Long-term fate of nitrate fertilizer in agricultural soils

Investigating isotopic markers for hazelnut geographical authentication: Promising variables and potential applications

Hazelnuts' features and price are influenced by their geographical origin, making them susceptible to fraud, especially counterfeit claims regarding their provenance. Stable isotope analysis is a recognised approach to establish the geographical origin of foods, yet its potential in hazelnut authentication remains unexplored. In this prospective study, we assessed multiple isotopic markers in hazelnuts from different origins and evaluated the most promising variables for geographical authentication by chemometric tools. Our findings indicate that bulk δ18O, along with δ2H and δ13C in the main fatty acid methyl esters, exhibit significant potential in discriminating geographical origins, and 87Sr/86Sr analysis could serve as a proficient confirmatory tool. Though no single marker alone can differentiate between all the studied origins, employing a multi-isotopic approach based on PLS-DA models achieved up to 92.5 % accuracy in leave-10 %-out cross-validation. These findings will probably lay the groundwork for developing robust models for hazelnut geographical authentication based on larger datasets.

Chaotic Transport of Solutes in Unsaturated Porous Media

Unsaturated porous media, characterized by the combined presence of several immiscible fluid phases in the pore space, are highly relevant systems in nature, because they control the fate of contaminants and the availability of nutrients in the subsoil. However, a full understanding of the mechanisms controlling solute mixing in such systems is still missing. In particular, the role of saturation in the development of chaotic solute mixing has remained unexplored. Using three-dimensional numerical simulations of flow and transport at the pore scale, built upon X-ray tomograms of a porous medium at different degrees of liquid (wetting)-phase saturation, we show the occurrence of chaotic dynamics in both the deformation of the solute plume, as characterized by computed chaos metrics (Lyapunov exponents), and the mixing of the injected solute. Our results show an enhancement of these chaotic dynamics at lower saturation and their occurrence even under diffusion-relevant conditions over the medium's length, also being strengthened by larger flow velocities. These findings highlight the dominant role of the pore-scale spatial heterogeneity of the system, enhanced by the presence of an immiscible phase (e.g., air), on the mixing efficiency. This represents a stepping stone for the assessment of mixing and reactions in unsaturated porous media.

Selective Reduction of Aqueous Nitrate to Ammonium with an Electropolymerized Chromium Molecular Catalyst

Nitrate (NO3-) is a common nitrogen-containing contaminant in agricultural, industrial, and low-level nuclear wastewater that causes significant environmental damage. In this work, we report a bioinspired Cr-based molecular catalyst incorporated into a redox polymer that selectively and efficiently reduces aqueous NO3- to ammonium (NH4+), a desirable value-added fertilizer component and industrial precursor, at rates of ∼0.36 mmol NH4+ mgcat-1 h-1 with >90% Faradaic efficiency for NH4+. The NO3- reduction reaction occurs through a cascade catalysis mechanism involving the stepwise reduction of NO3- to NH4+ via observed NO2- and NH2OH intermediates. To our knowledge, this is one of the first examples of a molecular catalyst, homogeneous or heterogenized, that is reported to reduce aqueous NO3- to NH4+ with rates and Faradaic efficiencies comparable to those of state-of-the-art solid-state electrocatalysts. This work highlights a promising and previously unexplored area of electrocatalyst research using polymer-catalyst composites containing complexes with oxophilic transition metal active sites for electrochemical nitrate remediation with nutrient recovery.

Nitrate leaching characteristics of red soils from different parent materials in subtropical China

Globally, nitrate (NO3-) leaching from agroecosystems has been of major concern. There is evidence that NO3- leaching exhibits intense seasonal variation in subtropical regions. However, influencing factors to the seasonal dynamics remain unclear. In this study, a two-year field lysimeters experiment was conducted with three red soils derived from different parent materials (Quaternary red clay (QR), red sandstone (RS), and basalt (BA)). An N fertilizer (15N-enriched urea, 10 atom% excess) of 200 kg N ha-1 yr-1 was applied for maize. The effect of parent material on NO3- leaching characteristics was examined in surface (0-20 cm) and subsoil (20-100 cm) layers. The results showed due to the weakening of abundant drainage, there was no significant effect of parent materials on NO3- leaching characteristics in surface layers. Environmental factors (precipitation and temperature) and fertilization together led to obvious seasonal characteristics, i.e. abundant NO3- leaching during both crop growth and fallow periods. In subsoil layers, NO3- leaching characteristics were completely different among three soils. The concentrations and δ15N of NO3- in QR and RS soils showed a continuous increase after first year's fertilization, while those in BA soil remained relatively stable after reaching peak levels around harvest in first year. Meanwhile, the NO3- leaching amount in BA soil was significantly lower than in the other two soils. These might be explained by different NO3- adsorption capacities caused by the differences in mineral composition and free iron and aluminium contents. These elucidated in subsoil layers, NO3- leaching characteristics highly depended on parent materials. Meanwhile, adsorption capacity was limited and cannot slow NO3- leaching in the long run. Our results suggest that seasonal variation of NO3- leaching in surface layers and temporary retardant effect from NO3- adsorption capacity in subsoil layers should receive much attention when calculating and predicting NO3- leaching in subtropical regions.

Sampling methods may drive short-term groundwater nitrate variability in an irrigated watershed connected to a coastal lagoon (Campo de Cartagena-Mar Menor, SE Spain)

This study highlights concerns regarding the reliability of groundwater nitrate data used in official surveys, such as within the EU-mandated Water Framework Directive (WFD). The focus is on the Campo de Cartagena - Mar Menor hydrosystem in Spain, a region known for its intensively irrigated watershed and eutrophicated lagoon, where monitoring the evolution of nitrate contamination in surface and groundwater is crucial but challenging due to the risk of inconsistent characterization leading to erratic remediation measures. The study employed an experimental approach in private wells that belong to a longstanding official nitrate survey network marked by irregular sampling practices. Importantly, these wells lacked comprehensive design documentation and were frequently used by farmers. The study aimed to evaluate the representativity of dissolved nitrate measurements in such an emblematic case, while investigating the source of the water using geochemical and isotope tracers. This assessment considered the effects of different sampling techniques (bailer or pumping) and sampling parameters (depth and time), acknowledging actual practices. The research highlights several key findings. Firstly, the bailer sampling method proved to account for a substantial portion of the observed variation in nitrate content. Secondly, in some cases, pumping introduced contributions from different water horizons, complicating the interpretation of nitrate data. Thirdly, alterations in the sampling protocol had a notable impact on the resulting nitrate measurements. Furthermore, the study emphasized a critical issue: the lack of analytical uncertainty estimation in official surveys introduces significant bias in result interpretation, with discrepancies exceeding 100 mg/L in four of the six wells analyzed. This underscores the pressing need for improved sampling protocols, dedicated borehole infrastructure and precise data interpretation. Given the potential unreliability of some official groundwater nitrate data shared under EU or other regulations, with corresponding economic and environmental impacts, the study recommends meticulous verification before transmitting data.

Survival of a microbial inoculant in soil after recurrent inoculations

Microbial inoculants are attracting growing interest in agriculture, but their efficacy remains unreliable in relation to their poor survival, partly due to the competition with the soil resident community. We hypothesised that recurrent inoculation could gradually alleviate this competition and improve the survival of the inoculant while increasing its impact on the resident bacterial community. We tested the effectiveness of such strategy with four inoculation sequences of Pseudomonas fluorescens strain B177 in soil microcosms with increasing number and frequency of inoculation, compared to a non-inoculated control. Each sequence was carried out at two inoculation densities (106 and 108 cfu.g soil-1). The four-inoculation sequence induced a higher abundance of P. fluorescens, 2 weeks after the last inoculation. No impact of inoculation sequences was observed on the resident community diversity and composition. Differential abundance analysis identified only 28 out of 576 dominants OTUs affected by the high-density inoculum, whatever the inoculation sequence. Recurrent inoculations induced a strong accumulation of nitrate, not explained by the abundance of nitrifying or nitrate-reducing microorganisms. In summary, inoculant density rather than inoculation pattern matters for inoculation effect on the resident bacterial communities, while recurrent inoculation allowed to slightly enhance the survival of the inoculant and strongly increased soil nitrate content.

