Full inversion tillage during pasture renewal to increase soil carbon storage: New Zealand as a case study.

As soils under permanent pasture and grasslands have large topsoil carbon (C) stocks, the scope to sequester additional C may be limited. However, because C in pasture/grassland soils declines with depth, there may be potential to sequester additional C in the subsoil. Data from 247 continuous pasture sites in New Zealand (representing five major soil Orders and ~80% of the grassland area) showed that, on average, the 0.15–0.30 m layer contained 25–34 t ha−1 less C than the top 0.15 m. High‐production grazed pastures require periodic renewal (re‐seeding) every 7–14 years to maintain productivity. Our objective was to assess whether a one‐time pasture renewal, involving full inversion tillage (FIT) to a depth of 0.30 m, has potential to increase C storage by burying C‐rich topsoil and bringing low‐C subsoil to the surface where C inputs from pasture production are greatest. Data from the 247 pasture sites were used to model changes in C stocks following FIT pasture renewal by predicting (1) the C accumulation in the new 0–0.15 m layer and (2) the decomposition of buried‐C in the new 0.15–0.30 m layer. In the 20 years following FIT pasture renewal, soil C was predicted to increase by an average of 7.3–10.3 (Sedimentary soils) and 9.6–12.7 t C ha−1 (Allophanic soils), depending on the assumptions applied. Adoption of FIT for pasture renewal across all suitable soils (2.0–2.6 M ha) in New Zealand was predicted to sequester ~20–36 Mt C, sufficient to offset 9.6–17.5% of the country's cumulative greenhouse gas emissions from agriculture over 20 years at the current rate of emissions. Given that grasslands account for ~70% of global agricultural land, FIT renewal of pastures or grassland could offer a significant opportunity to sequester soil C and offset greenhouse gas emissions. [ABSTRACT FROM AUTHOR]