Correcting Microtopography Effects on DRIFT Mapping Signals of Organic Matter at Intact Soil Aggregate Surfaces

Soil organic matter (OM) of aggregate coatings and biopore walls can affect the transport of reactive solutes during preferential flow in structured soil. As a nondestructive method, diffuse reflection infrared Fourier transform (DRIFT) spectroscopy, has been proposed for mapping the millimeter‐scale OM composition of intact flow path surfaces. The surfaces of such intact soil structures, e.g., aggregate and biopore surfaces, mostly have a distinctive microtopography, however, that affects the intensity of DRIFT signals. Thus DRIFT mapping data require geometric corrections for a quantitative interpretation. This study analyzed a digital terrain model (DTM)‐based approach for describing microtopography effects on DRIFT reflectance. A gypsum block model was first used for developing the concept. The surface of the gypsum block had defined channels and pores and was partly coated with defined humic acid (HA). A millimeter‐scale DTM of the DRIFT‐mapped gypsum block surface was obtained with a laser scanner. The signal intensities at specific wavenumbers were corrected for microtopography effects using surface elevation, slope, and aspect data of the sample surfaces. The corrections were found to be dependent on both the wavelengths and the measured substance (gypsum or HA). The method was then applied to determine the OM composition at intact structural soil surfaces. The DTM‐based approach reduced microtopography effects and was compared with an alternative approach in which spectral ratios between specific absorption bands were used for corrections. The results indicated differences in OM composition and local distribution between surfaces of worm burrows and crack walls. The results suggest that DTM correction of a DRIFT‐mapped intact soil aggregate surface enhanced the interpretation of the millimeter‐scale OM composition.