Liming and nitrification inhibitor affects crop N uptake efficiency and N loss through changing soil N processes

The form of inorganic N in soil is governed by N transformations, but the relationship between the soil N transformation dynamics and the N uptake by plants is unclear. The effect of liming and nitrification inhibitor on crop N uptake efficiency (NUE) and N loss due to altered soil N processes was investigated using a 15 N tracing approach. Crops with different N uptake strategies (sweet potato (Dioscorea esculenta L.); an ammonium-N preferring plant and pakchoi (Brassica chinensis L.); a nitrate–N preferring plant) were grown in two agricultural soils with different pH values with or without lime and nitrification inhibitor (3,4-dimethylpyrazole phosphate, DMPP). We monitored the effects of 15 N-nitrate or 15 N-ammonium on the plant biomass and NUE, residual soil N, leachate, and emitted gas. The NUE of pakchoi in acidic soil was greater with nitrate than with ammonium; however, when lime was applied, the NUE of pakchoi increased significantly compared to ammonium alone. The NUE of pakchoi was not affected by N source in the alkaline soil, but application of DMPP together with ammonium significantly reduced NUE of pakchoi compared to ammonium alone. The N source had no effect on the NUE of sweet potato in acidic or alkaline soils, while NUE increased significantly after application of lime and ammonium (31.3%) and with application of DMPP (32.4%) compared to ammonium alone (30% in acidic soil and 23% in alkaline soil). Pakchoi NUE was significantly (p < 0.05) related to gross mineralization rate and the inorganic N supply capacity. Nitrous oxide emissions after N fertilization were greater in the alkaline than in acid soil greater after ammonium than after nitrate application. Since DMPP together with ammonium reduced N2O losses from the alkaline soil, this indicates the importance of nitrification as an N2O production process. Nitrate leaching was greater with lime and ammonium application compared to ammonium alone in the acidic soil, whereas the opposite occurred for ammonium leaching. In summary, crop NUE can be significantly increased by lime treatment whereas the addition of DMPP also increased the NUE of sweet potato, which prefer ammonium, grown in the alkaline soil.