Cereals rhizosphere microbiome undergoes host selection of nitrogen cycle guilds correlated to crop productivity

Sustainable transformation of agricultural plant production requires the reduction of nitrogen (N) fertilizer application. Such a reduced N fertilizer application may impede crop production due to an altered symbiosis of crops and their rhizosphere microbiome, since reduced N input may affect the competition and synergisms with the plant. The assessment of such changes in the crop microbiome functionalities at spatial scales relevant for agricultural management remains challenging. We investigated in a field plot experiment how and if the N cycling guilds of the rhizosphere of globally relevant cereal crops - winter barley, wheat and rye - are influenced by reduced N fertilization. Crop productivity was assessed by remote sensing of the shoot biomass. Microbial N cycling guilds were investigated by metagenomics targeting diazotrophs, nitrifiers, denitrifiers and the dissimilatory nitrate to ammonium reducing guild (DNRA). The functional composition of microbial N cycling guilds was explained by crop productivity parameters and soil pH, and diverged substantially between the crop species. The responses of individual microbial N cycling guild abundances to shoot dry weight and rhizosphere nitrate content was modulated by the N fertilization treatments and the crop species, which was identified based on regression analyses. Thus, characteristic shifts in the microbial N cycling guild acquisition associated with the crop host species were resolved. Particularly, the rhizosphere of rye was enriched with potentially N-preserving microbial guilds - diazotrophs and the DNRA guild - when no fertilizer was applied. We speculate that the acquisition of microbial N cycling guilds was the result of plant species-specific acquisition strategies. Thus, the investigated cereal crop holobionts have likely different symbiotic strategies that make them differently resilient against reduced N fertilizer inputs. Furthermore, we demonstrated that these belowground patterns of N cycling guilds from the rhizosphere microbiome are linked to remotely sensed aboveground plant productivity.

Mapping the ratio of agricultural inputs to yields reveals areas with potentially less sustainable farming

Fertilisers and pesticides are major sources of the environmental harm that results from farming, yet it remains difficult to target reductions in their impacts without compromising food production. We suggest that calculating the ratio of agrochemical inputs to yield can provide an indication of the potential sustainability of farmland, with those areas that have high input relative to yield being considered as less sustainable. Here we design an approach to characterise such Input to Yield Ratios (IYR) for four inputs that can be plausibly linked to environmental impacts: the cumulative risk resulting from pesticide exposure for honeybees and for earthworms, and the amount of nitrogen or phosphorus fertiliser applied per unit area. We capitalise on novel national-scale data to assess IYR for wheat farming across all of England. High-resolution spatial patterns of IYR differed among the four inputs, but hotspots, where all four IYRs were high, were in key agricultural regions not usually characterised as having low suitability for cropping. By scaling the magnitude of each input against crop yield, the IYR does not penalise areas of high yield with higher inputs (important for food production), or areas with low yields but which are achieved with low inputs (important as low impact areas). Instead, the IYR provides a globally applicable framework for evaluating the broad patterns of trade-offs between production and environmental risk, as an indicator of the potential for harm, over large scales. Its use can thus inform targeting to improve agricultural sustainability, or where one might switch to other land uses such as ecosystem restoration.

The environmental and socioeconomic benefits of optimized fertilization for greenhouse vegetables

China produces more than half of global vegetables with greenhouse farms contributes approximately 35 % to the country's overall vegetable supply. The average nitrogen (N) application rate of greenhouse vegetable production exceeds 2000 kg N ha-1 yr-1, considerably contributing to global agricultural GHG emissions and reactive N (Nr) losses. Optimizing the N fertilizer utilization in greenhouse vegetable production is essential for mitigating environmental pollution and promoting sustainable development nationally and globally. In this study, we estimated the N footprint (NF), social costs (SC, which includes ecosystem and human health damage costs caused by Nr losses to the environment) and net ecosystem economic income (NEEI, which balances between the fertilizers input cost, yield profit, and social costs) of different greenhouse vegetables (tomato, pakchoi, lettuce, cabbage) under farmers' practice (FP) and reduced fertilization treatment (R). Results showed that compared with FP, the NF of tomato, pakchoi, lettuce and cabbage in the R treatment decreased by 61 %, 29 %, 46 % and 36 %, respectively, and the social costs were decreased by 60 %, 48 %, 57 % and 50 %, respectively. On the regional scale, the reduction in N fertilizer use for greenhouse vegetables in Beijing only could save the fertilizer input cost by 1-5 million USD, and avoided SC would increase by 1-14 million USD. As a result, this increased the NEEI by 2-19million USD. This study has demonstrated that adopting reduced fertilization practices represents a cost-effective measure that not only ensures yields but also decrease social costs, NF, and improve the benefits to help achieve sustainable development of greenhouse vegetable production.

Enhanced extracellular ammonium release in the plant endophyte Gluconacetobacter diazotrophicus through genome editing

IMPORTANCE

Our results demonstrate increased extracellular ammonium release in the endophyte plant growth-promoting bacterium Gluconacetobacter diazotrophicus. Strains were constructed in a manner that leaves no antibiotic markers behind, such that these strains contain no transgenes. Levels of ammonium achieved by cultures of modified G. diazotrophicus strains reached concentrations of approximately 18 mM ammonium, while wild-type G. diazotrophicus remained much lower (below 50 µM). These findings demonstrate a strong potential for further improving the biofertilizer potential of this important microbe.

Leaching of soil legacy nitrogen in intact soil columns and significance of soil macropore structure

Excessive application of chemical fertilizers accumulates nitrogen in soil (soil legacy nitrogen), and its release has a long-term impact on environmental quality. The information regarding leaching of soil legacy nitrogen and role of soil pore structure is scarce. Fifteen undisturbed soil cores with a depth of ~200 mm were collected from five paddy fields in a subtropical area of China, and soil pore structure was characterized with X-ray computed tomography. The batch leaching column experiments were conducted to investigate the nitrogen leaching from the soil cores. In the leachate, inorganic nitrogen (NO3--N and NH4+-N) accounted for 73-85 % of total dissolved nitrogen (TDN), which showed small variations among the five sampling sites. The NO3--N, NH4+-N and TDN concentrations in the leachate over the number of leaching times could be well fitted with the Gaussian, exponential or linear models, suggesting that different nitrogen forms showed variable leaching dynamics. NO3--N and TDN leaching losses continued to increase with the number of leaching times, whereas NH4+-N leaching from the soil had a threshold value. Combination of changes in soil nitrogen content after leaching, the lagging effect of soil legacy nitrogen may be partly attributed to the continuous transformation of NH4+-N and organic nitrogen into NO3--N in the soils. Besides the nitrogen content in the soil, the most important factor of soil pore structure controlling NO3--N and TDN leaching loss was the fractal dimension; NH4+-N was mainly controlled by connectivity. The present study highlighted the importance of soil pore structure and nitrogen format transformation in the leaching of soil legacy nitrogen.

Quantitative biodistribution of nanoparticles in plants with lanthanide complexes

The inefficient distribution of fertilizers, nutrients, and pesticides on crops is a major challenge in modern agriculture that leads to reduced productivity and environmental pollution. Nanoformulation of agrochemicals is an attractive approach to enable the selective delivery of agents into specific plant organs, their release in those tissues, and improve their efficiency. Already commercialized nanofertilizers utilize the physiochemical properties of metal nanoparticles such as size, charge, and the metal core to overcome biological barriers in plants to reach their target sites. Despite their wide application in human diseases, lipid nanoparticles are rarely used in agricultural applications and a systematic screening approach to identifying efficacious formulations has not been reported. Here, we developed a quantitative metal-encoded platform to determine the biodistribution of different lipid nanoparticles in plant tissues. In this platform lanthanide metal complexes were encapsulated into four types of lipid nanoparticles. Our approach was able to successfully quantify payload accumulation for all the lipid formulations across the roots, stem, and leaf of the plant. Lanthanide levels were 20- to 57-fold higher in the leaf and 100- to 10,000-fold higher in the stem for the nanoparticle encapsulated lanthanide complexes compared to the unencapsulated, free lanthanide complex. This system will facilitate the discovery of nanoparticles as delivery carriers for agrochemicals and plant tissue-targeting products.

Harnessing biological nitrogen fixation in plant leaves

The importance of biological nitrogen fixation (BNF) in securing food production for the growing world population with minimal environmental cost has been increasingly acknowledged. Leaf surfaces are one of the biggest microbial habitats on Earth, harboring diverse free-living N2-fixers. These microbes inhabit the epiphytic and endophytic phyllosphere and contribute significantly to plant N supply and growth. Here, we summarize the contribution of phyllosphere-BNF to global N cycling, evaluate the diversity of leaf-associated N2-fixers across plant hosts and ecosystems, illustrate the ecological adaptation of N2-fixers to the phyllosphere, and identify the environmental factors driving BNF. Finally, we discuss potential BNF engineering strategies to improve the nitrogen uptake in plant leaves and thus sustainable food production.

