Nitrogen addition decreases the soil cumulative priming effect and favours soil net carbon gains in Robinia pseudoacacia plantation soil

Carbon (C) and nutrient inputs regulate the soil carbon balance by affecting microbial growth and the priming effect (PE). However, we still lack a comprehensive understanding of the microbial mechanism of the soil carbon balance. To examine the regulatory effect of C or nutrient deficiency on microbial C turnover in forest systems of different ages, we collected soil from two Robinia pseudoacacia stands of different ages and conducted an incu-bation experiment using 13C-labelled glucose combined with nitrogen and/or phosphorus (N and/or P) addition. We determined 13C partitioning in CO2 and phospholipid fatty acids (PLFAs). Glucose and nutrient addition caused both positive and negative PEs during the 30-day incubation. The PE of the 15-year-old (15Y) stand changed from positive (0-15 days) to negative (15-30 days), while the PE of the 45-year-old (45Y) stand ranged from negative (0.5 days) to positive (after 1 day). Different microbial mechanisms play diverse roles during various decomposition stages. The 45Y stand led to a higher cumulative positive PE than the 15Y stand. For both stands, fungi assimilated a greater proportion of 13C-glucose in the glucose plus nitrogen (CN addition) treatment than in the glucose addition alone treatment, which alleviated the fungal N requirements and decreased the cumulative PE. However, the glucose plus phosphorous treatment (CP addition) had no significant influence on the cumulative PE. CN addition favoured high-yielding microbial strategists (Y-strategists) and glucose-derived SOC accumulation. This phenomenon was supported by the relatively high microbial carbon use efficiency (CUE), which increased the soil net C balance. In contrast, CP addition was conducive to resource-acquisition strategists (A-strategists), which increased soil organic carbon mineralization and decreased the soil net C bal-ance. This was evidenced by higher 13C incorporation into the PLFAs of gram-positive bacteria and the positive PE for C and nutrient acquisition under CP addition. Consequently, CN addition decreased the cumulative PE and increased 13C-fungi and CUE, which favoured the soil net C gain. Our study highlights the importance of N fertilization for soil C sequestration in Robinia pseudoacacia plantations.