Plant and microbial nitrogen use and turnover: Rapid conversion of nitrate to ammonium in soil with roots

Immobilization of ammonium (NH ₄ ⁺ ) by plants and microbes, a controlling factor of ecosystem nitrogen (N) retention, has usually been measured based on uptake of¹⁵NH ₄ ⁺ solutions injected into soil. To study the influence of roots on N dynamics without stimulating consumption of NH ₄ ⁺ , we estimated gross nitrification in the presence or absence of live roots in an agricultural soil. Tomato (Lycopersicon esculentum var. Peto76) plants were grown in microcosms containing root exclosures. When the plants were 7 weeks old,¹⁵N enriched nitrate (NO ₃ ⁻ ) was applied in the 0–150 mm soil layer. After 24 h, > 30 times more¹⁵NH ₄ ⁺ was found in the soil with roots than in the soil of the root exclosures. At least 18% of the NH ₄ ⁺ -N present at this time in the soil with roots had been converted from NO ₃ ⁻ . We estimated rates of conversion of NO ₃ ⁻ to NH ₄ ⁺ , and rates ofNH ₄ ⁺ immobilization by plants and microbes, by simulating N-flow of¹⁴⁺¹⁵N and¹⁵N in three models representing mechanisms that may be underlying the experimental data: Dissimilatory NO ₃ ⁻ reduction to NH ₄ ⁺ (DNRA), plant N efflux, and microbial biomass nitrogen (MBN) turnover. Compared to NO ₃ ⁻ uptake, plant NH ₄ ⁺ uptake was modest. Ammonium immobilization by plants and microbes was equal to at least 35% of nitrification rates. The rapid recycling of NO ₃ ⁻ to NH ₄ ⁺ via plants and/or microbes contributes to ecosystem N retention and may enable plants growing in agricultural soils to capture more NH ₄ ⁺ than generally assumed.     

Nitrogen-sodium concentrated complex fertilisers (CCF’s)versus nitrogen-sodium blends: Effects on ammonium and nitrate uptake by perennial ryegrass plants replete with potassium

There is ample experimental evidence that, Na, if supplied in separate fertiliser granules or crystals to N, i.e., in blended fertiliser form, can improve both the yield and the recovery of fertiliser N by grassland swards in situations of limited K supply, but not in situations of K abundance. There is some evidence, though, that in K-replete situations, Na, if supplied in the same fertiliser granule as N, i.e. in concentrated complex fertiliser (CCF) form, also improves dry matter production and N recovery by swards whilst lowering the risk of grass tetany in grazing animals. However, the mechanism for the latter effect of Na on N uptake has never been elucidated, nor has it been clarified whether Na stimulates NH ₄ ⁺ and NO ₃ ⁻ uptake by plants or simply NO ₃ ⁻ uptake alone. The aim of the present study was to see if supplying Na in the same fertiliser pellets (NNa-CCF) as NH₄NO₃ (differentially labelled with¹⁵N), or in separate pellets (NNa-blend), had any effect on the recovery of¹⁵N-labelled NH ₄ ⁺ and NO ₃ ⁻ -N by perennial ryegrass plants growing in a glasshouse under K-replete conditions. The results of the experiment confirmed that using an NNa-CCF was more beneficial to shoot production than using an NNa-blend. However, the differential in shoot production occurred without any corresponding difference in total N (i.e. NH ₄ ⁺ plus NO ₃ ⁻ -N) recovery in shoot tissue. Instead, Na, in the CCF appears to have stimulated NO ₃ ⁻ uptake at the expense of NH ₄ ⁺ absorption, thereby altering the balance between NH ₄ ⁺ and NO ₃ ⁻ -nutrition in favour of NO ₃ ⁻ -nutrition, and stimulating shoot production as a consequence. It was concluded that if grassland is already well supplied with K it would be more beneficial in terms of sward production to apply a Na and N-containing CCF than a blend of separate Na and N-containing granules or crystals.     

