On the fate of anthropogenic nitrogen

This article provides a synthesis of literature values to trace the fate of 150 Tg/yr anthropogenic nitrogen applied by humans to the Earth''s land surface. Approximately 9 TgN/yr may be accumulating in the terrestrial biosphere in pools with residence times of ten to several hundred years. Enhanced fluvial transport of nitrogen in rivers and percolation to groundwater accounts for ≈35 and 15 TgN/yr, respectively. Greater denitrification in terrestrial soils and wetlands may account for the loss of ≈17 TgN/yr from the land surface, calculated by a compilation of data on the fraction of N₂O emitted to the atmosphere and the current global rise of this gas in the atmosphere. A recent estimate of atmospheric transport of reactive nitrogen from land to sea (NO_x and NH_x) accounts for 48 TgN/yr. The total of these enhanced sinks, 124 TgN/yr, is less than the human-enhanced inputs to the land surface, indicating areas of needed additional attention to global nitrogen biogeochemistry. Policy makers should focus on increasing nitrogen-use efficiency in fertilization, reducing transport of reactive N to rivers and groundwater, and maximizing denitrification to its N₂ endproduct.     

Unexpected nondenitrifier nitrous oxide reductase gene diversity and abundance in soils

Agricultural and industrial practices more than doubled the intrinsic rate of terrestrial N fixation over the past century with drastic consequences, including increased atmospheric nitrous oxide (N₂O) concentrations. N₂O is a potent greenhouse gas and contributor to ozone layer destruction, and its release from fixed N is almost entirely controlled by microbial activities. Mitigation of N₂O emissions to the atmosphere has been attributed exclusively to denitrifiers possessing NosZ, the enzyme system catalyzing N₂O to N₂ reduction. We demonstrate that diverse microbial taxa possess divergent nos clusters with genes that are related yet evolutionarily distinct from the typical nos genes of denitirifers. nos clusters with atypical nosZ occur in Bacteria and Archaea that denitrify (44% of genomes), do not possess other denitrification genes (56%), or perform dissimilatory nitrate reduction to ammonium (DNRA; (31%). Experiments with the DNRA soil bacterium Anaeromyxobacter dehalogenans demonstrated that the atypical NosZ is an effective N₂O reductase, and PCR-based surveys suggested that atypical nosZ are abundant in terrestrial environments. Bioinformatic analyses revealed that atypical nos clusters possess distinctive regulatory and functional components (e.g., Sec vs. Tat secretion pathway in typical nos), and that previous nosZ-targeted PCR primers do not capture the atypical nosZ diversity. Collectively, our results suggest that nondenitrifying populations with a broad range of metabolisms and habitats are potentially significant contributors to N₂O consumption. Apparently, a large, previously unrecognized group of environmental nosZ has not been accounted for, and characterizing their contributions to N₂O consumption will advance understanding of the ecological controls on N₂O emissions and lead to refined greenhouse gas flux models.     

