Reduced nitrous oxide emissions and increased yields in California tomato cropping systems under drip irrigation and fertigation

Agriculture, Ecosystems and Environment 170 (2013) 16–27

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Reduced nitrous oxide emissions and increased yields in California tomato cropping systems under drip irrigation and fertigation Taryn L. Kennedy a,∗ , Emma C. Suddick b , Johan Six c a b c

Department of Plant Sciences, University of California, One Shields Avenue, Davis, CA, USA Woods Hole Research Center, 149 Woods Hole Road, Falmouth, MA, 02540, US Department of Environmental Systems Sciences, Institute of Agricultural Sciences, Swiss Federal Institute of Technology, Zurich, Switzerland

a r t i c l e

i n f o

Article history: Received 17 August 2012 Received in revised form 26 January 2013 Accepted 12 February 2013 Keywords: Nitrous oxide Drip irrigation Fertigation Furrow irrigation Processing tomatoes

a b s t r a c t Understanding the effect of various agricultural management practices on nitrous oxide (N2 O) emissions is crucial to advise farmers and formulate policies for future greenhouse gas (GHG) reductions. In order to estimate present N2 O emissions, annual N2 O budgets must be thoroughly and precisely quantified from current farms under conventional and alternative management, but subject to practical and economic constraints. In this study, field sites were located on two on-farm processing tomato (Lycopersicon esculentum) fields, under contrasting irrigation managements and their associated fertilizer application strategy: (1) furrow irrigation and sidedress fertilizer injection (conventional system) and (2) drip irrigation, reduced tillage, and fertigation (integrated system). Nitrous oxide emissions were monitored for seven to ten days following major events of cultivation, irrigation, fertilization, harvest, and winter precipitations. Total weighted growing season emissions (15 March–1 November 2010) were 2.01 ± 0.19 kg N2 O-N ha−1 and 0.58 ± 0.06 kg N2 O-N ha−1 in the conventional and integrated systems, respectively. The highest conventional system N2 O emission episodes resulted from fertilization plus irrigation events and the first fall precipitation. In the integrated system, the highest N2 O fluxes occurred following harvest and the first fall precipitation. Soil chemical and physical properties of soil moisture, inorganic nitrogen (N), and dissolved organic carbon (DOC) were low and less spatially variable in the integrated system. Used as an index of substrate availability, soil ammonium (NH4 + ) and nitrate (NO3 − ) exposures were significantly lower in the integrated system. Of great importance is that the drip irrigation water and fertilizer management of the integrated system also increased crop yield (119 Mg ha−1 vs. 78 Mg ha−1 ), highlighting the potential for decreasing N2 O emissions while simultaneously improving the use of water and fertilizer for plant production. Published by Elsevier B.V.

1. Introduction The causal relationship between anthropogenic increases in GHG concentrations and the current issue of climate change is irrefutable (IPCC, 2007). Since the Industrial Revolution, the concentrations of carbon dioxide (CO2 ), methane (CH4 ), and N2 O have increased in the atmosphere by 36%, 148%, and 20%, respectively (Keeling et al., 2005; Wuebbles, 2009). Of these three major, longlived GHGs, N2 O has the greatest radiative forcing on a per molecule basis, 296 times that of CO2 (IPCC, 2007). Nitrous oxide is largely produced naturally from agricultural soils during the microbially mediated processes of nitrification and denitrification (Firestone and Davidson, 1989; Pathak, 1999). The destructive potential of

∗ Corresponding author at: 1700 Wilson Court, Eugene, OR 97402, USA. Tel.: +1 928 830 2563. E-mail address: [email protected] (T.L. Kennedy). 0167-8809/$ – see front matter. Published by Elsevier B.V. http://dx.doi.org/10.1016/j.agee.2013.02.002

N2 O has been likened to that of chlorofluorocarbons, positioning it as the next most important GHG to monitor and regulate (Ravishankara et al., 2009). Although largely unregulated at the present time (Smith et al., 2007), efforts to measure, predict, and mitigate N2 O emissions are essential for the benefit of future generations and the sustainability of agriculture. The passage of the Global Warming Solutions Act of 2006 (AB32) in California, United States, has spurred widespread efforts to quantify N2 O emissions from major cropping systems in California. Gaseous emissions of ammonia (NH3 ) and N2 O have been identified as the main avenue of fertilizer N loss in agricultural systems (Peoples et al., 1995), especially in irrigated cropping systems (Freney, 1997). In California’s Mediterranean climate, irrigation systems are essential for crop production and also serve as the method for fertilizer delivery. In conventional processing tomato (Lycopersicon esculentum) cropping systems, fertilizer is applied via side-dress applications and dripped into furrow irrigation (FI) water runs. Alternative tomato management practices include subsurface drip irrigation (SDI) (O’Neill et al., 2008), which enables a

T.L. Kennedy et al. / Agriculture, Ecosystems and Environment 170 (2013) 16–27

very controlled administration of water and fertilizer, known as fertigation (Li and Zhang, 2003; Scheer and Wassmann, 2008). Subsurface drip irrigation systems allow for smaller, more precise, and more frequent inputs of water and fertilizer that reduce deep percolation (Amali et al., 1997) and early season evaporation (Hanson and May, 2006), meet plant N requirements more accurately (Li and Zhang, 2003), and result in reduced N loss as N2 O (SánchezMartín et al., 2008). However, further study is needed to elucidate and quantify how changes in management affect N2 O emissions from tomato and other major cropping systems in California. Despite the identified relationships between improved water management and N2 O emissions, few studies to date have compared the effect of different irrigation regimes on N2 O emissions in agroecosystems, where water is the most limiting factor controlling rates of N2 O production (Amos et al., 2005; Scheer and Wassmann, 2008). In addition, studies within single cropping systems are few (Kallenbach et al., 2010); making direct comparisons of irrigation/fertigation regimes difficult, since N2 O emissions are highly dependent on soil texture, climate, and crop type (Mosier et al., 1998). However, published results are promising; SánchezMartín et al. (2008, 2010) have consistently observed 30–70% reductions in N2 O emissions under melon crops when irrigated by SDI versus FI. In processing tomato systems, Kallenbach et al. (2010) reported significantly lower growing season N2 O and CO2 emissions under SDI than FI, with as much as a 75% and 35% reduction, respectively. In a Northern California almond orchard, Suddick et al. (2011) observed 7.5% lower N2 O emissions from subsurface drip irrigation compared to surface drip, following a fertigation event. Following the pivotal work of Kallenbach et al. (2010) at a longterm agricultural research station, we identified on-farm field sites to assess the effect of active, farm-field conditions and managements on GHG emissions. On-farm studies are an effective means to monitor and evaluate whole management systems. Equally, they are important for corroborating results from research-based trails like those of Kallenbach et al. (2010), yet under realistic farm conditions and management expertise. A farm is a complex system of interacting components that bridge natural and socioeconomic realms. By comprehensively monitoring N2 O emissions on active farms under different managements in close proximity, within the same cropping system, and on the same soil type, it is possible to characterize the soil chemical and physical properties that influence N2 O fluxes as expressed by each management system, increasing the rigor and efficiency of research station trials by basing research questions in the context of current and economically viable management approaches. In the following study, gas and environmental soil measurements were taken from processing tomato fields following major management events and winter precipitations. The aims of this study were to: (1) determine which management events produce the largest N2 O fluxes over the course of a year, (2) record and compare changes in soil chemical and physical properties under both management systems, and (3) assess if potential changes in irrigation and fertilizer management reduce N2 O emissions.

