Nitrogen Budget Research Articles

Benthic nitrogen cycling in the deep ocean of the Kuroshio Extension region

Benthic nitrogen cycling, including nitrification, N-loss, and other nitrogen transformations, plays a crucial role in the marine nitrogen budget. However, studies on benthic nitrogen cycling mainly focus on marginal seas, while attention to the deep ocean, which occupies the largest area of the seafloor, is severely lacking. In this study, we investigate the benthic nitrogen cycling in the Kuroshio Extension region (KE) of the northwest Pacific Ocean at water depths greater than 5,000 m through 15N enrichment slurry incubation and pore-water dissolved oxygen and inorganic nitrogen profiles. The slurry incubation indicates nitrification is the predominant process in benthic nitrogen cycling. The potential nitrification rates are nearly an order of magnitude higher than dissimilatory nitrate reduction. Nitrification and total N-loss flux estimated from pore-water nitrate and ammonium profiles are 6–42 and 5–30 μmol N m−2 d−1, respectively. Generally, anammox is the predominant N-loss process in KE sediment. The temperature gradient experiment indicates that the optimum temperature for anammox and denitrification is 13 and 41°C, respectively, partially explaining anammox as the dominant process for deep-ocean benthic N-loss. Both the low concentration of ammonium in pore-water and the discrepant results between anoxic incubation amended with 15NO3− and 15NH4++14NO3− suggest that ammonium is another limiting factor for benthic anammox. N-loss activity gradually declines with the distance from the Oyashio–Kuroshio transition zone. However, nitrification has the opposite trend roughly. This reveals that the sediment in KE transfers from nitrate sink to source from north to south. This trend is mainly caused by the variation of primary production and the supplement of active organic matter, which is the energy source for microbes and the potential source for ammonium through remineralization. Overall, our results highlight temperature and ammonium as two limiting factors for deep-ocean benthic N-loss and also exhibit a tight coupling relationship between pelagic primary production and the benthic nitrogen cycle in KE.

Reactive nitrogen budgets in human-nature coupling system in lakeside area with insufficient data - A case study of Mwanza, Tanzania

Nitrogen (N) is an essential nutrient element for life, and also a major element involved in the composition of greenhouse gases, surface water pollutants, air pollutants, etc. Quantifying and evaluating the nitrogen budget of a region is very important for effectively controlling the nitrogen discharge and scientifically managing the nitrogen cycle. In this paper, the urban Rural Complex N Cycling (URCNC) model was used to analyze the nitrogen budget of Mwanza region, a typical lakeside area with insufficient data, and the nitrogen flow process of livestock subsystem, cropland subsystem, human subsystem and landfill subsystem was clearly described and the nitrogen input sources of atmospheric subsystem and surface water subsystem were clarified. And the results demonstrated: (1) the cropland subsystem was the subsystem with the largest nitrogen flux, and the input, output and accumulation of nitrogen were 33,116 t of N, 31,925 t of N and 1191 t of N, respectively. Livestock subsystem was the second largest subsystem of nitrogen flux, and the input, output and accumulation of nitrogen were 31,013 t, 30,183 t and 830 t, respectively. The nitrogen flux of the human subsystem was also large, and the nitrogen input, output and accumulation were 17,905, 17,125 and 780 t, respectively. The nitrogen input, output and accumulation of the landfill subsystem were 3700 t, 770 t and 2930 t, respectively. (2) 8093 t of N, 6864 t of N, 3959 t of N, and 758 t of N emitted into the atmospheric subsystem from the livestock subsystem, cropland subsystem, human subsystem, and landfill subsystem, respectively. (3) The total Nr input of surface water subsystem increased from 18,545 t of N in 2010 to 20,174 t of N in 2020, with an increase of 8.78 % in the past decade. It was estimated that by 2030, the total Nr input of the surface water subsystem would reach 24,946 t of N with an increase of 23.65 % compared with 2020. The livestock subsystem was the largest source, the cropland subsystem was the second largest source and human subsystem was an important source. (4) Population growth, economic development and urbanization are the main nitrogen driving factor. (5) Technology and policy together have important contributions to the reduction of nitrogen pollution in surface water.

