Elsevier

Chemosphere

Volume 275, July 2021, 129928
Chemosphere

Redox-induced mobilization of phosphorus in groundwater affected arable soil profiles

https://doi.org/10.1016/j.chemosphere.2021.129928Get rights and content

Highlights

  • Concentrations of dissolved P were higher under low than under high EH.

  • The reductive dissolution of Fe-oxides caused an increase in P mobilization.

  • Phosphorus was more mobilized in the toe-than the mid- and upper-slope soil.

  • Phosphorus was more mobilized in the top-than the sub-soil samples.

  • The loss of P is higher under reducing than oxidizing conditions.

Abstract

Mobilization of phosphorus (P) in arable soils might be affected by groundwater fluctuations and the associated changes in redox potential (EH). However, the impact of systematic changes of EH on P mobilization in redoximorphic arable soils along a catena has not been studied so far. Therefore, we investigated P mobilization under different redox conditions in top- and sub-soil horizons of three groundwater affected arable soils along a slight slope (toe-, mid-, and upper-slope position) in Northern Germany using an automated biogeochemical microcosm system. The impact of pH, Al, Fe, Mn, and dissolved organic carbon (DOC) on P mobilization was also studied. The initial EH (+351 to +431 mV) and pH (6.5–7.0) decreased in all soil samples (EH = −280 mV; pH = 4.4) when creating a slurry. Thereafter, the pH increased to 7.1 and 6.4 with increasing EH in the mid-and toe-slope soil, respectively. Concentrations of dissolved P ranged between 20.8 mg L−1 under low EH in the toe slope topsoil and 0.69 mg L−1 under high EH in the toe- and mid-slop subsoil. Concentrations (mg L−1) of dissolved Fe (0.31–13.3) and DOC (92–2651) increased under low EH and decreased under high EH. The increase of P mobilization under low EH and pH in the soils might be due to the release of P via the reductive and acidic dissolution of Fe-(oxhydr)oxides and/or due to soil organic matter mineralization. The high mobilization of P under reducing conditions may increase its bioavailability; however, it may increase its loss in the soils, particularly in the toe slope profile.

Introduction

Phosphorus (P) is an essential nutrient for plant growth. However, the high mobilization of P in soil may increase its loss to the surface and groundwater. The aquatic environmental pollution caused by P has raised the interest in this element because it is considered to be the main element responsible for the eutrophication process (Abdulkareem et al., 2018; Barcellos et al., 2019).

Redox-induced mobilization of nutrients and pollutants in groundwater affected arable soils has large agro-environmental implications, because the redox processes affect the bioavailability of P and also affect its loss to ground and surface water (Sosa, 2018; Chen et al., 2019). Mobilization, bioavailability, and potential loss of P in soils could be affected by redox potential (EH) and pH changes via regulating P biogeochemical processes in soils (Gasparatos et al., 2019; Zhao et al., 2019; Baumann et al., 2020a). For example, the interactions between P and soil constituents (e.g., Fe–Mn-(oxhydr)oxides, calcium carbonates, organic matter) are affected by the dynamics of EH and pH (Gu et al., 2019; Bai et al., 2020).

In soils, P can be bound to soil organic matter (SOM), Fe, Al, and Mn -(oxhydr)oxides, and/or calcium compounds (Shaheen et al., 2007; Baumann et al., 2020b). These fractions can be influenced by soil properties and the changes of soil EH and pH (Yang et al., 2019). Therefore, these soil components are important factors in driving P mobilization in soils (Cui et al., 2019). The mobilization of P under anaerobic conditions has been studied in peat soils (Meissner et al., 2008). However, the impact of systematic changes of EH and the EH–dependent changes of governing factors such as pH, dissolved organic (DOC) and inorganic (DIC) carbon, and the Fe- and Mn- (oxhydr)oxides on the mobilization of P in groundwater affected arable soils is not known yet.

