Wet nitrogen deposition across the urban-intensive agricultural-rural transect of a small urban area in southwest China

Atmospheric organic nitrogen deposition around the Danjiangkou Reservoir: Fluxes, characteristics and evidence of agricultural source

Dissolved organic nitrogen (DON) deposition was the substantial component of dissolved total nitrogen (DTN) deposition in the world's nitrogen deposition hot spots areas. However, the information on the importance for DON deposition and its sources was still scarce, which limited the comprehensive assessment of the ecological threat from nitrogen deposition. Six sampling sites around the Danjiangkou Reservoir were set up to collect the dry and wet deposition samples from October 2017 to September 2021. The results showed that dry and wet DTN deposition averaged 34.72 kg ha-1 yr-1 and 22.27 kg ha-1 yr-1, respectively. Dry NH4+-N, NO3--N and DON deposition averaged 14.28 kg ha-1 yr-1, 5.91 kg ha-1 yr-1 and 14.53 kg ha-1 yr-1, respectively. Wet NH4+-N, NO3--N and DON deposition averaged 11.14 kg ha-1 yr-1, 3.89 kg ha-1 yr-1and 7.24 kg ha-1 yr-1, respectively. The contributions of DON to DTN were 41.85% (in dry deposition) and 32.50% (in wet deposition), respectively. Dry DON deposition varied between 26.44 kg ha-1 yr-1 and 9.11 kg ha-1 yr-1, and significantly differed among six sampling sites (P < 0.05). The different intensity of agricultural activities disturbance at the sampling sites was the important reason for the spatial variations of DON deposition. DON deposition was significantly correlated with ammonium nitrogen (NH4+-N) deposition (P < 0.05). According to the results of positive matrix factorization (PMF) model, agriculture source contributed significantly to the DON deposition, the contributions at six sampling sites ranged from 45.8% to 73.7% in dry deposition, and from 56.8% to 81.6% in wet deposition. In summary, our findings found that agricultural activities were the important factors influencing the spatial patterns of DON deposition around Danjiangkou Reservoir and provided new evidence for the anthropogenic source of DON deposition in China.

Nitrogen atmospheric deposition driven by seasonal processes in a Brazilian region with agricultural background

Understanding the seasonal patterns and influencing factors of nitrogen atmospheric deposition is essential to evaluate human impacts on the air quality and nitrogen biogeochemical cycle. However, evaluation of the nitrogen deposition flux, especially in South America agricultural regions, has not been fully investigated. In this paper, we quantified the atmospheric wet deposition fluxes of total dissolved nitrogen (TDN), dissolved organic nitrogen (DON), and dissolved inorganic nitrogen (DIN), in a region with agricultural and livestock predominance in the Southern Minas Gerais region, Brazil, from May 2018 to April 2019. Deposition fluxes of nitrogen species in the wet season (October-March) were on average 4.8-fold higher than those in the dry season, which revealed significant seasonal variations driven largely by the seasonality of rainfall and agricultural operations. We also found high NO3-/NH4+ ratios (average = 8.25), with higher values in dry season (NO3-/NH4+  = 12.8) in comparison with wet season (NO3-/NH4+  = 4.48), which revealed a higher relative contribution of NOx emissions from traffic sources in dry season. We also estimated the influence of atmospheric deposition of inorganic nitrogen (N-DIN) on environmental ecosystems, being 2.01 kgNha-1 year-1 with potential risk of acidification and eutrophication of 30%. Therefore, attention should be paid to the role of wet atmospheric deposition of nitrogen as a source of nitrogen environmental pollution in agricultural regions.

Long-term observations of the chemical composition, fluxes and sources of atmospheric wet deposition at an urban site in Xi'an, Northwest China

Atmospheric wet deposition (AWD) is closely related to air quality, and excessive deposition poses risks to ecological systems and human health. Seasonal and interannual variations in acidity, electric conductivity (EC), ionic composition, fluxes, sources, and atmospheric transport of AWD were analyzed at an urban site in Xi'an from 2016 to 2019. The annual volume-weighted mean (VWM) pH and EC values were 6.8 and 40.6 μS cm^-1, respectively. NO₃^- (47%) was the most dominant anion, while Ca²⁺ (34%) was the most dominant cation. The analysis of fractional acidity (FA) and neutralization factors (NFs) showed that 96% of the acidity was neutralized by alkaline constituents, especially Ca²⁺ and NH₄⁺. The annual AWD flux of total ions was 125.9 kg ha^-1 year^-1, and NO₃^-, NO₂^-, SO₄^2- and NH₄⁺ fluxes accounted for approximately 70%, indicating considerable sulfur (9.1 kg ha^-1 year^-1) and nitrogen (22.0 kg ha^-1 year^-1) deposition. Under dilution by precipitation, the EC and major ion concentrations were lower, while the pH and fluxes were higher, in summer and autumn, and the opposite results were observed in spring and winter. The source apportionment via by positive matrix factorization (PMF) revealed that the six sources of major ions were confirmed as follows: vehicular emissions (38.1%), agriculture (22.3%), fossil fuel combustion (13.8%), crust (12.9%), marine (9.6%), and biomass burning (3.3%). And on the basis of back trajectory analysis, the air masses of precipitation were primarily from the northwest in spring and winter, from the southeast in summer, and from various directions in autumn, and they transported different natural and anthropogenic pollutants along their paths, thereby affecting the chemical composition and fluxes of AWD.

