Total Inorganic N Research Articles

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.

Comparison of single vs composite soil electrical apparent conductivity-directed samples of irrigated soils for inorganic nitrogen estimation with electromagnetic induction sensors

The management of soil can be improved by precision agriculture. A way to achieve this is by quantifying NH4+–N and NO3−–N in soil. The sampling of soil for quantifying these N species is a laborious task. Electromagnetic induction (EMI) sensors could ease the task, providing a large quantity of data. The correlation of inorganic N with soil electrical apparent conductivity (ECa) in either single or composite samples is essential to accomplish this. Using an ECa-directed sampling approach, suitable sampling points were identified at four centre pivots. Single and composite samples were collected in 300 mm increments to 600 mm at Hofmeyr and 1500 mm at Douglas, Luckhoff and Empangeni. The samples were analysed for NH4+–N and NO3−–N from which total inorganic N (TIN) was calculated. All three inorganic N fractions were correlated insignificantly to in-field readings of ECa, implicating that EMI sensors are not suitable for inorganic N estimations in these irrigated soils. A comparison between single and composite samples, however, showed a good agreement of NH4+–N concentrations which was not true for NO3−–N or TIN concentrations. Composite samples are recommended for inorganic N measurements with an EMI sensor. Single samples could be used when only NH4+–N is analysed.

Post‐farmland conversion spatio‐temporal dynamics of net soil nitrogen mineralization and availability in a chronosequence of Zanthoxylum bungeanum plantations

AbstractAfforestation on cultivated farmlands causes major shifts in the nitrogen (N) cycle. The consequences of large‐scale Zanthoxylum bungeanum afforestation on spatio‐temporal patterns of soil N mineralization and inorganic N (ION) availability have not been reported. Moreover, the regulatory roles of microbial biomass and soluble organic N (MBN and SON) in the N cycle are poorly characterized in soils below 20 cm. To investigate the long‐term effects of afforestation on the governing mechanism of vertical N dynamics, a 0–100 cm soil profile was collected from a chronosequence of Z. bungeanum plantations aged 8‐year (H8), 15‐year (H15), 20‐year (H20) and 28‐year (H28), as well as adjacent farmland and abandoned‐land (28‐year) as controls in an arid valley in Southwest China. With increasing stand age, conversion of farmland to Z. bungeanum plantations significantly improved soil organic carbon, available nutrients, and all N forms. These impacts were more evident in the topsoil (0–20 cm) than in the subsoil layers. MBN and SON contents improved by 1.42‐fold and 1.34‐fold in H28, respectively, compared to H8. Net N mineralization (1.31‐fold), net nitrification rates (1.30‐fold), and total ION (1.31‐fold) content followed similar trends of increase along with the stand age. Correlation and redundancy analysis also established a positive relationship and demonstrated that increased ION availability is due to improved MBN, SON, and urease activity with the plantations age. Although the nitrate‐N content was highest in abandoned‐land, its content also increased steadily in Z. bungeanum plantations with the stand age. A relatively low ammonium/nitrate ratio (0.331) in H28 advocated improved N supply via nitrification as well as low N leaching risks from plantations. The spatial investigation provided novel insights into controls of N cycle and suggested converting farmland to Z. bungeanum plantations is a suitable approach for restoring soil N.

Soil Nitrogen Transformation Process Influenced by Litterfall Manipulation in Two Subtropical Forest Types.

Nitrogen (N) is often recognized as the primary limiting nutrient element for the growth and production of forests worldwide. Litterfall represents a significant pathway for returning nutrients from aboveground parts of trees to the soils and plays an essential role in N availability in different forest ecosystems. This study explores the N transformation processes under litterfall manipulation treatments in a Masson pine pure forest (MPPF), and Masson pine and Camphor tree mixed forest (MCMF) stands in subtropical southern China. The litterfall manipulation included litterfall addition (LA), litterfall removal (LR), and litterfall control (LC) treatments. The project aimed to examine how litterfall inputs affect the soil N process in different forest types in the study region. Results showed that soil ammonium N (NH4+-N) and nitrate N (NO3−-N) content increased under LA treatment and decreased under LR treatment compared to LC treatment. LA treatment significantly increased soil total inorganic N (TIN) content by 41.86 and 22.19% in MPPF and MCMF, respectively. In contrast, LR treatment decreased the TIN content by 10 and 24% in MPPF and MCMF compared to LC treatment. Overall, the soil net ammonification, nitrification, and N mineralization rates were higher in MCMF than in MPPF; however, values in both forests were not significantly different. LA treatment significantly increased annual net ammonification, nitrification, and mineralization in both forest types (p < 0.05) compared to LC treatment. LR treatment significantly decreased the values (p < 0.05), except for ammonification, where LR treatment did not differ substantially compared to LC treatment. Our results suggested that changes in litterfall inputs would significantly alter soil N dynamics in studied forests of sub-tropical region. Moreover, mixed forest stands have higher nutrient returns due to mixed litter and higher decomposition compared to pure forest stands.

