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Manual measurements of nitrous oxide (N2O) emissions with static chambers are commonly practised. However, they generally do not consider the diurnal variability of N2O flux, and little is known about the patterns and drivers of such variability. We systematically reviewed and analysed 286 diurnal data sets of N2O fluxes from published literature to (i) assess the prevalence and timing (day or night peaking) of diurnal N2O flux patterns in agricultural and forest soils, (ii) examine the relationship between N2O flux and soil temperature with different diurnal patterns, (iii) identify whether non‐diurnal factors (i.e. land management and soil properties) influence the occurrence of diurnal patterns and (iv) evaluate the accuracy of estimating cumulative N2O emissions with single‐daily flux measurements. Our synthesis demonstrates that diurnal N2O flux variability is a widespread phenomenon in agricultural and forest soils. Of the 286 data sets analysed, ~80% exhibited diurnal N2O patterns, with ~60% peaking during the day and ~20% at night. Contrary to many published observations, our analysis only found strong positive correlations (R > 0.7) between N2O flux and soil temperature in one‐third of the data sets. Soil drainage property, soil water‐filled pore space (WFPS) level and land use were also found to potentially influence the occurrence of certain diurnal patterns. Our work demonstrated that single‐daily flux measurements at mid‐morning yielded daily emission estimates with the smallest average bias compared to measurements made at other times of day, however, it could still lead to significant over‐ or underestimation due to inconsistent diurnal N2O patterns. This inconsistency also reflects the inaccuracy of using soil temperature to predict the time of daily average N2O flux. Future research should investigate the relationship between N2O flux and other diurnal parameters, such as photosynthetically active radiation (PAR) and root exudation, along with the consideration of the effects of soil moisture, drainage and land use on the diurnal patterns of N2O flux. The information could be incorporated in N2O emission prediction models to improve accuracy. [ABSTRACT FROM AUTHOR]

Abstract: Perennial bioenergy crops have significant potential to reduce greenhouse gas (GHG) emissions and contribute to climate change mitigation by substituting for fossil fuels; yet delivering significant GHG savings will require substantial land‐use change, globally. Over the last decade, research has delivered improved understanding of the environmental benefits and risks of this transition to perennial bioenergy crops, addressing concerns that the impacts of land conversion to perennial bioenergy crops could result in increased rather than decreased GHG emissions. For policymakers to assess the most cost‐effective and sustainable options for deployment and climate change mitigation, synthesis of these studies is needed to support evidence‐based decision making. In 2015, a workshop was convened with researchers, policymakers and industry/business representatives from the UK, EU and internationally. Outcomes from global research on bioenergy land‐use change were compared to identify areas of consensus, key uncertainties, and research priorities. Here, we discuss the strength of evidence for and against six consensus statements summarising the effects of land‐use change to perennial bioenergy crops on the cycling of carbon, nitrogen and water, in the context of the whole life‐cycle of bioenergy production. Our analysis suggests that the direct impacts of dedicated perennial bioenergy crops on soil carbon and nitrous oxide are increasingly well understood and are often consistent with significant life cycle GHG mitigation from bioenergy relative to conventional energy sources. We conclude that the GHG balance of perennial bioenergy crop cultivation will often be favourable, with maximum GHG savings achieved where crops are grown on soils with low carbon stocks and conservative nutrient application, accruing additional environmental benefits such as improved water quality. The analysis reported here demonstrates there is a mature and increasingly comprehensive evidence base on the environmental benefits and risks of bioenergy cultivation which can support the development of a sustainable bioenergy industry. [ABSTRACT FROM AUTHOR]

National governments and international organizations perceive bioenergy, from crops such as Miscanthus, to have an important role in mitigating greenhouse gas ( GHG) emissions and combating climate change. In this research, we address three objectives aimed at reducing uncertainty regarding the climate change mitigation potential of commercial Miscanthus plantations in the United Kingdom: (i) to examine soil temperature and moisture as potential drivers of soil GHG emissions through four years of parallel measurements, (ii) to quantify carbon (C) dynamics associated with soil sequestration using regular measurements of topsoil (0-30 cm) C and the surface litter layer and (iii) to calculate a life cycle GHG budget using site-specific measurements, enabling the GHG intensity of Miscanthus used for electricity generation to be compared against coal and natural gas. Our results show that methane ( CH4) and nitrous oxide (N2O) emissions contributed little to the overall GHG budget of Miscanthus, while soil respiration offset 30% of the crop's net aboveground C uptake. Temperature sensitivity of soil respiration was highest during crop growth and lowest during winter months. We observed no significant change in topsoil C or nitrogen stocks following 7 years of Miscanthus cultivation. The depth of litter did, however, increase significantly, stabilizing at approximately 7 tonnes dry biomass per hectare after 6 years. The cradle-to-farm gate GHG budget of this crop indicated a net removal of 24.5 t CO2-eq ha−1 yr−1 from the atmosphere despite no detectable C sequestration in soils. When scaled up to consider the full life cycle, Miscanthus fared very well in comparison with coal and natural gas, suggesting considerable CO2 offsetting per kWh generated. Although the comparison does not account for the land area requirements of the energy generated, Miscanthus used for electricity generation can make a significant contribution to climate change mitigation even when combusted in conventional steam turbine power plants. [ABSTRACT FROM AUTHOR]

