VIRTUAL ISSUE No. 6: Closing knowledge-gaps for quantifying, predicting and mitigating nitrous oxide emissions

VIRTUAL ISSUE No. 6: Closing knowledge-gaps for quantifying, predicting and mitigating nitrous oxide emissions

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We proudly present the 6th Virtual Issue of Plant and Soil:

VIRTUAL ISSUE No. 6

Closing knowledge-gaps for quantifying, predicting and mitigating nitrous oxide emissions

Authors:

Charlotte Decock (1), Klaus Butterbach-Bahl (2),(3), Hans Lambers (4), Johan Six (1)

In 1946, shortly after the presence of nitrous oxide (N2O) in the atmosphere had been discovered, it was first suggested that soil might be a major source of atmospheric N2O (Adel 1946). This postulation has been confirmed throughout the second half of the 20th century and several microbial and abiotic source processes have been revealed. It is now well-known that N2O fluxes from soil are generally about three orders of magnitude smaller compared to other nitrogen (N) transformation rates such as gross mineralization, nitrification and immobilization. Also with regard to annual ecosystem N losses, gaseous losses of N2O-N are relatively negligible (e.g., ca. 0.5-5% of N applied to cropland), especially from an agronomic point of view. Nonetheless, research interest in N2O emissions from soil has increased exponentially in the last three decades because N2O is a potent greenhouse gas (GHG) – on a 100 years time scale 296 times more powerful as compared to CO2 – and currently the most important ozone depleting substance (IPCC 2013; Ravishankara et al. 2009). The atmospheric N2O concentration is increasing rapidly since decades at a constant rate of approximately 1% per year, which is assumed to be mainly due to agricultural intensification, including the increased use of mineral N fertilizers (IPCC, 2013). Motivated by the environmental challenges ahead, contemporary research is often placed in a broader context of improving the quantification of current emissions, the development of mitigation practices, and the prediction of emissions under future scenarios. In this virtual special issue, we highlight progress and challenges associated with understanding, predicting and mitigating N2O emissions in a changing global environment (Fig. 1), featuring prominent papers that have been published in Plant and Soil.

New Content Item

Fig. 1 Interlocking gears illustrating the challenges and complexities associated with predicting and mitigating N2O emissions in a changing global environment

Direct quantification of N2O emissions from ecosystems can be achieved through field measurements, using flux chambers or micrometeorological techniques (Hensen et al. 2013; Rochette and Eriksen-Hamel 2008). Such measurements require special attention to hot spots or hot moments because N2O emissions show highly variable temporal and spatial patterns (Holst et al. 2007; Velthof et al. 1996); However, the importance of emissions occurring in between hot spots and hot moments should not be overlooked (Kim et al. 2013). Since in-situ measurements of N2O emissions for every location across the globe are impossible due to cost, labor and time constraints, statistical and biogeochemical process models are essential to extrapolate experimental observations and delineate appropriate conditions for model application (Chen et al. 2008). Model predictions can be greatly improved by knowledge on mechanisms underlying N2O emissions (Butterbach-Bahl et al. 2013; Farquharson and Baldock 2008). Current topics at the frontiers of research on mechanisms underlying N2O emissions include: Identification of the relative contribution of underlying source processes – predominantly nitrification, nitrifier denitrification and bacterial and fungal denitrification (Butterbach-Bahl et al. 2013) – to emitted N2O fluxes (Decock and Six 2012; Lan et al. 2013; Zhu et al. 2011); quantifying ratios of N2O to N2 and the importance of N2O production and reduction throughout the vadose zone (Butterbach-Bahl et al. 2002); the potential of soils as a sink for N2O (Jones et al. 2013); quantifying the importance of soil microsites for N2O production; effects of soil-plant interactions on N2O emissions (Sey et al. 2010); as well as effects of functional and phylogenetic diversity of soil microorganisms on N2O production (Inselsbacher et al. 2011; Vermue et al. 2013). Hereby, on-going advancements in the use of isotopically enriched tracers, variations in natural abundance isotope values of N2O (including the intramolecular distribution of 15N in N2O, also referred to as site preference (SP) or isotopomers), and molecular markers targeting functional genes involved in N2O production (e.g. amoA, nirK, nirS, nosZ), are paving the way to narrow the many outstanding mechanistic knowledge gaps (Decock and Six 2012; Decock and Six 2013; Pörtl et al. 2007; Vermue et al. 2013).