Historical trends of riverine nitrogen loading from land to the East China Sea: a model-based evaluation

East Asia is the one of the hotspot regions with too much reactive nitrogen (N) inputs from anthropogenic sources. Here, we evaluated historical total inorganic N (TIN) load from land to sea through the rivers surrounding the East China sea using biogeochemical model ‘VISIT’ combined with a newly developed VISIT Off-line River Nitrogen scheme (VISIToRN). VISIT calculated N cycling in both natural and agricultural ecosystems and VISIToRN calculated inorganic N transport and riverine denitrification through the river channels at half degree spatial resolution. Between 1961 and 2010, the estimated TIN load from land to the sea surrounding the East China Sea increased from 2.7 Tg-N Year−1 to 5.5 Tg-N Year−1, a twofold increase, while the anthropogenic N input to the East China Sea basin (N deposition, N fertilizer, manure, and human sewage) increased from 12.9 Tg-N Year−1 to 36.9 Tg-N Year−1, an increase of about 3 times. This difference in the rate of increase is due in large part to the terrestrial nitrogen budget, and the results of the model balance indicate that TIN load to rivers has been suppressed by improvements in fertilizer application rates, harvesting on agricultural land, and nitrogen accumulation in forests. The results of the model balance showed that the increase rate of nitrogen runoff from Chinese rivers has been declining since 2000. In our estimation by VISIToRN, the amount of nitrogen removed by river denitrification in the river channel before the mouth is not negligible, ranging from 1.6 Tg-N Year−1 to 2.16 Tg-N Year−1. The N load from agricultural sources is still significant and needs to be further reduced. TIN load tended to increase in years with high precipitation. In order to effectively reduce TIN load, it is necessary to consider climate change-adaptive agricultural N management.

Overlooked contribution of water column to nitrogen removal in estuarine turbidity maximum zone (TMZ)

Estuarine systems are important sites of eliminating reactive nitrogen (N) delivered from land to sea. Numerous studies have focused on N cycling in estuarine sediment. However, the N elimination role of suspended sediments in estuarine turbid water column, which might provide anaerobic microenvironment for N loss, is rarely considered. This study examined the community dynamics and activities of denitrifying and anaerobic ammonium oxidation (anammox) bacteria in the water column of the turbidity maximum zone (TMZ) of the Yangtze Estuary, using molecular and 15N isotope-tracing techniques. Results showed that the anammox bacterial community was dominated by Candidatus Kuenenia and Candidatus Brocadia in the TMZ water column, while the main nirS-harboring denitrifiers were affiliated with Rhodobacterales. The denitrifying nirS gene was two orders of magnitude more abundant than anammox bacterial 16S rRNA gene, ranging from 1.77 × 105 to 1.42 × 108 copies l−1 and from 7.68 × 104 to 4.27 × 106 copies l−1, respectively. Compared with anammox, denitrification, with rates of 0.88 to 20.83 μmol N l−1 d−1, overwhelmingly dominated the N removal in the TMZ water column and was significantly correlated to suspended sediment concentrations (SSC). Based on the measured N removal rates, it was estimated that about 2.5 × 105 ton N was annually removed from the TMZ water column, accounting for approximately 18.5% of the total inorganic N (TIN) discharged from the Yangtze River. Overall, this study implies the importance of estuarine turbid water column in controlling N budget, and also improves the understanding of N loss mechanisms in estuarine TMZ systems.

