Phosphorus accumulation and spatial distribution in agricultural soils in Denmark

Abstract

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 0.75–1.00m in the nationwide 7km Grid System in Denmark. Changes in soil P content between 1987 and 1998 at 0–0.25 and 0.25–0.50m were also examined in 337 and 335 agricultural soil profiles, respectively. Compared to forest soils, the agricultural soils contained more total P down to 0.75m depth (264mgPkg−1, or 88% more at 0–0.25m depth, 191mgPkg−1 or 82% more at 0.25–0.50m depth and 120mgPkg−1 or 63% more at 0.50–0.75m 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 0.25–0.50m depth. On average, the increase corresponded to an annual accumulation of c. 25kgPha−1, with the increase fairly equally divided between the two soil layers. The accumulation corresponds with the national P surplus of c. 20kgPha−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 0.75m depth. The paper discusses the potential importance of leaching, deep tillage, erosion and bioturbation for the observed accumulation of P in the subsoil.