High energy ultraviolet photo‐oxidation: a novel technique for studying physically protected organic matter in clay‐ and silt‐sized aggregates

Abstract

SUMMARYA 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.