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The C2 carbon-concentrating mechanism increases net CO2 assimilation by shuttling photorespiratory CO2 in the form of glycine from mesophyll to bundle sheath cells, where CO2 concentrates and can be re-assimilated. This glycine shuttle also releases NH3 and serine into the bundle sheath, and modelling studies suggest that this influx of NH3 may cause a nitrogen imbalance between the two cell types that selects for the C4 carbon-concentrating mechanism. Here we provide an alternative hypothesis outlining mechanisms by which bundle sheath NH3 and serine play vital roles to not only influence the status of C2 plants along the C3 to C4 evolutionary trajectory, but to also convey stress tolerance to these unique plants. Our hypothesis explains how an optimized bundle sheath nitrogen hub interacts with sulfur and carbon metabolism to mitigate the effects of high photorespiratory conditions. While C2 photosynthesis is typically cited for its intermediary role in C4 photosynthesis evolution, our alternative hypothesis provides a mechanism to explain why some C2 lineages have not made this transition. We propose that stress resilience, coupled with open flux tricarboxylic acid and photorespiration pathways, conveys an advantage to C2 plants in fluctuating environments. [ABSTRACT FROM AUTHOR]

[Display omitted] • S0&BM boosted NO 3 –-N removal by 15.4% and slashed N 2 O release by 59.0% vs. S0. • Boron and magnesium of S0&BM optimized the microenvironment and ecological niche. • Oxygen vacancies enabled a closed-loop sulfur cycle, converting chem-S0 to bio-S0. • S0&BM boosted gene expression for full denitrification and sulfur cycling. • S0&BM utilized for boron mud recycling supported nitrogen sink formation in CWs. Constructed wetlands (CWs) utilizing elemental sulfur (S0) as an electron donor are effective for nitrate (NO 3 –-N) removal from low C/N wastewater. However, the limited bioavailability of chemical S0 (chem-S0) due to low water solubility and inadequate mass transfer impedes complete denitrification. In this study, boron mud waste and S0 (S0&BM) were processed and combined to convert chem-S0 to biological S0 (bio-S0) in CWs. The employment of S0&BM improved NO 3 –-N and total nitrogen (TN) removal rates by 15.36 % and 10.55 % compared to S0 alone. The 15N isotope tracing experiment unveiled the pathways of N 2 O production and demonstrated that S0&BM facilitated full denitrification, resulting in a higher percentage of nitrogen being degraded in a non-polluting form (54.70 %). Releasing trace amounts of boron and magnesium in S0&BM mitigated the excessive acidification caused by the sulfur autotrophic process and promoted biofilm formation, thereby pronouncing vertical oxygen fluctuations. Meanwhile, oxygen vacancies generated in the S0&BM promoted the closed-loop sulfur cycle, starting from the oxidation of chem-S0 to sulfate (SO 4 2-), the important process of SO 4 2--reducing bacteria using partial organic compounds as electron donors to reduce SO 4 2- to sulfide (S2-) was realized, and finally the S2- was oxidized to bio-S0 with better properties. S0&BM enriched functional microbial communities in the biofilm related to nitrogen and sulfur transformation. Functional gene analysis showed an increase in the narG , nirS , norB , and nosZ genes supporting denitrification. The dsrA and sqr genes responsible for the sulfur cycle exhibited the highest abundance. This realization allowed for the utilization of waste materials for treatment, enhancing the energy efficiency of carbon reduction and pollution reduction in CWs. [ABSTRACT FROM AUTHOR]

Nitrogen and phosphorous loading drives eutrophication of aquatic systems. Lakes and reservoirs are often effective N and P sinks, but the variability of their biogeochemical dynamics is still poorly documented, particularly in tropical systems. To contribute to the extending of information on tropical reservoirs and to increase the insight on the factors affecting N and P cycling in aquatic ecosystems, we here report on a long-term N and P mass balance (2003–2018) in Valle de Bravo, Mexico, which showed that this tropical eutrophic reservoir lake acts as a net sink of N (−41.7 g N m−2 y−1) and P (−2.7 g P m−2 y−1), mainly occurring through net sedimentation, equivalent to 181% and 68% of their respective loading (23.0 g N m−2 y−1 and 4.2 g P m−2 y−1). The N mass balance also showed that the Valle de Bravo reservoir has a high net N atmospheric influx (31.6 g N m−2 y−1), which was 1.3 times the external load and likely dominated by N2 fixation. P flux was driven mainly by external load, while in the case of N, net fixation also contributed. During a period of high water level fluctuations, the net N atmospheric flux decreased by 50% compared to high level years. Our results outlining water regulation can be used as a useful management tool of water bodies, by decreasing anoxic conditions and net atmospheric fluxes, either through decreasing nitrogen fixation and/or promoting denitrification and other microbial processes that alleviate the N load. These findings also sustain the usefulness of long-term mass balances to assess biogeochemical dynamics and its variability. [ABSTRACT FROM AUTHOR]

Abstract: Local decision makers can influence land use practices that alter N loading and processing within the drainage basin of lower-order stream reaches. Because many practices reduce water retention times and alter the timing and pathways of water flow, local decisions regarding land use can potentially exert a major influence on watershed N export. We illustrate a geospatial approach for assessing the role of denitrification sinks in watershed N delivery at the local level using: (a) widely available geospatial data, (b) current findings from peer-reviewed literature, (c) USGS stream gage data, and (d) locally based data on selected stream attributes. With high resolution, high quality GIS data increasingly available to local communities, they are now in a position to guide local management of watershed N by targeting upland source controls and by identifying landscape sinks for protection and/or restoration. We characterize riparian wetlands, lentic water bodies, and stream reaches as N sinks in the landscape and use geospatial particle tracking to estimate flow paths from N sources and evaluate N removal within sinks. We present an example analysis of the Chickasheen drainage basin, RI, USA, comparing N flux from three equivalent hypothetical N source areas situated in different regions of the watershed and illustrating the role of each N sink type in mediating N flux. Because our goal is to generate a tool that is used by and useful to decision makers we are exploring methods to better understand how decision makers understand and respond to the manner in which information is presented. [Copyright &y& Elsevier]