Effect of Temperature on the Carbon, Nitrogen, and Phosphorus Nutrient Budgets of Steelhead Trout (Oncorhynchus mykiss) with Different Sizes

Effect of Temperature on the Carbon, Nitrogen, and Phosphorus Nutrient Budgets of Steelhead Trout (Oncorhynchus mykiss) with Different Sizes

Crab bioturbation affects competition between microbial nitrogen removal and retention in estuarine and coastal wetlands

As the important benthic animal in coastal wetlands, crab bioturbation may significantly affect the nitrogen (N) budgets by regulating microbial N transformation processes. However, the response of interaction between different microbial N processes to crab bioturbation remains poorly understood. Here, a 30-day microcosmic experiment was conducted using sediment collected from the Yangtze Estuary wetland, followed by the determination of temporal variations of physicochemical parameters, N removal (denitrification plus anammox, which is defined as N2 production) and retention rates (nitrate dissimilatory reduction to ammonium, DNRA) as well as relevant gene abundances in response to different crabs densities. The results showed that crab bioturbation simultaneously promoted the rates of N2 production and DNRA processes. These two process rates were positively associated with the intensity of crab bioturbation, which was supported by molecular analysis of relevant functional gene abundance. Crab bioturbation was more beneficial to DNRA than N2 production. Due to this disproportionate stimulation, crab bioturbation increased the importance of DNRA, indicating that N retention was becoming more significant under crab bioturbation in estuarine and coastal wetlands. The variations of sediment total organic carbon and oxygen availability driven by crab bioturbation were the critical factors mediating the changes in microbial N removal and retention. Overall, our findings highlighted that crab bioturbation can affect the N budgets in estuarine and coastal wetlands by altering the competition between microbial N removal and retention.

Nitrogen budgets of a lower subtropical forest as affected by 6 years of over-canopy and understory nitrogen additions

Although tropical and subtropical regions have replaced temperate regions as the global-change hotspots for increased atmosphere nitrogen (N) deposition, whether the regional forests reach N saturation is still unclear. Understory or floor N addition has been commonly used in N-deposition studies, but the results of such studies have recently been challenged because they fail to account for canopy interception, assimilation, and leaching processes. Here, we conducted a field experiment to quantify the effects of over-canopy and understory N addition on N budgets in a lower subtropical monsoon evergreen broadleaved (LSMEB) forest. We found that the LSMEB forest was not N saturated after receiving additional N at 25 and 50 kg ha−1 yr−1 for 6 years. Plants were able to absorb the added N by increasing the N concentrations in their organs, with 120–412 % increasing trend of plant N pools under N-addition treatments. Canopy absorption of N resulting from over-canopy N addition led to increases in N concentrations in tree organs but not to increases in tree biomass. Understory N addition could underestimate the effects of N deposition in forests due to neglecting canopy N interception and canopy effects on N redistribution. Additional experiments using over-canopy N addition are needed to assess the true effects of N deposition on different forest ecosystems in different climate zones.

Different variations in soil CO2, CH4, and N2O fluxes and their responses to edaphic factors along a boreal secondary forest successional trajectory

