The rhizosphere: a key hotspot for gross nitrogen transformations and nitrous oxide emissions in legume cropping systems

• N gross transformation and N 2 O fluxes quantified in rhizosphere and bulk soils. • Rhizosphere soil significantly increased N transformation rates compared to bulk soil. • Rhizosphere soil N 2 O emissions were 5.9–13.7 times those of bulk soil. • Denitrification and codenitrification were predominant N 2 O processes in the rhizosphere. • Rhizosphere N 2 O pathways and rate were more sensitive to field N addition than bulk soil. By facilitating biological nitrogen (N) fixation, legumes offer an eco-friendly strategy for increasing soil N. However, their root exudates alter rhizosphere N cycling, potentially increasing nitrous oxide (N 2 O) emissions—a critical greenhouse gas. To evaluate legume-mediated N cycling dynamics, we analyzed gross and net N transformations, as well as N 2 O pathways and emissions in the rhizosphere and bulk soils of hairy vetch ( Vicia villosa Roth.) using 15 N tracing techniques. These analyses were conducted under three field N fertilizer treatments-0 (N0), 50 (N1) and 100 (N2) kg N ha −1 -during the hairy vetch growing season. Our results revealed that the rhizosphere significantly enhanced gross N transformation rates and N 2 O emissions. Specifically, gross mineralization, nitrification, and immobilization rates in the rhizosphere were enhanced by 73.3–306.9 %, 38.4–62.7 %, and 8.0–310.6 %, respectively, while the net nitrification rate was significantly enhanced by 18.7–181.2 %. Moreover, the rhizosphere exhibited a greater sensitivity response of N 2 O emissions to N substrates, with cumulative N 2 O emissions ranging from 229.5 to 320.8 µg kg −1 , representing 5.9–13.7 times higher than those in bulk soil. The predominant N 2 O production pathways in both rhizosphere and bulk soils were denitrification and codenitrification. N amendment significantly boosted gross N transformations in the rhizosphere except for dissimilatory nitrate reduction to ammonium (DNRA), an effect not observed in the bulk soil. Notably, N amendment significantly increased cumulative N 2 O emissions in the rhizosphere (by 13.0–39.8 % across treatments), while exhibiting minimal effects on bulk soil N 2 O emissions. This highlights the importance of prioritizing research efforts to optimize rhizosphere N management and to develop methods to quantify N 2 O emissions from the rhizosphere in legume cropping systems.