Chemical and physiological aspects of various forms of phosphorus fertilizer
Ran Erel
General: Phosphorus (P) is an essential macronutrient with a central role in numerous structural and biochemical plant processes. Due to strong P fixation to soil constituents, P is limiting crop production in many arable soils and P fertilizer application is frequently needed to achieve high productivity (Hinsinger 2001; Lynch 2007). Unfortunately, rock phosphates that are mined for elaborating P fertilizers are a finite and depleted resource, and thus their prices are expected to further rise in the coming decades (Cordell et al. 2009). Therefore, there is a need to develop future P fertilization strategy with superior P fertilizer utilization and high P recovery.
P in soils. Although the total amount of P in the soil may be high, it is often present in unavailable forms (Schachtman et al. 1998). Hence, in most soils, P is strongly bound to soil particles or found in the organic form. In acidic soils, P predominantly reacts with iron and aluminum, and in alkaline soils with calcium to form poorly soluble complexes (Holford 1997). These P pools are not readily available for plants roots and only small fraction of soil P will become available. Thus, in many agricultural systems the application of P to the soil is necessary to ensure plant productivity. However, the recovery of applied P by crop plants in a growing season is very low, since ~80% of the applied P is adsorb, precipitates, or converted to the organic form (Holford 1997; Vance et al. 2003).
Several mechanisms allowing release of sparingly available P fixed in the soil. The main acknowledged means are: pH modification, competition on adsorption sites and phosphatases activity. Soil pH adjustment has large potential to affect P availability (Devau et al. 2011b; Hinsinger 2001; Hinsinger et al. 2003), and plant induced acidification is frequently measured in response to P deficiency. Additional way of releasing P is by competing on sorption sites. Several organic ions where shown to be enhance P solubility. Plants roots exudation of carboxylates was reported to potentially increase the solubilization of P compounds under P deficiency (Bolan et al. 1994; Veneklaas et al. 2003), depending on the carboxylate and soil type (Duputel et al. 2013). Of the organic acids, citric, oxalic and malic acids are the most frequently referred to for their potential effect on the rhizosphere (Hinsinger et al. 2003). A large portion of soil P occurs in organic forms, and a common response of plants and microorganisms to P-deficiency is the synthesis and exudation of acid phosphatase enzymes (Richardson et al. 2011; Simpson et al. 2011), while alkaline phosphatase enzymes are being produced and released only by microorganisms (George et al. 2002).
P fertilizers: In large portion of arable soils P is limiting plants growth (Lynch 2007) and thus, P fertilization is required for profitable agrosystem. Chemical P fertilizers are derived from phosphate rock. Two popular forms of P fertilizers are: single superphosphate (SSP) and triple superphosphate (TSP) which produced by reacting fine phosphate rock with liquid sulfuric or phosphoric acid. These P fertilizers are also good source for calcium and sulfur nutrition (Chien et al. 2011). When TSP or SSP dissolves in the soil solution, it is hydrolyzed and H3PO4 is released causing temporal decrease in soil pH.
Second group of water-soluble P fertilizers are produced from phosphoric acid and include: MAP, MKP and poly-P. The various forms of P fertilizers may affect P availability and consequently, P acquisition by plants roots. Yet, the interaction between P application form and chemical processes (i.e. soil P distribution, fixation dynamic and P availably) are highly dependent on environmental conditions and thus, are not simple to predict. Few of the factor acknowledged to play a role in soil P dynamics: clay mineralogy, soil texture, pH and moisture (Devau et al. 2011a; Marschner 2011). The mechanisms governing P availability of the various P fertilizers are not adequately understood. On top of that, plants roots significantly involve and interact with these processes adding extra level of complexity (Hinsinger 2001; Hinsinger et al. 2009). Only a limited amount of research has been reported in the literature regarding the agronomic effectiveness of different P fertilizers (e.g., SSP, TSP and MAP). Most published reports on this topic have focused on water-insoluble and partially water soluble P sources as rock P (Chien et al. 2011).
Polyphosphate: Polyphosphates (poly-P) composed of two or more orthophosphates (ortho-P) units condensed to linear or cyclic chains sharing oxygen atoms. In soils, poly-P are subject to hydrolysis to form several ortho-P or pyrophosphate. Poly-P based fertilizers are commonly used in agriculture (McBeath et al. 2007b). While various forms of poly-P are commonly applied fertilizers, ortho-P is the chief readily available form for plants. Poly-P hydrolysis typically occurs within few days or weeks subsequent to soil poly-P application with estimated half-life of 2-3 weeks (McBeath et al. 2007a). Conversely, poly-P is very stable in solutions as long as pH and temperature are not extreme (McBeath et al. 2007a). The hydrolysis reaction of pyrophosphate is shown below:
P2O7–4 + H2O → 2HPO42–
Several factor affects poly-P hydrolysis rate, such as chain length, pH, temperature, enzymes activity and soil moisture (Dick and Tabatabai 1986; Hons et al. 1986). In a detailed study, Torres-Dorante et al. (2005) demonstrated the substantial effect of soil type on hydrolysis dynamics and adsorption processes. In comparison to ortho-P, poly-P precipitation in soils is slower and thus, often referred as “slow-released” P fertilizers (Dick and Tabatabai 1987). This chemical property have the potential to increase P mobility in the soil and hence, may elevate P acquisition by plants roots (Torres-Dorante et al. 2006). Indeed, several studies indicated superior P up take or growth in response to poly-P compared to ortho-P fertilizer application (references within (Torres-Dorante et al. 2005)). Yet, numerous studies found no significant effect on P source type (e.g.(Engelstad and Terman 1980; Goh et al. 2013; Ottman et al. 2006)) or inconsistent response depending on poly-P type, soil type and the experiment duration (Dick and Tabatabai 1987). Furthermore, indirect benefits of poly-P utilization, mainly due to micronutrients chelation, may easily overshadow the direct impact. Overall, these studies are generally descriptive and lack mechanistic explanation.
Inorganic poly-P is naturally occurs in soils as a result of P polymerization by soil microorganisms (Harold 1966). Poly-P can accumulate to high concentrations in the microbial cells, these P pools probably used for storage and regulation of cytoplasmic pH (Pick et al. 1990). Thus, given the right conditions, soil ortho-P can be converted to poly-P by microorganisms (Pepper et al. 1976).
Plants response to P. Roots responses to environmental alternation by rhizosphere or/and morphological adjustment (Hinsinger 2001; Hinsinger et al. 2009; White et al. 2013). As P being poorly available and mobile in soils, total root length is often considered as an important feature of root system architecture to efficiently acquire soil P (Pagès 2011), while mycorrhizal symbiosis may provide an alternative option (Richardson et al. 2011), especially under low P supply. Roots systems exhibiting the largest root length, surface area and branching would be expected to exhibit a greater P acquisition efficiency. Thus, theoretically, factors stimulating root growth, branching and generally increasing the absorbing surface area will indirectly elevate P fertilizers utilization. Phosphate source was previously shown to alter roots growth (Torres-Dorante et al. 2006) and thus, the potentially accessible P pool. Yet, there are small number of studies examining such possible indirect effect.
Second root strategy for P acquisition is by enhancing bioavailability of poorly labile forms of P. In this regard, exudations of phosphatases enzymes by roots catalyze P hydrolysis and elevates P availability. Both plants and microbial oriented enzymes are participating in organic-P hydrolysis. Phosphorus deficiency was shown to stimulate phosphatases exudation (Engelstad and Terman 1980), thus, under stressed conditions PUE is expected to increase.