Agronomic assessment and 15N recovery of urea amended with the urease inhibitor nBTPT (N-(n-butyl) thiophosphoric triamide) for temperate grassland

Three field experiments were undertaken concurrently at one site to evaluate a range of surface-applied nBTPT-amended urea products (0.01, 0.05, 0.1, 0.25 and 0.5% nBTPT w/w) on NH₃ volatilization, grass yield and ¹⁵N recovery in the plant-soil system. Each experiment was repeated on five separate occasions over the 1992 growing season to cover a range of weather conditions. Total NH₃ loss from unamended-urea ranged from 5.5% in early May to 20.8% in June. The inhibitor was highly effective in reducing ammonia volatilization and delaying the time at which maximum rate of NH₃ loss occurred. Over all time periods the % inhibition was 50.4, 82.8, 89.0, 96.5 and 97.0% at the 0.01, 0.05, 0.1, 0.25 and 0.5% nBTPT levels respectively. There was no significant difference in the overall % inhibition in ammonia loss at different times suggesting that the effectiveness of the inhibitor was not dependent on climatic conditions.Over all times incorporation of nBTPT at the 0.05% level increased dry-matter yield by 9% compared to urea alone and increased the shoot recovery of N from 66.7% to 80.9%. Nitrogen saved from volatilization was taken up by the plant, however, the subsequent translation into dry-matter yield appeared to be adversely affected at the high inhibitor rates.There was no significant effect of inhibitor on ¹⁵N recovery in soil at any depth down to 15 cms. nBTPT significantly increased (p < 0.001) the % N derived from fertilizer (% N dff) in the shoot compared to unamended-urea and increased (p < 0.01) the shoot recovery of ¹⁵N from 32% up to 39%. Total ¹⁵N recovery in the soil-plant system was increased by up to 17% by amending urea with nBTPT. This urease inhibitor has been shown to improve the efficiency of urea however, its potential for the European market will be dependent on economic factors.Faculty of Agriculture and Food Science, The Queen's University of Belfast     

Response over two growing seasons of a Sitka spruce stand to 15N-urea fertilizer

Urea fertilizer labelled with ¹⁵N (2.5 atom %) was applied to a 20 year old Sitka spruce stand on a peaty gley at a rate equivalent to 160 kg N ha⁻¹. The application of urea resulted in increased biomass and N concentration of needles and enhanced development of the crown. Differences in N concentrations of the amended trees were also observed for new wood and bark. Analysis of ¹⁵N in tree biomass showed a continued influence of fertilizer N in the second growing season following urea application. The overall recovery of fertilizer N in the trees was estimated to be about 10%.     

Carbon distribution and nitrogen partitioning in a soil-plant system with barley (Hordeum vulgare L.), ryegrass (Lolium perenne) and rape (Brassica napus L.) grown in a14CO2-atmosphere

To examine the influence of plant-microorganism interactions on soil-N transformations (e.g. net mineralization, net immobilization) a pot experiment was conducted in a¹⁴C-labelled atmosphere by using different (two annuals, one perennial) plants species. It was assumed that variation in below-ground, microorganism-available C would influence N transformations in soil. Plant species were fertilized (low rate) with¹⁵N-labelled nitrogen and grown, during days 13 and 62 after germination, in a growth chamber with a¹⁴C-labelled atmosphere. Nitrification was inhibited by using nitrapyrin (N-Serve). During the chamber period, shoots were harvested, and associated roots and soil were collected on two sampling occasionm, e.g. after 4 and 7 weeks in the growth chamber.The distribution of net (%) assimilated¹⁴C was significantly affected by both plant and time factors, and there was a significant plant × time interaction. There were significant differences between plants in all plant-soil compartments examined as well as in the degree of the plant × time interaction.Differences in the¹⁴C distribution between plants were due to both interspecific and developmental variation. In general, when comparing¹⁵N and¹⁴C quantities between species, many of the differences found between plants can be explained by the differences determined in the weight of shoot or root parts. Despite the fact that amounts of C released were greater in ryegrass than in the other plant-treatments no unequivocal evidence was found to show that the effects of plant-microorganism interactions on soil-N mineralization were greater under ryegrass. Possible mechanisms accounting for the partitioning of N found among plant biomass, soil biomass and soil residues are discussed.     

