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CO 2 elevation stimulates plant growth, which in turn demands more nutrients to sustain. Since the increase of demand for nitrogen (N), phosphorus (P) and potassium (K) may be in different proportions, the optimal N–P–K ratios at elevated [CO 2 ] are likely different from those at the ambient [CO 2 ]. This study investigated the effects of various N supply levels with constant and variable (constant P and K concentrations) N–P–K ratios under ambient and elevated [CO 2 ] on black spruce ( Picea mariana Mill. BSP) seedlings. One-year-old seedlings were exposed to two [CO 2 ] (370 vs. 720 μmol mol −1 ), two nutrient ratio regimes (constant vs. variable N/P/K ratios) and six N concentrations (10, 80, 150, 220, 290 and 360 μmol mol −1 ) in four environmentally controlled greenhouses for 3.5 months. Growth response to N varied with [CO 2 ] and N/P/K ratios: under the elevated [CO 2 ], height growth increased with increasing N supply when P and K concentrations were kept constant across different N levels, but it only increased when increasing N from 10 to 150 μmol mol −1 , and started to decline with further increase in N supply when N/P/K ratios were kept constant at different N levels; at the ambient [CO 2 ], height growth was greatest at 150 μmol mol −1 N and was generally greater at 220–360 than at 10 and 80 μmol mol −1 N in both nutrient ratio treatments. The foliage to root ratio, shoot mass ratio and total biomass generally increased with increasing N supply but root mass ratio decreased. The smallest specific leaf area occurred at the lowest N supply when N/P/K ratios were kept constant but at 220 μmol mol −1 N when P and K concentrations were kept constant across different N supplies. The results of leaf nutrient concentrations suggest that the elevated [CO 2 ] increased demand for N, P and K and the increase for N was greater than P and K, altering the relationship between growth and nitrogen supply. Under the elevated [CO 2 ], high N supplies resulted in growth suppression by critical toxicity content only in the constant N/P/K ratios treatment but low N supplies led to growth suppression by critical deficiency content in both nutrient ratio treatments. At the ambient [CO 2 ], in contrast, N/P/K ratio treatments did not affect growth suppression by critical deficiency or critical toxicity content. Because elevated CO 2 causes unequal increases in N, P and K demands, N–P–K ratios should be considered when modeling plant growth responses to elevated CO 2 . |
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