Soil carbon changes and uncertainties with New Zealand land-use change

Content Partner: Manaaki Whenua – Landcare Research. An IPCC-based Carbon Monitoring System (CMS) is being implemented to monitor soil C stocks and flows for New Zealand. Geo-referenced soil C data from 1153 sites (0.3 m depth) are used in the current version to assign steady-state soil C stocks to various combinations of soil class, climate, and land use. Overall, CMS estimates of soil C stock are consistent with detailed, stratified soil C measurements at specific sites and over larger regions. Soil C changes accompanying land-use changes were quantified using a national set of land-use effects (LUEs) derived using a General Linear Model. Predicted and measured soil C changes for the grazing–forestry conversion agreed closely. Most soil C is stored in grazing lands (1480±60 Tg to 0.3 m depth), which appear to be at or near steady state; their 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. Major uncertainties arise from estimates of changes in the areas involved, the assumption that soil C is at steady state for all land-cover types, and from lack of soil C data for some LUEs. Other uncertainties in our current soil CMS include: spatially integrated annual changes in soil C for the major land-use changes; lack of soil-C-change estimates below 0.3 m; C losses from erosion; and the contribution of agricultural management of organic soils.