Mitigation options for N2O emissions from managed agroecosystems mostly focus on improving N efficiency, by optimizing the rate, source, time and placement of fertilizer nitrogen (Liu et al. 2006; Snyder et al. 2009; Van Groenigen et al. 2004). Hereby, enhanced efficiency fertilizers, including nitrification inhibitors and slow release fertilizers, have shown great promise for mitigating N2O emissions (Decock 2014; Saggar et al. 2008; Vallejo et al. 2005). Recently, the importance of considering stoichiometric nutrient relations was highlighted, as it was shown that not only the rate of nitrogen fertilization, but also the rate of phosphorous fertilization, affect N2O emissions (Baral et al. 2014). In irrigated cropping systems and rice systems, research has focused on optimizing water management, gaming on simultaneously reducing water usage and greenhouse gas emissions (Cai et al. 1997). The potential of biochar amendments to mitigate N2O emissions have been subject of research, but successes are mixed (Scheer et al. 2011; Zhang et al. 2012). Innovative strategies have been sought in the manipulation of microbial communities or plant traits. For example, great progress has been made in the inoculation of legumes with N2 fixing bacteria that have N2O reduction capabilities (Hénault and Revellin 2011). Another important topic of research is the effect of relatively well-established agricultural conservations practices such as reduced tillage intensity, improved crop residue management, cover cropping, and organic agriculture on N2O emissions, because these practices are already being promoted in the context of other ecological benefits (Baggs et al. 2003; Liu et al. 2006). However, their GHG mitigation potential, both in terms of reducing N2O emissions and sequestering carbon, remains highly controversial. In fact, when it comes to assessing responses of N2O emissions to any management practice, one of the major knowledge gaps lies within the understanding of trade-offs with other sustainability indicators such as nitrate leaching, NH3 volatilization and soil carbon storage, across different spatial and temporal scales (Loubet et al. 2011; Zhou et al. 2013). Besides managing existing ecosystems, significant impacts on N2O emissions can be expected from land-use change. Examples of land-use changes include the expansion of irrigated cropland, drainage and reconstruction of wetlands, and deforestation or afforestation; all of which need to be taken into account when assessing N2O emission in a global framework (Maljanen et al. 2001; Martikainen et al. 1995; Shvaleva et al. 2014; Werner et al. 2006).

While much research focuses on quantifying N2O emissions under business as usual scenarios and the development of mitigation strategies, other studies assess the effects that external drivers, such as N-deposition, elevated CO2, elevated O3, increased temperature, and drought, can exert on soil N2O emissions. Many experiments that simulate increased N-deposition show a general stimulatory effect on N2O emissions, but interactions with soil nutrient status and land-use history exist (Liu and Greaver 2009; Zhang et al. 2008). Elevated CO2, the most important anthropogenic GHG, can increase plant growth and reduce evapotranspiration, thereby increasing soil C inputs and soil moisture content, and hence N2O production (Decock and Six 2012; Ineson et al. 1998; Mosier et al. 2002). Effects of increased temperature on N2O emissions remain dubious and inconsistent across studies, likely because of interacting effects on plants and soil processes (Barnard et al. 2005; Dijkstra et al. 2012). Manipulative field experiments suggest that drought is likely to decrease N2O emissions, while increased intensity of rain causes an increase in emissions (Brown et al. 2012; Hartmann and Niklaus 2012). However, most uncertainty associated with altered rain patterns probably lies within the N2O pulses induced by rewetting events, which are often not well characterized. Biodiversity is an important topic under the sustainability umbrella, especially in terms of how creation and loss of biodiversity affects ecosystem services. Studies in managed grassland ecosystems have demonstrated that N2O emissions are dependent on plant community composition (Niklaus et al. 2006), but the effect of biodiversity loss in natural ecosystems on N2O emissions remains largely unknown. In general, any form of pollution or external driver that directly or indirectly affects plant growth, soil quality, or biogeochemical cycling, including tropospheric O3 concentrations, smog, acidification, and melting of glaciers and permafrost can in principle affect N2O emissions from soil. However, the importance of many of such drivers to N2O emissions remains largely unknown (Decock and Six 2012).

Finally, the most confidence in current knowledge relates to single factor effects of ecosystem management or external drivers on N2O emissions. Much uncertainty remains around interactive effects among and between various management strategies, external drivers, and existing environmental and agro-ecological conditions. Furthermore, the majority of research has focused on grain crops and grasslands in the temperate climates of industrialized countries. Data from developing countries, vegetable and fruit crops, and natural ecosystems susceptible to change, is still scarce or not available at all. Whether it comes to quantification for inventory purposes, assessment of management effects, or prediction of N2O emissions in a changing global environment, current research makes continuous efforts towards addressing the research gaps laid out in this editorial, and exciting new developments can be expected in future years.

Affiliations:

(1) Department of Environmental Systems Science, Swiss Federal Institute of Technology, CH-8092 Zurich, Switzerland.

(2) Institute of Meteorology and Climate Research, Atmospheric Environmental Research. Karlsruhe Institute of Technology, DE-82467 Garmisch-Partenkirchen, Germany.

(3) International Livestock Research Institute, 00100 Nairobi, Kenya

(4) School of Plant Biology, The University of Western Australia, Crawley WA 6009, Australia.

Corresponding author:

Charlotte Decock, email: charlotte.decock@usys.ethz.ch, telephone: +41 44 632 47 53

Keywords:

N2O, knowledge gaps, mitigation, agronomic management, global change.

