Plant species rather than elevated atmospheric CO₂ impact rhizosphere properties and phosphorus fractions in a phosphorus deficient soil

Abstract

By 2050, elevated atmospheric CO₂ (eCO₂) could stimulate plant growth, but dwindling phosphorus (P) stocks in the soil could limit growth. However, little is known about how eCO₂ could affect soil P availability and dynamics in P-poor soils. Here, we conducted a 6-week pot experiment where three plant species were grown in a low-P soil under ambient (390 ppm) and eCO₂ (700 ppm) to investigate plant growth, rhizosphere properties, and changes in soil P fractions. Our results showed that under P deficiency, plant biomass, P uptake, and rhizosphere properties did not respond to eCO₂. Changes were noted by plant species. Compared to the control soil (unplanted pots), rhizosphere pH decreased the most under wheat, while microbial biomass P was higher under blue lupin. Among plant species, the blue lupin rhizosphere exhibited higher acid and alkaline phosphatase activity as well as organic anion release. Soil P fractions were impacted by plant species but similar across CO₂ treatments. Blue lupin accumulated labile organic P while depleted moderately labile organic P. Accumulation of labile organic P could be ascribed to microbial P immobilisation, whereas the mineralization of moderately labile organic P was associated with higher phosphatase activity. Wheat depleted acid extractable inorganic P the most, probably due to soil acidification and higher root biomass. These results suggest that plants can mobilise different P fractions irrespective of their chemical availability using morphological and/or physiological adaptations. However, these adaptations to acquire P from a low-P soil were not affected by eCO₂. This implies that current P fertiliser recommendations to boost or maintain crop production in low-P soils would remain unchanged under future eCO₂.