Effect of land-use change on soil fertility characteristics within water-stable aggregates of two cultivated soils in northern Iran

Forest and pastureland soils in highland of northern Iran are being seriously degraded and destructed due to extensive agricultural activities. These land use changes are usually accompanied by decreasing in concentrations of soil organic carbon and nutrients and also deterioration of soil structure in these regions. However, studies on concentration of nutrients and carbohydrates in soils suggest that the location of nutrients within aggregates of each size should be determined. This study was to evaluate the nutrient elements and carbohydrate distributions within water-stable aggregates (WSA) of virgin forest and pasture lands under different land uses in Alborz maintain range, northern Iran. In September 2006, soil samples were collected from depths of 0–20 cm in Typic Haploxerolls soils. The overall pattern indicated that the mean weight diameter (MWD) and WSA were greater in the virgin pasture and forest soils compared with the adjacent cultivated (ex-forest and ex-pasture) soils and aggregates of >1.0 mm size were dominant in the virgin soils, whereas the cultivated soils comprised aggregates of the size ≤0.5 mm. Distribution of organic carbon, nitrogen, phosphorus and carbohydrates within the WSA showed preferential enrichment of these parameters in the macroaggregate fraction (4.75–1.0 mm) for the virgin soils and microaggregate fraction (>0.25 mm) for the exposed soils. Cultivation decreased the concentration of carbohydrates by 23.6% and 20.6% in ex-forest and ex-pasture lands, respectively. Average distribution of total exchangeable bases within WSA showed that the cultivation of forest and pasture soils significantly led to reduce in these nutrient in the 4.75–2.0 mm fraction and increase in concentration of these exchangeable cations in <0.25 mm fractions. In general, results indicated that cultivation significantly led to 71% and 6% reductions in total exchangeable bases within the 4.75–2.0 and <0.25 mm aggregate fraction, respectively, for virgin soil. Since smaller aggregates are preferentially removed by erosion, this study emphasizes the need for sustainable soil management practices that they will minimize nutrient loss when forest or pastures lands are converted to croplands.