Direct evidence on the impact of organic amendments on carbon stabilization in soil microaggregates

Direct evidence-based approaches are vital in understanding the involvement of abiotic/biotic factors and evaluating the newly proposed theories on soil carbon (C) stabilization. Microaggregates (150–250 µm) collected from a corn system (>22 years; Kansas, USA), which had been under no-till with different nitrogen (N) treatments were analyzed (N treatments: manure/compost, urea, zero fertilizer). We studied C stabilization in free soil microaggregates (with preserved aggregate architecture), directly using scanning transmission X-ray microscopy coupled with near edge X-ray absorption fine structure (STXM-NEXAFS) spectroscopy. Submicron scale findings were complemented with bulk chemical analysis. The STXM-NEXAFS analysis revealed soil organic carbon (SOC) preservation inside nano- and micro-pores and organo–mineral association, various degrees of humification, and high molecular diversity. The presence of microbial-derived C was found in manure-/compost-added microaggregates highlighting the contribution of organic amendments in facilitating microbial diversity. The incidence of aragonite-like minerals suggested the biologically/chemically active nature of microaggregate cores. Bulk analysis of free microaggregates showed a higher concentration of SOC (6.5%), ammonium oxalate extractable Fe/Al/Si), and higher aliphaticity of humic acid in manure-/compost-added soils compared to inorganic fertilizer (3% SOC) and control (2.7% SOC) treatments. The co-existence of elements (calcium [Ca]/C, iron [Fe]/N, Fe/C, aluminum [Al]/C, and silicon [Si]/C) was partially supported by bulk chemical analysis that indicated a strong association between ammonium oxalate extractable Fe/Al/Si and SOC (R² = 0.63—0.77). Overall, our study provided direct/indirect evidence for the complex and interactive involvement of chemical, mineralogical, and biological mechanisms that may have been stimulated by the long-term addition of compost/manure in stabilizing SOC.