Ectomycorrhiza: phosphorus source specific economy and potential in resource partitioning

Abstract

Many natural and anthropogenic soils are phosphorus (P) limited, as P is largely immobilized in forms of low bioavailability affecting the plant productivity. One of the strategies to overcome this shortage lies in the symbiosis of plants with mycorrhizal fungi that increases the plant P uptake of these hardly accessible sources in exchange for photoassimilates. Nevertheless, the required investment to acquire the free phosphate from various P sources increases with decreasing P source bioavailability, indicating a different carbon (C) sink potential. Ectomycorrhizae were shown to be able to acquire various P sources, but experimental evidence for their P source dependent C sink potential as well as for their role in resource partitioning for P is missing. The present study aimed to address following objectives: (i) to design a system to observe mycorrhizal mediated nutrient exchange (Study I); and to investigate (ii) whether the C investment from the host plant for the mycorrhizal mediated P derived from different P species is P source dependent (Study II); as well as the (iii) preferences of mycorrhizal plant to specific P sources from a mixed P pool (Study III). Following P sources were tested for the mycorrhizal mediated plant P uptake: PO43- (oP), the primary mineral apatite (AP; Study II) or hydroxyapatite (HAP; Study III), the organic P species (Porg) phytic acid (Phy; Study II) or adenosine monophosphate (AMP; Study III), and goethitebound PO43- (gP) as secondary mineral-P adsorption complex. The practical experience made in Study I revealed that, compared with the in vitro culture system, the construction and maintenance of the axenic rhizotrone and the mesocosm culture systems are less complicated and time consuming and at the same time more robust and very versatile systems that are also suitable for greenhouse conditions. In Study II a P source dependent trend in exchange of host C for ectomycorrhizal mediated P in roots was determined. The exchange of C for P in ectomycorrhizal roots of AP and gP treatments were of similar magnitude and could result from the applying the same mechanism, the exudation of LMWOAs and hyphal growth to acquire mineral P sources (mineral P effect). In contrast, the readily available P source oP, the soluble Phy, as well as the complete P limitation turn the ectomycorrhizal plant into a system of C retention in the mycorrhizal root interface. These results indicate a trend that the C allocation into the belowground is P source specific, which can have an impact on the C balance in soils of natural and anthropogenic ecosystems. The results of Study III show that an ectomycorrhizal plant is able to utilize all provided P sources via its mycorrhizal fungal associate. The acquisition timing was determined by the most bioavailable P sources, with oP and AMP over HAP and gP, and a mixed P pool over single P source. In contrast, the magnitude was defined by the amount of supplied P source and provision of additional nitrogen, hence AMP over oP and gP, as well as by P source complexity, with gP as the least favourable P form. Nevertheless, these results provide evidence that an ectomycorrhiza has the potential to occupy fundamental niches of various P sources differing in their bioavailability, indicating that being a generalist in P nutrition can facilitate adaptation to various nutritional settings in soil.