Nitrogen-15 labeling effectiveness of two tropical legumes

Nitrogen-15 foliar applications for the production of field-labeled plant tissues may achieve more effective labeling of plant shoot and root tissues and minimize directly labeling the soil N fraction as occurs when¹⁵ N is soil applied. Consequently, foliar-labeled plant tissues should be better suited for subsequent ¹⁵N mineralization studies. A field experiment was conducted to determine the effectiveness of ¹⁵N-labeling and the accumulation of ¹⁵N in various plant parts of two tropical legumes. Desmodium ovalifolium Guillemin and Perrottet and Pueraria phaseoloides (Roxb.) Benth., grown in 0.5 m² microplots, were labeled with foliar-applied urea containing 99 atom% ¹⁵N. Plants in each microplot received a total of 0.1698 g ¹⁵N that was applied all at once or split equally into two, three or four applications. Legume shoots and roots and soil were destructively harvested and analyzed for total ¹⁵N content. Averaged over both legumes and foliar application rates, total plant (shoots, flowers, leaf litter, and roots) recovery was approximately 79% of the ¹⁵N applied. The soil contained 3% of the ¹⁵N applied, of which 2.5 and 0.5% were in the inorganic and organic fractions, respectively. Nitrogen-15 recovery in shoots (76%) was sixty-five fold greater than in roots (1%) and about nineteen fold greater than the sum of roots and soil (4.1%), a much greater percent recovery than observed in other foliar labeling studies. Averaged over all four foliar split-application rates, ¹⁵N recovery by Desmodium shoots was greater than Pueraria. Results demonstrate that ¹⁵N foliar application to legumes is an effective method for labeling, resulting in atom% excess ¹⁵N levels and ¹⁵N recoveries comparable to those reported with the more traditional soil-labeling approach. Another advantage of this method is a nondestructive, in situ labeling method that permits separation of shoot and root residual N contribution to subsequent crops in N tracer studies.     

Conservation of urea-N by immobilization-remobilization in a rice-Azolla intercrop

Nitrogen losses are notoriously high in flooded rice fertilized with urea. An Azolla intercrop can reduce such losses by immobilizing urea-N during periods of potentially high N-loss. The reduction in N loss linked with the absorption and remobilization of urea-N by Azolla, was studied in two greenhouse experiments conducted in Goettingen (Germany). Grain yield and N recovery were positively influenced by Azolla more than doubling grain yield and N uptake as compared to the split application of 300 mg N pot^–1 alone (Exp. 1). In the second experiment, the yield increase was 78.3% with single applications of 97.5 and 68.4% after a split-application of a total of 195 mg N pot^–1. In both years the effect of urea and Azolla combined exceeded that of the sum of the factors alone, a clear positive synergistic effect on yield and N uptake by rice. Azolla effectively competed with the young rice plants for applied urea, capturing nearly twice the urea-N than the rice plants up to tillering in experiment 1. In the second experiment, 64.6 mg N of the 97.5 mg applied early in the season was immobilized by Azolla within 2 weeks. This represented 63.1% of the total N accumulated in the Azolla. The fraction of Azolla-N derived from urea sank to 36.4 mg within 4 weeks and only 27.2 mg at maximum tillering as a result of Azolla senescence and N-release. Of this 64.6 mg urea N immobilized 28.7% is eventually taken up by the standing rice plant, representing 43.1% of the remineralized, urea-derived Azolla N. Following the second urea application, only 17.9 mg N were immobilized in the Azolla biomass during the 2 weeks, of which 6.9 mg pot^–1 were still retained in the Azolla at maturity. At this stage, rice is the more effective competitor for applied N. As much as 42.1% of this immobilized N finds its way into the rice by maturity. Thus, Azolla contributed to the conservation of N in the system, particularly of the urea applied early in the season. Loss of N from the system amounted to no more than 15%. Although the early-applied N directly recovered by the rice plant was low (20%), 2/3 of the N captured by Azolla following this first urea application was released to the system by the time of rice harvest, over 40% of which was available to the rice plant. Azolla thus appears to act as a slow release fertilizer.     

