Abstract
The relationship between photosynthesis and leaf nitrogen concentration is often used to model forest carbon fixation and ratios of different nutrient elements can modify this relationship. However, the effects of nutrient ratios on this important relationship are generally not well understood. To investigate whether N/P/K ratios and CO2 concentration ([CO2]) influence relationships between photosynthesis and nitrogen, we exposed one-year-old black spruce seedlings to two [CO2] (370 and 720 μmol·mol-1), two N/P/K ratio regimes (constant (CNR) and variable (VNR) nutrient ratio) at 6 N supply levels (10 to 360 μmol·mol-1). It was found that photosynthesis (Pn) was more sensitive to nitrogen supply and N/P/K ratios under the elevated [CO2] than under ambient [CO2]; under the elevated [CO2], Pn declined with increases in N supplies above 150 μmol·mol-1 in the CNR treatment but was relatively insensitive to N supplies of the same range in the VNR treatment. Further, our data suggest that the nutrient ratio and the CO2 elevation effects on photosynthesis were via their effects on the maximum rate of carboxylation (Vcmax) but not electron transport (Jmax) or triose phosphate utilization (TPU). The results suggest that the CO2 elevation increased the demand for all three nutrient elements but the increase was greater for N than for P and K. The CO2 elevation resulted in greater photosynthetic use efficiencies of N, P and K, but the increases varied with the nutrient ratio treatments. The results suggest that under elevated [CO2], higher net photosynthetic rates demand different optimal N-P-K ratios than under the current [CO2].
Highlights
The close relationship between photosynthesis and nitrogen in plants
It was found that photosynthesis (Pn) was more sensitive to nitrogen supply and N/P/K ratios under the elevated [CO2] than under ambient [CO2]; under the elevated [CO2], Pn declined with increases in N supplies above 150 μmol·mol−1 in the constant N/P/K ratios (CNR) treatment but was relatively insensitive to N supplies of the same range in the variable N/P/K ratios (VNR) treatment
Our data suggest that the nutrient ratio and the CO2 elevation effects on photosynthesis were via their effects on the maximum rate of carboxylation (Vcmax) but not electron transport (Jmax) or triose phosphate utilization (TPU)
Summary
The close relationship between photosynthesis and nitrogen in plants Increasing nutrient supply between the CDC and CTC generally results in greater plant growth, but the pattern of the response varies with nutrient element [18] [21]. The objective of this study was to investigate how elevated CO2, nitrogen supply and their interactions with P, K and N/P/K ratios affect the relationship between N and photosynthesis in black spruce. Since the increases in nutrient demand induced by CO2 elevations are proportionally greater for N than for P and K, and CTC is reached faster for P and K than for N under elevated CO2 when N/P/K ratios are maintained constant, we hypothesize that N/P/K ratios and CO2 will interactively affect the N-photosynthetic relationship
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