Form and stability of soil organic matter: an integration of image analysis and SEM-EDS techniques with NaClO oxidative method of soil thin sections

The image analysis of soil thin sections is a very useful technique in studying the natural complexity of soil organic matter (SOM) and SEM-EDS technique can be further applied in mapping the distribution of the SOM forms on the basis of their chemical composition. On soil thin sections, bleaching with NaClO has been used to distinguish organic forms, and in the disturbed fine-earth fraction it is now a well established oxidative method used to isolate chemically stable SOM without affecting inorganic soil constituents. To investigate the relationship between microscale distribution of SOM and its stability in undisturbed soil samples, we combined optical and SEM-EDS analysis to the NaClO oxidation of soil thin sections. Moreover, to gain a better understanding on the distribution of different forms of SOM, we also took into account the soil pore system. The study involved five soils from Harwood Forest in Northumberland (UK) differently affected by water (two Gleysols and three Stagnosols), which is expected to influence SOM transformation in different ways. The image analysis indicated differences both in pore system and in SOM between the two soil types. In fact, unimodal distribution of pores was more evident for soil profiles showing stagnic properties, with one 10-50 μm modal pore class; while in the gleyic soils bimodality was detected, with an additional modal class around 200-500 μm. In both soils plant organs at different stage of decomposition and amorphous organic compounds were found, but in Stagnosols amorphous organic matter reddish in color was more frequent. The SEM-EDS elemental analysis confirmed that the reddish compounds were Fe-rich (mean Fe content 2.7%) and their distribution was related to the presence of pores smaller than 50 μm (r=0.90; p<0.01). Black and red amorphous compounds had higher O:C ratio than that of organs (0.32 and 0.30, respectively; p<0.01) likely because the acidic compounds increased in more transformed organic fractions. After oxidation with NaClO, both morphological observation and SEM-EDS analysis showed that the nature of the changes that had occurred in the organic features were linked to the SOM form. In all samples, after the NaOCl treatment, the slightly decomposed organs were almost totally removed, while the reddish amorphous organic compounds seemed less affected both morphologically and chemically, with Fe:C ratio not significantly different (p=0.976) before and after NaClO treatment. The proposed combined procedure seems to be a very promising approach to investigate the organic carbon dynamics and could provide new insights into the complexity of soil physical, chemical and biological processes.