Phosphorus (P) availability shows large differences among different soil types, affecting P nutrition of forest trees. Chemical binding of P to soil moieties affects partitioning of P between soil particles and solution, affecting soluble P concentrations upon which plants, their associated mycorrhizal symbionts, and microbes feed. The goal of this study was to characterize root P uptake by mycorrhizal and non-mycorrhizal root tips in competition with microbes in situ in the organic and mineral layer of a P-rich and a P-poor forest. We used intact soil cores (0.2m depth) from beech (Fagus sylvatica) forests to tracing the fate of ³³P in soil, plant and microbial fractions. We used the dilution of ³³P in the rhizosphere of each soil layer to estimate the enrichment with new P in mycorrhizal and non-mycorrhizal root tips and root P uptake. In soil cores from P-rich conditions, 25% and 75% of root P uptake occurred in the organic and mineral layer, respectively, whereas in the P-poor forest, 60% occurred in the organic and 40% in the mineral layer. Mycorrhizal P efficiency, determined as enrichment of new P in mycorrhizal root tips, differed between soil layers. Root P uptake was correlated with mycorrhizal P efficiency and root tip abundance but not with root tip abundance as a single factor. This finding underpins the importance of the regulation of mycorrhizal P acquisition for root P supply. The composition of mycorrhizal assemblages differed between forests but not between soil layers. Therefore, differences in P efficiencies resulted from physiological adjustments of the symbionts. Non-mycorrhizal root tips were rare and exhibited lower enrichment with new P than mycorrhizal root tips. Their contribution to root P supply was negligible. Microbes were strong competitors for P in P-poor but not in P-rich soil. Understory roots were present in the P-rich soil but did not compete for P. Our results uncover regulation of mycorrhizal P efficiencies and highlight the complexity of biotic and abiotic factors that govern P supply to trees in forest ecosystems.

Collection of forest soil cores

We collected ten intact soil cores in the HP (27.6.2017) and the LP (19.6.2017) forest, respectively, using PVC pipes of 120 mm diameter and a length of 200mm.

Radioactive labeling and harvest

We labelled five soil cores per forest by addition of 1850 kBq of H₃³³PO₄ (Hartmann Analytic GmbH, Braunschweig, Germany) in 40 ml of tap water to each soil core.

Analyses of root tips and mycorrhizal species

Fresh beech roots (10 fragments) were weighed and used to count the number of root tips under a stereomicroscope (Leica M205 FA, Wetzlar, Germany).

Determination of total and soluble phosphorus in roots and soil

Dry soil and root samples were milled in a ball mill (Retsch MN 400, Haan, Germany) to a fine powder.

Determination of ³³P by scintillation counting

The extracts of all soil, plant and root tip samples were used to measure ³³P.