Assessing the effect of forest degradation and slope position on soil quality index

 Soil quality is the capacity of soil function to sustain plant and animal productivities, to maintain or enhance water and air quality and to support human health. Slope position and deforestation are known to influence soil quality and assessing the soil quality degradation is important to soil management. Soil-quality indices are a common and easy way to quantify soil quality; they can improve understanding of soil ecosystems and allow more efficient soil management Two soil indicator selection approaches, total data sets and minimum data sets have been widely used to evaluate soil quality. The region of Marivan in Kurdistan province is one of the forested areas of Zagros which has been threatened due to population growth and increasing demand for food and, some parts are now under agriculture land use. Based on the present reports, deforestation and cultivation on the sloping areas have started almost 30 years ago. The aim of this research was to assess the effect of forest degradation and slope position on soil quality index. The study area is located in Kurdistan Province, about 10 km northeast of Marivan city, west Iran (46°24΄ 46°40΄E, 35°42΄ 35°50΄N). Two adjacent sites were selected, consisting of a natural forest and deforested cultivated land on a hill slope. Average annual precipitation and temperature are 813 mm and 13.8 °C, respectively. Soil moisture and temperature regimes are Xeric and Mesic, respectively. Forests of the study area are relative intensive and their main forest vegetation is oak. In this study, 24 soil samples (0–20 cm depth) were taken from four slope positions (shoulder, back slope, foot slope and toe slope) of forest and adjacent deforested cultivated soils. Eight profiles (on each slope positions of both land uses) were also described. Fifteen soil properties: pH, electrical conductivity, sodium adsorption ratio, organic carbon, cation exchange capacity, carbonat calcium equivalent, soil erodibility, soil porosity, mean weight diameter of aggregates, available water, soil microbial respiration, available phosphorous, available potassium, total nitrogen, bulk density, were measured for 24 soil samples (0–20 cm depth). These Fifteen soil properties were applied as the total data set. Then, seven soil properties were selected as minimum data set using principle component analysis. Weight and score of each property were found using communality and scoring function (including more is better, low is better and optimum) and finally weighted additive and nemoro soil quality indices was computed. Seven soil properties (including soil organic carbon, cation exchange capacity, bulk density, soil erodibility, plant available water content, available potassium and total nitrogen) were selected as total data set using principle component analysis. The soils formed in low slope positions had higher depth and evolution compared to high slope positions. The results also showed land use change of forest land to cropland has led to degradation of Mollisols. The results showed that the mean values for weighted additive and nemoro soil quality indices in the deforested were significantly lower compared to forest. The mean values for weighted additive and nemoro soil quality indices in the shoulder were significantly lower compared to other slope position. significantly Strong Pearson correlation coefficients (0.98) were obtained between computed weighted additive soil quality index using total data set and a minimum data set. The results showed that forest degradation in the Marivan region led to a decrease in weighted additive and nemoro soil quality indices through a significant reduction of organic carbon, microbial respiration, total nitrogen, CEC, soil porosity and available moisture and significant increasing of bulk density, pH, SAR and soil erodibility. Forest degradation and land use change also due to cultivation led to decrease in the organic carbon content and soil structure degradation of Mollic horizon. Therefore, Mollic horizon has converted to Ochric horizon and Entisols and Inceptisols have formed in cropland land use. Moreover, the results showed different slope positions affect weighted additive soil quality index and mark significant difference. The results also showed that using the weighted additive soil quality index and minimum data set method can adequately represent total data set (R2=0.98) and thus reduce the time and cost involved in evaluating soil quality. Slope positions and where forest was converted to agriculture were characterized by low values of weighted additive soil quality index, suggesting a recovery of soil quality through changing to sustainable practices.