Chemistry and Oxidation State of Soil Organic Matter Fractions Under Long-Term Mineral and Manure Amendments in Ferralic Cambisol

Chemical properties of organic matter are fundamental for soil fertility. However it is unclear how physical size and fertilization regime shifts the chemical composition and oxidation state of soil organic matter (SOM). Here we studied this issue among three physical size classes under four 32-years amendments: control (unfertilized), urea (N), urea+Ca(H2PO4)2+KCl (NPK), and NPK + pig manure (NPKM). Coarse particulate (cPOM, >250 μm), fine particulate (fPOM, 53–250 μm) and mineral-associated OM (MAOM, <53 μm) fractions were separated and analyzed by pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS). Over the 32-years, the SOM of control, N, NPK and NPKM increased by 10.6%, 14.2%, 23% and 52% compared with the initial level, respectively. The conversion efficiency from fertilization-induced input-C to soil organic carbon (SOC) was 6.8%. Physical size but not fertilization primarily regulated the molecular composition of SOM, with relative selective retention of aromatics and lignin in cPOM and fPOM, whereas N-containing compounds, especially amino-N, were enriched in MAOM because of their high chemical affinities to minerals. The C oxidation state was also mainly dependent on physical size, with the highest value in fPOM. The sources of SOM and its fractions, dominated by microbial-derived compounds (60-90%), were independent on physical size and fertilization. In conclusion, physical size, not amendment regime, primarily regulated the chemical composition and oxidation state of SOM, suggesting that physical fractionation (proxy of microbial decomposition degree) plays greater role in SOM chemistry than the quality of amendments.