Tracer studies on nitrogen immobilization-mineralization relationships in forest raw humus

Summary An investigation was conducted to study the effect of fertilizer-N, application rate, and incubation temperature on the immobilization-mineralization relationships of N in forest raw humus. Urea, ammonium chloride, and potassium nitrate enriched with N¹⁵ were used at rates of 0, 50, 100, 200, and 400 ppm N. During a 60 days' experimental period the soil was incubated at 12° and 20°C. The interchanges of N added to this highly acid humus material are to a great extent found to be governed by the N-carrier itself. The total recovery in the N¹⁵ inorganic pool at the end of the 60 days' period, increased with increasing fertilizer application rate regardless of the source of N added. With one exception only, the recovery of KCl-extractable N¹⁵ was larger in the ammonium treatment than in the urea-treated humus. The more striking differences between the interchanges of N¹⁵ in the urea and the ammonium treatments, as compared with the nitrate-treated humus, are revealed in the extremely high recovery of KCl-extractable N, and a concomitant low recovery of the added N in the non-extractable pool in the nitrate treatment. The immobilization and re-mineralization of N are found to be positively correlated with temperature. The total recovery of N¹⁵ was high in the nitrate treatment, and even higher in the ammonium-treated soil (96 to 104 per cent). However, the unaccounted-for losses reached a maximum of about 48 per cent of the added N at 20°C in the 400 ppm urea-N treatment. The results are consistent in showing a statistically highly significant effect of N-fertilizer, application rate, and temperature on the net mineralization of humus N. The release of soil N is discussed in relation to the highly significant two- and three-way interactions between the above-mentioned main variables. Contribution from the Forest Soil Fertilization Research Group, Vollebekk, Norway. This work was supported by the Agricultural Research Council of Norway.     

Soil drying and organic matter decomposition

Summary A study was made of the effects of drying the soil at various temperatures on the subsequent mineralization of carbon, nitrogen and phosphorus of native and added organic matter in the soil.Heating the soil, especially at 100°C was shown to increase the solubility of soil nitrogen, phosphorus and organic matter. On moistening dried soil and incubating, the mineralization of native soil organic matter (humus) increased with the drying temperature and with the length of drying period. Drying, especially at 100°C, reduced the decomposition of fresh organic matter added to the soil. In contrast it increased the mineralization of soil organic nitrogen, but while the bulk of the inorganic nitrogen so produced was converted to nitrate at the lower drying temperature, nitrification did not occur in the soil dried at 100°C.Addition of decomposable organic materials caused nitrate immobilization and retarded the nitrification of the ammonia produced.Drying the soil also caused an immobilization of soil phosphorus, but while this was short-lived at the lower temperatures, it persisted up to twelve weeks in the soil dried at 100°C. Addition of decomposable organic materials increased phosphorus immobilization.     

Non-exchangeable binding of ammonium and amino nitrogen by Norway spruce raw humus

Summary The capacity of an originally acid Norway spruce raw humus to fix isotopically labelled ammonium and amino nitrogen in a form resistant to cold 1N HCl treatment was studied. The amount fixed was determined after a reaction period of 24 hours (the humus pretreated with propylene oxide), using the amount of labelled N in the HCl-leached humus residue as a basis for calculating the amount of added N fixed. The nitrogen sources used were ammonium chloride, glycine and cyanamide. It was found that the fixation of added ammonium and glycine N was exceedingly low in the H⁺-saturated raw humus (pH 3.3–3.4), but the fixation rate was rapidly increased by increasing the pH during the aerobic incubation. Maximum fixation was obtained at a final pH of 10–11. Within the acid range the fixation was constantly higher for added glycine-N than ammonium-N. On the alkaline side of the neutral point the amount of fixation tended to be similar for ammonium and glycine. In treatments with N¹⁵-labelled ammonium, it was shown that small but fully detectable amount of added N were present in the soluble organic N fraction of the HCl extract, the quantities increasing with increasing soil pH during the incubation. The fixation was increased by increasing temperature and decreased by oxidative pretreatment of the humus before the addition of N. In the nitrogen gas atmosphere the fixation figures were 40 to 50 per cent lower than for corresponding treatments in air atmosphere. When various N compounds were added in equimolar concentrations the highest fixation was recorded for cyanamide. In studying the stability of fixed N to acid hydrolysis, it was found that 54 per cent of the fixed N resisted eight hours' refluxing with 6N HCl, the corresponding figure for the native raw humus N being 19 per cent. About one third of the fixed N was liberated as ammonia during the acid hydrolysis.     

