The effect of pH on morphological and physiological root traits of Lupinus angustifolius treated with struvite as a recycled phosphorus source

Abstract

Aims: Phosphorus (P) recovery from specific waste streams is necessary to develop environmentally sustainable and efficient fertilizers, achieving maximum productivity with minimum losses. A promising example of a P-recovery product is struvite (MgNH 4 PO 4 ⋅6H 2 O). Phosphorus availability from struvite is profoundly influenced by soil pH and/or processes in the rhizosphere. Root exudates (e.g., organic anions) and root morphology affect fertilizer bioavailability. The overall objective of our study was to identify root morphological and physiological traits of the narrow-leaf lupine (Lupinus angustifolius L. subsp. angustifolius, cultivar: blau “Boregine”) involved in the acquisition of P from struvite, compared with KH 2 PO 4 as a soluble P source. The study included different pH conditions, as soil pH is one of the main factors affecting P availability. Methods: Narrow-leaf lupine plants were grown in river sand under three pH conditions (4.5, 6.5 and 7.5). Three different P treatment conditions were used: 1) KH 2 PO 4 (KP); 2) MgNH 4 PO 4 ⋅6H 2 O (Struvite), both supplied at 15 μg P g −1 dry sand; and 3) no P addition (Nil-P), as control. Organic acids in the rhizosheath were collected. Root morphological parameters such as specific root length and root diameter were analyzed. Results: There was no significant difference in total plant biomass detected under any pH condition between struvite and KP treatments. In both acidic and alkaline conditions, the P-uptake efficiency (PUE: mg P plant −1 /cm 2 root surface area) with struvite was significantly greater than with KP. At neutral pH, there was no difference in PUE between plants supplied with KP or struvite. Plants growing at neutral pH showed greater root exudation of carboxylates (mainly citrate) when struvite was added compared with KP. At alkaline pH, the exudation per unit root surface area was greater than that at acidic or neutral pH. Plants growing in acidic pH had a higher specific root length (SRL) compared with those grown at alkaline or neutral pH. Conclusions: Similar P-uptake efficiency from struvite and KH 2 PO 4 at neutral pH in conjunction with the higher total biomass compared to the Nil-P treatment (70% higher) suggests very effective mobilization of P from struvite by carboxylate exudation. Application of struvite, while taking into account the different strategies for nutrient mobilization, can increase the use efficiency of this recovered P source.