Read the Virtual Issue

SPATIAL AND TEMPORAL PATTERNS AT FIELD AND LANDSCAPE SCALE + BACKGROUND EMISSIONS

Seasonal variations in nitrous oxide losses from managed grasslands in The NetherlandsImportance of point sources on regional nitrous oxide fluxes in semi-arid steppe of Inner Mongolia, ChinaBackground nitrous oxide emissions in agricultural and natural lands: a meta-analysis

NEED FOR MODELS

Concepts in modelling N2O emissions from land useN2O emissions from agricultural lands: a synthesis of simulation approaches

MECHANISMS / SOURCE PROCESSES

The contribution of nitrogen transformation processes to total N2O emissions from soils used for intensive vegetable cultivationEffects of elevated CO2 and O3 on N-cycling and N2O emissions: a short-term laboratory assessmentProcesses leading to N2O and NO emissions from two different Chinese soils under different soil moisture contents

QUANTIFYING N2O REDUCTION

Soil core method for direct simultaneous determination of N2 and N2O emissions from forest soils

PLANT SOIL INTERACTIONS

Root-derived respiration and nitrous oxide production as affected by crop phenology and nitrogen fertilization

MICROBIAL COMMUNITIES

Greenhouse gas fluxes respond to different N fertilizer types due to altered plant-soil-microbe interactionsInfluence of integrated weed management system on N-cycling microbial communities and N2O emissions

ISOTOPE AND MICROBIAL METHODS

Natural N-15 abundance of soil N pools and N2O reflect the nitrogen dynamics of forest soilsEffects of elevated CO2 and O3 on N-cycling and N2O emissions: a short-term laboratory assessmentInfluence of integrated weed management system on N-cycling microbial communities and N2O emissions

MITIGATION / N MANAGEMENT

Nitrous oxide emissions from silage maize fields under different mineral nitrogen fertilizer and slurry applicationsThe Impact of Nitrogen Placement and Tillage on NO, N2O, CH4 and CO2 Fluxes from a Clay Loam Soil

NUTRIENT STOICHIOMETRY

Liebig’s law of the minimum applied to a greenhouse gas: alleviation of P-limitation reduces soil N2O emission

WATER MANAGEMENT

Methane and nitrous oxide emissions from rice paddy fields as affected by nitrogen fertilisers and water management

ENHANCED EFFICIENCY FERTILIZERS

Soil-atmosphere exchange of nitrous oxide and methane in New Zealand terrestrial ecosystems and their mitigation options: a reviewComparison of N losses (NO −3, N 2O, NO) from surface applied, injected or amended (DCD) pig slurry of an irrigated soil in a Mediterranean climate

BIOCHAR

Effect of biochar amendment on the soil-atmosphere exchange of greenhouse gases from an intensive subtropical pasture in northern New South Wales, AustraliaEffect of biochar amendment on maize yield and greenhouse gas emissions from a soil organic carbon poor calcareous loamy soil from Central China Plain

MICROBIAL INOCULANTS

Inoculants of leguminous crops for mitigating soil emissions of the greenhouse gas nitrous oxide

CONSERVATION AGRICULTURE (COVER CROPS, TILLAGE INTENSITY)

Nitrous oxide emissions following application of residues and fertiliser under zero and conventional tillageThe Impact of Nitrogen Placement and Tillage on NO, N2O, CH4 and CO2 Fluxes from a Clay Loam Soil

TRADEOFFS WITH OTHER SUSTAINABILITY INDICATORS

Nitrous oxide emissions and nitrate leaching from a rain-fed wheat-maize rotation in the Sichuan Basin, ChinaCarbon, nitrogen and Greenhouse gases budgets over a four years crop rotation in northern FranceComparison of N losses (NO −3, N 2O, NO) from surface applied, injected or amended (DCD) pig slurry of an irrigated soil in a Mediterranean climate

LAND-USE CHANGE

Fluxes of N2O, CH4 and CO2 on afforested boreal agricultural soilsChange in fluxes of carbon dioxide, methane and nitrous oxide due to forest drainage of mire sites of different trophyN2O, CH4 and CO2 emissions from seasonal tropical rainforests and a rubber plantation in Southwest ChinaComparison of methane, nitrous oxide fluxes and CO2 respiration rates from a Mediterranean cork oak ecosystem and improved pasture

EXTERNAL DRIVERS

Emissions of nitrous oxide from three tropical forests in Southern China in response to simulated nitrogen deposition

ELEVATED CO2

Soil gas fluxes of N2O, CH4 and CO2 beneath Lolium perenne under elevated CO2: The Swiss free air carbon dioxide enrichment experimentSoil-atmosphere exchange of CH4, CO2, NOx, and N2O in the Colorado shortgrass steppe under elevated CO2Effects of elevated CO2 and O3 on N-cycling and N2O emissions: a short-term laboratory assessment

ALTERED RAIN PATTERNS/DROUGHT

Effects of simulated drought and nitrogen fertilizer on plant productivity and nitrous oxide (N2O) emissions of two pastures

BIODIVERSITY

Effects of Plant Species Diversity and Composition on Nitrogen Cycling and the Trace Gas Balance of Soils

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