Abstract
To test the effects of elevated CO2 and soil N deficiency on N resorption efficiency (NRE) from senescing leaves in two non-N2-fixing deciduous broadleaved tree species, Japanese oak (Quercus mongolica var. grosseserrata Blume) and Painted maple (Acer mono Maxim. var. glabrum (Lév. Et Van’t.) Hara), potted seedlings were grown in a natural daylight phytotron with either ambient or elevated CO2 conditions (36 Pa and 72 Pa CO2) and with two levels of N (52.5 and 5.25 mg N pot−1 week−1 for high N and low N, respectively). We examined the N content (Nmass) of mature and senescent leaves, as well as photosynthesis and the growth of plants, and calculated both the mass-based NRE (NREmass) and leaf area-based NRE (NREarea). In both species, the Nmass of mature leaves decreased with high CO2 and low N, whereas the leaf mass per area (LMA) increased under elevated CO2, regardless of N treatments. In Q. mongolica, both the maximum rate of carboxylation (Vcmax) and the maximum electron transport rate (Jmax) were reduced by elevated CO2 and low N, but Vcmax exhibited an interactive effect of N and CO2 treatments. However, in A. mono, both the Vcmax and Jmax decreased under elevated CO2, regardless of N treatments. The partitioning of N for the photosynthetic function within leaves was also significantly decreased by elevated CO2 in both species and increased under low N in A. mono. The Nmass of senesced leaves decreased under low N in both species and exhibited an increase (Q. mongolica) or no effect (A. mono) by elevated CO2. The NREarea of Q. mongolica was affected by CO2 and N treatments, with a decrease under elevated CO2 compared to ambient CO2 and under low N compared to high N. The NREarea of A. mono was also affected by CO2 and N treatments and decreased under elevated CO2; however, unlike in the case of Q. mongolica, it increased under low N. We speculate that these interspecific differences in the responses of leaf N allocation, indicated by the photosynthetic (Vcmax and Jmax) and morphological (LMA) responses to elevated CO2, may have affected the NRE during defoliation under high CO2 and soil N-deficient conditions.
Highlights
The 100-day relative growth rate of the whole plant weight showed significant effects of CO2 (p < 0.05) and N treatments (p < 0.001) in Q. mongolica, and 6 of an enhanced whole plant biomass was observed under elevated CO2 and under high N (Table S1)
Contrary to expectations, photosynthesis was downregulated during tertiary elongation in Q. mongolica, and an accumulation of starch was observed in the leaves under high CO2, we suggest that the source–sink balance could not be regulated by tertiary elongation alone
The range of change in the leaf mass per area (LMA) of green leaves from Q. mongolica and A. mono was greater under elevated CO2
Summary
The increasing atmospheric CO2 concentration is a crucial factor influencing global climate change [1,2]. Projections for atmospheric CO2 concentrations are provided by distinct representative concentration pathways (RCPs). Elevated CO2 stimulates photosynthesis, which can increase forest net primary production but, at longer timescales, may not necessarily increase plant biomass. Biomass increase depends on nutrient availability, rising temperatures, heat stress, and variation in precipitation and plant water availability [4]. A recent study reported that the degree to which N limitation diminished the effects of CO2 on plant growth varied among experiments, depending on the age of forests and distinct nutrient strategies, for example, those involving microbial associations [4,10,11]
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