Soil organic carbon as an indicator of environmental quality at the national scale: Inventory monitoring methods and policy relevance

Soil organic carbon (SOC) storage is an indicator of environmental quality for mineral soils because of the influence that organic matter has on key functional properties, such as fertility, soil structure and water relations. Historically, agricultural management has caused large losses of SOC relative to native ecosystems, leading to degradation. However, new technologies and conservation practices have been developed during the past few decades that can enhance SOC storage, and thus improve environmental quality. Our objective was to describe a national inventory procedure to estimate SOC storage for purposes of monitoring environmental quality. The major steps in this procedure include: (1) model selection/development, (2) model verification, (3) identification of model input data, (4) uncertainty assessment, (5) model implementation, and (6) validation of results. Applying this approach With a simple C accounting method, the upper 30 cm of US agricultural soils were estimated to have accumulated 10.8 Tg C yr(-1) between 1982 and 1997, with an uncertainty of +/- 40%. A simple index was developed to relate estimated SOC stocks to the potential amounts under native conditions and conventional agricultural management. An index value of 0% on the proposed scale would be equivalent to the SOC under conventional agricultural use, while an index value of 100% would be equivalent to native levels. With an estimated 1997 stock of 22400 Tg C, the index value for US agricultural soils was about 60%. Using this inventory procedure, environmental issues related to soil, water and air quality could be informed by SOC in combination with other key indicators, in addition to using the inventory for evaluating sustainability of agricultural lands for food and fiber production.