Where did all the nitrogen go? Fate of nitrogen inputs to large watersheds in the northeastern U.S.A.

To assess the fate of the large amounts of nitrogen (N) brought into the environment by human activities, we constructed N budgets for sixteen large watersheds (475 to 70,189 km²) in the northeastern U.S.A. These watersheds are mainly forested (48–87%), but vary widely with respect to land use and population density. We combined published data and empirical and process models to set up a complete N budget for these sixteen watersheds. Atmospheric deposition, fertilizer application, net feed and food inputs, biological fixation, river discharge, wood accumulation and export, changes in soil N, and denitrification losses in the landscape and in rivers were considered for the period 1988 to 1992. For the whole area, on average 3420 kg of N is imported annually per km² of land. Atmospheric N deposition, N₂ fixation by plants, and N imported in commercial products (fertilizers, food and feed) contributed to the input in roughly equal contributions. We quantified the fate of these inputs by independent estimates of storage and loss terms, except for denitrification from land, which was estimated from the difference between all inputs and all other storage and loss terms. Of the total storage and losses in the watersheds, about half of the N is lost in gaseous form (51%, largely by denitrification). Additional N is lost in riverine export (20%), in food exports (6%), and in wood exports (5%). Change in storage of N in the watersheds in soil organic matter (9%) and wood (9%) accounts for the remainder of the sinks. The presence of appreciable changes in total N storage on land, which we probably under-rather than overestimated, shows that the N budget is not in steady state, so that drainage and denitrification exports of N may well increase further in the future.     

Strategy differences of two potato species in response to nitrogen starvation. Do plants have a genetic switch for nitrogen signalling?

Survival responses to nitrogen starvation are well known in micro‐organisms but little studied in plants. To construct a framework for study of the plant responses, we investigated the strategy differences of tubers from two closely related potato species. Solanum tuberosum conserves tuber nitrogen by inhibiting shoot growth, but S. phureja mobilizes tuber nitrogen to grow shoots, flowers and seeds. Genetic analysis of progeny from S. phureja–haploid S. tuberosum crosses uncovered segregation of a single dominant gene for the S. tuberosum inhibition strategy. Within S. tuberosum, haploid progeny closely resembled their tetraploid parents, suggesting strong genetic control of the inhibition. Growth of the inhibited shoots was proportional to sub‐optimal levels of added nitrate, and was triggered by exogenous gibberellic acid (GA₃). These observations support the notion that potato plants can closely tie shoot growth to ambient nitrogen levels – probably by a root–shoot nitrogen signal transduction pathway, and that this can be overridden by emergency mobilization of nitrogen reserves, perhaps by GA signalling from the tuber. Furthermore, genes for such developmental switches can be identified by classical genetic analysis of closely related species, such as S. tuberosum and S. phureja, that exhibit opposite survival strategies.     

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.     

Short communication. Carbon:nitrogen ratios of Baltic Sea copepods-indication of mineral limitation?

The carbon (C) and nitrogen (N) content and the C:N ratio of two common calanoid copepods, Eurytemora affinis and Acartia bifilosa were measured during spring and summer at the SW coast of Finland, northern Baltic Sea. The C:N ratio of both copepod species was slow and stable (4-4.5), irrespective of sampling time, which implies N limitation at least during intermediate to high food concentrations in spring and early summer. In addition, experiments were conducted to reveal whether the diet of copepods affects their C and N content. Adding green algae Brachiomonas submarina in concentrations of 50-500 g C l^-1 to <100-m-filtered sea water did not significantly increase copepod C or N content in 3 day experiments. However, the C and N contents of E.affinis eggs were 2 times higher in high food concentration, which may increase survival of eggs and nauplii.      

Mycorrhizal improvement in non-leguminous nitrogen fixing associations with particular reference toHippophaë rhamnoides L.

