4.1 Changes in the biodiversity of steppe plants associated with aridization
Our results demonstrated that the biodiversity of the study steppes showed a similar range of plant families. However, the Poaceae family prevailed in the biomass of true steppes, whereas the proportion of forbs (Compósitae, Caryophyllaceae, Umbelliferae, and Asteraceae families) in the biomass increased to the south, and the Asteraceae and Amaranthaceae families dominated in dry steppes. This fact can be most likely attributed to climatic parameters, which vary quite significantly within the steppes under study. For instance, the average values of MAP, MAT and IDM in true steppes were 450, 6.3 and 23, respectively; in desert steppes – 350, 8 and 17, respectively, and in desertificated steppes – 250, 10 and 12, respectively (Fig. 1). The water limitation reduces the productivity of species because it reduces the ability of dominants to develop sufficient growth, even under nutrient-rich conditions (Zhu et al. 2019). The steppes of midnorthern hemisphere latitudes are more sensitive to variations in water availability, and there is a strong positive correlation between grassland plant community biomass and precipitation (Sun et al. 2010, Xu et al. 2018, Liu et al. 2019). For the steppes of Central Mongolia, it was also been shown that the accumulation of aboveground biomass depends on the moisture conditions of a particular year, and with an increase in moisture, plants of the Poaceae family predominate, while with a decrease, the proportion of Asteraceae increases (Bazha et al. 2008). It has been pointed out that a prolonged lack of moisture and increased anthropogenic pressure on steppes can result not only in a decrease in the ecological characteristics of dominant species, but also in irreversible death of the indigenous inhabitants of the steppes (Danzhalova et al. 2011).
Adaptive reactions of plants to climate aridization primarily depend on the physiological characteristics of the species (Waring et al. 2015). The small and prostrate grasses, due to the physiological buffering capacity for moisture variability, can maintain a high photosynthesis rate and biomass during dry times (Liu et al. 2010). In addition, the water use efficiency of plant species depends on the depth of the roots (Fry et al. 2018), because there are often reservoirs of water deep underground. Consequently, the grasses, due to a more branched root system, have a higher water use efficiency compared to forbs with a tap root system. For example, grass root systems are shallow and have many fibrous roots (Nippert and Knapp, 2007), which means they have a large area of distribution in the soil and can quickly use water on the soil surface for soybean growth and biomass maintenance. By contrast, root systems of forbs are straight and are usually deeper than those of grasses, and require absorbing deep soil water (Nippert, Knapp 2007). It is assumed that aridity affects community biomass mainly by regulating grasses in the steppe (Waring et al. 2015), which was also confirmed by our study.
We found that the vegetation of dry steppes may include species with type C4 photosynthesis, and their chemical composition showed a higher content of Ca, Sr, Br, S, K, and P. The variation in plant major and trace elements are genus and family dependent (Shtangeeva et al. 2009, Kudrevatykh et al. 2021) determined by their genotype (Willey, Fawcett 2006). Simultaneously, in true steppes, all plants had type C3 photosynthesis and a higher content of Mn, Fe, Al, Ti, Ba, and Zn. It was revealed that in true and dry steppes plants of the Poaceae family have higher concentrations of Ti, Fe, Al, V, Rb, Ba, Mn, Cr, Mg, Sr, Ni, Zn, K, P, and Ca as compared to other species, whereas the Asteraceae and Amaranthaceae families concentrate Ca, Mg, K, P, S, and Cl (Kudrevatykh et al. 2021). Note that the accumulation of elements by plants is largely determined by climatic parameters and the type of photosynthesis. Thus, the accumulation of elements by Stipa and Agropyro (C3 plants) correlated with precipitation, while the accumulation of elements by Cleistogenes (C4 plant) was more related to temperature (Luo et al., 2015). It is assumed that the response of plants to different climatic parameters is the main mechanism for differences between C3 and C4 species (Luo et al. 2013), which depends on the fundamental biochemical parameters of the plants (Taylor et al. 2010,Still et al., 2014). Wright et al. (2001) found that plants growing in dry environments had higher aboveground nutrient concentrations compared to those in humid environments. It is assumed that thise nutrient concentration to help plants improve photosynthesis under arid conditions (Prentice et al. 2014) and this plant response is interpreted as a functional adaptation to water stress, allowing higher plasticity in risking dehydration (Luo et al. 2015).
4.2 The supply of chemical elements with plants to the upper layers of soils
Our data showed that the supply of the elements under study with plants in the upper soil layers was higher in the wetter true steppe compared to that in desert and desertificated ones. We also found that the intake of P, Ca, K, Rb, and S with plants supply the upper soil layers correlated with MAP (r = 0.65–0.85, at P ≤ 0.05) and MAT (r = 0.72–0.83, at P ≤ 0.05), whereas that of Br, Ca, K, Mg, P, Sr, Mn, and S correlated with IDM (r = 0.76–0.85, at P ≤ 0.05). The difference in the content of the supply of these elements with plants in the soil varied significantly with an increase in IDM by every 5 units, MAT by every 2⁰С, and with a decrease in MAP by every 100 mm.
