Rhizosphere priming of grassland species under different water and nitrogen conditions: a mechanistic hypothesis of C-N interactions

Background and aims The rhizosphere priming effect (RPE) plays an important role in the decomposition of soil organic matter (SOM) and the global carbon cycle. Although plant, soil, and environmental factors affecting the RPE have been examined, it is still unclear how soil water and nitrogen (N) status together alter the RPE. Methods In order to examine plant, water, and N interacting effects on the RPE, we conducted a semi-natural experiment with two plant species (Leymus chinnensis and Medicago sativa) grown at two soil moisture levels (45 and 85% of water holding capacity) with or without N fertilization (0 and 10 g N m(-2) y(-1)). We used a natural C-13 abundance tracer method for measuring the RPE, and an N budgeting method for measuring net N mineralization or immobilization. Results Both positive (up to 253%) and negative (down to -21%) RPEs were observed in different treatment combinations. L. chinensis exhibited greater RPE than M. sativa under low soil moisture, possibly due to specific root architecture and root activity of L. chinensis. High soil moisture significantly reduced the RPE, while N fertilization significantly increased the RPE. To reconcile our current results and previous observations with existing individual hypotheses related to the RPE, we put forward a new hypothesis: when N mineralization dominates, the rhizosphere input enhances the RPE; when N immobilization dominates, the rhizosphere input reduces the RPE. Conclusion Overall, this study demonstrates that the RPE on soil organic carbon decomposition is intimately linked with N processes and soil water status.