2. Materials and methods 2.1. Site descriptions and study design Our field study was conducted at two processing tomato farms in Winters, Yolo County, CA (Lat. 38◦ 34 N; Long. 121◦ 56 W) from 15 March 2010 to 30 April 2011. This region has a semi-arid Mediterranean climate where most of the precipitation falls as rain between October and April. During the study period, the average maximum and minimum temperatures were 23.9 ◦ C and 9.9 ◦ C, respectively. Precipitation was 742 mm during the study period

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Fig. 1. Temperature and precipitation data from 1 March 2010 to 1 April 2011 recorded at a UC IPM weather station in Winters, CA.

(15 March 2010–30 April 2011) (Fig. 1); average precipitation from 2000 to 2011 (15 March–30 April) was 658 mm. Two field-sites were selected based on crop cultivar and soil texture similarities; clay content was of primary importance, as it would affect planting dates and influence the degree of soil anaerobicity (Rochette, 2008). Both sites were designated as a Brentwood silty clay loam (fine, montmorillonitic, thermic Typic Xerochrept) by the National Cooperative Soil Survey; however textural results indicated that the site soils were a clay loam. Processing tomatoes (L. esculentum, Var. AB 2) were grown in both fields during the 2010-growing season (15 March–1 November 2010). Precipitation was 160 mm during the 2010-growing season. The field sites were under two different management regimes: conventional (sidedress fertilizer injection, FI, and standard tillage) and integrated (fertigation, SDI, and reduced tillage). Extensive field operations, including tillage passes, bed cultivation, seedling transplant, and harvest, were conducted in both fields and are listed in Tables 1 and 2. The conventional field in this study was fallow from June 2009 to March 2010, planted with tomatoes from March 2010 to August 2010, and seeded with winter wheat from November 2010 to June 2011. The conventional field was fertilized six times over the course of the study for a total of 237 kg N ha−1 (Table 1). Starter fertilizer (8-24-6) was injected into the seedbed prior to planting during the final bed preparation pass. Additional fertilizer (3-3-18) was mixed with the transplant water and applied at the rooting zone when the tomatoes were transplanted. The bulk of the fertilizer (28-0-5) was applied as a sidedress injection, prepared as a liquid and shanked into each shoulder of the seedbed to a depth of 15 cm, approximately one month after transplanting. During the growing season, three applications of CAN-17 (17-00) were applied. CAN-17 is a calcium ammonium nitrate solution composed of 5.4% ammoniacal N, 11.6% NO3 -N, and 8.8% calcium. As a liquid fertilizer, CAN-17 can be applied in many ways. In the conventional system, CAN-17 was dripped into the head ditch of the surface irrigation system and delivered to the beds through the furrows. In the conventional system, water was delivered through furrow irrigation. Estimated at a rate of 6.4–7.6 cm of water per irrigation, approximately 64–76 cm of water was delivered during the growing season across ten irrigation events. The first irrigation was for 24 h and all others were 12 h in duration. The farmer scheduled irrigations using evapotranspiration, as determined from California Irrigation Management Information System (CIMIS) stations nearby and soil moisture probes in the field. Irrigation occurred every 7–10 days throughout the growing season (Table 1). The last irrigation event occurred on 30 July 2010 to allow

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Table 1 Management events that occurred in the conventional tomato system. Background gas and soil sampling dates between events are not included. Event

Date

Land remains Fallow

Summer 2009

Fall tillage 2X stubble disc Chisel plow (56 cm) Stubble DISC 2X triplane (landplane) List beds

Fall 2009

Tomato planting preparations Cultivate bed tops & apply starter Herbicide spray & mulcher incorporation Transplant tomato Sidedress injection & weed cultivation Run incorporator to smooth beds Irrigation Irrigation (w/fertilizer) Irrigation (w/fertilizer) Irrigation (w/fertilizer) Irrigation Irrigation Irrigation Irrigation Irrigation Irrigation Tomato harvest Fall tillage 2X Wilcox (Remakes furrows, chops, mixes top 5–8 cm) Roller First precipitation of season Seeded winter wheat Wheat fertilizations Wheat harvest Total tomato fertilization

3/22/10 3/23/10 3/30/10 5/5/10 5/8/10 5/13/10 5/27/10 6/3/10 6/13/10 6/20/10 6/29/10 7/8/10 7/15/10 7/23/10 7/30/10 8/27/10 9/4/10 9/4/10 10/25/10 11/4/10 3 events 6/5/11

Fertilizer type

kg N ha−1

8-24-6a

15

3-3-18b 28-0-5c

3 146

Yield Mg ha−1

Sampling dates

4/3–4/10 5/2–5/13 5/13–5/21

CAN-17d CAN-17 CAN-17

24 24 24

6/2–6/12

7/25–8/1 78

8/26–9/3 9/3–9/16 10/21–11/2 11/5–11/14

Urea

165 6.5 237

a

7.29% ammoniacal nitrogen, 0.23% nitrate nitrogen, 0.48% urea nitrogen. 75% ammoniacal nitrogen, 25% nitrate. c Ammoniacal nitrogen. d CAN-17 = Ca(NO3 )2 ·NH4 NO3 . UN-32 = (NH2 )2 CO·NH4 NO3 . b

the soil to dry out before harvest, which was on 12 August 2010. The conventional field was harvested using a mechanical harvester. Yield values were obtained from the farmer and measured at the field scale by weight (Table 1). Crop residues were returned whole to the field, allowed to dry, and incorporated prior to seeding of winter wheat (Triticum aestivum) on 4 November 2010. In preparation for winter wheat, the field was cultivated with two Wilcox passes, which remakes furrows, chops, and mixes the top 5–8 cm. Following the Wilcox, a roller was used to smooth the beds. Prior to the study period, the integrated field was managed under conventional tillage with the following crop rotation: alfalfa (Medicago sativa) from 2005 to 2007, processing tomato (L. esculentum) in 2008, and sunflower seed in 2009. In the fall of 2009, the farmer installed a drip irrigation system and changed the direction of the rows from east/west to north/south. At this time, the integrated field transitioned from standard to reduced tillage. Prior to SDI installation, the field was prepared with standard tillage practices (Table 2). The integrated field had a Triticale trios winter grain cover crop from Fall 2010 to Spring 2011. The integrated field was fertilized seven times over the course of the study for a total of 205 kg N ha−1 (Table 2). Starter fertilizer (8-24-6) was injected into the seedbed prior to planting during the pre-planting cultivation. Throughout the growing season, the tomato crop was fertilized with UN-32, also known as UAN, (32-0-0) via fertigation. A liquid fertilizer, UN-32 is composed of 16.8% urea, 7.6% ammoniacal N, and 7.6% NO3 -N. In addition to N fertilization with UN-32, potassium (0-0-14) was applied via fertigation, totaling 105 kg K ha−1 . Fertilization rates were determined by soil tests from the previous year and tissue tests taken during the 2010 growing season.