Forest succession is an important process regulating the carbon and nitrogen budgets in forest ecosystems. However, little is known about how and extent by which vegetation succession predictably affects soil CO2, CH4, and N2O fluxes, especially in boreal forest. Here, a field study was conducted along a secondary forest succession trajectory from Betula platyphylla forest (early stage), then Betula platyphylla-Larix gmelinii forest (intermediate stage), to Larix gmelinii forest (late stage) to explore the effects of forest succession on soil greenhouse gas fluxes and related soil environmental factors in Northeast China. The results showed significant differences in soil greenhouse gas fluxes during the forest succession. During the study period, the average soil CO2 flux was greatest at mid-successional stage (444.72 mg m-2 h-1), followed by the late (341.81 mg m-2 h-1) and the early-successional (347.12 mg m-2 h-1) stages. The average soil CH4 flux increased significantly during succession, ranging from -0.062 to -0.036 mg m-2 h-1. The average soil N2O flux was measured as 17.95 μg m-2 h-1 at intermediate successional stage, significantly lower than that at late (20.71 μg m-2 h-1) and early-successional (20.85 μg m-2 h-1) stages. During forest succession, soil greenhouse gas fluxes showed significant correlations with soil and environmental factors at both seasonal and successional time scales. The seasonal variations of soil GHG fluxes were mainly influenced by soil temperature and water content. Meanwhile, soil MBN and soil NO3--N content were also important factors for soil N2O fluxes. Structural equation modelling showed that forest succession affected soil CO2 fluxes by changing soil temperature and microbial biomass carbon, affected soil CH4 fluxes mainly by changing soil water content and soil pH value, and affected soil N2O fluxes mainly by changing soil temperature, microbial biomass nitrogen, and soil NO3--N content. Our study suggests that forest succession mainly alters soil nutrient and soil environment/chemical properties affecting soil CO2 and N2O fluxes and soil CH4 fluxes, respectively, in the secondary forest succession process.

Analysis on the evolution of nitrogen metabolism process and nitrogen supply hinterland pattern in Beijing's food system

As one of the main human activities affecting the global nitrogen cycle, food production and consumption caused a series of environmental issues with nitrogen, especially in densely populated cities. To comprehensively quantify these issues of cities, Beijing's food system was chosen as a case to build a refined 18-node nitrogen metabolic network model based on environmental and external support, its nitrogen budgets was innovatively accounted from the perspective of “bigger food system”, which contained the nitrogen flows both involved in and driving food metabolic processes. Then the key nodes were identified according to the principle of contributing more than 10% of total nitrogen inputs and emissions. Based on above, China Multi-Regional Input-Output table was used to further examine its nitrogen supply hinterlands and identify the key supply drivers from the sectoral perspective. The results show that the total nitrogen input of Beijing's food system increased by 32.6% but the nitrogen output increased by 64.8% between 1979 and 2019. The (1) Crop cultivation, (2) Animal husbandry, and (11) Urban residents are not only key nitrogen input nodes but also key nitrogen emission nodes. Besides, they drove the external nitrogen supply to Beijing's food system, thus should be the key managed objects. Hebei, Heilongjiang and Tianjin are the largest inland nitrogen supply hinterlands, while the foreign nitrogen supply increased by 15.8% from 2007 to 2019. Technical assistance, economic compensation and trade strategy adjustment are good ways to reduce hinterlands' nitrogen supply pressure. This study provides targeted management suggestions for the nitrogen sustainable of food system in cities and the nitrogen coordination among regions.

Agricultural practices regulate the seasonality of groundwater-river nitrogen exchanges

Soil System Budgets (SSB) of nutrients are generally performed annually over arable land to infer their use efficiency and water pollution risk in highly exploited agricultural watersheds. They are seldom partitioned into seasonal budgets and matched with seasonal nutrient transport in adjacent river reaches. We calculated seasonal soil nitrogen (N) budgets in a Mincio River sub-basin (Italy), and we analyzed the dissolved inorganic N net export in the river reach draining such sub-basin. Our results show seasonal differences of SSB with N excess in winter and even more in spring, equilibrium among sources and sinks during autumn and N deficit during summer. Seasonal inorganic N loads transported by the river were not correlated with SSB as they peaked in late summer and were at their minimum in early spring. Fertilization uncoupled to significant uptake supports N excess in winter and spring, whereas crop uptake uncoupled to N inputs supports summer N deficit. Nitrification cannot explain nitrate accumulation in the river reach, suggesting alternative dynamics driving the local hydrology. Flood irrigation results in large soil nitrate solubilization, transport and in upward migration of the groundwater piezometric head during spring and summer periods. River water is likely replaced by nitrate-rich groundwater when the groundwater recharge exceeds a certain threshold coinciding with late summer. Irrigation is then interrupted and the piezometric head, together with nitrate exchange, decreases. This work suggests that a deep understanding of N dynamics in agricultural watersheds with flooding irrigation on permeable soils needs the reconstruction of the vertical pathways of nitrate and of river-groundwater interactions. Moreover, the partitioning of annual into seasonal N budgets and their combination with irrigation practices allows the identification of hot moments in N cycling. Agricultural practices minimizing nitrate excess, its mobility and the risk of surface and groundwater pollution are suggested for this area.

