Denitrification Losses Research Articles

Fates of slurry-nitrogen applied to mountain grasslands: the importance of dinitrogen emissions versus plant N uptake

AbstractIntensive fertilization of grasslands with cattle slurry can cause high environmental nitrogen (N) losses in form of ammonia (NH3), nitrous oxide (N2O), and nitrate (NO3−) leaching. Still, knowledge on short-term fertilizer N partitioning between plants and dinitrogen (N2) emissions is lacking. Therefore, we applied highly 15N-enriched cattle slurry (97 kg N ha−1) to pre-alpine grassland field mesocosms. We traced the slurry 15N in the plant-soil system and to denitrification losses (N2, N2O) over 29 days in high temporal resolution. Gaseous ammonia (NH3), N2 as well N2O losses at about 20 kg N ha−1 were observed only within the first 3 days after fertilization and were dominated by NH3. Nitrous oxide emissions (0.1 kg N ha−1) were negligible, while N2 emissions accounted for 3 kg of fertilizer N ha−1. The relatively low denitrification losses can be explained by the rapid plant uptake of fertilizer N, particularly from 0–4 cm depth, with plant N uptake exceeding denitrification N losses by an order of magnitude already after 3 days. After 17 days, total aboveground plant N uptake reached 100 kg N ha−1, with 33% of N derived from the applied N fertilizer. Half of the fertilizer N was found in above and belowground biomass, while at about 25% was recovered in the soil and 25% was lost, mainly in form of gaseous emissions, with minor N leaching. Overall, this study shows that plant N uptake plays a dominant role in controlling denitrification losses at high N application rates in pre-alpine grassland soils.

Herbicide Applications Reduce Gaseous N Losses: A Field Study of Three Consecutive Wheat–Maize Rotation Cycles in the North China Plain

Herbicide residues in farmland soils have attracted a great deal of attention in recent decades. Their accumulation potentially decreases the activity of microbes and related enzymes, as well as disturbs the nitrogen cycle in farmland soils. In previous studies, the influence of natural factors or nitrogen fertilization on the soil nitrogen cycle have frequently been examined, but the role of herbicides has been ignored. This study was conducted to examine the effects of herbicides on NH3 volatilization- and denitrification-related nitrogen loss through three rotation cycles from 2013 to 2016. The four treatments included no urea fertilizer (CK), urea (CN), urea+acetochlor-fenoxaprop-ethyl (AC-FE), and urea+2,4D-dicamba (2,4D-DI) approaches. The results showed that the application of nitrogen fertilizer significantly increased the nitrogen losses from ammonia volatilization and denitrification in the soil. Ammonia volatilization was the main reason for the gaseous loss of urea nitrogen in a wheat–maize rotation system in the North China Plain (NCP), which was significantly higher than the denitrification loss. In the CK treatment, the cumulative nitrogen losses from ammonia volatilization and denitrification during the three crop rotation cycles were 66.64 kg N hm−2 and 8.07 kg N hm−2, respectively. Compared with CK, the nitrogen losses from ammonia volatilization and denitrification under the CN treatment increased 52.62% and 152.88%, respectively. The application of AC-FE and 2,4D-DI significantly reduced the nitrogen gas losses from the ammonia volatilization and denitrification in the soil. Ammonia volatilization reduction mainly occurred during the maize season, and the inhibition rates of AC-FE and 2,4D-DI were 7.72% and 11.80%, respectively, when compared with CN. From the perspective of the entire wheat–maize rotation cycle, the inhibition rates were 5.41% and 7.23% over three years, respectively. Denitrification reduction also mainly occurred in the maize season, with the inhibition rates of AC-FE and 2,4D-DI being 34.12% and 30.94%, respectively, when compared with CN. From the perspective of the entire wheat–maize rotation cycle, the inhibition rates were 28.39% and 28.58% over three years, respectively. Overall, this study demonstrates that herbicides could impact the nitrogen cycle of farmland soil ecosystems via the suppression of ammonia volatilization and denitrification rates, thus reducing gaseous N losses and mitigating global climate change.

