Spatial variability in the potential for symbiotic N2 fixation by woody plants in a subtropical savanna ecosystem.
Root infection by symbiotic N2-fixing Frankia and Rhizobium strains was quantified in relation to light and soil properties for seedlings of 12 woody species (7 Fabaceae, 4 Rhamnaceae, one Ulmaceae) from a subtropical savanna in southern Texas, USA. None of 4 rhamnaceous species (Colubrina texensis, Condalia hookeri, Karwinskia humboldtiana, Ziziphus obtusifolia) nodulated, despite the fact that bioassays with a known actinorhizal species yielded 13 nodules per seedling. Celtis pallida (Ulmaceae), Acacia greggii and A. berlandieri (Fabaceae) also failed to nodulate even though field populations of these species were characterized by high (2.7-4.2%) foliar nitrogen concentration. The other 5 leguminous species tested (Prosopis glandulosa, the dominant woody species in this subtropical savanna and throughout the SW USA, A. farnesiana, A. rigidula, A. schaffneri, Eysenhardtia texana) were nodulated. 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 suggested that all nodules were capable of N2-fixation. The extent of nodulation varied by species. Nodulated seedlings were taller, produced more biomass and allocated less biomass to root systems than their non-nodulated counterparts. Numbers of nodules on seedlings of P. glandulosa, were reduced by low light (15% full sunlight) regardless of soil N level; at medium and full sunlight nodule biomass expressed as a fraction of whole plant biomass decreased with increasing soil N. Nodulation of field-grown P. glandulosa appeared to be ephemeral, apparently varying with changes in soil moisture. Nodulation and N2 fixation among woody legumes occurred across a broad range of soil conditions and depths with significant impacts on local and regional N-cycles. Field levels of foliar N in species that failed to nodulate in the laboratory were comparable to or greater than those in species capable of nodulation, suggesting that leaf N is not a reliable indicator of N2 fixation.