Bicarbonates in irrigation water contribute to carbonate formation and CO2 production in orchard soils under drip irrigation

Geoderma 266 (2016) 120–126

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Bicarbonates in irrigation water contribute to carbonate formation and CO2 production in orchard soils under drip irrigation Kirsten D. Hannam a,⁎, Dan Kehila a, Peter Millard b, Andrew J. Midwood c, Denise Neilsen d, Gerry H. Neilsen d, Thomas A. Forge d, Craig Nichol e, Melanie D. Jones a a

Department of Biology, University of British Columbia Okanagan, Kelowna, BC V1V 1V7, Canada Landcare Research, PO Box 40, Lincoln 7640, New Zealand The James Hutton Institute, Craigiebuckler, Scotland AB15 8QH, United Kingdom d Agriculture and Agri-Foods Canada, Pacific Agri-Food Research Centre, Summerland, BC V0H 1Z0, Canada e Department of Earth & Environmental Sciences and Physical Geography, University of British Columbia Okanagan, Kelowna, BC V1V 1V7, Canada b c

a r t i c l e

i n f o

Article history: Received 22 July 2015 Received in revised form 15 December 2015 Accepted 16 December 2015 Available online 8 January 2016 Keywords: 13 C Bicarbonates Carbonates Carbon dioxide Irrigation Soil

a b s t r a c t Irrigated agriculture is conducted on approximately 257 million hectares worldwide and continues to expand, particularly in arid to semi-arid regions. Applications of water containing dissolved calcium and bicarbonate ions cause the precipitation of calcium carbonate in the soil and the release of carbon dioxide into the air. However, the contribution of inorganic C to CO2 emissions from the soil is rarely considered. Using a short-term incubation technique developed to examine changes in mineralizable organic C pools, we found that soils beneath drip emitters in an irrigated apple orchard released CO2 from both organic and inorganic C. Soils under drippers had higher concentrations of carbonates than soils that had not received direct inputs of irrigation water. The quantity of carbonates detected in the soil under the drippers at this site was small but may be greater on sites using irrigation water with higher concentrations of Ca2+ and HCO− 3 . Furthermore, site productivity may be reduced by unfavourable physical and chemical changes caused by carbonate deposition within the small soil volume occupied by tree roots in micro-irrigated orchards with dwarfing rootstocks. In order to better understand the implications for site productivity and for global C flux of carbonate precipitation in micro-irrigated systems, future work is required to quantify CO2 emissions during irrigation, and to characterize soil chemical and physical properties through the soil profile. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Approximately 257 million hectares of agricultural land were irrigated worldwide in 2007, with projections suggesting that another 20 million hectares will be equipped for irrigation by 2050 (Alexandratos and Bruinsma, 2012). Irrigation can affect soil carbon storage by altering both soil organic carbon (SOC) and soil inorganic carbon (SIC) cycling. In arid to semi-arid regions, irrigation water often contains a significant quantity of dissolved inorganic carbon (DIC) in the form of bicarbonate − (HCO− 3 ) (Suarez, 2000, 2006). When water containing dissolved HCO3 2+ is applied to the soil surface in the presence of sufficient Ca (and/or Mg2+) ions, inorganic carbonates such as calcite (CaCO3) or dolomite (CaMg(CO3)2) can form according to the following reaction (shown

Abbreviations: DIC, dissolved inorganic carbon; HWIC, hot water extractable inorganic carbon; HWOC, hot water extractable organic carbon; SIC, soil inorganic carbon; SOC, soil organic carbon. ⁎ Corresponding author. E-mail address: [email protected] (K.D. Hannam).

http://dx.doi.org/10.1016/j.geoderma.2015.12.015 0016-7061/© 2015 Elsevier B.V. All rights reserved.

for calcite; Bower et al., 1965; Suarez, 2000; Eshel et al., 2007; Sanderman, 2012): − Ca2þ ðaqÞ þ 2HCO3 ðaqÞ↔CaCO3ðsÞ þ H2 OðlÞ þ CO2ðgÞ

ð1Þ

The net effect of this reaction on SIC storage depends on several factors, including the quantity of irrigation water applied, the fraction of irrigation water that leaches out of the soil profile and the concentra2+ tions of HCO− (and/or Mg2+) dissolved in the water (Bower 3 and Ca et al., 1965; Suarez and Rhoades, 1977; Suarez, 2006; Sanderman, 2012). Modelling simulations suggest that the SIC pool could increase by up to 125 kg per hectare each year if Ca2+- and HCO− 3 -rich water is applied at a rate of 1.2 m per year and only 10% of the applied water is lost to leaching (Suarez and Rhoades, 1977; Suarez, 2006). Similar quantities of CO2 would be released into the atmosphere during this process (Schlesinger, 2000). Given that these values are on a comparable scale to the 300–500 kg/ha of soil organic C (SOC) that can be sequestered each year using intensive agricultural practices (Lal, 2007), there is a growing recognition that neither accurate predictions of global carbon flux nor development of effective climate change mitigation