Immobilization and mineralization of tracer nitrogen in forest raw humus

Summary An experiment was conducted to permit an assessment of the effect of the N-carrier on the interchange of fertilizer-N in humus maintained under waterlogged conditions. The experiment was carried out at 4°; 12°; and 20°C; and the incubation varied from 0 to 90 days. N¹⁵-enriched urea, ammonium chloride and potassium nitrate were used. The incorporation of fertilizer-N into a non KCl-extractable fraction occurred relatively fast in the urea treatment regardless of the temperature. The highest recovery of tracer-N in the mentioned fraction was attained at 20°C after 90 days' incubation. The urea was hydrolyzed at a high rate, and within 30 days of fertilizer application the inorganic N¹⁵-pool was almost stabilized at a level in the vicinity of 60% of the added N at 12° and 20°C. Nitrogen added as ammonium was incorporated into the non-extractable fraction at a lower rate than was the N added in the form of urea. The data reveal a continual N-immobilization, but at 20°C a substantial net re-mineralization took place during the last part of the experiment. The nitrate treatment was followed by a sizeable temporary net immobilization of tracer-N. The extremely great changes in the inorganic N¹⁵-pool and the net re-mineralization of tracer-N were positively correlated with temperature and incubation time. Within 30 days, about 95% of the added nitrate-N had disappeared from the inorganic pool at 12° and 20°C. Approximately 7.6% of the added NO₃-N was re-mineralized at 20°C after 90 days' incubation. The data reveal great losses of gaseous N from this waterlogged acid forest soil after treatment with nitrate. Contribution from the Forest Soil Fertilization Research Group, Vollebekk, Norway. This work was supported by the Agricultural Research Council of Norway.     

Immobilization,mineralization and the availability of the fertilizer nitrogen during the decomposition of the organic matters applied to the soil

Summary Immobilization and mineralization of the tracer nitrogen (K¹⁵NO₃) applied to the soil together with several organic matters during their decomposition was investigated in incubation experiments.After incubation for three months at 30°C, the decomposition rates of rice straw, hardwood bark, sawdust, softwood bark and peat moss were 41, 15, 7, 5, and 5%, respectively. After incubation for three months at 30°C, 100 and 80% of the fertilizer nitrogen were immobilized in the treatment with 2.0% of rice straw and sawdust carbon, respectively. These resulted in the lowered uptake of the fertilizer nitrogen by plants. In case of peat moss and barks, the amount of fertilizer nitrogen which transformed to the organic nitrogen fractions was quite small and the plant uptake of the nitrogen was hardly affected. Remineralization of the immobilized nitrogen was clearly observed after 2 months' incubation in case where rice straw carbon was added to the extent of 0.5 and 1.0%, but it was not observed in case where other organic matter carbon was added.The data showed that peat moss and barks were highly resistant to the action of microorganisms. As a results the immobilization process of the fertilizer nitrogen incubated with these organic matter was quite slow.     

The use of mean pool abundances to interpret 15N tracer experiments

A pulse dilution ¹⁵N technique was used in the field to determine the effect of the ammonium to nitrate ratio in a fertilizer application on the uptake of ammonium and nitrate by ryegrass and on gross rates of mineralization and nitrification. Two experiments were performed, corresponding approximately to the first and second cuts of grass. Where no substantial recent immobilization of inorganic nitrogen had occurred, mineralization was insensitive to the form of nitrogen applied, ranging from 2.1–2.6 kg N ha^-1 d^-1. The immobilization of ammonium increased as the proportion of ammonium in the application increased. In the second experiment there was evidence that high rates of immobilization in the first experiment were associated with high rates of mineralization in the second. The implication was that some nitrogen immobilized in the first experiment was re-mineralized during the second. Whether this was nitrogen taken up, stored in roots and released following defoliation was not clear. Nitrification rates in this soil were low (0.1–0.63 kg N ha^-1 d^-1), and as a result, varying the ratio of ammonium to nitrate applied markedly altered the relative uptake of ammonium and nitrate. In the first experiment, where temperatures were low, preferential uptake of ammonium occurred, but where >90% of the uptake was as ammonium, a reduction in yield and nitrogen uptake was observed. In the second experiment, where temperatures and growth rates were higher, the proportion of ammonium to nitrate taken up had no effect on yield or nitrogen uptake.     

