Stabilization mechanisms of organic matter in Andosols under long-term fertilization as revealed from structural, molecular, and stable isotopic signatures.

Purpose: The mechanisms responsible for soil organic matter (SOM) stabilization under long-term organic amendment (OA) in Andosols remain elusive. Thus, this study was aimed at unraveling the mechanisms underlying the stabilization of SOM in Andosols as a consequence of long-term OA application. Methods: The SOM in Andosols from three treatments, chemical fertilizer (CF), fish cake (FC), or rapeseed oil cake (ROC) applied for 32 years, was examined. The SOM was fractionated using heavy liquid separation into free SOM (fSOM), occluded light SOM (oSOM), weakly bound form SOM (wSOM), and strongly bound form SOM (sSOM) fractions. Each fraction was characterized using solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy and thermally assisted hydrolysis and methylation combined with gas chromatography/mass spectrometry (THM-GC/MS). The δ¹³C signatures were also determined. Results: The total C in ROC and FC soils was equivalent to 112% and 103% of that in CF soil, respectively. The δ¹³C suggested that OA-derived C had accumulated in fSOM, oSOM, and wSOM fractions (FC soil) or fSOM (ROC soil). ¹³C NMR spectra revealed that O-alkyl C was the major C group (24–47%), especially in fSOM and sSOM fractions. THM-GC/MS data indicated that the yield of N-containing compounds was the highest in sSOM fraction while that of aromatic compounds other than lignin-phenols was the highest in fSOM fraction. The yield of other aromatic compounds in SOM fractions was larger in ROC soil than in other soils. Conclusion: These findings suggest that the mechanisms responsible for stabilizing SOM in Andosols as influenced by the application of OA involve the selective preservation of OA-derived aromatic components in the fSOM and oSOM fractions and the inaccessibility of metabolic products due to the intimate association with mineral surfaces. [ABSTRACT FROM AUTHOR]