K.D. Hannam et al. / Geoderma 266 (2016) 120–126

practices is possible without a better understanding of inorganic C cycling (Rey, 2015). Critical insights into SOC turnover have been gained by monitoring the δ13C of CO2 released from soil samples during short-term (e.g., 250 to 480 min; Millard et al., 2010; Snell et al., 2014; Zakharova et al., 2014) laboratory incubations. Within the first few hours after their removal from the ground, the CO2 released from sampled soils has been observed to fall from approximately − 22‰ to − 26‰ (Millard et al., 2010; Snell et al., 2014; Zakharova et al., 2014). This phenomenon has been ascribed to the exposure of physically occluded organic C substrates that become available when aggregates are broken up during soil sampling and mixing (Millard et al., 2010; Snell et al., 2014; Zakharova et al., 2014). These studies were all conducted using SICpoor soils; however, recent work on SIC-rich soils suggests that as much as 80% of the CO2 released during soil incubations could originate from inorganic C (Bertrand et al., 2007; Tamir et al., 2011; Ramnarine et al., 2012). Although temporal changes in the δ13C of CO2 released from the soil during longer-term incubations have been used to demonstrate the effects of lime applications on the SIC pool (Bertrand et al., 2007), this technique has not, to our knowledge, been employed to examine the effects of irrigation on SIC dynamics. Here, we compared the δ13C of CO2 released during short-term incubations of soils collected directly under and 30 cm away from drip emitters in a micro-irrigated apple orchard. To confirm that observed differences were caused by inputs of HCO− 3 dissolved in the irrigation water, we conducted a fourmonth laboratory study in which intact soil cores were amended using deionized water or irrigation water collected from the study site. We then used an adapted version of our short-term incubation method to compare the effects of sample position and water source on the δ13CO2 released from these soils.

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Ponderosa pine (Pinus ponderosa). However, the site has been used for perennial horticultural crops (e.g., apples, wine grapes) since the early 1900s. Prior to establishment of the orchard, the site was managed for 15 years as a trickle-irrigated vineyard but all grapevines (Vitis vinifera L.) were removed in the late 1990s. The area was left fallow until 2003, when it was deep ripped to a depth of approximately 30 cm in order to homogenize the soil and remove the roots from the previous crop. Apple trees (M. domestica Borkh. cv. Ambrosia on M9 dwarfing rootstock) were planted at 0.9 m (within row) × 3.5 m (between row) spacing. A 1.5 m herbicide strip, centred along the tree row, was maintained with periodic applications of herbicide (usually glyphosate). Since establishment, the orchard has been irrigated from early May to mid-October using fully automated drip irrigation scheduled to meet evapotranspirative demand as described by Neilsen et al. (2008). Irrigation is applied using two 4 L hr− 1 drippers placed 30 cm away from each tree on opposite sides of the tree stem and perpendicular to the tree row. Approximately 0.7 m of water is typically applied to the orchard over the irrigation period each year. Irrigation water at this site is drawn from Okanagan Lake and stored in a closed tank prior to application. Okanagan Lake has a surface area of 351 km2 and a volume of almost 25,000 Mm3 (Nordin, 2005). Published values of alkalinity for −1 Okanagan Lake water range from 130 to 190 mg HCO− 3 equivalent L (Anonymous, 1974; Mackie, 2010). Published values of dissolved Ca2+ and Mg2+ concentrations for Okanagan Lake water collected near the irrigation water intake range from 20 to 35 mg L−1 and from 9 to 12 mg L−1, respectively; published values of pH vary between 7.3 and 8.5 but are usually above 8.0 (Wilcox, 1947; Wilcox and Mason, 1963; Anonymous, 1974; Mackie, 2010). 2.2. Experiment 1

2. Materials and methods 2.1. Study site The study was initiated in 2013 using soil collected from a 10 yearold apple (Malus domestica Borkh.) orchard located at Agriculture and Agri-Food Canada's Pacific Agri-Food Research Centre (PARC) in the Okanagan Valley near Summerland, British Columbia (lat. 49° 34′N, long. 119° 39′W). The climate in this region is semi-arid: average daily temperature and total precipitation are 16.0 °C and 178.6 mm, respectively, from September to April and 2.0 °C and 148.1 mm, respectively, from October to March (Environment Canada, 2015). Surface soils are aridic haploxerolls, characterized as Skaha loamy sands, which have a neutral pH, low organic matter content and very few coarse fragments (Wittneben, 1986; Table 1). These soils are not considered calcareous but evidence of pedogenic carbonates (i.e., fizzing in response to the application of dilute hydrochloric acid) was found at 85–90 cm depth. The native vegetation on the site was dominated by C3 plants, including big sagebrush (Artemisia tridentata), antelope brush (Purshia tridentata), bluebunch wheatgrass (Pseudoroegneria spicata) and scattered Table 1 Gravimetric moisture content, pH, δ13C, and concentrations of total carbon, and hot water extractable organic and inorganic carbon in soils (0–20 cm depth) collected away from drippers (‘Away’) and directly under drippers (‘Under’) in Experiment 1 (n = 5). Moisture content %

pH

Total carbon mg g−1

δ13C ‰

Hot water extractable carbon Organic

Inorganic⁎

μg g−1 Away Under SE p-Values

12.5 16.2 2.4 0.072

7.0 7.2 0.1 0.17

6.6 7.4 0.5 0.34

−26.46 −26.38 0.08 0.49

205.3 214.3 14.8 0.66

18.1b 28.1a 2.1 0.0097

⁎ Within columns, means with different letters indicate differences compared using t-tests at p b 0.05.

