N2O emissions from California farmlands: A review
California Agriculture 71(3):148-159. https://doi.org/10.3733/ca.2017a0026
Published online September 13, 2017
Of the greenhouse gases emitted from cropland, nitrous oxide (N2O) has the highest global warming potential. The state of California acknowledges that agriculture both contributes to and is affected by climate change, and in 2016 it adopted legislation to help growers reduce emissions of greenhouse gases, explicitly including N2O. Nitrous oxide emissions can vary widely due to environmental and agronomic factors with most emission estimates coming from temperate grain systems. There is, however, a dearth of emission estimates from perennial and vegetable cropping systems commonly found in California's Mediterranean climate. Therefore, emission factors (EFs) specific to California conditions are needed to accurately assess statewide N2O emissions and mitigation options. In this paper, we review 16 studies reporting annual and seasonal N2O emissions. This data set represents all available studies on measured emissions at the whole field scale and on an event basis. Through this series of studies, we discuss how such farm management and environmental factors influence N2O emissions from California agriculture and may serve as a basis for improved EF calculations.
The application of nitrogen (N) in the form of inorganic fertilizers, cover crops, manure, or compost is necessary to maintain economically viable yields without depleting soil N. However, increases in agricultural N application are not always balanced by plant N uptake or soil N storage, leading to an imbalance and potential loss of reactive N to the atmosphere or to other ecosystems where it significantly contributes to air and water pollution and global warming (Davidson et al. 2012; Galloway et al. 2003). The worldwide application of N has risen sharply in the past 70 years, and California is no exception to this trend (Rosenstock et al. 2013).
Automated gas flux chambers monitor N2O emissions in an almond orchard. Current estimates of emissions from cropland in California are based on the assumption that, in every crop system, 1% of the nitrogen applied as fertilizer is emitted as N2O. Findings from the studies reported in this review provide more nuanced estimates, reflecting the large differences in emissions factors among crop systems.
With a global warming potential 298 times greater than carbon dioxide (CO2), nitrous oxide (N2O) is the most potent of the three major agricultural greenhouse gases (CO2, methane [CH4] and N2O). Of anthropogenic sources, N2O emissions are also the largest contributor to ozone depletion (Ravishankara et al. 2009), with agriculture accounting for more than 60% of global N2O emissions (Mosier et al. 1998).
In California, N2O emissions accounted for 2.8% (on a CO2-equivalent basis) of statewide greenhouse gas emissions in 2014, of which agricultural soils made up 51% of emissions (CARB 2014). Current statewide emissions are calculated from global default emission factors (EFs) set by the Intergovernmental Panel on Climate Change (IPCC) based on a constant fraction of the amount of N applied. A default EF of 1.0% is typically applied, meaning that 1.0% of applied N is assumed to be lost as N2O.
Global default EFs for specific management and N sources do exist, for example, ranging from 0.03% to 2.0% for flooded rice and manure, respectively. Yet high uncertainty surrounds these estimates, particularly for systems where little empirical data is available. Direct N2O emissions generally do not represent an economically important loss to growers, but the high global warming potential of N2O means these emissions have significant environmental impacts.
Indirect N2O emissions may occur from leaching of dissolved N2O in soil and surface water and subsequent off-gassing or leaching of nitrate (NO3−), which may later be reduced to N2, producing N2O in the process. NO3 leaching may be extensive in irrigated systems that have periodic high N excess loads. Barum et al. (2016) calculated annual NO3−-N losses of 71 to 214 lbs per acre per year (80 to 240 kg per hectare per year) in a California almond orchard. Clearly the management of such N losses is important for both economic, environmental, and human health reasons far beyond the potential for this N to be a source of N2O. However, indirect emissions are beyond the scope of this review.
How is N2O produced?
In agricultural systems, N2O is primarily produced through two microbial pathways: nitrification, which converts ammonium (NH4+) to NO3−, and denitrification, which converts NO3− to N2 (Box 1). Both processes produce N2O as a byproduct and can occur simultaneously in soil. However, nitrification is an aerobic process that requires oxygen, while denitrification is an anaerobic process that is inhibited at high oxygen concentrations. In soil, the oxygen content is largely controlled by soil moisture; when soil moisture is high, oxygen content is low and vice versa. Soil oxygen content is also controlled by microbial respiration and is related positively to the moisture content up to levels near saturation when a lack of oxygen inhibits many microbial processes. During periods of high microbial activity, soil oxygen is consumed, leading to an increase in N2O production from nitrification (Zhu et al. 2013). Denitrifiers also consume N2O when soil moisture is very high (Firestone and Davidson 1989). Therefore, soil moisture plays a large role in determining which process occurs and how much N2O is eventually emitted from the soil. Soil bulk density, texture and structure also strongly influence soil moisture, oxygen and gas exchange, and therefore influence many microbial processes, including N2O production and consumption.