Summary The roots ofHippophaë rhamnoides which regularly bear actinomycete induced nodules when growing on Scottish sand dunes have also been found to support an endomycorrhizal association withGlomus fasciculatus. Ultrastructural and cytochemical studies carried out on the indigenous infections of establishedHippophaë mycorrhizal roots would support the postulate that transport is indeed occurring between the fungal symbiont and the host plant and vice versa in respect of phosphate and carbohydrate. Experiments using various inoculation regimes, demonstrated the significant improvement in the mycorrhizal/nodulated plants compared to the nodulated-only and the mycorrhizal-only plants with respect to plant growth, uptake of phosphate and nitrogenase activity, when grown in a medium poor in combined nitrogen and soluble phosphate. Preliminary work onAlnus andMyrica species growing in Central Scotland indicates that the mycorrhizae associated with these nodulated root systems exhibit a different interaction pattern which may be dependent on habitat type and associated angiosperm species.     

How does nitrogen availability alter rhizodeposition in Lolium multiflorum Lam. during vegetative growth?

The objective of this work was to determine if the impact of nitrogen (N) on the release of organic carbon (C) into the soil by roots (rhizodeposition) correlated with the effect of this nutrient on some variables of plant growth. Lolium multiflorum Lam. was grown at two levels of N supply, either in sterile sand percolated with nutrient solution or in non-sterile soil. The axenic sand systems allowed continuous quantification of rhizodeposition and accurate analysis of root morphology whilst the soil microcosms allowed the study of ¹⁴C labelled C flows in physico-chemical and biological conditions relevant to natural soils. In the axenic sand cultures, enhanced N supply strongly increased the plant biomass, the plant N content and the shoot to root ratio. N supply altered the root morphology by increasing the root surface area and the density of apices, both being significantly positively correlated with the rate of organic C release by plant roots before sampling. This observation is consistent with the production of mucilage by root tips and with mechanisms of root exudation reported previously in the literature, i.e. the passive diffusion of roots solutes along the root with increased rate behind the root apex. We proposed a model of root net exudation, based on the number of root apices and on root soluble C that explained 60% of the variability in the rate of C release from roots at harvest. The effects of N on plant growth were less marked in soil, probably related to the relatively high supply of N from non-fertiliser soil-sources. N fertilization increased the shoot N concentration of the plants and the shoot to root ratio. Increased N supply decreased the partitioning of ¹⁴C to roots. In parallel, N fertilisation increased the root soluble ¹⁴C and the ¹⁴C recovered in the soil per unit of root biomass, suggesting a stimulation of root exudation by N supply. However, due to the high concentration of N in our unfertilised plants, this stimulation was assumed to be very weak because no significant effect of N was observed on the microbial C and on the bacterial abundance in the rhizosphere. Considering the difficulties in evaluating rhizodeposition in non sterile soil, it is suggested that the root soluble C, the root surface area and the root apex density are additional relevant variables that should be useful to measure along with the variables that are commonly determined when investigating how plant functioning impacts on the release of C by roots (i.e soil C, C of the microbial biomass, rhizosphere respiration).     

Influence of nitrogen on the production of hypericins by St. John’s wort

The effect of nitrogen supply on the production of ‘hypericins’ (hypericin and pseudohypericin) in leaves of St. John’s wort (Hypericum perforatum L.) was examined with plants grown in sand culture and soil. In sand culture, 56-d growth of St. John’s wort plants with decreased nitrogen levels resulted in increased production of hypericins in leaves. A short-term low nitrogen stress in sand culture also resulted in increased production of leaf hypericins. While growth in a low nitrogen-containing soil resulted in elevated levels of hypericins, their production was decreased by supplementation of the soil with additional nitrogen. Increased production of hypericins in St. John’s wort leaves did not require the nitrogen supply to be decreased to levels that resulted in nitrogen deficiency symptoms. Moreover, alteration in the production of leaf hypericins occurring with changes in nitrogen supply did not alter the concentration ratio of pseudohypericin and hypericin. Increased production of leaf hypericins was not associated with any significant changes in the number of dark glands on the leaves and only a weak correlation was observed between leaf dark gland number and levels of leaf hypericins. These results are discussed in terms of the biochemistry of naphthodianthrone production by St. John’s wort plants and implications for growth environment effects during cultivated growth of this medicinal plant.     

