Organic matter fractions and stabilization potential in some Andosols of Iran

Abstract

Soils play a vital role in the carbon cycle of terrestrial ecosystems, serving as both a source and a sink for carbon. However, the carbon storage capacity of soils is not yet fully understood. This study was conducted to investigate the actual and potential carbon stabilization in 12 selected soil samples from four out of 17 pedons of Andosols studied in the Alborz Mountain range of central Iran. To evaluate the present stabilization of OM, organic carbon (OC) fractionation was performed through chemical oxidation a followed by mineral dissolution. Various OC fractions were extracted and analyzed for their total organic carbon (TOC), nitrogen (N), and C/N ratios. The fractions include water-extractable organic carbon (WEOC), labile OC released by NaOCl, mineral-associated OC released by HF, and the recalcitrant OC fraction. Furthermore, the samples were incubated for 180 days to evaluate CO₂ emissions. The relationships between various forms of Fe, Al, Si, plus allophane, and sub-fractions of OC in the topsoil and subsoil samples were investigated. The high amounts of WEOC suggest poor stabilization of the organic compounds particularly for small and aromatic moieties. The negative correlation between clay and NaOCl-labile fraction (r = −0.696, p < 0.01) implies a role of the mineral fraction in protecting C from oxidation. The organic matter (OM) released by the HF treatment accounted for small portion of OM with a very low C/N ratio suggesting that mineral-associated OM is mainly composed N-rich organic molecules. The recalcitrant fraction, instead, showed a higher C/N ratio. The amounts of OC released as CO₂ after 180 days incubation of the control samples represent an 8 to 18 % loss of the total OM. The stabilization potential was evaluated by adding Humic Acid (HA) to the samples and evaluating the CO₂ emissions upon incubation. Upon HA additions, CO₂ emission increased sharply until 30 days of incubation and then increased more smoothly. Interactions with minerals were found to be an important mechanism acting in the preservation of palatable, N-rich organic moieties, but this OM-pool did not dominate among OM fractions. Surprisingly, intrinsic recalcitrance seemed an important mechanism in the stabilization of organic matter, also thanks to the migration of small aromatic molecules in the subsoil horizons.