Emission of gaseous nitrogen oxides in soils of boreal forests (review)

Denitrification losses of nitrogen-containing gases in the biome of boreal forests are discussed. In the soils of coniferous and deciduous forests of Western Europe, 0.57 ± 0.2 and 1.0 ± 0.2 kg N-N2O/ha/ year are lost during denitrification. In North America this figure was 0.35 ± 0.29 kg N-N2O/ha/yr for all stands. The emission of N2O from forest soils correlated with the input of nitrogen from the atmosphere withr= 0.47 in coniferous forests and withr= 0.68 in deciduous plantations, returning to the atmosphere up to 30% of the nitrogen supplied with atmospheric precipitation. With a high input of nitrogen from the atmosphere, the emission of nitrogen-containing gases reached 20 kg N /ha /yr. Of these, NO, N2O, and N2accounted for 21, 15, and 64%. Measurements of NO and especially N2emissions remain very rare, leading to incomplete estimates of denitrification losses. Denitrification remains the most complex process in the nitrogen cycle, with no definitive methods for measuring it.

Denitrification Losses in Response to N Fertilizer Rates—Integrating High Temporal Resolution N2O, In Situ 15N2O and 15N2 Measurements and Fertilizer 15N Recoveries in Intensive Sugarcane Systems

AbstractDenitrification is a key process in the global nitrogen (N) cycle, causing both nitrous oxide (N2O) and dinitrogen (N2) emissions. However, estimates of seasonal denitrification losses (N2O + N2) are scarce, reflecting methodological difficulties in measuring soil‐borne N2 emissions against the high atmospheric N2 background and challenges regarding their spatio‐temporal upscaling. This study investigated N2O + N2 losses in response to N fertilizer rates (0, 100, 150, 200, and 250 kg N ha−1) on two intensively managed tropical sugarcane farms in Australia, by combining automated N2O monitoring, in situ N2 and N2O measurements using the 15N gas flux method and fertilizer 15N recoveries at harvest. Dynamic changes in the N2O/(N2O + N2) ratio (<0.01 to 0.768) were explained by fitting generalized additive mixed models (GAMMs) with soil factors to upscale high temporal‐resolution N2O data to daily N2 emissions over the season. Cumulative N2O + N2 losses ranged from 12 to 87 kg N ha−1, increasing non‐linearly with increasing N fertilizer rates. Emissions of N2O + N2 accounted for 31%–78% of fertilizer 15N losses and were dominated by environmentally benign N2 emissions. The contribution of denitrification to N fertilizer loss decreased with increasing N rates, suggesting increasing significance of other N loss pathways including leaching and runoff at higher N rates. This study delivers a blueprint approach to extrapolate denitrification measurements at both temporal and spatial scales, which can be applied in fertilized agroecosystems. Robust estimates of denitrification losses determined using this method will help to improve cropping system modeling approaches, advancing our understanding of the N cycle across scales.

Effects of herbicide butachlor application on the growth of periphytic biofilms and nitrogen loss in paddy systems

The application of butachlor as an herbicide in paddy fields is widely practiced, aiming to increase rice yield by directly or indirectly influencing the paddy environment. Periphytic biofilms, which form at the soil-water interface in paddy fields, are complex bioaggregates that play an important role in nitrogen (N) cycling. The objective of this study was to investigate the effect of butachlor on periphytic biofilm growth and N cycling under both light and dark conditions in the laboratory. The results revealed that butachlor application hindered the growth of periphytic biofilms and led to the dominance of Cyanobacteria as the primary prokaryotes, while inhibiting the development of eukaryotic Trebouxiophyceae. Furthermore, the application of butachlor reduced the richness and diversity of prokaryotes, but increased those of eukaryotes in periphytic biofilms. The light treatments exhibited higher total N loss because light favored periphytic biofilm growth and enhanced ammonium (NH4+) assimilation and nitrification. Additionally, butachlor application resulted in the increased retention of NH4+-N and nitrate (NO3-)-N and an increase in N loss via denitrification. The abundances of functional genes encoding enzymes such as ammonia monooxygenase, nitrite reductase, and nitrous oxide reductase were increased by butachlor application, favoring nitrification and denitrification processes. Overall, the results suggest that butachlor application leads to an increase in total N loss mainly through denitrification in paddy systems, particularly in the presence of periphytic biofilms. Thus, the results may provide valuable insights into the changes in periphytic biofilm growth and N cycling induced by butachlor, and future studies can further explore the potential implications of these changes in paddy soils.