Anthropogenic effects on global soil nitrogen pools

The amount of nitrogen stored in terrestrial soils, its "nitrogen pool", moderates biogeochemical cycling affecting primary productivity, nitrogen pollution and even carbon budgets. The soil nitrogen pools and the transformation of nitrogen forms within them are heavily influenced by environmental factors including anthropogenic activities. However, our understanding of the global distribution of soil nitrogen with respect to anthropogenic activity and human land use remains unclear. We constructed a meta-analysis from a global sampling, in which we compare soil total nitrogen pools and the driving mechanisms affecting each pool across three major classifications of human land use: natural, agricultural, and urban. Although the size of the nitrogen pool can be similar across natural, agricultural and urban soils, the ecological and human associated drivers vary. Specifically, the drivers within agricultural and urban soils as opposed to natural soils are more complex and often decoupled from climatic and soil factors. This suggests that the nitrogen pools of those soils may be co-moderated by other factors not included in our analyses, like human activities. Our analysis supports the notion that agricultural soils act as a nitrogen source while urban soils as a nitrogen sink and informs a modern understanding of the fates and distributions of anthropogenic nitrogen in natural, agricultural, and urban soils.

Physiological and Transcriptomic Analysis Provides Insights into Low Nitrogen Stress in Foxtail Millet (Setaria italica L.)

Foxtail millet (Setaria italica (L.) P. Beauv) is an important food and forage crop that is well adapted to nutrient-poor soils. However, our understanding of how different LN-tolerant foxtail millet varieties adapt to long-term low nitrogen (LN) stress at the physiological and molecular levels remains limited. In this study, two foxtail millet varieties with contrasting LN tolerance properties were investigated through analyses of physiological parameters and transcriptomics. The physiological results indicate that JG20 (high tolerance to LN) exhibited superior biomass accumulation both in its shoots and roots, and higher nitrogen content, soluble sugar concentration, soluble protein concentration, zeatin concentration in shoot, and lower soluble sugar and soluble protein concentration in its roots compared to JG22 (sensitive to LN) under LN, this indicated that the LN-tolerant foxtail millet variety can allocate more functional substance to its shoots to sustain aboveground growth and maintain high root activity by utilizing low soluble sugar and protein under LN conditions. In the transcriptomics analysis, JG20 exhibited a greater number of differentially expressed genes (DEGs) compared to JG22 in both its shoots and roots in response to LN stress. These LN-responsive genes were enriched in glycolysis metabolism, photosynthesis, hormone metabolism, and nitrogen metabolism. Furthermore, in the shoots, the glutamine synthetase gene SiGS5, chlorophyll apoprotein of photosystem II gene SiPsbQ, ATP synthase subunit gene Sib, zeatin synthesis genes SiAHP1, and aldose 1-epimerase gene SiAEP, and, in the roots, the high-affinity nitrate transporter genes SiNRT2.3, SiNRT2.4, glutamate synthase gene SiGOGAT2, fructose-bisphosphate aldolase gene SiFBA5, were important genes involved in the LN tolerance of the foxtail millet variety. Hence, our study implies that the identified genes and metabolic pathways contribute valuable insights into the mechanisms underlying LN tolerance in foxtail millet.

Distribution, sources, and potential health risks of fluoride, total iodine, and nitrate in rural drinking water sources of North and East China

Small-scale water sources serving villages and towns are the main source of drinking water in rural areas. Compared to centralized water sources, rural water sources are less frequently monitored for water quality and have poor post-treatment facilities, making them vulnerable to drinking health risks. To reveal the hydrochemical characteristics, contaminant sources, and health risks in rural water sources, 189 water samples were collected from lakes and reservoirs, rivers, and groundwater in North and East China for major ions, nutrient salts, microelements, and stable isotope analysis. Statistical analysis and isotopic tracing were performed, as well as human health risk assessment. The exceeding threshold rates for fluoride (F-) and nitrate (NO3-) in surface water were 1.8 % and 9.1 %, respectively. For groundwater, the exceeding threshold rates were 20.9 % for F-, 15.7 % for total iodine (TI), and 4.5 % for NO3-. F- and TI were mainly derived from the leaching of fluoride- and iodine-containing minerals by cationic exchange, and NO3- is mainly derived from nitrogen in the soil (31.7-43.9 %), the use of ammonia fertilizers (24.3-36.1 %), and the discharge of manure and sewage (19.4-31.9 %). Nitrogen in the soil can be an important source of nitrate in the aquatic environment, and soils with higher clay content have a greater retention effect on the migration of nitrogen pollutants from the surface to the groundwater. F- in water sources contributes most to human health risks for drinking, followed by NO3- and TI, and a higher proportion of groundwater (37 %) present health risks for drinking than surface water (14 %) for children. Authorities should give high priority to optimizing the choice of water sources and technology for water treatment, and rational measures should be taken to protect water sources from the threats of anthropogenic pollution.

Unraveling the impact of human perturbation on nitrogen cycling in terrestrial ecosystems of lower Himalaya, Pakistan

Terrestrial ecosystems are under the enormous pressure of land use management regimes through human disturbances, resulting in the disruption of biogeochemical cycles and associated ecosystem services. Nitrogen (N) in soil ecosystems is of vital importance for primary productivity, hence estimating the extent of these human interventions on N-cycling processes becomes imperative from economic and environmental perspectives. This work investigated the impacts of variable anthropogenic activities on N cycling in three different terrestrial ecosystems (arable, grassland, and forest) in three regions of lower Himalaya, Pakistan. Potential nitrification (PNA) and denitrification (DEA) enzyme activities, relative distribution of inorganic N species (NH4, NO3), and the role of inherent edaphic factors were assessed. Results revealed high nitrification potentials and increased nitrous oxide (N2O) emissions in the incubated soil microcosms, in the order as arable > grassland > forest ecosystems. Notably, higher rates of both studied processes (~ 30-50%) and elevated soil mineral nitrogen pool were observed in arable ecosystems. Forest soils, assumed as pristine ecosystems relying mainly on natural N fixation, produced (de)nitrification rates relatively lower than grasslands, followed by arable soils which were moderately disturbed through long-term fertilization and intensive land-use regimes. Linear regression modeling revealed that the inorganic N species (particularly NO3), and inherent edaphic factors were the key determinants of high (de)nitrification rates, hence warn of accelerated N losses in these ecosystems. The study highlights that elevated PNA and DEA being proxies for the altered N cycling in the studied terrestrial ecosystems are of great ecological relevance in view of predicted N2O budget in the lower Himalaya.

Contradictory Impacts of Nitrate on the Dissimilatory Arsenate-Respiring Prokaryotes-Induced Reductive Mobilization of Arsenic from Contaminated Sediments: Mechanism Insight from Metagenomic and Functional Analyses

Dissimilatory arsenate-respiring prokaryotes (DARPs) are considered to be a key impetus of the reductive dissolution of solid-phase arsenic. However, little is known about the interaction between nitrate and DARPs so far. In this study, we showed that nitrate either inhibited or promoted the DARP population-catalyzed reductive mobilization of As in sediments. Metagenomic analysis of the microbial communities in the microcosms after seven days of As release assays suggested that microbes mainly consisted of: Type-I DARPs having potential to reduce NO3- into NO2- and Type-II DARPs having potential to reduce NO3- to NH4+. We further isolated two cultivable DARPs, Neobacillus sp. A01 and Paenibacillus sp. A02, which represent Type-I and -II DARPs, respectively. We observed that nitrate suppressed A01-mediated release of As(III) but promoted A02-mediated release of As(III). Furthermore, we demonstrated that this observation was due to the fact that nitrite, the end product of incomplete denitrification by Type-I DARPs, suppressed the arrA gene expression per cell and growth of all DARPs, whereas ammonium, the end product of dissimilatory nitrate reduction to ammonium (DNRA) by Type-II DARPs, enhanced the arrA gene expression per cell and significantly promoted the growth of all DARPs. These findings suggest that the actual effects of nitrate on DARP population-catalyzed reductive mobilization of arsenic, largely depend on the ratio of Type-I to Type-II DARPs in sediments.

