Atmospheric nitrogen deposition and climatic warming both can potentially increase the amount of nitrogen available to plants in the soil. It is often assumed that increased nitrogen concentration in response to such factors will result in higher specific respiration of roots of woody plants. The objective of this study is to determine the effects of chronic N additions on root N concentrations and respiration rates in northern hardwood forests. Four study sites were located along a 500 km climatic gradient from southern Michigan to northern Michigan in a northern hardwood forest. Three of the six 30 m x 30 m plots at each study site have received N amendments (30 kg/ha/y) 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 nitrogen concentration.
Results/Conclusions
Root N and respiration were greater for smaller diameter roots and roots at shallow depths. Root nitrogen concentrations have increased significantly in response to the chronic N addition treatment, particularly for roots of the larger size classes. This is in contrast to measurements made in the early years of the study. Specific respiration rates, however, did not increase with the addition of nitrogen, resulting in lower respiration per unit N for the 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.
