Application of nitrogen-rich sunflower husks biochar promotes methane oxidation and increases abundance of Methylobacter in nitrogen-poor soil.

The area of sunflower crops is steadily increasing. A beneficial way of managing sunflower waste biomass could be its use as a feedstock for biochar production. Biochar is currently being considered as an additive for improving soil parameters, including the ability to oxidise methane (CH4) - one of the key greenhouse gases (GHG). Despite the high production of sunflower husk, there is still insufficient information on the impact of sunflower husk biochar on the soil environment, especially on the methanotrophy process. To fill this knowledge gap, an experiment was designed to evaluate the effects of addition of sunflower husk biochar (produced at 450-550 °C) at a wide range of doses (1-100 Mg ha-1) to Haplic Luvisol. In the presented study, the CH4 oxidation potential of soil with and without sunflower husk biochar was investigated at 60 and 100% water holding capacity (WHC), and with the addition of 1% CH4 (v/v). The comprehensive study included GHG exchange (CH4 and CO2), physicochemical properties of soil (pH, soil organic carbon (SOC), dissolved organic carbon (DOC), nitrate nitrogen (NO3--N), WHC), and the structure of soil microbial communities. That study showed that even low biochar doses (5 and 10 Mg ha-1) were sufficient to enhance pH, SOC, DOC and NO3--N content. Importantly, sunflower husk biochar was significant source of NO3--N, which soil concentration increased from 9.40 ± 0.09 mg NO3--N kg-1 for the control to even 19.40 ± 0.26 mg NO3--N kg-1 (for 100 Mg ha-1). Significant improvement of WHC (by 11.0-12.4%) was observed after biochar addition at doses of 60 Mg ha-1 and higher. At 60% WHC, application of biochar at a dose of 40 Mg ha-1 brought significant improvements in CH4 oxidation rate, which was 4.89 ± 0.37 mg CH4-C kg-1 d-1. Higher biochar doses were correlated with further improvement of CH4 oxidation rates, which at 100 Mg ha-1 was seventeen-fold higher (8.36 ± 0.84 mg CH4-C kg-1 d-1) than in the biochar-free control (0.48 ± 0.28 mg CH4-C kg-1 d-1). CO2 emissions were not proportional to biochar doses and only grew circa (ca.) twofold from 3.16 to 6.90 mg CO2-C kg-1 d-1 at 100 Mg ha-1. Above 60 Mg ha-1, the diversity of methanotrophic communities increased, with Methylobacter becoming the most abundant genus, which was as high as 7.45%. This is the first, such advanced and multifaceted study of the wide range of sunflower husk biochar doses on Haplic Luvisol. The positive correlation between soil conditions, methanotroph abundance and CH4 oxidation confirmed the multifaceted, positive effect of sunflower husk biochar on Haplic Luvisol. Sunflower husk biochar can be successfully used for Haplic Luvisol supplementation. This additive facilitates soil protection against degradation and has the potential to mitigate GHG emissions.

Three decades of shade trees improve soil organic carbon pools but not methane uptake in coffee systems.

The rapid expansion of coffee plantations in tropical area at the cost of natural forest may suppress the methane (CH4) uptake and change the soil fertility. However, observations on soil CH4 uptake rates and the ecological consequence studies on coffee-based plantations are sparse. The objectives of this study were to characterize the dynamics of CH4 uptake among natural forest, coffee monoculture (CM), and coffee intercropping with shade tree (CI), and to evaluate the key drivers of soil CH4 uptake. Results showed that the conversion of forest into 25-year and 34-year CM plantations significantly reduced the soil organic carbon (SOC) content by 57% and 76%, respectively, whereas CI plantation profoundly increased the SOC by 20%-76% compared with CM plantation. Although soils of forest, CM and CI functioned exclusively as CH4 sinks, the CM and CI plantations significantly decreased the ambient CH4 uptake rates by 64%-83% due to soil moisture shift and soil nitrate availability by using chemical fertilizer. Interestingly, the potential CH4 uptake of CM and CI plantations did not decrease and in some treatments, was even higher than that of the natural forest. Potential CH4 uptake showed a negative correlation with soil pH and SOC content, but a positive correlation with soil available phosphorus (AP). Collectively, although the SOC and soil pH were increased through intercropping with shade trees for decades, the inhibition of atmospheric CH4 uptake was still difficult to alleviate.

Microbially enhanced methane uptake under warming enlarges ecosystem carbon sink in a Tibetan alpine grassland.

