Biochemical composition of soil organic matter physical fractions under 32-year fertilization in Ferralic Cambisol

Abstract

AbstractBiochemical properties of soil organic matter (SOM) are fundamental for soil fertility and health. However, it is unclear how fertilization regime influences the biochemical compositions and oxidation states of SOM and physical fractions. In this study, this issue was studied under four 32-year amendment regimes: unfertilized control, urea (N), N + calcium dihydrogen phosphate + potassium chloride (NPK), and NPK plus manure (NPKM). Three physical fractions: coarse particulate (> 250 μm, cPOM), fine particulate (53–250 μm, fPOM) and mineral-associated OM (< 53 μm, MAOM) were separated and measured by pyrolysis gas chromatography/mass spectrometry (Py-GC/MS). Compared with the background in 1986, the SOM increased by 10.6%, 14.2%, 23% and 52% in unfertilized control, N, NPK, and NPKM, respectively. The red soil here had not reached carbon saturation, because of the low conversion efficiency (6.8%) from input-carbon to soil organic carbon (SOC). Physical size but not amendment type primarily regulated the SOM molecular composition, with relative selective retention of aromatics and lignin in both the cPOM and fPOM, whereas N-containing compounds (particularly amino-N) were enriched in MAOM due to their high abilities to adsorb soil 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 of physical size and fertilization. In conclusion, physical size arrangement (proxy of microbial decomposition degree) played a more important role in regulating the SOM biochemical features than initial quality of various amendments. Graphical Abstract