Fate of fertilizer nitrogen and residual nitrogen in paddy soil in Northeast China

The relationship between the fate of nitrogen (N) fertilizer and the N application rate in paddy fields in Northeast China is unclear, as is the fate of residual N. To clarify these issues, paddy field and 15N microplot experiments were carried out in 2017 and 2018, with N applications at five levels: 0, 75, 105, 135 and 165 kg N ha−1 (N0, N75, N105, N135 and N165, respectively). 15N-labeled urea was applied to the microplots in 2017, and the same amount of unlabeled urea was applied in 2018. Ammonia (NH3) volatilization, leaching, surface runoff, rice yield, the N contents and 15N abundances of both plants and soil were analyzed. The results indicated a linear platform model for rice yield and the application rate of N fertilizer, and the optimal rate was 135 kg N ha−1. N uptake increased with an increasing N rate, and the recovery efficiency of applied N (REN) values of the difference subtraction method were 45.23 and 56.98% on average in 2017 and 2018, respectively. The REN was the highest at the N rate of 135 kg ha−1 in 2017 and it was insignificantly affected by the N application rate in 2018, while the agronomic efficiency of applied N (AEN) and physiological efficiency of applied N (PEN) decreased significantly when excessive N was applied. N loss through NH3 volatilization, leaching and surface runoff was low in the paddy fields in Northeast China. NH3 volatilization accounted for 0.81 and 2.99% of the total N application in 2017 and 2018, respectively. On average, the leaching and surface runoff rates were 4.45% and less than 1.05%, respectively, but the apparent denitrification loss was approximately 42.63%. The residual N fertilizer in the soil layer (0–40 cm) was 18.37–31.81 kg N ha−1 in 2017, and the residual rate was 19.28–24.50%. Residual 15N from fertilizer in the soil increased significantly with increasing N fertilizer, which was mainly concentrated in the 0–10 cm soil layer, accounting for 58.45–83.54% of the total residual N, and decreased with increasing depth. While the ratio of residual N in the 0–10 cm soil layer to that in the 0–40 cm soil layer was decreased with increasing N application. Furthermore, of the residual N, approximately 5.4% was taken up on average in the following season and 50.2% was lost, but 44.4% remained in the soil. Hence, the amount of applied N fertilizer should be reduced appropriately due to the high residual N in paddy fields in Northeast China. The appropriate N fertilizer rate in the northern fields in China was determined to be 105–135 kg N ha−1 in order to achieve a balance between rice yield and high N fertilizer uptake.

Nanoattapulgite reduces the stimulation effect of nitrogen fertilizer on ammonia‐oxidizing microorganisms in salinized fluvo‐aquic soil

AbstractThe nitrification, which utilized ammonium nitrogen (NH4+‐N) as substrate, is susceptible to the bioavailability of NH4+‐N. However, it is unclear how the nitrification process responds to the decrease of NH4+‐N availability due to nanoattipulgite (NA) addition, especially in salt‐affected soils. A pot experiment was performed to study the effect of different NA rates (NA0, NA1, and NA2: 0, 25, 50 g kg−1) and nitrogen fertilizer (F) gradients (F1, F2, and F3: 54, 81, 108 mg N kg−1) on the net nitrification rate (NNR), abundance of ammonia monooxygenase (amoA) gene copies, and communities of ammonia‐oxidizing archaea (AOA) and bacteria (AOB) in a moderately salinized fluvo‐aquic soil. The concentrations of NH4+‐N and nitrate nitrogen in soils showed insignificant differences among various NA treatments. The addition of NA reduced NNR after topdressing but had no significant effect on NNR after base fertilizer. Nitrogen fertilizer could decrease the amoA gene abundance of AOA and increase the amoA gene abundance of AOB in soil. NA could maintain the relative stability of AOB amoA gene abundance. NA treatment only changed the community composition of AOB before topdressing at the F2 level but did not change the community composition of ammonia‐oxidizing microorganisms under other conditions. Overall, NA adsorbs more NH4+‐N and indirectly affects nitrification by reducing the stimulating effect of F on the activity of ammonia‐oxidizing microorganisms, which can play an important role in reducing nitrate pollution and denitrification loss from saline soil.