The alpine grasslands of the Tibetan Plateau store 23.2 Pg soil organic carbon, which becomes susceptible to microbial degradation with climate warming. However, accurate prediction of how the soil carbon stock changes under future climate warming is hampered by our limited understanding of belowground complex microbial communities. Here, we show that 4 years of warming strongly stimulated methane (CH4 ) uptake by 93.8% and aerobic respiration (CO2 ) by 11.3% in the soils of alpine grassland ecosystem. Due to no significant effects of warming on net ecosystem CO2 exchange (NEE), the warming-stimulated CH4 uptake enlarged the carbon sink capacity of whole ecosystem. Furthermore, precipitation alternation did not alter such warming effects, despite the significant effects of precipitation on NEE and soil CH4 fluxes were observed. Metagenomic sequencing revealed that warming led to significant shifts in the overall microbial community structure and the abundances of functional genes, which contrasted to no detectable changes after 2 years of warming. Carbohydrate utilization genes were significantly increased by warming, corresponding with significant increases in soil aerobic respiration. Increased methanotrophic genes and decreased methanogenic genes were observed under warming, which significantly (R2  = .59, p < .001) correlated with warming-enhanced CH4 uptakes. Furthermore, 212 metagenome-assembled genomes were recovered, including many populations involved in the degradation of various organic matter and a highly abundant methylotrophic population of the Methyloceanibacter genus. Collectively, our results provide compelling evidence that specific microbial functional traits for CH4 and CO2 cycling processes respond to climate warming with differential effects on soil greenhouse gas emissions. Alpine grasslands may play huge roles in mitigating climate warming through such microbially enhanced CH4 uptake.

Differential response of soil CO2 , CH4 , and N2 O emissions to edaphic properties and microbial attributes following afforestation in central China.

Land use change specially affects greenhouse gas (GHG) emissions, and it can act as a sink/source of GHGs. Alterations in edaphic properties and microbial attributes induced by land use change can individually/interactively contribute to GHG emissions, but how they predictably affect soil CO2 , CH4 , and N2 O emissions remain unclear. Here, we investigated the direct and indirect controls of edaphic properties (i.e., dissolved organic carbon [DOC], soil organic C, total nitrogen, C:N ratio, NH4+ -N, NO3- -N, soil temperature [ST], soil moisture [SM], pH, and bulk density [BD]) and microbial attributes (i.e., total phospholipid fatty acids [PLFAs], 18:1ω7c, nitrifying genes [ammonia-oxidizing archaea, ammonia-oxidizing bacteria], and denitrifying genes [nirS, nirK, and nosZ]) over the annual soil CO2 , CH4 , and N2 O emissions from the woodland, shrubland, and abandoned land in subtropical China. Soil CO2 and N2 O emissions were higher in the afforested lands (woodland and shrubland) than in the abandoned land, but the annual cumulative CH4 uptake did not significantly differ among all land use types. The CO2 emission was positively associated with microbial activities (e.g., total PLFAs), while the CH4 uptake was tightly correlated with soil environments (i.e., ST and SM) and chemical properties (i.e., DOC, C:N ratio, and NH4+ -N concentration), but not significantly related to the methanotrophic bacteria (i.e., 18:1ω7c). Whereas, soil N2 O emission was positively associated with nitrifying genes, but negatively correlated with denitrifying genes especially nosZ. Overall, our results suggested that soil CO2 and N2 O emissions were directly dependent on microbial attributes, and soil CH4 uptake was more directly related to edaphic properties rather than microbial attributes. Thus, different patterns of soil CO2 , CH4 , and N2 O emissions and associated controls following land use change provided novel insights into predicting the effects of afforestation on climate change mitigation outcomes.

Declines in methane uptake in forest soils.

Forest soils are a sink for atmospheric methane (CH4) and play an important role in modulating the global CH4 budget. However, whether CH4 uptake by forest soils is affected by global environmental change is unknown. We measured soil to atmosphere net CH4 fluxes in temperate forests at two long-term ecological research sites in the northeastern United States from the late 1990s to the mid-2010s. We found that annual soil CH4 uptake decreased by 62% and 53% in urban and rural forests in Baltimore, Maryland and by 74% and 89% in calcium-fertilized and reference forests at Hubbard Brook, New Hampshire over this period. This decrease occurred despite marked declines in nitrogen deposition and increases in atmospheric CH4 concentration and temperature, which should lead to increases in CH4 uptake. This decrease in soil CH4 uptake appears to be driven by increases in precipitation and soil hydrological flux. Furthermore, an analysis of CH4 uptake around the globe showed that CH4 uptake in forest soils has decreased by an average of 77% from 1988 to 2015, particularly in forests located from 0 to 60 °N latitude where precipitation has been increasing. We conclude that the soil CH4 sink may be declining and overestimated in several regions across the globe.

