Long term impact of organic amendments on soil organic matter molecular composition

Agriculture and food systems account for one-third of the anthropogenic GHG emissions. As the largest terrestrial carbon pool, soils play a significant role in climate change mitigation and adaptation processes. Due to different agricultural inputs and their intensive usage, agricultural soils are more vulnerable to carbon loss and act as a source of GHG emissions while diminishing the soil's inherent capacity to act as a carbon sink. The composition, stoichiometry, and quantity of applied soil amendments differ and significantly influence soil carbon retention in the long run. Combining organic amendments and inorganic fertilizers could affect soil organic matter (SOM) stabilization pathways differently. The latter is identifiable via SOM composition and overall compound diversity. It has been hypothesized that more persistent soil organic matter is characterized by high molecular compound diversity and complexity (Lehmann et al., 2020). The molecular composition of the soils can further be categorized into plant, microbial, and mixed-originated compounds to understand the preferential carbon stabilization pathways under different nutrient management conditions. Besides, the energy released during oxidative differential scanning calorimetry could be used to assess the degree of SOM transformation. Within this context, this study aims to assess the long-term impact of organic amendments under different inorganic N fertilizer levels on soil organic matter content, their molecular composition, and transformation. Here, we analysed soil sampled from a long-term field experiment in Switzerland spanning over 45 years, which contains treatments with varying levels of mineral N fertilization (no mineral N, recommended N application dose ± 40 N units) and organic amendments (no organic amendments, farmyard manure, and wheat straw). The bulk soils and organic amendments were analysed for their elemental composition, molecular composition using pyrolysis gas chromatography-mass spectrometry, and thermal stability using simultaneous thermal analysis. The initial results indicate that soils receiving no organic amendments and no inorganic N fertilizers exhibited significantly lower carbon content, more recalcitrant compounds, and lower molecular compound diversity than other treatments receiving organic amendments. Concurrently, soils receiving no organic amendments and no inorganic N fertilizers exhibited lower energy release during thermal analysis, indicating more transformed SOM. According to the principal component analysis, there was a clear demarcation between manure/straw-added treatments and not-added treatments related to the composition of soil organic matter. The manure-added treatments exhibited significantly higher carbon content, manure-derived sterols, and molecular diversity than the non-manure-added treatments. Furthermore, we expect to showcase the interaction effect of organic amendments and inorganic N fertilizers on the proportion of microbial and plant-derived compounds in soil organic matter.  Overall, the findings of this study will provide useful information for stakeholders to identify the optimum fertilization practices for croplands to increase the stable SOM fraction in the long run.

References: 
Lehmann, J., Hansel, C. M., Kaiser, C., Kleber, M., Maher, K., Manzoni, S., ... & Kögel-Knabner, I. (2020). Persistence of soil organic carbon caused by functional complexity. Nature Geoscience, 13(8), 529-534.