Microbial denitrification dominates nitrate losses from forest ecosystems

Denitrification removes fixed nitrogen (N) from the biosphere, thereby restricting the availability of this key limiting nutrient for terrestrial plant productivity. This microbially driven process has been exceedingly difficult to measure, however, given the large background of nitrogen gas (N₂) in the atmosphere and vexing scaling issues associated with heterogeneous soil systems. Here, we use natural abundance of N and oxygen isotopes in nitrate (NO₃⁻) to examine dentrification rates across six forest sites in southern China and central Japan, which span temperate to tropical climates, as well as various stand ages and N deposition regimes. Our multiple stable isotope approach across soil to watershed scales shows that traditional techniques underestimate terrestrial denitrification fluxes by up to 98%, with annual losses of 5.6–30.1 kg of N per hectare via this gaseous pathway. These N export fluxes are up to sixfold higher than NO₃⁻ leaching, pointing to widespread dominance of denitrification in removing NO₃⁻ from forest ecosystems across a range of conditions. Further, we report that the loss of NO₃⁻ to denitrification decreased in comparison to leaching pathways in sites with the highest rates of anthropogenic N deposition.Nitrogen (N) is an essential, although ecologically limiting, nutrient in many terrestrial ecosystems (1). Anthropogenic emissions of reactive forms of N due to fossil fuel combustion and modern agriculture practices have drastically increased N deposition inputs to forests globally (2). Atmospherically deposited N represents a new N input to terrestrial ecosystems and may enhance carbon dioxide sequestration, potentially reducing some global climate impacts (3). On the other hand, long-term N deposition could result in N saturation and increased nitrate (NO₃⁻) losses to leaching and denitrification (4, 5), pathways that have different consequences for the Earth system, including consequences on climate, biodiversity, and water and air quality for human health (6). Thus, in the context of global climate and other biogeochemical changes, it is critically important to understand terrestrial N balances and their responses to anthropogenic N inputs.Denitrification is considered the most poorly resolved pathway of N removal from the soil, owing to difficulties in quantifying denitrification rates using standard methods (7). Conventional approaches used to estimate denitrification rates are largely intrusive and challenged by issues of pattern and scale. Acetylene block, ¹⁵N tracer applications, and direct nitrogen gas (N₂) quantification are examples of approaches that have deepened our understanding of denitrification; however, they are inherently disruptive and cannot be applied at scales larger than individual soil cores without a high degree of extrapolation (7). Whereas watershed mass balance techniques are nonintrusive and can provide larger scale insights into gaseous N losses, this approach relies on difficult-to-measure N input fluxes and assumptions therein, and is geographically constrained to environments that lack ground water seepage (7).As an alternative, natural composition of N and oxygen (O) isotopes in NO₃⁻ provides nonintrusive, quantitative, and integrative constraints on denitrification across a myriad of space-time scales (8, 9). This approach takes advantage of the biological imprint of soil denitrification on the N cycle and the tendency for kinetic isotope effects to elevate ¹⁵N/¹⁴N and ¹⁸O/¹⁶O of NO₃⁻ systematically in forests (8). Here, we extend on this approach by developing a new way to estimate NO₃⁻ supply rates to denitrification, which involves a combined Δ¹⁷O, δ¹⁵N, and δ¹⁸O analysis. We hypothesize that denitrification rates examined via our multiple-isotope approach will be larger than denitrification rates based on conventional soil techniques because they are generally applicable to the upper part of soil [e.g., the upper 50 cm (9)] and are challenged by spatial and temporal complexities in the soil denitrification process. We examine this hypothesis across an array of sites spanning tropical (southern China) to temperate (central Japan) regions, as well as various stand ages and N input levels, including moderate (6.1 kg of N ha⁻¹⋅y⁻¹) to high (33.5 kg of N ha⁻¹⋅y⁻¹;

Overexpression of a pH-sensitive nitrate transporter in rice increases crop yields

Cellular pH homeostasis is fundamental for life, and all cells adapt to maintain this balance. In plants, the chemical form of nitrogen supply, nitrate and ammonium, is one of the cellular pH dominators. We report that the rice nitrate transporter OsNRT2.3 is transcribed into two spliced isoforms with a natural variation in expression ratio. One splice form, OsNRT2.3b is located on the plasma membrane, is expressed mainly in the phloem, and has a regulatory motif on the cytosolic side that acts to switch nitrate transport activity on or off by a pH-sensing mechanism. High OsNRT2.3b expression in rice enhances the pH-buffering capacity of the plant, increasing N, Fe, and P uptake. In field trials, increased expression of OsNRT2.3b improved grain yield and nitrogen use efficiency (NUE) by 40%. These results indicate that pH sensing by the rice nitrate transporter OsNRT2.3b is important for plant adaption to varied N supply forms and can provide a target for improving NUE.Intracellular pH is stringently regulated, because most metabolic enzymes can function only within a narrow range of pH. The cytosolic pH is maintained around neutrality, whereas in individual organelles, pH can range from 4.7 (lysosome) to 8.0 (mitochondria) (1). Mammalian cells balance cytosolic pH using Na⁺/H⁺ exchangers, Na⁺–HCO3− cotransporters, Cl^––HCO3−, or anion exchangers (AEs) (1). When bacteria face an acid challenge, the proton-pumping respiratory chain complexes or proton-coupled ATPases, and secondary active transporters, such as anion-proton antiporters like the Cl^–/H⁺ exchangers, are activated to maintain intracellular pH (2). In alkali conditions, bacterial Na⁺/H⁺ antiport and the generation and transport of CO₂, HCO3−, NH₃, and NH4+ are the main strategies for pH homeostasis (2).In plants, cytosolic pH can vary from 7.3 to 8.0 (3). Plant roots acquire mineral nitrogen (N) as the source for growth as nitrate, ammonium, or both; the total amount and the ratio of the two N forms can determine cellular pH. In plants, the phloem is an important tissue for nutrient, mRNA, and signal transport, acting like a neural network connecting the shoot and root (4–8). Phloem pH homeostasis is important for maintaining the physiological balance of the whole plant, as well as the transport and signaling functions of the tissue.Rice (Oryza sativa L.) is a major crop, feeding almost 50% of the world’s population. It has been traditionally cultivated under flooded anaerobic soil conditions, where ammonium is the main N source; however, specialized aerenchyma cells in rice roots can transfer oxygen from the shoots to the roots and release it to the rhizosphere, where bacterial conversion of ammonium to nitrate (nitrification) can occur (9). Nitrification in the waterlogged paddy rhizosphere can result in 25–40% of the total crop N being taken up in the form of nitrate, mainly through a high-affinity transport system (HATS) (10). The uptake of nitrate is mediated by cotransport with protons (H⁺) that can be extruded from the cell by plasma membrane H⁺-ATPases (11).In this study, we analyzed the function of a nitrate transporter, OsNRT2.3, with natural variation of its expression in rice cultivars and the cytosolic pH regulatory motif in the protein. The high expression of one of the two splice forms of this protein, OsNRT2.3b, in rice resulted in better adaptation to changes of N supply forms in the environment and strong improvements in growth, yield, and nitrogen use efficiency (NUE). Our results have significant implications for the understanding of cytosolic pH balance in plant adaptation and its importance for crop improvement.     

