Contributions to nitrogen leaching and pasture uptake by autumn-applied dairy effluent and ammonium fertilizer labeled with 15N isotope

The objective of this study was to compare the N leaching loss and pasture N uptake from autumn-applied dairy shed effluent and ammonium fertilizer (NH₄Cl) labeled with ¹⁵N, using intact soil lysimeters (80 cm diameter, 120 cm depth). The soil used was a sandy loam, and the pasture was a mixture of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). The DSE and NH₄Cl were applied twice annually in autumn (May) and late spring (November), each at 200 kg N ha^-1. The N applied in May 1996 was labeled with ¹⁵N. The lysimeters were either spray or flood irrigated during the summer. The autumn-applied DSE resulted in lower N leaching losses compared with NH₄Cl. However, the N applied in the autumn had a higher potential for leaching than N applied in late spring. Between 4.5–8.1% of the ¹⁵N-labeled mineral N in the DSE and 15.1–18.8% of the ¹⁵N-labeled NH₄Cl applied in the autumn were leached within a year of application. Of the annual N leaching losses in the DSE treatments (16.0–26.9 kg N ha^-1), a fifth (20.3–22.9%) was from the mineral N fraction of the DSE applied in the autumn, with the remaining larger proportion from the organic fraction of the DSE, soil N and N applied in spring. In the NH₄Cl treatments, more than half (53.8–64.8%) of the annual N leaching loss (55.9–57.6 kg N ha^-1) was derived from the autumn-applied NH₄Cl. DSE was as effective as NH₄Cl in stimulating pasture production. Since only 4.4–4.5% of the annual herbage N uptake in the DSE treatment and 12.3–13.3% in the NH₄Cl treatment were derived from the autumn-applied mineral N, large proportions of the annual herbage N uptake must have been derived from the N applied in spring, the organic N fraction in the DSE, soil N and N fixed by clover. The recoveries of ¹⁵N in the herbage were similar between the DSE and the NH₄Cl treatments, but those in the leachate were over 50% less from the DSE than from the NH₄Cl treatment. The lower leaching loss of ¹⁵N in the DSE treatment was attributed to the stimulated microbial activities and increased immobilization following the application of DSE. This revised version was published online in June 2006 with corrections to the Cover Date.     

Fate of nitrogen fertilizer applied on two main arables crops,winter wheat (Triticum aestivum) and sugar beet (Beta vulgaris) in the loam region of Belgium

Since 1986, the fate of fertilizer N (NH₄NO₃ or NaNO₃) applied in field conditions on two main arable crops, winter wheat (Triticum aestivum) and sugar beet (Beta vulgaris), has been studied using ¹⁵N. Up to a rate of 200 kg ha^-1 of N, mean recovery of fertilizer by winter wheat was 70%, provided it had been split applied. Single application (with or without dicyandiamid) was less effective. For sugar beet, in 1990, 1991 and 1992, 40% of fertilizer N was found in the crop at harvest when NH₄NO₃ had been broadcast at 100 to 160 kg N ha^-1 at sowing time. For the same N rate, recovery was 50% when row applied near the seeds and 60% for 80 kg N ha^-1. For the two experimental crops, residual fertilizer N in soil was exclusively organic. It ranged from 15 to 30% of applied N and was located in the 30 cm upper layer. Losses were generally lower with winter wheat (12%) than with sugar beet (20–40%) and could be ascribed to volatilization and denitrification. Soil derived N taken up by the plant was site and year dependent.     

