Chronic N deposition alters root respiration-tissue N relationship in northern hardwood forests

Abstract

Specific root respiration rates typically increase with increasing tissue N concentration. As a result, it is often assumed that external factors inducing greater root N concentration, such as chronic N deposition, will lead to increased respiration rates. However, enhanced N availability also alters root biomass, making the ecosystem-level consequences on whole-root-system respiration uncertain. The objective of this study was to determine the effects of chronic experimental N deposition on root N concentrations, specific respiration rates, and biomass for four northern hardwood forests in Michigan. Three of the six measurement plots at each location have received experimental N deposition (3 g NO3 --N m -2 yr -1) since 1994. We measured specific root respiration rates and N concentrations of roots from four size classes (<0.5, 0.5-1, 1-2, and 2-10 mm) at three soil depths (0-10, 10-30, and 30-50 cm). Root biomass data for the same size classes and soil depths was used in combination with specific respiration rates to assess the response of whole-root-system respiration. Root N and respiration rate were greater for smaller diameter roots and roots at shallow depths. In addition, root N concentrations were significantly greater under chronic N deposition, particularly for larger diameter roots. Specific respiration rates and root biomass were unchanged for all depths and size classes, thus whole-root-system respiration was not altered by chronic N deposition. Higher root N concentrations in combination with equivalent specific respiration rates under experimental N deposition resulted in a lower ratio of respiration to tissue N. These results indicate that relationships between root respiration rate and N concentration do not hold if N availability is altered significantly. For these forests, use of the ambient respiration to N relationship would over-predict actual root system respiration for the chronic N deposition treatment by 50%. © 2011 Blackwell Publishing Ltd.