Almond Orchards Research Articles

Contrasted Impact of Land Abandonment on Soil Erosion in Mediterranean Agriculture Fields

Abstract Abandonment of agricultural land results in on- and off-site consequences for the ecosystem. In this study, 105 rainfall simulations were carried out in agriculture lands of the Mediterranean belt in Spain (vineyards in Malaga, almond orchards in Murcia, and orange and olive orchards in Valencia) and in paired abandoned lands to assess the impact of land abandonment on soil and water losses. After abandonment, soil detachment decreased drastically in the olive and orange orchards, while vineyards did not show any difference and almond orchards registered higher erosion rates after the abandonment. Terraced orchards of oranges and olives recovered a dense vegetation cover after the abandonment, while the sloping terrain of almond orchards and vineyards enhanced the development of crusts and rills and a negligible vegetation cover resulted in high erosion rates. The contrasted responses to land abandonment in Mediterranean agricultural lands suggest that land abandonment should be programmed and managed with soil erosion control strategies for some years to avoid land degradation.

Compatibility of organic farming treatments against Monosteira unicostata with non-target arthropod fauna of almond trees canopy

Field trials had shown that 1-2 applications of kaolin and potassium salts of fatty acids combined with thyme essential oil (PSTEO) reduced the abundance of the lace bug Monosteira unicostata (Mulsant & Rey) (Hemiptera: Tingidae), an important pest of almond trees in the Mediterranean region. These products could be useful for the control of this pest in organic production of almonds, but higher number of applications could be necessary. However, the possible detrimental effects on the almond orchard ecosystem should be evaluated. In the present work, the effects observed on the non-target arthropod fauna of the almond trees canopy in those field assays are shown. First, a comprehensive report of the non-target arthropod fauna of the almond tree is provided. Regarding natural enemies, most of the predatory arthropods captured were spiders belonging to different families like Salticidae, Thomisidae, Philodromidae, Theridiidae, Araneidae or Oxyopidae. Other predatory families that appeared in significant numbers were Chrysopidae, Anthocoridae, Aeolothripidae, Coccinellidae, Phytoseiidae, Erythraeidae or Forficulidae. Among parasitoids, the most abundant families were Eulophidae, Scelionidae and Dryinidae. Kaolin reduced the abundance of natural enemies and other non-target arthropods as well as their diversity and number of species. On the contrary, PSTEO only produced a slight reduction in the number of natural enemies, whereas no effect was found on the diversity and species richness. These effects were observed despite the reduced number of applications, so greater effect is expected if its frequency is increased in order to achieve an efficient control of M. unicostata.

Nut set evaluation in inter-specific almond × peach backcross progenies for self-compatibility selection in almond breeding programme

Self-compatibility was examined in 22 almond samples that were selected based on their generic and physiological characteristics. Polymerase chain reaction was also done with consensus primers. Pollen tube growth and nut set analyses were carried out on partially bagged limbs after self-pollination treatment. In spite of the fact that all genotypes had the S f-allele and pollen tube growth were similar, nut set exhibited an alternate year characteristic that ranged from 16.2% to a maximum of 24.7%. This was the highest treatment after self-pollination. Difference existed between genotypes could be due to the genetic constitution of each genotype, is explored in set that is diminished by inbreeding, which diminish gene aggregation in different members of a progeny. In addition, flower morphology could also influence set in bagged limbs. Furthermore, with regard to flower sterility and bud density in relation to both pollination success and ecological condition availability, these traits must be considered when examining production in self-compatible almond orchards, e.g.,—the impacts accompanying inbreeding and effective autonomous self-pollination.