Strategies for mitigating N2O and N2 emissions from an intensive sugarcane cropping system

In sugarcane cropping systems, high rates of N fertiliser are typically applied as sub-surface bands creating localised zones of high mineral N concentrations. This in combination with high levels of crop residue (trash) retention and a warm and humid climate creates conditions that are known to promote soil denitrification, resulting in high emissions of the potent greenhouse gas N2O. These losses illustrate inefficient use of N fertilisers but total denitrification losses in the form of N2 and N2O remain largely unknown. We used the 15N gas flux method to investigate the effect of cane trash removal and the use of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) on N2 and N2O emissions on a commercial sugarcane farm at Bundaberg, Australia. High gaseous N losses were observed under the standard grower practice where cane trash retention and N fertiliser application (145 kg N ha−1 as urea) resulted in N2 and N2O emissions (36.1 kg N ha−1) from the subsurface N fertiliser band, with more than 50% of these losses emitted as N2O. Cane trash removal reduced N2 emission by 34% and N2O emission by 51%, but had no effect on the N2O/(N2 + N2O) ratio. The use of DMPP lowered N2 and N2O emission by 35% and 98%, respectively, reducing the percentage of these losses (N2 + N2O) emitted as N2O to only 4%. We conclude that the use of DMPP is an effective strategy to reduce N losses, minimise N2O emissions, while keeping the benefits of cane trash retention in sugarcane cropping systems.

Tracing the role of Arctic shelf processes in Si and N cycling and export through the Fram Strait: insights from combined silicon and nitrate isotopes

Abstract. Nutrient cycles in the Arctic Ocean are being altered by changing hydrography, increasing riverine inputs, glacial melt and sea-ice loss due to climate change. In this study, combined isotopic measurements of dissolved nitrate (δ15N-NO3 and δ18O-NO3) and silicic acid (δ30Si(OH)4) are used to understand the pathways that major nutrients follow through the Arctic Ocean. Atlantic waters were found to be isotopically lighter (δ30Si(OH)4=+ 1.74 ‰) than their polar counterpart (δ30Si(OH)4=+ 1.85 ‰) owing to partial biological utilisation of dissolved Si (DSi) within the Arctic Ocean. Coupled partial benthic denitrification and nitrification on Eurasian Arctic shelves lead to the enrichment of δ15N-NO3 and lighter δ18O-NO3 in the polar surface waters (δ15N-NO3= 5.44 ‰, δ18O-NO3= 1.22 ‰) relative to Atlantic waters (δ15N-NO3= 5.18 ‰, δ18O-NO3= 2.33 ‰). Using a pan-Arctic DSi isotope dataset, we find that the input of isotopically light δ30Si(OH)4 by Arctic rivers and the subsequent partial biological uptake and biogenic Si burial on Eurasian shelves are the key processes that generate the enriched isotopic signatures of DSi exported through Fram Strait. A similar analysis of δ15N-NO3 highlights the role of N-limitation due to denitrification losses on Arctic shelves in generating the excess dissolved silicon exported through Fram Strait. We estimate that around 40 % of DSi exported in polar surface waters through Fram Strait is of riverine origin. As the Arctic Ocean is broadly N-limited and riverine sources of DSi are increasing faster than nitrogen inputs, a larger silicic acid export through the Fram Strait is expected in the future. Arctic riverine inputs therefore have the potential to modify the North Atlantic DSi budget and are expected to become more important than variable Pacific and glacial DSi sources over the coming decades.