Mineralization of the carbon and nitrogen of plant material added to soil and of the soil humus during incubation following periodic drying and rewetting of the soil

Summary The production of mineralized carbon and nitrogen by a slightly acid sandy loam soil, in the presence and absence of finely chopped fresh plant material or powdered dry plant material, was followed by determination of the amounts of carbon and nitrogen mineralized at intervals during continuous incubation over a period of twelve weeks. Mineralization of carbon and nitrogen was also followed in parallel soil samples and soil plant material mixtures which were dried at 35°C or 105°C and then rewetted every two weeks during the incubation period.The amounts of carbon and nitrogen mineralized were determined at intervals during the incubation period.Mineralization of the carbon and nitrogen of the humus of the soil was stimulated by periodic drying of the soil and particularly when the soil was dried at 105°C.It was found that more mineral nitrogen was produced from fresh plant material than from dried plant material in all the treatments. Periodic drying of the soil-plant material mixtures did not stimulate the production of mineral nitrogen from the added plant material and reduced it considerably when the drying was carried out at 105°C.Periodic drying at 35°C did not stimulate the mineralization of the carbon of fresh or dried plant material. It is clear therefore that, at temperatures occurring in nature, it is unlikely that the decomposition of plant material added to the soil will be stimulated as a consequence of drying of the soil. Periodic drying of the soil-plant material mixture at 105°C increased the mineralization of the carbon of the dried plant material. The amounts of carbon mineralized in 12 weeks from the dried plant material did not, however, exceed the amounts from fresh plant material incubated continuously in fresh soil or in soil periodically dried at 35°C.     

Effect of temperature and sources of carbon and nitrogen on the conidial germination and appresoria formation in Colletotrichum capsici

The effect of temperatures and exogenous supply of different carbon and nitrogen sources on the conidial germination and appresoria formation inC. capsici has been studied. 25 °C was observed to be the best temperature for conidial germination. At 18 °C, conidia germinated only in hanging drops and did not germinate on 2 % agar. Amongst carbon sources, 1-rhamnose supported maximum conidial germination and appresoria formation. Potassium nitrate supported very good conidial germination and appresoria formation. Ammonium nitrate, ammonium sulphate and ammonium chloride were found to be inhibitory. A few aminoacids stimulated conidial germination but dl-alanine, dl-methionine, tyrosine and l-glutamine were found to be inhibitory.     

The course of 15N-ammonium nitrate in a spring barley cropping system

¹⁵N-labelled ammonium nitrate was applied to spring barley growing on a Cambisol soil in western Switzerland. Immobilization, plant uptake and disappearance of inorganic nitrogen were followed at frequent intervals. Fertilizer nitrogen disappeared shortly after its application, mainly through immobilization by soil microorganisms and absorption by the crop. Some of the added nitrogen was probably denitrified as a result of humid conditions during the first days after fertilizer application. At the end of the growing season, 31% of the added nitrogen was recovered from the aerial barley plants, and 56% was immobilized by microorganisms. Most of the fertilizer nitrogen not used by the crop was immobilized in the upper 0–30 cm soil layer. This prevented downward movement of nitrate and limited nitrogen losses. Fertilizer efficiency was mainly determined by the competition between crop uptake and microbial immobilization. Careful consideration of the time of fertilization, taking into account plant growth and weather conditions, can result in an increase in fertilizer efficiency and minimal pollution.     

