Phosphorus accumulation and spatial distribution in agricultural soils in Denmark

- Phosphorus enrichment was observed to 0.75 m depth in Danish agricultural soils.
- Half of the P surplus added in recent time had accumulated below the plow layer.
- Leaching, deep tillage, tillage erosion and bioturbation redistribute P in soil.

Over the past century phosphorus (P) has accumulated in Danish agricultural soils. We examined the soil P content and the degree of P saturation in acid oxalate (DPS) in 337 agricultural soil profiles and 32 soil profiles from deciduous forests sampled at 0-0.25, 0.25-0.50, 0.50-0.75 and 0.75-1.00 m in the nationwide 7 km Grid System in Denmark. Changes in soil P content between 1987 and 1998 at 0-0.25 and 0.25-0.50 m were also examined in 337 and 335 agricultural soil profiles, respectively. Compared to forest soils, the agricultural soils contained more total P down to 0.75 m depth (264 mg P kg− 1, or 88% more at 0-0.25 m depth, 191 mg P kg− 1 or 82% more at 0.25-0.50 m depth and 120 mg P kg− 1 or 63% more at 0.50-0.75 m depth). The mean degrees of phosphorus saturation (DPS) of the agricultural soils were 32, 23 and 15% in the three upper soil layers, which were approximately twice as high as at the corresponding depths of deciduous forest soils. Between 1987 and 1998 total soil P content in the agricultural soils increased at both 0-0.25 and 0.25-0.50 m depth. On average, the increase corresponded to an annual accumulation of c. 25 kg P ha− 1, with the increase fairly equally divided between the two soil layers. The accumulation corresponds with the national P surplus of c. 20 kg P ha− 1 calculated from national statistics. This investigation shows that long-term surplus P fertilisation of agricultural soils has resulted in P accumulation to at least 0.75 m depth. The paper discusses the potential importance of leaching, deep tillage, erosion and bioturbation for the observed accumulation of P in the subsoil.