In early October 2013, a series of short-term soil incubations were initiated. Although the soils at this site were relatively homogeneous, sample locations were systematically selected from across the orchard to represent as wide a range in soil properties as possible. Microirrigation using drippers induces spatial variability in soil moisture conditions across the soil surface, with a bulb-shaped wetted zone extending into the soil beneath the emitter; in sandy soils such as those found at the study site, this ‘wetted bulb’ is expected to be relatively narrow (e.g., approximately 30 cm diameter at its widest point; Hao et al., 2007). Changes in soil moisture and temperature regime, root biomass and turnover, and soil microbial activity, induced by ten years of irrigation, were expected to have caused differences in the amount and composition of soil carbon under and away from drippers. In order to capture some of this variability, samples were collected from two locations within the herbicide strip maintained around the trees: five samples were collected directly under drippers, and five samples were collected approximately 30 cm away from drippers (10 samples total). Each soil sample was collected to a depth of 20 cm using a 10 cm diameter PVC pipe. Within 7 min of removal from the ground, soil samples were ejected from the pipe, homogenized by hand, and visible roots and stones were removed. Soils were weighed, and approximately half the sample was placed in a Kynar® bag (Keika Ventures, Chapel Hill, NC, USA) which was then sealed, evacuated and flushed repeatedly with CO2-free air until the concentration of CO2 remaining in the headspace of the bag was less than 40 ppm, as determined using a portable infra-red gas analyser (IRGA; EGM-4 — PP Systems, Amesbury, MA, USA). Soils were incubated at ambient temperature inside the bags, and gas samples were collected over a six-hour period for δ13CO2 analysis by transferring the headspace gas from the incubating soils into empty Kynar bags that had been repeatedly flushed with CO2-free air and evacuated to remove all residual CO2. Headspace gas samples were collected at regular intervals: every 5 min for the first hour; every 10 min for the second hour; every 20 min for the third hour; and every 60 min for the remaining 3 h. After the headspace gas was

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collected for analysis, the bag containing the incubating soil was evacuated and re-filled with CO2-free air. The δ13CO2 of headspace gas samples was measured using a Picarro G2131-i wavelength-scanned cavity ring-down spectrometer (Santa Clara, CA, USA). Data were corrected using a gas standard with a known δ13CO2. Given the intensity of data collection required for each sample, only one or two soil cores were collected and incubated per day. Sampling began in early October and was completed by the end of the month. The half of the soil sample not incubated for δ13CO2 measurements was weighed and dried (65 °C) to determine moisture content and used for determination of other soil properties (described below). 2.3. Experiment 2 After several pairs of samples had been analysed for Experiment 1, it became clear that (i) the δ13C of CO2 produced from these soils was more enriched than observed in previous studies (e.g., Millard et al., 2010; Snell et al., 2014; Zakharova et al., 2014) and (ii) this enrichment was greater in samples collected from directly beneath drippers. A laboratory incubation experiment was initiated to characterize the effect on soil properties of irrigation water applications under controlled conditions. In mid-October 2013, 24 intact soil cores were collected using collars cut from PVC pipe (15 cm long; 10 cm diameter). Collars were driven to a 10 cm depth and carefully excavated from the orchard described above: 12 soil cores were collected from directly under drippers and 12 soil cores were collected approximately 30 cm away from drippers within the herbicide strip maintained around the trees. Locations that were visibly disturbed or that had dead or living weeds at the soil surface were avoided. After soil cores were excavated, a fine nylon mesh was secured around the bottom of each collar and the fresh weight of the soil sample was determined using a field scale. Irrigation water was collected in 20 L carboys and both the soil cores and irrigation water were transported to the University of British Columbia — Okanagan campus in Kelowna, BC. Two irrigation treatments (irrigation water or reverse-osmosis deionized water) were applied to the soil cores in a factorial design (sample position x irrigation treatment). Irrigation treatments were randomly assigned to soil cores within each sample position and replicated six times. Cores were stored in plastic crates with open bottoms, and held in a growth chamber set to 30 °C and constant darkness. This high temperature was selected to ensure that moisture would rapidly evaporate from the cores during the four-month water amendment stage of the experiment, and was representative of the period between May and September, when air temperatures at the field site frequently exceed 30 °C (Environment Canada, 2015). To account for any variation in air flow or temperature within the growth chamber, the position of the soil cores was re-randomized every two weeks. For the first 24 days of the experiment, samples were weighed every two to three days and re-wet to their initial field weight (50 to 100 mL was usually required for each core) using either deionized water or irrigation water, as appropriate. However, concerns arose that soil cores excavated from under the drippers were wetter than those excavated away from the drippers, even though irrigation had been turned off one week prior to removal of the soil cores from the field site. To eliminate this possible bias, volumetric soil moisture was determined on all samples using a Delta-T WET-3 moisture probe (Delta-T Devices Ltd., Cambridge, U.K.), and all soil cores were amended to the same volumetric moisture content as the wettest soil core (37%) during all subsequent irrigation events for the remaining 100 days of the experiment. In February 2014, we measured the δ13C of the CO2 released during short-term incubations of these soils. On the day prior to sampling, soil cores to be processed were randomly selected and amendments with irrigation or deionized water were stopped. At the time of processing, the average gravimetric moisture content of the incubated soil cores was 9.2%, and the effects of sample position (under or away from drippers) and water source (irrigation or deionized water) on soil moisture were not significant. Sample processing was conducted as described above