Fate of 15N-labelled fertilizer applied to spring barley grown on soils of contrasting nutrient status

An experiment with ¹⁵N-labelled fertilizer was superimposed on the Rothamsted Hoosfield Spring Barley Experiment, started in 1852. Labelled ¹⁵NH₄ ¹⁵NO₃ was applied in spring at (nominal) rates of 0, 48, 96 and 144 kg N ha^-1. The labelled fertilizer was applied to microplots located within four treatments of the original experiment: that receiving farmyard manure (FYM) annually, that receiving inorganic nutrients (PK) annually and to two that were deficient in nutrients: applications were made in two successive years, but to different areas within these original treatments. Maximum yields in 1986 (7.1 t grain ha^-1) were a little greater than in 1987. In 1987, microplots on the FYM and PK treatments gave similar yields, provided enough fertilizer N was applied, but in 1986 yields on the PK treatment were always less than those on the FYM treatment, no matter how much fertilizer N was applied. In plots with adequate crop nutrients, about 51% of the labelled N was present in above-ground crop and weed at harvest, about 30% remained in the top 70 cm of soil (mostly in the 0–23 cm layer) and about 19% was unaccounted for, all irrespective of the rate of N application and of the quantity of inorganic N in the soil at the time of application. Less than 4% of the added fertilizer N was present in inorganic form in the soil at harvest, confirming results from comparable experiments with autumn-sown cereals in south-east England. Thus, in this experiment there is no evidence that a spring-sown cereal is more likely to leave unused fertilizer in the soil than an autumn-sown one. With trace applications (ca. 2 kg N ha^-1) more labelled N was retained in the soil and less was in the above-ground crop. Where P and K were deficient, yields were depressed, a smaller proportion of the labelled fertilizer N was present in the above-ground crop at harvest and more remained in the soil.Although the percentage uptake of labelled N was similar across the range of fertilizer N applications, the uptake of total N fell off at the higher N rates, particularly on the FYM treatment. This was reflected in the appearance of a negative Added Nitrogen Interaction (ANI) at the highest rate of application. Fertilizer N blocked the uptake of soil N, particularly from below 23 cm, once the capacity of the crop to take up N was exceeded. Denitrification and leaching were almost certainly insufficient to account for the 19% loss of spring-added N across the whole range of N applications and other loss processes must also have contributed.     

The fate of labelled15N urea and ammonium nitrate applied to a winter wheat crop

Labelled urea or ammonium nitrate was applied to winter wheat growing on a loamy soil in Northern France. Two applications of fertilizer were given: 50 kg N ha^–1 at tillering (early March) and 110 kg N ha^–1 at the beginning of stem elongation (mid-April). The kinetics of urea hydrolysis, nitrification of ammonium and the disappearance of inorganic nitrogen were followed at frequent intervals. Inorganic nitrogen soon disappeared, mainly immobilized by soil microflora and absorbed by the crop. Net immobilization of fertilizer N occured at a very similar rate for urea and ammonium nitrate. Maximum immobilization (16 kg N ha¹) was found at harvest for the first dressing and at anthesis for the second dressing (23 kg N ha¹). During the nitrification period, the labelled ammonium pool was immobilized two to three times faster than the labelled nitrate pool. No significant net¹⁵N remineralization was found during the growth cycle.The actual denitrification and volatilization losses were probably more important than indicated from calculations made by extrapolation of fluxes measured over short intervals. However microbial immobilization was the most important of the processes which compete with plant uptake for nitrogen.     

Dynamics of soil microbial biomass N under zero and shallow tillage for spring wheat,using15N urea

Summary Field studies to determine the effect of zero and shallow (10 cm) cultivation on microbial biomass were conducted on several Chernozemic soils in western Canada. Using the CHCl₃ fumigation method, the distribution of microbial biomass N and the immobilization and subsequent release of added¹⁵N (¹⁵N-urea) from the microbial biomass were determined in the A horizon, at the 0 to 5 and 5 to 10 cm depth, during the growing season for spring wheat.Temporal variation in microbial biomass N, associated with the development of the rhizosphere, was characterized by an increase between Feekes stage 1 and 5 or 10 and decrease at Feekes stage 11.4. Over the long term, the variation in biomass N between tillage systems corresponded with crop residue distribution. Immobilization of fertilizer N was related to the increase in biomass N from Feekes stage 1, which in turn, was associated with the incorporation of recent crop residues or levels of labile organic matter in the surface soil. The study demonstrated the relatively rapid remineralization of immobilized fertilizer N under field conditions and emphasized the role of the microbial biomass N as both a sink and source of mineral N.     