The nitrogen budget of an urban watershed: Zeekoevlei, Cape Town

Urban watersheds receive excessively high Nitrogen (N) loads predominantly via N deposition, fertilizer use, stormwater runoff, treated effluent discharge and sewage leaks. Beyond engineered solutions, the capacity of these systems to remove this excess N is largely dependent upon the functioning of riparian and wetland ecosystems, which in urban areas are highly modified, fragmented and increasingly under pressure. Appropriate management of urban watersheds needs to consider N flux and process rates that occur within the urban landscape. This research provides a preliminary quantification of the relative magnitudes of N flux in the context of a small urban watershed comprising key features of cities in the Global South: a mosaic of informal and formal residential, peri-urban agriculture and stressed service delivery. For a watershed system of 88 km2, total N inputs amount to 2.82 Gg N yr−1, 78 % of which is in the form of household food. N outputs amount to 2.35 Gg N yr−1 with 86 % of that coming out of wastewater streams either as treated effluent or during the denitrification process. An estimated 0.17 Gg N yr−1 accumulates in the system as plant biomass, in groundwater, soils and landfill. The Zeekoevlei wetland system is hypothesised to remove 76 % of total N which drains off the urban landscape, indicating an important service provided to the City, in addition to other well documented ecosystem services. In this urban system, our estimates indicate N flux rates almost 3 times higher than what would be here naturally in a nutrient-deplete Fynbos system. This work daylights important monitoring and research needs in order to better understand the role of the urban landscape and wetland ecosystems in a highly human-mediated N cycle.

Nitrogen budget and critical load determination at a Sahelian grazed grassland site

Based on rare and original in situ measurements together with published data, we estimate a complete nitrogen (N) budget for a semi-arid Sahelian grazed grassland located in Dahra (Senegal) in 2014 and 2017. Nitrogen inputs include biological fixation, dry and wet atmospheric deposition, and input from livestock manure. Nitrogen outputs include nitric oxide (NO) and nitrous oxide (N2O) emissions from soils, NO and ammonia (NH3) emissions from biomass burning, NH3 volatilization from manure, ingestion from livestock grazing, uptake by trees and soil leaching. Nitrogen inputs ranged between 11.7 ± 0.5 and 34.4 ± 0.5 kg N ha−1 yr−1 for low and high estimates respectively, and N outputs ranged between 16.4 ± 1.5 and 45.7 ± 1.5 kg N ha−1 yr−1 for low and high estimates respectively, on average for both years. Nitrogen depletion was estimated between 4.7 ± 2.0 and 11.3 ± 2.0 kg N ha−1 yr−1, which involves N mining from soils. The budget is dominated by the impact of livestock through grazing (63% of the outputs), NH3 volatilization (15% of the outputs), manure (68% of the inputs) and atmospheric deposition (19% of the inputs). The N critical load (Steady State Mass Balance method) ranged from 16.7 ± 0.8 to 47.5 ± 1.7 kg N ha−1 yr−1, showing that the grazed grassland of Dahra was not yet threatened by an excess of N. The assessment of the critical load in Sahelian landscapes depends heavily on livestock participation to the ecosystem equilibrium.

Estimation of nutrient loads with the use of mass-balance and modelling approaches on the Wełna River catchment example (central Poland)

Nitrogen and phosphorus budgeting is considered to be a key tool for policy makers and stakeholders when dealing with nutrient contamination issues, however no unified method has been employed in countries affected by this eutrophication problem. The current study offers a detailed insight into the estimations of nutrient loads and their distribution between different sources for a middle-sized agricultural catchment, with the use of two approaches: mass balance (static) and modelling (dynamic). Both methods revealed similar contributions of analysed nutrient sources, although the final estimates in the chosen calculation profile were divergent due to the various reasons related to the methods’ specificity. The advantages and disadvantages of both approaches have been specified in our study, and a hybrid solution on a local and country wide scale has been proposed.