Notes on life history and field efficiency of Cybocephαlus fodori predator of Quαdrαspidiotus perniciosus in Northern Greece

Sampling results during 1978-1979 and 1981-1982 in two heavily-infested almond orchards in Northern Greece showed a high mortality of the San Jose scale. A considerable part of this mortality was attributable to indigenous entomophagous insects, especially predators. Among them, Cybocephalus fodori Endrody-Younga (Coleoptera: Nitidulidae) appeared to be the key species. Field observations showed that C. fodori hibernates as an adult, forming aggregations. C. fodori reared in outdoor cages showed full adult activity by end March. Given an abundant supply of prey, the insect develops two complete and a third partial generations per year. Under constant conditions (25±2° C, 65±5% R.H. and 16 hrs light per day), the development of its three larval instars was completed in about 19 days and the pupal stage lasted about 10 days. On average, 68 Quadraspidiotus perniciosus Comstock (Homoptera: Diaspididae) adult females were partly consumed per larva of C. fodori. Individually deposited eggs and young larvae were found under the covers of the San Jose scales.

Nathrius brevipennis (Mu1sant 1839) (Col.: Cerambycidae) in Greece

In the spring of 1975, the trees of an almond orchard [Prunus dulcis (Miller) D.A. Webb] in Avliotes Corfu, were found to be severely infested by a wood borer insect in the larval stage. Samples of infested with larvae twigs, preserved in the laboratory, and in the same year in September a number of adults emerged. After identification this insect was found to be Nathrius (Leptidea) brevipennis (Mulsant 1839). This species caused important damage in the almond trees of the Avliotes area of Corfu during the years 1975-1976. In 1977, N. brevipennis was found in Ioannina county on walnut trees (Juglans regia L.) and in 1980, it was found in Metsovo area on almond trees.

Evapotranspiration Estimate over an Almond Orchard Using Landsat Satellite Observations

California growers face challenges with water shortages and there is a strong need to use the least amount of water while optimizing yield. Timely information on evapotranspiration (ET), a dominant component of crop consumptive water use, is critical for growers to tailor irrigation management based on in-field spatial variability and in-season variations. We evaluated the performance of a remote sensing-based approach, Mapping Evapotranspiration at high Resolution with Internalized Calibration (METRIC), in mapping ET over an almond orchard in California, driven by Landsat satellite observations. Reference ET from a network of weather stations over well-watered grass (ETo) was used for the internal calibration and for deriving ET at daily and extended time period, instead of alfalfa based reference evapotranspiration (ETr). Our study showed that METRIC daily ET estimates during Landsat overpass dates agreed well with the field measurements. During 2009–2012, a root mean square error (RMSE) of 0.53 mm/day and a coefficient of determination (R2) of 0.87 were found between METRIC versus observed daily ET. Monthly ET estimates had a higher accuracy, with a RMSE of 12.08 mm/month, a R2 of 0.90, and a relatively small relative mean difference (RMD) of 9.68% during 2009–2012 growing seasons. Net radiation and Normalized Difference Vegetation Index (NDVI) from remote sensing observations were highly correlated with spatial and temporal ET estimates. An empirical model was developed to estimate daily ET using NDVI, net radiation (Rn), and vapor pressure deficit (VPD). The validation showed that the accuracy of this easy-to-use empirical method was slightly lower than that of METRIC but still reasonable, with a RMSE of 0.71 mm/day when compared to ground measurements. The remote sensing based ET estimate will support a variety of State and local interests in water use and irrigation management, for both planning and regulatory/compliance purposes, and it provides the farmers observation-based guidance for site-specific and time-sensitive irrigation management.

Grower Analysis of Organic Matter Amendments in California Orchards.

