A survey of soil phosphorus (P) and nitrogen (N) in Swedish horse paddocks

• Horse feeding and excretion areas inside the paddocks are enriched with P and N.
• Horse density and paddock age determines nutrient build-up in paddock soils.
• Around 34,000 ha of Swedish paddock soils are at risk for P and N leaching.

Of the EU countries, Sweden utilizes the highest proportion (10%) of its total agricultural land for horses. Horse paddocks commonly hold horses on a limited space, in the present study at a rate of 5–14 livestock units ha−1. Thus these paddocks receive significant amounts of phosphorus (P) and nitrogen (N) through feed residues and deposition of faeces and urine, which can lead to nutrient build-up in the soil. This study examined soil P and N status in different parts of horse paddocks (feeding, grazing and excretion areas) and compared it with that in adjacent, unmanaged reference soils. The paddock areas were then categorized with respect to environmental risk using the threshold concentrations of plant-available P extracted with ammonium acetate lactate solution at pH 3.75 (P-AL) and total N set by the Swedish Environmental Protection Agency. In total, seven horse farms, covering different grazing densities and soil textures representative of Swedish horse paddocks, were examined. The results showed that concentrations of water-soluble P (WSP), P-AL, and total N were highest in feeding and excretion areas within the paddocks. Weighted concentrations of soil P for the whole paddocks amounted to 2.9–10.5 mg WSP (kg−1) and 35–224 mg P-AL (kg−1), and were higher than in the corresponding reference fields (0.8–4.9 mg WSP (kg−1) and 17–102 mg P-AL (kg−1)). The WSP concentration in the paddocks was strongly correlated with horse density (R2 = 0.80***1, n = 13) and P-AL with years of paddock management (R2 = 0.78***, n = 13). Total organic C was significantly correlated with P-AL, total P and total N in the feeding and excretion areas. The degree of soil P saturation (DPS) concentrations was important soil parameters determining WSP concentration in the paddocks (R2 = 0.63***, n = 110), whereas total P concentration in soils was determining P-AL concentrations (R2 = 0.82***, n = 112). According to Swedish environmental guidelines, two of the seven farms studied posed a high risk and three a moderate risk of extensive P leaching losses, but the risk of extensive N losses was moderate for all farms studied. As regards the specific sections of the paddock, the feeding and excretion areas had the highest risk of P leaching losses. Thus paddock soils can be high-risk areas for P leaching comprising about 3.85% of the total high-risk land area in Sweden.