Evaluating foliar characteristics as early indicators of plant response to biochar amendments

Biochar amendments are widely promoted for increasing soil nutrient availability and plant yield, and although many plant species show positive growth responses, neutral to negative effects can also occur. Although biochar effects on plant yield may not yet be evident in the first growing season after amendment, foliar characteristics such as nitrogen (N) and chlorophyll concentrations are known to respond to changes in soil fertility and therefore may provide an early indication of plant performance. In this study, we investigated the effects of contrasting biochar treatments on soil nitrogen (N) forms and plant chlorophyll, and the relationships between soil and plant nitrogen status, for two economically important conifer species, Abies balsamea (L.) Mill and Picea pungens Englem., as well as forb and grass weeds. We applied two types of pyrolysis biochars (hereafter, USB and BGR) that differed in feedstock type (pine wood versus mixed forest harvesting residue) at contrasting application rates (low, high, and untreated control) to a fine sandy loam soil. We measured soil water content (SWC), pH, and inorganic nitrogen (NH4+, NO3−, and total inorganic N (TIN)), as well as plant foliar N, and chlorophyll content (Chl-a, -b, -ab), and fluorescence (Fv/Fm). SWC response was variable whereas all treatments except USB low increased pH. All treatments decreased inorganic N concentrations relative to the control, although differences diminished over the study period. USB high decreased Picea foliar N concentration whereas all treatments decreased grass foliar N. BGR decreased Abies Fv/Fm (low) and Picea Chl-a, Chl-ab (both levels), and Chl-b (low), whereas USB increased Abies Chl-ab (high). There was a weak positive relationship between foliar N and soil NH4+ (r2 = 0.14) and NO3− (r2 = 0.11) in Abies seedlings, and between foliar N and TIN (r2 = 0.16, 0.11) in both conifers. Abies foliar N showed a weak positive effect on Chl-a (r2 = 0.22), Chl-b (r2 = 0.10), and Chl-ab (r2 = 0.22). Soil NO3− (r2 = 0.14) and TIN (r2 = 0.14) showed a weak negative correlation with Picea foliar Fv/Fm. This study shows that biochar type and amendment level influence soil and plant nitrogen responses, as well as their relationships, and responses may differ between plant species. Evaluating foliar characteristics shows promise as an early indicator of plant response to biochar, and may provide additional important information for evaluating contrasting biochars as potential sustainable soil amendments.

Model Inter-Comparison Study for Asia (MICS-Asia) phase III: multimodel comparison of reactive nitrogen deposition over China

Abstract. Atmospheric nitrogen deposition in China has attracted public attention in recent years due to the increasing anthropogenic emission of reactive nitrogen (Nr) and its impacts on the terrestrial and aquatic ecosystems. However, limited long-term and multisite measurements have restrained the understanding of the mechanism of the Nr deposition and the chemical transport model (CTM) improvement. In this study, the performance of the simulated wet and dry deposition for different Nr species, i.e., particulate NO3- and NH4+, gaseous NOx, HNO3 and NH3 have been conducted using the framework of Model Inter-Comparison Study for Asia (MICS-Asia) phase III. A total of nine models, including five Weather Research and Forecasting models coupled with the Community Multiscale Air Quality (WRF-CMAQ) models, two self-developed regional models, a global model and a Regional Atmospheric Modeling System coupled with the Community Multiscale Air Quality (RAMS-CMAQ) model have been selected for the comparison. For wet deposition, observation data from 83 measurement sites from the East Asia Acid Deposition Monitoring Network (EANET), Chinese Ecosystem Research Network (CERN), China Agricultural University Deposition Network (CAUDN), National Acid Deposition Monitoring Network (NADMN) and Department of Ecological Environment (DEE) of China have been collected and normalized for comparison with model results. In general, most models show the consistent spatial and temporal variation of both oxidized N (Nox) and reduced N (Nrd) wet deposition in China, with the normalized mean error (NME) at around 50 %, which is lower than the value of 70 % based on EANET observation over Asia. Both the ratio of wet or dry deposition to the total inorganic N (TIN) deposition and the ratios of TIN to their emissions have shown consistent results with the Nationwide Nitrogen Deposition Monitoring Network (NNDMN) estimates. The performance of ensemble results (ENMs) was further assessed with satellite measurements. In different regions of China, the results show that the simulated Nox wet deposition was overestimated in northeastern China (NE) but underestimated in the south of China, namely southeastern (SE) and southwestern (SW) China, while the Nrd wet deposition was underestimated in all regions by all models. The deposition of Nox has larger uncertainties than the Nrd, especially in northern China (NC), indicating the chemical reaction process is one of the most important factors affecting the model performance. Compared to the critical load (CL) value, the Nr deposition in NC, SE and SW reached or exceeded reported CL values and resulted in serious ecological impacts. The control of Nrd in NC and SW and Nox in SE would be an effective mitigation measure for TIN deposition in these regions. The Nr deposition in the Tibetan Plateau (TP) with a high ratio of TIN ∕ emission (∼3.0), indicates a significant transmission from outside. Efforts to reduce these transmissions ought to be paramount due the climatic importance of the Tibetan region to the sensitive ecosystems throughout China.