Natural 15N abundances of maize and soil amended with urea and composted pig manure

To investigate the effect of inorganic fertilizer and composted manure amendments on the N isotope composition (delta ¹⁵N) of crop and soil, maize (Zea mays L.) was cultivated under greenhouse conditions for 30, 40, 50, 60, and 70 days. Composted pig manure (delta ¹⁵N= +13.9) and urea (-2.3) were applied at 0 and 0 kg N ha^–1 (C0U0), 0 and 150 kg N ha^–1 (C0U2), 150 and 0 kg N ha^–1 (C2U0), and 75 and 75 kg N ha^–1 (C1U1), respectively. The delta ¹⁵N of total soil-N was not affected by both amendments, but delta ¹⁵N of NH⁺ ₄ and NO^– ₃ provided some information on the N isotope fractionation in soil. During the early growth stage, significant differences (P < 0.05) in delta ¹⁵N among maize subjected to different treatments were observed. After 30 days of growth, the delta ¹⁵N values of maize were +6.6 for C0U0, +1.1 for C0U2, +7.7 for C2U0, and +4.5 for C1U1. However, effects of urea and composted manure application on maize delta ¹⁵N progressively decreased with increasing growth period, probably due to isotope fractionation accompanying N losses and increased uptake of soil-derived N by maize. After 70 days of growth, delta ¹⁵N of leaves and grains of maize amended with composted pig manure were significantly (P < 0.05) higher than those with urea. The temporal variations in delta ¹⁵N of maize amended with urea and composted manure indicate that plant delta ¹⁵N is generally not a good tracer for N sources applied to field. Our data can be used in validation of delta ¹⁵N fractionation models in relation to N source inputs.     

Fate of urea-15N in a soil-wheat system as influenced by urease inhibitor hydroquinone and nitrification inhibitor dicyandiamide

By applying labeled urea into a loamy meadow brown soil, a pot experiment with spring wheat as test crop was carried out. The results showed that at the end of this experiment, the plant recovery, the soil recovery and the total loss of applied urea ¹⁵N was 17.7–23.7%, 43.7–56.3% and 20.0–36.8%, respectively. ¹⁵N recovery by wheat grain in any treatment varied within a range of 9.0–14.7% of the applied ¹⁵N. A combined application of hydroquinone (HQ) and dicyandiamide (DCD) gave the lowest loss and the highest recoveries in both the plant and soil, while applying HQ or DCD alone had less effect on them. During the whole period of wheat growth, HQ+DCD induced an increasing ¹⁵N uptake by plant, and even promoted the translocation of absorbed ¹⁵N from stem to grain. In the presence of inhibitors, organic plus chemically fixed ¹⁵N occupied a large portion of soil ¹⁵N recovery at maturity stage of wheat growth (34.3–50.6%, in contrast to 9.9% in the absence of inhibitors), and DCD and DCD+HQ could remarkably reduce the remaining soil (NO₃ ^-+NO₂ ^-)-¹⁵N. In this pot experiment, the leaching loss of applied ¹⁵N was excluded, and hence, the gaseous loss was considered as the main part of the ¹⁵N loss. Regarding N loss, N₂O flux only occupied a very small part, and its main part was other gaseous N losses. DCD and DCD+HQ retarded N₂O flux from the soil-wheat system after treatment with urea and reduced the total N₂O flux during the whole period of wheat growth. Treatment with both inhibitors had much lower gaseous N losses than that with HQ or DCD alone. Hence, a proper combination application of HQ and DCD is an efficient way to improve urea-N efficiency and crop quality, while decreasing its loss to the environment. This revised version was published online in June 2006 with corrections to the Cover Date.     

