Abstract
Greenhouse gas (GHG) emissions are fueling global climate change, with methane and nitrous oxide being the primary agricultural gases emitted. It has been shown that N2O emissions correlate to moisture content fluctuations; however, emissions from agricultural fields in the semiarid regions of the Southwest where rewetting events occur regularly are not well established. The scope of this study was to quantify GHG emissions in correlation to soil moisture fluctuations and fertilizer application. The study was conducted continuously in two pecan [Carya illinoinensis (Wangenh.) K. Koch] orchards between Aug. 2010 and Aug. 2011 on a sandy loam soil (La Mancha) and a silty clay loam soil (Leyendecker), both under normal management practices. The small chamber technique was used to measure GHGs. Emissions varied greatly throughout the year. The largest flux of CO2 at La Mancha and Leyendecker both occurred during a drying event immediately following an irrigation event: 84,642.49 μg·m−2·h−1 and 30,338.24 μg·m−2·h−1, respectively. The net CH4 flux at Leyendecker and La Mancha was close to zero with the largest emissions occurring during wetting events. Results showed that N2O emissions were maintained near the baseline except for the few days following an irrigation event. The largest emission peak at La Mancha occurred after irrigation and nitrogen application: 322.06 μg·m−2·h−1. The largest emission peaks of 26.37 and 1.13 μg·m−2·h−1 at Leyendecker and La Mancha, respectively, occurred after irrigation, nitrogen application, and tillage. Nitrogen application was the driving factor affecting N2O emissions at La Mancha, whereas soil moisture content was the driving factor at Leyendecker. Emission factors (EFs) at La Mancha and Leyendecker were 0.49% and 0.05%, respectively. A thorough accounting of GHG emissions is necessary for budgeting and identifying mitigation policy.
Carbon dioxide, methane (CH4), and nitrous oxide (N2O) are major contributors to the increases in GHG emissions, fueling changes in the earth’s climate. The USEPA(2015) estimates that GHG emissions due to agriculture accounted for 7.6% of total emissions in 2013, with CH4 and N2O being the primary GHGs emitted. Agricultural practices contribute up to 74.4% of US N2O emissions and enteric fermentation and manure management contribute up to 34.6% of CH4 emissions (USEPA, 2015). N2O is 296 times and CH4 23 times more effective than CO2 in causing an immediate increase in global warming. It has been estimated that a 2-fold increase in atmospheric N2O would result in a 10% decrease in ozone and, therefore, cause a 20% increase in ultraviolet radiation reaching the earth (Crutzen and Ehhalt, 1977). Even though CO2 is the main GHG of concern for national and global budgeting, for agriculture, the most important is N2O (Snyder et al., 2009). Biotic and anthropogenic activities have caused an increase in atmospheric N2O (Andreae and Schimel, 1989; Rhode, 1990), with N2O from soils largely accounting for these increases.
Agricultural practices are altering the carbon and nitrogen cycles through combustion of fossil fuels, application of N fertilizers, cultivation of N-fixing legumes, and other actions, with agriculture being the main contributor to N2O in the atmosphere. N2O is increasing at a rate of 0.2% to 0.3% a year (Vitousek et al., 1997). In the past, land conversion to croplands caused major perturbations in soil N processes. Continued growth of atmospheric N2O has been partly attributed to increased N input into the soil system (Mosier et al., 1996). To continue global food production to meet the needs of an expanding population, soil N input is expected to continue over the next 100 years (Hammond, 1990). The Intergovernmental Panel on Climate Change currently recommends an EF (amount of N2O lost per unit of N applied) of 1.25% of the amount of N applied for the accounting of N2O emissions from agricultural soils (IPCC, 1997).
Greenhouse gas emission variations found in the literature are due to a variety of factors (Mosier et al., 1996; Smith et al., 2003) including crop, soil texture, temperature (Cates Jr. and Keeney, 1987; Hart, 2006; Johnson et al., 2010; Wagner-Riddle et al., 1994), fertilization and other management techniques (Alluvione et al., 2010; Amos et al., 2005; Dusenbury et al., 2008; Halvorson et al., 2008, 2010; Jacinthe and Dick, 1997; Ma et al., 2010; Mosier et al., 2006), and soil moisture (Adviento-Borbe et al., 2006; Cates Jr. and Keeney, 1987; Hernandez-Ramirez et al., 2008; Rochette et al., 2007; Russow et al., 2007; Welzmiller et al., 2008; Zhang et al., 2008). Few studies have been carried out in the semiarid region of the Southwest United States quantifying N2O emissions as they are related to biological and physical factors.
