Positive trends in organic carbon storage in Swedish agricultural soils due to unexpected socio-economic drivers

Abstract. Soil organic carbon (SOC) plays a crucial role in the global carbon cycle as a potential sink or source. Land management influences SOC storage, so the European Parliament decided in 2013 that changes in carbon stocks within a certain land use type, including arable land, must be reported by all member countries in their national inventory reports for greenhouse gas emissions. Here we show the temporal dynamics of SOC during the past 2 decades in Swedish agricultural soils, based on soil inventories conducted in 1988–1997 (Inventory I), 2001–2007 (Inventory II) and from 2010 onwards (Inventory III), and link SOC changes with trends in agricultural management. From Inventory I to Inventory II, SOC increased in 16 out of 21 Swedish counties, while from Inventory I to Inventory III it increased in 18 out of 21 counties. Mean topsoil (0–20 cm) SOC concentration for the entire country increased from 2.48 to 2.67% C (a relative increase of 7.7%, or 0.38% yr⁻¹) over the whole period. We attributed this to a substantial increase in ley as a proportion of total agricultural area in all counties. The horse population in Sweden has more than doubled since 1981 and was identified as the main driver for this management change (R² = 0.72). Due to subsidies introduced in the early 1990s, the area of long-term set-aside (mostly old leys) also contributed to the increase in area of ley. The carbon sink function of Swedish agricultural soils demonstrated in this study differs from trends found in neighbouring countries. This indicates that country-specific or local socio-economic drivers for land management must be accounted for in larger-scale predictions.