Organic matter quality defined by 13C NMR spectroscopy explains nitrogen mineralization and soil aggregation better than C/N ratio

Abstract

Soil organic matter (SOM) plays a key role in sustainable agricultural production by the improvement of physical, chemical and biological properties of soils. The decline in organic matter (OM) content of many soils is becoming a major process of soil infertility and degradation, particularly in European semi-arid Mediterranean regions. Organic amendments can increase SOM content, thus influence soil characteristics by the interdependent modification of biological, chemical and physical properties. Then, a better understanding of the impact of organic amendment on soil processes is required. The underlying hypothesis of this thesis is that the organic amendment can regulate soil processes which directly linked to its initial chemical characteristics. However, identifying and defining OM quality based on molecular composition is operationally difficult. In fact, OM contain multiple types of biomolecules with different aqueous solubility, and, hence, the different susceptibility to microbial decomposition (e.g. peptides, carbohydrates, lipids, lignin, organic acids, and polyphenols). Generally, lignin/N and C/N ratios are extensively used as descriptors of OM quality, but those simple indicators are not always give reliable information about their potential of effects on soil functions. In this context, several chemical throughput methods, such as pyrolysis-gas chromatography/mass spectrometry, near infrared reflectance spectroscopy and nuclear magnetic resonance (NMR) spectroscopy have been utilized to characterize OM at the molecular level. In particular, solid-state13C-CPMAS NMR spectra were found useful to provide an overview of the total organic chemical composition of complex matrices of SOM. This thesis showed that the chemical quality of OM, defined by solid-state 13C-CPMAS NMR spectra, can be an important indicator of soil functions, which explain nitrogen mineralization and soil aggregation process better than classical C/N and lignin/N ratio indices. Finally, modeling approach based on novel implementation of OM quality by 13C-CPMAS NMR, to purposely overcome the limitations of C/N as a single OM quality indicator and to explore the relationship between OM quality and soil structural stability. This thesis provides evidence for the importance of OM, and in particular its chemical quality, which influences soil processes by inter-depended modification of soil physical, chemical and biological parameters.