Positive growth response of Pinus pinaster seedlings in soils previously subjected to fertilization and irrigation

Nutrient availability, particularly phosphorus (P), and water are major factors limiting tree growth in plantations of maritime pine (Pinus pinaster) in southwestern France. Applied intensively, fertilization and irrigation could have differential effects on the actual nutrient availability to the trees, especially P. These practices could modify the geochemical cycling of P between mineral and organic forms in soil as well as the enzyme activities able to mobilize soil organic P, namely, acid phosphatases. The result is modified growth response and mineral nutrition of the trees. Our research objective was to evaluate the growth and mineral nutrition of P. pinaster seedlings, together with soil P cycling, in response to prior fertilization and irrigation practices performed in the field. Seedlings were grown in a growth chamber in rhizoboxes containing soil samples from a 13-year-old maritime pine forest stand, previously fertilized annually (C: no fertilization, P: phosphorus only, F: complete fertilization NPKCaMg) with and without irrigation for 7 years. Plants formed ectomycorrhizal roots (ECM) mainly with the basidiomycete Rhizopogon luteolus. Fertilization significantly increased bicarbonate and hydroxide extractable inorganic P (Pi) and organic P (Po) while it decreased ECM acid phosphatase activity. Plants were hardly able to acquire any P from control soils, despite a high phosphatase activity assayed in ECM. Seedlings grown in soil with previous complete fertilization and irrigation displayed the strongest ability to deplete Po and to produce biomass. Compared to control soils, P accumulations were significantly greater in seedlings from the fertilized treatments. Diagnostic approaches confirmed deficiency status of P in control soils resulting in increased plant N. In contrast, both P and N were sufficient in soils from the irrigated and complete fertilization treatment, and a steady growth response of seedlings was observed. We suggest that the significant differences observed in Po mobilization based on prior fertilization and irrigation practices could be due to differential turnover of microbial populations.