Factors influencing cropland N2O emissions
N2O emissions are determined by a combination of factors (below). The impact of a change in one factor depends on the values of the other factors.
Management implications for N2O mitigation
Increase nitrogen use efficiency. Irrigation and fertilization methods that allow for increased synchronization of N supply with plant demand increase plant N uptake and reduce N losses. Fall application of fertilizer likely decreases N use efficiency by increasing precipitation-induced N losses through nitrate leaching and N2O emissions.
Increase water use efficiency. Buried drip and microjet irrigation systems can increase water use efficiency and reduce N2O emissions.
Source of N does not matter. Both synthetic- and organic-derived N contributes to N2O emissions. The application of organic matter as an N source provides valuable soil C, but increases the likelihood of climatic interactions (e.g., exposure to precipitation) and increases spatial and inter-annual variability in N2O emissions. To the extent that is possible, incorporation of plant residues or N application before significant rainfall or irrigation should be avoided.
Importance of multiple variables in N2O emissions. In all systems covered in this review, fertilization induced N2O emissions, but no correlation between total N application rate and annual emissions was found. Thus, factors other than N application rate had a strong influence on emissions (e.g., soil type or irrigation method). In conclusion, default EFs based on N application rate may not be accurate for many California systems.
Year-round emissions. Fallow/winter season emissions are significant, representing between 29% and 64% of annual emissions. Both perennial and annual systems have the potential for high fallow/winter season emission pulses. Emissions occurring after the first seasonal fall rain dominate total winter/fallow season emissions; emissions shortly after fertilization dominate total growing season emissions.
Along with soil oxygen content, which is mostly determined by soil moisture and microbial activity, other soil environmental conditions (i.e., pH and temperature) and substrate availability (NH4+, NO3− and soil carbon [C]) control microbial N2O production and consumption rates (see Box 1). The magnitude of each of these controls is in turn subject to their own set of biological and abiotic controls. Thus, much of the difficulty in predicting, measuring and managing N2O emissions lies in understanding the interactions among these controlling factors.
California cropping systems and climate
The relatively arid, Mediterranean climate of California tends to favor nitrification, which occurs at lower soil moisture (Bateman and Baggs 2005). However, any irrigation event will increase soil moisture and microbial activity leading to the potential to increase N2O pulses from both nitrification and denitrification (Scheer et al. 2008). The release of N and C from sudden soil wetting such as in irrigation events has been shown to fuel N2O production from both nitrification and denitrification (Harrison-Kirk et al. 2013). In a review of N2O emissions in Mediterranean systems, Aguilera et al. (2013) reported mean emissions four times higher in irrigated compared to rain-fed systems. Warm soil temperatures, which occur often in California, also tend to increase N2O emissions (Smith et al. 1998). Denitrification derived N2O emissions generally increase with increases in soil organic matter and C inputs, and rates may be partially C limited in low soil C systems, which could be the case for many California agroecosystems (Harrison-Kirk et al. 2013; Kennedy, Decock and Six 2013).
Unique to California is the growing importance of perennial orchard and vineyard cropping systems, which cover roughly half of the irrigated production acreage (CDFA 2016; NASS 2014) but are underrepresented in the global body of scientific literature on N2O emissions. Perennial systems pose unique challenges to N2O emission quantification because of the discrete management practices in the tree/vine row (cropped area) versus the tractor row (noncropped area).
The data set we present here consists of 12 studies in which one or more of the authors of this article were involved and four additional studies that were found to meet our criteria for sampling frequency. Only studies with a minimum sampling frequency of two times per month were considered. All studies meeting this criterion utilized “event based” sampling, where sampling occurred daily for 3 to 7 days or until fluxes returned to background levels following fertilization, precipitation and selected additional management events dependent on the crop (i.e., tillage, irrigation, mowing, drainage, flooding). Three studies were found that did not meet these criteria for sampling frequency (Lee et al. 2009; Smukler et al. 2012; Townsend-Small et al. 2011). Together, this body of work comes from four research groups at UC Davis.