Transfer of nitrogen from damaged roots of white clover (Trifolium repens L.) to closely associated roots of intact perennial ryegrass (Lolium perenne L)

An experiment is described in which the magnitude of N transferred from damaged white clover roots to perennial ryegrass was determined, using ¹⁵N labelling of the grass plant. There was no effect on the growth and N-fixation of the clover plants after removing part of the root system. The ¹⁵N data suggested that N had been acquired by all grass plants, even in plants grown alone with no further N supplied after labelling. However, after quantifying the mobile and stored N pools of the grass plants it was evident that significant transfer of N from clover to grass only took place from damaged clover roots. Dilution of the atom% ¹⁵N in the roots of the grass plants grown alone, and in association with undamaged clover roots, was explained by remobilisation of N within the plant.     

Below-ground nitrogen transfer between different grassland species: Direct quantification by 15N leaf feeding compared with indirect dilution of soil 15N

Nitrogen (N) transfer from one species to another is important for the N cycling in low-input grassland. In the present work, estimates obtained by an indirect ¹⁵N dilution technique were compared with estimates obtained by a direct ¹⁵N leaf feeding technique over two complete growing seasons in red clover-ryegrass and white clover-ryegrass mixtures under field conditions. The direct technique confirmed that N transfer between clovers and ryegrass is a bi-directional process. The transfer of N from both clovers to ryegrass occurred within 25 days upon the first labelling event. A very high N transfer occurred from white clover to the associated ryegrass, 4.5 and 7.5 g m⁻² in the 1st and 2nd production year, respectively. The corresponding values for transfer from red clover to the associated ryegrass were 1.7 and 3.6 g m⁻². Quantified relatively to the total above-ground N content of white clover- ryegrass and red clover-ryegrass mixtures, the N transfer exceeded 50% and 10%, respectively, in three out of seven harvests. The N transfer from ¹⁵N labelled grass to associated clovers constituted a relatively constant proportion of approx. 8% of the above-ground N content of the mixtures. Estimates based on the soil ¹⁵N dilution technique generally underestimated the net N transfer by more than 50% compared to the direct ¹⁵N labelling technique. Furthermore, the indirect ¹⁵N dilution technique estimated only marginal differences between red and white clover in the quantities of N transferred, whereas the direct ¹⁵N labelling technique showed the N transfer from white clover to the associated ryegrass to be significantly higher than that involving red clover. It is concluded that N transfer is a much more dynamic and quantitatively important process in grassland than previously recognised. This revised version was published online in June 2006 with corrections to the Cover Date.     

Alley cropping with Senna siamea in South-western Nigeria: II. Dry matter,total N,and urea-derived N dynamics of the Senna and maize roots

Crop and tree roots are crucial in the nutrient recycling hypotheses related to alley cropping systems. At the same time, they are the least understood components of these systems. The biomass, total N content and urea-derived N content of the Senna and maize roots in a Senna-maize alley cropping system were followed for a period of 1.5 years (1 maize-cowpea rotation followed by 1 maize season) to a depth of 90 cm, after the application of ¹⁵N labeled urea. The highest maize root biomass was found in the 0–10 cm layer and this biomass peaked at 38 and 67 days after planting the 1994 maize (DAP) between the maize rows (112 kg ha^–1, on average) and at 38, 67 and 107 DAP under the maize plants (4101 kg ha^–1, on average). Almost no maize roots were found below 60 cm at any sampling date. Senna root biomass decreased with time in all soil layers (from 512 to 68 kg ha^–1 for the 0–10 cm layer between 0 and 480 DAP). Below 10 cm, at least 62% of the total root biomass consisted of Senna roots and this value increased to 87% between 60 and 90 cm. Although these observations support the existence of a Senna root `safety net' between the alleys which could reduce nutrient leaching losses, the depth of such a net may be limited as the root biomass of the Senna trees in the 60–90 cm layer was below 100 kg ha^–1, equivalent to a root length density of only < 0.05 cm cm^–3. The proportion of maize root N derived from the applied urea (%Ndfu) decreased significantly with time (from 21% at 21 DAP to 8% at 107 DAP), while %Ndfu of the maize roots at the second harvest (480 DAP) was only 0.6%. The %Ndfu of the Senna roots never exceeded 4% at any depth or sampling time, but decreased less rapidly compared to the %Ndfu of the maize roots. The higher %Ndfu of the maize roots indicates that maize is more efficient in retrieving urea-derived N. The differences in dynamics of the %Ndfu also indicate that the turnover of N through the maize roots is much faster than the turnover of N through the Senna roots. The recovery of applied urea-N by the maize roots was highest in the top 0–10 cm of soil and never exceeded 0.4% (at 38 DAP) between the rows and 7.1% (at 67 DAP) under the rows. Total urea N recovery by the maize roots increased from 1.8 to 3.2% during the 1994 maize season, while the Senna roots never recovered more than 0.8% of the applied urea-N at any time during the experimental period. These values are low and signify that the roots of both plants will only marginally affect the total recovery of the applied urea-N. Measurement of the dynamics of the biomass and N content of the maize and Senna roots helps to explain the observed recovery of applied urea-N in the aboveground compartments of the alley cropping system.     