The lignocellulosic perennial grass Miscanthus has received considerable attention as a potential bioenergy crop over the last 25 years, but few commercial plantations exist globally. This is partly due to the uncertainty associated with claims that land-use change ( LUC) to Miscanthus will result in both commercially viable yields and net increases in carbon (C) storage. To simulate what the effects may be after LUC to Miscanthus, six process-based models have been parameterized for Miscanthus and here we review how these models operate. This review provides an overview of the key Miscanthus soil organic matter models and then highlights what measurers can do to accelerate model development. Each model ( WIMOVAC, BioCro, Agro- IBIS, DAYCENT, DNDC and ECOSSE) is capable of simulating biomass production and soil C dynamics based on specific site characteristics. Understanding the design of these models is important in model selection as well as being important for field researchers to collect the most relevant data to improve model performance. The rapid increase in models parameterized for Miscanthus is promising, but refinements and improvements are still required to ensure that model predictions are reliable and can be applied to spatial scales relevant for policy. Specific improvements, needed to ensure the models are applicable for a range of environmental conditions, come under two categories: (i) increased data generation and (ii) development of frameworks and databases to allow simulations of ranging scales. Research into nonfood bioenergy crops such as Miscanthus is relatively recent and this review highlights that there are still a number of knowledge gaps regarding Miscanthus specifically. For example, the low input requirements of Miscanthus make it particularly attractive as a bioenergy crop, but it is essential that we increase our understanding of the crop's nutrient remobilization and ability to host N-fixing organisms to derive the most accurate simulations. [ABSTRACT FROM AUTHOR]

218 | Glob Change Biol. 2021;27:218-219. wileyonlinelibrary.com/journal/gcb Received: 24 September 2020 | Accepted: 28 September 2020 DOI: 10.1111/gcb.15381 C O MME N T A R Y The importance of nitrogen for net carbon sequestration when considering natural climate solutions Christian A. Davies 1,2 | Andy D. Robertson 1 | Niall P. McNamara 2 1 Shell International Exploration and Production Inc. (USA), Shell Technology Centre Houston, Houston, TX, USA 2 UK Centre for Ecology & Hydrology, Lancaster Environment Centre, Lancaster, UK Correspondence Christian A. Davies, Shell International Exploration and Production Inc., Shell Technology Center Houston, 3333 Highway 6 South, Houston, TX 77082, USA. Overall, the paper by Guenet et al. (2020) summarizes the agricultural management options for increasing SOC stocks and implications of associated N 2 O emissions on net GHG emissions. [Extracted from the article]

Intergovernmental Panel on Climate Change ( IPCC) Tier 1 methodologies commonly underpin project-scale carbon accounting for changes in land use and management and are used in frameworks for Life Cycle Assessment and carbon footprinting of food and energy crops. These methodologies were intended for use at large spatial scales. This can introduce error in predictions at finer spatial scales. There is an urgent need for development and implementation of higher tier methodologies that can be applied at fine spatial scales (e.g. farm/project/plantation) for food and bioenergy crop greenhouse gas (GHG) accounting to facilitate decision making in the land-based sectors. Higher tier methods have been defined by IPCC and must be well evaluated and operate across a range of domains (e.g. climate region, soil type, crop type, topography), and must account for land use transitions and management changes being implemented. Furthermore, the data required to calibrate and drive the models used at higher tiers need to be available and applicable at fine spatial resolution, covering the meteorological, soil, cropping system and management domains, with quantified uncertainties. Testing the reliability of the models will require data either from sites with repeated measurements or from chronosequences. We review current global capability for estimating changes in soil carbon at fine spatial scales and present a vision for a framework capable of quantifying land use change and management impacts on soil carbon, which could be used for addressing issues such as bioenergy and biofuel sustainability, food security, forest protection, and direct/indirect impacts of land use change. The aim of this framework is to provide a globally accepted standard of carbon measurement and modelling appropriate for GHG accounting that could be applied at project to national scales (allowing outputs to be scaled up to a country level), to address the impacts of land use and land management change on soil carbon. [ABSTRACT FROM AUTHOR]

Respiration of heterotrophic microorganisms decomposing soil organic carbon releases carbon dioxide from soils to the atmosphere. In the short term, soil microbial respiration is strongly dependent on temperature. In the long term, the response of heterotrophic soil respiration to temperature is uncertain. However, following established evolutionary trade-offs, mass-specific respiration ( Rmass) rates of heterotrophic soil microbes should decrease in response to sustained increases in temperature (and vice-versa). Using a laboratory microcosm approach, we tested the potential for the Rmass of the microbial biomass in six different soils to adapt to three, experimentally imposed, thermal regimes (constant 10, 20 or 30 °C). To determine Rmass rates of the heterotrophic soil microbial biomass across the temperature range of the imposed thermal regimes, we periodically assayed soil subsamples using similar approaches to those used in plant, animal and microbial thermal adaptation studies. As would be expected given trade-offs between maximum catalytic rates and the stability of the binding structure of enzymes, after 77 days of incubation Rmass rates across the range of assay temperatures were greatest for the 10 °C experimentally incubated soils and lowest for the 30 °C soils, with the 20 °C incubated soils intermediate. The relative magnitude of the difference in Rmass rates between the different incubation temperature treatments was unaffected by assay temperature, suggesting that maximum activities and not Q10 were the characteristics involved in thermal adaptation. The time taken for changes in Rmass to manifest (77 days) suggests they likely resulted from population or species shifts during the experimental incubations; we discuss alternate mechanistic explanations for those results we observed. A future research priority is to evaluate the role that thermal adaptation plays in regulating heterotrophic respiration rates from field soils in response to changing temperature, whether seasonally or through climate change. [ABSTRACT FROM AUTHOR]

