Dynamics in the phytomass of meadows in the middle Ob floodplain during the growing season

The article is related to the study of the dynamics in the phytomass in order to understand the mechanisms of circulation and establish the initial conditions for the retention of organic carbon in the floodplain ecosystem. During the 2023 growing season, the rhythmics of the primary production process of meadow phytocenoses of the middle Ob floodplain were studied. The research was carried out at the Kaibasovo stationary of Tomsk State University (57°14'44.0391"N, 84°11'00.2761"E), located within the subzone of the southern taiga, the Shegarsky segment of the Ob floodplain (see Fig. 1). 4 non-mowed phytocenoses were studied: mixed-grass, mixed-herb elecampane (ass. Poo angustifoliae-Festucetumpratensis Mirk. In Denisova et al., 1986), veiny-sedge (Ass. Thalictro-Phalaroidetum (Libbert 1931) Taran, 1995), mixed-grass-sod-sedge (ass. Sanguisorbo oficinalis-Caricetum cespitosae Taran 1995). Hayfields were also studied - two trial areas - analogues of grass-grass and veiny-sedge phytocenoses. The dynamics of the aboveground phytomass of meadows was considered by components - green mass, litter, standing dead plant biomass. Sampling of aboveground phytomass was carried out monthly from May to October. Geobotanical relives was carried out on permanent test areas with a size of 100 m2, samples of aboveground phytomass were taken from five squares with a size of 0.25 m2. In the wet state, the mowing was sorted by species, then dried to an air-dry state and weighed to identify species diversity, the composition of dominants, the ratio of biological (cereals, sedges, grasses, legumes) and ecological groups species (see Tables 2, 3). The change in underground phytomass was studied in a soil layer of 0-20 cm, the ratio of standing phytomass and mortmass was analyzed in three periods - early July, September, October. Soil monoliths with a volume of 10 cm3 were selected in the center of the sloping sites in layers 0-10 and 10-20 cm to determine the underground mass. The underground phytomass was washed from the soil using a sieve with a 0.25 mm hole, stained and dried in a drying cabinet for 24 hours at a temperature of 80 degrees, then weighed [13]. For each sample area, the average values were determined from all five repetitions, after which the reserves of all components of the plant substance were expressed in grams per square meter. The roots were divided into fractions - living (B) and dead (V) in appearance using certain features according to the method. Analysis of monthly average air temperatures and precipitation (see Table 1) during the growing seasons of 2022 and 2023, and comparing the data with the average for the previous 10 years showed that the weather conditions in spring and early summer of 2023 were cold and dry, and starting in July - warm and humid. The flood of 2023 was low and the meadows of the high manes were not flooded by flood waters, and the humidification of the meadows of the lower parts of the slopes of the manes was ensured by the rise of the groundwater level during the flood. It was found that phytomass of the studied communities ranges from 1288 to 3763 g/m2, the share of the aboveground part ranges from 18 to 23%. In all meadows, there is a significant excess of reserves of living underground mass over the mortmass at all observation periods (see Table 4). The ratio of green biomass to the biomass of living roots is 0.3-0.4 at high relief positions and from 0.1 to 0.3 at low positions. All indicators are lower than those published [15] on stocks of phytomass of floodplain meadows in the area of Kolpashevo. These low values indicate a significant excess of the underground mass over the aboveground mass, which reflects the peculiarities of the functioning of floodplain grass ecosystems. The revealed seasonal changes in phytomass indicate the role of the individual rhythm of development of dominant species in the formation of grassland phytomass reserves, whichmust be taken into account in the timing of sampling to determine primary productivity. In general, the ratio of phytomass fractions (litter, standing dead plant biomass, green mass) is determined both by the influence of weather conditions and the composition of the dominant meadows. Under the influence of changing hydro-climatic conditions in communities, significant changes in phytomass occur during the season, with the bulk of the plant matter accumulating in the mortmass. In arid conditions, there is a decrease in the mass of grasslands and the accumulation of dead plant residues. With increasing humidity, the total aboveground mass increases, while the proportion of dead plant matter decreases. There are differences in fluctuations of green phytomass for different meadows during the growing season: two rises of green mass (July and August-September) are characteristic for grasslands of various grasses, one maximum was revealed for veiny-sedge and grass-sod meadows in August. There are two maxima of standing dead plant biomass, formation - spring and autumn. Active decomposition of the litter occurs in August on mixed-herb (elecampane), veiny-sedge and mixed-grass-sod meadows, and on mixed-grass-cereal - decomposition of the litter begins in September. The processes of dying off of living underground phytomass in conditions of sufficient moisture proceed more intensively than in conditions of drier phytocenoses. The distribution of underground phytomass in the upper and lower soil layers may depend on the humidity of the habitat, which is associated with the position in the relief of the floodplain, as well as on the structure of the plant community, the greatest concentration of phytomass is observed in the upper soil horizon (see Table 4). The article contains 6 Figures, 4 Tables, 29 References. The work was carried out using the equipment of the Unique scientific installation "System of experimental bases located along the latitudinal gradient" TSU. The Authors declare no conflict of interest.

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