Atmospheric nitrogen deposition and its responses to anthropogenic emissions in a global hotspot region

Anthropogenic nitrogen (N) emission has been the major cause of rapidly increasing atmospheric N deposition in many regions of the world. However, fluxes, distributions, and physi-chemical compositions of regional N deposition and the response of N deposition levels to N emissions remain poorly characterized, particularly in high N-emission regions. To explore these issues, this study collated observations on N deposition fluxes in the Beijing-Tianjin-Hebei (BTH) region of northern China. We found that total inorganic N (TIN) deposition in the study region averaged 61.3 ± 13 kg-N/ha/yr based on observations from 1998 to 2017. Respectively, about 97%, 73%, 20%, 32%, and 59% of farmland, constructed land, forest, grassland, and water areas had TIN deposition above 50 kg-N/ha/yr. These results indicated substantial ecological risks of atmospheric N loading to these ecosystems. Dry deposition accounted for 59% of TIN deposition and gaseous N accounted for 73% in dry IN deposition. In gaseous N deposition, ammonia, N dioxides, and nitric acid accounted for 42%, 47%, and 11%, respectively. Ratios of NH3-N (N in ammonia) to (NO2-N + HNO3-N) (N in N dioxides, nitric acid) or NH4-N (N in ammonium) to NO3-N (N in nitrate) averaged 0.8, 2.0, 1.0, 1.9, and 1.3 in gaseous, particulate, dry, wet, and total deposition fluxes, respectively. These indicated different transformation and deposition mechanisms between NH4-N and NO3-N under high emissions. Moreover, we found a non-linear relationship between TIN deposition and anthropogenic IN emissions. All these results provide a systematic assessment of the N deposition and its responses to human activities in a high N-emission region, which is valuable for emission mitigation and effect evaluation of regional N pollution.

Soil C and N dynamics and their non-additive responses to litter mixture under different moisture conditions from an alpine steppe soil, Northern Tibet

Although there is a strong consensus that biodiversity can influence ecosystem functioning, how litter diversity affects soil ecological processes in different climatic contexts has been rarely studied. Here, we employed a mixed-litters’ experiment to examine how mixed-litters decomposition affects soil carbon (C) and nitrogen (N) dynamics under different moisture conditions from an alpine steppe soil in Northern Tibet. Litter of alpine steppe species, in monoculture or combination, were mixed with soil in 20%, 30% and 40% water holding capacities (WHC) in microcosms. During each microcosm incubation, we measured CO2 emissions, soluble organic carbon (SOC), total inorganic N (TIN) and microbial biomass C (MBC). Our results showed that CO2, SOC, TIN and MBC differed significantly among litter treatments and soil moisture. In the litter mixture, frequent and unsystematic non-additive effects could be found in CO2, SOC, TIN and MBC, and the non-additivities could be explained by the litter chemical traits. Higher soil moisture strengthened the synergistic effects of litter-mixing on CO2 and antagonistic effects on TIN, while weaken the synergistic effect on SOC and the antagonistic effects on MBC. This result indicated that, under multiple litter species decomposition, a changing soil moisture from dry to wet generally accelerate CO2 emission and increase MBC, while decrease the accumulation of SOC and TIN. Our work demonstrated that soil C and N dynamics and their non-additive responses to litter mixture were climatic context dependent. Soil moisture condition should be considered in understanding the effects of litter diversity on soil C and N processes.

Partial nitrification enhances natural attenuation of nitrogen in a septic system plume

Natural attenuation of nitrogen (N) was investigated in a well characterized septic system plume at a campground in Ontario, Canada. Total inorganic N (TIN) concentrations in deeper portions of the plume were about one third of the septic tank value of 40.7mgL−1. NH4+ and NO3− isotopic characterization were used to provide insight into potential attenuation processes. Concentrations of NH4+ and NO3− were highly variable in the plume, but approached the septic tank TIN value in some shallow zones and exhibited δ15N values like the tank value of +6‰. However, isotopic enrichment (up to +24‰ for NH4+ and +45‰ for NO3−) and declining TIN concentrations in the deeper zones indicated that anaerobic ammonium oxidation contributed to the TIN attenuation. The degree of isotopic enrichment increased at lower NH4+ concentrations and was consistent with Rayleigh-type distillation with an enrichment factor (Ɛ) of −5.1‰. Additionally, decreasing DOC values with depth and the concomitant enrichment of δ15NNO3 and δ18ONO3, suggested that denitrification was also active.The N attenuation observed in the Killarney plume was partly due to incomplete nitrification that occurred because of the shallow water table, which varied from only 0.2–0.7m below the tile bed infiltration pipes. Moreover, some of the monitoring locations with the shallowest water table distances from the infiltration pipes, had the highest degree of TIN attenuation (70–90%) in the plume. This behavior suggests that controlling water table distance from the infiltration pipes could be a useful mechanism for enhancing N attenuation in septic system plumes.