In the short-term heterotrophic soil respiration is strongly and positively related to temperature. In the long-term, its response to temperature is uncertain. One reason for this is because in field experiments increases in respiration due to warming are relatively short-lived. The explanations proposed for this ephemeral response include depletion of fast-cycling, soil carbon pools and thermal adaptation of microbial respiration. Using a > 15 year soil warming experiment in a mid-latitude forest, we show that the apparent ‘acclimation’ of soil respiration at the ecosystem scale results from combined effects of reductions in soil carbon pools and microbial biomass, and thermal adaptation of microbial respiration. Mass-specific respiration rates were lower when seasonal temperatures were higher, suggesting that rate reductions under experimental warming likely occurred through temperature-induced changes in the microbial community. Our results imply that stimulatory effects of global temperature rise on soil respiration rates may be lower than currently predicted. [ABSTRACT FROM AUTHOR]

Hartley et al. question whether reduction in Rmass, under experimental warming, arises because of the biomass method. We show the method they treat as independent yields the same result. We describe why the substrate-depletion hypothesis may not solely explain observed responses, and urge caution in interpretation of the seasonal data. [ABSTRACT FROM AUTHOR]

The oxygen minimum zone (OMZ) of the Arabian Sea is a significant source of nitrous oxide (N2O), yet the metabolism responsible for N2O production is unclear. High-resolution profiles identified peaks and troughs of N2O and NO2- in the top 500 m of the water column. The first peak in N2O was not in the oxycline, but deeper at the oxic-suboxic interface. Peaks and troughs were targeted with a suite of 15N incubations (15NO3-, 15NO2-, 15NH4+) to identify pathways of N2O and N2 formation. With 15NO2-, 15N-N2O was produced at all depths with a binomial distribution with respect to the NO2- pool. With 15NO3-, the 15N was not binomially distributed. NO3- is first reduced to NO2- before reduction to N2O, and NO2- - > N2O is the dominant metabolism responsible for N2O production. The N2O produced from 15NH4+ represented 2-5% of that from 15NO2- at the top of the OMZ. In addition, the production of 45N2O, but no 46N2O, at some depths with 15NH4+, suggested a novel source akin to Nitrosomonas spp. under O2 limitation. Unlike N2O, the production of N2 with 15NO2- or 15NO3- was not binomially distributed and therefore was not entirely derived from the same source as N2O. Although indicative of an alternative N2 source to denitrification, the lack of significant production of labeled N2 with 15NH4+ discounts anaerobic ammonium oxidation (anammox), as we understand it. Dissolved organic nitrogen or nitrate/nitrite reduction to ammonium are suggested as the additional sources of N in N2 production. [ABSTRACT FROM AUTHOR]

Perennial bioenergy crops have significant potential to reduce greenhouse gas (GHG) emissions and contribute to climate change mitigation by substituting for fossil fuels; yet delivering significant GHG savings will require substantial land-use change, globally. Over the last decade, research has delivered improved understanding of the environmental benefits and risks of this transition to perennial bioenergy crops, addressing concerns that the impacts of land conversion to perennial bioenergy crops could result in increased rather than decreased GHG emissions. For policymakers to assess the most cost-effective and sustainable options for deployment and climate change mitigation, synthesis of these studies is needed to support evidence-based decision making. In 2015, a workshop was convened with researchers, policymakers and industry/business representatives from the UK, EU and internationally. Outcomes from global research on bioenergy land-use change were compared to identify areas of consensus, key uncertainties, and research priorities. Here, we discuss the strength of evidence for and against six consensus statements summarising the effects of land-use change to perennial bioenergy crops on the cycling of carbon, nitrogen and water, in the context of the whole life-cycle of bioenergy production. Our analysis suggests that the direct impacts of dedicated perennial bioenergy crops on soil carbon and nitrous oxide are increasingly well understood and are often consistent with significant life cycle GHG mitigation from bioenergy relative to conventional energy sources. We conclude that the GHG balance of perennial bioenergy crop cultivation will often be favourable, with maximum GHG savings achieved where crops are grown on soils with low carbon stocks and conservative nutrient application, accruing additional environmental benefits such as improved water quality. The analysis reported here demonstrates there is a mature and increasingly comprehensive evidence base on the environmental benefits and risks of bioenergy cultivation which can support the development of a sustainable bioenergy industry.