Dr Roberto Calvelo-Pereira1, Dr Matteo Poggio2, Professor Michael J. Hedley1, Dr Michael Blaschek2, Dr Carolyn Hedley2, Dr Michael H. Beare3, Dr Samuel R. McNally3
1Massey University, Palmerston North, New Zealand, 2Manaaki Whenua-Landcare Research, Palmerston North, New Zealand, 3The New Zealand Institute for Plant and Food Research Limited, Lincoln, New Zealand
Sequestration of soil organic carbon (SOC) is one strategy to reduce atmospheric CO₂ concentrations and limit climate change. Permanently grazed pastures, such as those in New Zealand, accumulate large amounts of SOC because pastures allocate a high proportion of plant-fixed C to root turnover and rhizodeposition. Consequently, these soils show a strong vertical stratification of roots and SOC that limits the topsoil’s ability to sequester any additional carbon. Full inversion tillage at pasture renewal (FIT-renewal) has been proposed as a farm management practice suitable to accelerate SOC storage in pastoral soils showing a high degree of SOC stratification. Using a modified mouldboard plough, a one-off soil inversion event transfers carbon-rich topsoil into the subsoil while low-carbon mineral subsoil is brought to the surface where it is exposed to higher carbon inputs from the new pasture. The potential of increasing SOC stocks after FIT-renewal is under investigation at the field scale in New Zealand. In the North Island, two field trial sites were established, one on an Alfisol and the other on an Andosol. Pasture renewal via FIT involved full cultivation of a summer brassica crop followed by autumn re-grassing by direct drilling compared to other pasture renewal treatments by direct drill and shallow till. The sites were sampled through coring before renewal and after 5 months of crop growth to assess changes in SOC stocks and stratification using either (1) laboratory-based analytical methods or (2) diffuse reflectance spectroscopy. The one-time deep ploughing event redistributed large quantities of soil and soil C into depth (below 10 cm), altering its SOC stratification at varying degrees. The rapid reflectance scanning of the cores allows soil carbon content and stratification to be estimated with greater depth resolution helping researchers to make rapid decisions about the plough settings needed prior to FIT-renewal.