Nature of carbon in soil humic substances and incorporation of corn residuederived carbon to soil humic fractions as revealed by 13C natural abundance

Abstract

Most studies on humic substances are currently focused on chemical nature; however, Knowledge of the turnover of humic substances is essential in understanding the dynamics of soil organic matter under arable and forest soils. This study used13C natural abundance 0 3C) to reveal the nature of carbon in soil humic fractions and to determine the incorporation of com residue-derived carbon (C4-C) to soil humic fractions. Surface soil samples (0-5 cm) taken from selected cropping treatments o f a long-term field experiment involving com were used to extract humic substances. Continuous corn under conventional tillage (CT), continuous corn under Minimum tillage (MT) and alfalfa treatments were selected from the experimental site as Cropping treatments. Duration of corn (C4) and forage (C3) cropping in different treatments at the time of sampling ranged from 13-31 andl8 years, respectively. Soil samples were also taken from the nearby forest fo r comparison and to obtain background $ 3C values. Humic substances Were extracted from finely ground light fraction (LF) organic matter free soil using 0.1M Na4P2O7.10H2O. Humic substances were fractionated to humin,fulvic acid (FA) and humic acid (HA) using standard methods. A molecular fractionation using specific dialysis membranes were also carried out to separate humic fractions with molecular weights (MW) >50000 and >100000. 6>3C of bulk soil, LF free soil and separated humic fractions were analysed and C4-C estimated. The humin and FA fractions had the least and highest enrichments of l3C, respectively. These levels of enrichments may be related to the chemical nature of substances present in humin and FA. The humic fraction with >100000 MW was depleted in 13C compared to the fraction with >50000 MW. The incorporation of C4-C was highly variable among different humic fractions and humin fraction had the highest proportion of C4-C compared to all other Fractions. Continuous com cropping under conventional tillage fo r 31 years resulted in nearly 45% C4-C in humin compared to that of 26% in bulk soil. The proportions of C4-C in FA and HA Fractions were irregular among treatments with no distinct relationships observed