Effect of COD on methanotrophic growth and the anaerobic digestibility of its sludge as a further step for integration in WWTPS.

Within wastewater treatment plants (WWTPs), the anaerobically produced biogas is often underutilized. Fortunately, methanotrophic based biotechnologies can be the remedy for on-site exploitation and recovery of unused biogas. In this regard, efforts have been placed on evaluating the suitably of methanotrophs to be deployed in WWTPs. Even so, the effect of chemical oxygen demand (COD) on methanotrophic activity and methanotrophic sludge digestibility have not been studied, which is the focus of the present study. A methanotrophic culture enriched from activated sludge was exposed to four different COD concentrations (0-540 mg/L) to evaluate the effect of COD on the culture activity in batch mode. It was attained that the presence of COD concentrations up to 540 mg/L has limited influence on methanotrophic activity. This finding was supported by the similar average methane uptake rate (between 2.48 and 2.53 mgCH4/hr) and consumption (61.4 ± 1.5%) observed under the different COD concentrations. On the other hand, methanotrophic sludge was digested in comparison to waste activated sludge (WAS) collected from a WWTP for more than 40 days to evaluate its digestibility. It was obtained that the methanotrophic sludge had a methane specific yield of approximately 1.72 times greater than WAS and had a higher solids destruction rate. This research is another step demonstrating the feasibility of methanotrophs integration in WWTP.

Asymmetric response of soil methane uptake rate to land degradation and restoration: Data synthesis.

Land degradation and restoration profoundly affect soil CH4 uptake capacity in terrestrial ecosystems. However, a comprehensive assessment of the response of soil CH4 uptake to land degradation and restoration at global scale is not available. Here, we present a global meta-analysis with a database of 228 observations from 83 studies to investigate the effects of land degradation and restoration on the capacity of soil CH4 uptake. We found that land degradation significantly decreased the capacity of soil CH4 uptake, except the conversion of pasture to cropland where the soil CH4 uptake rate showed no response. In contrast, all types of land restoration significantly increased the capacity of soil CH4 uptake. Interestingly, the response of soil CH4 uptake rate to land degradation and restoration was asymmetric: the increased soil CH4 uptake rate in response to the land restoration was smaller compared to the decrease in CH4 uptake rate induced by the land degradation. The effect of land degradation on soil CH4 uptake rate was not dependent on the time since land use change, but the CH4 sink strength increased with the time since land restoration. The response of soil CH4 uptake rate to both land degradation and restoration was predominantly regulated by changes in the soil water-filled pore space, soil bulk density, and pH, whereas alterations in the substrate quantity and quality had negligible effect. Additionally, the effects of land degradation and restoration on soil CH4 uptake were strongly related to the mean annual precipitation and soil texture. Overall, our results provide novel insights for understanding of how land degradation and restoration can affect the CH4 sink strength of upland soils, and more importantly, our findings are beneficial to take measures to enhance the potential of soil CH4 uptake in response to global land use change.

Methane oxidation in contrasting soil types: responses to experimental warming with implication for landscape-integrated CH4 budget.

