Paper Reviewed
Sun, J., Dai, W., Peng, B., Liu, J., He, T., Jiang, P., Han, S. and Bai, E. 2018. Does the accelerated soil N cycling sustain N demand of Quercus mongolica after decade-long elevated CO2 treatment? Biogeochemistry 139: 197-213.

The progressive nitrogen limitation or PNL hypothesis contends that low concentrations of soil nitrogen will gradually reduce the growth-promoting strength of the aerial fertilization effect of atmospheric CO2 enrichment.

In a test of this hypothesis, Sun et al. (2018) examined the temporal dynamics of coupled carbon (C) and nitrogen (N) cycling processes, along with N availability, in a 10-year open-top chamber CO2 enrichment experiment conducted on oak (Quercus mongolica) trees at the Changbai Forest Ecosystem Research Station in Jilin Province of northeastern China. There, over the course of the past decade, researchers have exposed oak trees to ambient or enriched (ambient + 180 ppm) concentrations of atmospheric CO2 during daytime hours over the course of the growing season (May-Oct). It was the authors' hypothesis that "(1) elevated CO2 would accelerate soil N cycling due to increased microbial biomass and activity caused by increased C inputs; and (2) accelerated soil N cycling may sustain plant N demand and progressive N limitation to date may not have occurred under 10 years of elevated CO2 treatment."

So, did their hypothesis prove correct?

In discussing their findings, Sun et al. report, first of all, that ten years of CO2 enrichment caused a significant increase in oak photosynthesis, biomass and C and N stocks. Mean plant photosynthetic rates under elevated CO2 conditions were 35% higher. Leaf, branch, stem, root and total biomass of the oak trees in the elevated CO2 treatment were also enhanced by 22.1, 43.1, 28.5, 23.6 and 26.4%, respectively. Total C and N stocks also increased for each of these parameters, rising 22, 43, 28, 23 and 26% for C and 14, 25, 18, 16, and 16% for N leaf, branch, stem, root and whole-plant stocks, respectively. On the other hand, elevated CO2 did not change soil organic C, total N, pH or soil texture, but it did increase soil C/N by a small 3.4%.

Despite the observed stimulation of plant biomass and increases in C and N stocks, Sun et al. report that soil inorganic N "generally did not change, or even increased sometimes under elevated CO2," adding that "combined with increased plant photosynthesis, these results suggested that to date progressive N limitation has not happened in our studied system."

In light of these and other findings discussed in the authors' paper, the eight scientists conclude that, "consistent with our hypothesis, elevated CO2 increased photosynthesis and microbial biomass, which accelerated soil N cycling and supplied additional N for plant growth," revealing that "progressive N limitation for plant growth has not happened in this oak dominated system after 10 years of elevated CO2 treatment."