The amount of N2O released into the atmosphere is likely underestimated because the effect of N input is only partially traced through the environment and estimates do not include production from animal manure and biological fixation (Mosier et al., 1996). Because of water being limited, we believe that the potential to reduce N2O emissions is much higher than the potential to increase C sequestration as a GHG mitigation technique in the semiarid southwest than in the high plains and southeast. An array of studies is needed that take into account N2O emissions as affected by N application, crop, climate, and management technique throughout the year and not just the cropping season, and to create more accurate prediction models on an area, regional, and global scale (Mosier et al., 1996). To the best of our knowledge, no studies have quantified GHG emissions from irrigated pecan orchards of southern New Mexico. The objectives of this research are 1) to examine the effect of soil moisture, fertilization, climate, and soil texture on N2O emissions, 2) to quantify N2O, CH4, and CO2 emissions throughout the year, and 3) to report percent of N application lost as N2O for some pecan orchards in semiarid southern New Mexico. The hypothesis for this research is that N2O emissions are a direct function of moisture content fluctuations and fertilization events.
Materials and Methods
Experimental sites.
We established sites at two pecan [C. illinoinensis (Wangenh.) K. Koch] orchards: New Mexico State University Leyendecker Plant Science Research Center, located 14.5 km south of Las Cruces (lat. 32°11′56.66″N, long. 106°44′30.50″W, and altitude of 1,174 m above sea level), and a private orchard (La Mancha), located 12.7 km northwest of Las Cruces, NM (lat. 32°17′5.32″N, long. 106°50′3.85″ W, and altitude of 1,185 m above sea level). Soils at Leyendecker and La Mancha are primarily Arimjo clay loam and Brazito very fine sandy loam, respectively. Soil properties at the sites are referenced in Table 1 (Deb et al., 2013). The climate of the experimental areas is classified as arid with average annual temperature and precipitation of 17.7 °C and 29.7 cm, respectively (Gile et al., 1981). Both experimental sites are flood irrigated with groundwater and canal water. Both fields were managed under conventional farming techniques using shallow tillage. Fertilizer was applied as broadcast application followed by flood irrigation; La Mancha received three applications of 46–0–0: N–P–K (urea) and Leyendecker two applications of 21–0–0: N–P–K (ammonium sulfate) and 11–52–0: N–P–K (monoammonium phosphate).
Soil properties of La Mancha (Site 1) and Leyendecker (Site 2) at Las Cruces, NM. Ks = saturated hydraulic conductivity, FC = water content at field capacity (at 30 kPa), and WP = water content at wilting point (at 1,500 kPa) (Deb et al., 2013).
Field experiment.
We used the closed small chamber method to measure GHG fluxes to quantify point emission rates. The chamber method has been reported to underestimate the actual emission rates because of inherently nonlinear time series data (Venterea et al., 2009). Our chamber design was based on Mosier’s (1989) review of key issues related to chamber techniques for flux measurements. The apparatus consisted of two parts, an anchor of 20.32 cm diameter by 5.08 cm length PVC and the chamber of 20.32 cm diameter by 15.24 cm long PVC pipe and a 20.32 cm PVC endcap. Chambers were not left in place for longer than 1 h to prevent an increase of the temperature inside the chamber and to maintain linear regression of the flux.
We took gas samples using four replicates at each site by withdrawing gas from inside the chambers using a 20 mL syringe, 0, 30, and 60 min after the chambers were installed. We stored the samples with an overpressure of air in 12-mL soda glass flat-bottom evacuated vials (LabCo, High Wycombe, UK). The University of California Davis Viticulture and Enology laboratory conducted the GHG analysis using a gas chromatograph (GC-2014; Shimadzu Scientific Instruments, Columbia, MD) equipped with electron capture detector, thermal conductivity detector, and flame ionization detector to quantify N2O, CO2, and CH4, respectively. We collected samples between two and five times a week following fertilizer application and irrigation events to capture the rising and falling limbs of the CO2, CH4, and N2O emission curves accurately. During the rest of the year, we collected samples every 2–3 weeks with additional sampling conducted during periods of change in soil and environmental properties. We conducted sampling between 8 and 10 am. During sampling events, we took internal chamber temperature readings at 0, 30, and 60 min; soil temperature readings; and soil samples at each replicate from 10, 20, and 30 cm to determine soil moisture content and nitrate concentration.
Laboratory experiment.
We collected four replicate soil cores between 10 and 20 cm from each field to collect in vitro gas samples to study the effect of anaerobic conditions and fertilizer application on GHG emissions. We incubated cores, 5 cm in diameter, in the dark for 7 d at 20 °C in 1-L mason jars with a septa butyl stopper installed in the lid. Incubations took place at field capacity (Table 1), saturation, and at saturation with a surface fertilizer application (168.13 kg·ha−1 urea), respectively. Cores were initially saturated and then equilibrated to field capacity (–0.3 bar) using a pressure plate apparatus before the first incubation. The second incubation was conducted after re-saturation and the third after granular urea application with wetting. Gas samples were collected at time 0, 1, 3, 24, 72, and 168 h, with atmosphere equilibration at 24 and 72 h. We collected air samples from inside the jars using a 20 mL syringe. The samples were stored with an overpressure of air in 12-mL soda glass evacuated vials (LabCo) until analyzed.