Organic matter amendments supply crop nutrients and enhance soil health, yet information specific to orchards is lacking. A survey was conducted to analyze use of these materials by California almond [ (Mill.) D.A. Webb] growers. Significant differences were observed for benefits, concerns, and accessibility to manure and green waste sources and between users and nonusers. Use patterns were significantly influenced by heavy and light users, farm size, and geographic region. Enhanced soil biology was the main benefit attributed to organic matter amendments by both users and nonusers. Nonusers showed greater concern for food safety compared to users, and all growers reported greater concern for food safety from manure. The greatest adoption of organic matter amendments occurred on small farms (≤170 ha) located in the north San Joaquin Valley in California. Greater accessibility to manure correlated with presence of dairy farms. Poorer accessibility ratings by nonusers suggest access is a barrier to adoption, as opposed to nonusers having an undesirable view of the practice. Common management included applying organic matter amendments during tree dormancy from manure sources in composted forms with no-till. Heavy users on small farms exhibited the greatest year-to-year consistency and were more flexible with selection of sources and diverse in application methods. Large farms (>170 ha) were less likely to use organic matter amendments every year and less likely to apply them on all their farm area. This study identifies a number of strategies to fill knowledge gaps, increase practice awareness, and overcome barriers to adoption.

Wildflower Plantings Do Not Compete With Neighboring Almond Orchards for Pollinator Visits.

The engineering of flowering agricultural field borders has emerged as a research and policy priority to mitigate threats to pollinators. Studies have, however, rarely addressed the potential that flowering field borders might compete with neighboring crops for pollinator visits if they both are in bloom at the same time, despite this being a concern expressed by growers. We evaluated how wildflower plantings added to orchard borders in a large (512 ha) commercial almond orchard affected honey bee and wild bee visitation to orchard borders and the crop. The study was conducted over two consecutive seasons using three large (0.48 ha) wildflower plantings paired with control orchard borders in a highly simplified agricultural landscape in California. Honey bee (Apis mellifera L.) and wild bee visitation to wildflower plots were at least an order of magnitude higher than to control plots, but increased honey bee visitation to wildflower plots did not lead to any detectable shifts in honey bee visitation to almond flowers in the neighboring orchard. Wild bees were rarely observed visiting almond flowers irrespective of border treatment, indicating a limited short-term potential for augmenting crop pollination using wild bees in highly simplified agricultural landscapes. Although further studies are warranted on bee visitation and crop yield from spatially independent orchards, this study indicates that growers can support bees with alternative forage in almond orchards without risking competition between the wildflower plantings and the crop.

Season and location–specific nitrous oxide emissions in an almond orchard in California

Almonds are an important commodity in California and account for around 15% of the state’s fertilizer nitrogen (N) consumption. Motivated by strong correlations typically observed between fertilizer N inputs and emissions of the potent greenhouse gas and ozone depleting molecule nitrous oxide (N2O), this study aimed to characterize spatial and temporal patterns in N2O emissions in an almond orchard under typical agronomic management. N2O fluxes were measured for a total of 2.5 years, including 3 growing seasons and 2 dormant seasons. Measurements targeted two functional locations, defined as tree rows and tractor rows. In conjunction with the flux measurements, we determined driving variables including soil ammonium (NH4+) and nitrate (NO3−), dissolved organic carbon (DOC), soil water-filled pore space (WFPS), soil pH, air temperature and precipitation. Cumulative annual N2O emissions were low (0.65 ± 0.07 and 0.53 ± 0.19 kg N2O–N ha−1 year−1 in year 1 and 2, respectively), likely due to the coarse soil texture and microject sprinkler irrigation and fertigation system. Emission factors (EF), conservatively calculated as the ratio of N2O emitted to fertilizer N applied, were 0.25 ± 0.03% and 0.19 ± 0.07% for year 1 and 2, respectively, which is below the IPCC EF range of 0.3–3%. Correlation analyses between N2O and driving variables suggested that overall N2O production was limited by microbial activity and nitrification was likely the major source process, but specific drivers of N2O emissions varied between seasons and functional locations.

