Three challenges for legacy phosphorus in different catchment compartments

The loading of soils with P that exceeds crop requirements may have involved chemical fertilizer and manure P additions with certain P forms. Both the P source nature and in-situ conversion processes alter resulting P forms present in soils (Stutter et al., 2015). Forms include orthophosphate inorganic P as well as diverse P compounds where P or PO43− joins C-based molecules (so-called organically complexed P), necessitating hydrolysis and desorption to liberate plant available P into solution. The varying availability of these P forms is crucial for understanding P processes and guiding data collection (e.g., soil testing) appropriate to including wider P forms in inventories, agronomic strategies, and fertilizer planning (George et al., 2018). Monoester P is an often-dominant form of soil P, where its high charge density requires sorption and hydrolysis for crop-availability. Management must favor crops able to utilize monoester P if this large pool of the legacy P is going to be (i) drawn-down by cropping and (ii) not contribute to poor use efficiency for future added fertilizer P (Menezes-Blackburn et al., 2018). Used alongside in-field P source controls, edge-of-field mitigation measures are a core part of diffuse pollution mitigation on farms. Watercourse edges and field bottom corners occupy low-lying positions that receive and process runoff from larger upslope areas. Riparian margins are key locations for sediment, water, and nutrient retention zones, but this brings vulnerabilities because of their proximity and connection to stream ecosystems. High soil P may occur in riparian zones due to the buffer sediment deposition action and may also be present when a former cultivated field edge is subsequently brought into a buffer zones mitigation scheme. Buffer zone soils P enrichment has been shown to lead to enhanced P solubility (Stutter et al., 2009). This is difficult to reverse, since unlike N that can be lost to the gas phase, P drawdown can only occur via incorporation into biomass (Stutter et al., 2019). Hence, legacy P moved from the field to the buffer zone may be remobilized through biogeochemical P cycling processes associated with the often moister and higher organic matter situations than the adjacent field (Stutter et al., 2009). This leads to a need for a conceptual rethink of the size of the longer term storage potential for P in riparian buffer zones (Ramler et al., 2022) and potential for management and/or technical enhancements (Stutter et al., 2019) if such mitigation zones are to protect streams from available P. The catchment comprises a continuum of compartments for P from, from the soil-plant field system to edges of field and other semi-natural landscape elements, to stream channels and eventually oceans. As conduits for P exports, the stream channel system must be viewed as a storage compartment for legacy P, as well as a zone of P transformations. Riverbed P storage, comprising sediment P and biota, can vary considerably in space and time (Stutter et al., 2021). These P transformations between sediment and biota and the water column are part of stream eutrophication. Transformations between sediment, biota, and the water column may be particularly susceptible to climate change as alteration of catchment carbon sources, flow, temperature, and light alter biotic (e.g., algal and microbial uptake and decomposition) and abiotic (e.g., redox processes) rates. Physical disturbance that may comprise storms, or even restoration, can enhance downstream transfers of legacy P. Few catchment studies on legacy P include and contrast the inventories of P in both terrestrial and aquatic zones. Research should broaden to encompass the stocks, flows, and sensitivities of change to P across multiple terrestrial and aquatic landscapes compartments. Marc Stutter: Conceptualization; formal analysis; investigation; writing—original draft; writing—review and editing. The author declares no conflicts of interest.