except that headspace gas samples were collected for δ13CO2 analysis at 5, 20, 40, 60 and 80 min after removal from the PVC collars, allowing more soil cores to be processed in a single day. Thus, the short-term incubations for Experiment 2 were completed over a two-week period. The second half of each soil sample was weighed and dried (65 °C) to determine moisture content and the dried soils were used for determination of treatment effects on other soil properties (described below). The irrigation water used for Experiment 2 was held in cold storage (4 ± 2 °C); to minimize exchange with atmospheric CO2, carboys containing irrigation water were only opened to withdraw water for amending the soil cores. At the end of the experiment, the pH of the water was determined using a Fisher Scientific Accumet AB150 pH metre (Fisher Scientific Co., Toronto, Canada). The remaining irrigation water was placed in acid-washed and rinsed Nalgene bottles and stored at −20 °C for subsequent analysis. In preparation for analysis of the δ13C of HCO− 3 dissolved in the irrigation water, the samples were defrosted overnight; a 250 mL aliquot of irrigation water was withdrawn from each bottle and the HCO− 3 dissolved in solution was trapped by the addition of excess SrCl. The precipitated SrCO4 was recovered by filtration (0.45 μm; Singleton et al., 2012), dried at room temperature and placed in 12 mL exetainers (Labco Medical Supplies Inc., Ceredigion, U.K.) which were then tightly capped and evacuated to remove atmospheric air. To convert the Sr-trapped carbonates to CO2, 1 mL of 1.2 M HCl was injected into each exetainer and the CO2 released was transferred into a Kynar gas sampling bag pre-filled with 1 L of CO2-free air. The δ 13C of the carbonate-derived CO2 was then measured using the Picarro G2131-i wavelength-scanned cavity ring-down spectrometer; CO2 released from a known carbonate standard was employed to correct the data. Total alkalinity was measured on thawed samples using the Gran method (USEPA, 2012) by titrating 60 mL of water with 0.16 N sulphuric acid to a pH of 4.2. Total alkalinity has been shown not to change substantially with freezing (Canfield et al., 2002). To address the possibility that the δ13C composition of the HCO− 3 dissolved in the irrigation water may have shifted due to exchange with atmospheric air during the four-month laboratory amendment period, samples of irrigation water were collected on a biweekly basis through the growing season of the following year (2014) and analysed in October 2014 for pH, alkalinity and the δ13C of dissolved HCO− 3 as described above. A second set of water samples was collected through the growing season for analysis of dissolved cations. Concentrations of Ca, Mg and K were determined on acidified samples using a SpectroBlue ICP-OES (SPECTRO Analytical Instruments GmbH; Kleve, Germany).

2.4. Soil analyses Dried soil samples from both experiments were sieved (2 mm) to remove roots and stones and to break up larger soil aggregates. Soil pH was determined on all soil samples using a Fisher Scientific Accumet AB150 pH metre at a ratio of 1:2 (soil:distilled water), by mass. Total C was determined using thermal conductivity detection with a LECO CNS-2000 (Experiment 1: Leco Corporation, St. Joseph, MI) or a Costech 4010 Elemental Analyser (Experiment 2: Costech Analytical Technologies Inc., Valencia, CA). Hot water extractable organic C (HWOC) and inorganic C (HWIC) samples were prepared following the method of Ghani et al. (2003) but using a ratio of 1:2.5 (soil:water), by mass. HWOC and HWIC were determined on a Shimadzu TOC-5000A Total Organic Carbon analyser (Experiment 1: Shimadzu Scientific Instruments, Inc., Columbia, MD) or an Aurora 1030 W TOC analyser (Experiment 2: OI Analytical; College Station, TX). Soil carbonates were measured on samples from both experiments using an Eltra Helios CS analyser (Eltra Elemental Analysers, Haan, Germany) equipped with a CO3–C module; CO3–C was liberated using 70% phosphoric acid. The δ13C of whole soil samples collected in Experiment 1 was determined using a ThermoFinnigan Delta+ Advantage Continuous Flow Isotope Ratio Mass Spectrometer (CF-IRMS; Thermo Finnigan Corp, Bremen, Germany).

