Spring pasture renewal involving full inversion tillage and a summer crop can facilitate soil C storage, improve crop yields and lower N leaching

Abstract

Soil organic carbon (SOC) sequestration in pastoral soils could offset net greenhouse gas emissions by reducing global atmospheric carbon dioxide (CO2) concentrations and, thereby, slow climate warming. Long-term pastures typically have topsoils (0–10 cm) rich in SOC, with subsoils (10–30 cm) storing less than half as much SOC. In New Zealand, lowland farmers are advised to renew (reseed) their pastures every 7–10 years to improve pasture production. Renewal typically involves desiccating the old pasture followed by shallow tillage (or direct drilling) to establish a short season forage crop as a weed break, then direct drilling new pasture species. Minimum till at renewal maintains the vertical stratification of SOC, limiting the scope to increase SOC stocks under new pasture. During the spring of 2016 and 2017, two independent trials were established, on an Alfisol (trial 1) and on an Andisol (trial 2), to assess the effects of establishing a summer Brassica crop with either full inversion tillage (FIT; 30 cm furrow depth, as a one-off or infrequent (every 25–30 years) management), shallow tillage or no-till (direct drill) on SOC stocks and crop/pasture agronomic performance. In autumn, new pasture species were direct drilled into the stubble of the summer forage crop. Changes in SOC vertical distribution, plant growth, herbage quality (at both trials) and nitrogen (N) leaching (trial 1 only) were monitored. At both trials, FIT effectively buried SOC below 0–10 cm depth and increased crop yields compared to no-till treatment. After re-grassing pasture production was similar among all treatments. In trial 1, N leaching losses were 42% lower under FIT than under no-till. These results highlight the potential agronomic and environmental benefits of pasture renewal including FIT. Economic and N cycling benefits depend on the timely inclusion of a crop phase.