Development of a Continuous Leaf Monitoring System to Predict Plant Water Status

Abstract. There is increased demand for irrigation scheduling tools that support effective use of the limited supply of irrigation water. An efficient precision irrigation system requires water to be delivered based on crop needs by measuring or estimating plant water stress. Leaf temperature is a good indicator of water stress. In this study, a system was developed to monitor leaf temperature and microclimatic environmental variables to predict plant water stress. This system, called the leaf monitor, monitored plant water status by continuously measuring leaf temperature, air temperature, relative humidity, ambient light, and wind conditions in the vicinity of a shaded leaf. The system also included a leaf holder, a solar radiation diffuser dome, and a wind barrier for improved performance of the unit. Controlled wind speed and consistent light conditions were created around the leaf to reduce the effect of nuisance variables on leaf temperature. The leaf monitor was incorporated into a mesh network of wireless nodes for sensor data collection and remote valve control. The system was evaluated for remote data collection in commercial orchards. Experiments were conducted during the 2013 and 2014 growing seasons in walnut () and almond () orchards. The system was found to be reliable and capable of providing real-time visualization of the data remotely, with minimal technical problems. Leaf monitor data were used to develop modified crop water stress index (MCWSI) values for quantifying plant water stress levels. Keywords: Almonds, CWSI, Infrared sensor, Irrigation scheduling, Leaf temperature, Nut crops, Plant water stress, Precision irrigation, Stem water potential, Walnuts, Wireless mesh network.

Spur behavior in Almond trees (Prunus dulcis [Mill.] DAWebb): effects of flowers, fruit, and “June drop” on leaf area, leaf nitrogen, spur survival and return bloom

Effects of flowering, fruiting, and fruit drop on leaf area, leaf nitrogen, probability of spur survival and return bloom were studied in an 11 year-old highly productive almond orchard. Six different spur categories (spurs with no fruit, spurs with flowers removed at full bloom, spurs with fruit removed at 14days after full bloom (DAFB), spurs with fruit removed at 28 DAFB, spurs with one fruit, spurs with two fruits) were labeled on a total of forty-eight trees, variety Nonpareil. Spurs were tracked from winter 2011 to spring 2012. Spur survival and spur return bloom data were collected by visually inspecting each labeled spur in 2012. Results showed that the competition for nitrogen between fruit and leaves at the spur level occurs during the whole season. Fruiting spurs had 50% less leaf area than non-fruiting spurs because they had smaller and less leaves. Spurs whose fruit was removed early in the season reached similar survival probabilities to spurs that had no fruit throughout the season. However, the return bloom probabilities were lower on spurs whose fruit was removed than in non-fruiting spurs. Two fruiting spurs had the lowest chance to either survive or return bloom.

Effect of observation scale on remote sensing based estimates of evapotranspiration in a semi-arid row cropped orchard environment

Understanding in detail the spatial distribution of evapotranspiration (ET) in row cropped fruit production areas with diverse water requirements is vital for monitoring water use and efficient irrigation scheduling. Spatially distributed ET for these environments can be estimated using remote sensing (RS). However, the computation of RS based ET under such conditions is complicated because of the complex parameterizations that are required to derive ET for the mixed pixels comprising of bare soil and well-watered plants typical of row cropped areas. Also, the parameterization of these processes is not scale invariant, owing to change in the percentage of vegetation cover in the mixed pixels across remote sensing observation scales. In this study, our main objectives were (1) to isolate and evaluate the effect of varying spatial scales (comparable to canopy sizes and larger) of the remote sensing data on ET estimates; and (2) provide an operational method for estimating remote sensing based ET for row cropped conditions. ET was computed using an empirical technique (S-SEBI: Simplified-Surface Energy Balance Index Algorithm) for almond and pistachio orchards from remote sensing imagery collected at a scale comparable to the canopy sizes of the trees (5.8 and 7.2 m) and a scale that was much larger than the canopy size (120 m) using the MASTER and Landsat sensors, respectively. In order to account for the effect of mixed pixels, a Normalized Difference Vegetation Index based correction factor was applied to the derived ET values and the results averaged for different fields were validated with Penman–Monteith based ET estimates. It was found that the corrected mean ET estimates at 120 m were in agreement with the Penman–Monteith based ET estimates (RMSEaverage = 0.12 mm/h), whereas they were underestimated at the finer resolutions. Our results indicated that a remote sensing pixel resolution comparable to the row spacing and smaller and comparable to the canopy size overestimated the land surface temperature and consequently, underestimated ET when using operational models that do not account for vegetation and soil temperature separately. The results of the application of the NDVI correction factor indicates that good spatial estimates of crop ET can be made for crops growing in orchards using simple ET models that require minimal data and freely available Landsat imagery. These findings are very encouraging for the regular monitoring of crop health and effective management of irrigation water in highly water stressed agricultural environments.