In the integrated system, water was supplied to the crop via SDI, with drip tape positioned at the center of each bed, 23 cm below the soil surface. Irrigation was scheduled using evapotranspiration, as determined from nearby CIMIS stations, modified by a crop coefficient for tomato based on the vine size and a system efficiency modification. The farmer also had soil moisture monitors in the field. In July, irrigation events occurred every 16 h for 8-h duration. During the study, the integrated field was irrigated a total of 64 cm, determined precisely from the pump meter readings. The integrated field was harvested using a mechanical harvester with a vine shredder attachment. Crop residues were chopped and returned to the field as fine mulch. Yield values were obtained from the farmer and measured at the field scale. The field was cultivated with one Wilcox pass prior to seeding the wheat cover crop on 5 November 2010. No fertilizer was applied to the winter wheat crop. The experimental design was identical for both cropping systems. One field was selected for measurement within each management system. Each field had three plot replicates, positioned 20 m apart along the same row and 75 m from the field edge. Each plot contained three chambers for repeated sampling of GHG gas emissions from soil. The three chambers corresponded with functional locations relative to fertilizer and irrigation water placement: chambers were positioned at the center of the seedbed (Berm) between two rows of tomato plants, on the shoulder of the seedbed (Side), and in the furrow (Furrow) with a spacing of 2 m between each collar (Fig. 2). In both systems, two rows of tomatoes were planted 30 cm apart on top of a 1.5 m bed. The tomato population was 23,475 and 22,487 plants ha−1 in the conventional and integrated systems, respectively.

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Table 2 Management events that occurred in the integrated tomato system. Background gas and soil sampling dates between events are not included. Event

Date

Sunflower Seed Crop Fall tillage 2X disc Stubble disc Deep rip (61–66 cm) Disc 2X triplane (landplane) List beds Installed drip tape @ 20 cm depth

Summer 2009 Fall 2009

Tomato planting preparations Sweep cultivator Herbicide Spray & Mulcher Incorporation Transplant tomatoes Fertigation Close cultivate Fertigation Cultivate Fertigation Fertigation Fertigation Fertigation Vine train Tomato harvest/vines shredded Fall tillage Wilcox (remakes furrows, chops, mixes top 5–8 cm) First precipitation of season Drill Triticale trios Herbicide spray Total tomato fertilization a b

3/21/10 3/22/10 3/25/10 4/21/10 4/23/10 5/24/10 5/27/10 5/29/10 6/1/10 6/16/10 6/22/10 6/29/10 8/23/10

Fertilizer type

kg N ha−1

8-24-6a UN-32b

6 75

UN-32

14

UN-32 UN-32 UN-32 UN-32

20 29 29 29

Yield Mg ha−1

Sampling dates

3/23–3/31

4/23–4/30 5/21 5/25–6/13

6/22–6/30 8/23–9/3

10/2/10 10/25/10 10/30/10 2/10/11

10/21–11/2

205

53

7.29% ammoniacal N, 0.23% nitrate N, 0.48% urea N. UN-32 = (NH2 )2 CO·NH4 NO3 .

2.2. Sampling, analysis, and estimation of N2 O emissions Daily gas measurements were taken after significant tomato management events, i.e., for a period of seven days following irrigation and fertilization and for a period of ten days following individual cultivation events (e.g., bed preparation, planting, etc.) and harvest. Given the frequency of irrigation in this Mediterranean climate, one irrigation event was measured per month. Sampling campaigns also occurred following winter precipitation events. In order to comprehensively characterize event-induced N2 O emissions, gas samples were taken daily after significant management events rather than one to three times a week throughout the growing season (Halvorson et al., 2010). The intent was to fully characterize the peak in N2 O emissions induced by management events. If gases are not measured daily after events, the highest N2 O emission can occur on one day and the height of the N2 O rate curve is not accurately characterized. Following an event, gases were sampled every day and soils were sampled every other day.

Background emissions of N2 O were measured the day before each event and once every 12 days in between management events. For precipitation events, background measurements were taken in advance of the storm by one or two days, depending on weather forecast. Annual emissions were calculated for each system based on event-based and background measurements made from 15 March 2010 to 30 April 2011. Data collected were analyzed by season, by event, and by functional location. The growing season data presented here were measured from 15 March 2010 through 1 November 2010, prior to the first winter wheat fertilization event in the conventional system. In situ soil-surface N2 O fluxes were measured using vented, static flux chambers modeled after Hutchinson and Mosier (1981) and made out of polyvinylchloride (PVC) rings, 20.3 cm in diameter and 15 cm tall. Chamber lids were constructed from PVC irrigation caps and covered with aluminum to reflect sunlight and minimize temperature increases. Each chamber enclosed an approximate volume of 5.6 L. Chamber bases were installed to a depth of 10 cm at each

Fig. 2. Illustration of the three functional locations for chamber placement and gas sampling with the postulated soil moisture gradients resulting from furrow irrigation (a) and subsurface drip irrigation (b). Fertilizer is injected into the side of the seedbed and added to furrow irrigations (a). Alternatively, fertilizers are added via drip irrigations (fertigation) (b).

