Nutrient dynamics and carbon partitioning in nutrient loaded Picea mariana [Mill] BSP seedlings during hardening

Biomass and nutrient dynamics of black spruce seedlings during the hardening phase of the greenhouse rotation were examined after four pre-hardening fertilization regimes-conventional, constant-rate loading, exponential loading and high exponential loading (delivering respectively 15, 40, 40 and 60 mg N seedIing(-1))-in order to assess nutrient dilution and steady-state nutrition, Although height growth of seedlings had virtually ceased during hardening, shoot biomass increased 99-142% and root biomass increased five- to tenfold depending on treatment. The biomass increase compromised steady-state nutrient status by diluting internal nutrient pools as nutrient supply was unable to keep up with growth. Soil nutrient levels were rapidly depicted during this period because of plant uptake and lack of fertilizer supplementation. Pre-hardening nutrient loading partly countered and delayed dilution effects, especially when applied exponentially and at high dose levels. Plant nutrient uptake was increased, presumably drawing on residual fertilizer in the rooting media. The higher pre-plant nutrient reserves in the seedlings are expected to contribute to improved outplanting performance. Sustained steady-state nutrition in seedlings during hardening may not be achieved by nutrient loading alone, suggesting the need for supplemental fertilization during this period.