Nitrogen fixation makes biomass allocation to roots independent of soil nitrogen supply

Abstract

Biomass allocation patterns in plants are known to be affected by soil nitrogen availability. Since nitrogen availability can depress symbiotic nitrogen fixation, and nitrogen fixation can make plant growth independent of soil nitrogen availability but is energetically costly, it is unclear how allocation patterns in nitrogen-fixing species should respond to variation in soil nitrogen availability. We examined the effect of nitrogen source and concentration on the growth and allocation patterns in the nitrogen-fixing shrub Alnus viridis subsp. crispa (Aiton) Turrill. Plants were grown with either NH4+-N or NO3–-N at a range of low N concentrations, from 0 to 2 mmol·L–1, and either inoculated with Frankia or not. Plants without nodules had 25.l% lower biomass and had double the allocation to roots at all but the 2 mmol·L–1 nitrogen concentration. Even though nodulated plants increased growth with nitrogen concentration, allocation to roots as a fraction of total biomass did not vary in these plants, suggesting increased growth resulted from more efficient nitrogen acquisition. Allocation to roots was a significant predictor of plant growth in non-nodulated plants (r2 = 0.318, for linear least squares fit with log mass) but not for nodulated plants (r2 = 0.108). As nitrogen concentrations increased, allocation to nodules, specific nodule numbers, and the proportion of nitrogen fixed by the plants decreased, demonstrating a shift to soil nitrogen use.

Variation in moss-associated nitrogen fixation in boreal forest stands

“…On the other hand, high rates of inorganic nitrogen application have been found to decrease moss-associated nitrogen fixation (Zackrisson et al 2009). This phenomenon has been well documented for vascular plant symbiotic nitrogen fixation (Markham and Zekveld 2007). The lack of a canopy openness effect is also consistent with previous studies.…”

Field performance of alder-Frankia symbionts for the reclamation of oil sands sites

“…Since there was no detectable total nitrogen in the soil, nitrogen acquisition is most likely due to atmospheric nitrogen fixation by Frankia sp. (Markham and Zekveld, 2007). Both inoculated and non-inoculated alders had Frankia-nodules where nitrogen fixation could have occurred.…”

Do plants need nitrate? The mechanisms by which nitrogen form affects plants

“…Two studies since Andrews et al . () have argued that S : R is dependent on N form (Markham & Zekveld, ; Cambui et al ., ) but neither tested the relationship between S : R and leaf soluble protein concentration. It is emphasised that in other studies, S : R was strongly correlated with leaf soluble protein concentration across N forms (Andrews et al ., ).…”