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functional location prior to each management event, removed for the event, and re-installed for gas sampling. In between events, chamber bases remained in position and were removed only when required for machine operations. Furrow chamber bases in the conventional system were removed for irrigation events and CAN-17 fertilization events, so that the furrow chamber did not impede the flow of water. When chambers were removed for management events, care was taken to replace the chamber in the same location and to not sample immediately after reinstalling of the chambers (1–24 h). Chamber base heights were measured at regular intervals, to account for variations in headspace volume due to soil settling and chamber relocation. At the time of deployment, lids were manually sealed to the chamber bases with a rubber gasket. Air samples (35 mL) were taken from the headspace via a rubber septum using polypropylene syringes at 0, 30, and 60 min after chamber closing. Pressurized samples were stored for later analyses with a Shimadzu GC-2014 Gas Chromatograph equipped with an electron capture device (ECD) and a thermal conductivity detector (TCD) (Palo Alto, CA). Gas samples were analyzed within two weeks of collection for N2 O and CO2 . At the time of gas sampling, soil temperature at 15 cm depth and air temperature within each chamber were measured using a thermocouple (Hanna Instruments, HI 93512). In order to avoid high midday temperatures, all measurements were made between 10:00 and 12:00 h. Nitrous oxide fluxes were computed from the change in N2 O concentration with time by (1) using the Ideal Gas Law to convert from ppm to ␮mol L−1 , (2) confirming that the difference between the maximum and minimum flux concentration was greater than the detection limit of the Gas Chromatograph, (3) testing for linearity (Hutchinson and Mosier, 1981) to determine the best flux, (4) validating with CO2 linearity with R2 > 0.8, and (5) converting to g N2 O-N ha−1 day−1 . Best flux refers to the selection of the most linear increase in N2 O concentration from the measurements taken after chamber closure, evaluating the intervals of 0–30, 30–60, and 0–60 min. Carbon dioxide fluxes were computed similarly as the N2 O fluxes, but from the change in CO2 concentration with time and were converted to kg CO2 -C ha−1 day−1 . Seasonal cumulative N2 O and CO2 fluxes per field and functional location were determined by interpolating emissions between measuring days. When calculated at the field scale from chamber measurements, total field emissions were weighted according to the relative width of each functional location across the seedbed (Berm 40%, Side 40%, and Furrow 20%). Emission factors (EF) were calculated by dividing the cumulative N2 O emissions by the N applied as fertilizer. Emission factors were not corrected for background N2 O emissions due to the missing of a proper control. Processing tomatoes are never not fertilized, hence the lack of a control. In the on-farm research context, requesting that the farmers not fertilize an entire row, was not economically feasible. The EF factors calculated in this study are estimates based on the available information. 2.3. Soil analyses Soils were collected from each functional location/replicate every other day during the course of an event and analyzed for soil moisture and concentrations of NO3 − , NH4 + , and DOC using potassium sulfate (K2 SO4 ) extractions. Three surface soil (0–15 cm) samples were collected within 30 cm of each collar with a 2 cm diameter auger and composited into one soil sample, the day before each event and every two days during the sampling campaign. For inorganic N and DOC analyses, 15 g was sub-sampled from the bulked soil sample and extracted with 0.5 M K2 SO4 (3.33:1 K2 SO4 /Soil). Nitrate and NH4 + concentrations in the extracts were estimated colorimetrically (Doane and Horwath, 2003) with a Shimadzu UV PharmaSpec 1700 spectrophotometer. Dissolved

organic C was measured at the beginning and end of each management event, by combustion of K2 SO4 extracts using a total organic C analyzer (Shimdazu TOC-V). Nitrogen exposures were calculated as the linear interpolation of NO3 − and NH4 + concentrations between sampling dates (Burton et al., 2008). It is expressed in units of mg N (g soil)−1 per unit time, which combines both the magnitude of NO3 − and NH4 + concentrations and the duration they are present. This measure gives us a temporally integrated measure of the exposure of the soil microbial community to inorganic N substrates on a per event basis and over the growing season (Burton et al., 2008). Soil moisture was measured gravimetrically by drying a subsample of 50 g for 24 h at 105 ◦ C. Soil bulk density was determined by retrieving three replicates of undisturbed soil cores of known volume from each functional location and drying at 105 ◦ C until constant weight was reached. Bulk density was measured two times per year, prior to cultivation and in the fall after harvest. Percent soil WFPS was calculated using bulk densities measured at each functional location and assuming a mineral particle density of 2.65 g cm−3 (Table 2). At the beginning and end of each event, pH was measured in 1:1 deionized water:soil. Total soil C and N were measured by dry combustion (Costech Instruments ECS 4010). Nutrient use efficiency was calculated as partial factor productivity (PFP) (Snyder and Bruulsema, 2007). PFP =

Yield N applied

2.4. Statistical analysis The three plots in each field were replications of the functional locations (Berm, Side, and Furrow). Statistical comparisons were made across functional locations and between events within each management system using ANOVA. All data was tested for normality using the Shapiro–Wilk test; no data transformations were required. Differences between means within each management system were analyzed using Tukey–Kramer pair-wise comparisons with Proc Mixed in SAS (9.1 TS1M3 SAS Institute Inc., Cary, NC, USA). Significance was accepted at a level of probability of p < 0.05. Relationships among measured parameters were assessed using correlation analyses. Pearson correlation coefficients were calculated on a per event basis among cumulative N2 O emissions, soil inorganic N concentrations, N exposures, WFPS, and DOC. The correlations were performed using treatment means and the functional locations were considered separately. Because microbial response to changes in N availability are not immediate, we investigated a time lag response of N2 O emissions to available soil N by correlating inorganic N and N2 O data collected on the same day (linear correlation) and between inorganic N concentrations and N2 O fluxes measured one, two, and three days later. Linear correlations were made between inorganic N exposures, pH, and cumulative gas emissions across all events. Cumulative N2 O emissions were compared between functional locations across management regimes. 3. Results 3.1. N2 O emissions and tomato yield In the conventional system, total weighted growing season emissions (15 March–1 November 2010) were 2.01 ± 0.19 kg N2 ON ha−1 , which amounted to 0.85% of the N fertilizer applied to the tomato crop. The yield was 53 Mg ha−1 and the partial factor productivity was 0.37 Mg fresh fruit kg−1 N. Total weighted annual emissions were 3.06 ± 0.19 kg N2 O-N ha−1 , which equated to an overall 0.76% of N fertilizer applied to the tomato and winter wheat

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Fig. 3. Weighted N2 O emissions by event and management regime. Events labeled ‘fert’ are conventional fertilizations and integrated fertigations. Error bars represent ±1 standard error.

crops. The percent lost as N2 O declined in the annual figure because the fertilizer applied was not constant throughout the year. In the integrated system, total weighted growing season emissions (15 March–1 November 2010) were 0.58 ± 0.06 kg N2 ON ha−1 , which was 0.28% of the N fertilizer applied to the tomato crop. The yield was 53 Mg ha−1 and the partial factor productivity was 0.58 Mg fresh fruit kg−1 N. Total weighted annual emissions were 0.95 ± 0.05 kg N2 O-N ha−1 , which equated to an overall 0.46% of N fertilizer applied. Weighted N2 O emissions were calculated for each event to investigate the effect of specific field operations on emissions. In the conventional system, notable N2 O fluxes were produced by two summer fertilizations (124.9 ± 47.6 and 77.8 ± 18.4 g N2 ON ha−1 day−1 ), harvest (68.6 ± 1.6 g N2 O-N ha−1 day−1 ), and the first winter rain event (187.8 ± 38.6 g N2 O-N ha−1 day−1 ) (Fig. 3). The largest N2 O fluxes in the integrated system occurred after harvest (103.2 ± 16.2 g N2 O-N ha−1 day−1 ) and following the first rain event (108.6 ± 26.9 g N2 O-N ha−1 day−1 ) (Fig. 3). Distinct patterns of variability of N2 O emissions across functional locations were apparent in the conventional system; cumulative growing season emissions were highest at the furrow (3.3 ± 0.24 kg N2 O-N ha−1 season−1 ) and side (2.8 ± 0.33 kg N2 O-N ha−1 season−1 ) locations, followed by the berm (1.5 ± 0.10 kg N2 O-N ha−1 season−1 ) (Fig. 4). Growing season

Fig. 5. Comparison of the conventional berm, side, and furrow in (a) N2 O emissions, (b) WFPS, (c) NO3 -N, (d) NH4 -N, and (e) C from measurements taken during the growing season. Error bars represent ±1 standard error.

and annual emissions of N2 O from the berm were significantly lower than side and furrow emissions; however, side and furrow emissions were not significantly different in either time frame (Fig. 4). In the integrated system, growing season and annual N2 O emissions were not significantly different across any of the functional locations (Fig. 4). 3.2. Soil chemical and physical properties

Fig. 4. Cumulative N2 O emissions by management and functional location during the tomato-growing season and across the entire rotation (annual). Growing season dates spanned 15 March to 1 November 2010. Bars with the same letter are not significantly different (p < 0.05) within each management. Error bars represent ±1 standard error.

