Stratification of soil organic matter and biota dynamics in natural and anthropogenic ecosystems

Soil organic matter (OM) stratification and macro and micro fauna are both good indicators for the evaluation of soil ecological functioning, which is interrelated with nutrient cycles. To the best of the authors’ knowledge, responses of the degree of OM stratification with soil depth expressed as a ratio, and belowground biota to forest degradation and land cover changes have received little attention, particularly in northern Iran. In this study, five land covers were selected: virgin natural forest dominated by Carpinus betulus and Parrotia persica (natural forest), mono-species oak stand (plantation), land including Citrus sinensis and Citrus tangerina trees (garden), land dominated by Brachypodium pinnatum and Carex sylvatica (rangeland), and land including rice cultivation (agriculture) to examine soil OM stratification ratio, biological dynamics, and nutrients. The aim of this study was to evaluate the effect of natural and anthropogenic sites of Hyrcanian region on soil functions following 32 years of land cover changes. Four soil profiles (25 × 25 cm) were dug along four parallel transects in the central part of each land cover plot. Thus, 16 soil samples for each site were sampled at a depth of 0−10 cm for different soil properties and two depths of 0–10 and 10−20 cm to measure the soil OM and calculate OM stratification ratio (OM surface soil/OM subsurface soil). Seasonal measurements during one year were performed under studied land covers to find the temporal pattern of the soil biological activities. With a descending trend of OM with soil depth, OM stratification ratio was significantly enhanced under plantation and natural forest. Conversion of natural forest to anthropogenic sites significantly decreased the activities of different earthworm ecological groups (i.e., epigeic, anecic, and endogeic), Acarina, collembola, nematode, protozoa, bacteria, and fungi. Two-way ANOVA indicated that in addition to land cover, the season also affected the variability of soil earthworm activities. Compared to the other studied soil organisms, the role of land covers was nearly 1.5–2 times greater than that of the season factor. From the PCA output, the high soil biological activities, OM stratification ratio, and nutrients can be attributed to forest ecosystems (i.e., natural forest and plantation), while the low soil biological activities, OM stratification ratio, and nutrients were imposed by anthropogenic sites (i.e., rangeland, garden, and agriculture). The findings of this study support the importance of preserving natural forests for soil conservation and function.