From the Cover: Reducing environmental risk by improving N management in intensive Chinese agricultural systems

Excessive N fertilization in intensive agricultural areas of China has resulted in serious environmental problems because of atmospheric, soil, and water enrichment with reactive N of agricultural origin. This study examines grain yields and N loss pathways using a synthetic approach in 2 of the most intensive double-cropping systems in China: waterlogged rice/upland wheat in the Taihu region of east China versus irrigated wheat/rainfed maize on the North China Plain. When compared with knowledge-based optimum N fertilization with 30–60% N savings, we found that current agricultural N practices with 550–600 kg of N per hectare fertilizer annually do not significantly increase crop yields but do lead to about 2 times larger N losses to the environment. The higher N loss rates and lower N retention rates indicate little utilization of residual N by the succeeding crop in rice/wheat systems in comparison with wheat/maize systems. Periodic waterlogging of upland systems caused large N losses by denitrification in the Taihu region. Calcareous soils and concentrated summer rainfall resulted in ammonia volatilization (19% for wheat and 24% for maize) and nitrate leaching being the main N loss pathways in wheat/maize systems. More than 2-fold increases in atmospheric deposition and irrigation water N reflect heavy air and water pollution and these have become important N sources to agricultural ecosystems. A better N balance can be achieved without sacrificing crop yields but significantly reducing environmental risk by adopting optimum N fertilization techniques, controlling the primary N loss pathways, and improving the performance of the agricultural Extension Service.     

Effects of melatonin on seedling growth,mineral nutrition,and nitrogen metabolism in cucumber under nitrate stress

In China, excessive use of nitrogen fertilizers in glasshouses leads to nitrate accumulations in soil and plants, which then limits productivity. Melatonin, an evolutionarily highly conserved molecule, has a wide range of functions in plants. We analyzed the effects of melatonin pretreatment on the growth, mineral nutrition, and nitrogen metabolism in cucumber (Cucumis sativus L. “Jin You No. 1”) when seedlings were exposed to nitrate stress. An application of 0.1 mmol/L melatonin significantly improved the growth of plants and reduced their susceptibility to damage due to high nitrate levels (0.6 mol/L) during the ensuing period of stress treatment. Although excess nitrate led to an increase in the concentrations of nitrogen, potassium, and calcium, as well as a decrease in levels of phosphorus and magnesium, exogenous melatonin generally had the opposite effect except for a further rise in calcium concentrations. Pretreatment also significantly reduced the accumulations of nitrate nitrogen and ammonium nitrogen and enhanced the activities of enzymes involved in nitrogen metabolism. Expression of Cs‐NR and Cs‐GOGAT, two genes that function in that metabolism, was greatly down‐regulated when plants were exposed to 0.6 mol/L nitrate, but was up‐regulated in plants that had received the 0.1 mmol/L melatonin pretreatment. Our results are the first evidence that melatonin has an important role in modulating the composition of mineral elements and nitrogen metabolism, thereby alleviating the inhibitory effect on growth normally associated with nitrate stress.     

