Reply to Comment on "Fossil Fuel Combustion-Related Emissions Dominate Atmospheric Ammonia Sources during Severe Haze Episodes: Evidence from 15N-Stable Isotope in Size-Resolved Aerosol Ammonium"

Response of soil microbial communities and rice yield to nitrogen reduction with green manure application in karst paddy areas

Fertilizer application practices are one of the major challenges facing agroecology. The agrobenefits of combined application of green manure and chemical fertilizers, and the potential of green manure to replace chemical fertilizers are now well documented. However, little is known about the impact of fertilization practices on microbial communities and tice yield. In this study, the diversity of bacterial and fungal communities, symbiotic networks and their relationship with soil function were analyzed in five fertilization treatments (N: 100% nitrogen fertilizer alone; M: green manure alone; MN₆₀: green manure couple with 60% nitrogen fertilizer, MN₈₀: green manure couple with 80% nitrogen fertilizer; and MN₁₀₀: green manure couple with 100% nitrogen fertilizer). First, early rice yield was significantly higher by 12.6% in MN₁₀₀ treatment in 2021 compared with N. Secondly, soil bacterial diversity showed an increasing trend with increasing N fertilizer application after green manure input, however, the opposite was true for fungal diversity. Microbial interaction analysis showed that different fertilizer applications changed soil microbial network complexity and fertilizer-induced changes in soil microbial interactions were closely related to soil environmental changes. Random forest models further predicted the importance of soil environment, microorganisms and rice yield. Overall, nitrogen fertilizer green manure altered rice yield due to its effects on soil environment and microbial communities. In the case of combined green manure and N fertilizer application, bacteria and fungi showed different responses to fertilization method, and the full amount of N fertilizer in combination with green manure reduced the complexity of soil microbial network. In contrast, for more ecologically sensitive karst areas, we recommend fertilization practices with reduced N by 20-40% for rice production. Graphical Abstract.

Reactive N emissions from cropland and their mitigation in the North China Plain

Excessive application of chemical nitrogen (N) fertilizer and inefficient N management are still common in the North China Plain, leading to large reactive N (Nr) losses and pollution, threatening environmental security and public health. Three improved N management practices (33% reduction in N applied (OU), OU combined with partial organic fertilizer substitution (UOM) and the urea in UOM amended with a urease inhibitor (ULOM)) together with no N application (CK) and farmers' conventional practice (CU) were tested on a maize-wheat rotation at Quzhou, Hebei, North China Plain (NCP). Nr emissions were related to WFPS (Water Filled Pore Space), soil mineral N (NH₄⁺-N and NO₃^--N) and soil temperature. Nr emissions and yield-scaled Nr emissions were significantly reduced by partial substitution of organic fertilizer for chemical fertilizer: NH₃ emissions were reduced by 55.8-62.4%. Using a urease inhibitor (Limus®), further reduced NH₃ emissions by 40.2-64.5%. Yield-scaled NH₃ emissions were, on average, reduced by 60.0% and 55.2% in the maize and wheat growing season, respectively, relative to the UOM treatment. Long-term application of organic fertilizer had a significant positive effect on N use efficiency (NUE). Overall, the study shows that appropriated N management such as reducing the N application rate, partial substitution of chemical N by organic N and using a urease inhibitor can reduce Nr emissions and promote NUE in the North China Plain. The methods corresponding to the ULOM and UOM treatments were the most and second most effective, respectively, with high net economic benefits.

Molecular Characterization of Nitrogen-Containing Compounds in Humic-like Substances Emitted from Biomass Burning and Coal Combustion

N-containing organic compounds (NOCs) in humic-like substances (HULIS) emitted from biomass burning (BB) and coal combustion (CC) were characterized by ultrahigh-resolution mass spectrometry in the positive electrospray ionization mode. Our results indicate that NOCs include CHON+ and CHN+ groups, which are detected as a substantial fraction in both BB- and CC-derived HULIS, and suggest that not only BB but also CC is the potential important source of NOCs in the atmosphere. The CHON+ compounds mainly consist of reduced nitrogen compounds with other oxygenated functional groups, and straw- and coal-smoke HULIS exhibit a lower degree of oxidation than pine-smoke HULIS. In addition, the NOCs with higher N atoms (N₂ and/or N₃) generally bear higher modified aromaticity index (AI_mod) values and are mainly contained in BB HULIS, especially in straw-smoke HULIS, whereas the NOCs with a lower N atom (N₁) always have relatively lower AI_mod values and are the dominant NOCs in CC HULIS. These findings imply that the primary emission from CC may be a significant source of N₁ compounds, whereas high N number (e.g., N_2-3) compounds could be associated with burning of biomass materials. Further study is warranted to distinguish the NOCs from more sources.

Pollution characterization and source identification of nitrogen-containing species in fine particulates: A case study in Hefei city, East China

To identify the nitrogen sources in atmospheric particulate matter, the stable isotope technique has been proven as an effective method. In this study, PM_2.5 samples at different pollution levels were collected from March 2018 to February 2019 in Hefei to analyze and compare the chemical composition. The results showed that the concentrations of PM_2.5, total nitrogen (TN) and nitrogenous species, as well as the total nitrogen isotopic composition (δ¹⁵N) increased with the aggravation of pollution. Ammonium nitrogen (NH₄⁺-N, 54%) was the dominant nitrogen-containing specie during the whole campaign, followed by nitrate nitrogen (NO₃^--N, 34%) and organic nitrogen (ON, 12%). The δ¹⁵N was positively correlated with NH₄⁺-N/TN but negatively correlated with NO₃^--N/TN. NH₄NO₃ and NH₄HSO₄ were the dominant forms of the secondary inorganic aerosols. In addition, a significant positive correlation was observed between the temperature and δ¹⁵N. Nitrogen source identification of PM_2.5 was conducted using Positive Matrix Factorization (PMF) model, δ¹⁵N values and Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model. The results indicated that the contributions of the four main nitrogen sources were obtained and shown in descending order: combustion and industrial emission (42.06%) > secondary aerosols (24.04%) > vehicle exhaust (23.57%) > re-suspended dust (10.33%). The nitrogen aerosols might be mainly influenced by local emissions on normal and slight pollution days, while by both local emissions and transport from other areas on moderate and serious pollution days. Furthermore, nitrogen-containing species in PM_2.5 primarily originated from long/medium-distance transportation in two serious pollution events during the entire campaign.

Severe haze in northern China: A synergy of anthropogenic emissions and atmospheric processes

Regional severe haze represents an enormous environmental problem in China, influencing air quality, human health, ecosystem, weather, and climate. These extremes are characterized by exceedingly high concentrations of fine particulate matter (smaller than 2.5 µm, or PM_2.5) and occur with extensive temporal (on a daily, weekly, to monthly timescale) and spatial (over a million square kilometers) coverage. Although significant advances have been made in field measurements, model simulations, and laboratory experiments for fine PM over recent years, the causes for severe haze formation have not yet to be systematically/comprehensively evaluated. This review provides a synthetic synopsis of recent advances in understanding the fundamental mechanisms of severe haze formation in northern China, focusing on emission sources, chemical formation and transformation, and meteorological and climatic conditions. In particular, we highlight the synergetic effects from the interactions between anthropogenic emissions and atmospheric processes. Current challenges and future research directions to improve the understanding of severe haze pollution as well as plausible regulatory implications on a scientific basis are also discussed.