Effects of soil organic amendments on soil organic carbon content and molecular diversity at the aggregate level

Soil organic matter (SOM) is critical for agroecosystem functions and can be improved by organic amendments. However, the chemical composition of different SOM fractions contained in soil aggregates under soil organic amendments is still unclear. Herein, this issue was addressed with soils sampled from five short-term fertilization regimes including stable (i.e., biochar, C/N ratio of 38.5) and unstable amendments (i.e., manure, C/N ratio of 16.2; food waste compost (FWC), C/N ratio of 5.7) in a vegetable field. The treatments were: (1) 3.4 kg·m^-2 manure as control, (2) 5 kg·m^-2 biochar + 3.4 kg·m^-2 manure, (3) 5 kg·m^-2 biochar + 2.26 kg·m^-2 manure + 1.14 kg·m^-2 FWC, (4) 5 kg·m^-2 biochar + 1.7 kg·m^-2 manure + 1.7 kg·m^-2 FWC, (5) 5 kg·m^-2 biochar + 1.14 kg·m^-2 manure + 2.26 kg·m^-2 FWC. Light (LF) and heavy (HF) fractions of SOM in water-stable aggregates were separated by density fractionation and then characterized for molecular composition using pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS). Results showed that both the proportion of macroaggregates (> 0.25 mm) and aggregate stability (indicated by mean weight diameter) increased with an increasing substitution rate of FWC for manure (i.e., substitution with equal-mass principle). In addition, biochar addition enhanced the LF proportion and the SOC content in LF of both macro- and microaggregates (< 0.25 mm), and these enhancements were not further affected by the FWC substitution for manure. However, biochar reduced the molecular diversity of LF SOM, and this reduction can be offset by the FWC substitution for manure. Common and unique molecules in different treatments further revealed the effects of organic amendments were dominant in LF of macroaggregates. The co-application of biochar and higher-quality organic fertilizer (e.g., FWC) can be more effectively beneficial to soil structure, SOC content and quality.