Seasonally dry Mediterranean ecosystems present unique challenges for all perennial plants. Specifically, perennial legumes need to cope with the influence of a prolonged rainless season on their symbiotic N2 fixation (first evolved in the Mediterranean Basin prior to the formation of the dry-seasonal climate). Furthermore, the chronicle of symbiotic fixation in Mediterranean perennial legumes was shaped by a long history of livestock grazing and human disturbances, leaving a guild of heavily protected, disturbance-responsive species with a strong seasonal behavior (spiny brooms). We therefore evaluate whether N2 fixation is regulated in Calicotome villosa, the most abundant native perennial legume in the Mediterranean area of Israel, and what are the climatic, microbial, and biogeochemical factors that influence the strategy of fixation. We integrated field observation and controlled experiments to examine: (1) the seasonal pattern of fixation, (2) fixation strategy – i.e., the influence of nitrogen availability on rate of fixation, and (3) the effects of C. villosa on the local ecosystem nitrogen cycle.
In the three field sites C. villosa fixation changed between high fixation rates during the rainy season, at flowering and seed set times, to almost none in the rainless summer. Under controlled soil resource conditions saplings of native perennial legume species differed in fixation intensity, but without exception, all downregulated fixation in response to soil nitrogen availability (facultative strategy). This differs from native annual species, which presented a range of fixation strategies. Regulation of fixation involved reduction in both nodulation and fixation rate within nodules, in most cases simultaneously. Furthermore, we found no effect of phosphorus availability on N2 fixation in saplings of perennial species. Finally, we identified a significant influence of C. villosa plants on soil nitrogen content, in all our field sites. However, increased soil nitrogen cannot be attributed to fixation only. We conclude that the high physiological cost of fixation imposes the need to regulate fixation in perennial legumes in seasonal dry Mediterranean conditions. Regulation of N2 fixation and nitrogen conservation in C. villosa and other spiny brooms influence soil nitrogen cycling. Further research is evaluating how these plant-soil interactions affect the local niche and regional distribution of dominant perennial legume fixers in dry ecosystems.