Nitrogen fertilization affected microbial carbon use efficiency and microbial resource limitations via root exudates.

Root exudation and its mediated nutrient cycling process driven by nitrogen (N) fertilizer can stimulate the plant availability of various soil nutrients, which is essential for microbial nutrient acquisition. However, the response of soil microbial resource limitations to long-term N fertilizer application rates in greenhouse vegetable systems has rarely been investigated. Therefore, we selected a 15-year greenhouse vegetable system, and investigated how N fertilizer application amount impacts on root carbon and nitrogen exudation rates, microbial resource limitations and microbial carbon use efficiency (CUE _ST ). Four N treatments were determined: high (N3), medium (N2), low (N1), and a control without N fertilization (N0). Compared to the control (N0), the results showed that the root C exudation rates decreased significantly by 42.9 %, 57.3 % and 33.6 %, and the root N exudation rates decreased significantly by 29.7 %, 42.6 %, and 24.1 % under N1, N2, and N3 treatments, respectively. Interactions between fertilizer and plant roots altered microbial C, N, P limitations and CUE _ST ; Microbial C and N/P limitations were positively correlated with root C and N exudation rates, negatively correlated with microbial CUE _ST . Random Forest analysis revealed that the root C and N exudation rates were key factors for soil microbial resource limitations and microbial CUE _ST . Through the structural equation model (SEM) analysis, soil NH ₄ ⁺ content had significant direct effects on the root exudation rates after long-term N fertilizer application. An increase in root exudation rates led to enhanced microbial resource limitations in the rhizosphere soils, potentially due to increased competition. This enhancement may reduce microbial carbon use efficiency (CUE), that is, microbial C turnover, thereby reducing soil C sequestration. Overall, this study highlights the critical role of root exudation rates in microbial resource limitations and CUE changes in plant-soil systems, and further improves our understanding of plant-microbial interactions.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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