3.2.1. Soil inorganic nitrogen In the conventional system, soil inorganic N levels strongly increased following fertilization events and after the first rain; smaller increases occurred following irrigation, harvest, and residue incorporation events. Soil NO3 − levels were generally highest at the side position, followed by the berm, and then the furrow (Fig. 5c). Soil NH4 + levels were generally higher at the side and berm positions; following the first rain, high NH4 + levels were measured in the furrow position (Fig. 5d). In the integrated system, soil inorganic N levels increased after planting, following fertigation events, and after the first rain. Trends in soil inorganic N levels across functional locations were not distinguishable in the integrated system (Fig. 6c and d). Following fertilization events in the conventional system, inorganic N concentrations generally increased, with soil NO3 − levels ranging from 0 to 100 ␮g NO3 -N g−1 dry soil and soil NH4 + levels ranging from 0 to 20 ␮g NH4 -N g−1 dry soil (Fig. 5c and d). Soil NO3 − levels ranged from 0 to 40 ␮g NO3 -N g−1 dry soil in the integrated system. Ammonium levels were typically low in

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Fig. 7. Nitrate and ammonium exposure by functional location and management during the tomato-growing season and across the entire rotation (annual). Bars with the same letter are not significantly different (p < 0.05) within each management. Error bars represent ±1 standard error.

Fig. 6. Comparison of the integrated berm, side, and furrow in (a) N2 O emissions, (b) WFPS, (c) NO3 -N, (d) NH4 -N, and (e) C from measurements taken during the growing season. Error bars represent ±1 standard error.

the integrated system, with only five of the sixty sampling days exceeding 5 ␮g NH4 -N g−1 dry soil (Fig. 6d). Used as an index of substrate availability and expressed as mg N (g soil)−1 per unit time, soil NO3 − and NH4 + exposures (NE and AE) were compared across functional locations and managements. In the conventional system, NE and AE were highest at the berm and side locations and were significantly greater (p < 0.05) than the N exposures at the furrow (Fig. 7). The AE variability in the integrated system had a distinct spatial pattern; it was highest at the berm, followed by lower AE values at the side and furrow, which were not significantly different from each other (Fig. 7). In contrast, there was no spatial pattern of NE observed in the integrated system (Fig. 7). On a per event basis, NE was high in the conventional system during the first two fertilization events (0.22 ± 0.04 and 0.19 ± 0.01 mg NO3 -N (g soil)−1 event−1 ) and as the soil dried out prior to harvest (0.24 ± 0.04 mg NO3 -N (g soil)−1 event−1 ). Nitrate exposure values peaked following the first rain event (0.44 ± 0.04 mg NO3 -N (g soil)−1 event−1 ). Ammonium exposure was also highest following the first rain event (0.13 ± 0.03 mg NH4 N (g soil)−1 (event−1 ). By event, there were no significant correlations between cumulative event N2 O emissions and NE or AE, indicating that the high NO3 − CAN-17 fertilizer may influence N exposures in the conventional system; NE was higher than AE across all events (data not shown).

In the integrated system, NE increased after planting (0.21 ± 0.003 mg NO3 -N (g soil)−1 event−1 ) and the first rain (0.23 ± 0.01 mg NO3 -N (g soil)−1 event−1 ). Nitrogen exposures did not significantly vary across the fertigation events despite differences in N application rates per fertigation (Table 1). Nitrate exposure was positively correlated with cumulative event N2 O emissions during the first fertigation event (p = 0.010, r = +0.79) and during harvest (p = 0.025, r = +0.73). Ammonium exposure was highest after planting (0.01 ± 0.00 mg NH4 -N (g soil)−1 event−1 ). Ammonium exposure was positively correlated with cumulative event N2 O emissions during the first (p = 0.047, r = +0.67) and second fertigation event (p = 0.019, r = +0.75), indicating that the high NH4 + UN-32 fertilizer did not influence N exposures in the integrated system; NE was higher than AE across all events. In addition, AE values were consistently low during events in the integrated system (data not shown). 3.2.2. Soil moisture content We expected soil moisture to decrease with increasing distance from the location of water application, with the highest WFPS at the conventional furrow and integrated berm, and the lowest WFPS at the conventional berm and integrated furrow (Fig. 2). This soil moisture gradient was observed in the conventional system; however both the integrated berm and furrow exhibited high WFPS. Water-filled pore space ranged from 40 to 80% in the conventional system, with the highest soil moisture consistently occurring at the furrow position (Fig. 5b). Water-filled pore space was between 40 and 60% in the integrated system, with the largest values occurring at the berm and furrow positions (Fig. 6b). 3.2.3. Extractable C and pH In the conventional system, DOC ranged from 0.01 to 2.75 mg C g−1 soil (average = 0.57 ± 0.07 mg C g−1 soil, n = 125). Soil

T.L. Kennedy et al. / Agriculture, Ecosystems and Environment 170 (2013) 16–27

23

Table 3 Soil properties for each treatment and functional location. Total C and N were measured in mg g−1 soil. Bulk density (ˇd ) was measured in g cm−3 . Error represents ±1 standard error. Year - 2010

