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

Abstract

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.