Effect of temperature on nitrogen transformation in Hawaiian soils

Summary In a field experiment soil samples buried at the warmer temperature regime nitrified added ammonium faster than soils buried at the cooler temperature regime. Nitrification occurred more rapidly under both regimes in a soil which had developed in a warm climatic zone than in two other soils developed under cooler conditions.The rate of nitrification of added ammonium in soils incubated at 5, 15, 25 and 40°C in the laboratory increased with increase in the temperature up to 25°C in three out of four soils. In the fourth soil nitrification was as active at 40°C as at 25°C. The temperature range for appreciable nitrification to occur in a soil was related to the environmental conditions where the soil was formed.Mineralization of organic nitrogen occurred to a greater extent at 40°C than at three lower incubating temperatures of 5, 15, and 25°C. Rapid and active mineralization was associated with high organic matter and C/N ratio in soils     

Effect of temperature on humus respiration rate and nitrogen mineralization: Implications for global climate change

Respiration and nitrogen mineralization rates of humus samples from 7 Scots pine stands located along a climatic transect across the European continent from the Pyrenees (42°40) to northern Sweden (66°08) were measured for 14 weeks under laboratory conditions at temperatures from 5 °C to 25 °C. The average Q₁₀ values for the respiration rate ranged from about 1.0 at the highest temperature to more than 5 at 10 °C to 15 °C in the northernmost samples. In samples from more northern sites, respiration rates remained approximately constant during the whole incubation period; in the southern end of the transect, rates decreased over time. Respiration rate was positively correlated with incubation temperature, soil pH and CN ratio, and negatively with soil total N. Regressions using all these variables explained approximately 71% of the total variability in the respiration rate. There was no clear relation between the nitrogen mineralization rate and incubation temperature. Below 15 °C the N-mineralization rate did not respond to increasing temperature; at higher temperatures, significant increases were found for samples from some sites. A regression model including incubation temperature, pH, N_tot and CN explained 73% of the total variability in N mineralization. The estimated increase in annual soil respiration rates due to predicted global warming at the high latitudes of the Northern Hemisphere ranged from approximately 0.07×10¹⁵ to 0.13×10¹⁵ g CO₂ at 2 °C and 4 °C temperature increase scenarios, respectively. Both values are greater than the current annual net carbon storage in northern forests, suggesting a switch of these ecosystems from net sinks to net sources of carbon with global warming.     

Comparative growth of the thermal alga Cyanidium caldarium on nitrate and ammonia at different temperatures

Summary The growth of Cyanidium caldarium on nitrate and ammonia as nitrogen sources was studied at different temperatures from 21 to 54°C.Algal growth occurred at temperatures of 24° C or above when ammonia was the nitrogen source, whereas with nitrate, growth occurred at 30° C or above. The optimum and the maximum growth temperatures were 45 and 54° C respectively on both substrates.Arrhenius plots show that the logarithm of the growth rate is not linear with the reciprocal of absolute temperature, but exhibit sharply defined breaks at 30° C on ammonia and at 40° C on nitrate.It is assumed that below 40° C, when Cyanidium grows on nitrate, the utilization of this substrate represents the master reaction which controls the growth rate of the alga.     

The use of different soil nitrogen sources by young Norway spruce plants

 Three-year-old Norway spruce trees were planted into a low-nitrogen mineral forest soil and supplied either with two different levels of mineral nitrogen (NH₄NO₃) or with a slow-release form of organic nitrogen (keratin). Supply of mineral nitrogen increased the concentrations of ammonium and nitrate in the soil solution and in CaCl₂-extracts of the rhizosphere and bulk soil. In the soil solution, in all treatments nitrate concentrations were higher than ammonium concentrations, while in the soil extracts ammonium concentrations were often higher than nitrate concentrations. After 7 months of growth, ¹⁵N labelled ammonium or nitrate was added to the soil. Plants were harvested 2 weeks later. Keratin supply to the soil did not affect growth and nitrogen accumulation of the trees. In contrast, supply of mineral nitrogen increased shoot growth and increased the ratio of above-ground to below-ground growth. The proportion of needle biomass to total above-ground biomass was not increased by mineral N supply. The atom-% ¹⁵N was higher in younger needles than in older needles, and in younger needles higher in plants supplied with ¹⁵N-nitrate than in plants supplied with ¹⁵N-ammonium. The present data show that young Norway spruce plants take up nitrate even under conditions of high plant internal N levels. Received: 1 April 1998 / Accepted: 9 October 1998     

The combined relationship of temperature and molybdenum concentration to nitrogen fixation byAnabaena cylindrica

