Implications of CaCl₂ application to plants in LID facilities

Rapid industrialization and urbanization from the mid-20th century onwards have provided goods and spaces essential for human activity. However, the increase in impermeable surfaces such as roads, parking lots and buildings has led to problems such as interference with the natural hydrological cycle, the urban heat island effect, and increased non-point source (NPS) pollutants. Seoul recorded that 47.7% of its surfaces were impermeable in 2010, and most were found to be roads and houses with imperviousness rate of greater than 90% (Seoul Metropolitan Government 2013; Zhou et al. 2016). Increased impermeable areas interrupt the natural hydrological cycle by decreasing the underground infiltration rate and increasing surface runoff during heavy rainfall. NPS pollutants are released into water bodies, which causes water pollution and destroys aqua-ecosystems. The resulting decrease in green space and underground infiltration rate has been identified as the cause lower groundwater level and the increased probability of urban flooding.

Low impact development (LID) technologies are being applied to address the above-mentioned urban problems, and have been found to be effective in the restoration of the natural hydrological cycle, flood prevention, provision of habitats for organisms, removal of NPS pollutants and ecosystem restoration. LID refers to the method of efficiently managing stormwater runoff containing NPS pollutants while maintaining natural hydrological functions such as infiltration, subsurface flow and evapotranspiration (MOE 2014; Zhang et al. 2016). In general, LID include technologies such as constructed wetlands, vegetative filter strips, bioretention, tree box filters, rain gardens, planters, permeable pavements, green roofs and rainwater harvesting. These technologies achieve ecological, hydrological and environmental effects through interactions with plants, soil, filter media, and microorganisms. In particular, plants play a key role in LID facilities, contributing to the reduction of the heat island effect, reduction of carbon, removal of pollutants, establishment of an ecological environment for microorganisms, and enhanced aesthetics (Kim 2014; Seoul Metropolitan Government 2015). However, it was found recently that the use of de-icers to remove heavy snow during the winter hinders plant growth (Macadam & Parsons 2004; Korea Highway Corporation report 2007). The excessive use of de-icers changes the structure of soil particles, causing non-uniformity in soil infiltration and affecting photosynthesis among plants. An elevated salt content in soil accelerates drying, and the crusting of soil hinders germination and root growth. Moreover, excessive salt accelerates the mobility of heavy metals within the soil, and plants are threatened by heavy metal toxicity (Viskari et al. 2000; Transportation research board guideline 2007; Dubuque 2010; Shin et al. 2010; Oh 2014). While Korea has established guidelines on the use of de-icers, few studies exist on the effects of de-icers on different plants. Therefore this study assesses the effects of CaCl₂ de-icer on plants commonly used in LID facilities.

Eleven plant species that are commonly used in LID facilities in South Korea were selected to evaluate the effects of CaCl₂, and these are listed in Table 1: two shrubs, one tree, and eight herbaceous plants. More specifically, the deciduous broadleaf shrub, evergreen shrub and deciduous tree were Spiraea prunifolia var. simpliciflora (Nakai) Nakai, Rhododendron indicum (L.) Sweet, and Metasequoia glyptostroboides Hu & W.C. Cheng, respectively. The herbaceous plants were mostly perennials: Acorus gramineus Aiton, Aquilegia japonica Nakai & Hara, Dianthus chinensis L., Pratia pedunculata (R.Br.) Benth., Tagetes erecta L., Liriope platyphylla F.T. Wang & Tang, Sedum makinoi aurea (SA), and Hosta longipes (Franch. & Sav.) Matsum. The plants monitored were 2–3 years old.

The pilot plant used in this experiment was designed to have a soil depth ranging from 0.3 to 0.8 m. Because woody plants such as SJ, RI, MG, and AS grow rapidly with longer root depth, these plants were planted in pots measuring 1 m (L) × 0.4 m (W) × 0.6 m (H), while AF, DL, PB, and TL herbaceous plants were planted in pots measuring 1 m (L) × 0.4 m (W) × 0.4 m (H) as shown in Figure 1. Soil and sand compatible with LID technologies were used to ensure the restoration of the natural hydrological cycle, provide nutrients for plant growth, and contribute to the removal of pollutants. To ensure high drainage capacity and include sufficient nutrients, sand and original soils were mixed in a ratio of 3:7 (Korea Land & Housing Corporation report 2014).

Monitoring of plants was conducted from April 2015 to October 2016. The pilot test was conducted once or twice a month in consideration of antecedent dry days (ADD) and plant growth. The influent volume was 10 L, based on a reference rainfall of 20 mm, which is 80% of the accumulated rainfall occurrence frequency in Cheonan.

