Phosphatase activity and organic acids in the rhizosphere of potential agroforestry species and maize

Previous work has hypothesised that Tithonia diversifolia produces both organic acids and phosphatase. Simultaneous exudation of these compounds could increase both P solubility, by releasing bound Po, and Po mineralisation by increasing the rate of hydrolytic cleavage. The objectives of this work were to identify and quantify the P active root exudates released into the rhizosphere of agroforestry species compared to maize. Agroforestry species and maize were grown in a rhizopot system and thin soil slices were analysed for phosphatase activity using a fluorescent enzyme assay based on methylumbelliferone-phosphate (MUB-phosphate) and organic acid concentration by methylation and GCMS. Tithonia (137 μM) and Tephrosia (108 μM) had higher concentrations of citric acid in the rhizosphere than maize (96 μM). However, subsequent analysis of the citric acid chromatograph peak with a mass spectrum detector showed that peaks were actually attributable to a fatty acid. Therefore, there was no evidence for enhanced organic acid concentration in the rhizosphere. In contrast, phosphatase activity was greatly enhanced in the rhizosphere of all agroforestry species. Tithonia (29.64 nmol MUB min−1 g−1 soil) and Crotalaria (36.08 nmol MUB min−1 g−1 soil) had greater activity at 0–1 mm from their root than did maize (15.75 nmol MUB min−1 g−1 soil). Agroforestry species enhanced the activity of acid phosphatase, while maize enhanced the activity of alkaline phosphatase. Acid phosphatase activity increased exponentially with increasing proximity to the root mat of the agroforestry species. This suggests that agroforestry species are actively increasing rhizosphere phosphatase activity either directly by secretion or indirectly by stimulation of microbial activity and/or depletion of Pi. Overall, agroforestry species enhanced phosphatase activity in their rhizosphere. However, this does not necessarily lead to increased Po mineralisation unless there is a concurrent increase in the solubility of Po forms.