The joint effects of growth temperature, incubation temperature, and molybdenum concentration on the nitrogen fixation rate ofAnabaena cylindrica were determined using the acetylene-reduction technique. The nitrogen-fixation response to increased molybdenum concentration varied among three growth temperatures (15°, 23°, and 30° C). The pattern of rate change was similar within a growth temperature but increased overall in magnitude with the three incubation temperatures (also 15°, 23°, and 30° C). The maximum rate of nitrogen fixation occurred at 30°C regardless of previous growth temperature. The minimum molybdenum concentration necessary to yield substantial acetylene reduction varied with growth temperature: at 15°C, 15g 1^–1 was effective; at 23°C, less than 5g 1^–1 was effective; and at 30°C, 50g 1^–1 was effective. At all three growth temperatures, increases in molybdenum concentration above the minimum effective concentration produced increases in acetylene reduction. However, at higher molybdenum concentrations inhibition of nitrogen fixation occurred.     

Effect of temperature and benzalkonium chloride on nitrate reduction

The effect of temperature and benzalkonium chloride (BAC) on nitrate reduction was investigated in batch assays using a mixed nitrate reducing culture. Nitrate was transformed completely, mainly through denitrification, to dinitrogen at 5, 10, 15 and 22 °C. In the absence of BAC, reduction of individual nitrogen oxides had different susceptibility to temperature and transient nitrite accumulation was observed at low temperatures. When the effect of BAC was tested up to 100 mg/L from 5 to 22 °C, denitrification was inhibited at and above 50 mg BAC/L with transient nitrite accumulation at all temperatures. The effect of BAC was described by a competitive inhibition model. Nitrite reduction was the denitrification step most susceptible to BAC, especially at low temperatures. BAC was not degraded during the batch incubation and was mostly biomass-adsorbed. Overall, this study shows that low temperatures exacerbate the BAC inhibitory effect, which in turn is controlled by adsorption to biomass.     

Quantification of N2-fixation and survival of inoculated diazotrophs associated with roots of Kallar grass

White clover plants were grown for 97 days under two temperature regimes (20/15°C and 8/5°C day/night temperatures) and were supplied with either small amounts (a total of 80 mg N pot^–1) of ammonium (NH ₄ ⁺ ) or nitrate (NO ₃ ^– ) nitrogen, or received no mineral N and relied on N₂ fixation. Greatest growth and total leaf area of clover plants occurred in N₂ fixing and NO ₃ ^– -fed plants grown at 20/15°C and poorest growth occurred in NH ₄ ⁺ -fed plants grown at 8/5°C. Nodule mass per plant was greater at 8/5°C due to increased nodule numbers rather than increased dry weight per nodule. This compensated to some extent for the reduced N₂-fixing activity per unit dry weight of nodule tissue found at the low growth temperature up to 116 d after sowing, but thereafter both activity per nodule dry weight and activity per plant were greater at the low temperature. Highest nitrate reductase activity (NRA) per g fresh weight and total activity per leaf, petiole or root occurred in NO ₃ ^– -fed plants at 8/5°C. Low growth temperature resulted in a greater partitioning of total plant NRA to the roots of NO ₃ ^– -fed plants. The results are considered in relation to the use of N fertiliser in the spring under field conditions.     

Different nitrogen sources and growth responses of Spirulina platensis in microenvironments

Spirulina platensis was cultivated, in comparative studies, using several sources of nitrogen. The standard source used (sodium nitrate) was the same as that used in the synthetic medium Zarrouk, whereas the alternative nitrogen sources consisted of ammonium nitrate, urea, ammonium chloride, ammonium sulphate or acid ammonium phosphate. The initial nitrogen concentrations tested were 0.01, 0.03 and 0.05 M in an aerated photobioreactor at 30 °C, with an illuminance of 1900 lux, and 12 h-light/12 h-dark photoperiod over a period of 672 h. Maximum biomass was produced in medium containing sodium nitrate (0.01–0.03–0.05 M), followed by ammonium nitrate (0.01 M) and urea (0.01 M). The final biomass concentrations were 1.992 g l^–1 (0.03 M sodium nitrate), 1.628 g l^–1 (0.05 M sodium nitrate), 1.559 g l^–1 (0.01 M sodium nitrate), 0.993 g l^–1 (0.01 M ammonium nitrate) and 0.910 g l^–1 (0.01 M urea). This suggested that it is possible to utilize nitrogen sources other than sodium nitrate for growing S. platensis, in order to decrease the production costs of scaled up projects.     

