Salt contamination and mineral nutrition in the soil rooting zone and tree foliage
Given sampling by the end of vegetation season, the levels of salt contamination measured in the soil of Riga’s street sites in September were consistent with the long-term trends and seasonal patterns of soil pollution (Cekstere & Osvalde, 2013). The eight times-lower pollution levels of Cl versus Na could relate to chemical mobility and non-reactivity of Cl- anions with soil adsorption complexes, causing rapid leaching (White & Broadley, 2001). The Na+/Cl- pollution levels were similar to those also recorded in September at Opole (330/170 mg kg-1, Czerniawska-Kusza et al., 2004) or in Ontario (Na+: 51-115 mg kg-1, Eimers et al., 2015) but inferior to those reported for the same season in Warsaw (2392/3599 mg dm-3, Dmuchowski et al., 2014). They were also lower than those measured in spring in Riga (724/129 mg kg-1, Cekstere & Osvalde, 2013), Toronto (Na+: 450-600 mg kg-1, Ordóñez-Barona et al., 2018), Moscow (517.5/480 mg kg-1, Nikolaeva et al., 2019) and Northeastern China (352–513/577–2,353 mg kg-1, Fayun et al., 2015). In this one-year study, the contamination levels of NaCl in the soilversus lime tree foliage measured in September showed poor agreement, contrasting with earlier findings in Riga and other cities (Cekstere et al., 2008; Dmuchowski et al., 2014; Ordóñez-Barona et al., 2018). Besides the longer-term NaCl dynamic within tree organs, the discrepancy between concomitant salt leaching in the soil versusaccumulation in foliage during the vegetation season may form the main causal factor.
In addition to salt pollution, the soil conditions of street lines of lime trees were dystrophic in several other instances, with especially an alkaline pH and nutrient levels of N, K, S and B often below and those of Ca, Mg, Fe Zn, Cu above the sufficiency range. The main causal factors may include 1) the mostly sandy bedrock (Bouraoui et al., 2019), 2) N leaching and denitrification as frequently observed in urban conditions (Scharenbroch & Lloyd, 2004) and 3) construction debris typical of anthroposoils enhancing the Ca, Mg content and promoting alkalization (Jim, 1998; Oleksyn et al., 2007; Cekstere & Osvalde, 2013; Dmuchowski et al., 2019). The low K concentrations, below those measured in Seville (Ruiz-Cortés et al., 2005) and Moscow (Nikolaeva et al., 2019), were still higher than those recorded in Poznan (Oleksyn et al., 2007). The enhancement of Fe, Zn and Cu soil content at street versus NBG sites reflected the busy urban traffic (Jim, 1998; Dmuchowski et al., 2014, Nikolaeva et al., 2019). Dystrophy trends can be worsened by salt pollution, with Na+ dislodginge.g. NH4+, K+, Ca2+ or Mg2+ cations within the soil adsorption complexes (Dobson, 1991, Bryson & Barker, 2002, Eimers et al., 2015). However, the enhanced salt concentrations did not affect the physical structure of Riga’s sandy soils (Bouraoui et al., 2019). The dystrophic soil conditions in 2014 showed good agreement with previous surveys (Čekstere, 2011; Cekstere & Osvalde, 2013).
With foliar concentrations of Na+/Cl- up to 13600/16750 mg kg-1, the salt accumulation within Riga’s lime tree foliage reached values similar to those of major macro-nutrients. Hence a main reason for the reported salt sensitivity in the Tiliagenus (Dobson, 1991, Dmuchowski et al., 2013, Sera, 2017) appears to be its poorly developed salt exclusion strategies (Munns & Tester, 2008; Chen et al., 2018). This finding is corroborated by those on other lime tree species showing NaCl accumulation levels higher than in other sensitive (Paludan-Müller et al., 2002) or salt tolerant (Dmuchowski et al., 2013) ornamentals. Foliar injury threshold (1-5 % necrosis percentage area) ranging between 660-3100/3000-7570 mg kg-1Na+/Cl-, in good agreement with previous findings (Čekstere, 2011), indicated higher sensitivity in Riga’s lime trees versus otherTilia genus (Kopinga & van den Burg, 1995) or salt-tolerant species (Dmuchowski et al., 2019). Such intra- and interspecific variation in salt sensitivity is of interest, with a view to selecting better-tolerant ornamentals for street greeneries.
Despite the strongly dystrophic nutritional conditions and poorly structured soil substrate of Riga’s street greeneries, the nutrient balance within T. x vulgaris foliage remained astonishingly well conserved. Even within infertile habitats (Chapin, 1980), the adapted plants generally achieve sustainable mineral nutrition and theTilia genus shows a particularly large tolerance with regard to the soil type and site conditions (Radoglou et al., 2008). The sufficient (Zn) or excessive (Fe, Cu) foliar concentrations of anthropogenically-enhanced metal content in the soil were in good agreement with findings in other cities (Baycu et al., 2006; Dmuchowski et al., 2014). Mn deficiency in foliage despite a sufficient soil supply - similar to other cases of salt pollution (Dmuchowski et al., 2014), may relate to oxidation to Mn4+ forms hardly available for plants in neutral to slightly alkaline soil conditions (Marschner & Marschner, 2012). Foliar K deficiency could result from 1) low K content in sandy soils, 2) dislodging from the soil adsorption complexes by Na+ or 3) seasonal changes (Mengel & Kirbby, 2001). Hence, the missing negative/even paradoxically positive correlation between K and Na/Cl concentration, despite the Na+-K+ antagonism observed otherwise at tissue and cell level, may relate to complex and interfering salt, nutrient and foliage dynamics. In the case of less limited Ca supply, with opposite accumulation dynamic in foliage as compared to K (Marschner & Marschner, 2012), the negative correlation with both Na and Cl foliar concentrations were consistent with findings at tissue and cell level.