Gas flux chambers deployed in two functional locations — the tree row and tractor row — in a prune orchard. It is important to measure emissions from both locations because of differences in soil moisture, the availability of nitrogen compounds, soil temperature and other factors.
Within the 16 studies we identified 26 distinct treatment x year combinations (observations, n = 26) (table 1). Complete data and methodological details for 13 of the 16 studies are reported in individual papers (Adviento-Borbe et al. 2013; Alsina et al. 2013; Angst et al. 2014; Decock et al. 2017; Garland et al. 2011; Garland et al. 2014; Kennedy, Suddick, Six 2013; Lazcano et al. 2016; Pereira et al. 2016; Pittelkow et al. 2013; Schellenberg et al. 2012; Verhoeven and Six 2014; Zhu-Barker et al. 2015). Our intent was to report only data representing standard regional practices; thus, only values from treatments following established management and N application rates were used. Data for four additional observations are part of unpublished data sets (E. Verhoeven et al., unpublished; M. Burger, Department of Land, Air and Water Resources, UC Davis, unpublished).
TABLE 1. Management characteristics, measured annual emissions and calculated emission factors for the 16 studies reviewed
In each study, in-situ N2O measurements were taken using vented, static flux chambers as described by Parkin and Venterea (2010) and Hutchinson and Mosier (1981). Briefly, headspace air samples were collected at discrete intervals, injected into preevacuated Exetainer vials and later analyzed on a gas chromatograph. Mean annual emissions were linearly interpolated from daily flux values. When emissions were measured at multiple spatial locations in a given field, weighted averages based on spatial coverage were calculated and are reported in table 1. For full methodological details see Verhoeven and Six (2014). Comparisons between functional locations (fig. 1) or season (fig. 2) were done on studies where disaggregated data was available.
Fig. 1. Percent of annual emissions occurring from a given functional location. Values are means from studies reporting emissions at discrete functional locations.
Fig. 2. Percent of annual emissions occurring during the winter/fallow season (September/October through March/April) or active growing season (March/April through September/October).
Basic field site characteristics, including irrigation and fertilization rates and methods, are reported in table 1. The growing season was defined as April-September or March-August (i.e., budding/planting) and the fallow/winter season as September-March or October-April (i.e., harvest/dormancy). When fertilizer was applied through irrigation systems, it was termed “fertigation”. For all studies, we report system EFs uncorrected for background (zero N) emissions. Adviento-Borbe et al. (2013), Pittelkow et al. (2013) and Zhu-Barker et al. (2015) report fertilizer-induced emission factors (EFfertilizer) in their original papers; therefore, our calculated emission factors differ from these.
Farm management effects on N2O emissions
Agricultural management and cropping systems strongly affect N2O production by altering C and N availability and environmental soil conditions (Box 1). Excluding dairy systems, mean annual N2O emissions for the cropping systems reviewed ranged from 0.77 pounds N2O-N per acre per year for almonds to 10.16 pounds N2O-N per acre per year for dairy forage systems (fig. 3). Aguilera et al. (2013) also found similar values for Mediterranean horticulture systems, 1.34 pounds N2O-N per acre per year, but observed lower emissions, 2.68 pounds N2O-N per acre per year, for liquid slurry systems than our dairy systems. N2O emissions in the majority of systems reported here were only marginally higher than background agricultural emissions (uncropped agricultural soil) or emissions from natural systems at 0.83 pounds N2O-N per acre per year and 0.37 to 0.82 pounds N2O-N per acre per year, respectively (Kim et al. 2013; Stehfest and Bouwman 2006).
Fig. 3. Average annual N2O emissions for each cropping system. Error bars represent the standard error of the mean. n = number of observations reporting annual emissions; wine grape (n = 4), almond (n = 8), walnut (n = 2), prune (n = 1), tomato (n = 3), rice (n = 4), dairy systems (n = 4). Dairy systems were defined by the production of forage or pasture with high manure N inputs; they include sites with pasture ryegrass, corn + forage mix, corn + winter wheat, corn + ryegrass.
