Tracing deep P uptake potential in arable subsoil using radioactive 33P isotope

Little is known about phosphorus (P) uptake at deep soil layers, e.g. below 1 m of soil depth. We aimed at tracing P uptake up to 4.2 m of soil depth of different plant species in an arable field using 33P isotope combined with the core-labelling technique. Soil was packed into ingrowth-cores and labelled with H3 33PO4. The ingrowth cores were placed into field plots of intermediate wheatgrass, perennial lupine, rosinweed and curly dock at the depths of 0.2, 1.0, 2.5 and 4.2 m. 33P uptake into shoots was measured by liquid scintillation assay after 4, 6 and 8 weeks of labelling. Root-length and diameter classes were analyzed from the retracted ingrowth-cores. We observed 33P uptake from all the tested soil depths down to 4.2 m. Curly dock with higher root density exhibited greater 33P uptake. Roots below 1 m was responsible for nearly 50 % of the total 33P uptake from all four depth-levels. Also, roots below 1.0 m showed higher 33P concentration per unit root mass, in line with a higher proportion of fine roots at depth. Our data demonstrate that perennial crop species can access deep P beyond 4 m of soil depth, and that the share of total P uptake below topsoil layers can be substantial, possibly related to the shift towards finer roots in deep soil layers. The results also show that the capacity for deep P acquisition highly depends on crop species even under same field conditions. Therefore, future research should focus on identifying crops with efficient P acquisition capacity in deeper soil layers which can potentially improve crop P access, especially under P-limited conditions in the topsoil.