Phosphorus speciation in Swedish arable soils with high leaching potentialSchmieder, Frank
Eutrophication is threatening biodiversity and ecosystem functions in inland water bodies and estuaries world-wide. Phosphorus (P) export from arable land by e.g. leaching is a major contributor to eutrophication, with high P leaching losses from long-term manured mineral soils and cultivated organic soils (Histosols). The forms of P present in these soils and their role in P mobilisation were examined in this thesis. Spectroscopic techniques (P K-edge XANES and 31P-NMR) were combined with extensive analysis of chemical and physical soil properties to characterise P in two cultivated fen peats profiles and the profile of a long-term manured mineral soil . Risk of P leaching from these soils was determined in rainfall simulation studies using 20-cm topsoil columns.
Topsoil P content (P-pstot) was with around 40 mmol kg-1 similar in the three profiles and thereby clearly elevated as compared to the P-pstot content in the sub soil of the profiles (maximum 20 mmol kg-1). When accounting for the higher bulk density the mineral sol profile was however substantially richer in total P than the organic profiles. Organic profiles were dominated by organic P (P-org) and the highest P-pstot proportion (80%) was observed in the topsoil, which was attributed to peat subsidence and transformation of mineral fertiliser P into organic P by soil microbiota and crops. According to P K-edge XANES, 20-40% of P in the organic soils was inorganic and primarily adsorbed to Al-(hydr)oxides.
Long-term manuring resulted in accumulation of inorganic P in the mineral topsoil. Up to 70% of total P was adsorbed to Fe-(hydr)oxides and Al-minerals. Most remaining topsoil P was present as secondary amorphous calcium phosphates. In the subsoil, the dominant P species was crystalline apatite, which declined in abundance towards the soil surface, reflecting soil weathering. Organic P content was very low throughout the mineral profile, consisting primarily of phosphate monoesters.
The rainfall simulation experiments indicated a risk of high P leaching from the manured mineral soil and cultivated Histosol, with cumulative P load mobilised from topsoil of 10 and 15 kg P ha-1, respectively. Phosphorus leaching was found to be driven by processes involving mobilisation of inorganic phosphate adsorbed to Fe or Al mineral surfaces, which was high relative to the size of the leachable P pool. This may reflect competition between phosphate and organic anions for sorption sites.
There was no indication that P-org is currently contributing to P leaching from Histosols, but continued soil subsidence over time will eventually lead to microbial mineralisation of organic soil P, increasing the availability of leachable inorganic P forms.
Keywordsphosphorus; organic soil; manured mineral soil; P leaching; XANES; chemical P analyses; phosphate; iron and aluminium (hydr)oxides
Published inActa Universitatis Agriculturae Sueciae
2019, number: 2019:83
ISBN: 978-91-7760-484-6, eISBN: 978-91-7760-485-3
Publisher: Department of Soil and Environment, Swedish University of Agricultural Sciences
UKÄ Subject classification
Environmental Sciences related to Agriculture and Land-use
URI (permanent link to this page)