Figure 2 shows the monthly mortality rate of plants in relation to CaCl₂ concentration. In the year 2015, SJ, MG, RI, AF, DL, AS, PB, and TL plants were analysed while LP, SA, and HL were reselected and replanted for analysis in the year 2016 due to the death of some plants such as AS, PB and TL.

SJ had a higher mortality rate when the concentration of CaCl₂ was greater than 1,000 mg/L, and less than 40% at a concentration below 500 mg/L. The mortality rate of RI increased with CaCl₂ concentration, indicating that plant growth is influenced by CaCl₂. It is apparent that when water availability is restricted by salinity, all the factors which regulate or influence water absorption and water loss by plants contribute in some way to salt tolerance (Hayward & Bernstein 1958; Navarro et al. 2000; Kaya et al. 2002). MG was observed to have higher mortality at 1,500 mg/L CaCl₂ concentration, but showed a lower mortality rate of 17.5% compared to SJ and RI. Among the three types of trees and shrubs, MG showed the greatest tolerance to CaCl₂.

An analysis of the monthly mortality rates for the eight types of herbaceous plants showed that AS had the lowest mortality rate of 5%. Depending on the plant growing season, AF showed a high mortality rate after July. The longer sunshine exposure of 7.2 hours in the summer of 2015 caused PB to have a rapid increase in mortality rate. Both TL and DL had mortality rates of about 20% at the highest concentration of 1,500 mg/L, but very low mortality rates at lower concentrations. These species were rated as having a high tolerance to CaCl₂. LP was observed to have high growth despite CaCl₂ application. Meanwhile, some of SA died due to heavy rainfall (greater than 300 mm) in July 2016. HL mostly died after August. The relatively high mortality rate of plants which were not exposed to CaCl₂ indicated that plant death was caused not only by CaCl₂ but also was related to annual plant life cycle. The mortality of herbaceous plants in relation to CaCl₂ concentration was in the order of PB > AF > TL > SA > HL > DL > AS > LP. In particular, herbaceous plants have shallow roots that caused them to be more easily exposed to CaCl₂. As such, these plants were more affected by high concentrations of CaCl₂ than the shrubs and trees.

The change in moisture is an index that provides information on plant water stress (Han 2009; Shishi et al. 2015). Plants experience greater water stress when the water content in the soil decreases. Table 3 shows the plant biomass and DW of shoots, the biomass weight and DW of roots, the DM rate, and the root/shoot (R/S) ratio in relation to CaCl₂ concentration. The DM for the SJ control was 19.5% lower compared to the experimental group in which 1,500 mg/L CaCl₂ was applied, with a DM content of about 67.1%. RI had a DM rate of 79.0% for the control group, but 71.8% for the experimental group after exposure to a high concentration (1,500 mg/L) of CaCl₂. The R/S ratio decreased by 56% from 2.40 to 1.35 for SJ, and by 65% from 2.87 to 1.89 for RI. MG, on the other hand, was unaffected in terms of DM rate and R/S ratio. The DM rate and R/S ratio were analyzed for the eight types of herbaceous plants in relation to CaCl₂ concentration. A_.S was identified as the most affected herbaceous plant by CaCl₂ application, whereas the DM rates of AF, DL, PB, TL, and SA were unaffected. High concentrations of CaCl₂ influenced the DM rates of LP and HL. The R/S ratio decreased for AF and TL but remained largely unaffected for DL, PB, and LP. The roots were more influenced by CaCl₂ compared to shoots in the case of trees and shrubs (SJ, RI) and herbaceous plants (AS, AF, TL, and HL), which saw a decrease in the R/S ratio. CaCl₂ has a greater impact on the roots of herbaceous plants than trees and shrubs because calcium chloride causes membrane decay of plants, suppression of nitrogen uptake and disruption of ion transport, resulting in plant death (Cerda & Martinez 1988; Cachorro et al. 1994; Shen et al. 1994; Tuna et al. 2007).

