RESUMO
Forest soils are an important source of nitrous oxide (N2O), however, field observations of N2O emission have often exhibited large variabilities when compared with managed agricultural lands. In the last decade, the number of forest N2O studies has increased more than tenfold, but only a few of them have looked into the interannual flux variabilities from the regional scale. Here, we have collected 30 long-term N2O monitoring studies (≥ 2 years) based on a global database, and extracted variabilities (VARFlux) as well as relative variabilities (VAR%, in proportions) of annual N2O fluxes. The relationship of mean annual precipitation (MAP), mean annual temperature (MAT), and nitrogen (N) deposition with flux variabilities was examined to explore the underlying mechanisms for N2O emission on a long-term scale. Our results show that mean VARFlux is 0.43 kg N ha-1 yr-1 and VAR% is 28.68%. Across climatic zones, the subtropical forests have the largest annual N2O fluxes, as well as the largest fluctuations among annual budgets, while the tropics were the smallest. We found that the regulating factors for VARFlux and VAR% are fundamentally different, i.e., MAT and N input determine the annual fluxes as well as VARFlux while MAP and other limiting soil parameters determine VAR%. The relative contributions of different seasons to flux variabilities were also explored, indicating that N2O fluxes of warm and cool seasons are more responsible for the fluctuations in annual fluxes of the (sub)tropical and temperate forests, respectively. Overall, despite the limitation in interpretations due to few long-term studies from literature, this work highlights that significant interannual variabilities are common phenomena for N2O emission from different climatic zones forest soils; by unraveling the divergent drivers for VARFlux and VAR%, we have provided the possibility of improving N2O simulation models for constraining the heterogeneity of N2O emission processes from climatic zones forest soils.
RESUMO
Food waste is of great concern because it causes severe environmental pollution during disposal and contains many resources that should be well managed. Food waste quantification could clarify the resource value of wasted food and thus help to improve resource utilization efficiency, reduce water eutrophication potential, and reduce greenhouse gas emissions. By considering household food waste, out-of-home food waste, and food delivery waste in rural and urban regions, this paper quantifies the nitrogen, phosphorus, water, and carbon footprint embedded in China's food waste at the provincial level. The results indicate that food waste in China was 56.75 Mt. in 2018. Those wasted food cause 0.54 Mt. loss of phosphorus (5.12% of the phosphorus fertilizer consumption), 3.58 Mt. loss of nitrogen (10.43% of the nitrogen fertilizer consumption), and 120.25 billion tons loss of water (3.06 times of the storage capacity of the Three Gorges Reservoir). If ignoring the greenhouse gas emissions caused by land-use change, the carbon footprint caused by wasted food is 168.07 Mt. CO2eq, accounting for 1.44% of China's total GHG emission. Principal component analysis indicates that the per capita disposable income, urbanization rate, and personal consumption expenditure are critical factors for food waste volume variation in different provinces. Considering China's significant role in the global resource cycling, improving nutrient/resource utilization efficiency along the food supply chain, minimizing food waste volume, and developing economic-effective processes for food waste reuse and recycling are recommended to close the imbalanced resource cycle during the current food waste management.
