Estimating soil organic carbon in Mollisols and its particle-size fractions by loss-on-ignition in the semiarid and semihumid Argentinean Pampas

Abstract

Recent interest in soil quality assessment underscores the need for an accurate measurement of soil organic carbon (OC) and its labile fraction, i.e. particulate OC. The loss-on-ignition (LOI) method has been proposed as a rapid, inexpensive and accurate method for estimating OC. The objectives of this study were i) to test the LOI method for soil organic matter (SOM) and particulate organic matter (POM), and ii) to evaluate the use of an adequate conversion factor (CF) for predicting OC from organic matter (OM) in soil and different particle-size fractions in several Mollisols of the Argentinean Pampas. Several agricultural fields (140) under no-tillage were sampled before crop sowing at 0–20 cm. SOM was physically separated by wet sieving, obtaining a coarse particle-size fraction (CPF, 105–2000 μm) and a medium particle-size fraction (MPF, 53–105 μm). Organic C and OM were determined by dry combustion (DC) and by LOI in whole soil (WS) and in both particle-size fractions. The values of OC (0.7–3.6 g 100 g− 1) and OM (1.9–7.9 g 100 g− 1) varied greatly between the soils. High and significant relationships (P < 0.001) were found between the different soil fractions determined by DC and LOI, with OM (105–2000 μm):OC (105–2000 μm) (R2 = 0.94) > OM(WS):OC(WS) (R2 = 0.88) > OM(53–105 μm):OC(53–105 μm) (R2 = 0.82). These results indicated that the C content in each fraction was in the order of 45%, 50% and 41% for WS, CPF and MPF, respectively. The LOI method can predict OC in CPF (105–2000 μm) with reliable accuracy. The CF for predicting OC from OM was variable for each soil fraction considered: 2.23 for WS, 2.00 for CPF and 2.44 for MPF.