Late-season fertilization of Picea mariana seedlings:: intensive loading and outplanting response on greenhouse bioassays

Traditional greenhouse culture involves a late-season hardening period that withholds irrigation and fertilization from black spruce seedlings to promote frost-hardiness. Since nutrient uptake is limited without supplemental fertilization, growth during hardening may lead to internal nutrient dilution, a condition detrimental to field performance of seedlings. We examine whether late-season fertilization, applied as intensive loading, will counter dilution and build up nutrient reserves in seedlings reared conventionally or nutrient loaded before hardening. A mixed NPK fertilizer delivering 0, 12, 24, or, 48 mg N . seedling(-1) for 9 weeks after bud set was tested. Root and shoot dry mass increased as much as 104 and 42% during hardening. Seedling biomass, however, was unchanged by late-season fertilization, but N uptake was increased 44 - 167% signifying induced luxury consumption. Extra K supplementation of treatments averted K dilution in plant tissues often occurring with high N addition. A 13-week outplanting trial on intact soil bioassays retrieved from a boreal site showed that growth and nutrient allocation were significantly enhanced by larger N reserves built up after intensive nutrient loading. About 72 - 80% of N required for new shoot growth was met from internal cycling, demonstrating the capacity of loading to enhance retranslocation. Intermediate loading ( 24 mg N) was most effective in promoting N accumulation and outplanting growth of both seedling types. Survival was reduced (30%) only at the highest dose. Study results demonstrate the potential advantage of these practices to improve growth of newly planted seedlings on northern forest sites.