Monitoring and verifying changes of organic carbon in soil

Changes in soil and vegetation management can impact strongly on the rates of carbon (C) accumulation and loss in soil, even over short periods of tim e. Detecting the effects of such changes in accumulation and loss rates on the amount of C stored in soil presents many challenges. Consideration of t he temporal and spatial heterogeneity of soil properties, general environme ntal conditions, and management history is essential when designing methods for monitoring and projecting changes in soil C stocks. Several approaches and tools will be required to develop reliable estimates of changes in soi l C at scales ranging from the individual experimental plot to whole region al and national inventories. In this paper we present an overview of soil p roperties and processes that must be considered. We classify the methods fo r determining soil C changes as direct or indirect. Direct methods include field and laboratory measurements of total C, various physical and chemical fractions, and C isotopes. A promising direct method is eddy covariance me asurement of CO2 fluxes. Indirect methods include simple and stratified acc ounting, use of environmental and topographic relationships, and modeling a pproaches. We present a conceptual plan for monitoring soil C changes at re gional scales that can be readily implemented. Finally, we anticipate signi ficant improvements in soil C monitoring with the advent of instruments cap able of direct and precise measurements in the field as well as methods for interpreting and extrapolating spatial and temporal information.