Evaluation of the role of root exudates on phosphorus uptake by maize and groundnut in a low-P soil using a simulation model

Our work has hypothesized that maize produces organic acids, which could increase P solubility. The objectives of this research were to quantify the P active root exudates released in to the rhizosphere of maize and groundnut using a mechanistic model. To test this hypothesis, field experiments were conducted in an Alfisol. Treatments consisted of three P levels i.e., P-0 (no P), P-50 (50 mg P kg⁻¹ soil) and P-400 (400 mg P kg⁻¹ soil), and two levels of the fungicide benomyl application (0 and 500 kg ha⁻¹). Four harvests were made to cover whole growing season and at each harvest different soil, plant and microbiological parameters were determined. In the early growing season, the P influx of maize was dependent on soil solution P. At this growth stage and at low P supply the effect of arbuscular mycorrhiza (AM) was rather large. At high P supply, the contribution of AM to P influx showed a decrease. During middle and at the end of the growth cycle P influx increased sharply and was not dependent on P supply. At these growth stages, the effect of AM was largest at low P supply. Without or low AM infection and at low P level, the P influx was 62% of that with AM. Groundnut, during early growth period showed a behaviour similar to maize at middle growth stage and without AM reduction of P influx, which was to an extent of 67%. In middle and at the end of the growth cycle P influx was found to be related to P supply; degree of AM infection had no influence. In absolute terms AM is more important at maximum growth in the middle of the growth season for maize and only early growth season for groundnut. However, in relative terms AM is more important in early growth season for both the crops. It is evident from the present investigation that the higher measured than calculated P influx of maize could be explained by less than 10% through mass flow and diffusion, by about 35% through arbuscular mycorriza and the remaining 60% might be due to P deficiency-induced exudation.