The reaction of the urban soil-plant complex on abnormal weather conditions

. This research is devoted to the study of geochemical features in the soil-plant complex of green spaces in Saint-Petersburg and their changes in abnormal weather conditions, which was the first experience of such work in this region. It included the research of such aspects as accumulation of heavy metals by plants from the soil, photosynthetic pigment apparatus of plants and the content of organic carbon in the soil. The X-ray fluorescence analysis method was used to determine the content of heavy metals, the determination of organic carbon in the soil was carried out in accordance with the Tyurin method, the content of chlorophyll and carotenoids was determined by photometrical method. The study showed that in stressful conditions (abnormal hot weather) there are changes happening in the pigments system of the leaf and in the barrier function in relation to heavy metals.


Introduction
Global climate change, manifested, among other things, in summer temperature anomalies, causes changes in the physiological state of plants and their ecological functions in various conditions. A city green spaces are among the most sensitive ecosystems to climate change, and they have a key role in forming healthy urban environment, so their physiological stress, considered in the framework of this study, can affect entire urbanized ecosystems [1].
The summer season of 2021 in Saint-Petersburg had abnormally high temperature period in beginning of vegetation (in June and July, an excess of the average monthly temperature by 5 °C was recorded), when plant have not yet reached their maximal tolerance. Previous abnormally hot summer seasons (2010) started about 1 month later, than in 2021, what caused higher stability. In the next summer season -2022 -the heat, atypical for this region, fell at the end of the vegetation period (August), which meant that plants achieved greater stability compared to 2021, however, together, two stressful seasons affected the productivity and duration of vegetation of a number of the species considered. The ecological aspects of the reaction of city vegetation on high temperature stress was not studied before in the region.
Plants of urban green spaces -trees and herbaceous species -perform the most important ecological functions in urban ecosystems, ensuring the utilization of carbon dioxide from the air, enriching the air with oxygen, perform a barrier function against pollutants accumulating in soils as a result of anthropogenic impact. Photosynthesis of urban plants occurs when they grow on polluted soils, which means that it takes place under constant stress for plants. The resistance of plants to such adverse factors varies from species to species. The influence of a complex of adverse factors is more pronounced than the impact of individual factors, and can lead to irreversible physiological changes and, as a consequence, a decrease in the ecological barrier functions of vegetation cover [2,3].

Materials and Methods
The research was conducted in Admiralteysky and Petrogradsky districts of St. Petersburg in June-September 2021 and 2022. Five species of trees (European ash (Fraxinus excelsior L.), Horse chestnut (Aesculus hippocastanum L.), European white elm (Ulmus laevis Pall.), Ashleaved maple (Acer negundo L.), Silver poplar (Populus alba L.)), three species of herbaceous plants (Broadleaf plantain (Plantago major L.), Dandelion medicinal (Taraxacum officinale L.), Meadow bluegrass (Poa pratensis L.)) and one species of moss (Barbilophozia barbata), presented in various conditions: a street, a park and the space between houses, were considered as objects. The age of the studied trees was 30-50 years.
Two parameters were used to study the physiological state of urban vegetation: • Maximum daily photosynthesis intensity.
• The content of heavy metals and oxides in plant leaves and soils in growing areas (Sr, Pb, Zn, As, Ni, Cu, V, Cr, MnO, Fe2O3, TiO2). The study of these issues was carried out on samples of leaves and soils collected during the summer weather anomaly in 2021, and continued in the summer and autumn of 2022. The analysis of the objects was carried out using the methods of spectrophotometric determination of the content of photosynthetic pigments of leaves -chlorophylls A and B and carotenoids in accordance with the Lichtentaller method [4,5], measuring the intensity of photosynthesis using a CO2 analyzer based on touch sensors Libellium and studying the behavior of heavy metals (HM) in the soil-plant system by X-ray fluorescence spectroscopy, as well as determining the content of organic carbon in soils according to the Tyurin method [6]. The study was conducted during and after the climatic anomaly (May-September) in 2021 and continued during the growing season in 2022.

