Do nitrogen fertilizers stimulate or inhibit methane emissions from rice fields?

In rice cultivation, there are controversial reports on net impacts of nitrogen (N) fertilizers on methane (CH ₄) emissions. Nitrogen fertilizers increase crop growth as well as alter CH ₄ producing (Methanogens) and consuming (Methanotrophs) microbes, and thereby produce complex effects on CH ₄ emissions. Objectives of this study were to determine net impact of N fertilizers on CH ₄ emissions and to identify their underlying mechanisms in the rice soils. Database was obtained from 33 published papers that contained CH ₄ emissions observations from N fertilizer (28–406 kg N ha^?1) treatment and its control. Results have indicated that N fertilizers increased CH ₄ emissions in 98 of 155 data pairs in rice soils. Response of CH ₄ emissions per kg N fertilizer was significantly (P < 0.05) greater at < 140 kg N ha^?1 than > 140 kg N ha^?1 indicating that substrate switch from CH ₄ to ammonia by Methanotrophs may not be a dominant mechanism for increased CH ₄ emissions. On the contrary, decreased CH ₄ emission in intermittent drainage by N fertilizers has suggested the stimulation of Methanotrophs in rice soils. Effects of N fertilizer stimulated Methanotrophs in reducing CH ₄ emissions were modified by the continuous flood irrigation due to limitation of oxygen to Methanotrophs. Greater response of CH ₄ emissions per kg N fertilizer in urea than ammonia sulfate probably indicated the interference of sulfate in the CH ₄ production process. Overall, response of CH ₄ emissions to N fertilizers was correlated with N‐induced crop yield (r = +0.39; P < 0.01), probably due to increased carbon substrates for Methanogens. Using CH ₄ emission observations, this meta‐analysis has identified dominant microbial processes that control net effects of N fertilizers on CH ₄ emissions in rice soils. Finally, we have provided a conceptual model that included microbial processes and controlling factors to predict effects of N fertilizers on CH ₄ emissions in rice soils.     

Combined carbon and nitrogen removal from acetonitrile using algal–bacterial bioreactors

When compared with Chlorella vulgaris, Scenedesmus obliquus and Selenastrum capricornutum, C. sorokiniana presented the highest tolerance to acetonitrile and the highest O₂ production capacity. It also supported the fastest acetonitrile biodegradation when mixed with a suitable acetonitrile-degrading bacterial consortium. Consequently, this microalga was tested in symbiosis with the bacterial culture for the continuous biodegradation of acetonitrile at 2 g l^–1 in a stirred tank photobioreactor and in a column photobioreactor under continuous illumination (250 E m^–2 s^–1). Acetonitrile removal rates of up to 2.3 g l^–1 day^–1 and 1.9 g l^–1 day^–1 were achieved in the column photobioreactor and the stirred-tank photobioreactor, respectively, when operated at the shortest retention times tested (0.4 days, 0.6 days, respectively). In addition, when the stirred-tank photobioreactor was operated with a retention time of 3.5 days, the microbial culture was capable of assimilating up to 71% and nitrifying up to 12% of the NH₄⁺ theoretically released through the biodegradation of acetonitrile, thus reducing the need for subsequent nitrogen removal. This study suggests that complete removal of N-organics can be combined with a significant removal of nitrogen by using algal–bacterial systems and that further residual biomass digestion could pay-back part of the operation costs of the treatment plant.     

Responses of nitrogen and phosphorus resorption from leaves and branches to long-term nitrogen deposition in a Chinese fir plantation北大核心CSCD

