Land-use change alters New Zealand’s terrestrial carbon budget: uncertainties associated with estimates of soil carbon change between 1990–2000

In New Zealand, afforestation and reforestation of grazing land give rise to large (relative to national CO₂ emissions) vegetation carbon (C) sinks. These land-use changes may, however, lead to losses of mineral soil C. Full C accounting may, therefore, require including mineral soil C losses if credits are awarded for vegetation C. To monitor soil C stocks and changes, we developed an IPCC-based soil Carbon Monitoring System (CMS) in which New Zealand is stratified by soil type, climate, land-use and an erosivity index (slope × precipitation). Georeferenced soil C data were used to assign steady-state soil C stocks to various combinations of these factors (cells). We then used a General Linear Model to compare soil C between cells, and derived land-use effects (LUEs) from this analysis that quantify soil C changes that accompany land-use change. These LUEs were used to predict soil C changes resulting from land-use change between 1990–2000. We tested the CMS by comparing predicted soil C stocks, and changes in these stocks, against more detailed soil C data. Overall, soil C estimates obtained from the CMS are consistent with detailed, stratified soil C measurements at specific sites and over larger regions. However, for grazing-land to exotic-forest conversions, estimates of soil C changes are higher and more variable than those based on paired-site studies. Nationally, soil C losses of 0.9 ± 0.4 Tg C yr⁻¹ for all land-use changes over the period 1990–2000 appear likely, with uncertainties arising mainly from estimates of changes in the areas involved, and LUE values for cells with limited soil C data. Changes in soil C in reforested land are likely to be small, but precise area changes, soil C data and detailed paired-site studies are lacking for this key land-use change. By contrast, biomass C accumulation in new exotic plantation forests (afforestation) and native reforestation are 6–9 Tg C yr⁻¹, with C accumulation by afforestation being well quantified. Detailed site studies, process-based modelling and improved area-change estimates are needed to further reduce uncertainties in land-use-change effects on soil C. Our approach could be adapted to countries with country-specific land-use issues different from those in the IPCC default methodology.