K.D. Hannam et al. / Geoderma 266 (2016) 120–126

2.5. Statistical analyses 2.5.1. Experiment 1 The effect of sample position (under and away from drippers) on the δ13C of CO2 released from sampled soils was plotted over the six-hour incubation period. Changes in the δ13CO2 released during incubation were not analysed statistically because treatment effects were evident by viewing the graph and because the residuals did not demonstrate homogeneity of variance, a requirement of mixed-model analyses. The effect of sample position on soil moisture, soil pH, total soil C, δ13C, HWIC and HWOC were examined using t-tests. The folded F test was used to test for homogeneity of variance. When variances between the two positions were not homogeneous, the Satterthwaite approximation was used to compare the soil properties from the two positions; otherwise, a pooled t-test was used (SAS® version 9.2; SAS Institute Inc., Cary, NC). 2.5.2. Experiment 2 Effects of sample position and water source on the δ13C of CO2 released during short-term incubations were analysed as a repeated measures experiment using the PROC MIXED procedure and repeated statement (SAS® version 9.2; SAS Institute Inc., Cary, NC) for a completely randomized split-plot experimental design, with two sample positions applied as whole-plot factors, two water source treatments applied as split-plots and five measurements through time; all treatment combinations were replicated six times. A compound symmetry model was fitted to the covariance structure following the procedure for analysis of repeated measures data using unequally spaced measurements in time, as described in Littell et al. (2006). KenwardRoger's adjustment was used to estimate degrees of freedom (Kenward and Roger, 1997). Because of significant sample time × treatment interactions, data were re-analysed by individual sample times. Treatment differences in other soil properties were examined using the same procedure. PROC UNIVARIATE was performed on residuals to confirm homogeneity of variance (SAS® version 9.2; SAS Institute Inc., Cary, NC). 3. Results 3.1. Experiment 1 The δ13C of CO2 released during the short-term incubations was clearly dependent on sample location relative to the drip emitters (Fig. 1). The initial δ13C of CO2 released from soils collected away from drippers was approximately − 11‰ and declined to approximately

Fig. 1. δ13C of CO2 released from soils (0–20 cm) collected away from drippers and under drippers and measured over a 6-hour period after sampling (mean ± standard error of the mean; n = 5 except for 5 min measurements, when n = 3 for ‘Away’ and n = 4 for ‘Under’) (Experiment 1).

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− 21‰ within three hours of removal from the ground. By contrast, the δ13C of CO2 released from soil collected under drippers was initially more depleted (approximately −14‰), rose to approximately −10‰ within 20 min of removal from the ground and then gradually declined to approximately −21‰ within 5 h of removal from the ground. Soils collected under and away from drippers had a similar pH (overall mean 7.1), total carbon content (overall mean 7.0 mg g−1) and δ13C (overall mean − 26.42‰; Table 1). However, soils collected away from drippers had significantly lower concentrations of HWIC than soils collected under drippers, and also tended to be drier. Concentrations of carbonate C were at or below detection limits (10 μg g−1) in all soil samples and are, therefore, not presented. 3.2. Experiment 2 As observed in Experiment 1, the CO2 released from soil cores collected under drippers was generally enriched in 13C compared to that released from soil cores collected away from drippers (Fig. 2; Table 2). Four months of amendment with irrigation water also caused the CO2 released from the soil to be noticeably enriched in 13C, regardless of where soil cores had been collected relative to drippers. However, there were significant interactions between sample time, sample position relative to drippers and water source (Table 2). Five minutes after soils were removed from the collars, δ13CO2 was most enriched in soils amended with irrigation water, regardless of where soil cores had been collected relative to drippers, and lowest in samples collected away from drippers and amended with deionized water (Fig. 2; Table 3). A similar pattern was observed 20 min after soils were removed from the collars; however, this effect had disappeared within 40 min of removing soils from the collars. For the remainder of the monitoring period, the δ13C of CO2 released from soil cores collected under drippers was higher than that from soil cores collected away from drippers, and the δ13C of CO2 released from the soil cores amended with irrigation water was higher than that from soil cores amended with deionized water. Soil pH and concentrations of total carbon and HWIC were higher in soils collected under drippers than in soils collected away from drippers (Table 4). Carbonates also appeared to be higher in soils collected under drippers: there were no measureable amounts of carbonates detected in soils collected away from drippers, even after four months of amendment with irrigation water, but there were approximately 31.7 μg g−1 carbonate-C in soils collected under drippers, averaged across both watering treatments. By contrast, concentrations of HWOC were higher

Fig. 2. δ13C of CO2 released from soils (0–10 cm) collected away from drippers and under drippers and irrigated for four months with deionized water or irrigation water (Experiment 2). Gas samples were collected 5, 20, 40, 60 and 80 min after soils were removed from collars (mean ± standard error of the mean; n = 6). Italicized values adjacent to each datapoint are the estimated fraction of CO2 derived from inorganic C.

