Evidence and causes of recent decreases in nitrogen deposition in temperate forests in Northeast China.

High reactive nitrogen (N) emissions due to anthropogenic activities in China have led to an increase in N deposition and ecosystem degradation. The Chinese government has strictly regulated reactive N emissions since 2010, however, determining whether N deposition has reduced requires long-term monitoring. Here, we report the patterns of N deposition at a rural forest site (Qingyuan) in northeastern China over the last decade. We collected 456 daily precipitation samples from 2014 to 2022 and analysed the temporal dynamics of N deposition. NH4+-N, NO3--N, and total inorganic N (TIN) deposition ranged from 10.5 ± 3.5 (mean ± SD), 6.1 ± 1.6, and 16.6 ± 4.7 kg N ha-1 year-1, respectively. Over the measurement period, TIN deposition at Qingyuan decreased by 55 %, whereas that in comparable sites in East Asia declined by 14-34 %. We used a random forest model to determine factors influencing the deposition of NH4+-N, NO3--N, and TIN during the study period. NH4+-N deposition decreased by 60 % because of decreased agricultural NH3 emissions. Furthermore, NO3--N deposition decreased by 42 %, due to reduced NOx emissions from agricultural soil and fossil fuel combustion. The steep decline in N deposition in northeastern China was attributed to reduced coal consumption, improved emission controls on automobiles, and shifts in agricultural practices. Long-term monitoring is needed to assess regional air quality and the impact of N emission control regulations.

Large Seasonal Variation in Nitrogen Isotopic Abundances of Ammonia Volatilized from a Cropland Ecosystem and Implications for Regional NH3 Source Partitioning.

Ammonia (NH3) volatilization from agricultural lands is a main source of atmospheric reduced nitrogen species (NHx). Accurately quantifying its contribution to regional atmospheric NHx deposition is critical for controlling regional air nitrogen pollution. The stable nitrogen isotope composition (expressed by δ15N) is a promising indicator to trace atmospheric NHx sources, presupposing a reliable nitrogen isotopic signature of NH3 emission sources. To obtain more specific seasonal δ15N values of soil NH3 volatilization for reliable regional seasonal NH3 source partitioning, we utilized an active dynamic sampling technique to measure the δ15N-NH3 values volatilized from maize cropping land in northeast China. These values varied from -38.0 to -0.2‰, with a significantly lower rate-weighted value observed in the early period (May-June, -30.5 ± 6.7‰) as compared with the late period (July-October, -8.5 ± 4.3‰). Seasonal δ15N-NH3 variations were related to the main NH3 production pathway, degree of soil ammonium consumption, and soil environment. Bayesian isotope mixing model analysis revealed that without considering the seasonal δ15N variation in soil-volatilized NH3 could result in an overestimate by up to absolute 38% for agricultural volatile NH3 to regional atmospheric bulk ammonium deposition during July-October, further demonstrating that it is essential to distinguish seasonal δ15N profile of agricultural volatile NH3 in regional source apportionment.

Isotopic imprints of aerosol ammonium over the north China plain.

Atmospheric PM2.5 poses a variety of health and environmental risks to urban environments. Ammonium is one of the main components of PM2.5, and its role in PM2.5 pollution will likely increase in the coming years as NH3 emissions are still unregulated and rising in many cities worldwide. However, partitioning urban NH4+ sources remains challenging. Although the 15N natural abundance (δ15N) analysis is a promising approach for this purpose, it has seldom been applied across multiple cities within a given region. This limits our understanding of the regional patterns and controls of NH4+ sources in urban environments. Here, we collected PM2.5 samples using an active sampling technique during winter at six cities in the North China Plain to characterize the concentrations, δ15N and sources of NH4+ in PM2.5. We found substantial variations in both the concentrations and δ15N of NH4+ among the sites. The mean NH4+ concentrations across the six cities ranged from 3.6 to 12.1 µg m-3 on polluted days and from 0.9 to 10.6 µg m-3 on non-polluted days. The δ15N ranged from 6.5‰ to 13.9‰ on polluted days and from 8.7‰ to 13.5‰ on non-polluted days. The δ15N decreased with increasing NH4+ concentrations at all six sites. We found that non-agricultural sources (vehicle exhaust, ammonia slip and urban wastes) contributed 72%-94% and 56%-86% of the NH4+ on polluted and non-polluted days, respectively, and that during polluted days, combustion-related emissions (vehicle exhaust and ammonia slip) were positively associated with the proportion of urban area, population density and number of vehicles, highlighting the importance of local sources of particulate pollution. This study suggests that the analysis of 15N in aerosol NH4+ is a promising approach for apportioning atmospheric NH3 sources over a large region, and this approach has potential for mapping rapidly and precisely the sources of NH3 emissions.