Characterizing legacy nitrogen-induced time lags in riverine nitrogen reduction for the Songhuajiang River Basin: Source analysis, spatio-seasonal patterns, and impacts on future water quality improvement

Legacy nitrogen (N) originating from net N inputs (NNI) may pose ongoing threats to riverine water quality worldwide and even cause serious time-lags between water quality restoration and NNI declines. A better understanding of legacy N effects on riverine N pollutions in different seasons is essential to improve riverine water quality. Here, we investigated contributions of legacy N on riverine dissolved inorganic N (DIN) changes in different seasons and quantified spatio-seasonal time-lags in the Songhuajiang River basin (SRB), a hotspot of NNI with four distinct seasons, by exploring long-term (1978-2020) NNI-DIN relationships. Results firstly showed a significant seasonal difference in NNI, with the highest value observed in spring (average, 2184.1 kg/km2), 1.2, 5.0, and 4.6 times higher than that in summer, autumn, and winter, respectively. Cumulative legacy N had dominated riverine DIN changes, with a relative contribution of approximately 64% in 2011-2020, causing time-lags of 11-29 years across the SRB. The longest seasonal lags existed in spring (average, 23 years) owing to greater impacts of legacy N to riverine DIN changes in this season. Mulch film application, soil organic matter accumulation, N inputs, and snow cover were identified as the key factors that strengthened seasonal time-lags by collaboratively enhancing legacy N retentions in soils. Furthermore, a machine learning-based model system suggested that timescales for water quality improvement (DIN, ≤1.5 mg/L) varied considerably (from 0 to >29 years, Improved N Management-Combined scenario) across the SRB, with greater lag effects contributing to slower recovery. These findings can provide a more comprehensive insight into sustainable basin N management in the future.

Deep denitrification: Stream and groundwater biogeochemistry reveal contrasted but connected worlds above and below

Excess nutrients from agricultural and urban development have created a cascade of ecological crises around the globe. Nutrient pollution has triggered eutrophication in most freshwater and coastal ecosystems, contributing to a loss in biodiversity, harm to human health, and trillions in economic damage every year. Much of the research conducted on nutrient transport and retention has focused on surface environments, which are both easy to access and biologically active. However, surface characteristics of watersheds, such as land use and network configuration, often do not explain the variation in nutrient retention observed in rivers, lakes, and estuaries. Recent research suggests subsurface processes and characteristics may be more important than previously thought in determining watershed-level nutrient fluxes and removal. In a small watershed in western France, we used a multi-tracer approach to compare surface and subsurface nitrate dynamics at commensurate spatiotemporal scales. We combined 3-D hydrological modeling with a rich biogeochemical dataset from 20 wells and 15 stream locations. Water chemistry in the surface and subsurface showed high temporal variability, but groundwater was substantially more spatially variable, attributable to long transport times (10-60 years) and patchy distribution of the iron and sulfur electron donors fueling autotrophic denitrification. Isotopes of nitrate and sulfate revealed fundamentally different processes dominating the surface (heterotrophic denitrification and sulfate reduction) and subsurface (autotrophic denitrification and sulfate production). Agricultural land use was associated with elevated nitrate in surface water, but subsurface nitrate concentration was decoupled from land use. Dissolved silica and sulfate are affordable tracers of residence time and nitrogen removal that are relatively stable in surface and subsurface environments. Together, these findings reveal distinct but adjacent and connected biogeochemical worlds in the surface and subsurface. Characterizing how these worlds are linked and decoupled is critical to meeting water quality targets and addressing water issues in the Anthropocene.

Integration of environmental signatures and omics-based approaches on the European flounder to assist with health assessment of estuarine ecosystems in Brittany, France

This study aimed to develop a multidisciplinary approach to assess the ecological status of six moderate-sized French estuaries. For each estuary, we gathered geographical information, hydrobiological data, chemistry of pollutants and fish biology, including integration of proteomics and transcriptomics data. This integrative study covered the entire hydrological system studied, from the watershed to the estuary, and considered all the anthropogenic factors that can impact this environment. To reach this goal, European flounder (Platichthys flesus) were collected from six estuaries in September, which ensures a minimum residence time of five months within an estuary. Geographical metrics are used to characterize land use in each watershed. The concentrations of nitrite, nitrate, organic pollutants, and trace elements were measured in water, sediments and biota. All of these environmental parameters allowed to set up a typology of estuaries. Classical fish biomarkers, coupled with molecular data from transcriptomics and shotgun proteomics, highlighted the flounder's responses to stressors in its environment. We analysed the protein abundances and gene expression levels in the liver of fish from the different estuaries. We showed clear positive deregulation of proteins associated with xenobiotic detoxification in a system characterized by a large population density and industrial activity, as well as in a predominantly agricultural catchment area (mostly cultures of vegetables and pig breeding) mainly impacted by pesticides. Fish from the latter estuary also displayed strong deregulation of the urea cycle, most probably related to high nitrogen load. Proteomic and transcriptomic data also revealed a deregulation of proteins and genes related to the response to hypoxia, and a probable endocrine disruption in some estuaries. Coupling these data allowed the precise identification of the main stressors interacting within each hydrosystem.

Linking nitrate removal, carbon cycling, and mobilization of geogenic trace metals during infiltration for managed recharge

We present results from a series of laboratory column studies investigating the impacts of infiltration dynamics and the addition of a soil-carbon amendment (wood mulch or almond shells) on water quality during infiltration for flood-managed aquifer recharge (flood-MAR). Recent studies suggest that nitrate removal could be enhanced during infiltration for MAR through the application of a wood chip permeable reactive barrier (PRB). However, less is understood about how other readily available carbon sources, such as almond shells, could be used as a PRB material, and how carbon amendments could impact other solutes, such as trace metals. Here we show that the presence of a carbon amendment increases nitrate removal relative to native soil, and that there is greater nitrate removal in association with longer fluid retention times (slower infiltration rates). Almond shells promoted more efficient nitrate removal than wood mulch or native soil, but also promoted the mobilization of geogenic trace metals (Mn, Fe, and As) during experiments. Almond shells in a PRB likely enhanced nitrate removal and trace metal cycling by releasing labile carbon, promoting reducing conditions, and providing habitat for microbial communities, the composition of which shifted in response. These results suggest that limiting the amount of bioavailable carbon released by a carbon-rich PRB may be preferred where geogenic trace metals are common in soils. Given the dual threats to groundwater supplies and quality worldwide, incorporating a suitable carbon source into the soil for managed infiltration projects could help to generate co-benefits and avoid undesirable results.

Nitrate in shallow groundwater after more than four decades of manure application

Over-application of manure to agricultural fields can leach nitrogen below the root zone and contaminate groundwater. The goal of this study was to evaluate the factors affecting the spatial and temporal distribution of nitrate in shallow groundwater following 44 years of manure application to irrigated and non-irrigated long-term test plots. Sampling of 26 wells over an 18-month period revealed high spatial variability of groundwater nitrate concentrations, ranging from <0.1 mg-N/L to 1350 mg-N/L (mean = 118 mg-N/L). The highest concentrations were associated with the highest manure nitrogen loads, longer durations of manure application, and were generally located beneath irrigated land use. Regression modeling confirmed that cumulative manure loading had the greatest control on the spatial distribution of groundwater nitrate. A significant decreasing temporal trend was observed in selected wells downgradient of plots where manure application ceased more than a decade earlier. Isotopic analysis of ¹⁵N-NO₃ and ¹⁸O-NO₃ showed that denitrification occurred at 16 well locations, with evidence for dissolved organic carbon as the electron donor. The groundwater nitrate trends observed in this long-term study demonstrate that historical nutrient and water management practices will affect groundwater quality for many decades to come.

Nitrate with enriched heavy oxygen isotope linked to changes in nitrogen source and transformation as groundwater table rises

Nitrate is a significant constituent of the total nitrogen pool in shallow aquifers and poses an escalating threat to groundwater resources, making it crucial to comprehend the source, conversion, and elimination of nitrogen using appropriate techniques. Although dual-isotope dynamics in nitrate have been widely used, uncertainties remain regarding the asynchronously temporal changes in δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^- observed in hypoxic aquifers. This study aimed to investigate changes in nitrogen sources and transformations using temporal changes in field-based NO₃^- isotopic composition, hydro-chemical variables, and environmental DNA profiling, as the groundwater table varied. The results showed that the larger enrichment in δ¹⁸O-NO₃^- (+13‰) compared with δ¹⁵N-NO₃^- (-2‰) on average during groundwater table rise was due to a combination of factors, including high ¹⁸O-based atmospheric N deposition, canopies nitrification, and soil nitrification transported vertically by rainfalls, and ¹⁸O-enriched O₂ produced through microbial and root respiration within denitrification. The strong association between functional gene abundance and nitrogen-related indicators suggests that anammox was actively processed with nitrification but in small bacterial population during groundwater table rise. Furthermore, bacterial species associated with nitrogen-associated gradients provided insight into subsurface nitrogen transformation, with Burkholderiaceae species and Pseudorhodobacter potentially serving as bioindicators of denitrification, while Candidatus Nitrotogn represents soil nitrification. Fluctuating groundwater tables can cause shifts in hydro-chemical and isotopic composition, which in turn can indicate changes in nitrogen sources and transformations. These changes can be used to improve input sources for mixture models and aid in microbial remediation of nitrate.