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Table 2 δ13C of CO2 released from soils (0–10 cm) collected away from drippers (‘Away’) and under drippers (‘Under’) and irrigated with deionized water or irrigation water (Experiment 2). Gas samples were collected 5, 20, 40, 60 and 80 min after soils were removed from collars, and data from all sample times were analysed together using repeated measures (n = 6).

Table 4 pH and concentrations of total carbon, carbonate carbon, and hot water extractable organic and inorganic carbon in soils (0–10 cm depth) collected away from drippers (‘Away’) and under drippers (‘Under’) and amended with deionized water or irrigation water for four months under laboratory conditions (Experiment 2) (n = 6). pH

Least squares means Time 5 min 20 min 40 min 60 min 80 min Standard error Position Away Under Standard error Water Deionized Irrigation Standard error p-Values Time Position Water source Time × position Time × water source Position × water source Time × position × water source

−16.64 −19.27 −20.75 −21.37 −21.57 0.24 −20.99 −18.86 0.50 −22.96 −16.88 0.65 b0.0001 0.0082 b0.0001 0.012 b0.0001 0.19 b0.0001

5 min⁎

20 min

40 min

60 min

80 min

−17.65 −15.64 0.50

−19.97b −18.58a 0.37

−21.64b −19.86a 0.37

−22.58b −20.16a 0.42

−23.10b −20.05a 0.42

−19.79 −13.50 0.51

−23.01b −15.54a 0.48

−23.99b −17.5a 0.48

−24.11b −18.62a 0.46

−23.92b −19.23a 0.45

0.041 b0.0001 0.063

0.016 b0.0001 0.59

0.0062 b0.0001 0.83

Hot water extractable carbon Organic

Inorganic⁎

−1

6.9b 7.4a 0.04

7.7b 10.6a 0.8

n.d.ǂ 31.7 6.4

436.7a 347.5b 22.9

2.5b 13.6a 2.1

6.9b 7.4a 0.06

9.1 9.2 0.6

30.0§ 33.3§ 13.2

398.6 385.6 19.1

3.1b 13.0a 1.4

0.0003 0.0019 0.98

0.013 0.87 0.88

– – –

0.012 0.49 0.24

0.023 b0.0001 0.0050

⁎ Within columns, means with different letters indicate differences of least squares means at p b 0.05. ǂ Data could not be analysed statistically because carbonate concentrations in soils collected away from drippers were below detection limits (n.d.) of 10 μg g−1. § Mean values were calculated from soils collected under drippers only.

Table 3 δ13C of CO2 released from soils (0–10 cm) collected away from drippers (‘Away’) and under drippers (‘Under’) and irrigated with deionized water or irrigation water (Experiment 2). Gas samples were collected 5, 20, 40, 60 and 80 min after soils were removed from collars, and data were analysed by individual sample times (n = 6).

0.021 b0.0001 0.0012

Carbonate carbon μg g−1

μg g Position Away Under SE Water source Deionized Irrigation SE p-Values Position (P) Water source (W) P×W

in soils collected away from drippers than in soils collected under drippers. Overall, soils amended with irrigation water for four months had a higher soil pH and greater concentrations of HWIC than soils amended with deionized water. However, there was a significant interactive effect between water source and sample position relative to drippers on concentrations of HWIC: in soil cores amended with irrigation water, those collected under drippers had higher concentrations of HWIC than those collected away from drippers, but in soil cores amended with deionized water, there was no effect of sample position relative to drippers on HWIC. At the end of Experiment 2, the irrigation water used to amend the −1 soil cores had a pH of 8.1, and an alkalinity of 84.6 mg HCO− . The 3 L δ13C of the HCO− was −1.9 ‰. These values were similar to the mean 3 values measured on irrigation water samples collected through the −1 growing season the following year (2014: pH 8.3, 82.5 mg HCO− , 3 L and − 2.6‰, respectively). Concentrations of dissolved Ca2 + and Mg2 + were, on average, 27.8 mg L−1 and 8.7 mg L−1, respectively, over the 2014 growing season.

Position Away Under Standard error Water Deionized Irrigation Standard error p-Values Position Water source Position × water source

Total carbon mg g−1

0.0027 b0.0001 0.67

⁎ Within columns, means with different letters indicate differences of least squares means at p b 0.05.