Community structure of coral microbiomes is dependent on host morphology

BackgroundThe importance of symbiosis has long been recognized on coral reefs, where the photosynthetic dinoflagellates of corals (Symbiodiniaceae) are the primary symbiont. Numerous studies have now shown that a diverse assemblage of prokaryotes also make-up part of the microbiome of corals. A subset of these prokaryotes is capable of fixing nitrogen, known as diazotrophs, and is also present in the microbiome of scleractinian corals where they have been shown to supplement the holobiont nitrogen budget. Here, an analysis of the microbiomes of 16 coral species collected from Australia, Curaçao, and Hawai’i using three different marker genes (16S rRNA, nifH, and ITS2) is presented. These data were used to examine the effects of biogeography, coral traits, and ecological life history characteristics on the composition and diversity of the microbiome in corals and their diazotrophic communities.ResultsThe prokaryotic microbiome community composition (i.e., beta diversity) based on the 16S rRNA gene varied between sites and ecological life history characteristics, but coral morphology was the most significant factor affecting the microbiome of the corals studied. For 15 of the corals studied, only two species Pocillopora acuta and Seriotopora hystrix, both brooders, showed a weak relationship between the 16S rRNA gene community structure and the diazotrophic members of the microbiome using the nifH marker gene, suggesting that many corals support a microbiome with diazotrophic capabilities. The order Rhizobiales, a taxon that contains primarily diazotrophs, are common members of the coral microbiome and were eight times greater in relative abundances in Hawai’i compared to corals from either Curacao or Australia. However, for the diazotrophic component of the coral microbiome, only host species significantly influenced the composition and diversity of the community.ConclusionsThe roles and interactions between members of the coral holobiont are still not well understood, especially critical functions provided by the coral microbiome (e.g., nitrogen fixation), and the variation of these functions across species. The findings presented here show the significant effect of morphology, a coral “super trait,” on the overall community structure of the microbiome in corals and that there is a strong association of the diazotrophic community within the microbiome of corals. However, the underlying coral traits linking the effects of host species on diazotrophic communities remain unknown.2iUFYDy-sBVn2WqgoVCqfbVideo

Nutrient stocks, flows and balances for the Bolivian agri-food system: Can recycling human excreta close the nutrient circularity gap?

Analysis of the current state of nutrient stocks, flows, and balances of a territory is necessary to inform strategies that can transition the agri-food sector to a circular economy model. In this study, we quantified the nitrogen and phosphorus budgets for the Bolivian agri-food system at national and regional scales by way of agro-ecological zoning. We performed nutrient balances to calculate indicators for sufficiency (extent of nutrient deficit/surplus) and circularity (proportion of nutrients recirculated). We also evaluated the potential of renewable stocks (human excreta and livestock manure) to meet nutrient deficits in the system. Our results showed that there are apparent deficits of 32 kt N and 8 kt P in the system that cannot be accounted for using available data. We estimate the real deficits required to bring yields of 45 crops grown in Bolivia to parity with those of neighbouring countries to be 110 kt N and 33 kt P. About 44% of nitrogen and 74% of phosphorus is currently recirculated in the system, with the major nutrient inputs being biological nitrogen fixation, livestock manure, and crop residues. However, nutrient recycling is likely to decrease in the future because the national strategy to address nutrient deficits is to increase domestic production of synthetic fertilisers. Our analysis also shows that there is a sufficient stock of nutrients already available in human excreta (39 kt N and 5 kt P) to cover 100% of the nitrogen deficit and 64% of the phosphorus deficit. The low-altitude zone of Chiquitania-Pantanal alone accounts for 65% of cultivation and 80% of the nutrient demand in the country. Here, export-oriented crops like soybean and sorghum are grown, but less than 25% of the nitrogen is recirculated. In contrast, there are nutrient surpluses of 41 kt N and 34 kt P in agro-ecological zones like the Valleys and Altiplano where traditional agriculture is practiced, and the majority of food is grown for local consumption. Overall, we find that recycling of human excreta, combined with transfer of regional nutrient surpluses, could be an effective strategy to reduce the overall nutrient deficit in the system.

