Relating Microfeatures of Soil Organic Matter to C Stabilisation: Optical Microscopy, 2 Sem-eds, Abiotic Oxidation 3 4 Authors 5

1 We investigated the relationships between microscale distribution of soil organic matter 2 (SOM) features and their stability by combining optical microscopy, SEM-EDS analysis and 3 NaClO oxidation of soil thin sections on five soils from Harwood Forest in Northumberland 4 (UK) differently affected by water stagnation. 5 Plant organs at different stages of decomposition and amorphous organic matter were 6 observed by optical microscopy in all samples. SOM microfeature distribution, size of SOM 7 features and the relation with the C-to-N ratio suggested that amorphous features could be the 8 end-products of organ transformation. SEM-EDS elemental analysis showed that amorphous 9 material had higher Si:C, Al:C and Fe:C molar ratios than organs, clearly pointing to 10 interactions with the soil inorganic phases, which contributed to SOM stabilisation. Soil 11 porosity coupled with water stagnation seemed to affect the Fe-SOM interactions as a greater 12 proportion of small water retention pores (10-50 μm) was associated with higher abundance 13 of Fe-rich amorphous organic features. The higher chemical stability of amorphous features 14 was confirmed by oxidation. After NaClO treatment organs were almost totally removed, 15 while amorphous organic material was less affected both morphologically and chemically. 16 Our results demonstrate that in water-affected soils local environment defined by the 17 pore system affects the distribution of SOM microfeatures and that the highest resistance to 18 oxidation of the amorphous features is attributable to the formation of organic-inorganic 19 associations. The proposed combined approach seems to be a promising means to investigate 20 SOM dynamics by relating features to stability. 21 22