Ammonium versus Nitrate Nutrition of Plants Stimulates Microbial Activity in the Rhizosphere

Using an alkaline calcareous soil, pot experiments were conducted to elucidate the effects of NH ₄ ⁺ vs. NO ₃ ⁻ nutrition (50 or 100 mg kg⁻¹ soil) of wheat and maize on microbial activity in the rhizosphere and bulk soils. Dicyandiamide was used as nitrification inhibitor to maintain NH ₄ ⁺ as the predominant N source for plants grown in NH ₄ ⁺ -treated soil. While maize grew equally well on both N sources, root and shoot growth of wheat was higher under NH ₄ ⁺ than under NO ₃ ⁻ nutrition. Bacterial population density on roots, but not in the rhizosphere soil, was higher under NH ₄ ⁺ than under NO ₃ ^– supplied at 150 mg N kg⁻¹ soil; whereas at both N levels applied, NH ₄ ⁺ compared to NO ₃ ⁻ nutrition of wheat and maize significantly increased microbial biomass in the rhizosphere soil. Under both plant species, NH ₄ ⁺ vs. NO ₃ ⁻ nutrition also increased aerobic and anaerobic respiration, and dehydrogenase activity in the rhizosphere. As microbial activity in the planted bulk and unplanted soils was hardly affected by the N-source, we hypothesize that the stimulation by NH ₄ ⁺ of the rhizosphere microbial activity was probably due to higher availability of root exudates under NH ₄ ⁺ than under NO ₃ ⁻ nutrition.     

Micronutrients and nitrogen metabolism

A sand-culture experiment was conducted to study the influence of a deficiency of and an excess of micronutrients on the uptake and assimilation of NH ₄ ⁺ and NO ₃ ⁻ ions by maize. By studying the fate of¹⁵N supplied as¹⁵NH₄NO₃ or NH₄ ¹⁵NO₃, it was demonstrated that in maize plants NH₄−N was absorbed in preference to NO ₃ ⁻ −N. The uptake and distribution of N originating from both NH ₄ ⁺ and NO ₃ ⁻ was considerably modified by deficiency of, or an excess of, micronutrients in the growth medium. The translocation of NH ₄ ⁺ −N from roots to shoots was relatively less than that of NO ₃ ⁻ −N. Deficiency as well as excessive amounts of micronutrients, in the growth medium, substantially reduced the translocation of absorbed N into protein. This effect was more pronounced in the case of N supplied as NO ₃ ⁻ . Amino-N was the predominant non-protein fraction in which N from both NH ₄ ⁺ and NO ₃ ⁻ tended to accumulate. The next important non-protein fractions were NO ₃ ⁻ −N when N was supplied as NO ₃ ⁻ and amide-N when NH ₄ ⁺ was the source. The relative accumulation of¹⁵N into different protein fractions was also a function of imposed micronutrient levels.     

Carbohydrate and free amino acid contents in tomato plants grown in media with bicarbonate and nitrate or ammonium

Tomato plants were cultivated (from 2 to 23 days after germination) in media with NO ₃ ⁻ , NH ₄ ⁺ , or a mixture of both forms in different proportions used as the N source given with or without 5 mol dm⁻³ HCO ₃ ⁻ . The accumulation of soluble sugars (reducing sugars and sucrose) and free amino acids was higher in the roots and leaves of NH ₄ ⁺ -fed plants than in NO ₃ ⁻ -fed plants. Starch accumulation in NH ₄ ⁺ -fed plants was higher in leaves (about 28%) and lower in roots (about 37%) in comparison with that of NO ₃ ⁻ -fed plants. Plants cultivated in media containing a mixture of NO ₃ ⁻ /NH ₄ ⁺ were characterized by a lower content of sugars and amino acids accumulation in comparison with that in plants fed with NO ₃ ⁻ or NH ₄ ⁺ . An elevated HCO ₃ ⁻ concentration in the rhizosphere stimulated the accumulation of soluble sugars and free amino acids in all the experimental variants. There were only small differences in the starch content.     

