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

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, 0.51, 12, and 210 mm) at three soil depths (010, 1030, and 3050 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%.