4. Discussion Irrigation can be expected to alter soil C cycling via increased inputs of organic C from roots and litter, and enhanced soil biological activity. However, inorganic C can also contribute significantly to soil C cycling in many systems (e.g., Serrano-Ortiz et al., 2010; Rey, 2015; Ahmad et al., 2015). In this study, we observed differences in the δ13C of CO2 released from soils collected from two positions (under drippers and away from drippers) in a 10 year-old micro-irrigated apple orchard in the Okanagan Valley, British Columbia. Previous short-term incubation experiments conducted on soil samples that contained no SIC demonstrated that the δ13CO2 released from the soil typically falls from approximately − 22‰ to approximately − 26‰ within a few hours after soil samples are collected and homogenized (Millard et al., 2010; Snell et al., 2014; Zakharova et al., 2014). In this study, however, the CO2 released from field-collected soils tended to be more enriched in 13C (with mean values falling between − 8‰ and − 22‰) and remained particularly 13C-rich in samples collected under drippers. We propose that irrigation with water containing 13C-rich HCO− 3 resulted in the precipitation of carbonates at the soil surface and that some of this carbonate-C was released as CO2 during the short-term incubations. At orchard establishment, the soil composition was relatively homogeneous across the site because the soils were deep-ripped to a depth of 30 cm prior to planting. Nevertheless, ten years later, soils under drippers had higher concentrations of HWIC (Experiment 1) and carbonate-C (Experiment 2) than soils away from drippers. Given that carbonate concentrations were detectable when soils were sampled to a depth of 10 cm (Experiment 2) but were below detection limits when soils were sampled to a depth of 20 cm (Experiment 1), these newly precipitated carbonates appear to be concentrated near the soil surface directly under the drippers. The tendency of carbonates to precipitate out of irrigation water is highly dependent on the concentrations of Ca2 + (and/or Mg2 +) and HCO− 3 ions dissolved in the irrigation water (Bower et al., 1965; Suarez and Rhoades, 1977; Suarez, 2000). The mean concentrations of Ca2+ and Mg2+ in irrigation water sampled over the 2014 growing season were 28 mg L− 1 and 9 mg L−1 (equivalent to 1.4 meq L− 1 and 0.7 meq L−1), respectively. Using total alkalinity as an estimate of the concentration of HCO− 3 dissolved in the irrigation water, average

K.D. Hannam et al. / Geoderma 266 (2016) 120–126 −1 concentrations were approximately 85 mg HCO− (equivalent to 3 L 1.4 meq L− 1). In general, the levels of Ca2 + and HCO− 3 measured in this and previous studies of Okanagan Lake water are less than half those reported for the Colorado and Pecos Rivers, the Rio Grande, the Great Ouse and the Thames (Bower et al., 1965; Suarez, 2000; Neal, 2002; Wu et al., 2008), all of which are considered ‘calcite-saturated’ and, therefore, likely to cause the precipitation of CaCO3. Nevertheless, simulations conducted under laboratory conditions suggest that the dissolved Ca2 + and HCO− 3 concentrations present in irrigation water drawn from Okanagan Lake are sufficient to cause small quantities of CaCO3 to precipitate out of solution (Bower et al., 1965). 13 The HCO− 3 dissolved in the irrigation water at this site had a δ C of 13 approximately −2‰. By contrast, the δ C of the whole soil carbon fraction was −26.4‰ (Table 1), typical of soils with a C pool dominated by organic C originating from C3 plants (Cerling and Quade, 1993; Bertrand et al., 2007; Millard et al., 2010; Tamir et al., 2011; Zakharova et al., 2014). Although concentrations of soil carbonates were too low for reliable δ13C determination, isotopic fractionation during organic matter decay cannot explain the enriched 13CO2 released during the shortterm incubations in Experiment 1 (Nordt et al., 1996). Given that a similar pattern of 13CO2 release was observed during short-term incubations of soil cores amended with irrigation water (Experiment 2), regardless of where the soils had originally been collected within the orchard, it is likely that the irrigation water is the source of the 13Cenriched CO2 released during short-term soil incubations. Using the carbonate C data from soil cores collected under the drippers in Experiment 2, and assuming that there were negligible amounts of carbonates in the top 10 cm of soil prior to irrigation, our data suggest that irrigation increased the SIC content of the top 10 cm of soil directly under the drippers by 4.6 kg ha−1 yr.−1. If the soils under the drippers represent 5% of the surface area within the herbicide strip around the trees (Hao et al., 2007), this suggests that irrigation could result in the storage of 0.23 kg SIC ha− 1 yr.− 1 in the top 10 cm of soil and cause the simultaneous release of approximately 21 mol CO2 ha− 1 yr.−1. However, this is a conservative estimate of SIC formation because applications of acidic or acidifying fertilizers, and intense precipitation or snowmelt, for example, could cause the loss of SIC from the soil profile (Eshel et al., 2007; Wu et al., 2008). In fact, newly precipitated carbonates are more susceptible to dissolution and re-translocation than primary soil carbonates (Margaritz and Amiel, 1981; Khokhlova et al., 1997; Eshel et al., 2007). Furthermore, previous field studies have shown that many irrigation-caused changes in SIC storage can be found at depth, rather than at the soil surface. Therefore, it is not yet possible to estimate with any certainty the accumulation of SOC and SIC caused by 10 years of drip irrigation at this site; future work is required to characterize C pools deeper into the soil profile. It should also be noted that other micro-irrigation methods (e.g., microjet or microsprinkler irrigation), which induce more pronounced wetting and drying cycles (Smajstrla et al., 2002; Stanley and Toor, 2010), may cause greater rates of SIC formation. Finally, as noted above, the process of carbonate precipitation also results in the release of CO2. Therefore, there is a critical need to quantify CO2 emissions during irrigation with HCO− 3 containing water in order to better understand the implications of micro-irrigation for global carbon flux models. The short-term incubation technique used in this study was originally developed as a tool to examine treatment differences in easily mineralizable pools of SOC (Millard et al., 2010; Snell et al., 2014; Zakharova et al., 2014). When used in soils free of inorganic C, the declining δ13C of the CO2 released over several hours after soil disturbance (from approximately −22‰ to −26‰) is believed to be driven by the liberation of physically protected C substrates and their subsequent decay by soil microbes (Zakharova et al., 2014) A remarkably similar pattern in δ13CO2 was observed during short-term incubations of soil cores collected away from the drippers and amended with deionized water, suggesting that much of the CO2 released from these soils was derived from SOC (Fig. 2). In SIC-containing soils, however, both SOC and SIC can release

