The Effect of Deforestation and Vegetation Cover Change on the Ecological Indices of Soil Organic and Mineral Layers

Degradation of forest habitats and alteration of soil vegetation are efficient factors affecting the variability of ecological indices of organic and mineral layers of soils. In Iran, degradation of forest habitats and changes in habitat type, especially over the last century, affected soil quality, plant biomass production and environmental sustainability. Hence, in this study, the effect of different forest and rangeland vegetation types on the ecological parameters of soil organic and mineral layer has been investigated. To study and evaluate the effects of forest degradation and site change on soil organic and mineral ecological indices, four types of vegetation were selected in Gorgpas areas, southwest of Chalus city, Mazandaran Province. The land cover is as follows in the study area: (1) Less-degraded forest dominated by Carpinus betulus L.- Parrotia persica C. A. May (2) Fourty year's old plantation of Pinea abies (L.) Karst - Pinus nigra Arnold (3) Deforested areas including Carpinus betulus L. - Parrotia persica C. A. May (4) Exclosure rangeland dominated by Coronilla varia L. Physiographically similar land covers, were selected during a field research in the studied areas. Eight litter and soil samples (0-15 cm in depth and 30 cm × 30 cm in depth) were collected from each area in summer. In order to reduce the boundary effects, sampling was performed in the center of each land cover. The collected samples of organic layer (litter) and soil mineral transferred to the laboratory for analysis. The collected data was stored as a database in Excel. Then, to analyze and compare the data, the normality distribution of observations was evaluated by Kolmogorov-Smirnov test and variance homogeneity by Levene test. Analysis of variance used to investigate the significant/non-significant differences of different soil organic and inorganic layer characteristics in relation to the studied areas. Duncan test used for multiple mean comparisons. All statistical analyzes were performed by SPSS software version 23. Principal component analysis (PCA) was employed to study the relationship between soil organic matter and soil mineral quality in the studied land cover. According to the results, in the soil organic layer the highest carbon/nitrogen ratio (%) assigned to the rangeland, while the nitrogen (%) content was highest in the natural forest. The highest amount of carbon and organic layer thickness were also observed in rangeland and degraded forest cover, respectively. In the mineral soil layer, the highest value of sand (%), moisture (%), carbon (%) and carbon to nitrogen ratios (%) belonged to the rangeland cover, while the highest amount of clay v (%), pH (1:2.5 H2O), electrical conductivity (ds m-1), nitrogen (%), phosphorus (%), potassium (mg kg-1), calcium (mg kg-1) and magnesium (mg kg-1) were observed in the forest cover. The highest number and biomass of earthworms (n m-2), nematode population (In 100-gram soil), nitrogen mineralization (mg kg-1l), ammonium (mg kg-1), nitrate (mg kg-1), basal respiration (mg CO2 g−1 day−1), substrate induced respiration (mg CO2 g−1 day−1), microbial nitrogen biomass (mg kg-1) and metabolic coefficient (μg CO2-C mg-1 MBC day-1) observed in forest cover. There was no significant difference between the studied vegetations for bulk density (g cm-3) characteristics, silt (%), microbial biomass of carbon (mg kg-1) and microbial coefficient (μg CO2-C mg-1 MBC day-1). Higher nitrate density in natural forest and under cultivated soils are due to the presence of litter species with low carbon/nitrogen ratio, high pH and calcium. Conversion of natural broadleaf covers to needle leaf plantation and rangeland reduces the biochemical processes of ammonium. Nitrogen mineralization rates are strongly influenced by area management and forest canopy cover, so that under the broadleaf stands, this rate was more than the needle leaf stands. This probably was due to the greater nitrogen of litter, the lower carbon to nitrogen ratio and the faster rate of decomposition of organic matter in broadleaves. Most of the time the increase in pH increases the rate of mineralization of nitrogen. The present study indicated that forest habitat had the highest number and biomass of earthworms, soil nematode population, ammonium, nitrate, metabolic coefficient, basal and substrate induced respiration, carbon availability index, microbial biomass and nitrogen mineralization, while, there was no significant difference between the studied forests and rangelands in carbon microbial biomass and microbial coefficient. In general, the results of this study showed that the physicochemical and biological characteristics of soil organic matter in the forest habitats were better than other studied vegetations and the forest degradation and land-use changes reduced soil fertility and microbial indices.