Urban soil and mycorrhizal symbiosis – is it always lower?

Associations (symbiosis) with mycorrhizal fungi are necessary for a vast number of tree species, and for those where associations are not necessary for (mainly pioneer species such as willow and birch) then such a lack of association will usually lead to the tree having significantly stunted growth. In this sense, a symbiosis between tree roots and mycorrhizal fungi is very much crucial. Therefore, where associations are lacking of abundance, which may particularly be the case in urban environments (because of soil compaction, construction damage, changing pH levels due to pollution, and so on), trees may markedly suffer. In fact, I have looked at a paper quite recently on such a lack of mycorrhizal symbiosis with fine root hairs in urban locations.

Granted, certain tree species do fare rather well in the urban environment, and therefore it may be the case that these species are able to associate more successfully with mycorrhizal fungi (amongst other reasons). By a similar token, the properties of the fine root mass of trees in urban areas may differ from rural locations, which of course would have implications for mycorrhizal associations. As a means of testing this, the well-known and well-used tree species horse chestnut (Aesculus hippocastanum) was studied across four urban and two rural sites in late autumn in Poznan, Poland, with specific focus upon the extent of arbuscular mycorrhizal colonisation in the trees’ root masses, the properties of the fine rooting masses, and soil chemical properties. The authors note that the horse chestnut is an ideal tree species to use for such a study, because is is well-adapted to the urban environment, and has been cultivated for ornamental planting for many centuries.

Briefly touching upon the actual study sites, the first two urban sites (URB1 & URB2) features horse chestnuts planted in an avenue between 1909-1911. this avenue is 10m wide, and on either side sits some form of public highway. The avenue has always had poor soil conditions, and in 1998 the two URB sites were mulched in an attempt to improve soil conditions. Site URB1 has, since then, received routinely mulch applications, whilst site URB2 hasn’t. The third urban site (URB3) is in the city centre and near a main public highway, within the Henryk Wieniawski Park. This site was planted with horse chestnuts (amongst other species) in 1907 and 1910. URB4 (a closed cemetery in General Jan Henryk Dabrowski Park) consists of many tree species, including horse chestnut, which were planted somewhere between 1830-1930. The two rural locations were both where horse chestnuts were situated in Wielkopolska National Park, which is 20km to the south of Poznan. The two locations were within horse chestnut avenues, bordering agricultural fields, established during 1800-1850, making them some of the oldest horse chestnuts in the region.

The avenue where sites URB1 & URB2 were located. Source: Wikimedia Commons.

With regards to the results, soil analyses identified that lead and copper levels were significantly higher in urban environments, and that elements such as calcium, carbon, magnesium, phosphorus, potassium, and sodium, also varied between all six sites to rather marked extents. No significant difference was found between sites for elements such as nitrogen and sulphur and, overall, the toxicity of urban soils (associated with higher levels of lead, and sodium, in particular) was considered to be only of a low level, and rural sites had a level toxicity as well because of chlorine (associated with fertilisers).

Despite this, the rural sites were shown to be significantly different in soil characteristics to the urban park sites (URB3 & URB4), which were in turn significantly different to the street-side urban sites (URB1 & URB2). Similarly, fine root characteristics of the horse chestnuts studied varied significantly between the rural and urban sites. In sites URB1 & URB2, for example, the fine root biomass was 1.6-2.2x greater, as was fine root length, surface area, and volume. This was very likely because of the mulching undertaken on these sites, which would have, particularly in the case of URB1, consistently improved soil conditions in the upper layer, thereby enabling better fine root growth.

However, fine root tip density was 1.3-1.4x lower in urban sites. Conversely, in rural sites, fine roots were thinner, and specific fine root lengths (the ratio of fine root length and the dry mass of such fine roots), and also specific fine root areas, were greater. This suggests that those horse chestnuts in rural areas, because of the better soil conditions (reduced bulk density and associated soil compaction), had the ability to produce finer rooting masses that could disseminate out into the soil with greater ease. In turn, nutrient and moisture uptake is, theoretically, easier for the tree.

Interestingly, this is where the differences end. In relation to the rate of colonisation by arbuscular mycorrhizal fungi (specifically, vesicles, hyphae, and coils), and even fungal endophytes, there was no significant difference between those horse chestnuts in rural environments and those in urban environments (though urban settings did yield greater abundances of arbuscular mycorrhizae and fungal endophytes, on average – around 1.2x higher for arbuscular mycorrhizae). The horse chestnuts at URB1 & URB2 had the highest abundance of vesicles, for example, and those at URB3 had the highest abundance of hyphae and coils (as shown in the table below). The abundance of vesicles is considered to be because they are most routinely found during late autumn, which coincided with the timing of this study.

Colonisation rates for each site.

Such results are also interesting because they are vastly different to the study I looked at in this blog post, which presented arbuscular mycorrhizal fungi colonisation rates down in the 20% range for urban sites and the 40% range for rural sites for the horse chestnut. Perhaps, the stressors that could impact upon such colonisation rates are not so evident in Poznan, when compared to Ontario, Canada. Additionally, as the horse chestnuts studied here are all very much mature, perhaps their likely longer existence in situ has enabled for a greater rate of mycorrhizal symbiosis.

Furthermore, the authors did note that the horse chestnuts in the rural locations were more extensively defoliated by horse chestnut leaf miner (Cameraria ohridella) and leaf blotch (Guignardia aesculi). Compiled with the fact that agricultural fertilisers would have been applied in the vicinity of the rural horse chestnuts (as known by the high chlorine content of the soils), such factors may have lead to lower colonisation rates by arbuscular mycorrhizal fungi – for the latter factor, there is a vast pool of research outlining how fertilisers adversely impact upon such associations.

In essence, therefore, one can conclude that, at least in this case, urban soils are not poor enough to impact upon mycorrhizal associations with horse chestnut roots. From this, it can be posited that, if urban soils are looked after so that they are not hugely adverse in properties, then they can support similar, or higher) levels of mycorrhizal fungi associations than rural locations. Because such associations are critical for the health of trees, it may enable for urban trees to live healthy lives that are not prematurely terminated by damaging environmental factors. Therefore, focus should be paid to the soil environment, as such an environment may be a key (of perhaps many) to safeguarding our urban tree populations.

Source: Karliński, L., Jagodziński, A., Leski, T., Butkiewicz, P., Brosz, M., & Rudawska, M. (2014). Fine root parameters and mycorrhizal colonization of horse chestnut trees (Aesculus hippocastanum L.) in urban and rural environments. Landscape and Urban Planning. 127 (1). p154-163.

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Urban soil and mycorrhizal symbiosis – is it always lower?

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