Precipitation, submarine groundwater discharge of nitrogen, and red tides along the southwest Florida Gulf coast

Blooms of the dinoflagellate Karenia brevis occur almost every year along the southwest Florida Gulf coast. Long-duration blooms with especially high concentrations of K. brevis, known as red tides, destroy marine life through production of neurotoxins. Current hypotheses are that red tides originate in oligotrophic waters far offshore using nitrogen (N) from upwelling bottom water or, alternatively, from blooms of Trichodesmium, followed by advection to nearshore waters. But the amount of N available from terrestrial sources does not appear to be adequate to maintain a nearshore red tide. To explain this discrepancy, we hypothesize that contemporary red tides are associated with release of N from offshore submarine groundwater discharge (SGD) that has accumulated in benthic sediment biomass by dissimilatory nitrate reduction to ammonium (DNRA). The release occurs when sediment labile organic carbon (LOC), used as the electron donor in DNRA, is exhausted. Detritus from the resulting destruction of marine life restores the sediment LOC to continue the cycle of red tides. The severity of individual red tides increases with increased bloom-year precipitation in the geographic region where the SGD originates, while the severity of ordinary blooms is relatively unaffected.

Modified microbiology through enhanced denitrification by addition of various organic substances-temperature effect

Worldwide, the environmental nitrate (NO₃^-) problem is increasingly coming into focus. These increases in NO₃^- concentration result mainly from agricultural inputs and are further exacerbated by decreasing and finite geogenic NO₃^- degradation capacity in aquifers. Thus, treatment methods are becoming more and more important. In this study, the effects of enhanced denitrification with addition of organic carbon (C) on thereby autochthonous occurring microbiology and compared at room temperature as well as 10 °C were investigated. Incubation of bacteria and fungi was carried out using natural sediments without degradation capacity and groundwater with high NO₃^- concentrations. Addition of the four applied substrates (acetate, glucose, ascorbic acid, and ethanol) results in major differences in microbial community. Cooling to 10 °C changes the microbiology again. Relative abundances of bacteria are strongly influenced by temperature, which is probably the explanation for different denitrification rates. Fungi are much more sensitive to the milieu change with organic C. Different fungi taxa preferentially occur at one of the two temperature approaches. Major modifications of the microbial community are mainly observed whose denitrification rates strongly depend on the temperature effect. Therefore, we assume a temperature optimum of enhanced denitrification specific to each substrate, which is influenced by the microbiology.

Response of nitrate leaching to no-tillage is dependent on soil, climate, and management factors: A global meta-analysis

No tillage (NT) has been proposed as a practice to reduce the adverse effects of tillage on contaminant (e.g., sediment and nutrient) losses to waterways. Nonetheless, previous reports on impacts of NT on nitrate ( NO 3 - ) leaching are inconsistent. A global meta-analysis was conducted to test the hypothesis that the response of NO 3 - leaching under NT, relative to tillage, is associated with tillage type (inversion vs non-inversion tillage), soil properties (e.g., soil organic carbon [SOC]), climate factors (i.e., water input), and management practices (e.g., NT duration and nitrogen fertilizer inputs). Overall, compared with all forms of tillage combined, NT had 4% and 14% greater area-scaled and yield-scaled NO 3 - leaching losses, respectively. The NO 3 - leaching under NT tended to be 7% greater than that of inversion tillage but comparable to non-inversion tillage. Greater NO 3 - leaching under NT, compared with inversion tillage, was most evident under short-duration NT (<5 years), where water inputs were low (<2 mm day^-1 ), in medium texture and low SOC (<1%) soils, and at both higher (>200 kg ha^-1 ) and lower (0-100 kg ha^-1 ) rates of nitrogen addition. Of these, SOC was the most important factor affecting the risk of NO₃ ^- leaching under NT compared with inversion tillage. Globally, on average, the greater amount of NO₃ ^- leached under NT, compared with inversion tillage, was mainly attributed to corresponding increases in drainage. The percentage of global cropping land with lower risk of NO₃ ^- leaching under NT, relative to inversion tillage, increased with NT duration from 3 years (31%) to 15 years (54%). This study highlighted that the benefits of NT adoption for mitigating NO 3 - leaching are most likely in long-term NT cropping systems on high-SOC soils.

Deep accumulation of soluble organic nitrogen after land-use conversion from woodlands to orchards in a subtropical hilly region

Accumulation of soluble organic nitrogen (SON) in soil poses a significant threat to groundwater quality and plays an important role in regulating the global nitrogen cycle; however, most related studies have focused only on the upper 100-cm soil layers. Surface land-use management and soil properties may affect the vertical distribution of SON; however, their influence is poorly understood in deep soil layers. Therefore, this study assessed the response of SON concentration, pattern, and storage in deep regoliths to land-use conversion from woodlands to orchards in a subtropical hilly region. Our results showed that the SON stocks of the entire soil profile (up to 19.5 m) ranged from 254.5 kg N ha^-1 to 664.1 kg N ha^-1. Land-use conversion not only reshaped the distribution pattern of SON, but also resulted in substantial accumulation of SON at the 0-200 cm soil profile in the orchards compared to that in the woodlands (124.1 vs 190.5 kg N ha^-1). Land-use conversion also altered the SON/total dissolved nitrogen ratio throughout the regolith profile, resulting in a relatively low (<50 %) ratio in orchard soils below 200 cm. Overall, 76.8 % of SON (338.4 ± 162.0 kg N ha^-1) was stored in the layers from 100 cm below the surface to the bedrock. Regolith depth (r = -0.52 and p < 0.05) was found to be significantly correlated with SON concentration, explaining 17.8 % of the variation in SON, followed by total nitrogen (14.4 %), total organic carbon/total nitrogen ratio (10.1 %), and bulk density (9.3 %). This study provides insights into the estimation of terrestrial nitrogen and guidance for mitigation of groundwater contamination risk due to deep accumulation of SON.

Denitrification dynamics in unsaturated soils with different porous structures and water saturation degrees: A focus on the shift in microbial community structures

Despite its environmental significance, little is known about denitrification in vadose zones owing to the complexity of such environments. Here, we investigated denitrification in unsaturated soils with different pore distributions. To this end, we performed batch-type denitrification experiments and analyzed microbial community shifts before and after possible reactions with nitrates to clarify the relevant denitrifying mechanism in the microcosms. For quantitative comparison, pore distribution in the test soil samples was characterized based on the uniformity coefficient (C_u) and water saturation degree (SD). Micro-CT analysis of the soil pore distribution confirmed that the proportion of bigger-sized pores increased with decreasing C_u. However, oxygen diffusion into the system was controlled by SD rather than C_u. Within a certain SD range (51-67%), the pore condition changed abruptly from an oxic to an anoxic state. Consequently, denitrification occurred even under unsaturated soil conditions when the SD increased beyond 51-67%. High throughput sequencing revealed that the same microbial species were potentially responsible for denitrification under both partially (SD 67%), and fully saturated (SD of 100%) conditions, implying that the mechanism of denitrification in a vadose zone, if it exists, might be possibly similar under varying conditions.

Stable C and N isotope variation during anaerobic digestate composting and in the compost-amended soil-plant system

Although the use of composts derived from anaerobic digestates as soil amendments is likely to increase in the future, there is little information concerning the fate of their C and N compounds after their incorporation into soil. This work assesses C and N concentrations and the associated changes in δ¹⁵N and δ¹³C during the composting processes of cattle and pig slurry anaerobic digestates. In addition, the compost effect on C and N fractions and plant uptake were studied during a six-month pot experiment with rosemary plants. The results did not show δ¹³C and δ¹⁵N isotopic discrimination during composting, indicating a previous stabilization of cattle manure and pig slurry during the anaerobic digestion. This fact was also confirmed by the low C losses during the composting processes (1.2-fold and 1.05-fold for the composting piles with cattle and pig slurry anaerobic digestates, respectively). After soil addition, the composts augmented N values (from 0.41 g kg^-1 to around 0.56 g kg^-1 in low dose and 0.68 g kg^-1 in high dose compost amended soils) and δ¹⁵N soil values (increases in the range of 50%-156%), but showed only slight differences in C and δ¹³C values compared to unfertilised control and inorganic fertilized soils. Moreover, the rosemary leaves of the plants grown on the compost amended soils presented higher N and δ¹⁵N abundance than control and inorganic fertilized plants. We conclude that δ¹⁵N abundance of anaerobic digestate composts is useful to discern its N uptake and could thus be a useful tool to detect whether organic or mineral fertiliser types were used for agricultural production.