In the previous section, we explained how climatic factors determine the variability of the biodiversity of steppe plants and therefore influence the chemical composition of incoming organic matter. For example, the highest concentrations of most study elements are found in the root systems of Poaceae, which represent the greater part of the biomass of true steppes, whereas in desert and desertificated steppes Asteraceae and Amaranthaceae with lower concentrations of elements predominate. The level of NPP, which was 1.2 times higher in true steppes, also affects the fixation of elements in plants and the rate of their return to the soil. Therefore, even with the chemical elements being concentrated by individual plant species (for example, the Asteraceae family is a Rb concentrator), true steppes, due to a higher level of bio-productivity, show a higher intake of elements with plants in the soil.
It has been reported that steppe ecosystems have a low rate of decomposition of organic material being collected on the soil surface (Abaturov, Nukhimovskaya 2013), which is reflected in the level of intake of elements with plants into the soil. With climate aridization in the steppes, the rate of decomposition of plant residues begins to decrease, which is due to the insignificant number of phytophag invertebrates, especially in microhills with sodic soils (Vsevolodova-Perel’, Kolesnikova 2010). It was revealed that the intensification of organic matter mineralization in the steppes most often occurs in the month with rain or in the following month. During such periods, mineralization is very intense, and slows down drastically with the drying out of the soil (Titlyanova et al. 1996). In a setting lacking moisture and low biota activity, the photodegradation of plant residues under the influence of light plays an important role in decomposition (Kulakova 2020). It was also shown that the decomposition of plant matter on the soil surface is most intensive in steppes with predominance of Poaceae as compared to areas with a predominance of Asteraceae and Amaranthaceae (Titlyanova et al. 2018). The amount of incoming organic material also depends on the use of ecosystems. On a pasture plot, the annual intake of organic matter is lower than in ecosystems without grazing and is about 60% in steppe plant associations and 66% in desert ones (Kulakova 2020).
The rate of supply of chemical elements to the soil is affected by the physical and chemical properties of these soils. Our results showed that Endosalic Kastanozems (true and desert steppes) had higher levels of clay, SOC, CaCO3 and chemical elements than Luvic Calcisoils (desertificated steppes). Note that, in Kastanozems, due to the high content in the rock, the concentrations of anionic forms of Al, Ti, S, Mg, V, Mn and Fe are higher, and in Calcisols, due to the low content of organic matter-binding elements in an organo-mineral complex, the availability of anionic forms of Cl, S and Rb is higher (Kalinin et al. 2021). The steppes and semi-deserts have alkaline environmental conditions, which contribute to the mobility of anionic (S, Cl) and complexing elements (Al, Mg, Fe, Ti, Zn, Ni, and Mn). The presence of soluble complex compounds of hydrolysates increases the absorption of many elements by plant roots (Kalinin et al., 2021). As the literature analysis has shown, the parameters determining the supply of elements with plants into the soil are largely dependent on climatic factors. However, there is rather conflicting evidence on what are the factors that describe the dynamics, i. e. moisture and temperature (Liu et al. 2018, Zhu et al. 2019, Wu et al. 2021). Our study showed that the calculated indicator IDM is a reliable indicator of the influence of climate on the supply of most of the elements under study into the upper soil layers. Thus, it can be assumed that interaction in the plant-soil system is best reflected not by the measured climatic parameters (MAT, MAP), but by the calculated ones that characterize the type of climate.
4.3 The behavior of chemical elements in the profile of steppe soils
Having studied the distribution of vegetation-influenced elements in soil, we found that in the profile, Ca, Mg, Sr, and S are distributed according to the eluvial type, whereas K, P, Mn, and Rb are distributed according to the accumulative type. The migration of Ca, Mg, Sr, and S in steppe soils is largely related to the behavior of salts. Because salts are highly soluble compounds, the migration of these elements shows significant variation in the upper part of the profile. With an increase in the average level of MAP, these elements leach out into the lower part of the profile, where they accumulate on the evaporative barrier. With a decrease in the average level of MAP and with short-term wetting, solutions enriched with these elements are pulled into the upper soil horizons and deposited on the evaporative barrier, thereby reducing the depth of the carbonate and salic horizons. These processes can happen quite quickly (Aleksandrovsky, Aleksandrovskaya 2005). This recycling makes it rather difficult to determine the effect of biogenic mobilization on the accumulation of Ca, Mg, and Sr, since this process occurs on a smaller scale compared to the evaporative concentration. S behaves similarly, but in modern soils, it lies deeper than salts and carbonates. Therefore, its accumulation in the upper 10 cm of the soil may be associated with its biogenic mobilization in the roots of local vegetation, as well as accumulation with dead vegetation.