Rhizosphere soil nitrification ability controls nitrogen‐use efficiency in rice growth period

AbstractNitrification in the rhizosphere is a crucial process in controlling nitrogen‐use efficiency (NUE) in flooded paddy soils. To understand the relationship between the nitrification ability of the rhizosphere and NUE, pot experiments using 15N tracer technique were conducted to investigate the impacts of the rhizosphere soil net nitrification rate on NUE and denitrification losses at different rice growth stages in two paddy soils, which were sampled from Jurong (JR) and Yancheng (YC) in Jiangsu Province in China. The results showed that the nitrification rate in JR rhizosphere soil was lower than in YC rhizosphere soil at all rice growth stages. The abundance of ammonia‐oxidizing bacteria (AOB), ammonia‐oxidizing archaea (AOA), and pH in YC rhizosphere soils were always higher than in JR rhizosphere soils. Rice yield, biomass, NUE, leaf glutamine synthetase (GS) activity, and nitrate reductase (NR) activity were higher in JR soils with low nitrification rates than in YC soils with high nitrification rates (p < 0.05). In contrast, denitrification loss from JR soil (12.69%–23.41%) was lower than that from YC soil (26.83%–40.98%; p < 0.05) for each rice growth stage. The biomass and NUE decreased significantly as the net nitrification rate, the abundance of AOA and AOB of both the JR and YC rhizosphere soils increased (p < 0.05), and the denitrification loss was enhanced as the rhizosphere nitrification rate increased in the JR and YC soils (p < 0.05) during the rice growth period. The rhizosphere‐dominant AOB community Nitrosospira is the key factor affecting the nitrification rate and then decreasing rice NUE. In general, the rhizosphere nitrification rate in paddy soils is a primary factor controlling the rice NUE and denitrification loss.

Integrated nutrient management reduced the nutrient losses and increased crop yield in irrigated wheat

ABSTRACT Nitrogen (N), the most vital nutrient, undergoes a transformation in the soil and was subjected to loss through ammonia volatilization, nitrate leaching, and denitrification, which lower the soil fertility, crop yield and pollute the environment. Integrated nutrient management (INM) improves the nutrient supply system by integrating recommended dose fertilizers and biofertilizers, lessens the nitrogen losses, and synchronizes with the nutrient demand of the crop. To evaluate the impact of INM on minimization of N losses and improvement of wheat (Triticum aestivum L.) yield and growth parameters, a field experiment was conducted with different combinations of biofertilizers along with the recommended dosage of fertilizers. Application of NPK + Azotobacter + mycorrhiza (Tram) in combination was found to improve soil nitrogen dynamics, availability of P, K, microbial activity, root growth, urease activity, and nitrogen use efficiency as compared to NPK (Tr) only. The cumulative emission from ammonia flux in the INM plot varied from 10.2 to 10.6 kg ha−1 and denitrification losses varied from 2.4 to 2.5 kg ha−1. At the same time, these were recorded significantly higher in Tr plot. The yield obtained under the INM plot receiving Tram was recorded as 6.4 t ha−1, which is higher than other treatments.

The other side of tropical forest drought: do shallow water table regions of Amazonia act as large-scale hydrological refugia from drought?

Tropical forest function is of global significance to climate change responses, and critically determined by water availability patterns. Groundwater is tightly related to soil water through the water table depth (WT), but historically neglected in ecological studies. Shallow WT forests (WT < 5 m) are underrepresented in forest research networks and absent in eddy flux measurements, although they represent c. 50% of the Amazon and are expected to respond differently to global-change-related droughts. We review WT patterns and consequences for plants, emerging results, and advance a conceptual model integrating environment and trait distributions to predict climate change effects. Shallow WT forests have a distinct species composition, with more resource-acquisitive and hydrologically vulnerable trees, shorter canopies and lower biomass than deep WT forests. During 'normal' climatic years, shallow WT forests have higher mortality and lower productivity than deep WT forests, but during moderate droughts mortality is buffered and productivity increases. However, during severe drought, shallow WT forests may be more sensitive due to shallow roots and drought-intolerant traits. Our evidence supports the hypothesis of neglected shallow WT forests being resilient to moderate drought, challenging the prevailing view of widespread negative effects of climate change on Amazonian forests that ignores WT gradients, but predicts they could collapse under very strong droughts.

