Acid phosphomonoesterase and β-glucosidase activities in volcanic soils under permanent fertilized pastures: distribution profile and microbial effort toward P acquisition

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ABSTRACT This work focused on the amounts and distribution of organic C (OC), organic phosphorus (Po), and acid phosphomonoesterase (AcP) and β-glucosidase (βG) activities in volcanic soils, as well as on the effect of inorganic P (Pi) on potential P-acquiring effort. The OC and Po contents are correlated with the active aluminum (Alox) and iron (Feox) and with the stratified gradients of the enzyme activities. The AcP activity does not seem to be suppressed by soil Pi content, showing a positive linear relationship with Pi. High values for the AcP:βG activity ratio are observed, which suggests a high energy allocation (C consumption) to ensure P demand. With the increase of Alox and Feox, the AcP:βG ratios increase and the potential C:P acquisition (ln(βG):ln(AcP)) values decrease. Nevertheless, the average C:P molar ratio of the six soils (115) are below the critical value, above which soils may present low P bioavailability. The results suggest that the effort for P acquisition (AcP:βG) and the potential C:P acquisition are both affected by the size of the Pi pools and by Alox and Feox contents. If Pi retention increases due to high contents of Alox and Feox, energy allocation shifts toward AcP production, in detriment of microbial population and plant nutrition. Considering the OC accumulation in the studied soils, several hypotheses may be considered such as the energy demand toward P acquisition, and the ability of Alox and Feox metal complexes to protect organic molecules to depress βG activity.