Chemical Features of Urban Soils

Urban soils show chemical human impacts; compared with natural soils their pH, element cycling and nutrient availability are altered. Their node function as nutrient source and buffering system or as detoxification medium is restricted or imbalanced. Fertilizer or compost application, atmospheric pollution and outwash via stem-flow, heavy metal contamination from various sources, de-icer from winter maintenance, contaminated irrigation-water, debris from exotic species, litter removal etc. contribute to chemical deviations from the natural element budgets and have additive or antagonistic effects.

Fertility of Urban Soils

Usually the foliage of most urban trees is acceptably green even though nutrient deficiencies and nutrient imbalances or even toxicities may significantly limit their growth. Generalizing: the more natural the urban site and vegetation type, the less nutrient deficiencies and imbalances occur. For urban soils, it is difficult to ascertain the origin of parent material and vertical and spatial variability is high. Soil testing and fertilization to ensure good vegetation growth is also important. However, for arboriculture applications there is still an information lack concerning nutrient requirements of the respective species.

Organic matter is a major energy source of soil biota, which brings nutrients back into the cycle. Urban soils generally lack organic matter and its nutrient contribution, because nutrient-containing litter is removed during maintenance. This deteriorates not only carbon pools and fluxes but also influences nutrient cycling and has via changes of soil inhibiting organisms a severe impact on nutrient availability. Off-site litter disposal removes high amounts of valuable plant nutrients and accumulates them elsewhere. On the other hand, the positive side effect is the removal of toxic elements and biotic stressors. Because of frequent compost application and intensive tillage combined with a relatively closed nutrient cycle soils of urban gardens and parks show higher nutrient contents compared with street tree sites, which have a severely deteriorated nutrient cycle. The nutrient cycle of urban woodlands are nearer to the natural, but still show the impact of urbanization e.g. air pollution.

Without permanent remediation, the nutrient supply for urban street trees becomes insufficient or unbalanced. Soils of urban street tree sites have a restricted nutrient availability and potential for several elements; N, P, K, and Mg are frequently lacking elements; the humus contents are low. The majority of tree species prefer soil-pH between 4.5 and 7.0. There are exceptions: One should check for the respective tree species (see e.g., Warda 2002). Soil-pH influences many processes, such as the release, availability and uptake of nutrients and pollutants from the soil. A pH below 4.0 that is present in soils, increases availability and uptake of heavy metals and can cause manganese and aluminum toxicity. Soil-pH above 7.5 can cause shortage of Fe and Mn.

Calcium depletion is also typical in acidic soil. Depending on parent material all over Europe pH of urban soils is highly variable. In humid regions of the north acidic soils with low base saturation predominate, in middle and south Europe soil-pH is mostly too high for several tree species. High soil pH increases antagonistic phenomena and malnutrition of urban trees: in alkaline substrates, the plant availability of micronutrients like B, Fe, Zn,

Mn and Cu is reduced because they remain insoluble. The positive effect of soil-alkalinity is that toxic elements like Al or Pb remain unavailable for root uptake. High soil pH influences quality and quantity of mycorrhiza, alters fungi species composition, diversity and vitality; urban trees have less mycorrhiza, which enhances the nutrient deficiency. Fertilization or organic amendment fails as long as the pH has not been adjusted. The pH may be lowered by application of one of several sulfur compounds or by addition of acidic organic matter. Raising of pH by liming is the common practice, which also has a longtime effect. Both lime and sulfur-compound application is more effective when mechanically worked into the soil.

For improving nutrient levels of alkaline soils acidic, slow-release fertilizers are proposed, but changes in availability of mineral nutrients or toxic elements by simultaneous adaptation of soil pH have to be taken into account. Antagonistic and syner-gistic effects of the fertilizer constituents have to be kept in mind and basic knowledge of the respective soil nutrient concentrations are helpful. Technique and timing of amendment application are controversial topics. Both surface fertilization in the forms of application or injection into deep horizons have advantages depending on soil textural distribution and the respective element. Two examples: because of their high mobility soluble nitrogen- or potassium-fertilizer applied on top of a soil with high hydraulic conductivity will quickly reach the main rooting horizon, immobile phosphorous will remain on top. Slow release fertilizer injections into the soil or packed under tree balls at planting might guarantee sustainable nutrient supply and deep rooting. Because of maintenance methods, soils of urban tree sites are generally poor in nitrogen. As nitrogen is the most important element for tree growth, excessive nitrogen input to soils may cause nutrient imbalances. The uncontrolled application of agro-mineral fertilizers (e.g., NPK) may lead to high soluble nitrogen contents in urban soils.

