Chemical nature of residual phosphorus in Andisols
Introduction
Phosphorus (P) is the principle limiting nutrient in many agroecosystems and continued P inputs are required to increase and maintain production (Nash et al., 2014, Haygarth et al., 2013). However, most of the P applied to soils is not taken up by plants and accumulates in the soil as various forms of inorganic and organic P, which is commonly referred to as ‘legacy P’ (Stutter et al., 2012, Condron et al., 2005, Frossard et al., 2000). In many soils, a significant proportion of the P is accumulated in recalcitrant forms (Stutter et al., 2012, Gyaneshwar et al., 2002). Understanding the nature of recalcitrant P in soil could be helpful to develop new biological approaches and technologies designed to enhance the bioavailability and utilization of this valuable P resource in soil–plant systems, such as use of enzymes or P -solubilizing bacteria for mineralizing soil P (Calabi-Floody et al., 2012, Menezes-Blackburn et al., 2011).
Fractionation of soil P involving sequential extraction with a combination of neutral, alkali and acid reagents has been used extensively to investigate the dynamics and bioavailability of inorganic, organic and microbial P in soil–plant systems (Condron and Newman, 2011, Turner et al., 2005, Hedley et al., 1982). In particular, the fractionation developed by Hedley et al. (1982) has been widely used in its original and modified versions, to study P cycling in a wide variety of managed and natural ecosystems (Negassa and Leinweber, 2009, Cross and Schlesinger, 1995, Tiessen and Moir, 1993, Sanyal and De Datta, 1991). A limitation of the Hedley P fractionation method is that the unextractable fraction remaining after sequential extraction (‘residual P’) constitutes a significant proportion of the total P, generally more than 30% in moderately weathered soils, but > 80% in soils that are strongly weathered or derived from volcanic parent material (Condron and Newman, 2011, Turner et al., 2005).
The sparingly soluble nature of residual P is commonly assumed to indicate that it is recalcitrant and of limited availability to plants and microorganisms (Condron and Newman, 2011). However, there is some evidence that the residual P pool can be accessed by plants and/or their associated microflora. For example, significant long-term depletion of soil residual P was observed in response to plant uptake and removal in arable cropping and managed forest systems in the absence of P fertilizer inputs (Pierzynski and Gehl, 2005), while short-term depletion of residual P has also been observed in some rhizosphere soils (Chen et al., 2002, George et al., 2002). Also, Richter et al. (2006) showed depletion of residual pools over long-term plantation forest growth, and other studies have shown depletion of residual P close to roots (Gahoonia and Nielsen, 1992).
The limited information on the composition and bioavailability of residual P constrains our ability to fully understand P dynamics and manage soil P resources. To address this, we examined the detailed chemical composition of residual P in a series of Andisols from Chile by introducing an additional extraction step with NaOH–EDTA solution coupled with 31P nuclear magnetic resonance (NMR) spectroscopy.
Section snippets
Soils and chemical analysis
Six soil samples were collected from Andisols located in La Araucania and Los Rios regions in southern Chile belonging to the Piedras Negras (PN), Pemehue (PEH) and Puerto Fonck (PF) soil series (CIREN, 2003). These soils were developed originally under forest, but had been under grazed pasture for around 10 years. The agricultural management on the Pemehue soil was annual cropping (wheat) under conventional tillage, while the Piedras Negras and Puerto Fonck soils were under permanent grassland
Soil chemical properties of Andisols
Soil pH ranged between 4.9 and 5.9. Total P concentrations varied from 981 to 2473 mg P kg− 1, and all soils showed a significant increase in total P according with fertilization input. As expected, the Andisols contained significant quantities of total C (70–117 g C kg− 1) and total N (4.8–8.6 g N kg− 1) (Table 1), with C to N ratios between 13 and 15. In Pemehue and Piedras Negras soils, total C and N were significantly higher in fertilized soils.
Organic P ranged from 337 and 739 mg P kg− 1 and accounted
Discussion
Our results highlight the quantitative importance of the pool of P in the residual fraction remaining after sequential fractionation in Andisols. This agrees with previous studies of soils derived from volcanic ash and other parent materials (Negassa and Leinweber, 2009, Cross and Schlesinger, 1995). The proportion of residual P found in Andisols is related to the abundance of amorphous Al and Fe oxides, which are known to stabilize P through polyvalent bridging cations (Frossard et al., 2000,
Conclusion
This work highlights that a significant proportion of the total P in Andisols is not extracted by sequential extraction using the Hedley fractionation procedure. These results provide the first direct determination of the chemical nature of residual P in soils. Between 21 and 42% of residual P was organic P in the form of inositol hexakisphosphate, which might have been stabilized in organic macromolecules. Further analyses of different soil types and different climate conditions are now needed
Acknowledgments
The authors thank the FONDECYT project No 1141247 from Chilean Government for financial support and the ECOSSUD-CONICYT C13U02 project for financial support of French and Chilean research groups.
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