Distribution and vertical stratification of carbon and nitrogen in soil under different managements in the Pampean region of Argentina

One of the expected benefits of no-tillage systems is a higher rate of soil C sequestration. However, higher C retention in soil is not always apparent when notillage is applied, due e.g., to substantial differences in soil type and initial C content. The main purpose of this study was to evaluate the potential of no-tillage management to increase the stock of total organic C in soils of the Pampas region in Argentina. Forty crop fields under no-tillage and conventional tillage systems and seven undisturbed soils were sampled. Total organic C, total N, their fractions and stratification ratios and the C storage capacity of the soils under different managements were assessed in samples to a depth of 30 cm, in three layers [0-5, 5-15 and 15-30 cm]. The differences between the C pools of the undisturbed and cultivated soils were significant [p less than 0.05] and most pronounced in the top [0-5 cm] soil layer, with more active C near the soil surface [undisturbed greater than no-tillage greater than conventional tillage]. Based on the stratification ratio of the labile C pool [0-5/5-15 cm], the untilled were separated from conventionally tilled areas. Much of the variation in potentially mineralizable C was explained by this active C fraction [R 2 = 0.61] and by total organic C [R 2 = 0.67]. No-till soils did not accumulate more organic C than conventionally tilled soils in the 0-30 cm layer, but there was substantial stratification of total and active C pools at no till sites. If the C stratification ratio is really an indicator of soil quality, then the C storage potential of no-tillage would be greater than in conventional tillage, at least in the surface layers. Particulate organic C and potentially mineralizable C may be useful to evaluate variations in topsoil organic matter.