Atmospheric nitrate formation pathways in urban and rural atmosphere of Northeast China: Implications for complicated anthropogenic effects.

Effects of human activities on atmospheric nitrate (NO3-) formation remain unclear, though the knowledge is critical for improving atmospheric chemistry models and nitrogen deposition reduction strategies. A potentially useful way to explore this is to compare NO3- oxidation processes in urban and rural atmospheres based upon the oxygen stable isotope composition of NO3- (Δ17O-NO3-). Here we compared the Δ17O-NO3- from three-years of daily-based bulk deposition in urban (Shenyang) and forested rural sites (Qingyuan) in northeast China and quantified the relative contributions of different formation pathways based on the SIAR model. Our results showed that the Δ17O in Qiangyuan (26.2 ± 3.3‰) is significantly higher (p < 0.001) than in Shenyang (24.0 ± 4.0‰), and significantly higher in winter (Shenyang: 26.1 ± 6.7‰, Qingyuan: 29.6 ± 2.5‰) than in summer (Shenyang: 22.7 ± 2.9‰, Qingyuan: 23.8 ± 2.4‰) in both sites. The lower values in the urban site are linked with conditions that favored a higher relative contribution of nitrogen dioxide reaction with OH pathway (0.76-0.91) than in rural site (0.47-0.62), which should be induced by different levels of human activities in the two sites. The seasonal variations of Δ17O-NO3- in both sites are explained by a higher relative contribution of ozone-mediated oxidation chemistry and unfavorable conditions for the OH pathway during winter relative to summer, which is affected by human activities and seasonal meteorological condition change. Based on Δ17O, wintertime conditions led to a contribution of O3 related pathways (NO3 + DMS/HC and N2O5 hydrolysis) of 0.63 in Qingyuan and 0.42 in Shenyang, while summertime conditions led to 0.15 in Qingyuan and 0.05 in Shenyang. Our comparative study on Δ17O-NO3- between urban and rural sites reveals different anthropogenic effects on nitrate formation processes on spatial and temporal scales, illustrating different responses of reactive nitrogen chemistry to changes in human activities.

Multiyear Measurements on Δ17O of Stream Nitrate Indicate High Nitrate Production in a Temperate Forest.

Nitrification is a crucial step in ecosystem nitrogen (N) cycling, but scaling up from plot-based measurements of gross nitrification to catchments is difficult. Here, we employed a newly developed method in which the oxygen isotope anomaly (Δ17O) of nitrate (NO3-) is used as a natural tracer to quantify in situ catchment-scale gross nitrification rate (GNR) for a temperate forest from 2014 to 2017 in northeastern China. The annual GNR ranged from 71 to 120 kg N ha-1 yr-1 (average 94 ± 10 kg N ha-1 yr-1) over the 4 years in this forest. This result and high stream NO3- loss (4.2-8.9 kg N ha-1 yr-1) suggest that the forested catchment may have been N-saturated. At the catchment scale, the total N output of 10.7 kg N ha-1 yr-1, via leaching and gaseous losses, accounts for 56% of the N input from bulk precipitation (19.2 kg N ha-1 yr-1). This result indicates that the forested catchment is still retaining a large fraction of N from atmospheric deposition. Our study suggests that estimating in situ catchment-scale GNR over several years when combined with other conventional flux estimates can facilitate the understanding of N biogeochemical cycling and changes in the ecosystem N status.

Uptake Patterns of Glycine, Ammonium, and Nitrate Differ Among Four Common Tree Species of Northeast China.