Applying C:N ratio to assess the rationality of estimates of carbon sequestration in terrestrial ecosystems and nitrogen budgets

Carbon (C) sequestration in terrestrial ecosystems needs to ensure the reactive nitrogen (Nr) supply. However, the organic C:N ratio is rarely considered in both estimates of C sequestration in terrestrial ecosystems and N budget at regional and global scale. Here we propose an approach for extracting Nr sequestrated in terrestrial ecosystems with C (termed as Nrc) from N budget and then assessing the rationality of estimates of C sequestration in terrestrial ecosystems and N budgets by using the sequestrated organic C:N ratio as a criterion. We extracted Nrc from the N budget of IPCC AR5 at global scale and Chinese N budget (Proc Natl Acad Sci (USA) 112:8792, 2015) at regional scale based on the assumptions that there is no net Nr accumulation in agricultural products and no net Nr (except N2O) accumulation in the atmosphere at annual temporal scale, and N2O is not involved in biological processes. By taking the C sequestration in terrestrial ecosystems from C budget of IPCC AR5 (2.1 Pg C/yr during 2000–2009) to assess the N budget from the same report, the organic C:N ratios calculated by the Nrc extracted from both the upper ranges and averages of Nr input and output in global N budgets of IPCC AR5 fell outside the rational range of organic C:N ratio, but the organic C:N ratio (=31) calculated by the Nrc (68.1 Tg N/yr) extracted from the lower ranges of the N budget fell very well into the range of organic C:N ratio in global terrestrial ecosystems. Nrc extracted from Chinese N budget was 11.6 Tg N/yr in 2010, which could be combined with C into Chinese terrestrial ecosystems in a range between 0.14–0.66 Pg C/yr with a robust estimate of 0.36 Pg C/yr by applying the global terrestrial ecosystem sequestrated organic C:N ratio to calculation. The results indicate that the proposed approach for extracting Nrc from N budget works well. Checking the estimated organic C:N ratio in terrestrial ecosystems will help to assess whether the C and/or N budgets are rational or not.Graphical abstract

Turtle ants harbor metabolically versatile microbiomes with conserved functions across development and phylogeny.

Gut bacterial symbionts can support animal nutrition by facilitating digestion and providing valuable metabolites. However, changes in symbiotic roles between immature and adult stages are not well documented, especially in ants. Here, we explored the metabolic capabilities of microbiomes sampled from herbivorous turtle ant (Cephalotes sp.) larvae and adult workers through (meta)genomic screening and in vitro metabolic assays. We reveal that larval guts harbor bacterial symbionts with impressive metabolic capabilities, including catabolism of plant and fungal recalcitrant dietary fibers and energy-generating fermentation. Additionally, several members of the specialized adult gut microbiome, sampled downstream of an anatomical barrier that dams large food particles, show a conserved potential to depolymerize many dietary fibers. Symbionts from both life stages have the genomic capacity to recycle nitrogen and synthesize amino acids and B-vitamins. With help of their gut symbionts, including several bacteria likely acquired from the environment, turtle ant larvae may aid colony digestion and contribute to colony-wide nitrogen, B-vitamin and energy budgets. In addition, the conserved nature of the digestive capacities among adult-associated symbionts suggests that nutritional ecology of turtle ant colonies has long been shaped by specialized, behaviorally-transferred gut bacteria with over 45 million years of residency.