Figure 3 shows the changes in the amount of chlorophyll per unit area for the control group (0 mg/L) and experimental group before and after the introduction of CaCl₂ (1,500 mg/L) in spring and summer. In spring, the amount of chlorophyll decreased by 0.4 to 8.6 μg/cm² after the introduction of CaCl₂. For SJ and RI, the amount of chlorophyll increased by 0.5 μg/cm² for the control group, but was observed to have a greater increase in chlorophyll for the experimental group of about 1.0 to 8.6 μg/cm² when exposed to a high concentration of CaCl₂. The amount of chlorophyll decreased for AF, DL, and HL when CaCl₂ was introduced at a concentration of 1,500 mg/L, indicating that CaCl₂ has an influence on the amount of chlorophyll. In summer (July), when plants were observed to have the highest growth, SJ, MG and DL showed an increase in chlorophyll regardless of the concentration of CaCl₂. However, RI, LP, AF, and HL exhibited a decrease in chlorophyll in relation to the introduction of CaCl₂. From the higher concentration of chlorophyll per unit area in spring compared to summer, it can be observed that the concentration of chlorophyll is affected not only by the concentration of CaCl₂ but by environmental factors, specifically sunlight and rainfall. The rate of photosynthesis varies according to the genetic difference of plant species and the rate of photosynthesis is only useful as a selection criterion for salt tolerance in those species in which there is a close relationship between photosynthesis and growth under salt stress (Lakshmi et al. 1996; Tezara et al. 2002; Burman et al. 2003).

A higher calcium chloride concentration accelerates the coagulation of soil particles, which leads to a higher permeation coefficient and influences the water content. In addition, soil exhibits hydrophobic properties, resulting in increased cations and facilitating acidification. When the water content in soil decreases, the soil becomes non-uniform and causes problems for plant growth (Transportation Research Board guideline 2007). Figure 4 shows the change of soil infiltration rate in relation to the concentration of CaCl₂. The control group (0 mg/L) showed an infiltration rate of 2.8–12.2 mm/h after 30 min but was later decreased to 0.7–2.6 mm/h at 60 min. At a high concentration (1,500 mg/L), the infiltration velocity was 2.5–8.1 mm/h at 30 min, and 0.60–3.0 mm/h at 60 min. The initial infiltration rate of approximately 50 mm/h decreased to 0.15–3 mm/h at the end of the experiment. This can be attributed to the influent being introduced using the pulse input method, such that the saturation of the soil drops after a certain period. If the influent is introduced continuously, a higher and more stable infiltration rate can be achieved. The concentration of CaCl₂ is likely to have influenced the infiltration rate as the soil was sufficiently drained, providing a high drainage capacity. By plant species, herbaceous plants exhibited faster declines in infiltration rate compared to the tree and shrubs. This finding was attributed to the roots of plants and the activity of roots in relation to CaCl₂. In general, all species showed similar infiltration rates at 60–90 min because the soil saturation dropped with rapid drainage up to 60 min. Since it is essential for LID facilities to maintain an adequate level of water content within 3 days of the end of heavy rain, the change of infiltration constant in relation to CaCl₂ and type of plants can be used to calculate the adequate infiltration rate.

Plants are widely used in LID technologies because they establish natural water circulation based on undercurrent, infiltration, and evapotranspiration. However, de-icers (CaCl₂) used to melt snow during the winter change the soil content and affect the growth of plants. This study assessed the effects of CaCl₂ application to plants in LID technologies. Based on the results, the following conclusions were drawn.

1. The mortality rates of the tree and shrubs in relation to the application of CaCl₂ was in the order RI > SJ > MG, while that of herbaceous plants was in the order PB > AF > TL > SA > HL > DL > AS > LP. Herbaceous plants have shallow roots that caused more exposure to CaCl₂ and they were therefore more affected by high concentrations of CaCl₂ compared to the tree and shrubs.

2. An evaluation of plant activity (DM rate, R/S ratio and amount of chlorophyll) showed a decrease in the amount of chlorophyll when exposed to a high concentration of CaCl₂. This finding indicated that the amount of chlorophyll is significantly influenced by CaCl₂. Compared to herbaceous plants, the tree and, shrubs were found to have a higher tolerance to CaCl₂.

3. Both the DM rate and R/S ratio were found to decrease at increasing CaCl₂ concentrations for AS, SJ and RI. On the other hand, it was found that the DM rate and R/S ratio of MG, DL, SA and PB were not affected by the change in CaCl₂ concentration. The DM rate of LP and HL decreased with increasing CaCl₂ concentrations. Lastly, only the R/S ratio decreased for AF and TL at increasing CaCl₂ concentrations.

4. The control group (0 mg/L) showed an infiltration rate of 2.8–12.2 mm/h after 30 min, which later decreased to 0.7–2.6 mm/h at 60 min. At a high concentration (1,500 mg/L), the infiltration velocity was 2.5–8.1 mm/h at 30 min, and 0.60–3.0 mm/h at 60 min. The infiltration velocity was similar regardless of CaCl₂ concentration and plant species after 60–90 min because of the decline in soil saturation arising from rapid drainage up to 60 min.

This work was supported by the Korea Environment Industry & Technology Institute (KEITI) through the Public Technology Program based on an Environmental Policy Project, funded by the Korea Ministry of Environment (MOE)(2016000200002).