Leaf urea metabolism in potato. Urease activity profile and patterns of recovery and distribution of (15)N after foliar urea application in wild-type and urease-antisense transgenics

The influence of urease activity on N distribution and losses after foliar urea application was investigated using wild-type and transgenic potato (Solanum tuberosum cv Désirée) plants in which urease activity was down-regulated. A good correlation between urease activity and (15)N urea metabolism (NH(3) accumulation) was found. The general accumulation of ammonium in leaves treated with urea indicated that urease activity is not rate limiting, at least initially, for the assimilation of urea N by the plant. It is surprising that there was no effect of urease activity on either N losses or (15)N distribution in the plants after foliar urea application. Experiments with wild-type plants in the field using foliar-applied (15)N urea demonstrated an initial rapid export of N from urea-treated leaves to the tubers within 48 h, followed by a more gradual redistribution during the subsequent days. Only 10% to 18% of urea N applied was lost (presumably because of NH(3) volatilization) in contrast to far greater losses reported in several other studies. The pattern of urease activity in the canopy was investigated during plant development. The activity per unit protein increased up to 10-fold with leaf and plant age, suggesting a correlation with increased N recycling in senescing tissues. Whereas several reports have claimed that plant urease is inducible by urea, no evidence for urease induction could be found in potato.     

Fate of urea-N in floodwater

One day after application, urea-N remaining in the floodwater and determined as water-soluble N (urea-N + NH₄ ⁺-N) was used to calculate the potential N loss from lowland rice soils. Actual N loss calculated from ¹⁵N balance measurements using forced air exchange (airflow rate: 20 L min^-1) in greenhouse pots. Conditions for variable potential N loss were created by manipulating the method of urea application and duration of presubmergence or by selecting soils with diverse cation exchange capacities (CEC). Potential N loss tended to be lower than actual N loss; the differences were, however, nonsignificant. The method of urea application that led to the lowest potential N loss from a Guthrie silty clay loam (Typic Fragiaquult) also led to the least ¹⁵N loss and vice-versa (r=0.99^**). Duration of presubmergence did not alter the relationship between potential and actual N loss although it influenced the rate of urea hydrolysis in floodwater. The primary depencence of actual N loss on water-soluble N was maintained in soils differing in CEC (r=0.83^**). The association between potential and actual N loss was closer for high-CEC soils ( 20 cmol [+] kg^-1 soil, r=0.91^**) than for low-CEC soils (<20 cmol [+] kg^-1 soil, r=0.85^**). Ammonia volatilization could be more closely predicted by potential N loss than could apparent denitrification.The results of this study suggest that potential N loss calculated from one-time determination of water-soluble N in floodwater can be a good index of actual N loss from flooded, puddled rice soils. Notable exceptions are to be expected for soils in which water-soluble N gets lost from floodwater either before (soils with fast urea hydrolysis in floodwater) or after (soils with steady leaching) determination of potential N loss.     