Fundamental questions of how plant species within secondary forests and plantations in northeast China partition limited nitrogen (N) resource remain unclear. Here we conducted a 15N tracer greenhouse study to determine glycine, ammonium, and nitrate uptake by the seedlings of two coniferous species, Pinus koraiensis (Pinus) and Larix keampferi (Larix), and two broadleaf species, Quercus mongolica (Quercus) and Juglans mandshurica (Juglans), that are common in natural secondary forests in northeast China. Glycine contributed 43% to total N uptake of Pinus, but only 20, 11, and 21% to N uptake by Larix, Quercus, and Juglans, respectively (whole plant), whereas nitrate uptake was 24, 74, 88, and 68% of total uptake for these four species, respectively. Retention of glycine carbon versus nitrogen in Pinus roots indicated that 36% of glycine uptake was retained intact. Nitrate was preferentially used by Larix, Quercus, and Juglans, with nitrate uptake constituting 68∼88% of total N use by these three species. These results demonstrated that these dominant tree species in secondary forests in northeast China partitioned limited N resource by varying uptake of glycine, ammonium and nitrate, with all species, except Pinus, using nitrate that are most abundant within these soils. Such N use pattern may also provide potential underlying mechanisms for the higher retention of deposited nitrate than ammonium into aboveground biomass in these secondary forests.

Fate of atmospherically deposited NH4+ and NO3- in two temperate forests in China: temporal pattern and redistribution.

The impacts of anthropogenic nitrogen (N) deposition on forest ecosystems depend in large part on its fate. However, our understanding of the fates of different forms of deposited N as well as the redistribution over time within different ecosystems is limited. In this study, we used the 15 N-tracer method to investigate both the short-term (1 week to 3 months) and long-term (1-3 yr) fates of deposited NH4+ or NO3- by following the recovery of the 15 N in different ecosystem compartments in a larch plantation forest and a mixed forest located in northeastern China. The results showed similar total ecosystem retention for deposited NH4+ and NO3- , but their distribution within the ecosystems (plants vs. soil) differed distinctly particularly in the short-term, with higher 15 NO3- recoveries in plants (while lower recoveries in organic layer) than found for 15 NH4+ . The different short-term fate was likely related to the higher mobility of 15 NO3- than 15 NH4+ in soils instead of plant uptake preferences for NO3- over NH4+ . In the long-term, differences between N forms became less prevalent but higher recoveries in trees (particularly in the larch forest) of 15 NO3- than 15 NH4+ tracer persisted, suggesting that incoming NO3- may contribute more to plant biomass increment and forest carbon sequestration than incoming NH4+ . Differences between the two forests in recoveries were largely driven by a higher 15 N recovery in the organic layer (both N forms) and in trees (for 15 NO3- ) in the larch forest compared to the mixed forest. This was due to a more abundant organic layer and possibly higher tree N demand in the larch forest than in the mixed forest. Leachate 15 N loss was minor (<1% of the added 15 N) for both N forms and in both forests. Total 15 N recovery averaged 78% in the short-term and decreased to 55% in the long-term but with increasing amount of 15 N label (re)-redistributed into slow turn-over pools (e.g., trees and mineral soil). The different retention dynamics of deposited NH4+ and NO3- may have implications in environmental policy related to the anthropogenic emissions of the two N forms.

Seasonal pattern of ammonium 15N natural abundance in precipitation at a rural forested site and implications for NH3 source partitioning.

Excess ammonia (NH3) emissions and deposition can have negative effects on air quality and terrestrial ecosystems. Identifying NH3 sources is a critical step for effectively reducing NH3 emissions, which are generally unregulated around the world. Stable nitrogen isotopes (δ15N) of ammonium (NH4+) in precipitation have been directly used to partition NH3 sources. However, nitrogen isotope fractionation during atmospheric processes from NH3 sources to sinks has been previously overlooked. Here we measured δ15NNH4+ in precipitation on a daily basis at a rural forested site in Northeast China over three years to examine its seasonal pattern and attempt to constrain the NH3 sources. We found that the NH4+ concentrations in precipitation ranged from 5 to 1265 µM, and NH4+ accounted for 65% of the inorganic nitrogen deposition (20.0 kg N ha-1 yr-1) over the study period. The δ15N values of NH4+ fluctuated from -24.6 to +16.2‰ (average -6.5‰) and showed a repeatable seasonal pattern with higher values in summer (average -2.3‰) than in winter (average -16.4‰), which could not be explained by only the seasonal changes in the NH3 sources. Our results suggest that in addition to the NH3 sources, isotope equilibrium fractionation contributed to the seasonal pattern of δ15NNH4+ in precipitation, and thus, nitrogen isotope fractionation should be considered when partitioning NH3 sources based on δ15NNH4+ in precipitation.