Assessment Of Reactive Nitrogen Losses Induced By Cropping Systems In The Bohai Rim Region

The human use of reactive nitrogen(Nr,all nitrogen(N)species except N2)has dramatically boosted the crop production,meanwhile altered the global N cycle and caused a cascade of environmental changes,leading to negative impacts on both human and ecosystem health.To achieve both food security and environmental sustainability,it is of great importance to better quantify Nr losses caused by various entities on multiple scales and evaluate their potential environmental impacts,as well as communicate effectively these scientific advances to researchers,consumers and policy makers to help them minimize their role in the disruption of the N cycle and the resulting environmental consequences.The development and use of N footprint tools can provide the public and policy makers with these scientific advances and raise their environmental awareness.However,few studies have focused on the N footprint on regional scale in China,thus a lack of assessment approach and accurate validation.Based on multiple data sources(e.g.in-site measurements,field surveys,regional soil properties and meteorological data,statistical data on food and energy consumption),this study systematically assessed the Nr losses in the Bohai Rim region(BR),China through the integration of a process-based biogeochemical model,the DenitrificationDecomposition(DNDC)model and N footprint models,and explored the impact of ammonia(NH3)volatilization and its regional transport on the air quality in urban areas.The main findings include:(1)A new approach by integrating the biogeochemical DNDC model and the Nr spatial intensity(NrSI)framework was developed to identify hotspots of Nr losses induced by various cropping systems and through different pathways on regional scale.Altogether 29 types of cropping systems in 429 counties were simulated for the N balance,Nr losses and the NrSI associated with crop production in the BR in 2008.Regarding the total Nr losses in the BR,45%of the total N input was lost to the environment during crop production with NH3 volatilization and nitrate leaching the two main pathways,making up 24%and 19%of the total N input,respectively.Shandong province was the biggest contributor of the total Nr losses(45.6%)among regions,and winter wheat-summer maize,triple vegetables and spring maize cropping systems were the top three contributors(60.5%)among various cropping systems.(2)For Nr loss hotspots in the BR in 2008,there were substantial variations of NrSI across 29 types of cropping systems(41-1024 kg N ha-1 yr-1)and 429 counties(28-4782 kg N ha-1 yr-1).Vegetable systems had the highest average NrSI of 355 kg N ha-1 yr-1 among all the mono-cropping systems,and Beijing had the highest NrSI associated with crop production(307 kg N ha-1 yr-1)among five provinces.(3)The virtual N factor and product-based N footprint of 15 types of crop products were calculated based on the DNDC regional simulation and the N-Calculator model.Vegetable had both the highest virtual N factor(9.5)and product-based N footprint(7.4 kg N released(kg product N)-1).Regarding the ecosystem and human health damage cost associated with the product-based N footprint,vegetable presented the highest total damage cost of 159.8 CNY(kg product N)-1,while soybean presented the lowest total damage cost of 29.6 CNY(kg product N)-1.(4)In 2008,the per-capita N footprint in the BR was 32.6 kg N capita-1 yr-1,of which the per-capita N footprint due to energy consumption,food consumption and food production were 13.1 kg N capita-1 yr-1,2.7 kg N capita-1 yr-1,and 16.8 kg N capita-1 yr-1,respectively.The regional N footprint totaled 9.0 Tg N yr-1 in the BR,and the total N footprint associated with energy consumption,food consumption and food production were 2.4 Tg N yr-1,0.6 Tg N yr-1 and 6.0 Tg N yr-1.Among the five provinces in the BR,Shandong province contributed the largest to the total regional N footprint with a percentage of 44.8%.The regional NrSI,and the NrSI for the agricultural system and the settled system were 357 kg N ha-1 yr1,255 kg N ha-1 yr-1,646 kg N ha-1 yr-1,respectively.Beijing had the highest provincial NrSI,and NrSI for the agricultural system and the settled system.(5)The NH3 and PM2.5 concentrations and their influencing factors during late spring in BeijingTianjin-Hebei region were investigated by combining observations and simulations of air quality model.A significant positive correlation was found between the NH3 and PM2.5 concentrations in Beijing.The higher NH3 concentrations were primarily related to southwesterly winds.Since there are strong NH3 emissions in the southern plains adjacent to Beijing,the regional transport induced by the southerly prevailing winds would increase the NH3 concentrations in Beijing significantly,leading to adverse impacts on air quality in urban areas.