Aims Our objectives were to investigate differences in nutrient resorption between different plant organs (leaf and branch), among plants with different life spans (one-year old, two-year old and senesced), and under different duration of nitrogen (N) deposition treatments in a Chinese fir (Cunninghamia lanceolata) plantation. Methods The long-term N deposition experiment was conducted in a 12-year-old fir plantation of subtropical China. N deposition treatment was initiated in January 2004 until now, up-going 14 years. N deposition were designed at 4 levels of 0, 60, 120, and 240 kg·hm–2·a–1, indicated as N0, N1, N2, and N3, respectively, with 3 replicates for each treatment. The solution of CO(NH2)2 was sprayed on the forest floor each month. In the study, we measured N and phosphorus (P) concentrations and analyzed the pattern of nutrient resorption of mature and senescing leaves and branches. The different responses of needles N and P resorption after 7- and 14-year N deposition treatments were also compared. Important findings After 14 years of N deposition, (1) during the senescing process, leaf and branch C, N, and P content gradually decreased with increasing treatment duration, with higher content in leaf than in branch. N content decreased in the order of one-year old green leaf > two-year old green leaf > senescent leaf > one-year old living branch > two-year old living branch > senescent branch, and N3 > N2 > N1 > N0, with C:N showing the opposite trend. Senescent organs had higher C:N, N:P, and C:P than mature living organs. N deposition increased N, N:P, and C:P of mature living organs (except for the two-year old green leaf), while decreased P and C:N. (2) N resorption efficiency (REN) and P resorption efficiency (REP) of leaves and branches decreased gradually with increasing life span. REP was typically higher in leaf and branch than REN. Leaf had lower REN (28.12%) than branch (30.00%), but higher REP (45.82%) than branch (30.42%). A highly significant linear correlation existed between N:P and REN:REP in leaves and branches. (3) REN decreased but REP increased with the treatment duration of N deposition. The longer experimental duration (14 years) reduced REN by 9.85%, 3.17%, 11.71% under N1, N2, and N3 treatments, respectively, and increased REP by 71.98%, 42.25%, 9.60%, respectively, than the shorter treatment duration (7 years). In summary, the responses of essential nutrients resorption efficiency for different plant organs and life span varied with the levels and duration of N deposition treatment. REN:REP in leaf and branch were mostly driven by N:P of leaf and branch. The results highlight that nutrients resorption is significantly influenced by long-term N deposition. © Chinese Journal of Plant Ecology.     

Increased plant density of winter wheat can enhance nitrogen–uptake from deep soil

Background and Aims

Increased plant density improves grain yield and nitrogen (N)–use efficiency in winter wheat (Triticum aestivum L.) by increasing the root length density (RLD) in the soil and aboveground N–uptake (AGN) at maturity. However, how the root distribution and N–uptake at different soil depths is affected by plant density is largely unknown.

Methods

A 2–year field study using the winter wheat cultivar Tainong 18 was conducted by injecting ¹⁵?N–labeled urea into soil at depths of 0.2, 0.6, and 1.0 m under four plant densities of 135 m^?2, 270 m^?2,405 m^?2, and 540 m^?2.

Results

We observed significant RLD and ¹⁵?N–uptake increases at each soil depth as the plant density increased from 135 to 405 m^?2. ¹⁵?N–uptake increased with plant density as the soil depth increased, although the corresponding RLD value fell with depth. The ¹⁵?N–uptake at each soil depth was positively related to the RLD at the same depth. The total AGN was positively related to RLD in deep soil, especially at 0.8–1.2 m.

Conclusions

Increasing the plant density from 135 m^?2 to the optimum increases AGN primarily by increasing the RLD in deep soil and therefore increasing the plant density of winter wheat can be used to efficiently recover N leached to deep soil. Moreover, the total root numbers per unit area and RLD still increased at supraoptimal density while shoot number and N uptake stagnated.     

High-rate nitrogen removal from livestock manure digester liquor by combined partial nitritation–anammox process

In this study, combination of a partial nitritation reactor, using immobilized polyethylene glycol (PEG) gel carriers, and a continuous stirred granular anammox reactor was investigated for nitrogen removal from livestock manure digester liquor. Successful nitrite accumulation in the partial nitritation reactor was observed as the nitrite production rate reached 2.1 kg-N/m³/day under aerobic nitrogen loading rate of 3.8 kg-N/m³/day. Simultaneously, relatively high free ammonia concentrations (average 50 mg-NH₃/l) depressed the activity of nitrite oxidizing bacteria with nitrate concentration never exceeding 3% of TN concentration in the effluent of the partial nitritation reactor (maximum 35.2 mg/l). High nitrogen removal rates were achieved in the granular anammox reactor with the highest removal rate being 3.12 kg-N/m³/day under anaerobic nitrogen loading rate of 4.1 kg-N/m³/day. Recalcitrant organic compounds in the digester liquor did not impair anammox reaction and the SS accumulation in the granular anammox reactor was minimal. The results of this study demonstrated that partial nitritation–anammox combination has the potential to successfully remove nitrogen from livestock manure digester liquor.     

Can mineralization of soil organic nitrogen meet maize nitrogen demand?

Background and aims

It is generally assumed that very large herbivores, such as elephants, make foraging decisions at large spatial scales, but the extent to which seasonal foraging decisions are driven by soil quality, and its link to plant nutrient levels, is uncertain.

Methods

We studied the diet selection of African elephants Loxodonta africana in Ithala Game Reserve in northern KwaZulu-Natal, South Africa, using data on elephant feeding preferences and spatial distributions from a published paper. Elephants were present in the eastern half with granite soils in the wet season, and in the western half with sedimentary soils in the dry season. The quality of these two soil types and of seven key tree species for elephants was assessed in both seasons.

