Various methods were assessed to determine the impact of forest harvesting on mineral soil carbon (C) in Pinus radiata stands at Puruki catchment in the Central North Island of New Zealand. Previous work at this site reported a reduction in soil carbon of 3.6 Mg ha^-1 in the top 0.1 m following one rotation of pine on improved pasture (Beets et al., 2002). Harvesting was associated with a further reduction of 3.1 Mg ha^-1 (p = 0.05) in mineral soil carbon stocks to 0.1 m depth, but sampling by depth increments down to 2 m before and after harvesting found no significant effects of harvesting. Variation between plots was large, and it was estimated that to detect a 10% change in carbon content with 95% confidence 19 permanent plots were required for 0 – 0.1 m depth and 40 permanent plots required to detect changes to 1 m depth. Soil C content to 1 m depth in Puruki Rua subcatchment was estimated to average from 143 and 164 Mg.ha^-1, depending on the method used to measure bulk density.

In two paired site studies comparing pasture with second rotation pine, the difference between land-uses in mineral soil C content to 0.1 m depth was 19.6 Mg.ha^-1 in volcanic soil at Kaingaroa and 8.5 Mg.ha^-1 in high clay activity soil at Ngaumu. Significant differences in mineral soil C between pine and pasture were found with cumulative depth down to 2 m, well below the rooting depth of pasture, which suggests that historic factors were over riding influences of the current land-use and can account for the difference in soil carbon. At Kaingaroa, variability in soil carbon content was high at all depths measured, and to detect a 10% difference in soil carbon content with 95% confidence approximately 60 pits measured to 1 m depth would be required.