Data collection.
We determined nitrogen content of the soil by extracting the free NO3− with 2 m KCl, filtering, and freezing the mixture until analysis. We tested the liquid using an automated spectrophotometric method (Maynard and Karla, 1993).
We calculated annual CO2, CH4, and N2O emissions by taking the sum of the average daily flux. We calculated EF of N2O emissions as the annual emission per the nitrogen applied. This allows for direct comparisons across experimental sites. We calculated EFs without a correction for background flux as the experimental sites were managed under normal management practices and there were no non-fertilized controls.
Results and Discussion
Results from in vitro soil cores showed that CO2 emissions were highest (11,400.41 μg·m−2·h−1) under field capacity and CH4 emissions were highest under saturation (0.99 μg·m−2·h−1). N2O emissions from the in vitro soil cores were highest (115.12 μg·m−2·h−1) under saturation with a nitrogen application (Fig. 1). The average gas flux was higher at La Mancha than at Leyendecker (P = 0.05).
CO2, CH4, and N2O emissions from eight in vitro soil cores. Cores underwent three sequential treatments: field capacity, saturation, and saturation with 168.128 kg·ha−1 urea (N). Cores one to four were collected from each replicate point at La Mancha and five to eight from those at Leyendecker. The vertical bars denote the sd.
Citation: HortScience horts 53, 5; 10.21273/HORTSCI12773-17
Soil moisture content values are plotted along with flux data (Figs. 2 and 3). Soil water content values were consistently higher at Leyendecker than at La Mancha and varied between 8.18% and 31.73% and 8.97% and 21.02%, with an average of 18.82% and 13.53%, respectively, during the growing season. This is mainly because of the differences in soil texture as each site was irrigated at similar intervals.
CO2, CH4, and N2O emissions (n = 4) and moisture and NO3− content at La Mancha, Sept. 2010–Aug. 2011. Irrigation and irrigation with N application are indicated by black and black arrows with plus signs, respectively. The vertical bars denote the se.
Citation: HortScience horts 53, 5; 10.21273/HORTSCI12773-17
CO2, CH4, and N2O emissions (n = 4) and moisture and NO3− content at Leyendecker, Sept. 2010–Aug. 2011. Irrigation and irrigation with N application are indicated by black and black arrows with plus signs, respectively. The vertical bars denote the se.
Citation: HortScience horts 53, 5; 10.21273/HORTSCI12773-17
Looking at Figs. 2 and 3, it is clear that soil NO3− varied with rising and falling moisture content as well as fertilizer application. After the first fertigation event of the season soil, NO3− went from 7.5 to 45.0 mg N/kg dry soil and 6.3 to 21.6 mg N/kg dry soil on days 236 and 219 at Leyendecker and La Mancha, respectively. At Leyendecker, soil NO3− was highest a few days after an irrigation event and peaks ranged from 21.3 to 55.3 mg N/kg dry soil (Fig. 3). At La Mancha, soil NO3− was highest immediately following irrigation and peaks ranged from 23.8 to 61.4 mg N/kg dry soil (Fig. 2).
Linear regression modeling between soil water content and CO2 or CH4 emissions did not produce a significant correlation on accord with their wide fluctuations throughout 2010 and 11. The largest flux of CO2 at La Mancha (84,642.49 μg·m−2·h−1) and Leyendecker (30,338.24 μg·m−2·h−1) occurred during a drying event immediately following an irrigation event (Figs. 2 and 3). The net CH4 flux at Leyendecker and La Mancha was close to zero with the largest emissions occurring during wetting events (Figs. 2 and 3).
N2O emissions varied greatly throughout the year at each site. They increased immediately following irrigation events and then declined. The largest emission peak of 322.1 μg·m−2·h−1 at La Mancha occurred after irrigation and nitrogen application (Fig. 2). This supported the hypothesis of this study that N2O emissions are a direct function of moisture content fluctuations and fertilization events. The largest emission peak of 26.37 μg·m−2·h−1 at Leyendecker occurred after irrigation and tillage (Fig. 3). La Mancha had the highest annual flux of 1.129 kg N/ha and % nitrogen loss (EF) of 0.49% (Table 2).
Annual CO2, CH4, and N2O fluxes; fertilizer rates; and emission factors (EFs) at La Mancha and Leyendecker.