Results

Soil quality was higher on the sedimentary soils in terms of total nitrogen, soil respiration, water-holding capacity, organic carbon and pH. Leaf quality was higher on the sedimentary soils in the dry season, while in the wet season there was no significant difference in leaf quality of the seven key tree species growing on the two substrates.

Conclusion

Soil quality may explain elephants’ foraging decisions in the dry season, but not in the wet season. Elephants preferred trees with higher protein and lower concentrations of fibre on both granite and sedimentary soils.

     

Does nitrogen uptake affect nitrogen uptake efficiency,or vice versa?

The aim of the paper is to enter into a discussion concerning the title question. In our opinion it is N uptake efficiency that affects N uptake, but not vice versa, mainly because the former is a genotypic characteristic, and as such is not influenced by N uptake. To support the conclusions we also show a similarity between the model used for the problem in question and a yield component model.     

Gaseous nitrogen losses from 15N‐ammonium and plant material amended great basin desert surface soils

Gaseous nitrogen losses from ¹⁵NH₄ ⁺ that was exogenously applied to desert surface soils which were then incubated under aerobic conditions for 35 days at constant 25% moisture reached 81% with plant material added and 75% without an exogenous carbon source. At a decreasing moisture regime, the respective values were 79% and 87%. The losses were attributed to denitrification and, partially, to dissimilatory N₂O release during nitrification. Ammonia volatilization reached a plateau after 4 days, did not exceed 5% of the total ¹⁵NH₄ ⁺ added, and was significantly higher in the plant interspace soils than in the canopy soils.     

Foliar δ15N is affected by foliar nitrogen uptake, soil nitrogen, and mycorrhizae along a nitrogen deposition gradient

Foliar nitrogen isotope (δ¹⁵N) composition patterns have been linked to soil N, mycorrhizal fractionation, and within-plant fractionations. However, few studies have examined the potential importance of the direct foliar uptake of gaseous reactive N on foliar δ¹⁵N. Using an experimental set-up in which the rate of mycorrhizal infection was reduced using a fungicide, we examined the influence of mycorrhizae on foliar δ¹⁵N in potted red maple (Acer rubrum) seedlings along a regional N deposition gradient in New York State. Mycorrhizal associations altered foliar δ¹⁵N values in red maple seedlings from 0.06 to 0.74 ‰ across sites. At the same sites, we explored the predictive roles of direct foliar N uptake, soil δ¹⁵N, and mycorrhizae on foliar δ¹⁵N in adult stands of A. rubrum, American beech (Fagus grandifolia), black birch (Betula lenta), and red oak (Quercus rubra). Multiple regression analysis indicated that ambient atmospheric nitrogen dioxide (NO₂) concentration explained 0, 69, 23, and 45 % of the variation in foliar δ¹⁵N in American beech, red maple, red oak, and black birch, respectively, after accounting for the influence of soil δ¹⁵N. There was no correlation between foliar δ¹³C and foliar %N with increasing atmospheric NO₂ concentration in most species. Our findings suggest that total canopy uptake, and likely direct foliar N uptake, of pollution-derived atmospheric N deposition may significantly impact foliar δ¹⁵N in several dominant species occurring in temperate forest ecosystems.     

A break in the nitrogen cycle in aridlands? Evidence from δp15N of soils

We examined the content and isotopic composition of nitrogen within soils of a juniper woodland and found that a cryptobiotic crust composed of cyanobacteria, lichens, and mosses was the predominant source of nitrogen for this ecosystem. Disturbance of the crust has resulted in considerable spatial variability in soil nitrogen content and isotopic composition; intercanopy soils were significantly depleted in nitrogen and had greater abundance of ¹⁵N compared to intra-canopy soils. Variations in the ¹⁵N/¹⁴N ratio for inter- and intra-canopy locations followed similar Rayleigh distillation curves, indicating that the greater ¹⁵N/¹⁴N ratios for inter-canopy soils were due to relatively greater net nitrogen loss. Coverage of cryptobiotic crusts has been reduced by anthropogenic activities during the past century, and our results suggest that destruction of the cryptobiotic crust may ultimately result in ecosystem degradation through elimination of the predominant source of nitrogen input.     

Biological nitrogen fixation: An efficient source of nitrogen for sustainable agricultural production?

