Change in the content of total carbon, nitrogen and phosphorus in the boreal soils of the Republic of Karelia when used in agriculture
Recently, studies devoted to geochemical cycles of biogenic elements have been especially relevant in connection with global climate change. Since soil is the central link in biological cycle of substances, in many respects its condition determines the stability of terrestrial ecosystems. It is known that changes in land use lead to changes in carbon stocks and other nutrients in soils, therefore, monitoring of their content is required to implement effective control and management of soil resources. This paper presents the results of studying the effect of agricultural use on the content of total carbon, nitrogen and phosphorus in the topsoil of soils of different genesis. The studies were carried out in the Middle Taiga zone of Karelia (Eastern Fennoscandia) at four key sites (See Fig. 1). Soil profiles were located on farmlands and in the adjacent territories under forest. General sampling comprises 73 profiles. Samples were selected from the middle part of the topsoil. In thick horizons, a mixed sample was analyzed from a depth of 0-20 cm and 20 and > cm. In soil samples, the following parameters were analyzed: total nitrogen according to Kjeldahl, total phosphorus photometrically, total carbon by high-temperature catalytic combustion, and pH<sub>KCl</sub> potentiometrically. For statistical analysis, the PAST Statistics software was used. Comparisons of mean values were performed using Student's t-test and one-way ANOVA with post-hoc analysis according to Tukey's HSD test. It is known that the moderately cold and humid climate of Karelia and the predominance of forest vegetation foster the zonal podzolic process of soil formation and waterlogging. The soil cover is characterized by a complex structure, a high degree of patchiness and shallowness. To a large extent, the soil diversity of Karelia is due to differences in soil-forming rocks and strong relief roughness. Mineral soils of agricultural lands are mainly in relation to Quaternary sediments of the last glacial period. The profile structure and the properties of mineral soils depend, to a large extent, on the texture and origin of parent rocks. Soils on sands and sandy loams (Albic / Entic Umbric Podzols), on loams and clays (Umbric Retisols), and soils on schungites (Skeletic Umbrisols) were analyzed (See Table 1). The presented data suggest that anthropogenic transformation of mineral soils leads to significant changes in structure of the profile and properties of soils. The thickness of upper horizons increases from 5 to 25 cm, strongly acidic pH (3.7-3.9) rises to slightly acidic (5.1-5.3). The content of carbon, nitrogen and phosphorus significantly increases in soils on sands and sandy loams. Mineralization of organic matter intensifies, and the content of nitrogen and phosphorus also increases reliably in soils on loams and clays. Soils on schungite rocks are less susceptible to noticeable changes during agrotransformation. Peat soils represent drained farmlands with varying degrees of transformation of lowland peat (Drainic Eutric Histosols) (See Table 2). A part of these soils (Peat soils) has preserved an undisturbed natural structure of upper horizons with a strongly acidic pH (4.3), a high content of carbon (48%) and nitrogen (1.6%) and weak decomposition of organic matter. Soils with a highly disturbed topsoil (Torfozems) have an elevated pH (5.0), a lower content of carbon (p <0.001), nitrogen (p <0.005) and a C:N ratio (p <0.01) compared to peat soils. Peat-mineral soils with peat thickness <50 cm (Histic Gleysols) have a highly disturbed topsoil with pH 4.9 and a lower content of carbon (p <0.001) and nitrogen (p <0.005) than peat soils. Thus, in cold humid conditions of Karelia under anthropogenic influence in topsoils of Al-Fe-humus and texturally-differentiated soils, there is a tendency to increasing the content of the main biogenic elements. In soils with peat horizons, the opposite process dominates when an active human impact reduces greately