the distribution of organic carbon, nitrogen and carbohydrates in aggregates of some desert and cropland soils in central iran

abstract soil organic matter and its components are important factors in formation and stability of aggregates and subsequently soil structure. however data regarding organic matter distribution in soil aggregates is scarce, particularly in arid soils of central iran. the objective of this study was to investigate the distribution of organic carbon (oc), total nitrogen (tn), and carbohydrates in aggregates of different size classes (2–4, 1–2, 0.5–1, 0.25–0.5, 0.05–0.25 and <0.05 mm) in desert and cropland (alfalfa and wheat) soils in abarkooh plain, central iran. composite soil samples at 0–10, 10–20 and 20–30 cm depths were taken. at all depths, the cultivation of desert soils caused a significant increase in the amount of oc, tn, carbohydrates, and wet aggregate stability (mwd). after land use change, the proportion of aggregates <0.05 mm was significantly reduced from 28–43% in the desert to 6–14% in the cropland and the proportion of 2–4 and 1–2 mm aggregates was significantly greater in the cropland than in the desert soil. land use change also caused significant increases in oc and tn in water-stable aggregates. the distribution pattern of oc and tn shows preferential enrichment of the larger than the smaller aggregates in cropland soils. however in most soils of the desert, the oc and tn contents in aggregates <0.05 mm were significantly higher than those in other aggregate classes. the major part of total soil organic carbon was associated with the fractions <0.05 mm for desert and 0.25–2 mm for the cropland soils. unlike oc, the distribution pattern of carbohydrates indicated that there was no clear relationship between the size of aggregates and carbohydrates contents for most cropland soils.