Document: CHR-3-33-8

Differences in nitrogen uptake modified by microbiotic soil crusts and AM fungi for herbs in a xeric Florida shrubland.

HAWKES, C.V.*

University of Pennsylvania, Philadelphia, PA, USA ¹

Abstract:
Microbiotic soil crusts are widespread and known to affect nutrient cycling in arid regions of the world. How crusts interact with vascular plants is less well understood. The primary source of N for herbs in a xeric Florida shrubland was hypothesized to be microbiotic soil crusts that are dominated by cyanobacteria and algae. In order to test this, total nitrogen and delta ¹⁵N were measured in plant tissue, soil crusts, and soils. Delta ¹⁵N of herbs was significantly different for species associated with arbuscular-mycorrhizal (AM) fungi and those that were not. Comparison of isotopic signatures suggests that the two AM species, Eryngium cuneifolium and Hypericum cumulicola, utilize crusts as a primary source of nitrogen whereas the two non-mycorrhizal species, Polygonella basiramia and Paronychia chartacea, do not and may actually compete with crust organisms for limited N. Similar delta ¹⁵N values for Hypericum and Eryngium imply no fractionation differences by the AM fungi. However, Hypericum total N content was significantly lower and mycorrhizal infection rates were higher, suggesting that total transfer of N from mycorrhizae to the plant may be lower in Hypericum. Among all four species, total N was greatest for Paronychia and Eryngium yet they were found in microsites of different N levels: Paronychia was found in the most nitrogen-rich, and Eryngium in the most nitrogen-poor soils. In contrast, Hypericum and Polygonella had similar nitrogen tissue contents and occurred in similar microsites with intermediate levels of nitrogen, but have very different delta ¹⁵N values possibly due to use of crust vs. soil as a source of nitrogen, or differential uptake and/or loss of the heavy isotope.

Keywords: nitrogen, soil crusts, mycorrhizae, herbs, isotopes

This abstract is being presented at: 9:15 AM in session:
Oral Session #23: Soil Ecology.