Towards phosphorus recycling for agriculture by algae: Soil incubation and rhizotron studies using 33P-labeled microalgal biomass

Abstract

Algae effectively accumulate phosphorus (P) from the environment, qualifying them as a promising novel P fertilizer. We hypothesized that P in algae can be rapidly transformed in soil and mobilized for plant growth. To determine the fate of algal fertilizer in soil and to trace its efficiency for plant uptake, we labeled the algae Chlorella vulgaris with the radioisotope 33P. To optimize the labeling we studied P-uptake dynamics in detail using a pre-starved culture and additionally monitored polyphosphate (Poly-P) and organic carbon (C) reserve pools by Raman microscopy. Using an optimized labeling procedure, the concentrations and distribution of both algae-derived 33P and mineral fertilizer 33P (control) were characterized in incubation and rhizotron experiments. Soil incubation was performed with four major reference groups (Andosol, Alisol, Cambisol, and Vertisol). To assess 33P plant uptake we grew wheat in rhizotrons on Cambisol. Soil analyses at different incubation times demonstrated sequential 33P fractionation, while plant uptake of algae-derived 33P was followed using sequential autoradiographic imaging. We found that the algae increased labile P pools comprising Resin- and NaHCO3-extractable P in soils during the first 2 weeks of incubation, similar to the effects of NPK fertilizer. The soils with elevated concentrations of Fe- and Al-oxides (Andosol and Alisol) immediately bound 55 to 80% of the applied fertilizer 33P into the moderately available NaOH-P fraction, whereas the soils with lower concentrations of Fe/Al-oxides (Cambisol, Vertisol) stored 35–71% of the algal-P in the labile fraction. The rhizotron experiments visually supported the release and plant-uptake of algal 33P, thus verifying the suitability of algal-fertilizer for plant growth.