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.