There has been much debate about the uncertainties associated with the estimation of direct and indirect agricultural nitrous oxide (N2O) emissions in developing countries and in particular from tropical regions. In this study, we report an up-to-date review of the information published in peer-review journals on direct N2O emissions from agricultural systems in tropical and sub-tropical regions. We statistically analyze net-N2O-N emissions to estimate tropic-specific annual N2O emission factors (N2O-EFs) using a Generalized Additive Mixed Model (GAMM) which allowed the effects of multiple covariates to be modelled as linear or smooth non-linear continuous functions. Overall the mean N2O-EF was 1.2% for the tropics and sub-tropics, thus within the uncertainty range of IPCC-EF. On a regional basis, mean N2O-EFs were 1.4% for Africa, 1.1%, for Asia, 0.9% for Australia and 1.3% for Central & South America. Our annual N2O-EFs, estimated for a range of fertiliser rates using the available data, do not support recent studies hypothesising non-linear increase N2O-EFs as a function of applied N. Our findings highlight that in reporting annual N2O emissions and estimating N2O-EFs, particular attention should be paid in modelling the effect of study length on response of N2O.

The search for agricultural practices that mitigate N2O emissions, and the validation and development of models, would benefit if the uncertainty of emission estimates would be reduced relative to current levels. This uncertainty has different sources, such as the error of the flux measurements, the error caused by spatially scaling up measurements to the field or regional level, and the error caused by estimating emissions for the time intervals between measurements by interpolation. This paper focuses on the uncertainty of flux measurements and on flux spatial variability, which is a major cause of error when measured fluxes are scaled up spatially. The analysis focuses on 2415 flux measurements made with the closed chamber method over a monitoring campaign of approximately 3 years. Statistically significant nonlinearities in the changes of N2O concentrations during chamber closure were infrequent (8.3%). Further analysis of significant non‐linear concentration changes indicates that, for positive fluxes, nonlinearity might not always be an artifact caused by chamber placement, but that it can reflect natural temporal variability of the flux during chamber placement in a significant number of cases. The analysis of the coefficients of determination (R2) and of the normalized root mean squared errors (NRMSE) of the linear regressions shows that, below emission rates of 5 g N2O‐N ha−1 d−1, the uncertainty of flux estimates strongly increases. The flux detection limit was 3.5 g N2O‐N ha−1 d−1, which is consistent with the outcome of the analysis of the R2s and NRMSEs. Flux measurements based on less than five N2O concentrations per flux led to estimates with considerably larger confidence intervals. When the number of N2O concentrations per flux was reduced from five to four, the detection limit increased to 7.5 g N2O‐N ha−1 d−1. The individual fluxes of spatially replicated plots show strong dispersion around the mean: the average coefficient of variation for fluxes above 5 g N2O‐N ha−1 d−1 was 54.3% and the data suggest that the spatial variability of fluxes correlates positively with flux magnitude. Our results suggest that, for measurements performed with the closed static chamber method, (1) linear regression might generally lead to the best estimates of the average fluxes during closure time, and that the chamber sampling strategy might be designed accordingly, (2) there is considerable potential to reduce the uncertainty for fluxes lower than 5 g N2O‐N ha−1 d−1 by employing analytical instrumentation with higher precision, (3) flux uncertainty and detection limit are strongly affected by the number of concentrations measured for each flux, and (4) large numbers of replicated chambers could be particularly beneficial if high fluxes are expected.

