The paradigm that permafrost-affected soils show restricted mineral nitrogen (N) cycling in favor of organic N compounds is based on the observation that net N mineralization rates in these cold climates are negligible. However, we find here that this perception is wrong. By synthesizing published data on N cycling in the plant-soil-microbe system of permafrost ecosystems we show that gross ammonification and nitrification rates in active layers were of similar magnitude and showed a similar dependence on soil organic carbon (C) and total N concentrations as observed in temperate and tropical systems. Moreover, high protein depolymerization rates and only marginal effects of C:N stoichiometry on gross N turnover provided little evidence for N limitation. Instead, the rather short period when soils are not frozen is the single main factor limiting N turnover. High gross rates of mineral N cycling are thus facilitated by released protection of organic matter in active layers with nitrification gaining particular importance in N-rich soils, such as organic soils without vegetation. Our finding that permafrost-affected soils show vigorous N cycling activity is confirmed by the rich functional microbial community which can be found both in active and permafrost layers. The high rates of N cycling and soil N availability are supported by biological N fixation, while atmospheric N deposition in the Arctic still is marginal except for fire-affected areas. In line with high soil mineral N production, recent plant physiological research indicates a higher importance of mineral plant N nutrition than previously thought. Our synthesis shows that mineral N production and turnover rates in active layers of permafrost-affected soils do not generally differ from those observed in temperate or tropical soils. We therefore suggest to adjust the permafrost N cycle paradigm, assigning a generally important role to mineral N cycling. This new paradigm suggests larger permafrost N climate feedbacks than assumed previously.

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.

Tea-planted soils generally receiving high nitrogen (N) fertilizer doses are more vulnerable to acidification, and turn into significant sources of the potent greenhouse gas nitrous oxide (N₂O). However, little is known about the magnitude of soil N₂O emissions from global tea plantations. Based on a global meta-analysis of field experimental data collected from major tea growing countries, we quantify annual N₂O emissions, calculate direct emission factors (EF_d) and identify key environmental controls of emissions from tea plantations. However, most data are from China and Japan, which is to be expected given that tea plantations in these countries represent >60% of the global area and the vital environmental research community in both countries. Results suggest that annual N₂O emissions from soils of global tea plantations are on average 17.1 kg N ha⁻¹ (or 8008 kg CO₂-eq ha⁻¹), being substantially greater than those reported for cereal croplands (662–3757 kg CO₂-eq ha⁻¹). The global mean EF_d for N applications to tea plantations equals 2.31% (with a 95% confidence interval of 1.91%–2.71%), being two times higher than the Intergovernmental Panel on Climate Change default value of 1%. Across tea plantations worldwide, total N₂O emissions are estimated to be 57–84 Gg N yr⁻¹, or 1.5%–12.7% of total direct cropland N₂O emissions. Given that tea plantations account for only 0.3% of total cropland area, our finding highlights that tea-planted soils are global hotspots for N₂O emissions and that these systems might be prime targets for climate change mitigation in the agricultural sector. Considering that tea is a high price commodity for which consumers may be willing to apply pressure for more climate-smart production, possible mitigation efforts include use of controlled-release fertilizers or nitrification inhibitors, and application of biochar and/or lime for increasing soil pH; i.e. measures that increase N use efficiency while reducing the climate footprint of tea production.

To safeguard food security and preserve precious water resources, the technology of water-saving ground cover rice production system (GCRPS) is being increasingly adopted for rice cultivation. However, changes in soil water status and temperature under GCRPS may affect soil biogeochemical processes that control the biosphere–atmosphere exchanges of methane (CH₄), nitrous oxide (N₂O) and carbon dioxide (CO₂). The overall goal of this study is to better understand how net ecosystem greenhouse gas exchanges (NEGE) and grain yields are affected by GCRPS in an annual rice-based cropping system. Our evaluation was based on measurements of the CH₄ and N₂O fluxes and soil heterotrophic respiration (CO₂ emissions) over a complete year, and the estimated soil carbon sequestration intensity for six different fertilizer treatments for conventional paddy and GCRPS. The fertilizer treatments included urea application and no N fertilization for both conventional paddy (CUN and CNN) and GCRPS (GUN and GNN), and solely chicken manure (GCM) and combined urea and chicken manure applications (GUM) for GCRPS. Averaging across all the fertilizer treatments, GCRPS increased annual N₂O emission and grain yield by 40 and 9%, respectively, and decreased annual CH₄ emission by 69%, while GCRPS did not affect soil CO₂ emissions relative to the conventional paddy. The annual direct emission factors of N₂O were 4.01, 0.09 and 0.50% for GUN, GCM and GUM, respectively, and 1.52% for the conventional paddy (CUN). The annual soil carbon sequestration intensity under GCRPS was estimated to be an average of −1.33 Mg C ha⁻¹ yr⁻¹, which is approximately 44% higher than the conventional paddy. The annual NEGE were 10.80–11.02 Mg CO₂-eq ha⁻¹ yr⁻¹ for the conventional paddy and 3.05–9.37 Mg CO₂-eq ha⁻¹ yr⁻¹ for the GCRPS, suggesting the potential feasibility of GCRPS in reducing net greenhouse effects from rice cultivation. Using organic fertilizers for GCRPS considerably reduced annual emissions of CH₄ and N₂O and increased soil carbon sequestration, resulting in the lowest NEGE (3.05–5.00 Mg CO₂-eq ha⁻¹ yr⁻¹). Accordingly, water-saving GCRPS with organic fertilizer amendments was considered the most promising management regime for simultaneously achieving relatively high grain yield and reduced net greenhouse gas emission.