Long-term NO3- additions alter root respiration:tissue N relationships in northern hardwood forests but not ecosystem root respiration

Background/Question/Methods

Specific root respiration rates typically increase with increasing tissue N concentration. As a result, it is often assumed that external factors that have the potential to increase root N concentration, such as chronic N deposition, may lead to increased respiration rates. However, enhanced N availability also can lead to reductions in root biomass, so the ecosystem level consequences on root system respiration are uncertain. The objective of this study was to determine the effects of long-term N additions on root N concentrations, respiration rates and biomass in four northern hardwood study sites located along a 500 km climatic gradient in Michigan. At each site, three of six measurement plots have received N amendments (3 g NO₃^--N m^-2 y^-1) since 1994. In August 2009, we measured specific root respiration rates for roots in four size classes ( <0.5 mm, 0.5-1 mm, 1-2 mm, 2-10 mm ) from  three soil depths (0-10 cm, 10-30 cm, 30-50 cm). The samples were subsequently analyzed for N concentration. Root biomass data for the same size classes and soil depths was used in combination with the specific respiration rates to assess the response of ecosystem root respiration to the chronic N additions.

Results/Conclusions

Root N and respiration were greater for smaller diameter roots and roots at shallow depths. In addition, root N concentrations were significantly greater for the chronic N addition treatment, particularly for roots of the larger size classes. This is in contrast to measurements made in the earlier years of the experiment. Specific respiration rates and root biomass both were unchanged for all depths and size classes. As a result, ecosystem root respiration was not altered by the chronic N additions. The increase in root N concentrations with unchanged specific respiration rates resulted in lower respiration per unit N for the chronic N addition treatment. These results indicate that linear relationships between root respiration and N concentration for an ecosystem do not hold if N availability is significantly altered. For these study sites, the respiration:N relationship for the control treatment would result in a 39% over-prediction of ecosystem root respiration for the chronic N treatment.