Labeled nitrogen fertilizer research with urea in the semi-arid tropics

Summary As part of a research program to determine the fate of N fertilizers applied to dryland sorghum in the semi-arid tropics,¹⁵N balance studies were conducted with various N sources in the greenhouse. Two American soils, Houston Black clay (Udic Pellustert) and Windthorst sandy loam (Udic Paleustalf), similar in properties to the Vertisol and Alfisol in the semi-arid tropics of India, were employed. Experiments were conducted with large pots containing 20 or 60 kg of soil which was subjected to several watering regimes. The¹⁵N not accounted for in the plant and soil was presumably lost. Losses of N on calcareous Houston Black clay were greatest for broadcast urea, 16%–28%. Amendment of broadcast urea with 2% phenyl phosphorodiamidate, a urease inhibitor, reduced N losses only slightly to 15%–20%. Point placement of urea at a 6 cm soil depth reduced losses to 1%–11%. Granule size had no effect on N loss from point-placed urea. Ammonia volatilization was apparently the main N loss mechanism, since N losses from sodium nitrate were less than 7%, except when the soil surface was waterlogged. N losses on the Windthorst soil averaged 30% from urea and 11% from ammonium nitrate. Amendment of urea with urea phosphate to form a 27% N and 13% P product reduced fertilizer N losses but did not increase grain yield on Windthorst soil. N losses were also less from ammonium nitrophosphate than from urea. Band and point placement of urea 6 cm below the soil surface were equally effective in reducing N loss on Houston Black clay. The findings give credence to the recommendation of deep band placement for urea in the semi-arid tropics.     

Nitrogen uptake and recovery from urea and green manure in lowland rice measured by15N and non-isotope techniques

In the recent past considerable attention is paid to minimize dependence on purchased inputs such as inorganic nitrogen fertilizer. Green manure in the form of flood-tolerant, stem-nodulatingSesbania rostrata andAeschynomene afraspera is an alternative N source for rice, which may also increase N use efficiency. Therefore research was conducted to determine the fate of N applied to lowland rice (Oryza sativa L.) in the form ofSesbania rostrata andAeschynomene afraspera green manure and urea in two field experiments using¹⁵N labeled materials.¹⁵N in the soil and rice plant was determined, and¹⁵N balances established. Apparent N recoveries were determined by non-tracer method. ¹⁵N recoveries averaged 90 and 65% of N applied for green manure and urea treatments, respectively. High partial pressures of NH₃ in the floodwater, and high pH probably resulted from urea application and favoured losses of N from the urea treatment. Results show that green manure N can supply a substantial proportion of the N requirements of lowland rice. Nitrogen released fromSesbania rostrata andAeschynomene afraspera green manure was in synchrony with the demand of the rice plant. The effect of combined application of green manure and urea on N losses from urea fertilizer were also investigated. Green manure reduced the N losses from¹⁵N labeled urea possibly due to a reduction in pH of the floodwater. Positive added N interactions (ANIs) were observed. At harvest, an average of 45 and 25% of N applied remained in the soil for green manure and urea, respectively.Contribution from IRRI, Los Baños, Philippines and Justus-Liebig-University, Giessen, GermanyContribution from IRRI, Los Baños, Philippines and Justus-Liebig-University, Giessen, Germany     

Nickel-enriched seed and externally supplied nickel improve growth and alleviate foliar urea damage in soybean

Background and aims

The importance of seed Ni reserves for plant growth and N metabolism is poorly understood. This study investigated the effects of both seed Ni and externally supplied Ni on the impact of foliarly-applied urea and N-nutritional status of soybean.

Methods

Soybean seeds were produced by growing plants in nutrient solutions containing different Ni levels, and their urease activities were measured. Plants were then grown from these seeds with or without external Ni. After treating half of the plants with foliar urea, the urea damage symptoms, elongation rates and chlorophyll concentrations were followed over one week. Biomass and mineral concentrations of different plant parts were determined.

