Visible and infrared spectroscopy in soils under cultivated and natural systems

Abstract

The soil organic carbon (SOC) and its different fractions are soil parameters highly influenced by management practices. Studies based on spectroscopic techniques are currently used, since they constitute a valuable tool to obtain information on the structural elucidation of many molecules, which allow interpretation of organic matter characteristics. The objective of this study was to evaluate the effect of different soil management practices: to determine the molecular changes in the organic forms with spectral technical along a gradient of sites with different soil textures and climate conditions. In three study sites (Bengolea (BE), Monte Buey (MB) and Pergamino (PER), with documented history of NT management located across a West-East transect in the most productive region in the Argentinean pampas were applied three treatments studied: ‘‘Good Agricultural Practices’’ (GAP), ‘‘Poor Agricultural Practices’’ (PAP) and ‘‘Natural Environment’’ (NE). E4/E6 index were determined by UV–visible spectroscopy and with IR spectroscopy the main peaks were compared and determined the ratio between them. The E4/E6 ratios showed the tendency: NE>GAP>PAP for all studied sites, this order is associated with the increase in the molecular condensation and the aromaticity, which are proportional to the humification degree and the stability of the system studied. The sequence of the aromaticity degree (1650/2920) obtained from the absorbance ratios of the IR spectra in the samples analyzed was MB>BE>PER in natural and cultivated soils it could associates with contained of coarse particulated organic carbon fraction. The sequence of the polycondensation degree (1034/1540) obtained from the absorbance rate of the IR spectra in the samples analyzed was. BE>MB>PER, it could be indicated that polycondensation degree it could be related with fine particulate organic carbon fraction in natural and cultivate soils. Analyzing the 1380/1620 ratios, these bands correspond to different vibration modes of the carboxyl group. The values of such rate were similar among sites MB ≈ BE ≈ PER it could be associated with soil potential fertility (ratio soil organic carbon versus (silt+ clay)) in natural and cultivated soils. As a conclusion, soil management produced biochemical quality changes on organic forms, which would modify the predominant functional group. In all study sites, the E4/E6 relationships was in order NE/GAP/PAP indicating it was sensitive to management practices. The relationships among the main IR peaks could indicated the presence of determinate C fraction.