Phosphorus dynamics in soils irrigated with reclaimed waste water or fresh water — A study using oxygen isotopic composition of phosphate

Abstract

Transformations of phosphate (Pi) in different soil fractions were tracked using the stable isotopic composition of oxygen in phosphate (δ¹⁸O_p) and Pi concentrations. Clay soil from Israel was treated with either reclaimed waste water (secondary, low grade) or with fresh water amended with a chemical fertilizer of a known isotopic signature. Changes of δ¹⁸O_p and Pi within different soil fractions, during a month of incubation, elucidate biogeochemical processes in the soil, revealing the biological and the chemical transformation impacting the various P pools. P in the soil solution is affected primarily by enzymatic activity that yields isotopic equilibrium with the water molecules in the soil solution. The dissolved P interacts rapidly with the loosely bound P (extracted by bicarbonate). The oxides and mineral P fractions (extracted by NaOH and HCl, respectively), which are considered as relatively stable pools of P, also exhibited isotopic alterations in the first two weeks after P application, likely related to the activity of microbial populations associated with soil surfaces. Specifically, isotopic depletion which could result from organic P mineralization was followed by isotopic enrichment which could result from preferential biological uptake of depleted P from the mineralized pool. Similar transformations were observed in both soils although transformations related to biological activity were more pronounced in the soil treated with reclaimed waste water compared to the fertilizer treated soil.

Introduction

Phosphorus (P) is a required nutrient for all living organisms and low P availability could limit growth and productivity. Specifically, P limitation may impact agricultural yield (Khasawneh et al., 1980, Bakker et al., 2005). Therefore P is added to cultivated soils worldwide through chemical or organic (e.g., manure and sewage sludge) fertilization. Water scarcity in arid places (like Israel) has led to the use of reclaimed waste water (RWW) for irrigation. This RWW also serves as a source of nutrients, including P. However, fertilization with biosolids or RWW may be associated with accumulation of excess labile P in top soils (Sui et al., 1999, Hansen et al., 2004, Tarchitzky, 2004) and consequently could become an environmental threat to adjacent water bodies, causing ecological imbalance and eutrophication (Tunney et al., 1997).

The most bioavailable form of P is inorganic orthophosphate (H₂PO₄⁻/HPO₄²⁻ hereafter referred to as Pi), which is also the most abundant stable form of free dissolved inorganic P in neutral pH soil solution (Lindsay, 1979). While P has only one stable isotope (³¹P), oxygen has three stable isotopes (¹⁶O, ¹⁷O, ¹⁸O) which could be used as isotope tracers for tracking Pi sources and transformations. Under common surface temperature and pH conditions the P–O bond in PO₄³⁻ is relatively strong and resists inorganic hydrolization for long periods (Shemesh et al., 1983, Saaby Johansen et al., 1989). However, enzyme mediated biological activity could break the P–O bond in processes that involve isotopic fractionation (Longinelli et al., 1976, Blake et al., 1997, Paytan et al., 2002). Intracellular as well as extracellular enzymes are expressed by various organisms for the utilization and cycling of P and may play a role in determining the oxygen isotopic composition of Pi (δ¹⁸O_p, ¹⁸O/¹⁶O relative to VSMOW (Vienna standard mean ocean water) international reference standard). Different enzymatic processes induce different isotopic fractionation, allowing the δ¹⁸O_p to elucidate such processes as long as their collective isotopic imprints do not cancel each other out (e.g., as long as one process controls the overall isotopic signature), (Blake et al., 2005).

