Nitrogen assimilation pathways in native and exotic plant species

Background/Question/Methods

Coastal sage scrub (CSS) is endemic to Southern California and habitat for many rare taxa. Remaining CSS is often highly fragmented and subject to invasion and anthropogenic disturbance. CSS ecosystems are typically nutrient and water-limited, dominated by perennial woody shrubs, with blooms of annuals which are often dominated by exotic species. The exotic annuals become active earlier than the native shrubs, and may be less limited in their use different forms of nitrogen, providing a competitive advantage.

To examine species differences in nitrogen use, we harvested living plant tissue from representatives of the dominant native and exotic plant communities and analyzed their ability to actively process different forms of nitrogen. Natives tested were Artemisia californica, Eriogonum fasciculatum, and Salvia apiana. Exotics tested were Brassica nigra, Bromus madritensis ssp. rubens, and Centaurea melitensis.

Fresh root samples of all species were analyzed for direct uptake of ¹⁴C-labeled glycine, an index of potential direct uptake of dissolved organic nitrogen. Root samples of all species except C. melitensis were analyzed for glutamine synthetase activity, an indicator of potential ammonia assimilation. Root and leaf tissues of the same five species were analyzed for nitrate reductase activity, an indicator of potential nitrate assimilation.

Results/Conclusions

¹⁴C-glycine uptake was greatest in C. melitensis (2.20±0.58 (mean±SE) μmol·g^-1·h^-1); B. madritensis (1.31±0.38) and E. fasciculatum (0.74±0.43) were intermediate; A. californica, S. apiana and B. nigra were lowest and not statistically different (0.33±0.22).

Glutamine synthetase activity levels were highest in B. madritensis (3.95±0.55 absorbance units·g^-1·h^-1), B. nigra was intermediate (2.69±0.31), and A. californica, E. fasciculatum and S. apiana were lowest and not statistically different (1.46±0.28).

Nitrate reductase activity levels were dominated by zero values, but median levels showed species differences. S. apiana had the highest levels (median 0.94 μmol·g^-1·hr^-1), followed by B. madritensis (0.47), E. fasciculatum (0.19), B. nigra (0.00), and A. californica (0.00).

Two of the three exotic species showed the ability to rapidly assimilate one or more forms of nitrogen. B. madritensis had intermediate to high activity levels in all three pathways. Surprisingly, B. nigra had low to intermediate activity levels in all three pathways. S. apiana showed evidence of nitrate specialization, but all other native pathways had low activity levels. This overall pattern may enable the exotics to better exploit both ambient and pulse levels of nutrients. This would provide a competitive advantage over and decrease available nutrients for native shrubs, and over time may alter ecosystem-level nitrogen cycles.