Elsevier

Geoderma

Volume 266, 15 March 2016, Pages 120-126
Geoderma

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

https://doi.org/10.1016/j.geoderma.2015.12.015Get rights and content

Highlights

  • Contributions of inorganic C to soil CO2 release are often not considered.

  • Irrigation with water containing dissolved calcium and bicarbonate can cause calcium carbonates to precipitate in the soil

  • Soils in an irrigated apple orchard released CO2 that was enriched in 13C because of newly deposited soil carbonates

  • These carbonates contributed between 9% and 45% of the CO2 released during short-term incubations

Abstract

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 HCO3. 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.

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 (HCO3) (Suarez, 2000, Suarez, 2006). When water containing dissolved HCO3 is applied to the soil surface in the presence of sufficient Ca2 + (and/or Mg2 +) ions, inorganic carbonates such as calcite (CaCO3) or dolomite (CaMg(CO3)2) can form according to the following reaction (shown for calcite; Bower et al., 1965, Suarez, 2000, Eshel et al., 2007, Sanderman, 2012):Ca2 +(aq) + 2HCO3(aq) ↔ CaCO3(s) + H2O(l) + CO2(g)

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 concentrations of HCO3 and Ca2 + (and/or Mg2 +) dissolved in the water (Bower 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 HCO3-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 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 SIC-poor 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 HCO3 dissolved in the irrigation water, we conducted a four-month 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.

Section snippets

Study site

The study was initiated in 2013 using soil collected from a 10 year-old 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

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 − 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

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

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 HCO3. 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 short-term incubations. Given that the HCO3 dissolved in the irrigation water was more enriched in 13C

Acknowledgements

Funding for this project came from the Agricultural Greenhouse Gases Programme of Agriculture and Agri-food Canada (Project No. 1585-16-3-4-39). 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

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