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Nitrogenous Gas Emissions from Soils and Greenhouse Gas Effects
, H. Henry Janzen§
, Bobbi Helgason¶
and Ben Ellert§ *Agriculture and Agri-Food Canada, Central Experimental Farm, Ottawa, ON, Canada §Agriculture and Agri-Food Canada, Research Centre, Lethbridge, AB, Canada ¶Agriculture and Agri-Food Canada, Research Centre, Saskatoon, SK, Canada
1 Corresponding author: E-mail: Ed.Gregorich@AGR.GC.CA
The Haber-Bosch process for synthesizing fertilizer nitrogen (N) is among the most important modern discoveries because it has enabled us to grow enough food for several billion more of us. At the same time, however, profusion of added "reactive" N has become a prominent ecological threat, globally, because a large fraction of applied N is lost from agricultural ecosystems. Nitrogen added to agricultural soils, in organic forms or as synthetic N fertilizers, has one of four fates; it can be assimilated by plants, lost to surface- or groundwater, retained in the soil, or lost to the atmosphere. The last of these is particularly worrisome because of links to climate change and other threats to the biosphere. Our aim is to summarize briefly the processes of atmospheric N emissions to the air from agricultural ecosystems, and to consider how management practices might reduce those emissions. Nitrogen gases emitted from soil emanate from naturally occurring biological processes regulated largely by three interactive factors: substrate availability, aeration, and temperature. Although these factors are partly dictated by weather and intrinsic soil properties, they are also influenced by management so that emissions can be heavily influenced by practices imposed on the land. Variables to consider in devising systems with reduced emissions include: forms, rate, and timing of fertilizer; tillage and residue management; crop rotation, including the use of legumes; and manuring practices. All of these need to be considered together to devise systems, tuned to local conditions, which not only reduce emissions but also meet growing demands for agricultural yields. Developing such systems, based on holistic understanding from many disciplines, is now critical to sustain the long-term productivity and vitality of our ecosystems.
Atmospheric N gases; Denitrification; Greenhouse gases; Mitigation; N2O; Nitrification; Nitrogen cycle; NO2; NO
1. Overview of Nitrogen Cycle
If you ask 'what has been the most important invention of the past 100, 150 years?' it's been the synthesis of ammonia. If we could not synthesize ammonia by taking nitrogen from the air, hydrogen from natural gas and pressing them together in the Haber-Bosch cycle. if we could not do this to make nitrogen fertilizers, we could not grow enough food for about 40% of people. So you are talking about something like three billion people. In existential terms, that is the most important invention.
Nitrogen is one of the most important elements to all life on earth, including that of humans. It is abundant in the atmosphere, yet deficient for living things in most terrestrial ecosystems, because atmospheric nitrogen (N) occurs mostly as dinitrogen (N2), a molecule containing two N atoms with a triple bond between them. Only when this strong bond is severed can the N become reactive and available for use by organisms. In the early 1900s Haber and Bosch devised a practical industrial way of converting N2 to reactive N, removing constraints of N deficiency, and vastly increasing agricultural yields (Smil, 2001
). Since then N use in agricultural systems has increased, as has food production, but especially so in the last 50-60 years; from 1950 to 2000, fertilizer N applied globally increased 20-fold (Bouwman et al., 2013
). In the future, N fertilizers will be critical for producing the large quantities of food needed to feed the burgeoning world population, which is expected to reach more than nine billion in less than 40 years (Zavatarro et al., 2012
). But the bioavailability of reactive N that makes it an effective fertilizer also allows it to readily dissipate through the environment and cascade through the atmospheric, aquatic, and terrestrial ecosystems where it can have negative effects on human health, ecosystem services, and climate change (Erisman et al., 2013
) (Figure 1
). Thus reactive N is both a boon for humanity, and one of our most prominent threats to the biosphere. Some have argued that the goals of producing more food with low pollution (dubbed "Mo Fo Lo Po") will not be achieved solely by technological developments, but will also require policies that recognize the economic and social factors affecting decisions made by farmers (Davidson et al., in press
). Figure 1
The nitrogen cycle. Our purpose in this chapter is to review gaseous emissions from soil of N compounds contributing to radiative forcing. About 10% of total anthropogenic greenhouse gas emissions originate from agricultural activities (IPCC, 2013
), of which N gas emissions (mostly nitrous oxide, N2O) account for about 60%, largely from reactive N applied to soils (Reay et al., 2012
; Smith et al., 2007
). We will briefly summarize the contribution of agriculture to atmospheric N gases by describing the forms, sources, and microbial pathways of these gases and discuss how their emission is influenced by agricultural practices. Some mitigation strategies will be presented along with a short discussion of the trade-offs and opportunities involved in managing N in agroecosystems; this is to broaden the perspective and extend the focal plane of how we might best manage N in the future.
