Influence of Plants on Soil P Concentration

McGill and Cole (1981) suggested that the concentration of available P in the soil depended on biochemical mineralization, i.e., mineralization by extracellular enzymes, which does not provide energy to organisms and depends on the amount of enzymes present. This is controlled by the need for P. Thus, organic P input into the soil only influences the size of the total pool, while plants, microbes, and mycorrhiza can make P available by releasing phosphatases and phosphohydrolases into the soil. Phosphatase excretion has been used as an indicator of the P status of plants (Johnson et al. 1999; Phoenix et al. 2004).

Exudation occurs in response to environmental constraints, especially P deficiency (e.g., Jones 1998; Hinsinger et al. 2003) and differs depending on the P-form (Lambers et al. 2002) and plant species (Nuruzzaman et al. 2006). Banksia grandis exuded citrate, malate, and trans-aconitate when supplied with aluminium-phosphate. It exuded less of these tricarboxylates and dicarboxylates, but instead lactate and acetate, when supplied with iron-phosphate (Lambers et al. 2002). Plant species differ in their abilities to use various P species (van Ray and van Diest 1979), which can be due to differences in their exudation behavior (Nuruzzaman et al. 2006) and acidification of the root zone (Haynes 1992). This can influence the interspecific competition and coexistence of species, as we will discuss later.

Exuded carboxylic acids may form complex metal cations binding phosphate and cause exchange of phosphate from the soil matrix; phenolics and mucilage may serve similar purposes (Lambers et al. 2006). Organic acids are also exuded by so-called phosphate-solubilizing microorganisms that may increase the availability of P to plants in sustainable agriculture (Khan et al. 2007). Exudation of organic acids has often been referred to as a possible source of rhizosphere acidification (e.g., Hoffland et al. 1989), and enhanced proton release may occur as a response to P shortage (Bertrand et al. 1999; Neumann and Romheld 1999; Hinsinger et al. 2003). Soil pH is one of the main parameters determining adsorption/desorption equilibria of phosphate in soils (Hinsinger 2001). Decreased pH mediated by plants was invoked as a possible mechanism for the increased dissolution of P-containing minerals and thus of increased P availability shown for example for the exotic invasive plants Lepidium latifolium (Blank and Young 2002) and Solidago gigantea (Herr et al. 2007).

Besides their influence on P availability, plants also influence the size of the total organic P pool, mainly through the rate and quality of organic input from aboveground litter and root turnover. The rate of organic P input with litter depends on the size and P status of the plants. P-deficient plants usually have less aboveground biomass and less P per unit biomass. Input into the soil from litter of P deficient plants may also be reduced because they may be more efficient in reallocating P during senescing of leaves (Güsewell 2004). This may also have an effect on leaf longevity, which is normally increased in reaction to nutrient limitation, but may be reduced when nutrient translocation to young tissue plays a relevant role (Lajtha and Harrison 1995). Root turnover may be a major input for the soil P pool. Aerts et al. (1992) calculated that root turnover contributed 67% to the total litter production of a stand of Molinia caerulea, and even 84% to total litter P loss, since no resorption of P from senescing roots was observed. The rates were about two or three times smaller for stands of Deschampsia flexuosa and Calluna vulgaris, respectively. Root turnover itself seems to be influenced by P availability: In Hawaiian montane forests, old, P-deficient sites had an increased turnover of roots when fertilized with P and differently fertile sites also showed a correlation between root turnover and P availability (Ostertag 2001). This can be caused by different rates of mineralization and immobilization of P, which are influenced by litter quality, e.g, its P content (McGrath et al. 2000).

Of course, plant symbionts also influence the P cycle. Ectomycorrhizal fungi have been described to contribute to rock weathering, i.e., solubilizing P from minerals that would otherwise be inaccessible for plants, even through tunnels to the inside of the minerals (Landeweert et al. 2001). Van Schöll et al. (2006) have shown that the fungus Paxillus involutus can increase weathering of muscovite, but not hornblende. Two further tested fungi did not increase weathering, indicating that this ability seems to depend on the species of ectomycorrhizal fungus. Van der Heijden et al. (2008) suggest that mycorrhizal fungi are responsible for up to 75% of P acquired by plants annually. The role of free-living bacteria for P acquisition by plants is still unknown (van der Heijden et al. 2008).

Invertases, the enzymes catalyzing irreversible hydrolysis of sucrose to fructose and glucose, were upregulated in mycorrhizal roots in response to colonization by arbuscular mycorrhiza, not to P nutrition (Garcia-Rodriguez et al. 2007). Since mycorrhizal colonization is negatively related with P availability (Covacevich et al. 2007) and fructose and glucose increase the transcription of genes essential for P uptake (Liu et al. 2005), this coupling may have developed in response to P nutrition, but might have a similar fate as in Pavlovian conditioning, where the original stimulus need not be present any more to evoke a reaction.

Another possibility for plants to influence the P cycle is the hydraulic redistribution of water. This is the redistribution of water from wet to dry soil areas via the roots, which has been suggested to have an impact on the availability of P due to better mobility of inorganic P in wet soil (Lambers et al. 2006). McCulley et al. (2004) found that the concentration of extractable P was greater at depth than in the top meter of the soil in several arid and semi-arid systems in the southwestern USA and that nutrients were uplifted from this depth. They proposed that hydraulic redistribution of water from the soil surface to depths up to 10 m by roots was the mechanism by which P and other nutrients were mobilized and could be taken up by plants.

Thus, plants have a range of possibilities to influence the total P pool and the availability of P directly or via symbionts. The influence of plant species on

P concentration can be seen clearly in cases of invasive species. These have in several studies been shown to affect P turnover rates. For example, Centaurea maculosa, an invasive forb in Montana grasslands, was more efficient in P uptake than native species, but also apparently increased P availability in invaded fields (Thorpe et al. 2006). The same was found for S. gigantea in Belgium: It increased the concentrations of labile soil P in summer, probably by enhancing mineralization (Chapuis-Lardy et al. 2006). An increased P concentration in belowground organs was found in plots invaded by S. gigantea in autumn. This could lead to easily mineralizable root debris in spring and may have caused the higher content of plant-available P in the invaded stands (Herr et al. 2007).

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