Abstract
Phosphate (P) bioavailability is severely constrained in volcanic ash soils due to its high fixation rate. To overcome this problem the use of P-solubilizing fungi (PSF) has been proposed gaining recently great attention. However, despite encouraging results in the laboratory, their effectiveness under field conditions is debatable. To provide a better understanding of the dynamics of PSF in soils and to establish criteria for screening effective PSF a series of studies were conducted. PSF were isolated from coffee plantations grown in a Typic Udivitrand (QU), a Pachic Fulvundand (CH), and a Typic Melanudand (Ti). Fifty-four isolates (27 from CH, 19 from Ti, and 8 from QU) produced index of P solubilization among 16 and 106 mg L-1 using as P source phosphate rock. The results suggest that the microbial P solubilization was not only associated to the decrease in the culture medium pH, but also the production of organic acids associated with the most effective PSF. The higher production of organic acids seems to be associated lower fungal colony growth rate likely due to a carbon/energy drainage. The soil P-fixation capacity, soil organic content and degree of humification seems to control the relative abundance of PSF in the soils tested. In soils cultivated with coffee Phlebia subserialis is reported, for the first time, as a PSF. Key words: Colombian coffee zone; soil fertility; phosphorus fixation; organic acids; humificaction degree; Phlebia.
Highlights
Phosphorus (P) is the least available of all the essential nutrients required by plants in the different agricultural areas of the world, in volcanic ash soils, where their chemical properties cause a strong P fixation (Velásquez et al, 2016; Shoji; Takahashi, 2002)
Osorno and Osorio (2014) documented P solubilization levels of 80 and 120 x 10-6 kg dm-3 from phosphate rock inoculated with A. niger and Mortierella sp
Since the results showed that the uninoculated control had a pH 0f 6.2, while the P solubilizing fungi (PSF) inoculation decreased it down below 4.2 (Figure 2), this seems to validate the theory that a reduction in pH is a determining factor in the microbial solubilization of P from phosphate rock
Summary
Phosphorus (P) is the least available of all the essential nutrients required by plants in the different agricultural areas of the world, in volcanic ash soils, where their chemical properties cause a strong P fixation (Velásquez et al, 2016; Shoji; Takahashi, 2002). The organic and inorganic P fractions are abundant in these soils (Miller; Arai, 2017), they are not bioavailable for crop plants (Bünemann, 2015). To alleviate this the application of soluble phosphate fertilizers has been commonly used (Roy et al, 2017); this practice is ineffective, expensive, and carries the risk of water pollution (Tian et al, 2017). The solubilization of the soil organic P fraction is conducted through phosphatase enzymes (Lazo; Dyer; Alorro, 2017; Jain; Singh, 2016), whereas the dissolution of mineral phosphates is carried out via the secretion of organic acids from the oxidative respiration of carbonaceous compounds (Wyciszkiewicz et al, 2016; Acevedo et al, 2014; Osorio; Habte, 2014a; Osorno; Osorio, 2014b; Oliveira et al, 2014; Haq; Ali; Iqbal, 2003). The PSF have been reported to significantly promote plant growth and P uptake in plants grown in man-made substrates (soil:sand:organic materials) as well as in soils with low to high P sorption capacity (Entisols, Alfisols, Oxisols, and Ultisols), but they have been ineffective to do so in soils with very high P sorption capacity (Andisols) (Osorio, 2011)
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