派瑞松治疗急性湿疹疗效观察

目的观察派瑞松治疗急性湿疹的疗效。方法60例患者随机分为两组，一组外用派瑞松霜（简称A组），一组外用去炎舒松尿素霜（简称B组），用药2周后比较疗效。结果A组有效率86.67％，B组有效率63.34％，两组之间差异有统计学意义（x^2=4．3556，P〈0.05）。结论派瑞松疗效高，起效快，安全可靠，特别适合于合并细菌感染的急性湿疹。     

Attenuation of nitrate effect during an intermittent treatment regimen and the time course of nitrate tolerance

The long-term efficacy of transdermal nitrate therapy, in particularthe ability of a single patch to provide 24 hprophylaxis againstangina, has been questioned. Two mechanisms have been suggestedfor this loss of effect: the development of pharmacologicaltolerance, and premature patch exhaustion. This study was designedto investigate this problem, and in particular to investigatethe time course of treatment failure. It comprised a randomized,double-blind, cross-over comparison of transdermal glyceryltrinitrate and matching placebo transdermal patches. Significant treatment effects were demonstrated by several criteriafor 8 h of continuous therapy, with some limited effect persistingfor 15 h. Loss of effect began to develop very soon after treatmentwas initiated and progressed in a steady, linear fashion sothat there was virtually no treatment effect after 24 h. Incontrast, during intermittent therapy, treatment effects weremaintained on the second day following a nitrate-free interval.Significant benefit was demonstrated for up to 32 h (i.e. 8h of treatment on day 2). Both nitrate-free intervals (12 and16 h) seemed lobe equally effective in maintaining efficacyafter 3 h of treatment on the second day, although this wasstill somewhat attenuated compared with day 1. These resultsconfirm that loss of therapeutic efficacy of transdermal nitrateis due to the development of tolerance and not premature patchexhaustion. In contrast to previous studies, however, they suggestthat tolerance can only partly be reversed by intermittent therapyand also that the onset of tolerance is so rapid that it iswell established in less than a day's treatment.     

Attenuation of nitrate effect during an intermittent treatment regimen and the time course of nitrate tolerance

The long-term efficacy of transdermal nitrate therapy, in particularthe ability of a single patch to provide 24 hprophylaxis againstangina, has been questioned. Two mechanisms have been suggestedfor this loss of effect: the development of pharmacologicaltolerance, and premature patch exhaustion. This study was designedto investigate this problem, and in particular to investigatethe time course of treatment failure. It comprised a randomized,double-blind, cross-over comparison of transdermal glyceryltrinitrate and matching placebo transdermal patches. Significant treatment effects were demonstrated by several criteriafor 8 h of continuous therapy, with some limited effect persistingfor 15 h. Loss of effect began to develop very soon after treatmentwas initiated and progressed in a steady, linear fashion sothat there was virtually no treatment effect after 24 h. Incontrast, during intermittent therapy, treatment effects weremaintained on the second day following a nitrate-free interval.Significant benefit was demonstrated for up to 32 h (i.e. 8h of treatment on day 2). Both nitrate-free intervals (12 and16 h) seemed lobe equally effective in maintaining efficacyafter 3 h of treatment on the second day, although this wasstill somewhat attenuated compared with day 1. These resultsconfirm that loss of therapeutic efficacy of transdermal nitrateis due to the development of tolerance and not premature patchexhaustion. In contrast to previous studies, however, they suggestthat tolerance can only partly be reversed by intermittent therapyand also that the onset of tolerance is so rapid that it iswell established in less than a day's treatment.     

Attenuation of nitrate effect during an intermittent treatment regimen and the time course of nitrate tolerance.