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.     

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.     

The use of nitrogen fertiliser in agriculture. Where do we go practically and ecolotically?

Nitrogen fertilisers were produced in 72 countries in 1982, total world capacity being 99 mt of N, having been 50 mt in 1970. Consumption was 31.8 mt in 1970, rising to 60.3 mt in 1980 (Av. annual growth rate 7%). Forecasts suggest N use of 90 mt in 1990 rising to between 111–134 mt by the year 2000.The large amount of N added to only some 11% of the earth's land surface as fertilisers, coupled with concurrent increases in biological N fixation, mainly by grain legumes, is bound to result in increases in the total N content of soils, waters, crop residues and municipal wastes. The need to use N to produce sufficient food and fibre for the 7 billion humans must be set against the need to maintain a good and safe environment. Nitrate levels are increasing in both surface and ground water supplies. The amount of ammonia and oxides of nitrogen in the atmosphere produced by volatilisation and denitrification from soils and animal excreta is also rising. Such increases may have detrimental environmental effects to human health and to the ecology of downstream or polluted non-agricultural ecosystems though the severity and extent of these effects requires verification. As yet, there is little hard evidence of direct damage to human health due to high levels of nitrate in diet or of NH₃ and NO_x in the atmosphere, but effects on natural and forest ecosystems in some areas are proven.With this background, strategies are examined which should help to increase the efficiency with which N is utilised by crops and animals and so decrease losses of nitrogen from farmland.These include the selection of optimum N fertiliser practices based on knowledge of plant requirements, soil N supply, and the use of carefully chosen times, methods and forms of N fertiliser application. Other technological approaches such as use of slow release fertilisers, chemicals that inhibit certain biological transfers of N in soils and amendments added to N fertilisers, to soils or to animal excreta to alter their chemical properties could be developed. Greater use of legumes and enhanced levels of N₂-fixation may also help to lessen the need for N fertiliser.To achieve further improvement in the ways of using N in agriculture, more precise knowledge is needed of the dynamics of nitrogen turnover in soils, of translocation and assimilation in plants, and of interactive flows between soil, plants and animals, and the atmosphere. Only with full understanding of the many biological processes that affect N in ecosystems obtained by multidisciplinary research will it be possible to determine the guidelines for environmentally kind, socially acceptable and economically sound management of nitrogen utilisation in agriculture.     

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.     

Carbon and nitrogen accumulation and microbial activity in Mount St. Helens pyroclastic substrates after 25 years

Lupines (Lupinus lepidus var. lobbii) are important integrators of above and belowground development of Mount St. Helens (1980) pyroclastic substrates because they increase soil organic matter formation and microbial activity and influence other biotic processes. However, basic information is required to understand the unfolding pattern of soil development and to corroborate evidence for increasing rates of organic matter accumulation suggested by earlier work. Soil properties were measured in bare pyroclastic sites without lupines or other plants, under live and under dead L. lepidus. In 2005, pyroclastic substrates had low cation exchange capacity but appeared able to supply sufficient P for plant growth. Soil under live and dead lupines contained higher concentrations of soluble and total C and N, larger, more active microbial communities, more Bradford reactive soil protein and enzymes, and had higher mineralization potentials than bare soil. Comparatively, lupine soil was less dense and had lower C to N ratios and relative respiration rates. Soil microbial biomass-C, determined by substrate-induced respiration, had not increased under lupines since 1990, and was indistinguishable from hot water soluble-C, suggesting microorganisms were a predominant pool of labile-C in these developing soils. Evidence for an important soil glycoprotein, glomalin, in these pyroclastic substrates suggests it can form early during soil development. Concentrations of total C and N under lupines have risen to nearly 4 and 0.4 g kg^?1 respectively since the 1980 eruption, but 2005 data indicate little change since 1997 and imply inputs from lupines into soil are in equilibrium with losses.     

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.     

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.