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.