Advancing Watershed Legacy Nitrogen Modeling to Improve Global Water Quality

Despite widespread implementation of watershed nitrogen reduction programs across the globe, nitrogen levels in many surface waters remain high. Watershed legacy nitrogen storage, i.e., the long-term retention of nitrogen in soils and groundwater, is one of several explanations for this lack of progress. However scientists and water managers are ill-equipped to estimate how legacy nitrogen moderates in-stream nitrogen responses to land conservation practices, largely because modeling tools and associated long-term monitoring approaches to answering these questions remain inadequate. We demonstrate the need for improved watershed models to simulate legacy nitrogen processes and offer modeling solutions to support long-term nitrogen-based sustainable land management across the globe.

Fertilizer 15N balance in a soybean-maize-maize rotation system based on a 41-year long-term experiment in Northeast China

Global awareness of the need to enhance crop production and reduce environmental issues associated with nitrogen (N) fertilizer has increased. However, studies on how the N fate changed with manure addition are still limited. To explore efficient fertilization management for an improved grain yield, N recovery efficiency, and reduced N residual in the soil or that unaccounted for, a field 15N micro-plot trial in a soybean-maize-maize rotation was conducted to evaluate the effect of fertilization regimes on soybean and maize yields and the fertilizer N fate in the plant-soil system during 2017-2019 within a 41-year experiment in Northeast China. Treatments included chemical N alone (N), N and phosphorus (NP), N, P, and potassium (NPK), and those combined with manure (MN, MNP, and MNPK). Application of manure increased grain yield, on average, by 153% for soybean (2017) and 105% and 222% for maize (2018 and 2019) compared to no manure, with the highest at MNPK. Crop N uptake and that from labeled ¹⁵N-urea also benefited from manure addition, mainly partitioned to grain, and the average ¹⁵N-urea recovery was 28.8% in the soybean season with a reduction in the subsequent maize seasons (12.6%, and 4.1%). Across the three years, the fertilizer ¹⁵N recovery ranged from 31.2-63.1% (crop) and 21.9-40.5% (0-40 cm soil), with 14.6-29.9% unaccounted for, including N losses. In the two maize seasons, manure addition significantly increased the residual ¹⁵N recovery in crop attributed to the enhancing ¹⁵N remineralization, and reduced that in soil and unaccounted for compared to single chemical fertilizer, with MNPK performing the best. Therefore, applying N, P, and K fertilizers in the soybean season and NPK combined with manure (13.5 t ha^-1) in the maize seasons is a promising fertilization management strategy in Northeast China and similar regions.

Antibiotics, antibiotic resistance and associated risk in natural springs from an agroecosystem environment

This study investigates the occurrence, transport, and risks associated to antibiotic residues, antibiotic resistance genes (ARGs) and antibiotic resistant Escherichia coli (AR-E. coli) in eleven natural springs in an agroecosystem environment with intense livestock production, where groundwater nitrate concentration usually sets above 50 mg L^-1. Out of 23 multiple-class antibiotics monitored, tetracycline and sulfonamide residues were the most ubiquitous, and they were detected at concentrations ranging from ng L^-1 to μg L^-1. Five ARGs were monitored, conferring resistance to the antibiotic classes of major use in livestock production. Thus, genes conferring resistance to sulfonamides (sul1 and sul2) and tetracyclines (tetW) as well as a gene proxy for anthropogenic pollution (intI1) were present in most springs. sul1 was the most abundant, with absolute concentrations ranging from 4 × 10² to 5.6 × 10⁶ gene copies L^-1 water. AR-E. coli showing resistance to sulfonamides and tetracyclines was also detected, with a prevalence up to approximately 40 % in some sites but with poor correlations with the concentration of antibiotic residues and ARGs. The occurrence of antibiotics, ARGs and AR-E. coli was characterized by large seasonal variations which were mostly associated to both hydrological factors and reactive transport processes. Finally, a risk assessment approach pointed out towards low risk for both the groundwater environment and human health, when spring water is used for direct human consumption, associated with the occurrence of antibiotics, ARGs and AR-E. coli. However, long-term effects cannot be neglected, and proper actions must be taken to preserve groundwater quality.

Plant litter strengthens positive biodiversity-ecosystem functioning relationships over time

Plant biodiversity-productivity relationships become stronger over time in grasslands, forests, and agroecosystems. Plant shoot and root litter is important in mediating these positive relationships, yet the functional role of plant litter remains overlooked in long-term experiments. We propose that plant litter strengthens biodiversity-ecosystem functioning relationships over time in four ways by providing decomposing detritus that releases nitrogen (N) over time for uptake by existing and succeeding plants, enhancing overall soil fertility, changing soil community composition, and reducing the impact of residue-borne pathogens and pests. We bring new insights into how diversity-productivity relationships may change over time and suggest that the diversification of crop residue retention through increased residue diversity from plant mixtures will improve the sustainability of food production systems.

Quantitative identification of groundwater contamination sources by combining isotope tracer technique with PMF model in an arid area of northwestern China

Nowadays, groundwater quality has deteriorated because of intensive human activities. It is important to accurately identify the pollution source for controlling the deterioration of groundwater quality. However, the accuracy of the current source analysis method needs to be improved. In this study, we combined hydrochemical method, isotope tracing technique and PMF model, for the first time, to trace the source of groundwater pollution in Beichuan River basin, Qinghai Province, China. According to the results, there were 35.8% of Fe, 34.1% of total hardness, 24.3% of SO₄^2- and 8.09% of NO₃^- samples exceeded the Grade III standards for Groundwater quality in China, which indicated that the groundwater in the study area has been significantly affected by human activities. Hydrochemical method suggested that the chemical component originated from rock weathering, cation exchange and mineral dissolution. Based on isotope tracing technique (δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, δ³⁴S-SO₄^2- and δ¹⁸O-SO₄^2-), the primary sources of nitrate and sulfate in groundwater were soil nitrogen and oxidation of sulfide minerals in the forest area, domestic sewage and oxidation of sulfide minerals in the urban and industrial area, and mixed sources in the village and agricultural area. Finally, the pollution source of groundwater was distinguished by combining the PMF model, isotope tracing technique and hydrochemical method. Results showed that the main pollutant of groundwater is domestic sewage in the urban, village and industrial area. The contribution rates to groundwater pollution were 60.7%, 60.8% and 57.8%, respectively. However, in the forest and agricultural area, the main source changed to water-rock interaction and chemical fertilizer, and the contribution rates to groundwater quality were 53.5% and 61.0%, respectively. Our results suggested that the coupling tracing methodology can improve the accuracy of source resolution in the water environment and it can be applied to other areas of the world.

Mixing Controlled Adsorption at the Liquid-Solid Interfaces in Unsaturated Porous Media

The unsaturated zone, located between the soil surface and the phreatic level, plays an important role in defining the fate of any substance entering the subsoil. In addition to the processes of flow and transport taking place in the liquid phase, surface reactions such as adsorption to the solid phase may occur and increase the residence time of the substance entering the system. In this study, we aim to understand the pore-scale mechanisms that control adsorption in unsaturated systems. We combine 2D pore-scale experimental images with numerical simulations to analyze flow, transport, and adsorption under different liquid saturation degrees. We demonstrate the role of mixing on adsorption at the liquid-solid interfaces by analyzing the deformation in time of a pulse-injected surfactant. We also analyze the impact of the isotherm functional shape and the inclusion of the liquid-gas interfaces as adsorption sites on this surface reaction. The enhancement of mixing as saturation decreases is accompanied by a reduction in the amount of adsorbed mass, located mainly along preferential flow paths, where the solute is primarily transported. For the same isotherm, a nonlinear behavior of adsorption as a function of liquid saturation has been observed. This is explained by the nonlinear variation of the volume fraction of liquid behaving as preferential path or stagnation zone as liquid saturation decreases, despite the linear decrease in the surface area of solids accessible for adsorption.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11242-022-01747-x.

Do heavy metals affect bacterial communities more in small repeated applications or in a single large application?

Heavy metals from anthropogenic sources accumulate slowly but steadily, leading to high metal concentration levels in soil. However, the effect of each heavy metal on soil bacterial communities is usually assessed in laboratories by a single application of individually spiked metals. We evaluated the differences between single individual application and repeated individual applications of Cr, Cu, Ni, Pb, and Zn on bacterial communities, through pollution-induced community tolerance (PICT), using bacterial growth as the endpoint (³H-leucine incorporation method). We found that PICT development was higher when soil was spiked in individual single application than individual repeated applications for Cu, Ni and Zn. In contrast, bacterial communities did not show different tolerance between singly or repeatedly when soil was spiked with Cr. In the case of Pb any increase of bacterial community tolerance to this metal was found despite high doses applied (up to 2000 mg kg^-1). These results are relevant for the interpretation of the effects of heavy metals on soil microbes in order to avoid laboratory overestimations of the real effects of heavy metals on soil microbes.