Uptake of ammonium and nitrate by carob (Ceratonia siliqua) as affected by root temperature and inhibitors

Seedlings of carob ( Ceratonia siliqua L. cv. Mulata) were grown in nutrient solution culture for 5 weeks, with or without nitrogen at different root temperatures (10, 16, 22, 30, 35 or 40deg;C) and with the air temperature kept between 20 and 24°C. The nitrogen was given as either ammonium or nitrate. At all root temperatures studied, nitrogen-depleted plants developed higher net uptake rates for nitrogen than plants grown in the presence of nitrogen. Temperature affected the kinetic parameters of nitrate uptake more than those of ammonium uptake. With increasing root temperature, the K_m of ammonium uptake decreased, but to a lesser extent than the K_m for nitrate. The increase in V_max of ammonium uptake with temperature was also less noticeable than that for nitrate uptake. Ammonium and nitrate uptakes were inhibited in a similar way by respiratory or protein synthesis inhibitors. It may be noted that ammonium uptake in the presence of inhibitors at 40°C was higher than uptake at 10°C without inhibitors. Some similarities between the transport mechanisms for nitrate and ammonium are underlined in the present work. Components of both transport systems displayed saturation kinetics and depended on protein synthesis and energy. The following components of nitrate uptake were distinguished: (a) a passive net influx into the apparent free space; (b) a constitutive active uptake and (c) active uptake dependent on protein synthesis. We may similarly define three ammonium uptake systems: (a) a passive influx into the apparent free space; (b) passive diffusion uptake at high temperature and (c) active uptake dependent on protein synthesis. The possible role of the ratio between mechanism (c) and mechanism (b) as determinant of ammonium sensitivity is discussed.     

The response of white clover (Trifolium repens L.) seedlings to spring root temperatures: The relative roles of the plant and the Rhizobium bacteria

Three experiments are reported which examine the relative roles of host and Rhizobium genotypes as factors limiting clover (Trifolium repens L.) growth at low soil temperatures.In the first experiment un-nodulated clover and perennial ryegrass (Lolium perenne L.) were grown with non-limiting nitrate at root temperatures of 8, 10 and 12°C. The ryegrass had substantially better relative growth rates (RGR) than the clover with the biggest difference occurring at 8°C. Alterations in growth rate with temperature were more marked in clover than in ryegrass but the latter still produced several times more dry matter than clover at each temperature.In the subsequent experiments clover nodulated with different strains of rhizobia was grown with and without non-limiting additions of nitrate at root temperatures of 9, 12 and 15°C. Plants receiving nitrate generally produced more dry matter than those dependent upon Rhizobium for nitrogen but differences in yield between these treatments did not alter with temperature. This suggests that limitations imposed by nitrogen fixation are similar at both high and low temperatures. Indeed, there was some evidence that nitrogen limitations were rather more pronounced at the highest temperature. The first experiment clearly demonstrated that the clover genotype makes particularly poor use of nitrate at low root temperatures when compared to its common companion perennial ryegrass.It can be concluded that improvements in spring growth of clover will rest largely with alterations to the plant genotype and its ability to use combined nitrogen for growth at lower temperatures rather than with changes in rhizobia or any symbiotic characters.     

Microbial populations and sludge characteristics in thermophilic aerobic sewage sludge digestion

Summary Estimates of bacterial numbers from raw sewage sludge and sludge treated by thermophilic aerobic digestion were compared with simple indicators of sludge quality and concentrations of potential substrates. Significant differences were found between sludge types for all but one of the variables examined (frequency of dividing cells). During a stable period of digestor operation, the average number of viable obligate thermophiles present in digested sludge (1.63 × 10⁶ ml^–1) was approximately 10²-fold greater than in feed sludge (1.10 × 10⁴ ml^–1). Total numbers of bacteria were slightly greater in digested sludge (3.24 × 10¹⁰ ml^–1) than in feed sludge (2.39 × 10 ml^–10), as were viable counts of bacteria at incubation temperatures of 37°C and 55°C. Significant correlation was found between viable counts of bacteria at 37°C and 55°C for digested sludge, and 65°C and 55°C for feed sludge. The numbers of obligate thermophiles present and the total of bacteria present were related to the temperature and pH of the digested sludge and inversely related to the numbers ofEscherichia coli and coliforms present, which were not detected at temperatures greater than 50°C.