The most dominant enzymatic process controlling δ¹⁸O_p in the environment is the intracellular activity of pyrophosphatase (PPase) (Blake et al., 2005), which involves equilibrium isotopic exchange. The isotopic equilibrium of Pi has been described by Longinelli and Nuti (1973) in the following empirical equation:

$$\text{T}\left({}^{\text{°}}\text{C}\right)=\text{111}.\text{4}–\text{4}.\text{3}\left({\delta }^{\text{18}}{\text{O}}_{\text{p}}-{\delta }^{\text{18}}{\text{O}}_{\text{w}}\right)$$

where δ¹⁸O_w is the isotopic composition of oxygen in water and T is temperature in degrees Celsius. It follows from the equation that δ¹⁸O_p is enriched relative to δ¹⁸O_w, however, the difference between them decreases as temperature increases. This equilibrium relation has been observed in tissues of a variety of organisms, including fish and mammals (Kolodny et al., 1983), bacteria and algae (Blake et al., 1997, Paytan et al., 2002, Blake et al., 2005) and used for reconstruction of paleoclimates (e.g., Ayliffe & Chivas, 1990, Fricke et al., 1998). Recently, δ¹⁸O_p in aquatic systems has been used as a tracer of Pi sources as well as for deciphering the biological utilization and turnover of Pi in these systems (Longinelli, 1989, Colman et al., 2005, McLaughlin et al., 2006a, McLaughlin et al., 2006b, Elsbury et al., 2009). This is based on the assumption that extensive recycling and turnover will lead to isotopic equilibrium while deviation from equilibrium may reflect source signatures or other processes that do not result in isotopic equilibrium. Specifically, extracellular remineralization and hydrolization of organic P (Po) compounds to form Pi, by phosphohydrolase enzymes such as alkaline phosphatase (APase) and 5′-nucleotidase, involves incorporation of oxygen atoms from ambient water with an isotope fractionation of –10‰ to –30‰ (Liang and Blake, 2006). These enzymatic processes are expected to occur in soils through the activity of microorganisms and can impact dissolved Pi concentrations and δ¹⁸O_p values. Uptake and utilization of Pi by plants or soil microorganisms is also associated with isotopic fractionation where Pi with light isotopes is preferentially utilized leaving the residual pool enriched (Blake et al., 2005).

Isotopic composition of soil Pi may also be affected by geochemical processes. Precipitation of apatite minerals is accompanied by a small oxygen isotope fractionation in the range of + 0.7‰ to + 2‰ (Zheng, 1996, Blake et al., 1997). Similarly, adsorption or precipitation with sesquioxides and hydroxides imprints a small positive isotope effect (Jaisi et al., 2010). These geochemical processes may alter Pi isotope ratio in the soil and can be described by isotopic mass balance models (e.g., Markel et al., 1994).

Labile P concentrations, defined as P in the soil solution and P which is loosely bound (Tiessen & Moir, 1993, Falkiner & Polglase, 1999, Guggenberger et al., 2000), are primarily controlled by the soil's various binding agents such as sesquioxides, clay and organic matter surfaces (via adsorption/desorption processes) and apatite minerals (by precipitation/dissolution), while highly recalcitrant soil P does not contribute to the labile pool (Tiessen and Moir, 1993). Understanding P transformations among the various distinct soil fractions may shed light on P availability. Utilization of δ¹⁸O_p for tracking P transformations in soil has so far been limited to tracking P dissolved in the soil solution (Larsen et al., 1989, Middelboe & Saaby, 1998). This is primarily due to lack of a method that addresses the complexity of extracting and analyzing Pi from other fractions of soil for oxygen isotopes. Zohar et al. (submitted) described a method to produce silver phosphate from different soil extract solutions, removing the barrier for using isotope tracing for the study of P transformations in soil.

In this paper we applied δ¹⁸O_p to study P transformations in two soil samples, over one month of incubation in the laboratory under controlled conditions. In this experiment, the soil samples were irrigated with either RWW or freshwater amended with fertilizer (FWF) and Pi transformations were tracked by determining changes in δ¹⁸O_p of the various soils' Pi pools. Using this isotope tracing technique enabled elucidation of processes which could not have been tracked by conventional methods and thus significantly enhanced our understanding of the biogeochemical processes that control P fate in soil. Application of this isotope tracing technique may further elucidate soil P transformations, mobility and bioavailability in future research.