A high-resolution monitoring station for the in situ assessment of nitrate-related redox processes at an agricultural site

Biogeochemical redox processes control the chemical behavior of many major and trace elements, making their comprehension crucial for predicting and protecting environmental health. Nitrogen (N) is especially susceptible to changes in soil redox conditions and affects the cycles of other redox-sensitive species. Elevated N concentrations, in nitrate form, in agricultural soils and associated freshwater ecosystems constitute a problem in many parts of the world. Although a wide variety of measures have been adopted, their assessment through concentration measurements in groundwater and surface water of the different monitoring networks has shortcomings. Nitrate, as a non-point pollutant, is subject to several processes (e.g., transformation and retardation) before it is detected, making it impossible to evaluate measurements' effectiveness reliably. Thus, we designed and constructed a monitoring station featuring commercially available products and self-manufactured components at an agricultural site for the in situ assessment of nitrate-related processes by high-resolution monitoring of hydraulic (soil water content, matric potential, groundwater head) and hydrogeochemical variables (oxidation-reduction potential and groundwater and pore water chemistry) within the vadose zone and the shallow aquifer. The monitoring station has proven to be a reliable tool. Changes over depth and time of measured variables have been identified, allowing the detection of the transient behavior of the redox reactive zone and the interpretation of ongoing denitrification processes and other redox nitrate-triggered phenomena, such as uranium roll-front and selenium accumulation at the redox interface. Measuring both geochemical and soil water variables allows for the calculation of in situ solute inputs into the groundwater and their reaction rates.

Gluconacetobacter diazotrophicus Gene Fitness during Diazotrophic Growth

Plant growth-promoting (PGP) bacteria are important to the development of sustainable agricultural systems. PGP microbes that fix atmospheric nitrogen (diazotrophs) could minimize the application of industrially derived fertilizers and function as a biofertilizer. The bacterium Gluconacetobacter diazotrophicus is a nitrogen-fixing PGP microbe originally discovered in association with sugarcane plants, where it functions as an endophyte. It also forms endophyte associations with a range of other agriculturally relevant crop plants. G. diazotrophicus requires microaerobic conditions for diazotrophic growth. We generated a transposon library for G. diazotrophicus and cultured the library under various growth conditions and culture medium compositions to measure fitness defects associated with individual transposon inserts (transposon insertion sequencing [Tn-seq]). Using this library, we probed more than 3,200 genes and ascertained the importance of various genes for diazotrophic growth of this microaerobic endophyte. We also identified a set of essential genes. IMPORTANCE Our results demonstrate a succinct set of genes involved in diazotrophic growth for G. diazotrophicus, with a lower degree of redundancy than what is found in other model diazotrophs. The results will serve as a valuable resource for those interested in biological nitrogen fixation and will establish a baseline data set for plant free growth, which could complement future studies related to the endophyte relationship.

Nitrate contamination and associated health risks of the Benslimane groundwater, Morocco

Consumption of polluted water has harmful impacts on human health. This study examined the quality of groundwater in the Benslimane area for drinking purposes based on the Water quality index (WQI), Nitrate pollution index (NPI), and Total risk quotient (THQ) for different age groups. A total of 120 groundwater samples were collected for physicochemical analyses. The results showed WQI values ranging from 157.7 to 472.7 and an average of 279.4, with a total absence of water of excellent or good quality, and about 62.5% of the groundwater samples were of very poor quality for consumption. Nitrate concentrations ranged from 1 to 270 mg/L with an average of 64 mg/L, and 56.7% had values above the World Health Organization safety level of 50 mg/L. The NPI showed that 78.3% of the sampled sites showed very high pollution as a result of intense anthropogenic activities. High contamination is observed in the north and east of the region for arboriculture, grapes, maize, and vegetables as opposed to cereals. The health risk associated with nitrates, based on oral exposure, was much higher than dermal contact. The total risk quotient for both pathways was 0.02 to 6.58, 0.02 to 6.12, 0.06 to 17.06, and 0.05 to 13.35 for women, men, children, and infants, respectively. A total of 65, 63.3, 82.0, and 78.3% of groundwater samples presented a non-cancer health risk for women, men, children, and infants, respectively. Therefore, this study can help identify contaminated areas in order to track corrective safety measures to control groundwater quality in the region and improve sanitary conditions.

Lewis acid-assisted reduction of nitrite to nitric and nitrous oxides via the elusive nitrite radical dianion

Reduction of nitrite anions (NO2-) to nitric oxide (NO), nitrous oxide (N2O) and ultimately dinitrogen (N2) takes place in a variety of environments, including in the soil as part of the biogeochemical nitrogen cycle and in acidified nuclear waste. Nitrite reduction typically takes place within the coordination sphere of a redox-active transition metal. Here we show that Lewis acid coordination can substantially modify the reduction potential of this polyoxoanion to allow for its reduction under non-aqueous conditions (-0.74 V versus NHE). Detailed characterization confirms the formation of the borane-capped radical nitrite dianion (NO22-), which features a N(II) oxidation state. Protonation of the nitrite dianion results in the facile loss of nitric oxide (NO), whereas its reaction with NO results in disproportionation to nitrous oxide (N2O) and nitrite (NO2-). This system connects three redox levels in the global nitrogen cycle and provides fundamental insights into the conversion of NO2- to NO.

Integrating major agricultural practices into the TRIPLEX-GHG model v2.0 for simulating global cropland nitrous oxide emissions: Development, sensitivity analysis and site evaluation

Nitrous oxide (N₂O) emissions from croplands are one of the most important greenhouse gas sources while the estimation of which remains large uncertainties globally. To simulate N₂O emissions from global croplands, the process-based TRIPLEX-GHG model v2.0 was improved by coupling the major agricultural activities. Sensitivity experiment was used to measure the impact of the integrated processes to modeled N₂O emission found chemical N fertilization have the highest relative effect sizes. While the coefficient of the NO₃^- consumption rate for denitrification (COE_dNO3), controlling the first step of the denitrification process was identified to be the most sensitive parameter based on sensitivity analysis of model parameters. The model performed well when simulating the magnitude of the daily N₂O emissions for 39 calibration sites and the continental mean of the parameters were used to producing reasonable estimations for the means of the measured daily N₂O fluxes (R² = 0.87, slope = 1.07) and emission factors (EFs, R² = 0.70, slope = 0.72) during the experiment periods. The model reliability was further confirmed by model validation. General trend of modeled daily N₂O emissions were reasonably consistent with the observations of selected validated sites. In addition, high correlations between the results of modeled and observed mean N₂O emissions (R² = 0.86, slope = 0.82) and EFs (R² = 0.66, slope = 0.83) from 68 validation sites were obtained. Further improvement on more detailed estimations for the variation of the environmental factors, management effects as well as accurate model input model driving data are required to reduce the uncertainties of model simulations. Consequently, our simulation results demonstrate that the TRIPLEX-GHG model v2.0 can reliably estimate N₂O emissions from various croplands at the global scale, which contributes to closing global N₂O budget and sustainable development of agriculture.

Integrated organic and inorganic fertilization and reduced irrigation altered prokaryotic microbial community and diversity in different compartments of wheat root zone contributing to improved nitrogen uptake and wheat yield

The effect of long-term water and integrated fertilization on prokaryotic microorganisms and their regulation for crop nutrient uptake remains unknown. Therefore, the impact of soil water and integrated fertilization after eight years on prokaryotic microbial communities in different compartments of root zone and their association with wheat nitrogen (N) absorption and yield were investigated. The results showed that compared with fertilization treatments (F), water regimes (W) more drastically modulated the prokaryotic microbial community structure and diversity in bulk soil, rhizosphere and endosphere. The increase of irrigation improved the prokaryotic diversity in the rhizosphere and endosphere while decreased the diversity in the bulk soil. Application of organic fertilizers significantly improved soil organic matter (SOM) and nutrient contents, increased rhizosphere and endophytic prokaryotic microbial diversity, and elevated the relative abundance of aerobic ammonia oxidation and nitrification-related functional microorganisms in rhizosphere and endosphere. Increasing irrigation elevated the relative abundance of functional microorganisms related to aerobic ammonia oxidation and nitrification in the rhizosphere and endosphere. Soil water content (SWC) and NH₄⁺-N as well as NO₃^--N were key predictors of prokaryotic microbial community composition under W and F treatments, respectively. Appropriate application of irrigation and organic fertilizers increased the relative abundance of some beneficial bacteria such as Flavobacterium. Water and fertilization treatments regulated the prokaryotic microbial communities of bulk soil, rhizosphere and endosphere by altering SWC and SOM, and provided evidence for the modulation of prokaryotic microorganisms to promote nitrogen uptake and wheat yield under long-term irrigation and fertilization. Conclusively, the addition of organic manure (50 %) with inorganic fertilizers (50 %) and reduced amount of irrigation (pre-sowing and jointing-period irrigation) decreased the application amount of chemical fertilizers and water, while increased SOM and nutrient content, improved prokaryotic diversity, and changed prokaryotic microbial community structure in the wheat root zone, resulting in enhanced nutrient uptake and wheat yield.

