.37%, and 75.06%, respectively, whereas under W and WN CH₄ uptake increased by 16.19% and 7.56%, respectively. Soil CH₄ uptake was positively correlated with soil temperature and pmoA and negatively correlated with soil moisture and NH₄⁺-N content. CH₄ uptake was significantly affected by the sampling period. CH₄ uptake was significantly lower rates during peak growing season compared with those during the early and late stages of the growing season. Our results suggest that, (1) CH₄ fluxes of alpine grassland ecosystems are more sensitive to P than W or N, and (2) precipitation controls CH₄ flux response to increasing nitrogen deposition in alpine meadows on the Tibetan Plateau. Therefore, future research should focus on the response and feedback of CH₄ uptake to changes in precipitation.

Soil bacterial community responses to warming and grazing in a Tibetan alpine meadow

Warming and grazing significantly affect the structure and function of an alpine meadow ecosystem. Yet, the responses of soil microbes to these disturbances are not well understood. Controlled asymmetrical warming (+1.2/1.7°C during daytime/nighttime) with grazing experiments were conducted to study microbial response to warming, grazing and their interactions. Significant interactive effects of warming and grazing were observed on soil bacterial α-diversity and composition. Warming only caused significant increase in bacterial α-diversity under no-grazing conditions. Grazing induced no substantial differences in bacterial α-diversity and composition irrespective of warming. Warming, regardless of grazing, caused a significant increase in soil bacterial community similarity across space, but grazing only induced significant increases under no-warming conditions. The positive effects of warming on bacterial α-diversity and grazing on community similarity were weakened by grazing and warming, respectively. Soil and plant variables explained well the variations in microbial communities, indicating that changes in soil and plant properties may primarily regulate soil microbial responses to warming in this alpine meadow. The results suggest that bacterial communities may become more similar across space in a future, warmed climate and moderate grazing may potentially offset, at least partially, the effects of global warming on the soil microbial diversity.