Double cropping and manure management mitigate the environmental impact of a dairy farm under present and future climate

CONTEXTStrategies are needed to reduce the environmental impacts of dairy farms, and these may become more important as our climate continues to change. Double cropping small grain and corn silages provides a strategy that can benefit dairy farms in the northeastern U.S. as they adapt to a longer growing season. Subsurface application of manure may also reduce nitrogen (N) and phosphorus (P) losses as the region faces warmer temperatures and greater precipitation. OBJECTIVEEvaluate whole-farm production, environmental and economic impacts of adopting these strategies on a representative dairy farm in central Pennsylvania under recent historical and projected midcentury climate. METHODSFarm management strategies were simulated using the Integrated Farm System Model to determine effects on crop yields, feed production, volatile, leaching and denitrification losses of N, sediment erosion, sediment-bound and soluble losses of P, farm-gate life cycle greenhouse gas emissions, fossil energy use, and production costs. RESULTS AND CONCLUSIONSDouble cropping increased and stabilized feed production by providing forage from a winter rye crop with less dependency on the summer crops of corn silage and perennial grasses. Summer crops are susceptible to summer droughts, which are expected to increase in this region due to warmer temperatures and increased evapotranspiration. Double cropping was more beneficial in the midcentury climate due to the projected increase in growing season length. Double cropping and subsurface injection of manure reduced total N losses by 12–18% and total P losses by 16–19%. Double cropping using broadcast manure application had a neutral environmental and economic impact on the farm. Adoption of these strategies provided a feasible adaptation and mitigation approach for future climate by reducing projected increases in soluble P runoff and ammonia emission caused by warmer temperatures and greater precipitation while maintaining and potentially reducing total farm production costs. SIGNIFICANCEWhole-farm simulation provides a tool for evaluating potential benefits and tradeoffs of novel technologies and strategies as agriculture adapts to changes in climate. Although these results are specific to a dairy farm in central Pennsylvania, they generally apply to dairy farms throughout the Northeast U.S. and climates where similar changes in temperature and precipitation are projected by mid-century. Our analyses suggest that use of a more intensive crop rotation (double cropping winter small grain and corn silage) along with improved manure application technology (subsurface injection) can help mitigate dairy farm environmental impacts now and even more in the future.

Draining the Landscape: How Do Nitrogen Concentrations in Riparian Groundwater and Stream Water Change Following Milldam Removal?

AbstractDam removals are on the increase across the US with Pennsylvania currently leading the nation. While most dam removals are driven by aquatic habitat and public safety considerations, we know little about how dam removals impact water quality and riparian zone processes. Dam removals decrease the stream base level, which results in dewatering of the riparian zone. We hypothesized that this dewatering of the riparian zone would increase nitrification and decrease denitrification, and thus result in nitrogen (N) leakage from riparian zones. This hypothesis was tested for a 1.5 m high milldam removal. Stream, soil water, and groundwater N concentrations were monitored over 2 years. Soil N concentrations and process rates and δ15N values were also determined. Denitrification rates and soil δ15N values in riparian sediments decreased supporting our hypothesis but no significant changes in nitrification were observed. While surficial soil water nitrate‐N concentrations were high (median 4.5 mg N L−1), riparian groundwater nitrate‐N values were low (median 0.09 mg N L−1), indicating that nitrate‐N leakage was minimal. We attribute the low groundwater nitrate‐N to denitrification losses at the lower, more dynamic, groundwater interface and/or dissimilatory nitrate reduction to ammonium (DNRA). Stream water nitrate‐N concentrations were high (median 7.6 mg N L−1) and contrary to our dam‐removal hypothesis displayed a watershed‐wide decline that was attributed to regional hydrologic changes. This study provided important first insights on how dam removals could affect N cycle processes in riparian zones and its implications for water quality and watershed management.

Increasing nutrient influx trends and remediation options at Hartbeespoort Dam, South Africa: a mass-balance approach

