The article by Legay et al. [1] addresses two main issues: the links between belowground and aboveground plant traits and the links between plant strategies (as defined by these traits) and the capacity to absorb nitrate and ammonium. I recommend this work because these are important and current issues. The literature on plant traits is extremely rich and the existence of a leaf economic spectrum linked to a gradient between conservative and acquisitive plants is now extremely well established [2-3]. Many teams are now working on belowground traits and possible links with the aboveground gradients [4-5]. It seems indeed that there is a root economic spectrum but this spectrum is apparently less pronounced than the leaf economic spectrum. The existence of links between the two spectrums are still controversial and are likely not universal as suggested by discrepant results and after all a plant could have a conservative strategy aboveground and an acquisitive strategy belowground (or vice-versa) because, indeed, constraints are different belowground and aboveground (for example because in given ecosystem/vegetation type light may be abundant but not water or mineral nutrients). The various results obtained also suggest that we do not full understand the diversity of belowground strategies, what is at stake with these strategies, and the links with root characteristics.
Each time I give a conference on the work we are carrying out on African grasses that likely absorb ammonium preferentially because they inhibit nitrification [6-7], somebody asks me a question about the fact that plant essentially absorb nitrate because ammonium is toxic and nitrate more available in the soil. The present article confirms that this is not the case and that, though there are currently some teams working on the subject, we do not really know for the moment whether plants absorb nitrate or ammonium, in which proportion, how plastic this proportion is within individuals and within species. This subject seems to me crucial because it is linked to (1) the capacity of ecosystems to conserve nitrogen [8], because nitrate, much more than ammonium, goes out of ecosystems through leaching and denitrification, (2) to carbon cycling and plant energy budget because absorbing nitrate requires spending mucho more energy than absorbing ammonium because nitrate must be reduced before being incorporated in plant biomass, which is very energy costly. These two issues are naturally very relevant to develop efficient cropping systems in terms of carbon and nitrogen.
Interestingly, the present article, comparing three grass species in different sites, suggests that there is no trade-off between the absorption of nitrate and ammonium: more acquisitive individuals tend to absorb more ammonium and nitrate. This is contrary to hypotheses we made to predict the outcome of competition between plants absorbing nitrate and ammonium in different proportions [9] but should be tested in the future comparing many different types of plants. The results also suggest that more conservative plants absorb relatively more ammonium, which makes sense because this allows them to spare the energy necessary to reduce nitrate. This leads to the question of the effect of these strategies on nitrogen retention within the ecosystem. If nitrification is high (low), absorbing ammonium is not efficient and likely leads to high (low) nitrogen losses. This should be tested in the future. Moreover, the authors have measured the absorption of nitrate and ammonium through measurements at the root scale on cut roots. This should be complemented by measurements at the whole plant scale.

References

[1] Legay, N., Grassein, F., Arnoldi, C., Segura, R., Laîné, P., Lavorel, S. and Clément, J.-C. (2020). Studies of NH4+ and NO3- uptake ability of subalpine plants and resource-use strategy identified by their functional traits. bioRxiv, 372235, ver. 4 peer-reviewed and recommended by PCI Ecology. doi: 10.1101/372235
[2] Shipley, B., Lechowicz, M.J., Wright, I. & Reich, P.B. (2006) Fundamental trade-offs generating the worldwide leaf economics spectrum. Ecology, 87, 535-541. doi: 10.1890/05-1051
[3] Reich, P.B. (2014) The world-wide ‘fast-slow’ plant economics spectrum: a traits manifesto. J. Ecol., 102, 275-301. doi: 10.1111/1365-2745.12211
[4] Maire, V., Gross, N., Pontes, L.D.S., Picon-Cochard, C. & Soussana, J.F. (2009) Trade-off between root nitrogen acquisition and shoot nitrogen utilization across 13 co-occurring pasture grass species. Func. Ecol., 23, 668-679. doi: 10.1111/j.1365-2435.2009.01557.x
[5] Roumet, C., Birouste, M., Picon-Cochard, C., Ghestem, M., Osman, N., Vrignon-Brenas, S., Cao, K.F. & Stokes, A. (2016) Root structure-function relationships in 74 species: evidence of a root economics spectrum related to carbon economy. New. Phytol., 210, 815-826. doi: 10.1111/nph.13828
[6] Lata, J.-C., Degrange, V., Raynaud, X., Maron, P.-A., Lensi, R. & Abbadie, L. (2004) Grass populations control nitrification in savanna soils. Funct. Ecol., 18, 605-611. doi: 10.1111/j.0269-8463.2004.00880.x
[7] Srikanthasamy, T., Leloup, J., N’Dri, A.B., Barot, S., Gervaix, J., Koné, A.W., Koffi, K.F., Le Roux, X., Raynaud, X. & Lata, J.-C. (2018) Contrasting effects of grasses and trees on microbial N-cycling in an African humid savanna. Soil Biol. Biochem., 117, 153-163. doi: 10.1016/j.soilbio.2017.11.016
[8] Boudsocq, S., Lata, J.C., Mathieu, J., Abbadie, L. & Barot, S. (2009) Modelling approach to analyze the effects of nitrification inhibition on primary production. Func. Ecol., 23, 220-230. doi: 10.1111/j.1365-2435.2008.01476.x
[9] Boudsocq, S., Niboyet, A., Lata, J.-C., Raynaud, X., Loeuille, N., Mathieu, J., Blouin, M., Abbadie, L. & Barot, S. (2012) Plant preference for ammonium versus nitrate: a neglected determinant of ecosystem functioning? Am. Nat., 180, 60-69. doi: 10.1086/665997