Nitrogen accumulation rates in a boreal Scots pine monoculture in central Sweden and three groups of mixed Scots pine/Norway spruce/silver birch forests in northern Sweden are estimated by using measured and calculated litter fall data, calculated limit values for litter decomposition, and the nitrogen concentration at the limit value (Nlimit). Humus has been accumulating in these forests for 120–3000 years. The Nlimit value is determined by extrapolating a linear relationship between accumulated litter mass loss and the increasing litter N concentration. Values for foliar litter fall and Nlimit are used to calculate the quantity of an hypothesized stable N fraction remaining after accumulated mass loss has reached an asymptotic plateau (the limit value). This stable remaining amount of nitrogen is compared to measured amounts of N in the humus layers of the forests. Measured rates of N accumulation in humus range from 0.147 to 0.255 g N m–2 year–1 and differ from estimated rates of N accumulation in humus by 1.8–30.4%, the higher value possibly due to a slow succession from N-poor pine to N-rich birch, not accounted for in the model. Relating the nitrogen accumulated in these forests at maturity to a nitrogen budget based on estimated rates of nitrogen input, N2 fixation, denitrification and leaching to the mineral soil gives a good agreement of about 0.30–0.35 g N m–2 year–1 stored in humus and vegetation at forest maturity. In these forests, this low rate of N accumulation and storage can apparently continue for thousands of years until the system is “reset” by forest fire. We conclude that in undisturbed boreal forests, once N is bound in recalcitrant fractions it is stable, and that the humus is thus a long-term nitrogen sink. [Copyright &y& Elsevier]

The aim of this study was to investigate whether symbiotic nitrogen fixation in white clover nodules limits nitrogen supply and hence clover growth by repeated defoliation at two cutting heights. Other possible factors governing symbiotic nitrogen fixation in the field were also elucidated. Using 15N, a 2-year field experiment including white clover (Trifolium repens L. cv. Ladino) and perennial ryegrass (Lolium perenne L. cv. Bastion) in monocultures and in mixtures was conducted in Eschikon, Switzerland. The effect of two cutting heights (4 cm and 10 cm above ground level) on the performance of symbiotic nitrogen fixation of white clover in the different sward-types was investigated. After each harvest, the plots were fertilized with 3 g N m-2 (equivalent to 30 kg N ha-1 cut-1 or 210 kg N ha-1 year-1). In both years, white clover grown in a mixture with grass received a significantly higher percentage of nitrogen from symbiotic fixation compared with clover grown in monoculture. This phenomenon is attributed to the strong competitiveness of ryegrass in soil nitrogen uptake. Consequently, white clover in the clover--ryegrass mixture was more dependent on symbiotic nitrogen fixation than that grown in monoculture. The cutting height did not preferentially influence symbiotic nitrogen fixation, as opposed to the uptake of mineral nitrogen from the soil. From this finding it is suggested that symbiotic nitrogen fixation did not limit the supply of nitrogen to clover and hence its growth. It is proposed that symbiotic nitrogen fixation in white clover is regulated by the demand for nitrogen rather than by the availability of carbohydrate reserves in the stolons. Symbiotic nitrogen fixation should thus be looked upon as an integrated plant growth factor and not as an isolated phenomenon. [ABSTRACT FROM AUTHOR]

Regular bi-weekly additions of NH4NO3, equivalent to a rate of 3 g N m-2 yr-1, were applied to cores of Sphagnum capillifolium, inhabiting hummocks and S. recurvum a pool and hollow colonizer, in a raised bog in north east Scotland. Microbial biomass C and N, both measured by chloroform extraction,showed similar seasonal patterns and, for most depths, the effects of added N on microbial biomass C and N changed with time. The addition of inorganic N had greatest effect during October when the water table had risen to the surface and microbial C and N in the untreated cores had decreased. Microbial C and N were maintained at 75 g C m-2 and 8.3 g N m-2 above the values in the untreated cores and far exceeded the amounts of N that had been added up tothat date (1 g N m-2) as NH4NO3. This increased microbial biomass was interpreted as leaching of carbonaceous material from the NH4NO3 treated moss resulting in greater resistance of the microbial biomass to changes induced by the rising water table. Treatment with N also caused significant reductions in extractable dissolved organic N (DON) at 10-15 cm depth, beneath the surface of the moss, but at lower depths to 25 cm nochanges were observed. Extracted dissolved organic carbon (DOC) was not affected by N treatment and showed less seasonal variation than DON, such that the C:N ratio of dissolved organic matter (DOM) in all depths increased from approximately 4 in July to around 30 in December. [ABSTRACT FROM AUTHOR]

"Knowledge of the N accumulation rate in soil is crucial for the calculation of 'critical loads' of N in forest ecosystems. Calculations of an average rate is based on an assumed constant rate of N accumulation, which may not be correct. In the present study, variations in Naccumulation in forest soil due to i) stand age; and ii) N deposition have been investigated. An increase in total amount of N in the morlayer with increasing forest stand age was observed, indicating a constant N accumulation rate of approximately 0.16 g N m-2 yr-1 in this layer. However, the data showed that the totalaccumulation of N in the soil down to about 70 cm may vary during the forest succession. No significant effect of increased N deposition on the total amount of N was observed. Most of the extra N applied seemed to be taken up by the plants. [ABSTRACT FROM AUTHOR]