A fundamental shift has taken place in agricultural research and world food production. In the past, the principal driving force was to increase the yield potential of food crops and to maximize productivity. Today, the drive for productivity is increasingly combined with a desire for sustainability. For farming systems to remain productive, and to be sustainable in the long-term, it will be necessary to replenish the reserves of nutrients which are removed or lost from the soil. In the case of nitrogen (N), inputs into agricultural systems may be in the form of N-fertilizer, or be derived from atmospheric N₂ via biological N₂ fixation (BNF).Although BNF has long been a component of many farming systems throughout the world, its importance as a primary source of N for agriculture has diminished in recent decades as increasing amounts of fertilizer-N are used for the production of food and cash crops. However, international emphasis on environmentally sustainable development with the use of renewable resources is likely to focus attention on the potential role of BNF in supplying N for agriculture. This paper documents inputs of N via symbiotic N₂ fixation measured in experimental plots and in farmers' fields in tropical and temperate regions. It considers contributions of fixed N from legumes (crop, pasture, green manures and trees), Casuarina, and Azolla, and compares the relative utilization of N derived from these sources with fertilizer N.     

Can mushrooms fix atmospheric nitrogen?

It is generally reported that fungi likePleurotus spp. can fix nitrogen (N₂). The way they do it is still not clear. The present study hypothesized that only associations of fungi and diazotrophs can fix N₂. This was testedin vitro. Pleurotus ostreatus was inoculated with a bradyrhizobial strain nodulating soybean andP. ostreatus with no inoculation was maintained as a control. At maximum mycelial colonization by the bradyrhizobial strain and biofilm formation, the cultures were subjected to acetylene reduction assay (ARA). Another set of the cultures was evaluated for growth and nitrogen accumulation. Nitrogenase activity was present in the biofilm, but not when the fungus or the bradyrhizobial strain was alone. A significant reduction in mycelial dry weight and a significant increase in nitrogen concentration were observed in the inoculated cultures compared to the controls. The mycelial weight reduction could be attributed to C transfer from the fungus to the bradyrhizobial strain, because of high C cost of biological N₂ fixation. This needs further investigations using¹⁴C isotopic tracers. It is clear from the present study that mushrooms alone cannot fix atmospheric N₂. But when they are in association with diazotrophs, nitrogenase activity is detected because of the diazotrophic N₂ fixation. It is not the fungus that fixes N₂ as reported earlier. Effective N₂ fixing systems, such as the present one, may be used to increase protein content of mushrooms. Our study has implications for future identification of as yet unidentified N₂ systems occurring in the environment.     

Cascading costs:An economic nitrogen cycle

The chemical nitrogen cycle is becoming better characterized in terms of fluxes and reservoirs on a variety of scales. Galloway has demonstrated that reactive nitrogen can cascade through multiple ecosystems causing environmental damage at each stage before being denitrified to N2. We propose to construct a parallel economic nitrogen cascade (ENC) in which economic impacts of nitrogen fluxes can be estimated by the costs associated with each stage of the chemical cascade. Using economic data for the benefits of damage avoided and costs of mitigation in the Chesapeake Bay basin, we have constructed an economic nitrogen cascade for the region. Since a single tonne of nitrogen can cascade through the system, the costs also cascade. Therefore evaluating the benefits of mitigating a tonne of reactive nitrogen released needs to consider the damage avoided in all of the ecosystems through which that tonne would cascade. The analysis reveals that it is most cost effective to remove a tonne of nitrogen coming from combustion since it has the greatest impact on human health and creates cascading damage through the atmospheric, terrestrial, aquatic and coastal ecosystems. We will discuss the implications of this analysis for determining the most cost effective policy option for achieving environmental quality goals.     

Exuviae eating: a nitrogen meal?

Many insects eat their cast cuticle (exuviae) after moulting. The functional significance of this behaviour has not been addressed experimentally. I tested the hypothesis that exuviae eating constitutes a meal, so the animal recycles its nitrogen content. Nitrogenous compounds (protein and chitin) are major components of the cuticle in Periplaneta americana, accounting for as much as 87% of the total weight. It was found that insects almost invariably ate their exuviae during their larval life. The frequency of the behaviour decreased in newly emerged adults and varied between the sexes, males eating their exuviae less frequently than females. This may be due to the extra nitrogen endowment which females need for reproduction. Aposymbiotic animals, which lack the supply of essential amino acids from endosymbiotic bacteria, always ate their exuviae regardless of sex. When animals were reared on different diets throughout their larval life protein level in the diet correlated with exuviae eating. Animals reared on a low protein diet showed the highest levels of exuviae eating; animals reared on a high protein diet showed the highest levels of exuviae rejection. Analysis of the frass produced after exuviae meals showed that over 58% of the nitrogen present in the exuviae was recycled. This demonstrated that cockroaches digested nitrogenous compounds contained in the cuticle. The possibility that the exuviae meal has other functions is discussed, although the evidence supports a nutritional role.     