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Reliable quantification of nitrous oxide emission is a key to assessing efficiency of use and environmental impacts of N fertilizers in crop production. In this study, N2O emission and yield were quantified with a database of 853 field measurements in 104 reported studies and a regression model was fitted to the associated environmental attributes and management practices from China's croplands. The fitted emission model explained 48% of the variance in N2O emissions as a function of fertilizer rate, crop type, temperature, soil clay content, and the interaction between N rate and fertilizer type. With all other variables fixed, N2O emissions were lower with rice than with legumes and then other upland crops, lower with organic fertilizers than with mineral fertilizers. We used the subset of the dataset for rice-covering a full range of different typical water regimes, and estimated emissions from China's rice cultivation to be 31.1 Gg N2O-N per year. The fitted yield model explained 35% of the variance in crop yield as a function of fertilizer rate, temperature, crop type, and soil clay content. Finally, the empirical models for N2O emission and crop yield were coupled to explore the optimum N rates (N rate with minimum N2O emission per unit yield) for combinations of crop and fertilizer types. Consequently, the optimum N application rate ranged between 100 kg N ha-1 and 190 kg N ha-1 respectively with organic and mineral fertilizers, and different crop types. This study therefore improved on existing empirical methods to estimate N2O emissions from China's croplands and suggests how N rate may be optimized for different crops, fertilizers and site conditions.

Soils can naturally be a source of the potent greenhouse gas nitrous oxide (N 2 O). By contrast, the largest anthropogenic source of N 2 O is the application of nitrogen (N) fertilizer on agricultural soil, but it is unclear if fertilizer-supported N 2 O emission only originates from the fertilizer N directly or through additionally stimulated N 2 O production from native soil N. Even though native soil N also includes mineral N already in soil before fertilizer application, organic N is the principal native N pool and thereby provides for mineral N cycling and N 2 O emission. Here, we tested (1) the contribution of native soil N to N 2 O emission after mineral N fertilizer application and (2) whether it is affected by different soil organic matter (SOM) contents by conducting a laboratory 15 N-tracing experiment with agricultural soil from a long-term field trial with two treatments. Both field treatments are fertilized with mineral N, whereas only one of the two receives liquid manure causing higher SOM content. Soil sampling was conducted in March 2016 shortly before fertilizer application in the field. The application of 15 N-labeled fertilizer more than doubled the N 2 O production from native N sources compared to the non-fertilized control incubations. This primed N 2 O production contributed by 5-8% to the fertilizer-induced N 2 O emission after one week of incubation and was similar for both field treatments regardless of liquid manure application. Therefore, further research is needed to link N 2 O priming to its potential production pathways and sources. While the observed effect may be important in soils, the amount of applied N fertilizer remains the largest concern being responsible for the majority of N 2 O emission.

Nitrogen fertilizers are supposed to be a major source of nitrous oxide (N2O) emissions from arable soils. The objective of this study was to compare the effect of N forms on N2O emissions from arable fields cropped with winter wheat (Triticum aestivum L.). In three field trials in North‐West Germany (two trials in 2011/2012, one trial in 2012/2013), direct N2O emissions during a one‐year measurement period, starting after application of either urea, ammonium sulfate (AS) or calcium ammonium nitrate (CAN), were compared at an application rate of 220 kg N ha−1. During the growth season (March to August) of winter wheat, N2O emission rates were significantly higher in all three field experiments and in all treatments receiving N fertilizer than from the non‐fertilized treatments (control). At two of the three sites, cumulative N2O emissions from N fertilizer decreased in the order of urea > AS > CAN, with emissions ranging from 522–617 g N ha−1 (0.24–0.28% of applied fertilizer) for urea, 368–554 g N ha−1 (0.17–0.25%) for AS, and 242–264 g N ha−1 (0.11–0.12%) for CAN during March to August. These results suggest that mineral nitrogen forms can differ in N2O emissions during the growth period of winter wheat. Strong variations in the seasonal dynamics of N2O emissions between sites were observed which could partly be related to weather events (e.g., precipitation). Between harvest and the following spring (post‐harvest period) no significant differences in N2O emissions between fertilized and non‐fertilized treatments were detected on two of three fields. Only on one site post‐harvest emissions from the AS treatment were significantly higher than all other fertilizer forms as well as compared to the control treatment. The cumulative one‐year emissions varied depending on fertilizer form across the three field sites from 0.05% to 0.51% with one exception at one field site (AS: 0.94%). The calculated overall fertilizer induced emission averaged for the three fields was 0.38% which was only about 1/3 of the IPCC default value of 1.0%.