Arctic ecosystems are characterized by a wide range of soil moisture conditions and thermal regimes and contribute differently to the net methane (CH4 ) budget. Yet, it is unclear how climate change will affect the capacity of those systems to act as a net source or sink of CH4 . Here, we present results of in situ CH4 flux measurements made during the growing season 2014 on Disko Island (west Greenland) and quantify the contribution of contrasting soil and landscape types to the net CH4 budget and responses to summer warming. We compared gas flux measurements from a bare soil and a dry heath, at ambient conditions and increased air temperature, using open-top chambers (OTCs). Throughout the growing season, bare soil consumed 0.22 ± 0.03 g CH4 -C m-2 (8.1 ± 1.2 g CO2 -eq m-2 ) at ambient conditions, while the dry heath consumed 0.10 ± 0.02 g CH4 -C m-2 (3.9 ± 0.6 g CO2 -eq m-2 ). These uptake rates were subsequently scaled to the entire study area of 0.15 km2 , a landscape also consisting of wetlands with a seasonally integrated methane release of 0.10 ± 0.01 g CH4 -C m-2 (3.7 ± 1.2 g CO2 -eq m-2 ). The result was a net landscape sink of 12.71 kg CH4 -C (0.48 tonne CO2 -eq) during the growing season. A nonsignificant trend was noticed in seasonal CH4 uptake rates with experimental warming, corresponding to a 2% reduction at the bare soil, and 33% increase at the dry heath. This was due to the indirect effect of OTCs on soil moisture, which exerted the main control on CH4 fluxes. Overall, the net landscape sink of CH4 tended to increase by 20% with OTCs. Bare and dry tundra ecosystems should be considered in the net CH4 budget of the Arctic due to their potential role in counterbalancing CH4 emissions from wetlands - not the least when taking the future climatic scenarios of the Arctic into account.

Spatial variability of CH4 uptake in aerated soils of Yellow River Delta.

The widespread occurrence of aerated plain soils underscores their significant role in the global soil methane (CH4) sink budget. However, plain soils are poorly characterized in terms of spatial variability of CH4 uptake and the relevant control. We investigated the intra- and inter-site spatial variability of CH4 uptake through flux measurements in intact soil cores from five non-wetland sites within the Yellow River Delta, each representing a distinct land use/cover type. Methane uptake rates were highest in undisturbed forest cores. The rates were very low, often falling below the detection limit, in cores from the other four sites. The significant correlation between CH4 uptake and bulk density across sites suggests the integrative role of bulk density for the effects of different disturbances (including salt stress and succession) on CH4 uptake. Methane uptake was heterogeneous at the within-site scale as indicated by large coefficients of variations (CVs). Soil texture variation manipulated the within-site pattern of CH4 uptake in the low-salinity sites. Salt affected the spatial variation of CH4 uptake only at high level of salinity. Neither Potter's nor Ridgwell's models effectively captured the within-site variation of CH4 uptake due to a texture-associated bias in the models. Establishing a quantitative relationship between CH4 uptake and clay content at the field scale in alluvial plain soils will facilitate the refinement of model parameters linked to texture and rectify biases in CH4 estimation. These results provide an insight for the biogeochemical control of CH4 uptake in alluvial plain soils and have important application for improving CH4 models.

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.

The impacts of nitrogen addition on upland soil methane uptake: A global meta-analysis.

Elevated nitrogen (N) addition from anthropogenic activities has great impacts on soil methane (CH4) uptake, which could interrupt the existing global CH4 balance and cause feedbacks to climate and biogeochemical processes. Previous studies have come to inconsistent conclusions on both the quantification of the response of CH4 uptake to N addition and understanding of its underlying mechanisms, probably due to the lack of experimental data. Here, we conduct a broad meta-analysis of 90 papers to quantify the responses of CH4 uptake to N addition in upland soil. The results show that N addition has a significant negative impact on soil CH4 uptake (-19.25%), which is termed the N inhibition effect. Soil pH is identified as the dominant factor, with the other factors affecting the CH4 uptake through the alteration of soil pH. The N inhibition effect is observed to be large and significant in forest and grassland, but small and insignificant in farmland, because of the distinct composition of their methanotrophic communities. A threshold of the N addition level is identified at about 68 kg N ha-1 year-1, which indicates the lowest N inhibition effect. Furthermore, the convex relationship between response ratio of CH4 uptake (negative) and N addition duration indicates that a medium level of N addition duration has the largest N inhibition effect, and longer or shorter durations will both reduce the effect. Our analysis of the N inhibition effect implies that controlling the N addition level could effectively reduce the CH4 concentration in the atmosphere and thus relieve global warming.

Effects of nitrogen addition on soil methane uptake in global forest biomes.

