Contrasting priming effects in soils subjected to long-term urea versus inorganic nitrogen addition

Priming effects play an important role in controlling soil organic matter (SOM) decomposition and global carbon (C) cycle. Although numerous studies have investigated the effect of inorganic nitrogen (N) addition on priming effects, large uncertainties remain regarding whether different N forms (e.g., organic versus inorganic) have divergent impacts. To examine how long-term N addition with different N forms and application rates alters soil properties and thereby influences the priming effects, 13 C-labelled glucose was added to soils from a semi-arid grassland that had received inorganic- (ammonium nitrate and ammonium sulfate) or urea-N of varying rates (0, 5, and 50 g N m –2 yr –1 ) for five years. Our results showed that effect of N application history on priming depends on application rates. Only the high-rate (50 g N m –2 yr –1 ) inorganic N addition significantly decreased the cumulative priming effect, likely due to elevated N availability and soil acidification. In contrast, high-rate urea amendment exerted no significant effect on the cumulative amount of primed C. Across all treatments, the cumulative priming effect was negatively correlated with soil total N concentration, but positively correlated with pH and soil C:N. Importantly, net N mineralization rates were positively correlated with the magnitude of priming, reinforcing the role of N availability in regulating the priming effect. We conclude that different forms and rates of long-term N addition result in distinct soil properties (e.g., soil C:N, N availability, and soil acidity) that ultimately regulate the susceptibility of SOM to priming. Our findings underscore that predicting soil C cycle in scenarios of increased N deposition requires accounting for form-specific alterations in soil properties. • Soils subjected to high-rate inorganic N addition exhibited a significantly decreased cumulative priming effect. • Soils with a history of high-rate urea addition exhibited no change in cumulative priming. • The priming effect was negatively correlated with soil N concentration, but positively correlated with pH and soil C:N.