How does management legacy, nitrogen addition, and nitrification inhibition affect soil organic matter priming and nitrous oxide production?

Long‐term management of croplands influences the fluxes and sources of nitrous oxide (N 2 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 N 2 O emissions was hay > FB > CB > CF ( P  < .001). We quantified the 15 N‐site preference to assess the N 2 O‐producing processes. Denitrification was the predominant source, contributing 77.4% of the N 2 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 N 2 O emissions by 16 and 25%, respectively. We used urea labeled with 15 N to trace N to N 2 O emissions, aboveground plant N uptake, and N retention by soils. Total 15 N‐recovery (N 2 O + plant + soil) was highest under FB (86%) and lowest under CB (29%). We further separated the N 2 O derived from urea versus N 2 O from soil organic matter (SOM). The inhibitor DMPSA reduced the N 2 O derived specifically from added urea‐N by more than half ( P  < .001). With the addition of urea, N 2 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 N 2 O production when averaged across the four management legacies. The CB soil had the highest proportion of priming‐derived N 2 O (24%). Management legacies clearly differed in soil carbon and N, which governed N 2 O production from denitrification and SOM priming.