The long-term efficacy of transdermal nitrate therapy, in particular the ability of a single patch to provide 24 h prophylaxis against angina, has been questioned. Two mechanisms have been suggested for this loss of effect: the development of pharmacological tolerance, and premature patch exhaustion. This study was designed to investigate this problem, and in particular to investigate the time course of treatment failure. It comprised a randomized, double-blind, cross-over comparison of transdermal glyceryl trinitrate and matching placebo transdermal patches. Significant treatment effects were demonstrated by several criteria for 8 h of continuous therapy, with some limited effect persisting for 15 h. Loss of effect began to develop very soon after treatment was initiated and progressed in a steady, linear fashion so that there was virtually no treatment effect after 24 h. In contrast, during intermittent therapy, treatment effects were maintained on the second day following a nitrate-free interval. Significant benefit was demonstrated for up to 32 h (i.e. 8 h of treatment on day 2). Both nitrate-free intervals (12 and 16 h) seemed to be equally effective in maintaining efficacy after 3 h of treatment on the second day, although this was still somewhat attenuated compared with day 1. These results confirm that loss of therapeutic efficacy of transdermal nitrate is due to the development of tolerance and not premature patch exhaustion. In contrast to previous studies, however, they suggest that tolerance can only partly be reversed by intermittent therapy and also that the onset of tolerance is so rapid that it is well established in less than a day's treatment.     

硝酸银和滑石粉产生胸膜固定术的效果比较

目的 比较硝酸银和滑石粉浆在大鼠中产生胸膜固定术的效果。方法 健康成年雄性Sprague—Dawley大鼠60只,随机分为两组：T组（滑石粉组）和A组（硝酸银组）,在造成大鼠气胸模型后,A组注入硝酸银（0．1％硝酸银1ml）,B组注入200mg／kg滑石粉＋1．0ml 0．9％生理盐水。A组和T组分别在注射药物后第1、4,7d,2、4w时,进行剖胸探查收集标本,观察大鼠胸膜黏连积分、胸膜及肺组织炎性积分、胸膜纤维化积分及检测肺组织或胸膜组织中髓过氧化物酶（MPO）的活性。结果：滑石粉浆组产生胸膜黏连在初期较硝酸银组快,但从第二周开始,硝酸银组产生胸膜黏连效果好于滑石粉浆组,到第四周,滑石粉浆组黏连积分为1．8±0．1,而硝酸银组积分为3．3±0．2,显著高于滑石粉浆组;炎性积分和纤维化积分均为硝酸银组高于滑石粉浆组,两组肺组织内MPO活性随着注入硬化剂的时间延长而逐渐增加,其活性均在注入硬化剂后开始增高,第2周时达到高峰值,到第四周时降至正常水平。结论 0．1％硝酸银1ml产生胸膜固定术的效果好于200mg／kg滑石粉,具有临床使用价值。     

Plasma nitrate concentrations in neutropenic and non-neutropenic patients with suspected septicaemia

Summary. Nitric oxide (NO) is an important physiological mediator of vascular tone and is thought to be involved in the pathogenesis of septic shock. Plasma nitrate is the stable end product of NO oxidation and in part reflects endogenous NO production. We measured plasma nitrate levels in 47 episodes of suspected septicaemia in 43 in-patients (16 male and 27 female, age 15–63 years). Nitrate concentrations were significantly higher (P < 0.01) compared to healthy controls. Further analysis revealed that significantly elevated levels occurred only in the septic patients who had normal or elevated numbers of neutrophils in the peripheral blood and were hypotensive on presentation. Failure of plasma nitrate concentrations to rise significantly in patients with neutropenia suggests that this cell type may be important in the activation of the arginine-NO system in severe sepsis in man.     

Nitrogen deposition accelerates soil carbon sequestration in tropical forests

Terrestrial ecosystem carbon (C) sequestration plays an important role in ameliorating global climate change. While tropical forests exert a disproportionately large influence on global C cycling, there remains an open question on changes in below-ground soil C stocks with global increases in nitrogen (N) deposition, because N supply often does not constrain the growth of tropical forests. We quantified soil C sequestration through more than a decade of continuous N addition experiment in an N-rich primary tropical forest. Results showed that long-term N additions increased soil C stocks by 7 to 21%, mainly arising from decreased C output fluxes and physical protection mechanisms without changes in the chemical composition of organic matter. A meta-analysis further verified that soil C sequestration induced by excess N inputs is a general phenomenon in tropical forests. Notably, soil N sequestration can keep pace with soil C, based on consistent C/N ratios under N additions. These findings provide empirical evidence that below-ground C sequestration can be stimulated in mature tropical forests under excess N deposition, which has important implications for predicting future terrestrial sinks for both elevated anthropogenic CO₂ and N deposition. We further developed a conceptual model hypothesis depicting how soil C sequestration happens under chronic N deposition in N-limited and N-rich ecosystems, suggesting a direction to incorporate N deposition and N cycling into terrestrial C cycle models to improve the predictability on C sink strength as enhanced N deposition spreads from temperate into tropical systems.