In perennial systems, management of the tractor row (noncropped area) is particularly variable across regions, farms and seasons. Tractor rows typically are not deliberately irrigated, but they may be wetted to varying degrees depending on the irrigation system (substantial wetting with overhead sprinkler or furrow irrigation versus little or no wetting with surface/subsurface drip or microjet sprinkler). Tractor rows also may be planted to a leguminous or grass cover crop, or allowed to self-seed with noncultivated vegetation, and they may be tilled or mowed with varying frequency. Since the management of these areas is not as time sensitive nor critical to crop production, the practices are inherently more variable and often no management records are kept for these activities. Among the studies with defined distinct functional locations, the tractor row accounted for 40%, 50%, 73%, and 70% to 82% of spatial coverage and corresponded to 31%, 62%, 57%, and 85% of total weighted emissions for almonds, walnuts, prunes, and wine grapes, respectively (fig. 1). Significantly different patterns of emissions between functional locations imply that both cropped and noncropped locations must be managed to effectively mitigate N2O emissions. Among the perennial systems, tree or vine row emissions peaked at fertilization events while tractor row emissions were most influenced by climatic (i.e., first fall rain) events and were coupled with plant residue management.
Many annual systems are also characterized by distinct spatial heterogeneity between functional locations, typically in relation to how irrigation and fertilizer is applied. For example, working in a tomato system, Kennedy, Suddick and Six (2013) defined three distinct functional locations: berm, side and furrow. The authors observed higher variation in N2O emissions between functional locations in a furrow-irrigated versus drip-irrigated system.
Photos show gas flux chambers and vegetation growth in the tractor row of a vineyard (A) early in cover crop growth, (B) at peak growth and (C) after mowing (with vine row in background). The images illustrate the dramatic differences in vegetation between functional locations and at different points in the year, and thus the need for field measurements of N2O emissions across functional locations and throughout the year.
Author Gina Garland (left) records chamber temperatures and (right) takes chamber gas samples in a vineyard.
A total of six irrigation practices are represented in our data set: furrow, flood, overhead sprinkler, microjet sprinkler, surface drip and subsurface drip. In all of the microjet sprinkler and drip irrigation systems, fertilizer was applied through the drip system. For the remainder of the systems, fertilizer N was banded, dissolved in flood water, or spread as compost or residue (table 1). Irrigation with microjet or drip irrigation may improve water use efficiency by applying small amounts of water to match daily soil/crop evaporation. However, effects can be crop dependent (Bryla et al., 2003; Sharmasarkar et al. 2001).
In almonds, Alsina et al. (2013) observed a significant reduction in N2O emissions in a microjet-versus drip-irrigated system. However, emissions across all almond studies were low compared to other crops. Kennedy, Suddick and Six (2013) reported significant reductions for buried drip irrigation versus furrow irrigation in tomatoes, namely due to increased fertilizer and water use efficiency with fertigation techniques via the drip. While we do not have sufficient coverage across crops and irrigation systems to draw broad conclusions, irrigation techniques that allow for dosing of N and water to match daily crop requirements appear to reduce N2O emissions.
It has been well established that N2O emissions increase with increasing fertilizer N application (Cole et al. 1997). However, a nonlinear relationship has often been observed, and emissions increase most rapidly when N rate exceeds crop demand (McSwiney and Robertson 2005; Van Groenigen et al. 2010). The challenge remains of better predicting the extent and timing of crop N uptake and finding a balance of reduced N input without sacrificing yield, thereby mitigating N pollution losses, including N2O. However, reduced N input may not be necessary in micro-irrigation systems that dose N and water inputs and generally have higher yields. Fertilizer form and placement also influence emissions. Fertilizers that lead to increased soil pH and/or highly concentrate N application, such as drip versus microjet irrigation or knife injection versus banding of ammonium or urea, have been found to increase emissions. Zhu-Barker et al. (2015) found that injection of anhydrous ammonium increased seasonal N2O emissions by 44% compared to application of banded ammonium sulfate. We found that fertilization with organic and synthetic N both resulted in N2O emission pulses. During fertigation, emissions pulses were immediate but typically short lived, lasting between one and two days (fig. 4) and only measurable in the tree or vine row. In contrast, organic inputs from cover crops typically caused the highest fluxes at subsequent rain or irrigation events.
Fig. 4. Examples of temporal and spatial dynamics of N2O emissions from a prune orchard, illustrating the effects of fertigation and precipitation events. Tree row = green dots, tractor row = orange dots.
Reduced- and no-till systems can alter N2O emissions by modifying N and C availability, soil structure, microbial community structure and activity and, most profoundly, soil moisture. In dry climates, such as California, van Kessel et al. (2013) found that no-till and reduced tillage increased N2O emissions during the first 10 years after switching from conventional tillage, but decreased emissions once the practice was in place for longer than 10 years. In our data set, only one study examined the role of tillage and found no effect of tillage on growing season emissions in a vineyard (Return to top