Chemical diversity and incubation time affect non-additive responses of soil carbon and nitrogen cycling to litter mixtures from an alpine steppe soil

The mechanisms of litter-mixing effects on soil carbon (C) and nitrogen (N) cycling in alpine ecosystems remain inconclusive. In this study, we employed a four-month litter-mixing experiment to examine the relationship between litter chemical diversity, incubation time and litter-mixing effects on soil C and N fluxes from an alpine steppe ecosystem in Northern Tibet. Litter mixtures caused predominantly non-additive effects on soil C and N fluxes, with more synergistic effects for carbon dioxide (CO2)/nitrous oxide (N2O) emissions while more antagonistic effects for soluble organic C (SOC), total inorganic N (TIN), microbial biomass C (MBC) and urease activity (UA). Litter-mixture would largely increase the fluxes of CO2, N2O emission and SON, while decrease SOC, TIN, MBC and UA concentration. We calculated six chemical diversity indices, and found litter chemical diversity correlated with the strength of litter-mixing effect on soil C and N, but the indices we chose may be influenced our understanding of the relationship. Our results also showed that models including the chemical diversity indices and incubation time generally gave better explanation on variations of litter-mixing effects. This work demonstrated a general relationship between litter chemical diversity and non-additive responses of soil C and N cycling, and suggested that incubation time is an important factor in understanding the litter-mixing effects.

Cover Crop Management in a Chardonnay/99 Richter Vineyard in the Coastal Region, South Africa. 3. Effect of Different Cover Crops and Cover Crop Management Practices on Organic Matter and Macro-nutrient Content of a Medium-textured Soil

The trial was conducted over a period of 10 years (1993/94 to 2002/03) on a medium-textured soil in a Chardonnay/99 Richter vineyard near Stellenbosch (33°55’S, 18°52’E), which is situated in the Coastal wine grape region of the Western Cape, South Africa. Sixteen treatments, consisting of three cereals and five legumes, managed according to two cover crop management practices, were included. These treatments were compared to a control, in which no cover crop was sown and the weeds were controlled mechanically in the work row and chemically in the vine row from bud break to harvest (approximately the first week of February). A treatment in which no cover crop was sown and full-surface post-emergence chemical weed control was applied from before bud break to harvest (BB) (weedchem) was also included. After five seasons, the soil organic matter (SOM) content in the 0 to 300 mm soil layer increased in all the cover crop management treatments. In weedchem and in the control, SOM remained unchanged and decreased by 16% respectively. The SOM content in the 0 to 150 mm soil layer of the cover crop treatments was, with the exception of Vicia dasycarpa Ten. (grazing vetch), significantly higher than that of the mechanically-cultivated control after a period of 10 years. The SOM content in the 0 to 300 mm soil layer of Secale cereale L. v. Henog and the treatments in which the N-fixing cover crops were sown (with the exception of grazing vetch) was significantly higher than that of weedchem. The total inorganic N (TIN) concentration of the 0 to 150 mm soil layer in the BB treatments of the two Medicago species and Trifolium subterraneum L. v. Woogenellup, as measured for the 1996/97 season during full bloom of the grapevines, was significantly higher than that of the control, weedchem, and the treatments in which full-surface chemical control was applied after bud break (AB). The TIN concentration of the 0 to 600 mm soil layer in the AB treatment of a species, measured after harvest in 2002/03, tended to be higher than that of the BB treatment of that species. The applied treatments had no significant effect on the exchangeable K, Ca and Mg.

