Diagnostic Role of Microrelief in Studying the Agrogenic Transformation and Sustainability of the Soil Cover.
Natalia P. Sorokina and Inna V. Kuznetsova. V.V. Dokuchaev Soil Science Institute, Pyzhevskii per. 7, Moscow, Russia
Knowledge of the agrogenic transformation of the soil cover and its differentiation in a landscape is of great importance for predicting the sustainability of agricultural landscapes. The microrelief is a determining factor and an indicator of the agrogenic transformation of soils. The diagnostic role of microrelief in studying the agrogenic changes in the soil cover has been proved for the areas of loamy soddy-podzolic soils in the center of European Russia. The large-scale and detailed soil mapping and the repeated detailed surveys of soils and microrelief on croplands and under natural forest vegetation, as well as detailed studies of soil catenas, have been performed. The study was based on the concept of the soil cover pattern (V.M. Fridland, 1977) and on the theory of structural and functional organization of the soil cover as proposed by F.I. Kozlovskiy (1992). On the slightly convex and smooth surfaces of gently sloping watersheds, where microrelief is virtually absent, the soil cover pattern under the cropland was relatively stable, being identical to that in the forest. It consisted of zonal soil combinations without gleyed and eroded components. The bulk density values in the plow layer were close to the optimum, in the subsoil they were within the range typical of the virgin soddy-podzolic soils (1.43-1.52 g/cm3). On the slightly concave surfaces with the clearly expressed microrelief, the soil cover under the croplands was more heterogeneous: the initial zonal soil combination were transformed into semihydromorphic erosional combinations with participation of differently gleyed and eroded soils. A detailed study of the morphological and physical properties of soils along a 32-m-long transect enabled us to determine the mechanism responsible for the soil cover heterogeneity: the increase in the contrast between soils in microhollows and on the main surface is due to consecutive changes in the soils of microhollows: additional moistening--compaction--moistening--gleyzation. The redistribution of surface runoff under the impact of soil plowing enhances the contrasts in the degree of soil moistening. Heavy machines induce additional soil compaction in the plow horizon and the subsoil. The increase in the degree of soil compaction also has an uneven pattern. When the bulk density of the subsoil exceeds 1.6 g/cm3, a local aquifuge occurs, thus increasing the soil erosion on slopes and gleying in the bottom of microhollows. The difference in the bulk density between the plow layer and the subsoil is important. The stability of the network of microhollows and rills is crucial for the further heterogenization of the soil cover. The repeated topographic survey after 6 years (1:500, 10-cm contour interval) proved that there are both stable and unstable elements of the microrelief. Three types of evolutionary dynamics of soil areas can be distinguished: (1) stable combinations of mesomorphic and gleyed soddy-podzolic soils within the network of stable microhollows, (2) quasistable stable combinations of differently eroded soils in the areas with unstable microtopography, and (3) local hotspots of soil degradation (erosion + gleying) on the slopes and in the bottoms of stable microhollows. The transformation of the microrelief consists of the deepening of stable microhollows and some leveling of the surface with unstable microrelief. Stable microhollows with a depth of 15-20 cm and deeper affect the pattern of surface runoff. The soil cover transformation on the slopes and in the bottoms of stable microhollows has been registered: eroded soils appear on the slopes and microareas of strongly gleyed soils and inwashed (aggraded) soils appear in the bottoms. Thus, the soil cover complexity increases considerably. Under the forest, these changes are not seen. A different pattern of the agrogenic transformation of the soil cover is seen on the slopes with combinations of differently eroded and aggraded soils. Their distribution depends on the slope steepness and on the features of mesotopography. The microtopography is indicative of recent soil changes. Linear and, especially, dendritic microrelief patterns are indicative of active heterogenization of the soil cover. The presence of lengthwise waves in the microrelief (alteration of loci with inclinations) attests to the quasi-stability of the soil cover with some redistribution of humus and clay particles within the given soil combinations. Some tendency toward increasing erosion or accumulation of sediments can be traced against this general background. Thus, the presence, clear expression and dynamics of the microrelief are informative indicators of the agrogenic transformation of the soil cover in different landscapes; the study of the microrelief is important to judge the tolerance of the soil cover toward agrogenic impacts.