Results

Nickel supply at increasing rates improved seed yield by up to 25 %. Seeds with Ni concentrations varying between 0.04–8.32 mg.kg^?1 were obtained. Depending on the Ni concentration, the seed urease activities differed up to 100-fold. Leaf damage due to foliar urea spray was significantly alleviated by higher seed Ni as well as external Ni supply. Higher Ni also promoted shoot elongation and improved chlorophyll concentrations. Nickel was 10-times more concentrated in the youngest part than in older leaves. In the absence of foliar urea, Ni enhanced the N concentration of the growing part of the shoot by up to 30 %.

Conclusion

A better utilization of foliarly-applied urea-N is achieved in soybean when adequate Ni is supplied to plants by seed reserves and/or externally. High seed Ni levels are also required for preventing foliar urea damage and improving N remobilization.     

Cyanobacterial inoculation and nitrogen fertilization in rice

During three rice-growing seasons in Uruguay, field experiments were conducted to study the contribution of cyanobacterial inoculation and chemical N fertilization to rice production. Neither grain yield nor fertilizer recovery by the plant were affected by inoculation with native cyanobacterial isolates. A low fertilizer use efficiency (around 20%) was observed when labelled (NH₄)₂SO₄ was applied at sowing. Recovery of applied ¹⁵N by the soil–plant system was 50%. Inoculation did not modify ¹⁵N uptake by the plant when the fertilizer was three-split applied either. The total N-fertilizer recovery was higher when the fertilizer was split than when applied in a single dose. Plant N-fertilizer uptake was higher when the fertilizer was applied at tillering. Uptake of ¹⁵N from cyanobacteria by rice was studied in a greenhouse pots experiment without chemical nitrogen addition. Recovery of ¹⁵N from labelled cyanobacteria by rice in greenhouse growth conditions was similar to that of partial recovery of (NH₄)₂SO₄ applied at sowing in the field. Cyanobacterial N mineralization under controlled conditions was fast as cyanobacterial N was detected in plants after 25 days. Moreover 40 days after inoculation non-planted and inoculated soil had more inorganic N than the non-inoculated one.     

Fate and efficiency of N fertilizers applied to pearl millet in Niger

Field studies were conducted in Niger using ¹⁵N-labeled fertilizers to assess the fate and efficiency of fertilizer N in pearl millet (Pennisetum glaucum [L.] R.Br.) production. Total plant uptake of fertilizer N was low in all cases (20%–37%), and losses were severe (25%–53%). The majority of N remaining in the soil was found in the 0- to 15-cm layer though some enrichment at lower depths was found when the N fertilizer was calcium ammonium nitrate (CAN). In a comparison of urea placement methods (band, broadcast, or point placement), no significant differences in ¹⁵N uptake or yield were noted though point placement did exacerbate ¹⁵N loss. The mechanism of N loss is believed to have been ammonia volatilization. Yields were similar whether urea or CAN was used, but ¹⁵N uptake from CAN was higher. A statistical model was developed relating millet yield and N response to midseason rainfall. In drought years, no N response was found, whereas in years of good rainfall a response was found of 15 kg grain for each kilogram of N applied (at 30 kg N ha^-1 rate).     

Nitrogen-15 balances for urea and neem coated urea applied to lowland rice following two cowpea cropping systems