Quantification of nitrogen dry and wet deposition in Fujian tobacco planting area

In this study, the wet/bulk and dry atmospheric deposition of different inorganic nitrogen (N) species was investigated in the Fujian tobacco planting area from 2010 to 2014. Wet/bulk N deposition flux of inorganic N was monitored at five sites (Fuzhou, Shanghang, Changting, Taining, and Wuyishan) of Fujian by using precipitation gauges. While dry deposition fluxes of gaseous NH3, NO2, and HNO3 (gNH3, gNO2, and gHNO3), and particulate NH4+ and NO3− (pNH4+ and pNO3−) were estimated only at the Fuzhou site through multiplying monthly mean Nr concentrations obtained from DELTA (DEnuder for Long-Term Atmospheric sampling) system (gNH3, gHNO3, pNH4+, pNO3−) and passive samplers (gNO2) and the modeled monthly mean dry deposition velocities provided by the GEOS-Chem global chemical transport model. Annual mean NH4+-N, NO3−-N, and total inorganic N (TIN) concentrations in precipitation (rainwater and snow) were 0.21–1.05, 0.33–0.74, and 0.59–1.68mgNL−1, respectively, and the averages over the monitoring period were 0.55, 0.49, and 1.04mgNL−1, respectively. Across the five monitoring sites, the average annual wet/bulk N deposition was 17.6kgNha−1a−1, while that of NH4+-N and NO3−-N amounted to 9.2 and 8.4kgNha−1a−1, respectively. Wet/bulk deposition was lowest at Changting, but no significant differences were found between the other four sites. Annual mean atmospheric concentrations of gNH3, gNO2, gHNO3, pNH4+, and pNO3− were 1.87, 2.95, 0.45, 2.36, and 1.23μgNm−3, respectively. Gaseous NH3 concentrations were highest in summer and lowest in winter. In contrast, seasonal fluctuations in other measured Nr species (e.g., gNO2) were not as large as those of gNH3.The annual mean total dry deposition was estimated to be 9.6kgNha−1a−1, and those of the individual species, gNH3, gNO2, gHNO3, pNH4+, and pNO3− were estimated to be 4.1, 2.2, 2.0, 0.8, and 0.4kgNha−1a−1, respectively. Based on the similarities in wet deposition, if we assume a similar level of dry deposition at all the sites, the total (dry and wet) N deposition was 27.2kgNha−1a−1 on average, which twice the critical load (10–15kgNha−1a−1) for forest ecosystems, suggesting a risk of “N saturation” in the local natural environment.

Inorganic nitrogen wet deposition in eastern China: Comparison of different land use-based monitoring sites in north and south regions

China is one of the global hotspots of atmospheric nitrogen (N) deposition. This is especially true for eastern China, an economically developed and densely populated area, which emits large quantities of anthropogenic reactive N species to the air and experiences high levels of atmospheric N deposition. Few studies, however, have examined spatial variations of N deposition across different ecosystems in northern and southern regions of eastern China. Using the traditional rain gauge (SDM6A) method, we conducted a three-year (2011-2013) study to quantify wet N deposition fluxes at twelve sites (six northern and six southern monitoring sites, covering urban, rural and background land use types) in China. The annual volume-weighted mean (VWM) concentrations of NH4+-N, NO3--N and total inorganic N (TIN) atmonitoring sites were in the ranges of 0.62-2.76, 0.54-2.50 and 1.25-4.92 mg N·L-1, and averaged 1.4, 1.5 and 2.9 mg N·L-1, respectively. Northern sites showed higher concentrations of N species in precipitation than southern sites. Annual deposition fluxes of NH4+-N, NO3--N and TIN were in the ranges of 7.0-18.3, 6.9-18.9 and 14.9-32.6 kg N·hm-2·a-1, averaging 11.5, 12.2 and 23.7 kg N·hm-2·a-1, respectively. There were significant differences in total N deposition fluxes among different land use types in the north, showing the decreasing order of urban sites (26.3±6.4 kg N·hm-2·a-1)> rural sites (21.8±3.5 kg N·hm-2·a-1)> background (15.5±1.3 kg N·hm-2·a-1). Total N deposition fluxes at urban, rural and background sites in the south were (26.8±2.7), (25.5±2.9) and (20.5±2.4) kg N·hm-2·a-1, respectively, showing no significant spatial variation. While urban sites did not show obvious regional differences, total N wet deposition fluxes at rural and background sites in the south were significantly higher than those at corresponding sites in the north. Our results revealed that both south and north regions of eastern China (including background areas) are receiving high levels of wet N deposition and associated ecological and environmental risks should be considered.

