Analysis of organic fractions as indicators of soil quality under natural and cultivated systems

Soil organic matter (SOM) or carbon (SOC) is the most important component of the soil and it is composed of fractions with different lability. Particulate organic carbon (POC) and carbohydrates (CH), among others, are the most sensitive to changes in crops and soil management practices. The purpose of this study was to analyze different soil parameters aimed for the evaluation of management practices effects under widely different soil and climatic conditions. Soils were located along a West–East transect in the most productive region of the Argentinean pampas: [West] Bengolea and Monte Buey (Córdoba), Pergamino (Buenos Aires) and Viale (Entre Rios) [East]. Three treatments were defined according to land use: “Good agricultural practices” (GAP): sustainable agricultural management under no-till; “Poor agricultural practices” (PAP): non-sustainable agricultural management under no-till; “Natural environment” (NE): rangelands long as reference situation. Samples were taken at 0–10 and 10–20cm depths. SOC was determined in different particle size fractions: 105–2000μm (coarse particulate organic carbon, POCc), 53–105μm (fine particulate organic carbon, POCf), and 53μm (mineral-associated organic carbon, MOC). Total (CHt) and soluble (CHs) carbohydrate contents were also determined. The SOC level in NE was decreasing from the East (27.3gkg−1 in Viale) to the West (13.3gkg−1 in Bengolea), following the rainfall and texture gradient among sites. The POCc/SOC and POCc+POCf/SOC ratios in the NE showed differences among sites, suggesting different dynamic depending on the environmental characteristics at the different locations. The SOC levels in the upper layer of agricultural soils were 16–44% lower than natural ones. Carbon stocks were estimated for an equivalent mass of soil (950 and 2350Mgha−1) in order to consider differences in bulk densities among different treatments. Mean values were significantly different (p<0.001) for the different management practices: NE (26.6Mgha−1)>GAP (20.1Mgha−1)>PAP (16.3Mgha−1). In general, labile organic fractions showed differential sensitivity. Fractions with an intermediate dynamic, as POCf (53–100μm) and CHt, seem to be better indicators to detect the short- and medium-term management effects than more dynamic fractions.