Elements with an accumulative type include K, P, Mn, and Rb. The accumulation of K in the upper horizons may be due to the accumulation of illite and K-feldspar in the process of weathering (Alekseeva et al. 2010). K is an important biophile element, therefore, an increase in its content in the upper 20 cm is also associated with the mobilization of local vegetation. These processes are most likely interrelated and unidirectional. Despite the basic conditions, during wetter periods, chemical weathering of silicates and the formation of K available to the plant occurs in the rhizosphere, after which said K is fixed in root systems and is not carried out down the profile in solutions. Mn is usually present in soil in the form of oxides and hydroxides. Ferromanganese concretions rarely form in Kastanozems and Calcisoils, but manganese inclusions are typical of humus soil horizons and loesses (Borisov et al. 2020). Plants can also absorb the available manganese and pull it into the upper soil horizons. Accumulation of Rb in the upper horizons may be due to its fixation in clay minerals after phyllosilicates (Salminen, 2005). Another possible mechanism is the residual concentration in weathering-resistant K-feldspar, muscovite, and illite, while other minerals (for example, biotite) are destroyed. The abovementioned accumulation of Rb by plants may occur due to the process of alkalinization, when silt migrates down the profile. Here, in the upper 10 cm there is a decrease in clay content and a decrease in sorption potential (Simonsson et al. 2016), which makes fixation by local vegetation possible. P is one of the main biophile elements, so its accumulation in the upper soil horizons can be expected. However, the second peak at a depth of 30–80 cm is associated with salts. Phosphorous mineral compounds are contained in soils in the form of calcium and magnesium salts and accumulate in carbonate horizons (Ubugunov et al. 2015). Therefore, the distribution of P in Kastanozems is largely similar to the distribution of Ca (Kalinin et al. 2021).
Previous research (Kudrevatykh et al. 2021) has shown that these elements are actively absorbed by the roots of Poaceae and Asteraceae, so they can rise from the lower part of the profile to the upper horizons. Soil PO4, Cl, SO4, and K accumulate under desert shrubs, whereas Rb, Na, Li, Ca, Mg, and Sr are usually more concentrated in the intershrub spaces (Schlesinger et al. 1996). Another study has also shown that the growth of some species of the family Salsola with Asteraceae contributes to the accumulation of Ca, K, Na, P, S and Cl in soil solutions of steppes (Glazovskaya, Gorbunova 2004). Therefore, the geochemical activity of plants in arid landscapes aggravates the negative effects in soils caused by climate aridization. In the setting of sparse vegetation cover caused by both desertification and anthropogenic human impact, capillary rise of salt cations and anions with soil solutions into the upper soil horizons occurs (Perelman, Kasimov 1999). They are deposited on the evaporative barrier and react, forming carbonates, gypsum and highly soluble salts and thereby causing soil degradation. The plant activity is an alternative to the radial migration, where root systems act as ascending soil solutions. The accumulation of Ca, Mg, and Sr by Asteraceae and Amaranthaceae and their pulling into the upper soil horizons activate the process of salinization and alkalinization. It has been reported that the influence of plants on the content of macronutrients (N, P, K, Mg, and Ca) in the soil is stronger than their influence on physico-chemical properties (such as pH, bulk density and moisture) (Waring et al. 2015). Relatively high effect values for macronutrients may be due to a relatively strong control of plants over the distribution of essential and non-essential elements (Jobba´gy, Jackson 2004). Therefore, we emphasize that the absolute values of the effects of individual plants do not necessarily correlate with the magnitude of environmental effects caused by changes in the soil chemical composition due to the influence of plants.
4.4 The effect of plant biodiversity changes on the chemical composition of steppe soils
During aridization in the steppe zone, there is a change in plant families in the family range grass → grass-forbs → grass-wormwood → wormwood-grass → wormwood-goosefoot association. Within this range, the biomass and bio-productivity of steppe landscapes decrease, and chemical elements associated with salts accumulate in soils, i. e. Ca, Mg, Na, S, and Sr. Thus, climate aridization results not only in a decrease in the bio-productivity of landscapes and the development of degradation processes such as erosion and desertification, but also in a change in landscape biogeochemical properties. The accumulation of a narrow range of chemical elements (Ca, K, Na, S, Cl) associated with salts by arid species leads to further degradation of the soil cover and a decrease in its sustainability. A decrease in precipitation leads to salinization of the middle part of the soil profile, while biogenic migration of salt cations and anions increases their accumulation in humus horizons. Mobilization of such elements leads to numerous degradation processes, such as landscape salinization and the formation of a cambic horizon in soil.
In the wetter true steppes, where the grass-wormwood association is common, these processes occur to a lesser extent. Here plants accumulate a wider range of elements (Ti, Fe, Al, V, Rb, Ba, Mn, Cr, Mg, Sr, Ni, Zn, K, P, and Ca). However, it has been reported that livestock grazing (Hurka et al. 2019), which is widespread in these territories, contributes to a decrease in the biomass of grasses; as a result, poorer landscapes, similar to the southern ones, begin to form, in which wormwood prevails. Climate aridization has a similar influence: it determines the change of grass associations first to forbs, and then to wormwood-goosefoot ones. This, in turn, leads to the salt anions and cations being pulled into the upper horizons, which causes degradation associated with salinization. Therefore, climate aridization and anthropogenic impact lead to the degradation of steppe landscapes, which manifests both in a changes in the qualitative and quantitative composition of vegetation cover and in changes in the physico-chemical properties of soils. The result is that global warming and anthropogenic activities trigger a mechanism in which all components of arid landscapes show a trend toward degradation.