Effects of aluminium and mineral nutrition on growth and chemical composition of hydroponically grown seedlings of five different forest tree species

Forest die-back and impaired tree vitality have frequently been ascribed to Al-toxicity and Al-induced nutritional disorders due to increased acidification of forest soils. Therefore, in this experiment effects of Al were studied on growth and nutrient uptake with seedlings of five different forest tree species. During growth in culture solutions with and without Al all five species proved to be very Al-tolerant, despite high accumulation of Al in roots. In the coniferous evergreens Douglas-fir and Scots pine shoot as well as root Al concentrations were significantly higher than in the deciduous broad-leaved species oak and birch. Larch showed intermediate Al levels. In none of the five species did Al reduce nutrient concentrations or the Ca/Al ratio to values below the critical level. Besides differences in Al accumulation, coniferous and broad-leaved species also differed with respect to uptake and assimilation of nitrogen. Due to extra NH ₄ ⁺ uptake, oak and birch showed a much higher N uptake and higher NH ₄ ⁺ preference than the coniferous species. Especially with oak this high NH ₄ ⁺ preference in combination with a low specific root surface area resulted in a high root proton efflux density. In comparison to both broad-leaved trees and Scots pine the NO ₃ ⁻ reduction capacity of larch and Douglas-fir was extremely low. This may have important consequences for both species if grown in NO ₃ ⁻ -rich soils.     

Seasonal patterns of pore-water nutrients, benthic chlorophyll a and sedimentary AVS in a macrobenthos-rich tidal flat

Monthly field investigations were carried out at controlled low-tide in an estuarine intertidal sandflat of the Seto Inland Sea (Japan) between January 1995 and April 1996. We assessed the spatial and temporal distribution of pore-water nutrient [NH ₄ ⁺ -N, (NO ₃ ⁻ + NO ₂ ⁻ )-N, PO ₄ ³⁻ -P and Si(OH)₄-Si], chlorophyll a (chl a) and acid-volatile sulphide (AVS) concentrations in the uppermost 0–10 cm sediment horizon, and evaluated their relationships with macrobenthic assemblages. Monthly hydrological data, nutrient and chl a concentrations in low-tide creek water adjacent to the flat were used as a complementary environmental characterisation of the study area. All different pore-water nutrients showed a 10 to > 30-fold variability between different layers and periods. NH ₄ ⁺ -N, PO ₄ ³⁻ -P and Si(OH)₄-Si concentrations were lowest in winter, progressively increased throughout spring and summer, and were highest between September and October, with a major increase at intermediate (4–8 cm) layers. In contrast, (NO ₃ ⁻ + NO ₂ ⁻ )-N concentrations, correlated positively with chl a, peaked in winter and sharply decreased with depth in all different seasons and in most occasions. Depth-integrated NH ₄ ⁺ -N, PO ₄ ³⁻ -P and Si(OH)₄-Si pools were correlated positively with biogenic nutrient flux calculated from the nutrient excretion rates of dominant bivalves Ruditapes philippinarum and Musculista senhousia. These relationships indicated rapid removal pathways of P, Si and N (i.e., every 0.1, 0.24 and 0.34 day, respectively), implying highly dynamic sediments. Based on the mass balance P = F + I, biogenic production (P) vs. diffusive flux (F) estimates suggested a predominant contribution of animal bioirrigation (I) to the upward flux of nutrients. AVS concentrations, correlated negatively with both (NO ₃ ⁻ + NO ₂ ⁻ )-N and chl a, showed a progressive increase from late summer throughout winter, subsequent to that of NH ₄ ⁺ -N, PO ₄ ³⁻ -P and Si(OH)₄-Si. The results indicate that in our study area the spatial and temporal distribution of pore-water nutrients, chl a and AVS in sediments are strongly interlinked and suggest that the metabolic processes (e.g., excretion, bio-deposition) and behavioural activities (e.g., particle mixing, bioirrigation) of abundant macrobenthos play an important role in the year-round biogeochemical processes occurring on this flat.     

