N addition affects the heterotrophic respiration of bulk and rhizosphere soils through changes in root traits and C-degrading functional genes in semi-arid grassland.

Background and aims: It is unclear how atmospheric nitrogen (N) deposition influences CO2 release from grassland by affecting heterotrophic respiration of bulk and rhizosphere soils and how the heterotrophic rhizosphere respiration of different grass species responds to N deposition. Methods: In this study, we investigated the effects of N addition on heterotrophic respiration of bulk and rhizosphere soils associated with the dominant species (Bothriochloa ischaemum) and subdominant species (Artemisia sacrorum and Stipa capillata) in the grassland treated with N fertilizer (0,3,9 g N m−2 y−1) for eight years. Results: Low-N addition significantly increased cumulative CO2-C emissions of bulk soil by 22.91% and rhizosphere soil of A. sacrorum by 72.26%, while high-N addition significantly increased that of rhizosphere soil of (A) sacrorum by 37.38% and S. capillata by 13.71%. There was no clear response to N addition for (B) ischaemum. Heterotrophic rhizosphere respiration of B. ischaemum was most strongly related to stable-C-degrading functional genes, whereas that of A. sacrorum and S. capillata was most strongly related to labile-C-degrading functional genes. C-degrading functional genes in rhizosphere soil were significantly related to root morphological characteristics, especially specific root length, specific root surface area, and root average diameter rather than root exudation rates. Conclusion: The present study revealed that N addition could affect heterotrophic soil respiration by changing labile-C and stable-C degrading functional genes, which in the rhizosphere of different plant species were regulated by their root functional traits. [ABSTRACT FROM AUTHOR]