Inorganic nitrogen deposition in China's forests: Status and characteristics

Nitrogen (N) deposition in China has been dramatically enhanced by anthropogenic emissions and has aroused great concerns of its impacts on forest ecosystems. This study synthesized data on ammonium (NH4+) and nitrate (NO3−) contents in bulk precipitation and throughfall from 38 forest stands in published literature to assess the status and characteristics of N deposition to typical forests in China between 1995 and 2010. Our results showed that ammonium dominated N deposition in this period, with a mean NH4+–N:NO3−–N ratio of ∼2.5 in bulk deposition and throughfall. Mean throughfall N deposition in China's forests was as high as 14.0 kg N ha−1 yr−1 for ammonium, 5.5 kg N ha−1 yr−1 for nitrate and 21.5 kg N ha−1 yr−1 for total inorganic N (TIN), respectively. Mean bulk deposition was 9.4 kg N ha−1 yr−1 for ammonium, 3.9 kg N ha−1 yr−1 for nitrate and 14.0 kg N ha−1 yr−1 for TIN, respectively. Canopy captured dry deposition, calculated as the difference between throughfall and bulk deposition, was thus approximately half of the bulk deposition. Spatial patterns of N deposition were in accordance with our urban hotspot hypothesis, showing a strong power-law reduction of ammonium with increasing distance to large cities but only slightly lower nitrate deposition. Our results suggest that high N deposition, especially of ammonium, exceeds critical N loads for large areas of China's forests.

Soil carbon and nitrogen dynamics in the first year following herbicide and scalping in a revegetation trial in south-east Queensland, Australia

During revegetation, the maintenance of soil carbon (C) pools and nitrogen (N) availability is considered essential for soil fertility and this study aimed to evaluate contrasting methods of site preparation (herbicide and scalping) with respect to the effects on soil organic matter (SOM) during the critical early establishment phase. Soil total C (TC), total N (TN), hot-water extractable organic C (HWEOC), hot-water extractable total N (HWETN), microbial biomass C and N (MBC and MBN), total inorganic N (TIN) and potentially mineralizable N (PMN) were measured over 53 weeks. MBC and MBN were the only variables affected by herbicide application. Scalping caused an immediate reduction in all variables, and the values remained low without any sign of recovery for the period of the study. The impact of scalping on HWETN and TIN lasted 22 weeks and stabilised afterwards. MBC and MBN were affected by both herbicide and scalping after initial treatment application and remained lower than control during the period of the study but did not decrease over time. While scalping had an inevitable impact on all soil properties that were measured, that impact did not worsen over time, and actually improved plant growth (unpublished data) while reducing site establishment costs. Therefore, it provides a useful alternative for weed control in revegetation projects where it is applied only once at site establishment and where SOM would be expected to recover as canopy closure is obtained and nutrient cycling through litterfall commences.

Effects of nitrogen on the removal of dibutyl phthalate from surface water in the presence of Potamogeton crispus L.

Experiments in 5 surface waters with different levels and compositions of nitrogen were performed to investigate the effects of nitrogen on the removal of dibutyl phthalate (DBP) from surface water in the presence of Potamogeton crispus L., a submerged aquatic plant. The removal of DBP using P. crispus with (Group A) and without (Group B) associated microorganisms was studied and compared with that in the non-autoclaved surface water samples (Group C). The first-order rate constants for DBP loss from the aqueous medium in Groups A, B, and C were in the range of 0.016–0.227d−1, 0.008–0.112d−1 and 0.015–0.204d−1, respectively, and were mostly in the order of Group A>Group C>Group B. In Group B, a positive link between NO3−-N concentration and rate constant for DBP loss, as well between NO3−-N concentration and plant activity (chlorophyll a, soluble protein and nitrate reductase) was found. In addition, at the end of 10-day incubation, only small amount of DBP taken up by P. crispus was retained in P. crispus, which was no more than 11.5%, suggesting that DBP in P. crispus was mostly degraded. Different from Group B, rate constant for DBP loss in Group C increased with the increase of total inorganic N (TIN) concentration. The rate constants for DBP loss in Group A were 5.9–45.9% higher than those in Group C, and the enhancement was significantly related to the NO3−-N:TIN ratio. The most possible reason is that surface waters with higher NO3−-N:TIN ratio facilitate the activity of P. crispus, and consequently, removal of DBP in the presence of P. crispus is more significantly enhanced.

