Elevated CO2 and nitrogen interactively affect the rhizosphere priming effect of Cunninghamia lanceolata

Abstract

The rhizosphere priming effect (RPE), referring to the acceleration or retardation of soil organic matter (SOM) decomposition by plant roots, plays a vital role in influencing atmospheric CO2 concentration and regulating the feedback of terrestrial ecosystems to future global change. However, little is known about the interactive effect of elevated CO2 and nitrogen (N) addition on the RPE. Here, we investigated the RPE of Chinese fir (Cunninghamia lanceolata) seedlings under two concentrations of atmospheric CO2 (400 and 800 ppm) and two levels of N addition (0 and 100 kg N ha−1 yr−1) in a 543-day chamber experiment. Results showed that RPEs were significantly positive for 134 and 543 days after planting, with the magnitude ranging from 27% to 390%. Elevated CO2 had no effect on the RPE on day 134, but it significantly decreased the RPE on day 543, possibly because the competition for N between plants and microbes constrained microbial activities at the later stage of planting. Nitrogen addition significantly increased the RPE under ambient CO2 but did not affect the RPE under elevated CO2 on day 543. The interaction between elevated CO2 and N addition on the RPE on day 543 suggests that N addition could alleviate plant-microbe competition for N under ambient CO2 but not the stronger competition for N under elevated CO2. Moreover, positive relationships of the RPE with soil mineral N and microbial enzyme activities under different CO2 and N treatments were observed only on day 543. Such findings indicate that plant-microbe competition for N is the predominant mechanism of the RPE at the later stage of planting. Overall, this study provides empirical evidence for the interactive effect of elevated CO2 and N addition on the RPE, and it emphasizes the significance of the RPE for soil organic carbon decomposition and global carbon cycling.