High carbon storage in a previously degraded subtropical soil under no-tillage with legume cover crops

The effect of no-tillage (NT) on soil organic carbon (SOC) storage may help Brazil meet its 37% greenhouse gas emissions reduction target by 2025. When combined with legume cover crops, NT could result in even greater SOC storage than NT alone. The objective of this study was to evaluate the SOC storage potential of NT and the contribution of legume cover crops and nitrogen (N) fertilization to this potential in both the surface and subsurface soil layers of a previously degraded subtropical Acrisol of Southern Brazil. Using a split-plot design, the long-term field experiment compared the effect of NT and conventional tillage (CT), with or without legume cover crops, and with or without mineral N fertilization. Thirty years of contrasting management systems resulted in large differences (up to 35 Mg ha−1) in SOC stocks in the whole soil profile (0-100 cm). The combination that provided the greatest increase in SOC was NT combined with two legume cover crops and N fertilization (1.15 Mg ha−1 year−1 compared to CT, with no N fertilization or legume cover crop). Legume cover crops were twice as efficient in storing SOC as N fertilization, with 1 kg of residue input being converted to 0.15 kg of SOC. Overall, the variation in SOC stocks was explained largely by plant carbon input (R2 = 80%) which varied with N fertilization and cropping system. About half of the SOC storage that occurred in this 30-year-old NT system was attributable to the increase in SOC stocks in the subsurface layer (30-100 cm), which was confirmed by the contribution of C3 cover crop residues using carbon isotope signature (from 14.8 to ∼17.5‰ in the 75-100 cm layer). Thus, the legume cover crop made a strong contribution to the potential of SOC storage in NT, and high rates of C storage occurred over a longer period in subsurface soil layers than previously believed.