Epidemiology of wheat take-all as influenced by soil pH and temporal changes in inorganic soil N

Summary Soil pH, NH ₄ ⁺ and NO ₃ ⁻ concentrations in soil, and take-all root rot of winter wheat grown in the field were measured concurrently from sowing to anthesis in order to relate disease development to liming and N fertilization practices. Experimental variables included soil pH (5.5 and 6.0) and three N sources (NH₄NO₃, (NH₄)₂SO₄, NH₄Cl) banded with the seed at sowing in factorial combination with the same three N sources topdressed in the spring. Take-all severity was increased by increasing soil pH and by fertilization with NO ₃ ⁻ . Disease severity on crown roots increased exponentially following spring N fertilization and was affected more by soil pH and N-form than was severity on seminal roots. Grain yield ranged from 4.70 Mgha⁻¹ with spring NH₄NO₃ at soil pH 6.0 to 7.65 Mgha⁻¹ with spring NH₄Cl at soil pH 5.5. Sixty-six percent of the variability in grain yield was explained by the number of take-all infected crown roots per tiller at anthesis. Oregon Agric. Exp. Stn. technical paper no. 7707.     

Colonization of acidic mining lakes: <Emphasis Type="Italic">Chydorus sphaericus</Emphasis> and other Cladocera within a dynamic horizontal pH gradient (pH 3−7) in Lake Senftenberger See (Germany)

Spatio-temporal distribution of nitrogen was examined along the gradient from open water to lakeshore in Lake Taihu, China. Two types of undulating littoral zones were selected: natural reed belt and bare lakeshore. The reed belt affected nitrogen transformation and was sink for internal-lake nitrogen, whereas the bare lakeshore showed little effect. During the growing season, NO ₃ ⁻ -N concentration increased by up to 3–5 times from open water to reed belt, while NH ₄ ⁺ -N concentration decreased. It suggested that nitrification was the main nitrogen process in reed belt. Total dissolved nitrogen (TDN) showed little spatial variation, indicating that most of nitrogen released from sediment did not move into open water again. Significant temporal variation of dissolved nitrogen occurred and was similar in both the littoral zone and the open water. Maximum TDN and NH ₄ ⁺ -N concentrations occurred in January, and NO ₃ ⁻ -N in March. Minimum NH ₄ ⁺ -N and NO ₃ ⁻ -N concentrations occurred in July and August, respectively. An increasing total soil nitrogen was found in the surface sediments from reed belt to open water. This further suggested that the reed-covered littoral zone had strong nitrogen transformation potential.     

Evidence for a periodic excretion of nitrogen by roots of grass-legume associations

Diurnal variation in ion content of the solution bathing roots of two plants growing together in sand culture was analysed for three pairs of grass-legume species (Lolium multiflorum andTrifolium pratense; Zea mays andGlycine hispida; Avena sativa andVicia sativa) and their monospecific controls. Biomass and nitrogen content of plants were determined. Ion concentration (NO ₃ ⁻ , NO ₂ ⁻ , NH ₄ ⁺ , and K⁺) and pH of root solutions were measured for Lolium-Trifolium plant pairs and controls at 6 hours intervals over 36 h, starting at 8 am within a circadian cycle. Root solutions were regularly depleted in NO ₃ ⁻ by the grasses (Lolium-Lolium control) throughout the cycle. For associations involving the legume (Lolium-Trifolium and Trifolium-Trifolium), NO ₃ ⁻ depletion was followed by NO ₃ ⁻ enrichment at night, from late afternoon to early morning; the enrichment was more marked for the Lolium-Trifolium association. Solutions which did not contain NO ₂ ⁻ ions, were enriched by trace amounts of NH ₄ ⁺ ions, largely depleted in K⁺ and alkalanized for all associations throughout the cycle. Repeating the experiment with the three pairs of species at the vegetative phase of development confirmed the previous results: NO ₃ ⁻ enrichment during the night for associations with legumes. When the experiment was repeated with older plants which had almost completed their flowering stage, depletion only was observed and no NO ₃ ⁻ enrichment. These data suggest that NO ₃ ⁻ enrichment results from N excretion from active nodulated roots of the legume, accounting for the increase in both biomass and nitrogen content of the companion grass in grass-legume association. The quantitative importance and periodicity of nitrogen excretion as well as the origin of nitrate enrichment are discussed.     