pH

Total C

Total N

␤d : Berm

␤d : Side

␤d : Furrow

% Sand

% Silt

% Clay

Conventional Integrated

6.61 ± 0.04 7.22 ± 0.03

7.4 ± 0.09 9.2 ± 0.10

0.9 ± 0.01 1.1 ± 0.02

1.28 ± 0.11 1.15 ± 0.09

1.21 ± 0.03 1.24 ± 0.03

1.52 ± 0.02 1.29 ± 0.00

28 27

38 41

34 32

DOC followed a cyclic pattern, with increasing levels and greater variability in the late summer months prior to and following harvest (Fig. 5e). In general, DOC levels were slightly higher at the side position during the tomato-growing season and at the berm position during the winter. In the integrated system, DOC ranged from 0.00 to 1.0 mg C g−1 soil (average = 0.62 ± 0.08 mg C g−1 soil, n = 107). Concentrations of DOC peaked at ∼1 mg C g−1 soil in late April, about a month after planting during the largest fertigation event. In general, DOC levels were uniform across the functional locations (Fig. 6e) in the integrated system. In the conventional system, values of pH ranged from 5.5 to 7.4 and were the greatest about a month after planting in May. The lowest pH values of 5.5–6.0 occurred around harvest in August at the side position (data not shown). Other than the aforementioned low pH at the side position, pH at the berm was consistently lower than the other functional locations. Within the integrated system, pH ranged from 6.8 to 7.7 and greater variability across the in-field replicates was exhibited. Temporally, pH increased following harvest and remained higher through April of 2011. Variability across the functional locations was not apparent in this system. 4. Discussion 4.1. Total N2 O emissions Our results indicate that drip irrigation and fertigation in the integrated system reduces direct N2 O emissions without yield penalties. Emissions factors calculated by the IPCC are higher than the 0.85% and 0.29% loss of N applied in the conventional and integrated systems, respectively. Although the EF factors in the current study were not corrected for background emissions, it remains useful to compare EFs calculated from field data to the IPCC default EF that 1% of applied N is lost as N2 O. In the current case, using the IPCC methodology would overestimate emissions from these tomato-cropping systems. In addition, if the EFs in this study were corrected for background emissions, the value would be even lower. The conventional system falls into the range of EFs that previous studies have shown (0.5–3% of N applied) (Stehfest and Bouwman, 2006; Linquist et al., 2012). However, the lower cumulative emissions under the integrated management suggest that substrates for nitrification (e.g., NH4 + ) and denitrification (NO3 − and C) were available less often within this system. The lower emissions and EFs in the integrated system compared to other systems could be due to (1) patterns and duration of soil water saturation as related to irrigation type (Amali et al., 1997), (2) differential fertilizer placement (Veldkamp and Keller, 1997; Hultgreen and Leduc, 2003; Drury et al., 2006; Engel et al., 2010) and type (Velthof et al., 1996; Del Prado et al., 2006), (3) frequency (Burton et al., 2008) and rate (Dusenbury et al., 2008; Ma et al., 2010) of fertilizer application, or 4) more efficient plant N uptake as indicated by yield and partial factor productivity (Cassman et al., 2002) in the integrated system. In addition, long-term effects of management type on soil properties, such as increases in bulk density due to soil compaction by agricultural equipment (Ball et al., 2008; Beare et al., 2009) and variations in total soil C levels in cropped compared to fallow soils (Collins et al., 1992; Burger et al., 2005), could influence emissions under different management practices. In the current study, bulk density

was highest at the furrow position in both systems, especially in the conventional system. Total soil C was higher in the integrated system, potentially due to the incorporation of a winter cover crop in the rotation. 4.2. Management effects on N2 O emissions The temporal variability of N2 O fluxes is closely linked to the effect of agricultural managements on substrate availability, causing N2 O emissions to be event based (Zebarth et al., 2008; Kallenbach et al., 2010). It has been shown that irrigation events immediately following fertilization events (F + I) generally produce greater N2 O emissions than irrigation events alone (Scheer and Wassmann, 2008), suggesting that inorganic N availability, not soil moisture, is mostly limiting the production of N2 O. Scheer and Wassmann (2008) reported that N2 O was produced from NO3 − fertilizers but not NH4 + fertilizers, concluding that denitrification was the primary process at work in the irrigated cotton systems. In contrast, Garland et al. (2011) observed the greatest N2 O fluxes in a vineyard after a rain event that followed the incorporation of a leguminous cover crop, implying that water limited N2 O production and therefore likely favored nitrification. In the current study, we observed high fluxes following conventional system fertilization events (Fig. 3). Similar to the findings of Scheer and Wassmann (2008), there was a significant difference between N2 O emissions following F + I events and those following irrigation events alone. These results are in line with numerous studies reporting fertilizer-induced emissions (Bouwman et al., 1996; Cole et al., 1997; MacKenzie et al., 1998; Smith et al., 1998; Ma et al., 2010), especially with the addition of water via irrigation or rainfall (Van Kessel et al., 1993; Burger et al., 2005). Major N2 O effluxes in the integrated system were not associated with fertigation events. The largest management induced fluxes occurred after the shredding of crop residues at harvest. During the first rain event after harvest, the increase in soil moisture caused the second highest flux in the integrated system. Similarly, Barton et al. (2011) observed the highest N2 O fluxes from a semiarid grain-legume cropping system during postharvest and after rainfall events. They attributed these emissions to favorable soil conditions of high soil moisture, increased inorganic N and DOC concentrations, and the absence of plant competition for inorganic N sources. Similar conditions were likely present in our integrated system, especially due to the use of a mulcher at the time of harvest. Wet crop residues were shredded and left on the soil surface, providing moisture, inorganic N, and available C. Conditions likely favored net mineralization over net immobilization, such that inorganic N was available for nitrification under the low WFPS (30–40%) conditions and in the absence of plant competition. Correlations between N2 O emissions and soil chemical and physical properties were few across both management systems (Tables 3 and 4). This is likely due to the inability of soil and gas sampling methods to accurately depict the high spatial and temporal variability of N2 O substrates and emissions. Within different cropping rotations under three tillage managements, Jantalia et al. (2008) found no correlations between measured N2 O fluxes and %WFPS and soil inorganic N. They attributed the lack of relationship to N hotspots (Parkin, 1987), uncharacterized by the sampling strategy, where inorganic N was measured every 2 weeks from

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T.L. Kennedy et al. / Agriculture, Ecosystems and Environment 170 (2013) 16–27

Table 4 Pearson correlation between N2 O emissions and soil NO3 -N, NH4 -N, and dissolved organic carbon (DOC) by conventional management events and location. Missing values are not significant.a Event-related N2 O emissions

Position: lag

Irrigation 5/13/10–5/21/10

Berm: Linear Berm: 2 days Side: 2 days

NO3 −

NH4 +

p = 0.018 r = −0.60 p = 0.027 r = −0.57

Furrow: 2 days

p < 0.001 r = +0.93

Irrigation 7/25/10–8/1/10

Furrow: Linear

p < 0.001 r = +0.62

Sidedress Fertilization 5/2/10–5/13/10

Furrow: Linear

CAN17 Fertilization 6/2/10–6/12/10

Furrow: Linear

p < 0.001 r = +0.70

Harvest 8/26/10–9/3/10

Berm: Linear

p =0.009 r = −0.56 p < 0.001 r = +0.95 p < 0.001 r = +0.81 p = 0.039 r = −0.69 p = 0.046 r = +0.68

p = 0.011 r = +0.58

Furrow: Linear Furrow: 1 day Furrow: 2 days a

p < 0.001 r = +0.83 p = 0.034 r = −0.85

Berm: 1 day Side: 1 day

DOC

p < 0.001 r = +0.63 p = 0.011 r = −0.64

p = 0.006 r = +0.93

p = 0.004 r = +0.81

There were no correlations between percent water-filled pore space (WFPS) and N2 O emissions. Hence, the results for WFPS are not shown.

