Long-term phosphorus (P) fertilizer application can lead to an accumulation of uranium (U) in agricultural soil, potentially posing risks on the environment and human health. In this study, we found that such risks could be severe in two long-term grasslands (Andosol) in Iceland (Sámstaðir and Geitasandur) after about 50 years of P fertilization. At Sámstaðir, where P fertilizers were applied at an annual rate of 39.3 kg ha⁻¹ year⁻¹, the soil U concentration increased from 0.65 mg kg⁻¹ in the unfertilized soil to 6.9 mg kg⁻¹ in the fertilized surface soil (0–5 cm). At Geitasandur with P fertilization rate at 78.6 kg ha⁻¹ year⁻¹, the soil U concentration reached 15 mg kg⁻¹. The average annual U accumulation rates were 130 and 310 µg kg⁻¹ year⁻¹, respectively. These values were larger, by up to a factor of ten, than any previously reported rates of fertilizer-derived U accumulation. However, the U concentration in one of the applied P fertilizers was 95 mg U kg⁻¹ fertilizer, similar to the median value of those reported in previous studies, and thus unlikely to be the only factor leading to the high U accumulation rates. By contrast, as our Andosols had low bulk density within a range of 0.2 to 0.5 g cm⁻³, the annual U inputs to the 0–5 cm soil were 19 g ha⁻¹ year⁻¹ and 32 g ha⁻¹ year⁻¹ at the two sites, respectively, within the range of to-date reported values in agricultural systems. In addition, we found that U was mostly retained in the surface soil rather than mobilizing to deeper soil. This was likely due to the fact that the Andosols were rich in organic matter which promoted U retention. Therefore, the observed high U accumulation rates were a result of the combination of (i) the large amounts of the applied P fertilizers and (ii) the soil properties of the Andosols with low bulk density and elevated organic matter content concentrating U in the upper surface soil. Our study shows that agricultural production systems on Andosols may have already suffered from severe U contamination due to P fertilization. We are therefore calling for future checks and regulations on P fertilizer-related soil U accumulation in these and certain comparable agroecosystems.

中文翻译：

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冰岛草原安多索尔的化肥衍生铀临界积累

长期施用磷（P）肥料会导致农业土壤中铀（U）的积累，从而可能对环境和人类健康构成威胁。在这项研究中，我们发现，经过大约50年的磷肥施用后，冰岛的两个长期草原（Andosol）（Sámstaðir和Geitasandur）的此类风险可能非常严重。在Sámstaðir，每年以39.3 kg ha - ¹年^-1的速率施用P肥料，土壤U浓度从未施肥的土壤中的0.65 mg kg ^-1增加到施肥的表土中的6.9 mg kg ^-1（0 –5厘米）。在Geitasandur，P施肥率为78.6 kg ha - ¹年^-1时，土壤U浓度达到15 mg kg^-1。每年的平均U累积速率分别为130和310 µg kg - ¹年^-1。这些值比以前报道的任何肥料衍生的U积累速率大十倍。但是，其中一种施用的P肥料中的U浓度为95 mg U kg ^-1肥料，与先前研究中报道的中值相似，因此不可能是导致高U积累率的唯一因素。相比之下，由于我们的Andosols在0.2至0.5 g cm ^-3的范围内具有较低的堆积密度，因此每年向0-5 cm土壤输入的U分别为19 g ha - ¹年^-1和32 g ha ^{- 1}年^{- 1个}在两个地点分别处于农业系统最新报告值范围内。此外，我们发现U大部分保留在表层土壤中，而不是迁移到更深的土壤中。这可能是由于Andosols富含促进U保留的有机物这一事实。因此，观测到的较高的U积累速率是由于以下因素的综合结果：（i）大量施用的P肥料和（ii）堆积密度低且有机质含量升高的Andosols的土壤特性使U集中在上部表土。我们的研究表明，Andosols上的农业生产系统可能已经因磷肥而遭受严重的U污染。