Design and results from a national soil carbon stock benchmarking and monitoring system for agricultural land in New Zealand

• Designed a soil carbon monitoring system for New Zealand’s agricultural land. • A total of 504 sampling sites spread across five broad land use classes. • Benchmark sampling to 60 cm depth completed. • Mean stocks were 101.3 Mg.ha −1 and 138.2 Mg.ha −1 for the 0–30 cm and 0–60 cm layers. • Data provide a crucial baseline to determine if soil carbon stocks are changing. Despite the importance of soil organic carbon (SOC) stocks and stock changes, few countries are monitoring changes through time with direct soil measurements. National-scale changes of SOC stocks in New Zealand’s mineral soils, reported to meet international requirements, are currently predicted based on transitions of land use using a statistical model calibrated with historic data. However, historical data were often originally collected for purposes other than SOC monitoring and are not fully spatially representative and the current model is also based on the assumption that SOC stocks do not change through time, if land use does not change. Here, we outline the design and report benchmark results from New Zealand’s National Soil Carbon benchmarking and Monitoring programme (NSCM), where 504 sites were established to determine a robust baseline of SOC stocks for agricultural land in New Zealand, with spatially representative sampling across five broad land use classes. Mean slope-corrected SOC stocks for the 0–30 cm layer for all agricultural land on mineral soils in New Zealand were 101.3 Mg.ha −1 , and 138.2 Mg.ha −1 for the 0–60 cm layer. SOC stocks in the 0–30 cm layer were highest under dairy pasture (110.7 Mg.ha −1 ) followed by hill country drystock pasture (104.3 Mg.ha −1 ), flat-rolling drystock pasture (98.7 Mg.ha −1 ), perennial horticulture (84.8 Mg.ha −1 ) and lowest under cropland (80.1 Mg.ha −1 ). Differences between land uses could not be attributed to land use alone, as the location of different land uses is often related to soil type and climate. Results provide spatially representative data that will feed into national soil carbon inventory reporting and is a crucial baseline against which to compare future sampling to determine if SOC stocks are changing in New Zealand’s agricultural land.