Nutrient utilization in liquid/membrane system for watermelon micropropagation

Watermelon [Citrullus lanatus (Thunberg) Matsumura and Nakai] proliferating shoot meristems from established shoot cultures were inoculated on modified Murashige and Skoog salts medium supplemented with 10 μM 6-benzyladenine (BA) for shoot proliferation and on similar medium supplemented with 1 μM BA and 10 μM gibberellic acid (GA₃) for shoot elongation. Agar-solidified medium and microporous polypropylene membrane rafts in liquid medium were used to support the tissues. Growth over culture time of proliferating and elongating tissues in liquid and agar-solidified media were compared. Nutrient depletion in liquid medium was monitored and quantified using ion selective electrodes. Tissue fresh weights in both proliferation and shoot elongation media were greater in liquid than in agar-solidified medium. Relative dry matter content, however, was greater in agar-solidified than in liquid medium. More shoots elongated in agar-solidified than in liquid medium. The numbers of buds or unelongated shoot meristems, however, were comparable for both the liquid and agar-solidified medium. Proliferating and elongating tissues in liquid medium used Ca⁺⁺ and K⁺ minimally. NO ₃ ⁻ was utilized but not depleted by proliferating tissues. NH ₄ ⁺ , however, was depleted. Most of the NH ₄ ⁺ was utilized by the proliferating tissues within 21 days of culture when growth rate was greatest. At 35 days, residual Ca⁺⁺, K⁺, NO ₃ ⁻ , and NH ₄ ⁺ in proliferation medium were 81.0%, 67.8%, 55.7%, and 1.2% of initial levels, respectively. NO ₃ ⁻ and NH ₄ ⁺ in shoot elongation medium were depleted. The greatest NO ₃ ⁻ and NH ₄ ⁺ utilization was observed during the first 14 days of culture when the largest growth rate was obtained. The residual Ca⁺⁺, K⁺, NO ₃ ⁻ , and NH ₄ ⁺ in shoot elongation medium at 38 days were 63.5%, 37.9%, 21.2%, and 24.3% of initial concentrations, respectively. At the end of experiment, 72.3% and 42.8% of initial sugars were still remaining in the shoot proliferation and shoot elongation medium, respectively. Technical Contribution No. 3236 of the South Carolina Agricultural Experiment Station.     

The influence of temperature and nitrogen source on growth and nitrogen uptake of two polar-desert species,Saxifraga caespitosa and Cerastium alpinum

Polar-desert plants experience low average air temperatures during their short growing season (4–8 °C mean July temperature). In addition, low availability of inorganic nitrogen in the soil may also limit plant growth. Our goals were to elucidate which N sources can be acquired by polar-desert plants, and how growth and N-uptake are affected by low growth temperatures. We compared rates of N-uptake and increases in mass and leaf area of two polar-desert species (Cerastium alpinum L. and Saxifraga caespitosa L.) over a period of 3 weeks when grown at two temperatures (6 °C vs. 15 °C) and supplied with either glycine, NH₄ ⁺ or NO₃ ⁻. At 15 °C, plants at least doubled their leaf area, whereas there was no change in leaf area at 6 °C. Measured mean N-uptake rates varied between 0.5 nmol g⁻¹ root DM s⁻¹ on glycine at 15 °C and 7.5 nmol g⁻¹ root DM s⁻¹ on NH₄ ⁺ at 15 °C. Uptake rates based upon increases in mass and tissue N concentrations showed that plants had a lower N-uptake rate at 6 °C, regardless of N source or species. We conclude that these polar-desert plants can use all three N sources to increase their leaf area and support flowering when grown at 15 °C. Based upon short-term (8 h) uptake experiments, we also conclude that the short-term capacity to take up inorganic or organic N is not reduced by low temperature (6 °C). However, net N-uptake integrated over a three-week period is severely reduced at 6 °C. This revised version was published online in June 2006 with corrections to the Cover Date.     