Contribution of atmospheric N deposition to riverine N load in a forest-dominated watershed through field monitoring for three years

Increased atmospheric nitrogen (N) deposition significantly impacts N cycling in freshwater ecosystems. Relative to lakes, the importance of N deposition in riverine N load is less studied. Thus, this study monitored N deposition and riverine N load for three years and then used the export coefficient model to explore N deposition's contribution to riverine N load in a forest-dominated watershed. It is found that the annual export of total N (TN) deposition could explain 17.4%-19.2% of riverine TN load. The contribution of TN deposition to riverine TN load was significantly higher (P < 0.05) during the crop production period (recorded as CPP, lasting from June to September, 22.7%) than the non-crop production period (Non-CPP, 13.8%). The application of chemical fertilizer and manure and the high precipitation were assumed as the primary reason for the increased N deposition and increased riverine TN load during CPP. This study shows that inland plain agriculture practices might considerably influence the nearby forest-dominated watershed, and it is necessary to develop sustainable agriculture programs for reducing riverine N load.

Optimum Water and Fertilizer Management for Better Growth and Resource Use Efficiency of Rapeseed in Rainy and Drought Seasons

Optimum water–fertilizer management in rainfed agriculture is an important factor in improving crop productivity and the ecological environment under fluctuating climate conditions, especially in Southwest China, where seasonal drought and waterlogging occur frequently. In order to investigate the effects of different cultivation technologies on growth and the water and fertilizer use efficiency of rapeseed (Brassica napus L.), a two-year field study was conducted in rainy (2016–2017) and drought (2017–2018) seasons which included three cultivation patterns: (1) conventional flat planting (FP); (2) straw mulching (SM); (3) ridge-furrow rainfall harvesting system (RF), and three fertilization patterns: (1) conventional fertilization (CF); (2) reduced slow-release fertilizer (SR); and (3) no fertilizer as a control treatment. The results indicated that the yield and its composition values were lower in the rainy year than in the seasonal dry year. The single water-saving technology had no significant effect on yield increase when seasonal drought occurred. The two technologies (SM + SR and RF + SR) improved the height, leaf SPAD value and dry matter of the rapeseed and adjusted the root–shoot ratio under two different climate conditions. In the rainy season, these technologies reduced the loss of nutrients, while in the seasonal drought year, it increased the soil moisture. The SM + SR and RF + SR increased the yield of rapeseed by 7.71% and 29.93% and enhanced oil content by 4.64% and 7.91%, respectively, compared with the local cultivation pattern. Meanwhile, these treatments decreased the total water consumption during whole growth stages and promoted water use efficiency by 14.84% and 28.71%, respectively. The combination of SM + SR and RF + SR also increased the accumulation of N, P, and K and significantly promoted the utilization efficiency of fertilizer. In the future, the adverse effects of environmental factors could be relieved, and the goal of cost savings and increasing efficiency could be achieved by adopting the optimal cultivation technologies in rapeseed production of Southwest China.

Soil nitrogen and phosphorous dynamics by in situ soil experiments along an urban-rural gradient in Shanghai, China

An in situ soil experimental system was designed to determine how urbanization impacts soil nitrogen and phosphorus dynamics. Variables including the road density, normalized difference vegetation index, distance to the nearest highway and industrial area from the soil experimental site, land use impact index, population density, population change index, total population, and percentage of water area were used to quantitatively explain the soil nitrogen and phosphorous contents. The results showed that the total phosphorous in the soil increased slowly after September 2013, indicating a phosphorous accumulation phenomenon in the soil in urban areas. The nitrate nitrogen in the soil had a higher value in September 2013, while the soil ammonium nitrogen content was higher during the winter. Moreover, the soil ammonium nitrogen content was higher than the nitrate nitrogen content during most of the experimental period. The distance from the urban centre, road density, proportion of built-up land, and population density can explain the soil nutrient dynamics quantitatively, showing that 45.4% of the soil nitrate nitrogen content, 84.1% of the soil ammonium nitrogen content, 44.6% of the ratio of NO₃/NH₄, 58.1% of the ratio of total inorganic nitrogen (TIN)/total phosphorous (TP), and 81.6% of the TIN could be explained by one of these variables at most. The potential factors affecting the changes in soil N contents include changes in human dietary habits as more people migrate to cities and industrial wastewater discharge. This study is helpful in quantitatively understanding the urbanization process and associated environmental impacts.

Emission of nitrous oxide from plain multi-ditch system and its impact factors

Multi-level ditch area is a major component of the hydrographic net of plain area, China. Given the high concentration of nitrogen (N) in the surface water and vigorous biogeochemical interactions, ditch is likely to be the hot spots of N₂O emission. However, N₂O emission flux and emission factor (EF_5r) of multi-level ditches have not been determined. To address this knowledge gap, a 1-year field work in three ditches with different levels in Chengdu Plain was conducted. It is found that the annual flux of N₂O emission and EF_5r was higher in the lateral (0.0020 and 83.94 μg m^-2 h^-1) and field ditches (0.0019 and 110.75 μg m^-2 h^-1) than in the branch ditch (0.0016 and 46.38 μg m^-2 h^-1, P < 0.05). It is found that parameters of groundwater level, discharge, precipitation, and NH₄⁺ were the primary factors, and these parameters can model the N₂O flux well. Furthermore, the content of NH₄⁺ in the surface water of ditches presented better correlation with the emission of N₂O than the content of NO₃^-. Therefore, controlling NH₄⁺ emission and lessening fertilizer usage in summer may be key solutions for indirect reduction of N₂O in Chengdu Plain.