Cultivated organic soils make a significant contribution to phosphorus (P) leaching losses from agricultural land, despite occupying a small proportion of cultivated area. However, less is known about P mobilisation processes and the P forms present in peat soils compared with mineral soils. In this study, P forms and their distribution with depth were investigated in two cultivated Histosol profiles, using a combination of wet chemical extraction and P K-edge X-ray absorption near-edge structure (XANES) spectroscopy. Both profiles had elevated P content in the topsoil, amounting to around 40 mmol kg(-1), and P speciation in both profiles was strongly dominated by organic P. Topsoils were particularly rich in organic P (P-org), with relative proportions of up to 80%. Inorganic P in the profiles was almost exclusively adsorbed to surface reactive aluminium (Al) and iron (Fe) minerals. In one of the pro-files, small contributions of Ca-phosphates were detected. A commonly used P saturation index (PSI) based on ammonium-oxalate extraction indicated a low to moderate risk of P leaching from both profiles. However, the capacity of soil Al and Fe to retain P in organic soils could be reduced by high competition from organic compounds for sorption sites. This is not directly accounted for in PSI and similar indices. Accumulation of P-org in the topsoil may be attributable by microbial peat decomposition and transformation of mineral fertiliser P by both microbiota and crops. Moreover, high carbon-phosphorus ratio in the surface peat material in both profiles suggests reduced net mineralisation of P-org in the two soils. However, advancing microbial peat decomposition will eventually lead to complete loss of peat horizons and to mineralisation of P-org. Hence, P-org in both profiles represents a huge potentially mobilised P pool.

Phosphorus derived from agricultural systems has been found to cause eutrophication of surface waters. To combat this, the specific location of soil profile P release is necessary for development of effective mitigation strategies. This paper describes a P leaching study of two Swedish arable soils, an organic (Typic Haplosaprist) and a mineral soil (Typic Hapludalf), both with high P content. Undisturbed soil columns isolated 0‐ to 20‐, 20‐ to 40‐, 40‐ to 60‐, and 60‐ to 80‐cm depth intervals. These were placed in a rainfall simulator and subjected to four 50‐mm rainfall events to identify the origin of P leachate as a function of soil depth interval and physicochemical properties. Phosphorus losses were greatest from the two uppermost layers of both soils after 200 mm of artificial rainfall was applied at 5 mm h −1 . Total P concentration in leachate from the 0‐ to 20‐cm layer ranged from 2.1 to 8.8 mg L −1 for the mineral and 3.7 to 10.3 mg L −1 from the organic soil, with most (95–100%) in dissolved reactive P form. Degree of P saturation correlated well with total P leaching losses from the organic soil ( R = 0.84) but not the mineral soil ( R = 0.69), suggesting that the presence of Al and Fe (hydr)oxides has a stronger influence on P leaching in the organic soil. Results indicate that both soils have the potential to contribute concentrations of P above those known to cause eutrophication of surface waters. Core Ideas The majority of P leached from both soils was from the top 20 cm, in DRP form. Phosphorus concentrations in leachate were higher from the organic than the mineral soil. Degree of P saturation correlated well with P leached from the organic soil. High rainfall application promoted losses of P potentially complexed to DOC and Fe or Al. Both soils leached concentrations of P above those known to cause eutrophication.

Recent research suggests that Swedish organic arable soils have been under-recognized as a potential source of phosphorus (P) loading to water bodies. The aim of this study was to compare P losses through leaching from organic and high-fertility mineral soils. In addition, the effectiveness of a magnesium-salt-coated biochar applied below the topsoil as a mitigation strategy for reducing P losses was evaluated.

Agriculture is one of the largest anthropogenic contributors of phosphorus (P) to surface waters and reducing these P loads is a target set by many countries. One potential mitigation strategy could be to utilize biochar, functionalized by co-precipitating metal ions onto its surface to adsorb phosphates. We evaluated such an approach in a series of laboratory experiments and a three-year field lysimeter study. Leached phosphates were trapped in a 3-cm layer of a commercially available 20% hardwood/80% Norway spruce biochar, coated with magnetite and placed under the soil root zone. Langmuir maximum adsorption potential (Qmax) was 3.38 mg P g(-1) biochar, as determined from batch adsorption studies performed at pH 6.5. Further laboratory experiments revealed that adsorption was strongly negatively correlated with pH (R-2 = 0.995). In a laboratory column study using high flow rates, no difference was found in phosphate adsorption between coated biochar and the control (no biochar), after application of ammonium phosphate at a rate corresponding to 22 kg P ha(-1). However, differences were observed at higher application rates (285 or 570 kg P ha(-1)). Calculation of a Fe:P molar ratio to evaluate the magnetite treatment method from laboratory (2:1) to lysimeter (248:1) suggested a relatively effective laboratory performance, but overall poor field efficiency. Nevertheless, biochar in the lysimeter study still reduced phosphate leaching from two organic soils, by 62% (P < 0.05) and 35% (NS). This suggests that, even though the method of magnetite-coating biochar is not necessarily resource-efficient, it is effective in removing P from soil leachate.

Long-term application of manure can lead to an enrichment of phosphorus (P) in agricultural soils. To which extent this P leaches into drainage systems and thereby potentially contributing to eutrophication of surface waters, depends on the distribution and speciation of P present in the soil. In this study the P speciation and related soil characteristics were investigated for a sandy loam soil that had been receiving manure for >40 years. A combination of solution-state 31P-NMR, P K-edge XANES and wet chemical extractions was applied.

Catch crops have been found to decrease leaching of nitrates into surface water and groundwater, but they also have the potential to increase P loadings to natural waters due to plant destruction during freezing–thawing events. An indoor lysimeter experiment was performed using a clay and a sand soil with four different plant species applied: perennial ryegrass ( Lolium perenne L.), honey herb ( Phacelia tanacetifolia Benth.), chicory ( Cichorium intybus L.), and oilseed radish ( Raphanus sativus L.). These plants were exposed to simulated rainfall and freezing events in two separate experiments, one using topsoil columns with plant material added and one with plant material only. The sand and clay soils had significantly different control total‐P leaching loads after all freezing events, with a mean of 0.32 kg ha –1 for the clay and 0.88 kg ha –1 ( P < 0.001) for the sand. The combined (soil including plant material) total‐P leaching loads from the clay soil were in the order: chicory (2.6 kg ha –1 ) > ryegrass (2.3 kg ha –1 ) > oilseed radish (2.2 kg ha –1 ) > honey herb (1.3 kg ha –1 ), with considerably smaller loads from the sand. Phosphorus losses were greater from the plant‐only experiment, with chicory (51.7 kg ha –1 ) > oilseed radish (43.2 kg ha –1 ) > honey herb (18.4 kg ha –1 ) > ryegrass (10 kg ha –1 ). The results indicate that soil texture and plant choice can have a large impact on P leaching loads entering natural waters in cold regions and that soils act as an efficient filter for P released from catch crop residues.