Does soil nitrogen influence growth, water transport and survival of snow gum (Eucalyptus pauciflora Sieber ex Sprengel.) under CO2 enrichment?

Abstract

Eucalyptus pauciflora Sieber ex Sprengel. (snow gum) was grown under ambient (370 L-1) and elevated (700 L-1) atmospheric CO2 in open-top chambers (OTCs) in the field and temperature- controlled glasshouses. Nitrogen applications to the soil ranged from 0.1 to 2.75 g N per plant. Trees in the field at high N levels grew rapidly during summer, particularly in CO2-enriched atmosphere, but suffered high mortality during summer heatwaves. Generally, wider and more numerous secondary xylem vessels at the root-shoot junction in CO2-enriched trees conferred fourfold higher below-ground hydraulic conductance. Enhanced hydraulic capacity was typical of plants at elevated CO2 (in which root and shoot growth was accelerated), but did not result from high N supply. However, because high rates of N application consistently made trees prone to dehydration during heatwaves, glasshouse studies were required to identify the effect of N nutrition on root development and hydraulics. While the effects of elevated CO2 were again predominantly on hydraulic conductivity, N nutrition acted specifically by constraining deep root penetration into soil. Specifically, 15-40% shallower root systems supported marginally larger shoot canopies. Independent changes to hydraulics and root penetration have implications for survival of fertilized trees under elevated atmospheric CO 2, particularly during water stress.