Comparative responses of a non-N-fixing shrub and an actinorhizal N-fixing shrub to N fertilization

Abstract

Variations in responses to soil N between a non-N-fixing shrub, Baccharis halimifolia L., and a N-fixing shrub, Morella cerifera (L.) Small, were tested over 12 weeks to determine whether N availability is the sole cause of persistent dominance of M. cerifera on barrier islands. Plants were supplied increasing levels of soil N up to 200 mg kg−1. Measurements included gas exchange and chlorophyll fluorescence parameters across treatments, species, and time. Tissues were analyzed for differences in biomass and nutrients. Baccharis halimifolia had reduced physiological responses across all treatment levels, but M. cerifera had comparatively few variations. Across all treatments B. halimifolia photosynthesis and stomatal conductance were reduced by 62 and 76 %, respectively,by week 12. Increasing foliar δ15N values across treatments for M. cerifera indicated a shift from utilizing fixed N to available soil N. Biomass was highest at 200 mg kg−1 N for both species. Baccharis halimifolia showed indications of stress response and resource limitation based on physiological responses, nutrient contents, and isotope effects. Baccharis halimifolia showed signs of co-limitation of both N and P whereas M. cerifera was limited by neither, suggesting that dominance of M. cerifera is only partially explained by actinorhizal symbiosis and N availability.