Assessing soil organic carbon stocks and sequestration opportunities on a Taranaki dairy farm : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Agricultural Science at Massey University, Manawatū, New Zealand

Abstract

Increasing soil organic carbon stocks (SOCS) on farms is widely accepted as a viable strategy to reduce agriculture’s net carbon footprint. Despite this opportunity, unlike in other countries such as Australia, there are no government-led incentives for farmers to monitor or increase SOCS in New Zealand (NZ). This lack of incentives, combined with the high cost of SOCS benchmarking and monitoring systems, has hampered the monitoring of farm-scale SOCS in NZ. With the variety of climates and farming systems across the country, this lack of data has meant that the scope to increase SOCS has not been well characterised. Consequently, the strategies to enhance SOCS in soils which already have medium to high SOCS have not been explored thoroughly. This study designed and implemented a practical, cost-effective farm-scale SOCS benchmarking and monitoring system based on a combination of international standards and the NZ Ministry for Primary Industries (MPI) soil carbon monitoring methods and applied this to the Dairy Trust Taranaki Kavanagh Farm in South Taranaki. The influence of soil drying and sample storage on possible changes in soil carbon respiration was also explored in this process and found some carbon respiration in both air-dried and imperfectly dried soil samples while in storage. The results of the benchmarking survey found that Kavanagh farm has 219.6 t C ha-1 to a depth of 60 cm and 162.14 t C ha-1 to a depth of 30 cm which is significantly above average for Allophanic pasture soils measured previously in NZ. The results also highlighted the importance of stratification of the farm as a way of reducing future survey cost. The final stratification of the farm reduced the number of samples required in the survey by 59% when compared to random sampling with no stratification. Finally, the study reviewed NZ-based research to identify farm management practices which may increase SOCS on a dairy farm like the Kavanagh farm. Our review highlighted inversion tillage and biochar applications as practices with the most potential to increase SOCS in Kavanagh’s high SOCS Allophanic soil by approximately 0.56 and 1.1 t C ha-1 year-1 respectively over the next 20 – 30 years. If these sequestration rates could be achieved throughout the entire farm, then net greenhouse gas emissions from the farm could be offset by 46%. However, practical limitations and the financial viability of implementing these practices across the entire farm without financial incentives will likely result in a lower net rate of sequestration. Further research into the viability and efficacy of these practices will be needed for more widespread adoption of carbon sequestration enhancing practices in the NZ pastoral industry.