Carbon sequestration capacity in no-till soil decreases in the long-term due to saturation of fine silt plus clay-size fraction

The capacity of soils to stabilize carbon (C) may decrease over time, limiting the potential of no-till soil to act as a C sink in the long-term. Our objectives were to evaluate the effects of long-term no-till cropping systems on (i) C storage in soil, (ii) C stabilization in the fine silt plus clay-size (<20 μm) fraction and its relationship with the decrease of C saturation deficit (CSD) in this fraction, and (iii) on C accumulation in labile fractions of soil organic matter (SOM) in 0–2.5, 2.5–5, 5–10 and 10–20 cm layers of a subtropical Acrisol. The study was based on a long-term (36 years) no-till experiment where five cropping systems, with variable annual C inputs, were assessed: [i] bare-soil, [ii] black oat/maize, [iii] black oat + vetch/maize + cowpea, [iv] lablab + maize and [v] pigeon pea + maize. Cropping systems including maize and tropical legumes (lablab, pigeon pea and cowpea) with high C input led to the highest C storage in the top layers (up to 10 cm depth) of this no-till soil. Also, a decrease of CSD in fine silt plus clay-size fraction was observed in all soil layers to 20 cm depth, but the most expressive impact on CSD occurred in the topsoil (0–2.5 cm), where the capacity to further stabilize more carbon decreased by 90–97% when compared to bare soil. Considering the full C saturation level of the silt plus clay-size fraction and the current C contents in the soil, the remaining capacity of C sequestration up to 20 cm was estimated as ranging from 22.5 to 32.8 Mg C ha−1, and much of it (58–75%) was in the 10–20 cm layer. Our results highlight the importance of diversified cropping systems with high input (quantity and quality) crop residues to C sequestration in soil. Moreover, although the mineral-associated SOM of the top layer reached a C stabilization limit, C accumulation continues in non-saturated labile fractions, and in non-saturated fine silt plus clay-size fraction in deeper layers of subtropical no-till soils.