Nutrient budgets drive the changes in shoot N and P concentrations of plants in Inner Mongolia's grasslands over the past 40 years

The nutrient budgets of grassland ecosystems have been extensively disturbed by human activity. The aims of this study were to quantify nitrogen (N) and phosphorus (P) budgets, and evaluate their contributions to changes in shoot nutrient concentrations of dominant plants in Inner Mongolia's grasslands over the past 40 years. N and P budgets were assessed using a nutrient budget model based on flowing intensity of nutrients in and out of the grassland. Meta-analysis was then conducted to quantify changes in shoot nutrient concentrations. The N budget remained positive and continued to increase throughout the study period, while enhanced N deposition and increased supplementary feeding dominated N input (76% of the total in 2017). In contrast, the P budget was negative until 2003, and became positive thereafter. The P input was mainly attributed to supplementary feeding (88% of the total in 2017). The mean shoot N concentration in 1979–1986 was 2.25%, while an increase to 2.53% was observed in 2006–2016. In contrast, the mean shoot P concentration was 0.17% in 1979–1991, subsequently leveling off at 0.17% in 2006–2016. The mean shoot N: P ratio basically remain unchanged over time from 16.72 to 15.85. The N surplus caused major increases in the shoot N concentration of the grassland plants; also, the increased P budget to compensate for past P deficiency resulted in no significant change of shoot P concentrations. Consequently, the grassland system had been in the joint N and P co-limitation over the past 40 years.

Flows and budgets of nutrients and potentially toxic elements on four Swedish organic farms using digestate from agricultural residues

Few fertilizers are permitted for organic farming, which is a challenge when securing nutrient availability, particularly of nitrogen (N). Digestate from biogas production could be a valuable fertilizer for increasing crop yields, through its high content of plant-available nitrogen (NH4-N), but is rarely used in practice. This study evaluated how anaerobic digestion of manure and use of digestate affected inflows and outflows of nutrients and potentially toxic elements on four organic farms with different solutions for digestate production. Mass flows and element concentrations were documented 3 years on three dairy farms and one crop farm and used for calculating farm budgets. Nitrogen and phosphorus (P) budgets were also calculated for biogas reactor and storage pits on three farms. Nitrogen surplus exhibited large variation (18–87 kg N ha−1 year−1) at farm level, with purchased digestate or poultry manure giving major N inputs. The risk of process losses was high, with up to 40% of N and P in feedstock entering farm biogas reactors not recovered in digestate. The proportion of NH4-N in total N in digestate was slightly higher (2–9%) or lower (37%) than in feedstocks entering farm biogas reactors. Improved stirring in farm biogas reactors and storage pits to decrease N and P sedimentation, particularly when digesting poultry manure, would directly increase digestate value. Two farms purchasing digestate from central biogas plants received a digestate causing significant cadmium inputs. Keeping records on element flows can help to tailor the use of digestate for organic farms to achieve a sustainable use of nutrients.

Streambank and floodplain geomorphic change and contribution to watershed material budgets

Stream geomorphic change is highly spatially variable but critical to landform evolution, human infrastructure, habitat, and watershed pollutant transport. However, measurements and process models of streambank erosion and floodplain deposition and resulting sediment fluxes are currently insufficient to predict these rates in all perennial streams over large regions. Here we measured long-term lateral streambank and vertical floodplain change and sediment fluxes using dendrogeomorphology in streams around the U.S. Mid-Atlantic, and then statistically modeled and extrapolated these rates to all 74 133 perennial, nontidal streams in the region using watershed- and reach-scale predictors. Measured long-term rates of streambank erosion and floodplain deposition were highly spatially variable across the landscape from the mountains to the coast. Random Forest regression identified that geomorphic change and resulting fluxes of sediment and nutrients, for both streambank and floodplain, were most influenced by urban and agricultural land use and the drainage area of the upstream watershed. Modeled rates for headwater streams were net erosional whereas downstream reaches were on average net depositional, leading to regional cumulative sediment loads from streambank erosion (−5.1 Tg yr−1) being nearly balanced by floodplain deposition (+5.3 Tg yr−1). Geomorphic changes in stream valleys had substantial influence on watershed sediment, phosphorus, carbon, and nitrogen budgets in comparison to existing predictions of upland erosion and delivery to streams and of downstream sediment loading. The unprecedented scale of these novel findings provides important insights into the balance of erosion and deposition in streams within disturbed landscapes and the importance of geomorphic change to stream water quality and carbon sequestration, and provides vital understanding for targeting management actions to restore watersheds.

