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

Abstract. In soils from a long-term agricultural experiment (LTE) installed in Uruguay, integrated crop-pasture rotational systems promote greater soil organic carbon (SOC) accumulation than continuous cropping 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 in the same LTE in Uruguay. To answer this question, we analyzed the vertical profile of SOC, Δ¹⁴C and δ¹³C in fractions representing compartments of different stability at two points in time of two contrasting agricultural treatments (continuous cropping and integrated crop-pasture rotational system). Additionally, using site-specific data we fit dynamic compartmental models to describe the temporal trajectory of C stocks and Δ¹⁴C signature in the stable SOC fractions at depth on the integrated crop-pasture system. Based on these models, we used the estimated age distribution for soil C to assess whether it is possible to sequester C in these compartments on time scales relevant in terms of climate change mitigation. We found that the SOC fractions associated with the mineral phase (MAOM - HF) are highly isolated with respect to inputs of recent atmospheric CO₂ and that this isolation increases substantially with depth in the soil profile. This degree of isolation was characterized by a progressive increase in the difference between Δ¹⁴C profile between MAOM-HF and LF-POM (more labile) fractions 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 crop-pasture rotational 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 inclusion of a vertical transfer mechanism for previously stabilized material was not necessary to explain the general age structure and the capacity of these systems to sequester new C inputs in deep stable layers. The results of this work imply that integrated crop-pasture rotational 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.