Carbon Sequestration in Soils by Hydrophobic Protection and In Situ Catalyzed Photo-Polymerization of Soil Organic Matter (SOM): Chemical and Physical–Chemical Aspects of SOM in Field Plots

Abstract

The application of innovative practices of soil organic matter (SOM) management, based on either soil amendment with hydropohobic mature compost or with a water-soluble biomimetic catalyst, has shown to enhance organic carbon sequestration in agricultural soils. In three different agricultural sites in Italy, field plots under maize were treated with traditional deep tillage, minimum tillage, green manuring and mature compost, whereas field plots under wheat were added with the iron–porphyrin catalyst. In terms of soil physical quality, mature compost additions improved soil aggregate stability by favoring the increase of water-stable macroaggregates. After 3 years of field experiments, both minimum tillage and green manuring treatments confirmed short-term and even negative effects on organic carbon (OC) accumulation, as compared to traditional tillage. Conversely, the hydrophobic protection exerted by compost amendments on SOM successfully fixed OC in soil from 3 to 22 ton ha−1 more than for traditional tillage, depending on the experimental site. Solid-state CPMAS-NMR spectra of humic substances (HS) extracted from treated soils allowed the molecular characterization of the stable organic matter pool. NMR data combined with chemometric methods revealed that compost-added soils progressively incorporated aliphatic and aromatic hydrophobic components into soil humic fractions over the experimental period. Though more variable among experimental sites, the treatment with biomimetic catalyst positively affected both total soil organic carbon (SOC) content and molecular characteristics of humic extracts. An increased aromaticity and hydrophobicity was shown in the spectra of HS from soils treated with the biomimetic catalyst, thus suggesting an effective photo-polymerization of soil aromatic components and their progressive inclusion into the humic pool. In the first and second year of treaments with the water-soluble iron–porphyrin catalyst, the in situ catalyzed photo-polymerization of SOM effectively occurred since soils were found to have sequestered from 4 to 24 ton ha−1 more than the control.