With the globalization of anthropologically elevated nitrogen (N) deposition (1–3), ecosystem C sequestration can be stimulated in many places, because N limitation of net primary productivity (NPP) is widespread (4, 5). However, N is relatively abundant in many tropical forests, and experiments demonstrate that N supply does not limit NPP in such N-rich forests (1, 4). Moreover, previous studies on forest C sequestration overwhelmingly emphasized plant productivity rather than soil stocks as sink for C (6–11). Soil is the largest pool of terrestrial organic C in the biosphere, and more than half of soil C is stored in forest ecosystems (12). Accordingly, C sequestration as soil organic matter could be quantitatively more important than vegetation for forest C budgets (8).Our current understanding of soil C sequestration in response to N deposition in forests has several limitations. First, unlike biomass C sequestration, responses of forest soil C sequestration to N deposition remain inconclusive. Many studies on soil C dynamics indicate that N deposition can increase soil C sequestration by reducing the decomposition of plant litter and soil organic matter (13–15), inhibiting soil respiration (16), or changing microbial enzymatic activity (14, 17). Conversely, other studies reported that long-term N application did not affect soil C sequestration (18), whereas Van Miegroet and Jandl (19) suggested that N addition can deplete soil C pool through microbial respiration linked to transformation of excess N. The contradictory evidence points to the necessity of further examination of how soil C sinks respond to increased N deposition (3, 20). Second, most studies have been conducted in mid-to-high latitudes in the Northern Hemisphere (16, 18, 20, 21), where most forest ecosystems are N-limited, and an increase in N supply can enhance NPP and aboveground litter production (4, 9, 11, 21, 22). Until now, however, we have lacked data about changes in soil C stocks with increased N supply in tropical forests, where ecosystems are more often N-rich (1, 4). This lack of information has led to a debatable assertion of ecosystem C neutrality with N deposition in many tropical forests (21, 23–25). In fact, we do not know what N will do to C storage in N-rich ecosystems because there is a large class of such systems for which there is little information.Here, we experimentally tested the influence of elevated N deposition on soil C sequestration in an N-rich tropical forest, using more than a decade of N addition to experimental plots established in a lowland primary forest at the Dinghushan Biosphere Reserve (DHSBR) in southern China, which has received high rates of ambient N deposition (e.g., >30 kg N⋅ha⁻¹⋅y⁻¹ in precipitation) for several decades (26). We first quantified the contribution of long-term N addition to below-ground soil C stocks. To further clarify the mechanisms on which soil C storage was changed, we evaluated soil density fractions, because these fractions are closely related to soil C stability and its potential for long-term preservation. The protection of organic matter in soils generally increases with the increasing density of SOM fractions, or with increasing association with mineral particles in the heavy fraction of soils (27). To characterize the biochemical composition of organic matter residing in mineral soil fractions, we employed solid-state ¹³C-NMR spectroscopy to determine the relative abundance of alkyl C, O-alkyl C, aromatic C, and carboxyl C. Moreover, to identify whether there is a general pattern of N-induced soil C sequestration in the tropics, we further combined this field evidence with a meta-analysis of N addition experiments to quantify the responses of soil C storage in tropical forests to N additions.     

Increased plant growth from nitrogen addition should conserve phosphorus in terrestrial ecosystems

Inputs of available nitrogen (N) to ecosystems have grown over the recent past. There is limited general understanding of how increased N inputs affect the cycling and retention of other potentially limiting nutrients. Using a plant–soil nutrient model, and by explicitly coupling N and phosphorus (P) in plant biomass, we examine the impact of increasing N supply on the ecosystem cycling and retention of P, assuming that the main impact of N is to increase plant growth. We find divergent responses in the P cycle depending on the specific pathway by which nutrients are lost from the ecosystem. Retention of P is promoted if the relative propensity for loss of plant available P is greater than that for the loss of less readily available organic P. This is the first theoretical demonstration that the coupled response of ecosystem-scale nutrient cycles critically depends on the form of nutrient loss. P retention might be lessened, or reversed, depending on the kinetics and size of a buffering reactive P pool. These properties determine the reactive pool's ability to supply available P. Parameterization of the model across a range of forest ecosystems spanning various environmental and climatic conditions indicates that enhanced plant growth due to increased N should trigger increased P conservation within ecosystems while leading to more dissolved organic P loss. We discuss how the magnitude and direction of the effect of N may also depend on other processes.     

