How does management legacy, nitrogen addition, and nitrification inhibition affect soil organic matter priming and...

Long-term management of croplands influences the fluxes and sources of nitrous oxide (N₂ O). We examined this premise in a greenhouse study by using soils collected from a 38-yr-old field experiment. The sampled treatments were continuous barley (Hordeum vulgare L.; CB), continuous fescue (Festuca rubra L., F. arundinacea Schreb; CF), and two phases of an 8-yr rotation: faba bean (Vicia faba L.; FB) and alfalfa (Medicago sativa L.)-bromegrass (Bromus inermis Leyss) hay. Barley was grown as a test crop in the greenhouse in each soil. The ranking of N₂ O emissions was hay > FB > CB > CF (P < .001). We quantified the ¹⁵ N-site preference to assess the N₂ O-producing processes. Denitrification was the predominant source, contributing 77.4% of the N₂ O production. We also evaluated nitrogen (N) additions: urea alone or urea with a nitrification inhibitor (nitrapyrin or DMPSA). Compared with urea alone, nitrapyrin and DMPSA reduced N₂ O emissions by 16 and 25%, respectively. We used urea labeled with ¹⁵ N to trace N to N₂ O emissions, aboveground plant N uptake, and N retention by soils. Total ¹⁵ N-recovery (N₂ O + plant + soil) was highest under FB (86%) and lowest under CB (29%). We further separated the N₂ O derived from urea versus N₂ O from soil organic matter (SOM). The inhibitor DMPSA reduced the N₂ O derived specifically from added urea-N by more than half (P < .001). With the addition of urea, N₂ O production from mineralization of SOM-N accelerated over the control (without urea), termed the priming effect. This priming of SOM-N contributed with 13% of the total N₂ O production when averaged across the four management legacies. The CB soil had the highest proportion of priming-derived N₂ O (24%). Management legacies clearly differed in soil carbon and N, which governed N₂ O production from denitrification and SOM priming.