Changes in Soil Organic Carbon Fractions Across a Loess Toposequence

Abstract

Soil organic carbon fractions may vary with soil development and soil genesis controls its vertical distribution. The objectives of this study were to determine organic carbon fractions in soil derived from loess parent material and the role of relief in its vertical distribution. Five soil pedons, i.e., Typic Ustorthents (Rajar), Typic Calciustepts (Missa), Udic Calciustepts (Basal), Udic Haplustalfs (Guliana), and Typic Hapludalfs (Mansehra), were selected along a loess toposequence and sampled at genetic horizon level. Soils were characterized for texture, pH, calcium carbonate (CaCO3), and organic carbon fractions including total organic carbon (TOC), dissolved organic carbon (DOC), particulate organic carbon (POC), HCl-insoluble organic carbon, and density fractions, i.e., heavy fraction (HF) and light fraction (LF). Total organic carbon content ranged from 0.03 to 0.98% in these soils and significantly differed with soil depth within each soil and soil type. Mansehra and Guliana had 0.04 and 0.031% POC, respectively, which was greater compared to remaining soils. Dissolved organic carbon was greater in the Basal followed by Mansehra, Rajar, Missa, and Guliana soil profiles, and distribution remains uniform throughout the profile’s depth. Insoluble C fraction was 0.214% in Mansehra profiles followed by 0.156% in Rajar, 0.078% in Basal, 0.063% in Guliana, and 0.091% in Missa soil profiles. The Mansehra soil profiles had the highest (0.008 and 0.30%) content of both light and heavy fractions. Total organic carbon had little correlation with clay. Particulate organic carbon, DOC, and HCl-insoluble organic carbon had a significant correlation with TOC. The HF had the strongest correlation with TOC, while LF had a negative correlation. The soil organic fractions significantly differed with soil type and soil genesis. The soils at gentle slopes are dominant in most of the organic carbon fractions due to higher development stage.