Salt contamination and mineral nutrition in the soil rooting zone and tree foliage
In 2014 by the end of vegetation season, the root zone soil in Riga’s street lines showed sizable but very variable salt contamination. In comparison to the unpolluted NBG site (Na: 13.30 ± 0.30, mg kg-1, range 12.75-13.75; Cl: 6.48 ± 0.36 mg kg-1, range 6.12-7.21), the highest NaCl concentrations in the soil were observed at sites with slight foliage contamination (Na: 373.08 ± 158.28, mg kg-1, range 27.81-940.80; Cl: 43.4 ± 16.31 mg kg-1, range 11.62-115.20). With 28.69/6.69 times more Na/Cl on average than at NBG, soil pollution thus exceeded that found in the case of severe foliage contamination, with soil concentrations of Na/Cl then only 13.45/2.49 times higher than at NGB (Na: 178.87 ± 73.20, mg kg-1, range 34.86-712.50; Cl: 16.16 ± 3.16 mg kg-1, range 9.20-39.90). Similarly, the sites with intermediate versus slight foliage contamination also showed lower Na/Cl soil pollution (Na: 60.39 ± 10.51, mg kg-1, range 28.06-89.04; Cl: 20.78 ± 1.73 mg kg-1, range 15.84-26.88) and exceedance of contamination threshold (4.54/3.21 times). At the end of summer, there was thus little agreement between the trends for NaCl soil and foliage contamination. Irrespective of the NaCl soil pollution, the nutrient supply for street trees was dystrophic because of imbalances in several mineral elements. All sites (including NBG) showed low levels of N, with regard to the sufficiency range in the case of deciduous trees (Table 2). The soil concentration of K, S and B also showed frequent suboptimal levels. In contrast, that of other elements sometimes exceeded the tree requirements. This was especially the case of Ca and Mg, which led to high soil pH at sites showing the largest exceedances (Valdemara, Barona, Gertrudes and Aspazijas). Elevated metal concentrations (Fe, Zn, Cu) appeared in priority related to the urban location of study sites.
The sub-optimal site conditions were only partly reflected by the foliar chemistry. Compared to foliage from the unpolluted NBG site (Na: 86.00 ± 7.57, mg kg-1, range 72-98; Cl: 760.00 ± 120.97 mg kg-1, range 530-940), the average foliar concentration of salts in 2014 gradually increased from the slightly (Na: 895.00 ± 476.89, mg kg-1, range 118-3100; Cl: 4743.33 ± 643.31 mg kg-1, range 3000-7570) to moderately (Na: 4506.67 ± 534.41, mg kg-1, range 2660-6600; Cl: 8655.00 ± 1168.30 mg kg-1, range 5000-11500) and severely polluted (Na: 8000.00 ± 793.03, mg kg-1, range 660-13600; Cl: 92.65.56 ± 1340.18 mg kg-1, range 3000-16750; Fig. 1) sites along the selected gradient of foliage contamination. Versus levels at NBG, the Na/Cl concentrations were thus 10.41/6.24, 52.40/11.39 and 93.02/12.19 times higher respectively. However, the foliage versus root zone soil of studied trees showed less nutrient imbalances on average, noteworthy with regard to N, S and B (soil concentration below the sufficiency range) or Ca, Mg, Zn or Cu (soil concentrations showing exceedances; Table 3 versus Table 2). Some elements locally in excess in the root zone soil (Mn) showed, nevertheless, frequent deficiencies in the foliage, particularly at sites with high soil pH. In the case of K and similar to observations in the root zone soil, the deficiencies in foliage were frequent, especially at the salt-contaminated sites. In a direct or indirect way, the elevated salt levels in foliage played a driving role regarding the observed foliar concentrations of several nutrients (Table S1A). The foliar concentration of Ca thus dropped in response to higher accumulation of Na and Cl in foliage, whilst that of K, Mo increased with higher Cl – but not Na – contamination (Fig. 3, Table S1A). Foliar salt accumulation also led to increased Zn levels, whereas the relationship may be rather correlative than causal.