Inhibition of Nitrification Alters Carbon Turnover in the Patagonian Steppe

Human activities are altering biodiversity and the nitrogen (N) cycle, affecting terrestrial carbon (C) cycling globally. Only a few specialized bacteria carry out nitrification—the transformation of ammonium (NH ₄ ⁺ ) to nitrate (NO ₃ ⁻ ), in terrestrial ecosystems, which determines the form and mobility of inorganic N in soils. However, the control of nitrification on C cycling in natural ecosystems is poorly understood. In an ecosystem experiment in the Patagonian steppe, we inhibited autotrophic nitrification and measured its effects on C and N cycling. Decreased net nitrification increased total mineral N and NH ₄ ⁺ and reduced NO ₃ ⁻ in the soil. Plant cover (P < 0.05) and decomposition (P < 0.0001) decreased with inhibition of nitrification, in spite of increases in NH ₄ ⁺ availability. There were significant changes in the natural abundance of δ¹⁵N in the dominant vegetation when nitrification was inhibited suggesting that a switch occurred in the form of N (from NO ₃ ⁻ to NH ₄ ⁺ ) taken up by plants. Results from a controlled-condition experiment supported the field results by showing that the dominant plant species of the Patagonian steppe have a marked preference for nitrate. Our results indicate that nitrifying bacteria exert a major control on ecosystem functioning, and that the inhibition of nitrification results in significant alteration of the C cycle. The interactions between the C and N cycles suggest that rates of C cycling are affected not just by the amount of available N, but also by the relative availability for plant uptake of NH ₄ ⁺ and NO ₃ ⁻ .     

Dynamics of Inorganic Nitrogen in Nitrate and Glucose-amended Alkaline–saline Soil

Induction of assimilatory NO ₃ ⁻ reduction through the application of an easily decomposable substrate in alkaline–saline soils of the former lake Texcoco (Mexico) resulted in a fast immobilization of NO ₃ ⁻ in excess of N required for metabolic activity and the release of large concentrations of NO ₂ ⁻ and smaller amounts of NH ₄ ⁺ . We postulated that this was regulated by the amounts of NO ₃ ⁻ and glucose added, and affected by the specific characteristics of soil from the former lake Texcoco. This was investigated by spiking soils of different electrolytic conductivity (EC) 56.0 dS m⁻¹ (soil A of Texcoco) and 11.6 dS m⁻¹ (soil B of Texcoco) with different concentrations of NO ₃ ⁻ and glucose while dynamics of CO₂, NH ₄ ⁺ , NO ₂ ⁻ and NO ₃ ⁻ were monitored in an aerobic incubation for 7 days. For comparison reasons (control) an agricultural soil with low EC (0.3 dS m⁻¹) was included as well. In the agricultural soil, 67% of the added glucose mineralized within 7 days, but only 15% in soil A of Texcoco and 20% in soil B of Texcoco. The application of NO ₃ ⁻ to the agricultural soil added with glucose increased cumulative production of CO₂ 1.2 times, 1.5 times in soil A of Texcoco and 1.8 times in soil B of Texcoco. Concentration of NO ₂ ⁻ increased to > 100 mg NO ₂ ⁻ -N kg⁻¹ when 1000 mg glucose-C kg⁻¹ and 500 mg NO ₃ ⁻ -N kg⁻¹ were added to soil A and B of Texcoco, but remained < 3 mg NO ₂ ⁻ -N kg⁻¹ in the agricultural soil. The ratio between the cumulative production of CO₂ and the decrease in concentration of NO ₃ ⁻ was approximately one in soil A and B of Texcoco, but 10 in the agricultural soil after 3 days. It was found that micro-organisms in the alkaline–saline soil of the former lake Texcoco were capable of immobilizing large quantities of NO ₃ ⁻ when an easy decomposable substrate was available in excess of what might be required for metabolic activity while producing large concentrations of NO ₂ ⁻ , but these phenomena were absent in an agricultural soil. In soil of Texcoco, concentrations of NO ₂ ⁻ and NH ₄ ⁺ increased with increased salinity and availability of NO ₃ ⁻ . This ability to remove large quantities of NO ₃ ⁻ under these conditions and then utilize it at a later time might benefit micro-organisms of the N limited alkaline–saline soils of Texcoco.     