Impacts of Fire and Fire Surrogate treatments on forest soil properties: a meta‐analytical approach

The soils underlying the 12 Fire and Fire Surrogates Network include six soil orders and >50 named soil series. Across the network, pretreatment soils varied from 3.7 to 7.1 in pH, and exhibited ranges of twofold in bulk density, fourfold in soil organic C (SOC) content, 10-fold in total inorganic N (TIN), and 200-1000-fold in extractable Ca and K. Nonmetric multidimensional (NMS) ordination of pretreatment soil conditions arrayed the FFS sites along gradients of pH/base cation status, net N transformation rates, bulk density, and SOC. At the network scale, mineral soil exposure was significantly greater in fire-only (mean of 9.2%) and mechanical + fire (5.0%) treatments than in the controls (1.5%) during the first posttreatment year, and this persisted through the later sampling year (second through fourth year, depending on site) in the fire-only treatment (fire 4.1%, control 1.1%). Bulk density was not affected significantly at the network scale. TIN concentrations during the first posttreatment year increased after all three manipulative treatments, but this effect did not persist to the later sampling year. Neither SOC content nor soil C:N ratio was affected by any of the treatments at the network scale. At the individual site scale, the combined mechanical + fire treatment produced more significant site x treatment x year effects than did the fire-only or mechanical-only treatments, though in most cases even the statistically significant differences produced by the manipulative treatments were modest in magnitude. Ordination of first-year standardized effect sizes produced no discernable separation of the three manipulative treatments but did separate the three sites with the greatest fire severity (based on proportional fuel consumption) from the majority of the network sites, with changes in pH, TIN, SOC content, and soil C:N ratio correlating most strongly with this separation. Ordination of the effect sizes from the later sampling year produced somewhat clearer separation of treatments than did the first-year ordination, though fewer sites were represented in this second ordination. Overall, the network-wide effects of the FFS treatments on soil properties appear to have been modest and transient.

Influence of Trypaea australiensis population density on benthic metabolism and nitrogen dynamics in sandy estuarine sediment: A mesocosm simulation

Laboratory mesocosm incubations were undertaken to investigate the influence of natural densities of the thalassinidean shrimp, Trypaea australiensis (marine yabby) on sediment oxygen demand (SOD), inorganic nutrient fluxes, and the N-cycle processes of nitrification, denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Mesocosms (~ 0.1 m 2 × 55 cm deep) of sieved, natural T. australiensis inhabited sands were continually flushed with fresh seawater and pre-incubated for two weeks prior to being assigned to one of three treatments; control (no additions), low yabby density (40 T. australiensis m − 2 ) or high yabby density (80 T. australiensis m − 2 ). Thereafter, SOD and sediment–water column inorganic nutrient fluxes were determined periodically over a 38 day period. On the final day rates of denitrification and dissimilatory nitrate reduction to ammonium (DNRA) were also determined using the 15N-isotope pairing technique. Yabbies consistently and significantly ( p < 0.001) stimulated SOD over the entire 38 day incubation period (mean values: 4.92, 9.21 and 14.9 mmol m − 2 day − 1 for control, low and high density treatments, respectively). The increased organic matter mineralisation rates greatly enhanced nitrogen regeneration rates in the sediment and fuelled significantly higher effluxes of dissolved inorganic nitrogen with NH 4 + and total DIN effluxes in the low and high density treatments respectively, being 617 and 1534%, and 269 and 565% higher than those in the controls, despite sediment bioavailable (porewater + exchangeable) NH 4 + pools being approximately 2 and 4-fold lower in the low and high density yabby treatment sediments compared to the controls, measured at the end of the 37 day experiment. Mass balance calculations based on the final day nutrient flux and nitrate reduction rate data demonstrated that yabbies stimulated benthic nitrification rates by 31 and 46% in the low and high density treatments. However, somewhat surprisingly T. australiensis population density had no effect on rates of denitrification and DNRA despite the higher rates of nitrification and higher equilibrium water column nitrate concentration. Indeed, nitrate reduction processes became an increasingly unimportant element with increasing yabby density with for example, N 2 generated by coupled nitrification–denitrification representing 11.5, 5.2 and 2.8% of the total inorganic-N recycled to the water column in the control, low density and high density yabby treatments, respectively. Overall, the major influence of T. australiensis in the studied low organic matter content, sandy sediments was to enhance coupling between the benthic and pelagic systems through increased rates of inorganic nitrogen regeneration in the sediment and enhanced export of this nitrogen to the water column. Our results also suggest that the influences of organisms such as T. australiensis which form deep, extensive and complex burrow systems where irrigation rates differ greatly between different burrow sections, may be more complex than those recorded for infauna which form simple U-shaped burrows. Additionally, there may be a strong interaction between faunal effects and the sediment physical and biological environment and thus the same species may have contrasting influences in different sediment types.