The effects of warming and nitrogen addition on fine root exudation rates in a young Chinese-fir stand

Despite the perceived importance of root exudates in forest ecosystem function, few studies have simultaneously examined the effects of elevated temperature and nutrient availability on root exudates, especially in the tropical and subtropical regions. This limits our ability to predict belowground C allocation and nutrient cycling in response to global change at a global scale. In this study, we used a complete randomized block design with factorial soil warming (ambient, ambient + 4 ℃) and nitrogen (N) addition (ambient, ambient + 40 Kg N ha⁻¹ yr⁻¹) to examine their effects on in situ fine root exudates and sapling growth in a young Chinese fir (Cunninghamia lanceolata) stand. We found that soil warming and N addition each had negative effects on root exudation rates. Moreover, there was a negative interactive effect of soil warming and N addition on fine root exudation rates (i.e., further reduction), likely due to altered fine root morphological and chemical properties, soil characteristics and belowground C allocation. Root exudation rates negatively related to soil inorganic N concentrations, but positively related to fine root diameter, specific root length, N concentration and non-structural carbohydrate concentration. Reducing root exudation rates may be a physiological adjustment of the Chinese fir stand to high soil nutrient availability associated with warming and N addition. Collectively, the results indicate that the effects of warming on root exudation rates are dependent on soil fertility and moisture. Reduced exudation rates under warming plus N addition may decrease the flux of labile C from the roots to the soil suggesting that N deposition may mitigate warming-enhanced SOM decomposition. These findings provide new insights into belowground C dynamics and root-soil interactions in response to soil warming and N addition.