Use of diffuse reflectance infrared Fourier transform (Drift) to study the composition of four size fractions of soils subjected to different management

This study investigates the effects of different tillage (conventional (CT), minimum (MT) and no tillage (NT)), fertilization (fertilization vs no fertilization), and soil depth (0-30, 30-60, and 60-90 cm) on the abundance and composition of four size fractions of soils and on their organic carbon (OC) content. Fraction-size separation was conducted by ultrasonic dispersion and wet sieving according to Amelung and Zech (1999), obtaining four size fractions (A: 2000–250 μm; B: 250–53 μm; C: 53–20 μm; D: <20 μm). The OC content and the quality composition of each fraction were determined by elemental and DRIFT analyses, respectively. Results suggest that tillage and fertilization influenced the amount of each soil size fraction. In fact, CT favored the increment of the finest fraction (D) while the fertilization, promoting the microbial community, increased the amount of soil micro-aggregates (B). Clearly, soil depth influenced the amount of each soil size fraction, with higher amounts of fraction A and fraction D in the upper and deepest soil layer, respectively. The OC content was influenced mainly by the size of fraction and the soil depth since a higher OC content was found in the top soil layer (0-30 cm) and in the finest soil fraction (D). The DRIFT analysis suggested that the size of the fractions, and secondarily the depth, were the only factors responsible for the composition of soil fractions. In particular, quartz was identified in A fraction, while the signal of kaolinites appeared in the other fractions. The signals of aromatic and aliphatic organic components were present in the spectra of the finest fractions, while the nitrogen compounds were mainly related to the coarser fraction of soil (A). With depth, a reduction of the signal of polisaccharides structures was observed only in A fraction. Different managements influenced only the quantity of each soil fractions and not their composition.