Effect of Sesbania,Azolla and rice straw incorporation on the kinetics of NH4, K,Fe, Mn,Zn and P in some flooded rice soils

In a greenhouse study, with and without rice plants, of five flooded Philippine rice soils whose organic C (OC) content varied from 0.5 to 3.6%, incorporation ofSesbania rostrata, Azolla microphylla and rice straw affected the kinetics of soil solution NH ₄ ⁺ −N, K⁺, Fe²⁺, Mn²⁺, Zn²⁺, and P. Sesbania and Azolla increased NH ₄ ⁺ −N concentration above the control treatment, whereas rice straw depressed it. In all soils Azolla released less NH ₄ ⁺ −N than Sesbania. The apparent net N release depended on the soil and ranged from 44–81% for Sesbania and 27–52% for Azolla. These effects persisted throughout the growth of IR36. Soil solution and exchangeable NH ₄ ⁺ −N increased initially but levelled off between 30 to 80 days and between 20 to 40 days after flooding (DF), respectively. With rice, soil solution NH ₄ ⁺ −N concentration, reached a peak at 15–40 DF and declined to very low levels (<4mg L⁻¹). In the 3 soils of low OC content nitrogen derived from green manure ranged from 34–53% and the apparent revovery of added green manure N varied from 29–67%. Almost all N released from both Azolla and Sesbania were recovered in the rice plant in all soils except Concepcion with only 77%. The concentration of K⁺, Fe²⁺, Mn²⁺ and P in the soil solution were higher with rice straw than Sesbania and Azolla in all soils except Hanggan which showed no change in Fe²⁺ and Mn²⁺ but increased K⁺ and P. In general, rice straw, Sesbania and Azolla decreased Zn²⁺ concentration in all soils.     

Nitrate and ammonium accumulation in bermudagrass in relation to nitrogen fertilization and season

Seasonal changes in nitrate and ammonium concentrations were studied inCynodon dactylon (L.) Pers. plants grown for one year in the field in a Mediterranean area. Plants cultivated in a sandy loam soil were fertilized with nitrate-N or ammonium-N at two application rates (250 and 1000 kg N ha⁻¹ year⁻¹) and compared to controls with no added N. Plots were harvested every three weeks from May to November. Shoots were separated into leaves and stems and analyses carried out in both fractions. Nitrogen applications generally led to elevated nitrate concentrations both in leaves and stems at all sampling dates but had little influence on the ammonium concentrations of the tissues. Higher nitrate and ammonium concentrations were found in stems than in leaves, although no levels higher than 0.22% NO ₃ ⁻ −N and 0.10% NH ₄ ⁺ −N were detected in either fraction. Nitrate tended to accumulate mostly in autumn and spring whereas low accumulations were found in summer. Ammonium showed both in leaves and stems a progressive but limited accumulation throughout the period with a peak in October, followed by a strong decrease in November.     

Effect of nitrogen form and phosphorus source on the growth,nutrient uptake and rhizosphere soil property of Camellia sinensis L.

