Linking plant traits to rhizosphere priming effects across six grassland species with and without nitrogen fertilization
The rhizosphere priming effect (RPE), i.e., the acceleration of soil organic matter decomposition (SOM) by plant roots, plays a key role in the soil carbon (C) cycle and its feedback to climate change. However, how plant traits regulate the RPE has rarely been investigated. Here we selected six grassland species with and without nitrogen (N) fertilization to examine the relationships between the RPE and plant traits, and also the interactive effect of plant traits and N fertilization on the RPE. All six species showed positive RPEs, ranging from 83% to 297% above the unplanted control. Among plant traits, specific rhizosphere respiration was the best predictor for the interspecific variation in the RPE, with the higher RPE related to higher specific rhizosphere respiration. The RPE was also positively linked to root N concentration across species. In contrast, leaf traits and root morphological or architectural traits could not explain the interspecific variation in the RPE. Nitrogen fertilization differentially affected the RPEs of six species through its interaction with plant traits and the effect on soil processes. Nitrogen fertilization increased leaf N concentration and specific rhizosphere respiration, causing higher photosynthesis and possibly root exudation. On the other hand, N fertilization increased mineral N, and decreased soil pH, β-glucosidase and β-cellobiohydrolase, causing lower microbial activities. The effect of N fertilization on the RPE was thus attributed to the balance between the changes in microbial activities and in plant traits. Overall, our results demonstrate that the variation of RPE across six species could be mainly explained by plant biomass and traits. It is promising that plant traits have important indicative functions in predicting the SOM decomposition through plant-soil interactions.