Do xerophytic and mesophytic perennial grasses differ in soil resource capture and allocation? A field ¹⁵N experiment

Abstract

In rangelands, pattern and processes below‐ground are important for a complete understanding of their ecology. Recent studies in grass‐dominated communities in Europe, North America and South America indicate that root systems of several species may overlap their root systems. This may indicate that, in these communities, resource acquisition and allocation strategies could play a relevant role in community assemblage. Resources absorption and allocation may be related to ecological strategies relevant in an arid steppe. Here, we explore if coexisting xerophytic and mesophytic perennial grasses differ in resource absorption and allocation to below‐ and above‐ground portions. We conducted a field labelling experiment to trace 15N (nitrogen) to compare acquisition and relative below‐ and above‐ground nitrogen allocation among four perennial grass species. We worked with two mesophytic (fast‐growing) and two xerophytic (slow‐growing) species. Our hypothesis was that mesophytic species will allocate greater proportions of N and produce a higher amount of biomass than xerophytic species. The four species showed similar 15N concentration, which changed over time in roots, while it remained constant in leaves. On average, all species presented 0.9% 15N atom excess in roots and 1.1% in green leaves. Mesophytic species showed higher reproductive over total tiller ratio than xerophytic species. Bromus pictus reached 60% while Poa ligularis only 20%. In xerophytic species (Pappostipa speciosa and Pappostipa humilis), this ratio was close to 0%. Despite their differences in growth rates, these mesophytic and xerophytic grass species do not differ in the estimated nitrogen acquisition. Nevertheless, there are ecological trade‐offs between allocating resources to above‐ground leaf biomass and reproductive tillers which could be related to species organisation in the community.

Arid community responses to nitrogen and carbon addition depend on dominant species traits and are decoupled between above‐ and below‐ground biomass

“…This is supported by a previous study, with the same four species at the same site, where it was observed that the 15 N absorbed by roots was rapidly allocated to the above-ground biomass (Reyes & Aguiar, 2021). Also, the four species included in our plots do not differ in resource acquisition rates (Reyes & Aguiar, 2021).…”

“…Our results suggest that in this Patagonian steppe belowground plant accumulation might be decoupled from the availability and uptake of soil resources (Kulmatiski et al, 2017;Dybzinski et al, 2019). This is supported by a previous study, with the same four species at the same site, where it was observed that the 15 N absorbed by roots was rapidly allocated to the above-ground biomass (Reyes & Aguiar, 2021). Also, the four species included in our plots do not differ in resource acquisition rates (Reyes & Aguiar, 2021).…”

“…Our study evaluated not only the total and specific above‐ground responses of a perennial‐grass‐dominated patches in an arid community but also its total and specific below‐ground biomass, an aspect that has been left out because of the methodological difficulties involved (Lauenroth, 2000; Wilson, 2014; Reyes & Aguiar, 2017a, 2021; López‐Mársico et al, 2020). The overrepresentation of pot experiments (Schmidt et al, 1997a, 1997b; Bleier & Jackson, 2007) compared to field experiments underscores the importance and strength of our plot experiment, performed in the field without manipulating the species present (including wild animals but excluding domestic sheep).…”