The Hartbeespoort Dam, located 40 km west of Tshwane on the Crocodile River, is an extremely eutrophic water body. Situated in one of the most economically active areas of South Africa, it receives a high nutrient input from wastewater treatment works (WWTW), leaking sewers, as well as urban and agricultural runoff. The Metsi a Me programme, which ran from 2006 to 2016, aimed to mitigate in-lake nutrient stocks using biomanipulation, including the physical removal of Eichhornia crassipes (water hyacinth) and Microcystis aeruginosa (blue-green algae). Using Department of Water and Sanitation water quality and flow data, the annual influxes and outfluxes of total nitrogen (TN) and total phosphorus (TP) to the Hartbeespoort Dam were calculated. Through literature review and comparison with previous studies, the relative importance of nutrient removal from biomass harvesting in relation to retained nutrients was assessed. The average nutrient influx from rivers during hydrological years 2010/11 to 2016/17 was 582 t∙a−1 TP and 4 687 t∙a−1 TN, with trends for both TN and TP being significantly positive over this period. TP influx increased by 77.8 t∙a−1 every year and TN influx increased by 456 t∙a−1, reversing a long-term negative trend. Average annual dam retention + removal (calculated as the difference between river inputs and outputs, i.e., including sedimentation, biomass removal and denitrification losses) was 358 t P and 2 195 t N. A best estimation of nutrient removal from water hyacinth and algal harvesting was 2.1 t∙a−1 P and 11.5 t∙a−1 N, and 3.9 t∙a−1 P and 40 t∙a−1 N, respectively. An estimated 341 t∙a−1 P and 674–1 288 t∙a−1 N was sedimented. Denitrification losses are poorly quantified but are possibly comparable to sedimentation. River outfluxes increased by 28.4 t∙a−1 TP and 110 t∙a−1 TN, smaller rates than the influxes, suggesting increasing retention per annum. Upgrading WWTWs in the catchment and refurbishing leaking and overflowing sewers is the most appropriate long-term solution.

Simulating nitrogen transport in paddy crop irrigated with alternate wetting and drying practice

Alternate wetting and drying (AWD) practice in rice cultivation can save a significant amount of irrigation water compared to conventional practice. This practice involves a soil moisture stress-based irrigation scheduling and possibly affects the nitrogen transport in the farm fields. In this study, non-weighing lysimeter plots were used to understand the nitrogen transformation, transport and balance of rice soil under AWD practice. The lysimeter data sets were used to simulate the nitrogen transport for no soil moisture stress (NS: 0 cm), mild soil moisture stress (MS: 450 cm) and severe soil moisture stress (SS: 700 cm)-based irrigation scheduling by using Hydrus-1D model. The Hydrus-1D model was able to simulate N transport ( $${NH}_{4}^{+}-N$$ and $${NO}_{3}^{-}-N$$ ) from the ponding water to the soil–plant system for both calibration and validation datasets. The simulation results indicated that the applied N was mostly concentrated within the puddled (top) layer but slightly decreased with the increase in the soil moisture stress. The N crop uptake and leaching loss were decreased by 6% and 38%, in MS and by 15% and 52% in SS, respectively, when compared to NS (flooded rice). Both volatilization and denitrification losses were increased by 14 and 7% in MS, and by 17 and 7% in SS, respectively, when compared to NS. The Hydrus model can be used as a management tool to determine the appropriate soil moisture stress in the puddled layer for which the required N nutrients are sufficiently available to the rice crop and the N loss to the environment can be controlled.

Estimating nitrogen fates and gross transformations in bioretention systems with applications of 15N labeling methods

Two batches of 15NH4+ and 15NO3− labeling experiments were conducted to understand the complex nitrogen (N) fates and transformations in bioretention systems, respectively. The fates of 15NH4+ were first traced in six bioretention systems with different wet-dry regimes and submerged zone settings during four months, indicating: (1) 15N was mainly leached during the second storm events following the 15NH4+ addition during the first storm events, suggesting nitrification during the dry period; (2) the main 15NH4+ fates after four-month exposure were: soil media 59.6%–80.0%, outflow 5.3%–16.4%, plants 2.3%–8.9%, denitrification losses 0–28.4%; (3) longer antecedent dry weather period and submerged zone could help alleviate outflow NO3− leaching. The occurrence time, positions and rates of major N transformation processes were later examined by the 15NO3− labeling experiment in a bioretention system over an 8 d wet-dry cycle, indicating: (1) during the brief wet period, hydraulic mixing of “old” water and “new” inflow mainly occurred; (2) during the subsequent dry period, gross rates of nitrification, denitrification and mineralization showed “pulse effects”, i.e. peaking at 24–48 h and decreasing significantly within 72 h; (3) denitrification became more dynamic with soil media depth, especially in submerged zone. This study evidenced the feasibility of 15N labelling method in studying N dynamics in bioretention systems and would inform future engineering and stormwater management practices.

