Soil aggregation and organic carbon as affected by topography and land use change in western Iran Shamsollah Ayoubi a, *, Parisa Mokhtari Karchegani b , Mohammad Reza Mosaddeghi a,1 , Naser Honarjoo b a Department of Soil Science, College of Agriculture, Isfahan University of Technology, 84156-83111 Isfahan, Iran b Department of Soil Science, College of Agriculture, Islamic Azad University, Khorasgan Branch, Isfahan, Iran 1. Introduction The importance of soil structure for soil tillage, water retention, root penetration and erosion potential has attracted a lot of attention for agricultural productivity and environmental quality (Jastrow et al., 1998). Knowledge on the formation and stabiliza- tion of soil aggregates in natural and disturbed ecosystems is necessary to address a variety of environmental concerns, ranging from the fate and transport of hazardous pollutants to the use of soils as potential C sink. Also, the conceptual links between aggregate stability and associated SOC are essential in the soil organic matter dynamics (Jastrow, 1996; Jastrow et al., 1998). Physical fractionation techniques consisting of size fraction- ation (primary and secondary particles) and density fractionation emphasize the role of physical fractions (i.e. soil minerals and aggregates) in SOC stabilization and turnover. These methods are considered less destructive than chemical fractionation proce- dures, and the results are expected to be better related to the soil structure and function of SOC (Christensen, 1992). The dynamics of aggregate formation seems to be closely linked with SOC storage in soils (Golchin et al., 1997). It is frequently observed that native soils usually have lower bulk density (BD), and higher SOC content, aggregate stability and saturated hydraulic conductivity when compared with the cultivated counterparts. Among several studies, are those of Celik (2005) on Mediterranean soils in Turkey, and Khormali et al. (2009) on loessial soils in semi-arid region of northern Iran, who reported diminishing soil quality indices upon cultivation of native soils. The land use is an important factor affecting SOC accumulation and storage in soils, which controls the magnitude of SOC stock and also greatly influences the composition and quality of organic matter in soils (Six et al., 2002; John et al., 2005; Helfrich et al., 2006). Land use and soil cultivation not only affect the total amount of SOC, but also influence the organic carbon distribution in physical fractions and the SOC protecting processes. The knowledge of SOC in aggregate-size fractions (i.e. secondary particles) can help to assess and predict the effects of land use on SOC storage and/or pools and soil aggregate soil stability. Soil & Tillage Research 121 (2012) 18–26 A R T I C L E I N F O Article history: Received 10 October 2011 Received in revised form 3 December 2011 Accepted 19 January 2012 Keywords: Soil organic carbon Total nitrogen Physical fractionation Slope gradient Aggregate stability Micromorphology A B S T R A C T The study was conducted to investigate the effects of slope gradient and land use change on soil structural stability, and soil organic carbon (SOC) and total nitrogen (TN) pools in aggregate-size fractions in western Iran. Three land uses in the selected site were natural forest (NF), disturbed forest (DF) and cultivated land (CL); and three classes of slope gradient (0–10%, S 1 ; 10–30%, S 2 ; and 30–50%, S 3 ) were used as a basis for soil sampling. The results showed that DF and CL treatments significantly decreased soil structural stability indices in the three slope classes. The highest percentages of macro- aggregates (i.e. 2.00–4.75 mm) and meso-aggregates (0.25–2.00 mm) were found in the lowest slope class (S 1 ) which was related to high SOC stock in this position. The highest percentage of macro- aggregates was observed in the NF soil; but the highest percentages of micro-aggregates (0.053– 0.25 mm) were observed in the CL treatment. Micromorphological observations confirmed that topsoil under natural forest mainly consisted of highly-porous crumb microstructure, excremental pedo- features or passage features, which are indicators of enhanced SOC and biological activity. The lowest values of SOC and TN were observed at the steep slope class (S3) presumably coincided with accelerated soil erosion. Overall, enhanced aggregation and aggregate-associated organic carbon pools were observed in the forest soils on the steep slopes indicating the importance of land management on C sequestration in natural environments. ß 2012 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +98 311 3913475; fax: +98 311 3913471. E-mail addresses: ayoubi@cc.iut.ac.ir (S. Ayoubi), mokhtaripari22@yahoo.com (P. Mokhtari Karchegani), mosaddeghi@cc.iut.ac.ir (M.R. Mosaddeghi), nhonarjoo@yahoo.com (N. Honarjoo). 1 Tel.: +98 311 3913470; fax: +98 311 3913471. Contents lists available at SciVerse ScienceDirect Soil & Tillage Research jou r nal h o mep age: w ww.els evier .co m/lo c ate/s till 0167-1987/$ – see front matter ß 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.still.2012.01.011