Tree centric localisation in almond orchards

Tree centric localisation in almond orchards

Establishing an almond water production function for California using long-term yield response to variable irrigation

The consumptive use of water by almond trees in California has become a controversial topic due to a worsening drought that began in 2011. The research reported herein was designed to provide information that could be used in irrigation decision-making including how to evaluate the risks/rewards of seeking/acquiring additional supplies in a drought. Ten irrigation levels ranging from 1000 to 1350 mm per season were applied in a mature southern San Joaquin Valley ‘Nonpareil’ almond orchard over a 5-year period. Nut load was invariant over the range of irrigation regimes with no evidence of any sustained negative impacts. Individual kernel weight was reduced commensurate with the level of deficit irrigation; the lowest irrigation resulted in a 15% lighter kernel. Maximum yield (3900 kg/ha) was achieved with 1250 mm of applied water. There was statistically significant tree stress in the 1250-mm application regime with midday shaded leaf water potential about 0.2 MPa more negative than trees receiving additional water. Maximum marginal water productivity (WPM) was 0.30 kg/m3 at 1080 mm of applied water, falling to zero at 1290 mm. With almond prices of $8/kg, this is equivalent to water valued at $2/m3. Estimated annual statewide almond orchard evapotranspiration based on this study is 1100–1350 mm which is about 25% higher than estimates made four decades ago. However, yields have increased in the 250% range since that time. This tenfold difference between yield and evapotranspiration means that WPM is much higher now than in the past, and at current prices, it demonstrates that almond trees are suited for high water cost areas.

Soil carbon sequestration rates under Mediterranean woody crops using recommended management practices: A meta-analysis

Mediterranean woody crops, such as olive and almond farming, and vineyards are usually cultivated in soils low in organic matter, with limited water availability and frequently on medium to steep slopes. Therefore, when conventionally cultivated, soils of these cropping systems are net sources of CO2 (throughout soil erosion and organic carbon mineralization). A promising option to sequester carbon (C) in these cropping systems is the implementation of recommended management practices (RMPs), which include plant cover in the inter-row area, minimum or no tillage and off- and on-farm organic matter amendments. However, the effects of RMPs on soil organic carbon (SOC) stocks in these cropping systems are widely overlooked, despite the critical importance of estimating their contribution on CO2 emissions for policy decisions in the agriculture sector in Mediterranean regions. We therefore conducted a meta-analysis to derive a C response ratio, soil C sequestration rate and soil C sequestration efficiency under RMPs, compared to conventional management of olive and almond orchards, and vineyards (144 data sets from 51 references). RMPs included organic amendments (OA), plant cover (CC) and a combination of the two (CMP). The highest soil C sequestration rate (5.3tCha−1 yr−1) was observed following the application OA in olive orchards (especially after olive mill pomace application), whereas CC management achieved the lowest C sequestration rates (1.1, 0.78 and 2.0tCha−1 yr−1, for olive orchards, vineyards and almond orchards, respectively). Efficiency of soil C sequestration was greater than 100% after OA and CMP managements, indicating that: i) some of the organic C inputs were unaccounted for, and ii) a positive feedback effect of the application of these amendments on SOC retention (e.g. reduction of soil erosion) and on protective mechanisms of the SOC which reduce CO2 emissions. Soil C sequestration rate tended to be highest during the first years after the change of the management and progressively decreased. Studies performed in Mediterranean sub-climates of low annual precipitation had lower values of soil C sequestration rate, likely due to a lower biomass production of the crop and other plant cover. Soil C sequestration rates in olive farming were much higher than that of vineyards, mainly due to the application of higher annual doses of organic amendments. The relatively high sequestration rate combined with the relative large spatial extent of these cropping system areas suggests that the adoption of RMPs is a sustainable and efficient measure to mitigate climate change.