Worldwide, arable land is covering around 1.5 billion hectares (Hens and Quynh, 2016). Approximately 40% of this arable land is naturally acidic, which may have developed with intensive agriculture, particularly when rainfall exceeds evapotranspiration (Kamprath and Smyth, 2005). Further, around 80 million hectares are affected by water logging (Hens and Quynh, 2016). In arable soils, particularly in Northern Germany, the closure of drainage systems and high precipitation may cause an increase in the water table (Svoboda et al., 2015; Zimmer et al., 2016), which can lead to reductive conditions. Arable soils on a single slope (catena) display different characteristics depending on slope position. For example, the toe slope position soil may have longer periods of water saturation due to rising groundwater level than the upper slope soils. Also, the subsoil might be more affected by water saturation than the topsoil. These in situ environmental conditions do not only affect soil properties but may also affect the mobilization of P in arable soils. Particularly under intensive applications of P fertilizers and/or manure the risk of P loss in these soils under changing environmental conditions may be increased. Consequently, in this study we investigated soil samples from the top- and subsoil of a toe-, middle-, and upper-slope arable soil profile to study P mobilization under different redox conditions.

We hypothesize that redox changes from reducing to oxidizing conditions and vice versa in groundwater affect arable soils as a result of water table fluctuations governing the release of P and its mobilization, through the direct impact of EH and/or the associated changes of soil pH, Fe–Mn oxides and DOC/DIC. We also hypothesize that if soil conditions become reducing, the P bound to the Fe and Mn-(oxhydr)oxides might be released due to the dissolution of these oxides. Also, we assume that redox-induced changes in soil organic and inorganic carbon affect P solubility and that the decomposition of SOM under reducing/oxidizing cycles leads to the release of P into soil solution. Moreover, we hypothesize that the slope position affects the water table level and thus affects the redox-induced mobilization of P in a soil catena differently. Therefore, our aim was to study the impact of rising groundwater level and the associated changes in EH, pH, Fe–Mn–Al-(oxhydr)oxides, DOC and DIC concentrations on P mobilization in the top- and sub-soil horizons of toe-, mid-, and upper-slope arable soil profiles.

Section snippets

Soil sampling and characterization

In a field at Dummerstorf, near Rostock, Germany, soil samples were collected from three soil profiles excavated by drilling along a slight slope at different positions (toe-, mid-, and upper-slope) i.e. different distances to groundwater (Baumann et al., 2020b). At each soil profile, four soil replicates were sampled from three horizons at different depths (Appendix A; Table S1). Soil classification, basic soil properties, and total element content as well as content of poorly crystalline

EH/pH dynamics

The EH of all soil samples ranged from −280 mV to +485 mV (Fig. 1). The toe slope soil samples showed a wider range of EH (−280 to +471 mV) than the mid slope (−272 to +442 mV) and the upper slope topsoil (UST) samples (−230 to +485 mV) (Fig. 1; Table 1). The EH range differed between the soil horizons; the toe slope subsoil (TSS) sample reached a lower EH value (−280 mV) than the toe-slope topsoil (TST) sample (−232 mV), while both the mid slope top- (MST) and sub-soil (MSS) sample showed

Conclusions

The release of P in groundwater affected arable soils might increase under high precipitation and surface irrigation if drainage is restricted, which may increase the ground water level and cause reductive conditions. Reducing acidic conditions increased P mobilization up to 30 folds and thus may cause increased leaching if the groundwater level increases in the soils under study. Therefore, mitigating the potential loss of P under reducing acidic conditions using low cost and environmentally

Credit author statement

Sabry M. Shaheen: Performing the experiment, Writing – original draft. Jianxu Wang: Performing the experiment, Software and visualization. Karen Baumann: Editing and proof reading. Shan-Li Wang: Editing and proof reading. Peter Leinweber: Review, editing, and proof reading. Jörg Rinklebe: Concept, Supervision, review, editing, and foundation

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.

Acknowledgements

We thank the German Federal Ministry of Education and Research (BMBF) who funded the InnoSoilPhos-project (http://www.innosoilphos.de/default.aspx), in the frame of the BonaRes-program (No. 521 031B0509A). Also, we thank the team of Laboratory of Soil- and Groundwater-Management at Wuppertal University for the technical support.

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