Organic carbon and short-range order minerals responsible for cementation of the spodic horizon (ortstein): a new proposal of chemical extractions in undisturbed samples

Abstract

Context

The cementation of spodic horizons (ortstein) has long been studied through chemical analysis in disturbed soil samples of <2 mm (soil structure destructuring).

Aims

Use of the undisturbed samples and selective chemical extractions to study the soil organic matter (SOM) and short-range order phase (SROP) cementing the spodic horizon.

Methods

The study was carried out in a Spodosol (ortstein) from southern Brazil in undisturbed cubes (1.5 cm × 1.5 cm × 1.5 cm). The undisturbed cubes were separated in two visual colour standards (yellow colour (YC – 10YR 6/8) and yellow dark colour (YDC – 10YR 4/3)) and were submitted to four SROP chemical extractions (pyrophosphate (PYR), ammonium oxalate (AO), NaOH 0.5 mol L⁻¹, and water).

Key results

The choice of selective SROP extraction in two colour standards of undisturbed samples enabled the identification of cementation details of the ortstein : (1) SOM illuviation also increases Fe, Al, Mn, Si, Ca, and Mg contents; (2) the SRO-Fe and Al oxides cover the SOM and/or there is a larger proportion of mineral in relation to OC in the organo/mineral association; (3) SRO-Al oxides were the main compounds responsible for cementation; and (4) the illuvial SOM associated with SRO-Fe oxides did not contribute to ortstein physical stability.

Conclusions and implications

The best extractions to study the samples were AO for YC colour and NaOH for YDC colour. We recommend the use of undisturbed samples and the inclusion of NaOH 0.5 mol L⁻¹ in the chemical protocol for analysis to better understand which organic and mineral phases clog the ortstein pores in different pedogenetic conditions around the world.