David S. Reay

Climate-smart soils

Soils are integral to the function of all terrestrial ecosystems and to food and fibre production. An overlooked aspect of soils is their potential to mitigate greenhouse gas emissions. Although proven practices exist, the implementation of soil-based greenhouse gas mitigation activities are at an early stage and accurately quantifying emissions and reductions remains a substantial challenge. Emerging research and information technology developments provide the potential for a broader inclusion of soils in greenhouse gas policies. Here we highlight ‘state of the art’ soil greenhouse gas research, summarize mitigation practices and potentials, identify gaps in data and understanding and suggest ways to close such gaps through new research, technology and collaboration.

Identification of active methylotroph populations in an acidic forest soil by stable- isotope probing

Stable-isotope probing (SIP) is a culture-independent technique that enables the isolation of DNA from micro-organisms that are actively involved in a specific metabolic process. In this study, SIP was used to characterize the active methylotroph populations in forest soil (pH 3.5) microcosms that were exposed to (13)CH(3)OH or (13)CH(4). Distinct (13)C-labelled DNA ((13)C-DNA) fractions were resolved from total community DNA by CsCl density-gradient centrifugation. Analysis of 16S rDNA sequences amplified from the (13)C-DNA revealed that bacteria related to the genera Methylocella, Methylocapsa, Methylocystis and Rhodoblastus had assimilated the (13)C-labelled substrates, which suggested that moderately acidophilic methylotroph populations were active in the microcosms. Enrichments targeted towards the active proteobacterial CH(3)OH utilizers were successful, although none of these bacteria were isolated into pure culture. A parallel analysis of genes encoding the key enzymes methanol dehydrogenase and particulate methane monooxygenase reflected the 16S rDNA analysis, but unexpectedly revealed sequences related to the ammonia monooxygenase of ammonia-oxidizing bacteria (AOB) from the beta-subclass of the PROTEOBACTERIA: Analysis of AOB-selective 16S rDNA amplification products identified Nitrosomonas and Nitrosospira sequences in the (13)C-DNA fractions, suggesting certain AOB assimilated a significant proportion of (13)CO(2), possibly through a close physical and/or nutritional association with the active methylotrophs. Other sequences retrieved from the (13)C-DNA were related to the 16S rDNA sequences of members of the Acidobacterium division, the beta-Proteobacteria and the order Cytophagales, which implicated these bacteria in the assimilation of reduced one-carbon compounds or in the assimilation of the by-products of methylotrophic carbon metabolism. Results from the (13)CH(3)OH and (13)CH(4) SIP experiments thus provide a rational basis for further investigations into the ecology of methylotroph populations in situ.

Large-Scale Controls of Methanogenesis Inferred from Methane and Gravity Spaceborne Data

Measuring Methanogenesis After carbon dioxide, methane is the second most important greenhouse gas, and an important species in terms of its role in atmospheric chemistry. The sources and sinks of methane, particularly the natural ones, are too poorly quantified, however, even to explain why the decades-long, steady increase of its concentration in the atmosphere was interrupted between 1999 and 2006. Bloom et al. (p. 322) use a combination of satellite data, which indicate water table depth and surface temperature, and atmospheric methane concentrations to determine the location and strength of methane emissions from wetlands, the largest natural global source. The constraints placed on these sources should help to improve predictions of how climate change will affect wet-land emissions of methane. Satellite measurements allow the strength of wetland emissions of methane to be determined. Wetlands are the largest individual source of methane (CH4), but the magnitude and distribution of this source are poorly understood on continental scales. We isolated the wetland and rice paddy contributions to spaceborne CH4 measurements over 2003–2005 using satellite observations of gravity anomalies, a proxy for water-table depth Γ, and surface temperature analyses TS. We find that tropical and higher-latitude CH4 variations are largely described by Γ and TS variations, respectively. Our work suggests that tropical wetlands contribute 52 to 58% of global emissions, with the remainder coming from the extra-tropics, 2% of which is from Arctic latitudes. We estimate a 7% rise in wetland CH4 emissions over 2003–2007, due to warming of mid-latitude and Arctic wetland regions, which we find is consistent with recent changes in atmospheric CH4.