Treatment with gallium nitrate: evidence for interference with iron metabolism in vivo.

Gallium, when bound to transferrin, has been previously shown to cause tumor cell cytotoxicity by preventing cellular uptake of transferrin bound iron in vitro. Patients treated with constant infusion gallium nitrate for carcinoma show a rise in serum iron within 6 hr of the start of treatment. Serum iron returns to baseline by 24 hr post-infusion. Atomic analysis of iron and gallium content of Sephadex G-150 fractions of treatment sera indicate that about an equimolar amount of gallium and iron are associated with transferrin. These gallium and iron concentrations result in inhibition of transferrin mediated iron uptake in vitro, and in vivo allow for > 90% saturation of transferrin with metal. All seven patients who completed two courses of gallium therapy exhibited hypochromic microcytic anemia (mean fall in hemoglobin 3.5 grams %). Evidence for red cell iron depletion was confirmed by an increase (mean 3.3-fold) in zinc protoporphyrin levels. Since transferrin receptor increases on gallium treated iron requiring cells in vitro, we assessed cell surface transferrin receptor on peripheral blood lymphocytes by measuring fluorescent transferrin receptor antibody binding. A population of highly transferrin receptor positive cells peaks at 48 hr into the infusion. DNA analysis as well as double staining indicate the majority of transferrin receptor positive cells are unstimulated B lymphocytes. These studies provide the first documentation that constant infusion gallium treatment results in significant interference with iron metabolism and evidence for tissue iron depletion in vivo. These changes may correlate with therapeutic effects of gallium such as tumor response.     

Brief Report: The biological carbon pump in CMIP6 models: 21st century trends and uncertainties

The biological carbon pump (BCP) stores ∼1,700 Pg C from the atmosphere in the ocean interior, but the magnitude and direction of future changes in carbon sequestration by the BCP are uncertain. We quantify global trends in export production, sinking organic carbon fluxes, and sequestered carbon in the latest Coupled Model Intercomparison Project Phase 6 (CMIP6) future projections, finding a consistent 19 to 48 Pg C increase in carbon sequestration over the 21st century for the SSP3-7.0 scenario, equivalent to 5 to 17% of the total increase of carbon in the ocean by 2100. This is in contrast to a global decrease in export production of –0.15 to –1.44 Pg C y^–1. However, there is significant uncertainty in the modeled future fluxes of organic carbon to the deep ocean associated with a range of different processes resolved across models. We demonstrate that organic carbon fluxes at 1,000 m are a good predictor of long-term carbon sequestration and suggest this is an important metric of the BCP that should be prioritized in future model studies.     

Iron conservation by reduction of metalloenzyme inventories in the marine diazotroph Crocosphaera watsonii

The marine nitrogen fixing microorganisms (diazotrophs) are a major source of nitrogen to open ocean ecosystems and are predicted to be limited by iron in most marine environments. Here we use global and targeted proteomic analyses on a key unicellular marine diazotroph Crocosphaera watsonii to reveal large scale diel changes in its proteome, including substantial variations in concentrations of iron metalloproteins involved in nitrogen fixation and photosynthesis, as well as nocturnal flavodoxin production. The daily synthesis and degradation of enzymes in coordination with their utilization results in a lowered cellular metalloenzyme inventory that requires ～40% less iron than if these enzymes were maintained throughout the diel cycle. This strategy is energetically expensive, but appears to serve as an important adaptation for confronting the iron scarcity of the open oceans. A global numerical model of ocean circulation, biogeochemistry and ecosystems suggests that Crocosphaera's ability to reduce its iron-metalloenzyme inventory provides two advantages: It allows Crocosphaera to inhabit regions lower in iron and allows the same iron supply to support higher Crocosphaera biomass and nitrogen fixation than if they did not have this reduced iron requirement.