Results
The first part of the study concerned photosynthesis and utilization of carbon dioxide by urban vegetation during the hot period and the subsequent period of stabilization of air temperature within the climatic norm. [7] It is shown that in the hot period the photosynthetic activity of all the studied plant species decreases regardless of the tier, photosynthesis practically stops and in some species during the daytime hours of the hottest days. Thus, in the leaves of the petiolate oak (Quercus robur L.) there was a transition to respiration with the release of carbon dioxide in the hot period, at an air temperature above 34°C, which is shown in Figure 1. Plants showed different species' abilities to utilize CO2 at this time. After the normalization of weather conditions, they only partially restored photosynthetic activity [8]. The maximum photosynthetic activity was observed in trees represented in the park, the minimum -in trees growing near busy streets and in ordinary plantings on boulevards. Changes in the utilization of carbon dioxide by urban vegetation caused by extremely hot and dry weather have led to an increase in the concentration of CO2 in urban areas, including recreational and residential areas [9].
The study of an important ecological function of urban vegetation and soilsbioaccumulation of pollutants -included studies of the content of HM in the leaves of woody and herbaceous plants and soils from places of their growth for the content of heavy metals and oxides. The effectiveness of the barrier function of vegetation in relation to heavy metals depends on many factors, including the species, physiological state of plants, the level of soil contamination and their geochemical features [10]. This indicator changes in extreme weather conditions, which was shown in the study. The degree and nature of the changes are species-specific, moreover, the barrier function in relation to various heavy metals varies differently in the same plant species ( fig. 2-5).    The analysis of the coefficients of the biological transition of heavy metals in the soilplant system showed the greatest vulnerability of the barrier function of the studied species to arsenic and zinc. Their content in the leaves significantly exceeds its content in the soil. This is due to the fact that during the period of stress caused by weather conditions abnormal The coefficients of biological transition for other heavy metals (strontium, lead, nickel, chromium, vanadium) are less than one, that is, their accumulation undergoes less changes when barrier functions are violated [11,12]. The greatest increase in biological transition coefficients is observed in southern introducers -white poplar (Populus alba L.) and horse chestnut (Aesculus hippocastanum L.). In St. Petersburg, these plants are more vulnerable, therefore, a more severe reaction to stress was observed in their attitude [13].
In the summer season of 2022, there was practically no violation of barrier functions, which can be explained by the later beginning of the period of atypically high temperatures for the region [14]. However, at the same time, an early exit from the vegetation of horse chestnut (Aesculus hippocastanum L.) and ash maple (Acer negundo) was noted. Moss Barbilophosia barbata did not grow after the stressful period of 2021, which leads to the conclusion about the impact of abnormally high temperatures of the previous season on these species.
The study of the content of photosynthetic pigments in leaves showed the presence of non-specific stress reactions in all species: an increase in the proportion of chlorophyll B and carotenoids to chlorophyll A, in some species (lawn grasses, small-leaved linden) there was a general decrease in the content of photosynthetic pigments, which does not recover after the end of the hot period. Figures 6 and 7 show the results of the analysis of the total chlorophyll content in the plants of the green spaces of St. Petersburg in weather dynamics. Figures 8-15 show fluctuations in the ratios of chlorophylls A and B and the ratio of the amount of total chlorophyll to the amount of carotenoids from species to species. At the same time, it was noted that the content of total chlorophyll was unchanged in the southern introducers due to the fact that for their native regions, temperatures, abnormal for the St. Petersburg, are typical.         The study of the composition of intra-block soils, conducted in accordance with the Tyurin methodology, showed a reduced content of organic carbon relative to the soils of the park massif. Figures 16 and 17 below show changes in the content of organic carbon in soils depending on weather conditions: abnormally high temperatures fell in June, rains and a drop in temperature fell in July, August results were presented under conditions of temperature normalization, and then cooling.  It was shown that the content of total chlorophyll was unchanged in the southern introducers due to the fact that for their native regions, the temperatures abnormal for St. Petersburg, are typical.

Discussion
The study of the geochemical features of the reaction of the soil-plant complex to abnormal conditions, expressed in the form of abnormally high temperatures, in the conditions of St. Petersburg showed that physiological stress is expressed in a change in the photosynthetic pigments concentration in leaves, in the CO2-exange level in leaves, as well as the barrier function in relation to HM. These indicators, which did not recover after the end of the stressful period, led to an early exit from the vegetation of a number of plants, for example, heart-shaped linden (Tilia cordata Mill.). The summer season of 2022, characterized by abnormally high temperatures in August, coupled with the previous anomaly of 2021, provoked an early completion of vegetation in other plants, for example, ash maple (Acer negundo L.), and negatively affected the viability of herbaceous plants and mosses [15].

Conclusions
Thus, the study showed, that prolonged periods of hot and dry weather lead to physiological stress in urban vegetation species. As a result, their main ecological functions -CO2 utilization and blocking the transition of heavy metals from the soil (preventing circulation in nutrition chains) -are changing -the ability to preserve the barrier function decreases, and as a result, plants, regardless of the tier, accumulate heavy metals. This leads to an increase in the level of pollution of the urban ecosystem.
The stress response was also manifested in a violation of the classical behavior of photosynthetic pigments: an increase in the proportion of carotenoids and chlorophyll B was observed with a general decrease in the content of photosynthetic pigments. However, this effect was noted only in plants native to the northern part of Russia, no such effect was found in southern introducers.