Ultraviolet radiation drives methane emissions from terrestrial plant pectins

Recent studies demonstrating an in situ formation of methane (CH(4)) within foliage and separate observations that soil-derived CH(4) can be released from the stems of trees have continued the debate about the role of vegetation in CH(4) emissions to the atmosphere. Here, a study of the role of ultraviolet (UV) radiation in the formation of CH(4) and other trace gases from plant pectins in vitro and from leaves of tobacco (Nicotiana tabacum) in planta is reported. Plant pectins were investigated for CH(4 )production under UV irradiation before and after de-methylesterification and with and without the singlet oxygen scavenger 1,4-diazabicyclo[2.2.2]octane (DABCO). Leaves of tobacco were also investigated under UV irradiation and following leaf infiltration with the singlet oxygen generator rose bengal or the bacterial pathogen Pseudomonas syringae. Results demonstrated production of CH(4), ethane and ethylene from pectins and from tobacco leaves following all treatments, that methyl-ester groups of pectin are a source of CH(4), and that reactive oxygen species (ROS) arising from environmental stresses have a potential role in mechanisms of CH(4) formation. Rates of CH(4 )production were lower than those previously reported for intact plants in sunlight but the results clearly show that foliage can emit CH(4) under aerobic conditions.

Can biochar reduce soil greenhouse gas emissions from a Miscanthus bioenergy crop

Energy production from bioenergy crops may significantly reduce greenhouse gas (GHG) emissions through substitution of fossil fuels. Biochar amendment to soil may further decrease the net climate forcing of bioenergy crop production, however, this has not yet been assessed under field conditions. Significant suppression of soil nitrous oxide (N2O) and carbon dioxide (CO2) emissions following biochar amendment has been demonstrated in short‐term laboratory incubations by a number of authors, yet evidence from long‐term field trials has been contradictory. This study investigated whether biochar amendment could suppress soil GHG emissions under field and controlled conditions in a Miscanthus × Giganteus crop and whether suppression would be sustained during the first 2 years following amendment. In the field, biochar amendment suppressed soil CO2 emissions by 33% and annual net soil CO2 equivalent (eq.) emissions (CO2, N2O and methane, CH4) by 37% over 2 years. In the laboratory, under controlled temperature and equalised gravimetric water content, biochar amendment suppressed soil CO2 emissions by 53% and net soil CO2 eq. emissions by 55%. Soil N2O emissions were not significantly suppressed with biochar amendment, although they were generally low. Soil CH4 fluxes were below minimum detectable limits in both experiments. These findings demonstrate that biochar amendment has the potential to suppress net soil CO2 eq. emissions in bioenergy crop systems for up to 2 years after addition, primarily through reduced CO2 emissions. Suppression of soil CO2 emissions may be due to a combined effect of reduced enzymatic activity, the increased carbon‐use efficiency from the co‐location of soil microbes, soil organic matter and nutrients and the precipitation of CO2 onto the biochar surface. We conclude that hardwood biochar has the potential to improve the GHG balance of bioenergy crops through reductions in net soil CO2 eq. emissions.

Global methane emission estimates from ultraviolet irradiation of terrestrial plant foliage

SUMMARY *Several studies have reported in situ methane (CH(4)) emissions from vegetation foliage, but there remains considerable debate about its significance as a global source. Here, we report a study that evaluates the role of ultraviolet (UV) radiation-driven CH(4) emissions from foliar pectin as a global CH(4) source. *We combine a relationship for spectrally weighted CH(4) production from pectin with a global UV irradiation climatology model, satellite-derived leaf area index (LAI) and air temperature data to estimate the potential global CH(4) emissions from vegetation foliage. *Our results suggest that global foliar CH(4) emissions from UV-irradiated pectin could account for 0.2-1.0 Tg yr(-1), of which 60% is from tropical latitudes, corresponding to < 0.2% of total CH(4) sources. *Our estimate is one to two orders of magnitude lower than previous estimates of global foliar CH(4) emissions. Recent studies have reported that pectin is not the only molecular source of UV-driven CH(4) emissions and that other environmental stresses may also generate CH(4). Consequently, further evaluation of such mechanisms of CH(4) generation is needed to confirm the contribution of foliage to the global CH(4) budget.

