Organically fertilized tea plantation stimulates N₂O emissions and lowers NO fluxes in subtropical China

Abstract. Tea plantations are rapidly expanding in China and other countries in the tropical and subtropical zones, but so far there are very few studies including direct measurements of nitrogenous gas fluxes from tea plantations. On the basis of 2-year field measurements from 2012 to 2014, we provided an insight into the assessment of annual nitrous oxide (N₂O) and nitric oxide (NO) fluxes from Chinese subtropical tea plantations under three practices of conventional urea application, alternative oilcake incorporation and no nitrogen fertilization. Clearly, the N₂O and NO fluxes exhibited large intra- and inter-annual variations, and furthermore, their temporal variability could be well described by a combination of soil environmental factors including soil mineral N, water-filled pore space and temperature, based on a revised "hole-in-the-pipe" model. Averaged over a 2-year study, annual background N₂O and NO emissions were approximately 4.0 and 1.6 kg N ha⁻¹ yr⁻¹, respectively. Compared to no nitrogen fertilization, both urea and oilcake application significantly stimulated annual N₂O and NO emissions, amounting to 14.4–32.7 kg N₂O–N ha⁻¹ yr⁻¹ and at least 12.3–19.4 kg NO–N ha⁻¹ yr⁻¹, respectively. In comparison with conventional urea treatment, on average, the application of organic fertilizer significantly increased N₂O emission by 71 % but decreased NO emission by 22 %. Although the magnitude of N₂O and NO fluxes was substantially influenced by the source of N, the annual direct emission factors of N fertilizer were estimated to be 2.8–5.9, 2.7–4.0 and 6.8–9.1 % for N₂O, NO and N₂O+NO, respectively, which are significantly higher than those defaults for global upland croplands. This indicated that the rarely determined N₂O and NO formation appeared to be a significant pathway in the nitrogen cycle of tea plantations, which are a potential source of national nitrogenous gases inventory.