Editorial: Phosphorus Along the Soil-Freshwater-Ocean Continuum

There is a strong link between soil P stores and P mobilization and transfer to receiving waters, termed the P transfer continuum. Ensuring adequate P for crop production while minimizing water quality degradation requires consideration of this continuum and an international, interdisciplinary approach.

section_content: Phosphorus Along the Soil-Freshwater-Ocean Continuum Phosphorus (P) is an essential element for all organisms. However, there is a P paradox, whereby P concentrations considered deficient in some environments such as in agricultural soils are considered excessive in freshwater, where they trigger eutrophication (e. g., Sims and Sharpley, 2005 and references therein; Elser and Bennet, 2011; Lougheed, 2011). Geographical imbalances also occur, with excesses in Western Europe and North America and deficiencies in regions with highly weathered soils, such as sub-Saharan Africa. There is a strong link between soil P stores and P mobilization and transfer to receiving waters, termed the P transfer continuum (Haygarth et al., 2005). Ensuring adequate P for crop production while minimizing water quality degradation requires consideration of this continuum and an international, interdisciplinary approach. This research topic brings together P studies in soil science, lakes, rivers, estuaries, and oceans, with 74 authors from 12 countries in Asia, Europe, and North America, and identifies key priorities for future research. 

In all ecosystems, P exists in many chemical forms. Inorganic P compounds include phosphate, pyrophosphate, and polyphosphate, with organisms directly using phosphate (H 2 PO - 4 or HPO 2- 4 at the pH range of most natural ecosystems). Organic P compounds contain a link to carbon (C), and include orthophosphate monoesters [e. g., glucose 6-phosphate and myo-inositol hexakisphosphate (myo-IHP or phytate)]; orthophosphate diesters (e. g., nucleic acids, phospholipids); phosphonates (e. g., aminoethyl phosphonate), and biological polyphosphates (e. g., ATP). In all environments, P cycles geochemically and biologically. Geochemical processes include adsorption/desorption of organic and inorganic P compounds with mineral particles, and precipitation with cations in solution. In biological cycling, phosphate is taken up by organisms and converted to organic P forms such as nucleic acids or is stored in cells as phytate or polyphosphate. This P can be released by secretion or after cell death, and converted back to phosphate by mineralization, primarily an enzymatic process involving P-specific enzymes (phosphatases). For more information on P cycling, a number of review papers are available (e. g., Condron et al., 2005; Pierzynski et al., 2005; Paytan and McLaughlin, 2007; Baldwin, 2013; Karl, 2014; Orihel et al., 2017). 

There are similarities and differences in P cycling in terrestrial and aquatic ecosystems. Terrestrial organisms obtain phosphate from the pool that is dissolved in the soil solution, in the space between the solid components of soil (minerals and organic matter). Here, P availability is limited by moisture. Easily-obtained (labile) phosphate is taken up from the soil solution, then replenished by desorption, dissolution, or mineralization. 

Because terrestrial plants are fixed in place, strategies have evolved to improve their P acquisition. This includes changes in rooting structure or associations with rhizosphere microbes and symbionts, which can enhance access to phosphate, or production of phosphatases or organic acids to desorb P compounds from mineral surfaces (Richardson et al., 2011). Aquatic environments include sediments, benthic biofilms, and the water column. In sediments, organic and inorganic P compounds will be associated with mineral particles, algal and bacterial biomass (particularly at the sediment-water interface) and organic matter, or occur in aqueous form in pore water. Phosphate and other P forms can exchange with the water column through adsorption/desorption and precipitation/dissolution, and through uptake and remineralization by organisms in the sediment and in the water column. Within the water column, organic and inorganic P compounds can be dissolved, associated with colloids, or associated with larger particulates including plankton (e. g., Paytan and McLaughlin, 2007; Orihel et al., 2017; Taylor and Lean, 2018). 

Although it can be the most limiting nutrient, or second to nitrogen (N) in many environments, P cycling in terrestrial and aquatic ecosystems in still poorly understood relative to C or N, because P concentrations are orders of magnitude lower than those of C and N, and due to methodological limitations. Unlike C and N, P has no significant natural gaseous forms, and only one stable isotope ( 31 P), which can hamper tracing P sources in the environment, but it does have two radioisotopes ( 32 P and 33 P) that occur naturally or can be added as tracers. Total P is determined by digestion of solids, or by inductively coupled argon plasma optical emission spectroscopy (ICP-OES) of liquids, and phosphate concentrations in liquids or extracted samples are analyzed by colorimetric techniques such as the molybdate blue method (Murphy and Riley, 1962). However, characterization of specific P compounds requires advanced techniques such as 31 P-nuclear magnetic resonance (P-NMR) spectroscopy or P-X-ray Absorption Near Edge Structure (P-XANES) spectroscopy that are not readily available to all researchers. Information about biological P cycling is also limited, particularly the roles of specific organisms. The links between the P cycle and those of other elements, including C and N, are also poorly understood. 

The papers assembled here provide important new information to address knowledge gaps, and cover all aspects of the soil-freshwater-ocean continuum. Thus, the articles in this research topic provide current and interdisciplinary information about P forms and their cycling along the soil-freshwater-ocean continuum, which is essential for environmentally sustainable P use.
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section_content: All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.
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section_content: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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