Influences of summer monsoon variations on the terrigenous influx, bioproductivity and early diagenetic changes in the southwest Bay of Bengal during Late Quaternary

The sediment core (MGS11/02), collected from the lower boundary of the Bay of Bengal (BoB) oxygen minimum zone (890 m) is studied to understand the influences of summer monsoon variations on the terrigenous sediment influx and its relation to bioproductivity and early diagenetic changes in the southwest BoB during the Late Quaternary. The high content of terrigenous controlled metals increased, kaolinite/chlorite increased, and K/Rb decreased after 19 cal kyr BP, indicating a simultaneous increase in the terrigenous influx and intensity of chemical weathering during this period. The variations in summer monsoon and land influx observed in the core sediments are also reflected in the sources of organic matter. High values of δ13C observed at 40 to 19 cal kyr BP suggests higher influence of marine organic matter and/or presence of C4 terrestrial plant. The negative shift of δ13C between 19 and 6 cal kyr BP could be the result of the change in terrestrial plant biomass to C3 photosynthesis and increased marine organic matter. Rock magnetic properties and redox-sensitive metals distribution suggests the prevalence of reduced environment during 52 to 19 cal kyr BP and oxic condition during 19 cal kyr BP to present. The low sedimentation during glacial period allowed sufficient time for pyritization which resulted in the low magnetic signals during 52 to 20 cal kyr BP and led to reducing conditions in the sediment interval. The δ15N values in the core sediments are suggesting the absence of denitrification in the water column during last 52 cal kyr BP. Previous studies suggest that the BoB is a geochemical ‘tipping point’ because the slight changes in water column denitrification by anthropogenic or climatic impact can accelerate BoB nitrogen contribution to the global nitrogen budget. This study indicates the absence of water column denitrification signals during last 52 cal kyr BP even though drastic climatic changes happened in the BoB in the Late Quaternary.

A nitrate budget of the Bohai Sea based on an isotope mass balance model

Abstract. The Bohai Sea (BHS) is a semi-closed marginal sea impacted by one of the most populated areas of China. The supply of nutrients, markedly that of reactive nitrogen, via fluvial and atmospheric transport has strongly increased in parallel with the growing population. Therefore, it is crucial to quantify the reactive nitrogen input to the BHS and understand the processes and determine the quantities of nitrogen eliminated in and exported from the BHS. The nitrogen budget and in particular the internal sources and sinks of nitrate were constrained by using a mass-based and dual stable isotope approach based on δ15N and δ18O of nitrate (NO3-). Samples of water, suspended matter, and sediments were taken in the BHS in spring (March and April) and summer (July and August) 2018. The Yellow River (YR) was sampled in May and July to November, and Daliao River, Hai River, Luan River, and Xiaoqing River were sampled in November of 2018. In addition to nutrient, particulate organic carbon, and nitrogen concentrations, the dual isotopes of nitrate (δ15N and δ18O), δ15N of suspended matters, and sediments were determined. Based on the available mass fluxes and isotope data an updated nitrogen budget is proposed. Compared to previous estimates, it is more complete and includes the impact of interior cycling (nitrification) on the nitrate pool. The main nitrate sources are rivers contributing 19.2 %–25.6 % and the combined terrestrial runoff (including submarine fresh groundwater discharge of nitrate) accounting for 27.8 %–37.1 % of the nitrate input to the BHS, while atmospheric input contributes 6.9 %–22.2 % to total nitrate. An unusually active interior nitrogen cycling contributes 40.7 %–65.3 % to total nitrate via nitrification. Nitrogen is mainly trapped in the BHS and mainly removed by sedimentation (70.4 %–77.8 %), and only very little is exported to the Yellow Sea (YS) (only 1.8 %–2.4 %). At present denitrification is active in the sediments and removes 20.4 %–27.2 % of nitrate from the pool. However, a further eutrophication of the BHS could induce water column hypoxia and denitrification, as is increasingly observed in other marginal seas and seasonally off river mouths.