Understanding Soil Organic Carbon Dynamics in Integrated Crop-Pasture Systems: Insights into Deep Carbon

Integrated crop-pasture rotational (R) systems promote greater SOC accumulation than continuous cropping (CC) systems at the soil surface (0-20 cm) by avoiding losses of centennial C. Here, we explore whether old-C losses due to continuous cropping extend to deeper layers or whether other factors control deep carbon dynamics. We measured the vertical profile of SOC, Δ¹⁴C and δ¹³C in SOC fractions at two points in time of two contrasting agricultural treatments (CC and R system), in a long-term experiment (>60 yr) on Vertic Argiudolls of Uruguay. Using site-specific data we fit dynamic compartmental models to describe the temporal trajectory of C stocks and Δ¹⁴C in the stable SOC at depth on the R system.  MAOM - HF was highly isolated with respect to inputs of recent atmospheric CO₂ and this isolation increased substantially with depth. The differences found in the C stock between management systems could be explained by different losses of old legacy C and the high stability and isolation of the compartments associated with the mineral phase in the R system. The high stability of MAOM-HF in this agricultural system was reflected in the very old ages of these C pools, from approximately 700 years at surface to a value of several thousand years at depth. The results of this work imply that R management systems have not been able to sequester significant amounts of new C along the profile (and in particular at depth), although they are relevant to preserve the natural C legacy of these soils.