Soil phosphorus forms and storage in stormwater treatment areas of the Everglades: Influence of vegetation and nutrient loading
Graphical abstract
Introduction
Phosphorus (P) storage and transformations in wetlands are influenced by organic and mineral matter inputs from external and internal sources (Kadlec and Wallace, 2009; Reddy and DeLaune, 2008; Reddy et al., 2005). Phosphorus retained in wetlands is present both in organic and inorganic forms and stored for short-term in vegetation and periphyton and for long-term in soils. A series of biotic and abiotic biogeochemical processes regulate reactivity and stability of these P forms stored in soils and the water column (Noe et al., 2002, Noe et al., 2001; Reddy et al., 1999; Reddy and DeLaune, 2008; Ruttenberg, 2003). Understanding the functional nature of different P pools is critical to developing strategies to improve the long-term sustainability of wetlands to retain P and associated elements (Condron et al., 2005; Kadlec and Wallace, 2009; Reddy and DeLaune, 2008; Turner et al., 2006).
Inorganic P (Pi) and organic P (Po) forms in soils are usually determined by sequential extractions including acid and alkaline reagents (Bhomia and Reddy, 2018; Hedley and Stewart, 1982; Hieltjes and Lijklema, 1980; Ivanoff et al., 1998; Olila et al., 1995; Reddy et al., 2013; Ruttenberg, 1992; Yang and Post, 2011). These schemes typically identify P in the following groups: (1) highly reactive P pools as loosely-bound Pi, microbial biomass P, and labile Po; (2) moderately reactive P as; Pi associated with metals including Fe, Al, Ca, and Mg; and acid and alkali-extractable organic P (fulvic- and humic-bound P); and (3) non-reactive residual P. Because fractionation procedures are based on different solubilities of various P forms in different extractants, the chemically separated fractions are considered operationally defined. However, despite the limitations of P fractionation techniques, these approaches can provide semi-quantitative information to assess the potential stability and importance of different soil P fractions, particularly when combined with complementary approaches (Condron and Newman, 2011). These complementary approaches include 31P nuclear magnetic resonance (NMR) spectroscopy, P K-edge X-ray absorption near edge structure (XANES) spectroscopy and two solid-state techniques – X-ray diffraction (XRD) and scanning electron microscopy (SEM) augmented with energy-dispersive X-ray fluorescence spectroscopy (EDS). These approaches provide confirmation of specific organic and inorganic P forms identified via P fractionation (Cade-Menun and Liu, 2014a; Cheesman et al., 2013; Harris and White, 2008; Koch et al., 2018).
Wetlands such as Stormwater Treatment Areas (STAs) accrete organic and mineral matter that contain organic and inorganic P forms and the interaction between these two pools results in release or retention of P in the system (Bhomia et al., 2015; Bhomia and Reddy, 2018; Reddy et al., 2005). The relative proportion and lability of organic and inorganic P pools are influenced by vegetation type, nutrient loading, and chemical composition of accreted material (Andersen et al., 2017; Bhomia and Reddy, 2018; Reddy et al., 1998; Yang and Post, 2011). Long-term P storage in wetlands relies upon the ability of soils to retain P in both organic and inorganic forms. Therefore, to determine the performance and sustainability of treatment wetlands, it is important to understand the stability and reactivity of various P pools in soils and its potential influence on surface water quality.
The primary question posed in this study is how and to what extent do the type of vegetation and nutrient loading influenced the forms, reactivity and storage of P in accreted soils. It is hypothesized that (1) emergent aquatic vegetation (EAV) and submerged aquatic vegetation (SAV) systems support different biogeochemical processes that affected the relative proportion of organic and inorganic P forms in the accreted material and (2) steady external nutrient loads result in higher labile pools of soil P in upstream areas of the STA.
We tested these hypotheses by determining the forms and distribution of P in floc and recently accreted soil (RAS) along two parallel FWs in STA-2, one of the five STAs established in strategic locations at the interface of the Everglades Agricultural Area and the Water Conservation Areas to reduce total P concentrations in surface water prior to discharging that water into the Everglades Protection Area (Chimney, 2019).
Section snippets
Site description
The study was conducted in STA-2 FWs 1 and 3. These are two good performing FWs (having achieved outflow TP concentration of 20 μg L−1 or lower) with different vegetation communities and soil characteristics. The FW 1 was never farmed while most of the FW 3 was previously farmed except for 25% of the treatment area in the southeastern portion of the FW. STA-2 FW 1 is a single cell FW with a treatment area of 743 ha. It has predominantly emergent aquatic vegetation (EAV) consisting primarily of
Physico-chemical properties
Floc depth decreased with distance from inflow in the EAV FW while no consistent trend was observed in the SAV FW (Table 2). RAS depth in both FWs was highest at the inflow station and was similarly low at the midflow and outflow locations (Table 2). Floc and RAS bulk densities were significantly higher in SAV than EAV (Table 2, P < 0.001). Pre-STA soils at both depths generally showed higher bulk densities than RAS and floc layers in EAV and SAV systems (Table 2, Table 3). Soil organic carbon
Phosphorus enrichment in recently accreted material (floc and RAS)
Phosphorus concentrations (expressed on dry weight basis, mg kg−1) in floc and RAS were significantly higher in EAV than SAV systems (Fig. 3). This distinct difference between two vegetation types was due to accretion of predominantly organic matter in EAV, while accretion of mineral matter supported by the production of CaCO3 and other associated metals in SAV, resulting in significantly lower bulk density of accreted material in EAV than SAV (Table 2). Although P concentrations of floc and
Conclusions
Phosphorus pools determined by conventional fractionation methods provided information on discrete pools of inorganic and organic P as influenced by vegetation and nutrient loading. Steady loading of P into the STAs has increased the relative proportion of all forms of soil P, with the largest proportion stored in slowly available and refractory forms of organic P. No phosphate minerals were detected by XRD, but SEM-EDS elemental dot maps and point spectra showed small discrete Ca-P particles.
CRediT authorship contribution statement
K.R. Reddy: Conceptualization, Methodology, Writing - original draft, Writing - review & editing, Resources, Funding acquisition. Lilit Vardanyan: Investigation, Writing - review & editing. Jing Hu: Investigation, Writing - original draft, Writing - review & editing. Odi Villapando: Writing - review & editing. Rupesh Bhomia: Investigation, Writing - review & editing. Taylor Smith: Investigation. W.G. Harris: Writing - review & editing, Investigation. Sue Newman: Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research work was conducted during 2015-2019 in compliance with the agreement between the South Florida Water Management District (SFWMD) and the University of Florida and (UF Contract #4600003031-WO01). It is part of the UF Collaborative Research Initiative Science Plan with the SFWMD for the Everglades Stormwater Treatment Areas (CRESTA) and supports the Restoration Strategies Science Plan for the Everglades STAs. A portion of this work was performed at the National High Magnetic Field
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