Structural characterization using 2D NMR spectroscopy and TMAH-GC x GC-MS: Application to humic acids from soils of an integrated agricultural system and an Atlantic native forest

Author(s)

Tadini, Amanda M. ; Goranov, Aleksandar I. ; Martin-Neto, Ladislau ; Bernardi, Alberto C. C. ; Oliveira, Patricia P. A. ; Pezzopane, Jose R. M. ; Hatcher, Patrick G.

• Document type: Journal article
• Source: Science of The Total Environment; v. 815, p. 14-pg., 2022-01-10.

Abstract

Understanding the chemical make-up of soils and their structure is critical for constraining the role of soil organic matter (SOM) into the global biogeochemical cycles, as well as to understand the capability of SOM to sequester carbon and mitigate greenhouse gas emissions. Here, we use two-dimensional H-1-C-13 heteronuclear single quantum coherence nuclear magnetic resonance (2D H-1-C-13 HSQC NMR) spectroscopy to structurally characterize the most refractory component of SOM, the humic acid (HA). The observations from 2D H-1-C-13 HSQC NMR were coupled with lignin phenol and fatty acid measurements using tetramethylammonium hydroxide (TMAH) thermochemolysis - two-dimensional gas chromatography - mass spectrometry (TMAH-GC x GC-MS). We studied humic acids extracted from an integrated Crop - Livestock - Forest System (CLFS) agricultural area and an undisturbed Atlantic Native Forest (NF) area. We evaluated soils from two different depths: the topsoil (0-20 cm) and subsoil (60-100 cm) layers, and reveal the presence of oxidized ligninaceous phenols as we had previously hypothesized. Collectively, our results indicate that there are significant oxidative processes with increasing soil depth which are more pronounced in the CLFS relative to the NF area. Degradation of stearic acid with increasing depth in the CLFS soils indicated that the CLFS is more microbiologically active than NF. Therefore, CLFS is less biochemically stable than we originally perceived. The enhanced bio-reactivity of CLFS is likely driving the enhanced carbon sequestration in the CLFS soils. This is perhaps due to the diversity of biomass remnants available at the CLFS soil rhizosphere which allows for more different types of biomass to be sequestered as oxidized ligninaceous phenols. Our results employing structural characterization with 2D H-1-C-13 HSQC NMR and TMAH-GC x GC-MS provide a new layer of knowledge about the practice of integrated agricultural systems and allow us to understand the structure and fate of sequestered carbon in soils from different soil environments. (AU)

• FAPESP's process: 17/20084-5 - Strategic practices for mitigating greenhouse gas emissions in grassland systems of the Brazilian Southeast
• Grantee: Paulo Henrique Mazza Rodrigues
• Support Opportunities: Research Program on Global Climate Change - Thematic Grants
• FAPESP's process: 19/02939-9 - Structural and molecular evaluation of soil organic matter from integrated crop-livestock-forest systems
• Grantee: Amanda Maria Tadini
• Support Opportunities: Scholarships abroad - Research Internship - Post-doctor
• FAPESP's process: 17/22950-1 - Dynamics, reactivity and structural aspects of soil organic matter under integration crop-livestock-forest systems
• Grantee: Amanda Maria Tadini
• Support Opportunities: Scholarships in Brazil - Post-Doctoral