A novel approach to the study of organic-matter distribution in soil microaggregates (<20 μm) using high-energy ultraviolet (UV) radiation in the presence of oxygen (photo-oxidation) is reported. The method quantitatively destroyed complex organic materials through oxidation, even in the presence of clay, provided the organic materials were directly exposed to the UV radiation.

Photo-oxidation of clay and silt fractions for periods up to 8 h demonstrated that a considerable proportion of the organic matter was physically protected within clay- and silt-sized aggregates. In some clay fractions, up to 23% of the organic carbon could be considered as physically protected whereas in silt fractions this was as high as 36%.

Infrared spectroscopy demonstrated that the materials external to both clay- and silt-sized aggregates were largely proteinaecous in nature, while the materials in the interior of the aggregates resembled humic acids. These humic materials appeared to be physically shielded against photo-oxidation, rather than being chemically recalcitrant. Using the clay- and silt-sized fractions from one soil, 14C accelerator mass spectrometry demonstrated that, although both clay and silt fractions contained essentially modern carbon, after 4 h of photo-oxidation much older organic carbon with a mean resonance time (MRT) of between 200±80 and 320±80 years before the present (BP) remained. This protection from photo-oxidation, therefore, appears to mirror the process which physically protects organic substances in soils against microbial degradation. Photo-oxidation of the clay-plus-silt fractions also resulted in a considerable reduction in particle size as the organic-cementing agents, consisting of proteinaecous and humic materials, were oxidized.

Using data from the photo-oxidation method along with infrared spectroscopy, radiocarbon dating and scanning electron microscopy, a simple model is proposed that spatially relates the various organic structures present to their positions in the mineral aggregates.