An International Journal on Plant-Soil Relationships
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
Welcome to Plant and Soil VIRTUAL ISSUES
A new initiative in providing a service for readers of Plant and Soil
In view of the rapid development of studies on soil-plant interactions, we are pleased to announce our new feature: “Virtual Special Issues”. These Special Issues bring together a selection of papers on a specific topic that were recently published in Plant and Soil.
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
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.
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.
(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.
Charlotte Decock, email: firstname.lastname@example.org, telephone: +41 44 632 47 53
N2O, knowledge gaps, mitigation, agronomic management, global change.
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SPATIAL AND TEMPORAL PATTERNS AT FIELD AND LANDSCAPE SCALE + BACKGROUND EMISSIONS
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