© 2010 19 th World Congress of Soil Science, Soil Solutions for a Changing World 1 – 6 August 2010, Brisbane, Australia. Published on DVD. 1 Aggregation, carbohydrate, total and particulate organic carbon changes by cultivation of an arid soil in Central Iran Mohammad Ali Hajabbasi A and Jaber Fallahzade B A University of Technology, Isfahan 84156-83111, Iran, Email Hajabbas@cc.iut.ac.ir, Jaber84023@yahoo.com Abstract The objectives of this paper were to determine the response of soil quality indicators and organic carbon (OC) distributions within different aggregate classes to changes in land use from desert soils to cropland occurring in the Abarkooh plain, Central Iran. Composite soil samples of the desert soil, wheat and alfalfa fields were taken from three different depths and soil quality indicators, including aggregate stability (MWD), OC, carbohydrate, particulate organic carbon existent in macro (POC mac ) and microaggregates (POC mic ) in these soils were determined. Conversion of desert soils to croplands resulted in a significant decrease in electrical conductivity (EC), but an increase in MWD, OC, carbohydrate, POC mac and POC mic contents. OC content increased with increased aggregate size and the aggregate; OC ratio in cultivated and desert soils was highest in the 1-2 and <0.05 mm aggregates, respectively. The improvement soil quality in cropland is due to the long-term use of irrigation water and soil fertilization and also the poor SOM in desert soils. The results of this study further indicate the POC mac , POC mic and POC mac /POC mic ratio are sensitive parameters that reflect differences of soil aggregation, SOM quality and tillage intensity in soils of this region. These parameters are more reliable indicators of soil quality than OC in reclaimed desert soils. Key Words Soil quality, desert soils, cultivation, irrigation, soil aggregate, particulate organic carbon (POC). Introduction The desert soils in arid regions of Central Iran are characterized by low rainfall, low fertility, high evaporation and salinity. Also, soil water and salinity are crucial factors influencing crop production in these regions. Numerous field trials have demonstrated the effectiveness of leaching for salt removal. For example, Fullen et al. (1995) reported that use of irrigation water resulted in distinct and rapid improvements in the physical and chemical properties of reclaimed desert soils. Soil aggregate structure and stability are important factors that contribute to sustainable soil quality (Shepherd et al. 2002). Numerous studies have shown that conversion of native ecosystems to agriculture, especially in tropical and temperate regions have led to a negative impact on soil quality (Islam and Weil 2000). Nonetheless, information about the soil quality of desert soils and marginal cropland is scarce. Thus, the main objectives of this study were: (1) to analyze the effects of changes in land use from desert soils to croplands on soil quality indicators (2) to determine aggregate organic carbon (OC) ratio (OC in aggregate/OC in total aggregates) and also the distribution of OC in the aggregates of soils under desert and cropland. Methods Study area The study area was located in Abarkooh plain at an elevation of around 1500 m above sea level, nearly 140 km southwest of Yazd, Iran (31° 18′ N, 53° 17′ E). The climate of the region is arid with a mean annual rainfall of 60 mm, potential evapotranspiration of 2800 mm and the temperatures ranging from 40 °C in summer down to -13 °C in winter. In the southeast Abarkooh plain, groundwater is mainly saline (1700– 2500 μmho cm -1 ), but could be exploited to meet crop water requirement. A flood-irrigation system was developed during 1979–1981 to reduce salinity levels in the root zone and increase water availability. Therefore, this research was conducted to understand the changes of soil quality and aggregation resulting only from cultivation of desert soils. For this purpose, three different land uses were chosen in the study area including virgin desert, alfalfa (Medicago sativa L.) in rotation with wheat, and wheat (Triticum aestivum L.) in rotation with fallow and barley. At the time of sampling, the vegetation of the desert soils was dominated by Tamarix hispida. In the cropland, mineral N (urea) and P (di-ammonium phosphate) fertilizers are usually applied for improving soil productivity.