composite soil samples taken from 0 to 10 cm depth. Soil sampling frequency was greater in our study; however, it is impossible to take sequential, destructive soil samples from the same location or from within the gas chamber. Soil measurements taken every 2 days depict soil conditions at temporal scales much larger than the microbial environments producing N2 O. When microbial N transformation rates are high, NH4 + and NO3 − pools turn over several times a day, making N availability difficult to quantify (Jackson et al., 2008) (Table 5). In concert with a scarcity of correlations, the overall trends observed in soil inorganic N concentrations and N exposures were not always identical to the variability of N2 O emissions across functional locations. Nitrogen exposures were low at the conventional

furrow position (Fig. 7), whereas furrow N2 O emissions were high (Fig. 4). Similarly, conventional berm N concentrations and exposures were high, but berm N2 O emissions were relatively low. The pronounced peak of AE at the integrated berm position was not reflected in N2 O emissions at that location (Figs. 6a and 7). The paucity of correlations and spatial relationships between soil chemical and physical properties and gas fluxes demonstrate that N2 O emissions result from a complex interaction between driving variables, which occur on different spatial and temporal scales (Baggs, 2008; Ma et al., 2010), and through multiple microbial pathways (Pathak, 1999). In addition, other driving factors could be influential, such as soil nitrite, which was shown to be important in another California tomato field (Venterea and Rolston, 2000).

Table 5 Pearson correlation between N2 O emissions and soil NO3 -N, NH4 -N, and percent water-filled pore space (WFPS) by integrated management events and location. Missing values are not significant.a Event-related N2 O emissions

Position: Lag

Fertigation 6/7/10–6/22/10

Berm: 1 day

NO3 − -N

NH4 + -N p = 0.008 r = +0.60 p = 0.003 r = +0.66

Berm: 2 days Furrow: 1 day Fertigation 4/23/10–4/30/10

Berm: 1 day

p = 0.016 r = +0.68 p = 0.039 r = −0.60 p = 0.010 r = −0.71

p = 0.026 r = +0.63

Furrow: 1 day Fertigation 6/22/10–6/30/10

Berm: 1 day Berm: 2 days

p = 0.011 r = +0.79 p = 0.029 r = +0.72

Side: 2 days Furrow: 2 days a

WFPS

p = 0.007 r = +0.82 p = 0.003 r = +0.86

p = 0.010 r = +0.79

There were no correlations between dissolved organic carbon (DOC) and N2 O emissions. Hence, the results for DOC are not shown.

T.L. Kennedy et al. / Agriculture, Ecosystems and Environment 170 (2013) 16–27

4.3. Conventional management and emissions of N2 O During conventional management events, significant relationships between N2 O emissions and soil chemical and physical properties were dominated by positive correlations between soil NH4 + levels and N2 O emissions (Table 3). Following irrigation, negative correlations were observed between NO3 − and N2 O with one and two day lags; positive correlations between emissions and NO3 − were only observed following harvest, with a lag between N availability and N2 O efflux. These correlations suggest that nitrification is the dominant source of N2 O in this system. Burger et al. (2005) reported the highest emissions in association with high NH4 + concentrations in organic and conventionally managed tomatoes. Similarly, NH4 + pools were high following conventional fertilization and irrigation events. The time lag between NH4 + availability and N2 O efflux suggests that nitrification was delayed and occurred as the soil dried out, where initially high WFPS inhibited nitrification. Denitrification could have occurred briefly, at the onset of furrow irrigation, when soil moisture contents were high in the furrow; but rather N2 than N2 O was most likely emitted. During periods of significant N2 O emissions in an irrigated sorghum cropping system of Arizona, Welzmiller et al. (2008) identified that N2 rather than N2 O was the primary product of denitrification. They also reported that soil NO3 − and soil organic C were weakly correlated with N2 O fluxes, concluding that these factors alone did not limit N2 O production. Rather, N2 O fluxes occurred in the sorghum system at a WFPS greater than 55%. In our study, the absence of correlations between %WFPS and N2 O emissions across the seven days following irrigation events suggests that soil moisture conditions were either too high or too low for N2 O emissions derived from denitrification. Hence, moisture rather than soil inorganic N or available C most likely limited denitrification. When considered in concert with soil moisture conditions, the form of N fertilizer can strongly influence N2 O emissions derived from denitrification (Del Prado et al., 2006). During wet conditions, Velthof et al. (1996) found that N2 O emissions were lower from NH4 + fertilizers than NO3 − fertilizers from grassland, attributing greater N loss to denitrification of the NO3 − fertilizer or preferred microbial immobilization of NH4 + rather than NO3 − . In our conventional system, high WFPS (55–100%) and increased bulk density at the furrow, most probably favored anoxic microsites primed for denitrification of the high NO3 − , low NH4 + CAN-17 fertilizer (Ruser et al., 2006). As a highly mobile anion, NO3 − likely moved toward the seedbed as indicated by high soil NO3 − concentrations at the side and berm positions. Soil moisture contents at these locations, however, likely favored nitrification over denitrification. Ammonium from fertilizer or mineralization of soil organic matter would be available for nitrification in the furrow as the soil dried out after water run fertilizations. At anaerobic microsites where denitrification was possible, high NO3 − levels potentially decreased the N2 O reduction to N2 favoring a higher proportion of N2 O to N2 (Blackmer and Bremner, 1978); however, plant uptake of NO3 − and soil water would limit the duration of denitrification conditions, making high N2 O effluxes from incomplete NO3 − reduction brief. In the conventional system, NO3 − applied in excess of plant uptake rates was lost during brief yet high N2 O effluxes that were largely limited by moisture availability; short-lived N2 fluxes likely occurred as well, lowering fertilizer use efficiency. 4.4. Integrated management and emissions of N2 O In the integrated system, positive N2 O correlations were equally associated with soil NO3 − and NH4 + levels (Table 4). The spatial position of these correlations suggests the occurrence of nitrification at the berm and side and denitrification at the berm and furrow. Denitrification potential is likely high at the emitters of the

