Sf. Zitzer et al., SPATIAL VARIABILITY IN THE POTENTIAL FOR SYMBIOTIC N-2 FIXATION BY WOODY-PLANTS IN A SUBTROPICAL SAVANNA ECOSYSTEM, Journal of Applied Ecology, 33(5), 1996, pp. 1125-1136
1. Root infection by symbiotic N-2-fixing Frankia and Rhizobium strain
s was quantified in relation to light and soil properties for seedling
s of 12 woody species from a subtropical savanna in southern Texas, US
A. 2. None of four rhamnaceous species nodulated, despite the fact tha
t bioassays with a known actinorhizal species yielded 13 nodules per s
eedling. Celtis pallida (Ulmaceae), Acacia greggii and Acacia berlandi
eri (Leguminosae) also failed to nodulate even though field population
s of these species were characterized by high (2.7-4.2%) foliar nitrog
en concentration. 3. Infective rhizobia occurred in all soils studied
regardless of soil depth, distance from a host plant or type of plant
cover. Plant growth in N-free media and acetylene reduction activity s
uggested that all nodules were capable of N-2-fixation. 4. The extent
of nodulation varied by species. However, nodulated seedlings were tal
ler, produced more biomass and allocated less biomass to root systems
than their non-nodulated counterparts. 5. Numbers of nodules on seedli
ngs of Prosopis glandulosa, the dominant woody species in this subtrop
ical savanna and throughout the south-western USA, were reduced by low
light (15% full sunlight) regardless of soil N level; at medium and f
ull sunlight nodule biomass expressed as a fraction of whole plant bio
mass decreased with increasing soil N. Nodulation of held-grown P. gla
ndulosa appears to be ephemeral, apparently varying with changes in so
il moisture. 6. Nodulation and N-2 fixation among woody legumes in sub
tropical savannas can occur across a broad range of soil conditions an
d depths with significant impacts on local and regional N-cycles. 7. F
ield levels of foliar N in species that failed to nodulate in the labo
ratory were comparable to or greater than those in species capable of
nodulation, suggesting that leaf N is not a reliable indicator of N-2
fixation.