Nano-structural and chemical characterization of charred organic matter in a fire-affected Arenosol

- 129Xe NMR patterns reveal black carbon (BC) domains in the soil PyOM.
- BC is mainly formed in the Ah horizon from carbohydrates and n-alkanes.
- Micro- and mesoporous regions co-exist within the same PyOM particle.
- Structural heterogeneity of PyOM is due to chemical variety of charred species.
- Fire-produced short-range order minerals promote interaction with PyOM.

The objective of this study was to characterize the formation and properties of black carbon (BC) domains within pyrogenic organic material (PyOM) of an Arenosol derived from a fire-affected site in Portugal (Algarve, Faro). Both the particulate organic matter (POM) and organo-mineral particle size fractions were investigated by means of 13C CPMAS NMR (composition of the soil organic matter, SOM) and 129Xe NMR of adsorbed Xe atoms (nano-structure of PyOM). Fire-induced alteration of the soil mineral phase was revealed by the X-ray diffraction and chemical extraction methods. An enhanced contribution of aromatic species combined with depletion in carbohydrates and alkyl functionalities (13C NMR) was observed in the surface soil horizon (Ah), suggesting that the dehydration of carbohydrates and thermal breakdown of n-alkanes was mainly responsible for the formation of BC species. As a measure of a charring degree of PyOM, we suggested to use the relative intensity of the down-field 129Xe NMR signal (130-160 ppm) characterizing micropores (< 2 nm) that were formed by the fused aromatic units of BC. The revealed microporous BC regions and the low-dense domains with larger pores (supposedly formed by the alkyl functional groups) exist within the same PyOM particle, as this was supported by the calculated mean Xe displacement distance between these two regions. The ascertained micropore heterogeneity within the BC domains is supposedly caused by the varying size of aromatic clusters that approach each other at a varying distance. This nano-scale heterogeneity may have important implications for the specific adsorption behavior and persistence of BC in a soil.In the organo-mineral soil fractions, the BC species mainly occurred in a highly dispersed (mineral-associated) form. Supposedly, the association “PyOM-soil minerals” did not necessarily require the aromatic ring opening, as we detected rather high content of aromatic 13C NMR moieties in these fractions. An increased content of the short-range order Fe- and Al oxyhydroxides and mixed layer silicates (supposedly fire-induced) appeared to promote the interactions “PyOM-soil minerals” via an enhanced extent of hydroxylated mineral surfaces.