N2O emissions were lower from Leyendecker than from La Mancha in both the laboratory and the field (Fig. 1; Table 2), although the difference in the field results (P = 0.008) was much more dramatic than that in the laboratory (P = 0.05). The lower N2O emissions were probably a result of low soil mineral N content because of lower N fertilizer rates at Leyendecker; this is in agreement with other studies that N input is more important than other factors as a predictor of N2O emissions (Dusenbury et al., 2008; Halvorson et al., 2008; Jacinthe and Dick, 1997; Ma et al., 2010). Higher N2O emissions at Leyendecker are expected because of its higher clay content and soil bulk density (Table 1) than La Mancha (Linn and Doran, 1984; Rochette et al., 2007).
CO2 emissions were lower and CH4 emissions were higher at Leyendecker, where the bulk density was higher despite having a higher percentage of clay content (Table 1) than that at La Mancha. This is consistent with decreased soil respiration and increased anaerobic conditions reported on compacted soils (Linn and Doran, 1984).
Factors affecting GHG emissions from agricultural soils have been shown to be a function of soil moisture, fertilizer application, other management techniques, temperature, crops, and soil texture (Halvorson et al., 2008; Hargreaves et al., 1994; Jacinthe and Dick, 1997; Mosier et al., 1996; Smith et al., 2003). N2O emissions are directly related to soil moisture content (Cates Jr. and Keeney, 1987; Dungan et al., 2016; Halvorson et al., 2016; Hernandez-Ramirez et al., 2008) and exhibit large peaks immediately after the rewetting of dry soils (Davidson, 1992; Leitner et al., 2017). This indicates the importance of observing drying–wetting conditions that occur in irrigated agricultural fields of semiarid regions. N2O emissions are reliant on both moisture content and soil NO3−; when available NO3− is not a limiting factor, the relationship between moisture content and EF has been shown to be linear (Dobbie and Smith, 2003). Higher N2O emissions as moisture content increases and its correlation to increased soil NO3− suggest that denitrification is the primary source of N2O emissions. The results from this project support these conclusions. Denitrification as the primary source of N2O emissions has been confirmed using other techniques (Ostrom et al., 2010; Russow et al., 2007).
Because of any one of the controlling factors becoming limiting, the analysis of flux response to a single variable has proved to be more useful than the use of multiple regression techniques (Dobbie and Smith, 2003). At La Mancha, an exponential relationship was found between soil NO3− and N2O flux (P < 0.09) (Fig. 4), although results are too weak to provide a direct correlation. At Leyendecker, a significant exponential relationship was found between N2O flux and W% (P < 0.01) (Fig. 5). Although not always consistent, the results of these relationships support moisture content and N application being key controlling factors for N2O emissions; these results are consistent with those reported in other studies.
Relationships between CO2, CH4, and N2O vs. water content (%W) and N2O vs. NO3− at La Mancha.
Citation: HortScience horts 53, 5; 10.21273/HORTSCI12773-17
Relationships between CO2, CH4, and N2O vs. water content (%W) and N2O vs. NO3− at Leyendecker.
Citation: HortScience horts 53, 5; 10.21273/HORTSCI12773-17
EFs varied widely among the two test sites but were still well below the 1.25% IPCC coefficient factor recommended in calculating N2O emissions. If the 1.25% coefficient factor was used, annual emissions would be several times higher than those observed: 0.88 kg N/ha and 2.9 kg N/ha, respectively, for Leyendecker and La Mancha. Studies have shown low N2O emissions in arid regions (Snyder et al., 2009). This study confirms the conclusion of Dusenbury et al. (2008) that a lower loss predictor should be adopted by the IPCC for semiarid regions. A thorough accounting of N2O emissions is imperative for determining the true mitigation potential of management practices (Gregorich et al., 2005).
Mitigating agricultural N2O emissions requires the consideration of matching spatial and nutritional needs of crops through limiting and regulating mineral N and water to maximize resource uptake. Possible nitrogen management practices include the following: minimize excess N application, split nitrogen application, precision irrigation application, and application of slow release N sources (Dajal et al., 2003). Improving irrigation efficiency is of particular importance in the semiarid southwest to manage N2O emissions. These techniques would limit excess mineral N in the soil, reduce the time soil is saturated, and ensure that resources are spatially available to crops.
Conclusion
In the semiarid region of Southern New Mexico, N2O emissions were highest from pecan orchards immediately following a fertigation application. The increased N2O emissions as soil nitrate and moisture content increase suggest that anaerobic denitrification is the dominant mechanism of N2O production in the field. This study further supports the adoption of a lower coefficient factor by the IPCC for semiarid regions. Further research on the effect of alternative irrigation techniques after fertilizer application could be beneficial in the future for the reduction of N2O emissions and to further a thorough emissions accounting system.
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