Regional and national changes in soil carbon stocks with land-use change from 1990 to 2016 for New Zealand

In accord with the Paris Agreement, New Zealand has committed to reducing national greenhouse gas emissions to 30% below 2005 levels by 2030 and further reductions by 2050. Exports of products from agriculture, horticulture, and forestry are important industries, but these industries also contribute 49% of the national greenhouse gas emissions. Research to develop and adopt mitigation practices is underway, but New Zealand also supports the aims of the 4 per 1000 Initiative to increase soil organic carbon (SOC) stocks and remove atmospheric CO2 globally. We used the Soil Carbon Monitoring System statistical model, which is specific to New Zealand, to estimate the effects of land-use change on spatial changes in SOC stocks to a depth of 0.3 m at regional and national scales for the period 1990–2016. During this period, land-use change occurred on only 8.45% of the national land area, and this led to a mean decrease in SOC stocks of 3.3 tC ha−1 on the area where land use changed. This was attributable mainly to the known reductions in SOC stocks that occur when grassland is converted to post-1989 forest, but the effects were dependent on regional land use and slope class. Although the estimated national decrease in SOC stocks of 8.435 MtC was only 0.45% of the total national SOC stocks and 0.35% of the carbon stored in vegetation, our analysis highlights the need for changes to land management practices for New Zealand to increase its SOC stocks.