Preliminary estimates of contemporary soil organic carbon stocks in Denmark using multiple datasets and four scaling-up methods

Accurate information regarding soil organic carbon (SOC) stocks is crucial to understanding the terrestrial carbon cycle and for setting environmental policies concerning C, such as those governing land use conversion and soil management. Soils may act as a sink or source of the greenhouse gas CO2 through exchange with the atmosphere. As soils contain a very large stock of organic C even small stock changes represent a large CO2 flux. The contemporary stocks of SOC in Denmark to 1 m depth were estimated by combining data from two soil and one land use database using four different scaling-up methods, which take into account land use and soil textures. The estimated stocks vary from 563 to 598 Tg C, with 579 Tg C as the average, when urban areas, lakes and open fjords are excluded. Wetland soils have the highest average SOC density (35.6 kg m(-2)), followed by soils under forests (16.9 kg m(-2)), agricultural soils (14.0 kg m(-2)), and soils under natural vegetation (14.4 kg m(-2)). Nationwide, 60% of the total SOC is found within 28 cm depth, which is the median ploughing depth, and 78% within 50 cm depth. Sixty-nine percent of the total SOC stock is under agricultural land and 40% is found in the plough layer. The Danish CO2 reduction commitment under the Kyoto Protocol corresponds to 0.57% of the total SOC stocks in Denmark, meaning that verification of C sequestration by C accounting will be difficult over the relatively short period set by the protocol. Adoption of 'improved best' management practices and conversion of arable land to forests or wetlands will contribute to increased C sequestration, but the biophysical conditions set finite limits for the amounts of C that can potentially be sequestrated. Additionally, the effects on other greenhouse gas release processes also need to be evaluated. (C) 2003 Elsevier Science B.V. All rights reserved.