Effects of Land-Use Change on Soil Organic Carbon and Nitrogen

Although the literature is full of references to soil degradation under forest cultivation, very little information is available on changes in soil properties following deforestation of the Hyrcanian area in northern Iran. Also, the literature provides little information on the effects of conversion from deforested cropland to grazing, a likely direction of land-use change in northern Iran. The objective of this study was to assess the effect of conversion of native forests into farmlands and/or grazing lands on soil properties and nutrients in the Hyrcanian forest. The dominant land uses are native forest, dryland farming of barley and wheat, and grazing. Three transects were established on hill slopes of the study area. Transects represented three land-use changes that occurred from 1967 to 2004 and included mature forest (Fagus orientalis and Acer velotinom) converted to dryland farming (barley and wheat) in 1970s, dryland farming converted to irrigated rice farming in mid-1980s, and dryland farming not cultured from 1986 to now. Average transects slope gradients were 23 degrees (T-1), 15 degrees (T-2), and 4 degrees (T-3). Three positions (top, middle, and bottom of hill slope) were selected on each transect to represent each land-use type. Therefore, nine sites were selected to analyze land-use change effects on soil properties. Twenty-m vertical transects were established perpendicular to main transects. At each vertical transect, three surface soil samples (0-25 cm) were collected to measure soil properties. Total nitrogen (N) was determined by the Kjeldahl method and soil organic carbon (SOC) content was measured using the modified Walkley-Black wet oxidation procedure. Comparisons of mean differences among land uses revealed that soils under native forest contained 4.73% SOC and 0.474% total N, which were significantly greater (P < 0.01) than contents under grazing (1.76% SOC, 0.323% total N) or dryland faming (1.37% SOC and 0.248% total N). Differences between grazing and dryland farming were not statistically significant.