Environmental impacts, human health, and energy consumption of nitrogen management for maize production in subtropical region

Over-application of fertilizers could not improve crop yield and agronomic efficiency, but result in increasing nitrogen (N) surplus and adverse effects on the ecosystem sustainability. Although some previous studies have addressed one or a few environmental aspects in crop production, an integrated assessment for the effects of N fertilizer on multiple environmental impacts, and the optional steps of normalization and weighting is required. A consecutive 2-year plot-based field experiment was conducted with five N fertilizer levels (0, 90, 180, 270, and 360 kg N ha-1) in maize production at three sites in Southwest China, to evaluate the environmental performance and sustainability through joint use of life cycle assessment (LCA) and energy consumption analysis. Results demonstrated that the optimal N rate (180 kg N ha-1) showed greater potential for maintaining high yield (achieved 86% of the yield potential) and reducing the global warming (- 31%), acidification (- 47%), eutrophication (- 44%) compared to farmers' practice, and energy depletion potentials, by reducing pollutants emission during the production and transportation of N fertilizer and Nr losses at farm stage. Optimal N treatment indirectly reduced the land use, life-cycle human toxicity, aquatic eco-toxicity, and terrestrial eco-toxicity potentials by improving grain yield and agronomic efficiency. In addition, the optimal N treatment reduced the energy consumption by enhancing the energy use efficiency (EUE) (+ 74%) and reducing non-renewable energy form (- 45%) than the farmer's practice. This study will provide comprehensive information for both scientists and farmers involved in maize production and N management in subtropical region.

A Spectroscopically Observed Iron Nitrosyl Intermediate in the Reduction of Nitrate by a Surface-Conjugated Electrocatalyst

We report an iron-based graphite-conjugated electrocatalyst (GCC-FeDIM) that combines the well-defined nature of homogeneous molecular electrocatalysts with the robustness of a heterogeneous electrode. A suite of spectroscopic methods, supported by the results of DFT calculations, reveals that the electrode surface is functionalized by high spin (S = 5/2) Fe(III) ions in an FeN₄Cl₂ coordination environment. The chloride ions are hydrolyzed in aqueous solution, with the resulting cyclic voltammogram revealing a Gaussian-shaped wave assigned to 1H⁺/1e^- reduction of surface Fe(III)-OH surface. A catalytic wave is observed in the presence of NO₃^-, with an onset potential of -1.1 V vs SCE. At pH 6.0, GCC-FeDIM rapidly reduces NO₃^- to ammonium and nitrite with 88 and 6% Faradaic efficiency, respectively. Mechanistic studies, including in situ X-ray absorption spectroscopy, suggest that electrocatalytic NO₃^- reduction involves an iron nitrosyl intermediate. The Fe-N bond length (1.65 Å) is similar to that observed in {Fe(NO)}⁶ complexes, which is supported by the results of DFT calculations.

Enhanced cycling of nitrogen and metals during rapid infiltration: Implications for managed recharge

We present results from a series of plot-scale field experiments to quantify physical infiltration dynamics and the influence of adding a carbon-rich, permeable reactive barrier (PRB) for the cycling of nitrogen and associated trace metals during rapid infiltration for managed aquifer recharge (MAR). Recent studies suggest that adding a bio-available carbon source to soils can enhance denitrification rates and associated N load reduction during moderate-to-rapid infiltration (≤1 m/day). We examined the potential for N removal during faster infiltration (>1 m/day), through coarse and carbon-poor soils, and how adding a carbon-rich PRB (wood chips) affects subsurface redox conditions and trace metal mobilization. During rapid infiltration, plots amended with a carbon-rich PRB generally demonstrated modest increases in subsurface loads of dissolved organic carbon, nitrite, manganese and iron, decreases in loads of nitrate and ammonium, and variable changes in arsenic. These trends differed considerably from those seen during infiltration through native soil without a carbon-rich PRB. Use of a carbon-rich soil amendment increased the fraction of dissolved N species that was removed at equivalent inflowing N loads. There is evidence that N removal took place primarily via denitrification. Shifts in microbial ecology following infiltration in all of the plots included increases in the relative abundances of microbes in the families Comamonadaceae, Pseudomonadaceae, Methylophilaceae, Rhodocyclaceae and Sphingomonadaceae, all of which contain genera capable of carrying out denitrification. These results, in combination with studies that have tested other soil types, flow rates, and system scales, show how water quality can be improved during infiltration for managed recharge, even during rapid infiltration, with a carbon-rich soil amendment.

Effects of biochar and chemical fertilizer amendment on diazotrophic abundance and community structure in rhizosphere and bulk soils

Diazotrophs carry out biological nitrogen (N) fixation process that replenishes available soil N; it is unclear how soil diazotrophic communities respond to biochar and chemical fertilizer amendment in agricultural ecosystem. Herein, we studied the impacts of biochar and chemical fertilizer amendment on diazotrophic communities in rhizosphere and bulk soils using nifH gene. The field experiment included four treatments: control (CK), biochar (B), chemical NPK fertilizer (CF), and biochar + chemical fertilizer (B + CF). nifH gene abundance in rhizosphere soils ranged from 9.00 × 10⁷ to 2.57 × 10⁸ copies g^-1 dry soil among the different treatments, which was 1.42-2.68 times higher compared with the bulk soils ranging from 5.83 × 10⁷ to 1.19 × 10⁸ copies g^-1 dry soil. Single application of biochar increased the abundance of nifH gene, whereas chemical fertilizer addition significantly decreased it in the bulk and rhizosphere soils. Single biochar addition affected diazotrophic community composition in rhizosphere soil, but not in the bulk soil. However, both CF and B + CF treatments obviously changed the community structure of diazotrophs in both soils. Moreover, rhizosphere effect enhanced nifH gene abundance and significantly altered the diazotrophic community structure compared to bulk soil. Phylogenetic analysis showed that all nifH sequences were affiliated to the cyanobacteria, α-, β-, γ-, and δ- subclasses of the proteobacteria group. Soil nutrient availability rather than pH had significant impacts on diazotrophic community structure based on mantel test and redundancy analysis. Overall, biochar improves the diazotrophic abundance, while chemical fertilization negatively affects it by altering nutrient availability, and combined application of biochar and chemical fertilizer does not counteract the adverse influences of chemical fertilizer on nitrogen-fixing microorganisms.

How different nitrogen fertilizers affect arsenic mobility in paddy soil after straw incorporation?

In straw return fields, nitrogen-fertilizers are added to mitigate microbial competition for nitrogen with plants. However, in arsenic (As)-contaminated paddy fields, the specific effects of different nitrogen fertilizers on As mobility after straw incorporation and the interactions among iron(Fe)/carbon(C)/nitrogen(N)/As are not well understood. In the reported microcosm experiment we monitored As-mobility as a function of different dosages of KNO₃, NH₄Cl and rice straw incorporation. Addition of both KNO₃ and NH₄Cl significantly inhibited the As mobilization induced by straw incorporation. Following the KNO₃ addition, the As concentration in porewater dropped by 51-66% after 2 days of the incubation by restraining Fe reduction and enhancing Fe oxidation. High-dose NH₄Cl addition reduced As in porewater by 22-43% throughout the incubation by decreasing porewater pH. High-throughput sequencing results demonstrated that KNO₃ addition enriches both the denitrifying and Fe-oxidizing bacteria, while diminishing Fe-reducing bacteria; NH₄Cl addition has the opposite effect on Fe-reducing bacteria. Network analysis revealed that As and Fe concentrations in porewater were positively correlated with the abundance of denitrifying and Fe-reducing bacteria. This study broadens our insight into the As biogeochemistry associated with the N/C/Fe balance in soil, which are of great significance for agronomic management and mitigation the risk of As-contaminated paddy fields.