Biol Fertil Soils (2008) 44:707–715 DOI 10.1007/s00374-007-0253-3
ORIGINAL PAPER
An examination of potential extraction methods to assess plant-available organic phosphorus in soil R. W. McDowell & L. M. Condron & I. Stewart
Received: 13 February 2007 / Revised: 28 October 2007 / Accepted: 1 November 2007 / Published online: 30 November 2007 # Springer-Verlag 2007
Abstract The role of soil organic phosphorus (P) in plant nutrition was assessed using data from a glasshouse pot experiment carried out on seven soil types using two contrasting plant species (Lolium perenne, Pinus radiata) and 12 different extractants (five salts (0.025 M ethylenediaminetetraacetic acid (EDTA), 0.025 M EDTA pH 7, Olsen, Mehlich-III, and 6% NaOCl pH 7.5) and seven exchange resins (Hampton chelating resin, Bio-Rad Chelex100, Dow MAC-3, Amberlite IRC76, Diaion WT01S, Lewatit MP500A, Diaion WA30)). The contribution from mineralization of soil organic P was inferred by consistent increases in correlation coefficients between extractable P and plant P uptake when organic P was considered in addition to inorganic P. The best correlated extractants for combined inorganic and organic P were NaOCl (r=0.84), Hampton chelating resin (r=0.78), and MP500A resin (r= 0.73), which compared favorably with Olsen P (r=0.66) and EDTA (r=0.72). 31P nuclear magnetic resonance analysis of selected extracts from two soils confirmed that the Hampton-chelating-resin-extractable P was mainly monoester and diester forms of organic P, while there was no monoester or diester organic P in the IRC76 resin extract—poorly correlated with plant uptake. The findings R. W. McDowell (*) AgResearch, Invermay Agricultural Centre, Private Bag 50034, Mosgiel, New Zealand e-mail:
[email protected] L. M. Condron Agriculture and Life Sciences, Lincoln University, P.O. Box 84, Lincoln 7647, New Zealand I. Stewart Department of Chemistry, University of Otago, P.O. Box 56, Mosgiel, New Zealand
of this study suggest that readily extractable forms of organic P in soil contribute to short-term plant P uptake, and this P should be considered for inclusion in routine tests for soil P availability. Keywords Bioavailability . Extractant . Nuclear magnetic resonance . Resin
Introduction A key criterion of optimal plant nutrition is the adequate supply of phosphorus (P) by soil. Although plants are known to only take up inorganic P, this pool can be replenished with time by the mineralization of organic P. The mechanisms for the mineralization of organic P are numerous but include the exudation of organic acids or phosphatase enzymes by plant roots or mycorrhiza (Gaume et al. 2001; Jones and Darrah 1994). Their exudation can be inversely related to the supply of inorganic P: when sufficient inorganic P is available for plant requirements, then the mineralization of organic P to inorganic P is less either by decreasing or changing the production of enzymes or organic acids (Veneklaas et al. 2003). In addition to this negative feedback loop, organic P can be protected from mineralization by soil organic matter and Al and Fe hydrous oxides (Celi and Barberis 2005). Organic P species such as myoinositol hexakisphosphate sorb to the same sites as inorganic P but can be bound tighter than orthophosphate due to the presence of six phosphate groups. However, the strength of sorption depends on the type of oxide present. For example, Anderson and Arlidge (1962) found that boehmite was very effective at sorbing myoinositol hexakisphosphate from solution while gibbsite was not. Phosphorus compounds can be associated with
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organic matter by weak van der Waals interactions or hydrophobic forces that aggregate organic molecules and this may make these organic molecules insoluble in water. This aggregation effect can cause organic matter to be insoluble in water and poorly extractable. For instance, Schlichting and Leinweber (2002) and Schlichting et al. (2002) found that drying 29 high- organic-matter soils and extracting them via the Hedley P fractionation procedure caused large increases in residual P thought to be mainly organic P. This was attributed to the binding and inclusion of P in pedogenic oxides and the structural rearrangement of humic macromolecules increasing hydrophobicity and exposing hydrophobic surfaces to extractants causing poor extractability or availability. Ideally, a soil test would account for differences in mineralogical, biological, and chemical properties of soils. As a consequence, much work has used plant growth parameters such as plant P concentrations, percent yield, or P uptake and correlated these to extractable P via known soil P tests (Edmeades et al. 2006). For example, Kamprath and Watson (1980) presented a summary of correlations that ranged from −0.10 to 0.94 between Bray-I extractable P and P uptake. Similarly, Sharpley et al. (1994) presented data for correlation coefficients between plant P uptake and an anion-exchange membrane, Olsen, Bray-I, Fe-oxide strip, ammonium bicarbonate-diethylenetriaminepentaacetic acid, acetic acid, and diluted H2SO4-extractable P that ranged from 0.41 to 0.93. However, given the many soil tests available to predict plant yield response to P, none have been able to do so for a wide range of soils or P concentrations. A possible reason for this variation is that many of these soil tests often only consider inorganic P, neglecting the influence of organic P. Predicting plant uptake or yield can be problematic at low soil-P concentrations where the importance to plant nutrition of organic P and mineralization is greater than supply of P from inorganic P alone (Condron and Tiessen 2005). At present, no one test is able to account for the mineralization of organic P or, at best, able to predict a plant labile organic P fraction. Recent work has focused both on the mechanisms the plant can use either alone or in association with mycorrhiza to liberate P and on quantifying the labile organic P fraction. As mentioned earlier, these not only include organic acids and phosphatase but also include diffusion gradients and anion exchange. The action of enzymes on P forms is well documented (for a summary, see Quiquampoix and Mousain (2005) ), while work on diffusion gradients and ion exchange have created techniques such as Fe-oxide gels that act as infinite sinks (e.g., Lookman et al. 1995) and anion-exchange membranes (usually bicarbonate form) that exchange with P from the soil or macroporous exchange
Biol Fertil Soils (2008) 44:707–715
resins that also allow large molecules, thought to house organic P, to be sequestered (Rubæk and Sibbesen 1993). More recently, work on organic acids has also focused on the organic P forms liberated (e.g., Hens et al. 2003). While the development of these techniques are based on sound reasoning, namely to mimic plant root actions, few have looked at relating the P forms (inorganic and organic) extracted to plant uptake as the ultimate expression of labile P (i.e., P that is available to influence plant growth). Our objective was to assess techniques for the extraction of labile P, inorganic or organic, and correlate this to the concentration of P in plant tissue. A diverse range of soils and two plant species (perennial ryegrass—Lolium perenne L. and radiata pine—Pinus Radiata L.) were chosen.
Materials and methods Soils and treatments The selected soils were a subset of those used by Chen et al. (2003). Briefly, Chen et al. (2003) chose soils from sites under long-term pasture and representing a wide range of parent materials and development (Table 1). These soils (0–7.5 cm in depth) were dried, sieved