Nitrogen (N) deposition has been conventionally thought to decrease forest soil methane (CH4) uptake, while the biome specific and dose dependent effect is poorly understood. Based on a meta-analysis of 63 N addition trials from 7 boreal forests, 8 temperate forests, 13 subtropical and 4 tropical forests, we evaluated the effects of N addition on soil CH4 uptake fluxes across global forest biomes. When combining all N addition levels, soil CH4 uptake was insignificantly decreased by 7% in boreal forests, while N addition significantly decreased soil CH4 uptake by 39% in temperate forests and by 21% in subtropical and tropical forests, respectively. Meta-regression analyses, however, indicated a shift from a positive to a negative effect on soil CH4 uptake with increasing N additions both in boreal forests (threshold = 48 kg N ha-1 yr-1) and temperate forests (threshold = 27 kg N ha-1 yr-1), while no such shift was found in subtropical and tropical forests. Considering that current N deposition to most boreal and temperate forests is below the abovementioned thresholds, N deposition likely exerts a positive to neutral effect on soil CH4 uptake in both forest biomes. Our results provide new insights on the biome specific and dose dependent effect of N addition on soil CH4 sink in global forests and suggest that the current understanding that N deposition decreases forest soil CH4 uptake is flawed by high levels of experimental N addition.

Heavy grazing reduces grassland soil greenhouse gas fluxes: A global meta-analysis.

Grazing degrades worldwide grasslands and possibly suppresses soil greenhouse gas (GHG: CO2, CH4 and N2O) fluxes. However, the global patterns of these three gas fluxes in response to grazing and the general mechanisms remain poorly understood. Here, we performed a meta-analysis of 63 independent grazing studies that measured soil GHG fluxes across global grasslands. Our results revealed that light and moderate grazing had no significant effect on soil CH4 uptake, N2O and CO2 emission, but heavy grazing consistently reduced them. The magnitudes of their responses to grazing were regulated by grazing duration and precipitation. In comparison with CO2 emission, soil CH4 uptake and N2O emission were reduced much more under heavier grazing, longer grazing duration or less precipitation. The decrease in soil CO2 emission was possibly caused by grazing-induced reduction in root biomass and soil moisture, while the decline in soil CH4 uptake and N2O emission was due to decreased soil moisture and substrate availability. Overall, this study provides the first large-scale evaluation on three main soil GHG fluxes in response to grazing, highlighting grazing inhibition of GHG emission but at the cost of plant productivity and soil fertility. We call for future efforts to identify an appropriate grazing intensity that is optimal to balance these complicated impacts.

Determination of the phytoremediation efficiency of Ricinus communis L. and methane uptake from cadmium and nickel-contaminated soil using spent mushroom substrate.

Spent mushroom substrate (SMS) as an organic amendment to plant production has received increasing attention on soil phytoremediation. However, organic amendments are known to contribute to greenhouse gas (GHG) emission from soils. Castor oil plant has a high biomass production and phytoremediation potential for heavy metal-contaminated soils. In the present study, the roles of SMS on phytoremediation efficiency of castor oil plant (Ricinus communis L.) from cadmium (Cd) and nickel (Ni)-contaminated soils were investigated, and the impact of SMS application on methane emission from the contaminated soil were evaluated. Pot experiments with SMS-amended and unamended contaminated soils were conducted to investigate Cd and Ni accumulation in R. communis and CH4 emission. After growing for 3 months in soils with the addition of Cd (10 mg/kg) and Ni (at rates of 200 and 600 mg/kg), the dry biomass and the concentrations of Cd and Ni in the R. communis were measured, and the mobility factors for Cd and Ni were calculated. To assess methane emission, CH4 fluxes and potential rates of CH4 production and oxidation were measured pre- and post-incubation. SMS addition significantly improved the growth of R. communis and gave 19.15~82.46% more dry weight as compared to the single plant cultivation in the contaminated soils. SMS also increased plant Cd uptake and the total amount of Cd accumulation in R. communis increased by 28.1-152.1%, respectively, in signal Cd treatment and Cd-Ni complexation treatment, as compared to the single plant cultivation. The high values of mobility factor for Cd in single plant cultivation and co-application of SMS and R. communis pointed to the potential of R. communis to the Cd mobilization from the contaminated soils. Moreover, the addition of SMS tended to stimulate CH4 uptake that the average increases in CH4 uptake rate were 3.84-fold (in controls) and 2.91-fold (in single Cd treated soils) by the co-application of SMS and R. communis as compared to the single plant cultivation. The results suggested that the application of SMS could improve the growth of R. communis in Cd and Ni-contaminated soil, enhance heavy metal bioaccumulation, and stimulate soil CH4 uptake. Therefore, SMS might be useful for enhancing phytoremediation of heavy metals and mitigate CH4 emission from the contaminated soil. In addition, results in the study implied that implementing carefully designed management strategies (e.g., application of organic residues) during contaminated soil remediation is a promising solution for agricultural waste management and soil phytoremediation.