25

drip system due to WFPS being higher than 60% within a few centimeters of the drip tape (Kallenbach et al., 2010; Sánchez-Martín et al., 2010). Nevertheless the concentration of NO3 − would be low because: (1) the form of N in the fertilizer is predominately NH4 + (e.g., UN-32) and (2) NO3 − , if present, would move quickly from the point source as a highly mobile anion. The UN-32 supplies a greater proportion of NH4 + to a zone with high WFPS, which is more conducive to denitrification. At the microbial scale, the NH4 + fertilizer would first require nitrification to become a substrate for denitrification. Another delay in NH4 + availability would result from immobilization as microbes preferentially take up NH4 + to NO3 − (Jackson et al., 2008). At the plant scale, the NH4 + fertilizer is delivered to the active root zone, providing an accessible source of NH4 + for plant uptake. With fertigation events delivering small amounts of N during periods of rapid crop growth, the tomato plants would effectively compete with nitrifying bacteria for available NH4 + , reducing the amount of substrate available for nitrification in the upper layers of the soil profile. While tomatoes have demonstrated a slight preference for NO3 − over NH4 + (Smart and Bloom, 1988), the crop appears to depend on NO3 − at depth and NH4 + at the soil surface (Jackson and Bloom, 1990). In summary, the N2 O produced is likely sourced from nitrification of the high NH4 + fertilizer that has not been oxidized to NO3 − due to anaerobic soil conditions, but N2 O fluxes are generally low because of the effective plant uptake of the NH4 + . As indicated above, NO3 − sourced from the fertilizer or from nitrification that is not immediately denitrified in conditions of high WFPS around the emitter, would move quickly out of that zone. The deposition of NO3 − at the wetting front (Li and Zhang, 2003) would position the substrate for denitrification in soil moisture conditions more conducive to nitrification; in addition, furrow WFPS did not exceed 60% except on three sampling days. In terms of the crop plant, NH4 + can inhibit plant uptake of NO3 − (Siddiqi et al., 2002), increasing the likelihood that NO3 − would move out of the denitrifying zone around the emitter. Plant roots along the wetting front would take up available soil NO3 − , competing with denitrifying microbes in the event that WFPS was suitable for NO3 − reduction. In unfertilized conditions, Jackson and Bloom (1990) found that tomatoes slightly preferred NO3 − to NH4 + . Hence, at the microbial scale, the substrates for nitrification and denitrification are asynchronous in time and space. Ammonium is available at the emitter where conditions are best suited for denitrification, and NO3 − accumulates at the wetting front, where soil moisture conditions are most favorable for nitrification. Aiming to reduce the number of tractor passes and CO2 emissions, a vine shredder was added to the tomato harvester in the integrated system in order to combine two management events. This study demonstrated that the mulching of crop residues at the time of harvest added moisture, N, and C to the soil, producing a large peak in N2 O and negating the reduction in CO2 emissions from one less tractor pass. Additional studies could investigate the effect of (1) the amount of chopping and subsequent size of mulch or (2) immediate incorporation of mulched residues into the soil, on N2 O emissions. The findings of this study could be elaborated on by investigating the effect of nitrification inhibitors in SDI and FI systems on N2 O emissions. For example, if nitrification was the dominant source of N2 O production in the integrated system, slowing rates of nitrification could potentially reduce N loss further. Given the higher cost of specialized fertilizers, further reduction in N2 O loss may not be significant enough, and therefore not cost effective, like that of drip versus furrow irrigation. A nitrification inhibiting fertilizer may be more economically advantageous in FI systems where transitioning over to SDI is not feasible. Fertilizers with nitrification inhibitors may be effective in reducing N2 O emissions in both production systems, but may not have a yield benefit.

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5. Conclusions Based on the findings of this study, we conclude that improvements in fertilizer and water use efficiency by better coordination of N availability and crop demand can reduce N loss via N2 O emissions. The fact that conventional fertilization events produced significantly higher N2 O emissions than irrigation events indicate that water and N were available in excess of crop demand; subsequently enabling N to be lost as N2 O in the conventional system. Low N2 O emissions and high tomato yields in the integrated system suggest improved synchrony between N availability and crop demand and more efficient plant N uptake, which result in reduced N loss as N2 O. In both management systems, the first precipitation event in the fall contributed considerable N2 O emissions and therefore should be included in the calculation of N2 O budgets for agricultural systems, especially in regions with a distinct dry summer and wet fall/winter. On-farm research has led to the identification of a feasible improved technology that increases resource use efficiencies and crop yield, while reducing N2 O emissions. While only one year of data was presented here, a similar study was conducted in 2011 measuring N2 O emissions from irrigation and fertilization events in both management systems; lower N2 O emissions were observed in the integrated system during major events (Kennedy et al., submitted). Nevertheless, more years of data would be beneficial to strengthen the conclusion that SDI with fertigation is effective in reducing N2 O emissions compared to a conventional tomato production system. In general, more quantitative, on-farm evaluations of improved technologies and measurements of N losses are needed to provide reliable estimates of the effect of improvements in fertilizer and water use on GHG emissions over the long term. Acknowledgements The Packard Foundation generously funded this study and made this project possible. We greatly appreciate the time, effort, and generosity of the Yolo County farmers who graciously let us use their fields for this study, as well as Robert Rousseau, Ben Wilde, Engil Isadora Pujol Pereira, and Julian Herszage for their help in the field and lab. References Amali, S., Rolston, D.E., Fulton, A.E., Hanson, B.R., Phene, C.J., Oster, J.D., 1997. Soil water variability under subsurface drip and furrow irrigation. Irrigation Sci. 17, 151–155. Amos, B., Arkebauer, T.J., Doran, J.W., 2005. Soil surface fluxes of greenhouse gases in an irrigated maize-based agroecosystem. Soil Sci. Soc. Am. J. 69, 387–395. Baggs, E.M., 2008. A review of stable isotope techniques for N2 O source partitioning in soils: recent progress remaining challenges and future considerations. Rapid Commun. Mass Spectromet. 22, 1664–1672. Ball, B., Crichton, I., Horgan, G., 2008. Dynamics of upward and downward N2 O and CO2 fluxes in ploughed or no-tilled soils in relation to water-filled pore space compaction and crop presence. Soil Tillage Res. 101, 20–30. Barton, L., Butterbach-Bahl, K., Kiese, R., Murphy, D.V., 2011. Nitrous oxide fluxes from a grain-legume crop (narrow-leafed lupin) grown in semiarid climate. Global Change Biol. 17, 1153–1166. Beare, M., Greogorich, E., St-Georges, P., 2009. Compaction effects on CO2 and N2 O production during drying and rewetting of soil. Soil Biol. Biochem. 41, 611–621. Blackmer, A.M., Bremner, J.M., 1978. Inhibitory effect of nitrate on reduction of N2 O to N2 by soil microorganisms. Soil Biol. Biochem. 10, 187–191. Bouwman, A., 1996. Direct emission of nitrous oxide from agricultural soils. Nutr. Cycl. Agroecosyst. 46, 53–70. Burger, M., Jackson, L.E., Lundquist, E.J., Louie, D., Miller, R.L., Rolston, D.E., Scow, K.M., 2005. Microbial responses and nitrous oxide emissions during wetting and drying of organically and conventionally managed soil under tomatoes. Biol. Fertil. Soils 42, 109–118. Burton, D.L., Zebarth, B.J., Gillam, K.M., MacLeod, J.A., 2008. Effect of split application of fertilizer nitrogen on N2 O emissions from potatoes. Can. J. Soil Sci. 88, 229–239. Cassman, K., Dobermann, A., Walters, D., 2002. Agroecosystems nitrogen-use efficiency, and nitrogen management. AMBIO J. Hum. Environ. 31, 132–140.

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