Effects of nitrogen fertilization on the rhizosphere priming

It is known that nitrogen (N) input modulates the rhizosphere priming effect (RPE); however, the magnitude and driving mechanisms of priming under increasing rates of fertilizer application remain unclear. 15N-urea (control, 75 (N75), 150 (N150), 225 (N225), and 300 (N300) kg N ha−1) was applied to a plant (maize)-soil (rice paddy) system and the RPE was monitored during the trumpet period (the most active stage) of plant growth. Addition of N decreased soil-derived CO2 emission by 21.1–49.3% in the presence of plants. The RPE declined following N input in the control−N150 as a result of low microbial C:N imbalance, which decreased enzyme activities due to low microbial N mining and microbial activation, and high microbial metabolic efficiency (MME). In contrast, the RPE increased following N input in the N150 − N300, which was attributed to the high microbial C:N imbalance causing low MME, rather than the promotion of microbial N mining or microbial activation mechanisms. The microbial C:N imbalance was the result of N competition between microorganisms and plants. Thus, the combination of enzyme activities and MME mediates N-regulated RPE, while N competition drives the switching of RPE mechanisms.