Sustainable remedies to improve the nutritional situation for urban trees are in situ recycling of urban vegetation litter. This has to be handled very carefully because of possible chemical and biological contamination of vegetation debris. Recycling of urban tree litter in situ contributes to biological urban soil improvement, delivers adequate organic material for soil fauna and helps to save money. Under-planting with shrubs can provide litter protection, humus layers are formed by development of 'more natural' structured vegetation, which has the positive side effect of decreasing temperature gradients. It improves soil gas balance and living conditions for soil biota. E.g. trees with grass-covered disks are better nitrogen-supplied despite of root concurrence, because of lower denitrification-rates due to less soil compaction (Nossag 1971).

Nutrient losses are partly compensated by other sources like fertilization, compost application, mulching, particular deposition or construction debris. Provided quality control they are appreciable soil amendments. Organic soil conditioners function as slow release fertilizers and improve deteriorated soil physical properties. Because of maintenance methods, soils of urban tree sites are generally poor in nitrogen. As nitrogen is the growth-determining element, excessive nitrogen input to soils may cause nutrient imbalances. The uncontrolled application of nitrogen-containing fertilizers may lead to high contents of soluble nitrogen in the soils, which may be washed out to the groundwater table.

Nitrogen emission to urban soils coming from intensified agricultural production and animal husbandry in the agricultural-urban interface represent the type of a point source. E.g. in recreation forests near poultry farms N-inputs of 80-120 kg N ha-1 yr-1 were measured (Sieghardt and Hager 1992). This induced rapid growth and depletion of other nutrients and had a severe impact on nutrient balance of trees; tree vigor and frost-resistance were reduced. Some fertilizers like urea or ammonium-sulfate are used as additives for de-icing products for paved pathways or pedestrian areas. N-containing de-icers induce over-fertilization of soils and eutrophication of water and enhance nutrition imbalances for vegetation. They increase transpiration and deteriorate the soil nitrogen cycle. In alkaline soils they contribute to gaseous nitrogen losses. For Vienna (A) these additives are already prohibited. Tree sites where people frequently walk their dogs are severely affected. Dog urine contains high amounts of P and NH3 and has a low C/N-ratio. It is alkaline and highly corrosive for above ground plant parts. Via stem flow the trunks and root systems are osmotically affected, bark-colonizing algae die as well as the tissue under the bark. Wood decaying fungi penetrate into the trunk. Later nitrification of NH4 causes soil acidification and an increase of water-soluble mineral compounds (heavy metals release), and unbalances plant nutrition (Balder 1998; Balder et al. 1997). Even if these sources are just selective and affect trees locally, they can be severe stressors for urban greenery in high-populated urban areas.

Chemical Pollution of Urban Soils

In urbanized areas a large number of inorganic and organic elements and compounds deriving from different point or non-point sources end up in the soil. Depending on the respective pollutant, concentrations and exposure time, soil contents may go beyond tox-icity levels, contaminate, and degrade the soil. They contribute to unfavorable soil conditions for urban vegetation. The special features of urban climate and urban water cycle influence the distribution of pollutants, emitted as gases, particulates or liquids. Direct contamination by discharging from sewer systems is another possible source. Sometimes chemically polluted soils require remediation not only to provide an appropriate growing medium but because they have become medical threats for urban people at least for young children with their 'hand-to-mouth-activity'. The sources and pathways of hazardous substances in urban soils are generally as described in Table 11.2.

Table 11.2. Sources and pathways of hazardous substances in urban soils

Pathway via

Form

Source

Atmosphere

Solid particles (dust), liquids (aci

Industry, manufacture, traffic,

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