CURRENT APPROACHES TO SOIL CARBON MONITORING IN NEW ZEALAND

In New Zealand, terrestrial carbon (C) stocks are dominated by remnant indigenous forests and soils Accordingly, national vegetation and soil C monitoring systems (CMS) are being developed to track changes in New Zealand's forest and soil C. To monitor New Zealand's soil C stocks and changes, we have developed an IPCC-based Carbon Monitoring System (CMS). Geo-referenced soil C data for 1153 sites (0.3 m depth) were used to assign steady-state soil C stocks to various combinations of soil class, climate, and land use. Afforestation and reforestation of grazing land are major contemporary land-use changes that result in large (relative to national CO2 emissions) vegetation carbon sinks, with some associated mineral soil C losses. These changes in soil C storage, with associated above-ground changes, need to be accurately accounted for under the Kyoto Protocol, if C credits are to be claimed for vegetation C. Overall, CMS soil C stock estimates are consistent with detailed, stratified soil C measurements at specific sites and over larger regions. We used a General Linear Model to include the effects of numeric predictors (e.g. slope x rainfall) to derive a national set of land-use effects (LUEs) that quantify soil C changes accompanying land-use changes. Uncertainties arise from estimates of changes in the areas involved, from the assumption that soil C is at steady state for all land- cover types, and from lack of soil C data for some LUEs. We used the LUEs to estimate national soil C stocks for 0-0.1, 0.1-0.3, and 0.3-1 m depths of 1300±20, 1590±30, and 1750±70 Tg, respectively. Most soil C is stored in grazing lands (1480±60 Tg to 0.3 m depth), which appears to be at or near steady state. Grazing land conversion to exotic forests and shrubland contributed most to the predicted national soil C loss of 0.7±0.3 Tg C yr -1 during 1990-2000. Predicted and measured soil C changes for the grazing land-forestry conversion agreed closely. Other uncertainties in our current soil CMS include: spatially integrated annual changes in soil C for the major land-use changes, soil C changes below 0.3 m and losses from erosion, and the contribution to the national soil C budget of organic soil C changes with agricultural management. Our approach could be adapted for use by other countries with land-use-change issues that differ from those in the IPCC default methodology.