Effects of land-use on the activity and diversity of methane oxidizing bacteria in forest soils

Abstract Methane is an important greenhouse gas and CH 4 oxidation in soil represents a significant sink for this gas. High capacity CH 4 oxidation potentials and molecular profiles of CH 4 oxidizing bacteria in soil were compared for five land-use treatments at a fully replicated experimental site within the Gisburn Forest Experiment, to assess the effects of land-use on both the potential activity of CH 4 oxidizing bacteria and their diversity. Forestry land-use was found to have a highly significant effect on CH 4 oxidation potentials. Highest CH 4 oxidation potentials were found in soils collected under stands of oak, in grassland plots, and in one soil under Norway spruce. A negative relationship between soil water nitrate concentration and CH 4 oxidation capacity was evident across the experimental site, with the high nitrate soils under stands of alder exhibiting little or no capacity for CH 4 oxidation even at optimal temperature and water content. Molecular profiles indicated that a diverse range of bacteria with the potential to oxidize CH 4 were present in all soils, however no clear correlation with CH 4 oxidation potential was identified.

Biochar diminishes nitrous oxide and nitrate leaching from diverse nutrient sources.

Manure generated by intensive livestock operations poses potential ecological risk in the form of water pollution and greenhouse gas emission. To assess the impact of biochar on coarse-textured soils under contrasting nutrient management regimes, a 55-d incubation was conducted using unplanted soil columns amended with manure, slurry, or fertilizer (plus unamended control), each with or without biochar applied at 2% soil mass (dry weight basis). Under repeated leaching, the cumulative NO emission from the columns was significantly affected by the presence of biochar ( < 0.0001), although these data were not normally distributed. Results indicated that the biochar-amended soils emitted significantly less NO than their unamended counterparts, with the exception of manure-amended soils. The presence of biochar increased the pH of column leachate by 0.08 to 1.70 and significantly decreased the cumulative amount of mineral N leached from the soil. The presence of biochar significantly increased the amount of PO-P in soil leachate, but there was no significant difference between the means for any of the amendments used on their own relative to their biochar-amended counterparts. The data demonstrate that biochar could potentially aid in the mitigation of NO emissions from certain soils and in N loss in leachate from soil amended with slurry, manure, or fertilizer used in livestock systems.

Indirect nitrous oxide emissions: Revised emission factors

Abstract Indirect nitrous oxide (N2O) emissions arising from ground and surface waters are thought to be significant, but considerable uncertainty surrounds the emission factors for dissolved nitrate in aquifers, drainage ditches (EF5-g), rivers (EF5-r) and estuaries (EF5-e). Through measurements of nitrate and N2O concentrations in aquifers and field drains, the existing IPCC default emission factor EF5-g has been thrown into doubt. Here, we present a synthesis of the work of two independent UK-based research groups working as part of the Global Nitrogen Enrichment (GANE) initiative. We put forward a revised emission factor for EF5-g based on these findings. We suggest the downward revision of the indirect N2O emission factor EF5-g from 0.015 to 0.002, and a consequent decrease in EF5 from 0.025 to 0.012. Such radical downward revision would halve current estimates of N2O emissions associated with N leaching and runoff from agriculture. In the UK, estimated annual N2O emissions via this route would be cut from around 14 Gg N y−1 to less than 7 Gg y−1. Globally, the EF5 emission estimate would come down from 1.6 Tg N y−1 to less than 0.8 Tg N y−1.

Mitigation of methane emissions from constructed farm wetlands

Constructed wetlands are increasingly used for water pollution treatment but may also be sources of the greenhouse gas CH(4). The effect of addition of two potential inhibitors of methanogenesis - iron ochre and gypsum - on net CH(4) emissions was investigated in a constructed wetland treating farm runoff in Scotland, UK. CH(4) fluxes from three 15-m(2) wetland plots were measured between January and July 2008 in large static chambers incorporating a tunable diode laser, with application of 5tonha(-1) ochre and gypsum in May. CH(4) fluxes were also measured from control and ochre- and gypsum-treated wetland sediment cores incubated at constant and varying temperature in the laboratory. Ochre addition suppressed CH(4) emissions by 64+/-13% in the field plot and >90% in laboratory incubations compared to controls. Gypsum application of 5tonha(-1) in the field and laboratory experiments had no effect on CH(4) emissions, but application of 10tonha(-1) to a sediment core reduced CH(4) emissions by 28%. Suppression of CH(4) emissions by ochre application to sediment cores also increased with temperature; the reduction relative to the control increased from 50% at 17.5 degrees C to >90% at 27.5 degrees C. No significant changes in N removal or pH and potentially-toxic metal content of sediments as the result of inhibitor application were detected in the wetland during the study.