Combining Soil C and N Spatial Variability and Modeling Approaches for Measuring and Monitoring Soil Carbon Sequestration

The main objective of the study was to investigate soil carbon and nitrogen stocks in five soil types within a 63-ha Amazon degraded pasture area using the Century model and to combine this information with soil C and N spatial variability at Nova Vida ranch in the western Brazilian Amazon. Samples for soil chemical, physical, and biological analyses were collected at eight different georreferenced points in each of five studied soil types. Century was used to simulate soil C and N contents in the 0- to 20-cm layer in each soil type within a 63-ha degraded pasture area. Century accurately simulated the level of total soil C and N for the five soil types, but it was not successful in predicting seasonal variations in microbial biomass C. The influence of soil texture and hydrological process affecting the ability of soils to accumulate soil organic matter were discussed. Scenarios for soil C and N stocks with varying pasture management were examined through a period of 40 years after deforestation. In conclusion, modeling and geostatistical techniques can be used for measuring and monitoring soil C and N stocks. Modeling provides a flexible and powerful way to assess how different scenarios for pasture management and land use change can affect soil C and N dynamics, but it is important to highlight that even the best models (as Century is) need to be used with caution for Kyoto Protocol projects at plot scale and that benchmark sites are needed to augment model predictions.