2. The Contribution of Agriculture to Atmospheric N Gases (NO2, NO, N2O)
Intensive cropping systems that produce large quantities of food, forage, biofuel, and fiber require adequate N, the nutrient that most often limits the rate of net primary production, and attainment of maximum yields. Most of the reactive N applied is fixed industrially, via the Haber-Bosch process, or biologically, via the legume-Rhizobium
symbiosis (Fowler et al., 2013
). Synthetic N fertilizer is the biggest source of N added to intensive cropping systems. Ideally, fertilizer is applied to match temporal plant N demands, but rarely is such synchrony attained because of logistical, economic, and biological contraints, so that a large fraction of this reactive N is typically lost. Large amounts of N are also added in animal manures-globally this amount exceeds that of N fertilizer (Bouwman et al., 2013
)-but this N primarily represents recycling of N originally fixed industrially or biologically. Gaseous N escaping from soil affects radiative forcing directly through the emission of N2O and indirectly because emissions of N oxides, such as NO2 and NO (together known as NOx
), affect atmospheric concentrations of other greenhouse gases, especially ozone (O3) and methane (CH4). Ammonia emissions from soil (especially soils receiving urea fertilizers or livestock manure) also contribute to aerosol formation, and eventually may be converted to NOx
or N2O. The emissions of N2O come primarily from agricultural soils and contribute to climate warming on short- and long-timescales. In this review, we focus primarily on emissions of N2O, the dominant nitrogenous greenhouse gas, and NOx
, because of its close biochemical links to N2O. Only a small portion (about 0.5-3%) of N applied to cropped soils is emitted as N2O (Stehfest and Bouwman, 2006
; Linquist et al., 2012
) but these emissions make a major contribution to the overall greenhouse gas budget (Robertson et al., 2013
) because N2O is such a potent contributor to radiative forcing, with each molecule about 265 times more effective than a molecule of CO2 in absorbing out-going long-wave solar radiation on a 100-year time frame (IPCC, 2013
). The estimated loss of N, as a proportion of that applied, varies widely because of soil variability, interactions with weather, and uncertainty of measurements.
2.1. Atmospheric Fates and Consequences of N Gases
Behavior of N2O and NO varies among layers of the atmosphere. N2O is inert in the lower atmosphere (i.e., troposphere) where it has a mean residence time of more than 100 years, but in the upper atmosphere (i.e., stratosphere) it participates in photochemical reactions that destroy ozone (O3), which absorbs most of the incoming high-frequency UV radiation (Ravishankara et al., 2009
). In contrast, NO participates in tropospheric photochemical reaction that produce O3 and its mean residence time there is too short for it to reach the stratosphere where it might destroy O3. In the troposphere NOx
is a key factor in the creation of O3, which reacts with other atmospheric chemicals that affect crop productivity and human health. As a strong oxidant, O3 can cause severe leaf damage (e.g., chlorosis, necrosis, and lesion formation (Vainonen and Kangäsjarvi, 2014
) and interferes with the ability of plants to produce and store carbohydrates (Betzelberger et al., 2012
). Continued exposure to O3 can make sensitive plant species more susceptible to damage from disease, insects, and severe weather (EPA, 2014