Monitoring and Modelling Soil Organic Carbon Stocks and Flows: a New Zealand Perspective

We developed further an IPCC-based Carbon Monitoring System (CMS) to monitor soil C stocks and flows for New Zealand. Geo-referenced soil C data were extended from 340 to 1119 sites (0.3 m depth) and used to assign steady-state soil C stocks to various combinations of soil class, climate, and land use. 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 angle) 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, 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 1280±20, 1540±30, and 1650±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±0.4 Tg C yr during 1990–2000. Predicted and measured soil C changes for the grazing–forestry conversion agreed closely. Major environmental issues linked to agricultural soil C are soil loss from erosion, and methane and nitrous oxide emissions from ruminant animals in grazing land. Our approach could be adapted for use by other countries with land-use-change issues that differ from those in the IPCC default methodology.