Yield in almond is related more to the abundance of flowers than the relative number of flowers that set fruit

Almond tree yield is a function of the number of flowers on a tree and the percentage of flowers that set fruit. Almonds are borne on spurs (short proleptic shoots that can have both leaves and flowers). Almond tree spur dynamics research has documented that previous year spur leaf area is a predictive parameter for year-to-year spur survival, spur flowering and to a lesser extent spur fruiting, while previous year fruit bearing has a negative impact on subsequent year flowering. However, a question remained about whether yields are more dependent on flower numbers or relative fruit set of the flowers that are present. The aim of the present work was to compare the importance of flower abundance with that of relative fruit set in determining the productivity of a population of tagged spurs in almond trees over a 6-year period. Overall tree yield among years was more sensitive to total number of flowers on a tree rather than relative fruit set. These results emphasize the importance of maintaining large populations of healthy flowering spurs for sustained high production in almond orchards.

Estimating Nitrate Leaching to Groundwater from Orchards: Comparing Crop Nitrogen Excess, Deep Vadose Zone Data‐Driven Estimates, and HYDRUS Modeling

Core Ideas Leaching below the root zone is estimated based on eight sites of intensive vadose zone monitoring. Across methods N losses estimated at the annual orchard scale were in the same order of magnitude. Simple N mass balance provided a good proxy of the orchard scale annual N accumulation in the soil Under current BMP N load to groundwater is likely in the range of 60 to 100 kg N ha–1. Large spatial and temporal variability in water flow and N transport dynamics poses significant challenges to accurately estimating N losses form orchards. A 2‐yr study was conducted to explore nitrate (NO3−) leaching below the root zone of an almond [Prunus dulcis (Mill.) D. A. Webb] orchard. Temporal changes in water content, pore water NO3− concentrations and soil water potential were monitored within and below the root zone to a soil depth of 3 m at eight sites, which represented spatial variations in soil profiles within an almond orchard in California. Orchard monthly average NO3− concentrations below the root zone ranged from 225 to 710 mg L−1 with mean annual concentration of 468 and 333 mg L−1 for the 2014 and 2015 growing seasons, respectively. Despite the huge variability in pore water NO3− concentration between sites, the larger spatiotemporal scale N losses estimated at the annual orchard scale from surface N mass balance, vadose zone based water and N mass balance, flow calculations, and HYDRUS modeling were all on the same order of magnitude (80–240 kg N ha−1 yr−1). All methods indicated that most of the N losses occur early in the growing season (February–May) when fertilizer is applied to wet soil profiles. Simple mass balance (i.e., N load applied minus N load removed) provided a good proxy of the annual N accumulation in the soil profile at the orchard scale. Reduction of N losses at the orchard scale would require alternative fertigation and irrigation practices to decrease the difference between the N load removed and the N load applied to orchards.

