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Both spikelet removal and low temperature stress largely reduced photosynthate allocation to wheat grain; the former stimulated the average N2O emissions by 31.3% and 33.2% under the field and pot conditions; the latter increased average N2O emission by 19.9% under pot condition. Spikelet removal and low temperature stress significantly reduced total plant N uptake, thereby increasing soil NO3–-N content, and increased root biomass. Our findings indicated that photosynthate allocation affects the soil N2O emissions from wheat fields through altering plant N uptake and root growth, and suggest that promoting photosynthate allocation to grain may not only benefit higher wheat yield but also mitigate greenhouse gas emissions. [ABSTRACT FROM AUTHOR]

Ammonia (NH) is an important atmospheric pollutant that threatens ecosystem and human health. Synthetic nitrogen (N) fertilizer applications are a major source of atmospheric NH. Most of current bottom-up estimates assume that the NH emission response to increasing N application rates is linear, and thus constant emission factors (EFs) are used. However, increasing evidence suggests that NH emissions increase exponentially with increasing N inputs. In the present study, we conducted a meta-analysis to generalize the relationship between N inputs and NH emissions. Overall, the change in EF per unit of additional N fertilizer input (ΔEF) was positive from 70 experiments with at least three N application rates, suggesting that NH emissions in response to increasing N additions grow at a rate higher than linear. Compared to our ΔEF model, the 10% EF model used by Intergovernmental Panel on Climate Change overestimated NH emissions when fertilizer N is applied at low levels, but underestimated NH emissions when N is applied in excess. Therefore, our results suggest that replacing the constant EF with the N-rate-dependent EF could improve the accuracy of NH emission estimates. [ABSTRACT FROM AUTHOR]