Water management impact on denitrifier community and denitrification activity in a paddy soil at different growth stages of rice

Rice is one of the largest water-consuming crops. Alternate wetting and drying (AWD) is now an effective water-saving practice for rice cultivation that has been widely applied across the world. AWD may increase denitrification loss. However, the underlying mechanism remains unclear. This study investigated denitrifier activity, abundance and community structure in the rice rhizosphere and bulk soil in response to water management at the internode elongation (IE) and flowering (F) stages. The results revealed that at the IE stage, AWD significantly increased denitrification capacity (DC) in the rhizosphere compared with continuously flooded practice (CF). DC was significantly and positively related to electrical conductivity (EC) and the abundance of nirS-harboring denitrifiers. AWD led to higher EC in the 0–20 cm soil layer than CF. Although AWD lowered nirS abundance by 15 % compared with CF, the activity of nirS-harboring denitrifiers was enhanced by 1.89-folds of CF. At the F stage, AWD significantly decreased DC relative to CF. AWD had lower dissolved organic carbon (DOC) than CF, and DOC was significantly correlated with DC. Compared with CF, AWD also markedly changed the structure and network stability of nirS and nirK communities, resulting in sharp decreases in the total number of links in nirK and nirS networks of rhizosphere. The soil pH and EC were the key factors shaping the structure of nirS and nirK communities. Overall, the increase in denitrification losses due to AWD occurred primarily at the IE rather than F stage, and this increase was attributed to a rise in EC and changes in the activity and structure of nirS community. These findings provide a new insight into the mechanism of impact of the water-saving practice on denitrification loss for rice cultivation.

Soil anammox and denitrification processes connected with N cycling genes co-supporting or contrasting under different water conditions

The anaerobic ammonium oxidizing bacteria (anammox) are the hidden culprit behind the excessive nitrogen loss under a favorable environment, since their detection and abundance get disturbed by several unknown factors. This study intends to find the gap between actual anammox working capacity under different water conditions and fertilizers in the laboratory. The 15N-isotopic tracer technique was used to measure anammox and denitrification rate, and anammox community structure was analyzed through high-throughput sequencing with cytochrome cd-1 nitrite reductase functional gene (an_nirS gene, initially found in Candidatus Scalindua). The experiment consisted of four treatments, i.e., (I) CK_ Control, (II) UR_Urea, (III) PM_Pig Manure, and (IV) SRF_ Slow release fertilizer, under two water conditions, i.e., (a) Continuous flooding_ CF, (b) Alternate wetting and drying_ AWD. Results showed that anammox under CF decreased over time by −40.24%, and denitrification increased up to 39.25%. However, anammox activity under AWD increased up to 10.62% with the availability of NO2–, and surprisingly accompanied by the reduction in denitrification loss (-31.97%), being the most critical factor. We found that soil nifH and AOB genes were strongly favorable for anammox activity, while we observed the presence of anammox and AOB genes co-existing at the same time in paddy soil. The high-throughput sequencing with an_nirS functional gene showed a much higher diversity of anammox genera ever reported, mostly uncultured and unidentified. We concluded that water management is more prominent than fertilizer for anammox, and the most critical factor is the duration of AWD cycle, because of short term air supply could boost anammox activity and gene abundance, and could reduce denitrifier activity as well as nirK gene abundance.

Import, export, and recycling of dissolved nutrients in the Ogeechee River estuary (Georgia, USA)

We constructed an empirical mass balance model of nutrient fluxes in the Ogeechee River estuary (Georgia, USA) from eight surveys of seasonal estuarine nutrient concentrations during 2015 and 2016. The model results indicated a net removal of dissolved phosphorus and a net production of dissolved nitrogen (N) within the estuary over an annual cycle. During summer and autumn low flow periods, much of the dissolved N discharged to the ocean seems to be recycled into the estuary in the form of phytoplankton biomass. As a result, the outwelled N is not new nitrogen fueling coastal production but is nitrogen trapped within a recycling loop across the ocean–estuarine boundary. Higher flows in the fall and winter lead to direct discharge of nutrients with minimal recycling. A balanced N budget for the Ogeechee River estuary requires that estuarine N-fixation must exceed burial and denitrification losses within the estuary.