Little is known about whether the high N losses from inorganic N fertilizers applied to lowland rice (Oryza sativa L.) are affected by the combined use of either legume green manure or residue with N fertilizers. Field experiments were conducted in 1986 and 1987 on an Andaqueptic Haplaquoll in the Philippines to determine the effect of cowpea [Vigna unguiculata (L.) Walp.] cropping systems before rice on the fate and use efficiency of¹⁵N-labeled, urea and neem cake (Azadirachta indica Juss.) coated urea (NCU) applied to the subsequent transplanted lowland rice crop. The pre-rice cropping systems were fallow, cowpea incorporated at the flowering stage as a green manure, and cowpea grown to maturity with subsequent incorporation of residue remaining after grain and pod removal. The incorporated green manure contained 70 and 67 kg N ha⁻¹ in 1986 and 1987, respectively. The incorporated residue contained 54 and 49 kg N ha⁻¹ in 1986 and 1987, respectively. The unrecovered¹⁵N in the¹⁵N balances for 58 kg N ha⁻¹ applied as urea or NCU ranged from 23 to 34% but was not affected by pre-rice cropping system. The partial pressure of ammoniapNH₃, and floodwater (nitrate + nitrite)-N following application of 29 kg N ha⁻¹ as urea or NCU to 0.05-m-deep floodwater at 14 days after transplanting was not affected by pre-rice cropping system. In plots not fertilized with urea or NCU, green manure contributed an extra 12 and 26 kg N ha⁻¹, to mature rice plants in 1986 and 1987, respectively. The corresponding contributions from residue were 19 and 23 kg N ha⁻¹, respectively. Coating urea with 0.2g neem cake per g urea had no effect on loss of urea-N in either year; however, it significantly increased grain yield (0.4 Mg ha⁻¹) and total plant N (11 kg ha⁻¹) in 1987 but not in 1986.     

Nitrogen accumulation in sole and mixed stands of sweet-blue lupin (Lupinus angustifolius L.), ryegrass and oats

Nitrogen fixation was measured in monocropped sweet-blue lupin (Lupinus angustifolius), lupin intercropped with two ryegrass (Lolium multiflorum) cultivars or with oats (Avena sativa) on an Andosol soil, using the ¹⁵N isotope dilution method. At 117 days after planting and at a mean temperature below 10°C, monocropped lupin derived an average of 92% or 195 kg N ha⁻¹ of its N from N₂ fixation. Intercropping lupin with cereals increased (p<0.05) the percentage of N derived from atmospheric N₂ (% Ndfa) to a mean of 96%. Compared to the monocropped, total N fixed per hectare in intercropped lupin declined approximately 50%, in line with the decrease in seeding rate and dry matter yield. With these high values of N₂ fixation, selection of the reference crop was not a problem; all the cereals, intercropped or grown singly produced similar estimates of N₂ fixed in lupin. It was deduced from the ¹⁵N data that significant N transfer occurred from lupin to intercropped Italian ryegrass but not to intercropped Westerwoldian ryegrass or to oats. Doubling the ¹⁵N fertilizer rate from 30 to 60 kg N ha⁻¹ decreased % Ndfa to 86% (p<0.05), but total N fixed was unaltered. These results indicate that lupin has a high potential for N₂ fixation at low temperatures, and can maintain higher rates of N₂ fixation in soils of high N than many other forage and pasture legumes.     

15N示踪控释氮肥的氮肥利用率及去向研究

选用¹⁵N同位素标记的新型回收塑料包膜控释肥和大颗粒尿素,采用池栽试验研究夏玉米-冬小麦轮作体系中肥料氮的去向及利用率。结果表明,整个轮作体系中,控释肥处理（PCU）作物吸收的肥料氮为241.03 kg/hm,高于尿素处理（Urea)的211.02 kg/hm。控释肥处理施用的肥料氮主要残留在0~40 cm土层,而尿素处理则残留在0~60 cm土层,控释肥延缓了肥料氮向土壤深层迁移的趋势。在夏玉米和冬小麦轮作体系中,控释肥处理的氮肥利用率（32.86%,32.47%）高于尿素处理（28.23%,30.16%）。在冬小麦季,控释肥处理损失率相比尿素处理从36.07% 降至28.75%,而夏玉米季,控释肥处理损失率相比尿素处理从37.17%降至29.50%。玉米季控释肥处理与尿素处理差异不显著,但在冬小麦季控释肥处理的产量显著高于尿素处理。因此,在玉米和小麦整个生长季,新型回收塑料包膜控释肥的养分释放与作物养分需求吻合,既提高氮肥利用率,也降低了肥料氮的损失。