The effects of nitrogen form and phosphorus source on the growth, nutrient uptake and rhizosphere soil property of tea (Camellia sinensis L.) were investigated in a pot experiment. The experiment was performed with a compartmental cropping device, which enables the collection of rhizosphere soil at defined distances from the root of tea plant. Nitrogen was supplied as nitrate or ammonium in combination with soluble phosphorus as Ca(H₂PO₄)₂ or insoluble P as rock phosphate. The leaf dry matter production of tea was significantly greater in the treatments with NH₄ ⁺ than NO₃ ^-, whereas dry matter production of root and stem was not significantly affected. Addition of phosphorus as either source did not influence the dry matter production. The concentrations of K in root, Mg and Ca in both the shoot and root supplied with NO₃ ^- were significantly higher than in NH₄ ⁺ and influence of P sources was minor. On the contrary, Al and Mn concentrations were significantly larger in NH₄ ^--fed plants which could be attributed to remarkably increased availability of Al and Mn caused by acidification of the rhizosphere soil (the first 1-mm soil section from the root surface) with NH₄–N nutrition. The concentration of N in shoot was also significantly higher in NH₄- than in NO₃-fed plants, indicating higher use efficiency of NH₄–N. Whatever the phosphate source, rhizosphere pH declined in ammonium compared to in nitrate treatment. The pH decrease was much larger when no P or soluble P were applied and reached 0.85–1.30 units which extended to 3–5 mm away from the root surface. Exchangeable acidity, content of exchangeable Al and Mn were also considerably higher in the rhizosphere soils of NH₄ ⁺ fed tea plants. Significant amounts of P dissolved from rock phosphate accumulated in rhizosphere of NH₄ ⁺, not NO₃ ^-, suggesting that the dissolution of rock phosphate was induced by the proton excreted by tea root fed with ammonium. With soluble P addition, shoot and root P concentrations were greater in NH₄ ⁺ than in NO₃ ^- treatment and it appeared that this difference could not be sufficiently explained by the available P content in soil which was only slightly higher in NH₄ ⁺ treatment. With rock phosphate addition, the shoot and root P concentrations were hardly affected by nitrogen form, although the available P content was much higher and accumulated in the rhizosphere soil supplied with ammonium. The reason for this was discussed with regard to the inter-relationship of Al with P uptake. This revised version was published online in June 2006 with corrections to the Cover Date.     

Toxicity of nitrapyrin to sunflower under different N nutrition regimes

Summary The purpose of this study was to investigate the phytotoxicity of nitrapyrin 2-chloro-6-(trichloromethyl)pyridine to sunflower (Helianthus annuus L.) under different N regimes and to see if N forms affect the phytotoxicity of nitrapyrin. Sunflower was grown in pot culture for 21 days and was fertilized with (NH₄)₂SO₄, NH₄NO₃ and NaNO₃ to provide 0, 100 and 200 ppm N and with nitrapyrin application of 0 and 20 ppm. All N-treated sunflower plants in all N regimes and regardless of titrapyrin treatment produced more root and shoot dry weights and contained a significantly higher N than untreated check. Nitrapyrin toxicity appeared as a curling of leaf margin and a tendril type of stem growth, the visible toxicity symptoms decreased in the order: (NH₄)₂SO₄>NH₄NO₃>NaNO₃. Furthermore nitrapyrin addition suppressed sunflower growth in each N regime, the suppressing effect being greater with (NH₄)₂SO₄ and NH₄NO₃ than as with NaNO₃. Although, shoot growth from plants receiving nitrapyrin was not significantly affected by any N regime, root growth of nitrapyrin-treated plants was somewhat restricted by NH₄ ⁺−N nutrition relative to NO₃ ⁻−N nutrition.     

Interactions between environmental variables on growth rate and carrageenan content of Solieria chordalis (Solieriaceae,Rhodophyceae) in culture

Using vegetative propagules (ramuli) of the iota carrageenan producing red seaweed Solieria chordalis, a maximum growth rate of 6.8% d⁻¹ was achieved when cultured at 20 °C and 100 μmol photon m⁻² s⁻¹ in seawater supplemented with 20 μM NO₃-N or 10 μM NO₃-N plus 10 μM NH₄-N. Ramuli grew less well when nitrogen was supplied solely as NH ₄ ⁺ . Maximum carrageenan content was observed at the same temperature, irradiance and salinity as growth, but at lower nitrogen concentrations. These findings are discussed in relation to results obtained from studies on other iota-carrageenan producing carrageenophytes.