Global nitrogen fertilizer supply and demand outlook

This paper presents a brief overview of the world nitrogen fertilizer demand, highlights trends in the global and regional developments of production capacity and provides a medium-term perspective of the global nitrogen supply/demand balance.     

Increased inorganic nitrogen leaching from a mountain grassland ecosystem following grazing removal: a hangover of past intensive land-use?

Heathlands and grasslands occur in montane regions, naturally or due to anthropogenic land-use. These are typically nutrient-poor but exposure to elevated nitrogen deposition and intensive livestock grazing causes large-scale ecological change. We studied the long-term implications of grazing removal on soil and drainage water biogeochemistry and the implications for nitrogen cycling in 50-year replicated grazing exclosures on a montane grassland exposed to high rates of ambient nitrogen deposition. Evidence of ‘ecosystem recovery’ represented by successional change from graminoid to shrub-dominance after cessation of grazing was not reflected in the soil biogeochemistry. Cessation of grazing had a negative impact, with increased soil extractable and soil solution nitrate concentrations; an apparent shift towards a more nitrogen-rich, bacterially dominated microbial community; and the acidification of soils and leachate. The increase in nitrate leaching appears to have been counterbalanced by a decrease in dissolved organic nitrogen leaching, approximately maintaining the overall nitrogen balance of the system, whilst apparently altering ecosystem functioning. High rates of organic matter cycling and inorganic nitrogen uptake in grazed grassland may have sustained ecosystem N limitation under elevated nitrogen deposition. Grazing removal caused long-term over-supply of nitrogen from mineralisation of enriched organic matter, exacerbated by continued high nitrogen deposition, exceeding the uptake demand of heath vegetation and resulting in nitrification and nitrate leaching. This disequilibrium between vegetation and soil following grazing removal has implications for restoration after periods of intensive grazing. Grazing may not simply leave a legacy of nutrient enrichment but its cessation may trigger nitrogen saturation and soil and freshwater eutrophication and acidification which counteract the immediate benefits of natural vegetation recovery. Long term, nitrogen saturation of abandoned grasslands is likely to reduce ecosystem resilience to invasion by nitrophilous species, pathogen attack and vulnerability to environmental pressures such as climate change. We conclude that partial and/or phased reduction in grazing levels may permit the more synchronised recovery of soils and vegetation, thereby avoiding imbalances between nitrogen supply and nitrogen demand and detrimental ecological effects.     

Metagenomic marine nitrogen fixation--feast or famine?

Natural populations of bacteria in different environments can be astonishingly diverse, as was revealed graphically by large-scale sequencing of samples of their so-called metagenomes. Among the sequence datasets from four different samples of marine bacterial metagenomes, we noted that nitrogen fixation (nif) genes were conspicuous by their absence from three of them. However, in one sample, more than one-third of the bacteria appeared to have a complement of these genes. Here, some reasons behind this site-to-site variability and their implications for how molecular methods, involving large-scale sequencing and/or functional metagenomics, can best be used to describe bacterial diversity in natural environments are discussed.     

Does geographic origin dictate ecological strategies in Acacia senegal (L.) Willd.? Evidence from carbon and nitrogen stable isotopes

Background and aims

Acacia senegal, a leguminous dryland tree, is economically and ecologically important to sub-Saharan Africa. Water-use efficiency (WUE) and biological nitrogen fixation (BNF) are fundamental to plant productivity and survival. We quantify provenance differences in WUE, BNF, photosynthesis, biomass and gum arabic production from A. senegal assessing genetic improvement potential.

Methods

Using stable isotope ratios, we determined WUE (δ¹³C) and BNF (δ¹⁵N) from provenances of mature A. senegal in field-trials (Senegal), sampling leaves at the beginning (wet) and end (dry) of the rainy season. Seedling provenance trials (UK) determined photosynthesis, and biomass and δ¹³C in relation to water table. Environmental data were characterised for all provenances at their sites of origin.

Results

Provenances differed in both δ¹³C and δ¹⁵N. Gum yield declined with increasing WUE. Virtually no BNF was detected during the dry season and seedlings and mature trees may have different WUE strategies. Wind speed and soil characteristics at provenance origin were correlated with isotope composition and gum production.

Conclusion

Provenance differences suggest that selection for desirable traits, e.g., increased gum production, may be possible. As ecological strategies relate to native locality, the environmental conditions at plantation site and provenance origin are important in assessing selection criteria.