Can highly weathered soils under conservation agriculture be C saturated?

Soil organic carbon (SOC) plays an essential function in global agroecosystems. Conservation agriculture (CA) associated with diverse and high C-input is an important tool to increase SOC, improve soil quality, and increase agronomic productivity. However, the information about the potential of highly weathered soils under CA to accumulate SOC, and when the SOC saturation may occur, is scarce. This study was based on the hypothesis that in highly weathered soil from tropical and subtropical agro-ecosystems, the potential to store SOC lies more in the sub-soil than in surface layers and is determined by nutrient scarcity. Thus, the aim of this study, performed in a long-term incubation experiment (30months), was to: (i) assess the SOC flow and mineralization based on CO2-C emissions for estimating SOC accumulation; (ii) evaluate the impact of nutrient scarcity on C accumulation efficiency by soil layers; and (iii) determine when C saturation occurs in these soils. The incubation study was performed in three Brazilian Oxisols under long-term CA, and was comprised of four amounts of C-inputs (0, 6, 12 and 24MgCha−1) added at 0, 10 and 20-months to three soil layers (0–20, 20–40 and 40–100cm). The CO2-C emission was 19.0, 9.0 and 7.0% higher in the 0–20cm than that in 40–100cm layer for Ponta Grossa, Londrina and Lucas do Rio Verde sites, respectively, which was associated with higher antecedent SOC content and fertility status. A higher SOC accumulation efficiency was observed for the 0–20cm layer than in deeper layers. Nutrient scarcity in deep soil layers; especially that of P, Ca2+ and Mg2+; was the driving force limiting SOC accumulation. Carbon saturation was not achieved indicating a high SOC storage capacity in these soils. Because these and similar soils cover a large global area, they possess a large C sink to mitigate atmospheric CO2-C. The potential SOC storage estimated for 20–100cm layer based on this study and upscaling for 1/3 Brazilian Oxisols (100millionha) may offset 0.06 to 0.36PgCyr−1 or 5.5% to 32.7% of the global annual greenhouse gas emissions by land use change.