Effects of nitrogen addition on root respiration of trees and understory herbs at different temperatures in Pinus tabulaeformis forest

Aims Nitrogen (N) addition had differential effects on root respiration. However, the reasons and mechanism have not yet been analyzed. We speculated that the differential effects of N addition are related to the N addition rate, root diameter, and temperature. Meanwhile, tree and understory herb in forest ecosystems maybe another reason that root respiration has different responses. Methods N addition was performed in a Pinus tabulaeformis forest for six years. P. tabulaeformis and Carex lanceolata fine roots with three size classes (< 0.5 mm; 0.5–1.0 mm; 1.0–2.0 mm) were sampled for respiration and temperature sensitivity measurement at three temperatures and chemical analysis. Results (1) N addition increased the root respiration of P. tabulaeformis and had the maximum values in 3, 6, and 9 g N m −2 y −1 at 1 °C, 14 °C, and 18 °C, respectively. (2) The root respiration of C. lanceolata significantly decreased under N addition only at 18 °C. (3) The effect of N addition on temperature sensitivity of root respiration was also varied due to N addition rate. (4) N and carbon/nitrogen ratio were the main driving factors of root respiration of P. tabulaeformis , while chemical properties had a slight driving effect on root respiration of C. lanceolata . Conclusions N addition rate and temperature were the reasons for the differential effect of N addition on root respiration, but not root diameter. Furthermore, root respirations of tree and understory herb have varied changing mechanism, which demonstrated an apparent species specificity.