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CO2. Indeed, several recent studies have demonstrated that SIC can be an important source of CO2 in calcareous soils and in soils amended with lime (Stevenson and Verburg, 2006; Bertrand et al., 2007; Biasi et al., 2008; Tamir et al., 2011; Ramnarine et al., 2012). SIC accounted for a very small fraction (approximately 0.3%; Table 4) of the total C pool in the soils examined in this study. Using a simple two-source mixing model (Millard et al., 2010) and assuming (i) that the CO2 released from soils collected away from drippers and irrigated with deionized water originated solely from SOC, and (ii) that the CO2 released from bicarbonates in these soils had an approximate δ13C of −2‰, we estimate that between 9% and 45% of the CO2 released from irrigated soils in Experiment 2 was derived from inorganic C (Fig. 2). Therefore, soil disturbances caused by routine agricultural activities, such as tillage, planting, application of acidic or acidifying fertilizers, etc., (Entry et al., 2004; Eshel et al., 2007; Wu et al., 2008) could stimulate the release of irrigation water-derived SIC that has accumulated in the soil. Lastly, it should be noted that drip irrigation on coarse textured-soils and using trees with dwarfing rootstocks tends to restrict tree root development to the narrow wetted bulb of soil directly under the drippers (BCMAFF, 2001; Hao et al., 2007). Consequently, changes in soil porosity or Ca2 + availability caused by the precipitation of irrigation waterderived carbonates within this small wetted volume of soil (Bower et al., 1965; Margaritz and Amiel, 1981; Amundson and Smith, 1988; Khokhlova et al., 1997; Eshel et al., 2007) could have large consequences for site productivity.

5. Conclusions In a 10 year-old drip-irrigated apple orchard on non-calcareous soil, we examined changes in soil properties and CO2 release caused by applications of irrigation water that contained measureable concentrations of Ca2 +, Mg2 + and HCO− 3 . We found that soils under drippers had higher concentrations of HWIC (0–20 cm), higher concentrations of carbonates (0–10 cm), and released 13C-enriched CO2 during shortterm incubations. Given that the HCO− 3 dissolved in the irrigation water was more enriched in 13C (δ13C of approximately − 2‰) than the bulk soil (δ13C of approximately −26‰), we propose that irrigation with HCO− 3 containing water caused the precipitation of small quantities of carbonates under the drippers and that CO2 originating from this material had a strong influence on the 13CO2 released from the soil. Precipitation of irrigation water-derived carbonates may be even more critical on sites that use irrigation water with higher concentrations of dissolved Ca2 +, Mg2 + and HCO− 3 . Chemical and physical changes associated with the precipitation of carbonates could have implications for long-term orchard productivity in drip irrigated systems with small rooting volumes. In order to better understand the effects of micro-irrigation on global C flux, future work is required to quantify CO2 emissions during irrigation, and to characterize irrigation-induced changes in soil chemical and physical properties across the soil surface and deeper into the soil profile.

Acknowledgements Funding for this project came from the Agricultural Greenhouse Gases Programme of Agriculture and Agri-food Canada (Project No. 1585-16-34-39). The authors gratefully acknowledge the capable assistance of Mathilde Bezard, Harveer Singh Dhupar, Aaron Godin, Ed Helfenbein, Paul Randall, Todd Redding, Tal Shalev, Doris Stratoberdha and Valerie Ward, and the fastidious work of David Dunn (Natural Resources Canada), Alan Harms (Natural Resources Analytical Laboratory — University of Alberta) and Dean Babuin (Agriculture and Agri-Food Canada), who performed the various soil C analyses. We are also grateful to the Pacific Agri-food Research Centre for providing access to the experimental plots.

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