Investigation of the effects of the conversion of forests and rangeland to cropland on fertility and soil functions in mountainous semi-arid landscape

Soil fertility and (micro)biological activities are prominent indicators of soil ecological functions interrelated with nutrient cycles and stocks. The responses of soil functions and belowground biota to degradation of natural ecosystems and land use changes have received little attention, especially in semi-arid ecosystems of mountainous areas that are sensitive and fragile habitats. Soils were sampled from two natural vegetation types: forest (Carpinus orientalis - Zelkova carpinifolia) and rangeland (Berberis integerrima - Crataegus melanocarpa), and two cultivated croplands (under Triticum aestivum), located in a mountainous semi-arid region of northern Iran, that were converted from the forests and rangelands thirty years ago. Soil properties, C and N stocks and fractions, and related microbial activities were analysed in 0–10 and 10–20 cm. C stocks in the upper 10 cm soil were affected by land use, while no differences were found in the lower depth. C stocks in the topsoil reached 49, 37, 33 and 29 Mg ha−1 for forest, rangeland, croplands (converted from forest and converted from rangeland), respectively. Soil N stocks under forest (5.2 Mg ha−1 for top 10 cm and 4.8 Mg ha−1 for 10–20 cm depths) were higher compared to the other land use types. Particulate and dissolved organic matters under croplands were almost half of that under forest and rangeland. High soil respiration, microbial biomass, ammonium and nitrate content, as well as N mineralization in 0–10 cm under forest were 2 times larger than in 10–20 cm. Concluding, conversion of forest and rangeland to croplands (converted from forest > converted from rangeland) has decreased the functions of soil due to removal of tree cover, decreasing litter layer, organic matter input and soil moisture and also increase of soil temperature. Consequently, more care should be taken by preserving natural vegetation in mountainous semi-arid ecosystems, and higher organic inputs are necessary to maintain microbial properties and functions.