δ13C as a tool to determine the origin of soil organic carbon: Case study of a restored sloping orchard.

The effect of drip irrigation on the origin and size fraction of soil organic carbon was studied in the soils of an apple orchard (Malus domestica Borkh.) on hilly (20%) terrain in northeastern Slovenia in three slope positions (upslope, midslope and downslope), comparing irrigated with non-irrigated soils. Physical fractionation of soil organic carbon was performed on three soil layers (0-0.05, 0.05-0.15 and 0.15-0.30 m) in three size fractions: fraction A (> 0.0002 m), fraction B (0.0002±0.00005 m) and fraction C (< 0.00005 m). Fraction A was the richest in soil organic carbon (7.7%), but fraction C was the dominant fraction in the total soil volume (86±92%), making it the largest source of soil organic carbon (73%). The δ13C signature was performed to determine the existence of two different types (origins) of soil organic carbon: fresh and sedimentary. Fresh organic carbon dominates in the A fraction, while sedimentary organic carbon dominates in the C fraction and may contribute to higher structural stability, besides higher carbonates in the finest fraction. Irrigation mainly contributes to the higher stock of soil organic carbon (predominantly fresh and less sedimentary) in the coarse A fraction (21.14 t/ha in irrigated and 14.17 t/ha in non-irrigated soils). [ABSTRACT FROM AUTHOR]