Mapping almond orchard canopy volume, flowers, fruit and yield using lidar and vision sensors

This paper present a mobile terrestrial scanning system for almond orchards, that is able to efficiently map flower and fruit distributions and to estimate and predict yield for individual trees. A mobile robotic ground vehicle scans the orchard while logging data from on-board lidar and camera sensors. An automated software pipeline processes the data offline, to produce a 3D map of the orchard and to automatically detect each tree within that map, including correct associations for the same trees seen on prior occasions. Colour images are also associated to each tree, leading to a database of images and canopy models, at different times throughout the season and spanning multiple years. A canopy volume measure is derived from the 3D models, and classification is performed on the images to estimate flower and fruit density. These measures were compared to individual tree harvest weights to assess the relationship to yield. A block of approximately 580 trees was scanned at peak bloom, fruit-set and just before harvest for two subsequent years, with up to 50 trees individually harvested for comparison. Lidar canopy volume had the strongest linear relationship to yield with R2=0.77 for 39 tree samples spanning two years. An additional experiment was performed using hand-held photography and image processing to measure fruit density, which exhibited similar performance (R2=0.71). Flower density measurements were not strongly related to yield, however, the maps show clear differentiation between almond varieties and may be useful for other studies.

Characterization and Pathogenicity of Botryosphaeriaceae Species Isolated from Almond Trees on the Island of Mallorca (Spain).

In this study, 31 almond orchards with trees showing severe decline symptoms were surveyed from 2009 to 2014 on the island of Mallorca (Spain). In all, 45 Botryosphaeriaceae isolates were collected and characterized based on phenotypical features and comparisons of DNA sequence data of the nuclear ribosomal DNA-internal transcribed spacer region and elongation factor 1-α gene. Five species were identified as Diplodia olivarum, D. seriata, Neofusicoccum luteum, N. mediterraneum, and N. parvum. Pathogenicity tests were performed on four cultivars ('Pons', 'Vivot', 'Jordi', and 'Ferragnes') under field conditions for two consecutive years (2013 to 2014), and confirmed that all five species cause canker and dieback of almond, with Neofusicoccum spp. more virulent than Diplodia spp. in both years. Jordi was less sensitive to fungal infection in 2013. First reports from almond in Spain include N. mediterraneum and N. luteum.

Effects of drip fertigation frequency and N-source on soil N2O production in almonds

Little work has been done on nitrous oxide (N2O) emissions from nitrogen-fertigation systems, despite the greater degrees of control allowed over fertilizer N fate. More fertigation users are adopting high-frequency (HF) N application schedules to improve nitrogen use efficiency and reduce nitrate (NO3−) leaching. The possibility is raised that this practice may also lower N2O emissions, through effects on soil N concentrations and soil microbial populations. At the same time, NO3-based N-fertilizers are frequently selected for use in HF systems, where they should be more immediately available to plants than ammoniacal fertilizers. This choice of N-source is likely to affect N2O emissions. We monitored surface emissions of N2O in an almond orchard (Prunus dulcis [Mill.] DA Webb) in Belridge, California. Fertigation treatments were 4xyear−1 (Standard) urea ammonium nitrate (UAN), 20xyear−1 (HF) UAN and 20xyear−1 (HF) CaNO3+KNO3. Estimated surface emissions were HF UAN>Standard UAN>HF NO3, with differences only significant between the HF treatments, where HF UAN emitted 2.0 times the N2O seen from HF NO3. Net production was also monitored by depth in the soil using sampling tubes and Fickian diffusion calculations. UAN typically had highest N2O production at 10–15cm depth, while N2O was generally reduced to N2 below 20cm in all treatments. Differences were seen in the distribution of NH4+ and NO3− on the soil exchange complex and in soil solution, with data from 60cm suggesting that leaching hazards could be greater from Standard UAN than from HF NO3. Multiple linear regression of N2O production with predictors had the best fit at 15cm, where extractable NH4+, WFPS and temperature together accounted for an adjusted R2 of 0.68. Persistent soil microbial changes were seen in denitrification capacity, with HF UAN=HF NO3>Standard UAN, while 3% O2 assays suggested high contributions of N2O from nitrifier denitrification in the fertigation context. High-frequency fertigation with ammoniacal fertilizers did not mitigate N2O emissions, but nitrate-based fertilizers did, suggesting that N sources for fertigation warrant careful study.