The partitioning of fertilizer-N between soil and crop: Comparison of ammonium and nitrate applications

Field experiments were carried out in 1987 on winter wheat crops grown on three types of soil. ¹⁵N-labelled urea, ¹⁵NH₄NO₃ or NH₄ ¹⁵NO₃ (80 kg N ha^-1) was applied at tillering. The soils (chalky soil, hydromorphic loamy soil, sandy clay soil) were chosen to obtain a range of nitrogen dynamics, particularly nitrification. Soil microbial N immobilization and crop N uptake were measured at five dates. Shortly after fertilizer application (0–26 days), the amount of N immobilized in soil were markedly higher with labelled urea or ammonium than that with nitrate in all soils. During the same period, crop ¹⁵N uptake occurred preferentially at the expense of nitrate. Nitrification differed little between soils, the rates were 2.0 to 4.7 kg N ha^-1 day^-1 at 9°C daily mean temperature. The differences in immobilization and uptake had almost disappeared at flowering and harvest. ¹⁵N recovery in soil and crop varied between 50 and 100%. Gaseous losses probably occurred by volatilization in the chalky soil and denitrification in the hydromorphic loamy soil. These losses affected the NH₄ ⁺ and NO₃ ^- pools differently and determined the partitioning of fertilizer-N between immobilization and absorption.     

Evaluation of N2-fixation and nitrogen economy of a maize/cowpea intercrop system using15N dilution methods

Yields of above ground biomass and total N were determined in summer-grown maize and cowpea as sole crops or intercrops, with or without supplementary N fertilizer (25 kg N ha⁻¹, urea) at an irrigated site in Waroona, Western Australia over the period 1982–1985. Good agreement was obtained between estimates of N₂ fixation of sole or intercrop cowpea (1984/85 season) based on the¹⁵N natural abundance and¹⁵N fertilizer dilution techniques, both in the field and in a glasshouse pot study. Field-grown cowpea was estimated to have received 53–69% of its N supply from N₂-fixation, with N₂-fixation onlyslightly affected by intercropping or N fertilizer application. Proportional reliance on N₂-fixation of cowpea in glasshouse culture was lower (36–66%) than in the field study and more affected by applied N. Budgets for N were drawn up for the field intercrops, based on above-ground seed yields, return of crop residues, inputs of fixed N and fertilizer N. No account was taken of possible losses of N through volatilization, denitrification and leaching or gains of N in the soil from root biomass. N₂-fixation was estimated tobe 59 kg N ha⁻¹ in the plots receiving no fertilizer N, and 73 kg N ha⁻¹ in plots receiving 25 kg N ha⁻¹ as urea. Comparable fixation by sole cowpea was higher (87 and 82 kg N ha⁻¹ respectively) but this advantage was outweighed by greater land use efficiency by the intercrop than sole crops.     

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.     

施肥对麦田土壤可溶性有机氮的影响

利用长期定位试验,研究施肥和小麦生长对土壤可溶性有机氮(EON)的影响。长期不同施肥土壤包括不施肥(No-F)、施用化肥(NPK)和有机肥与化肥配施(MNPK)3种。EON含量范围为7.5—29.3 kg/hm~2,No-F、NPK和MNPK土壤中EON分别占可溶性总氮的40%、56%和56%。长期有机肥与化肥配施显著提高0—15 cm土层EON含量,但对30 cm以下土层EON含量无影响。在小麦开花期,可溶性有机氮的含量及其相对含量显著高于拔节期和收获期。虽然施用氮肥对当季EON含量无显著影响,但同位素示踪微区试验表明,土壤耕层(0—15 cm)中仍有0.4%—2.8%的可溶性有机氮来源于当季施入的肥料氮。可见,化学氮肥向可溶性有机氮的转化缓慢,但农田土壤中可溶性有机氮含量与矿质态氮含量相当,发生淋溶损失的风险大。     

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%.     

Crop uptake and leaching of 15N applied in ruminant slurry with selectively labelled faeces and urine fractions

Two animal slurries either labelled with ¹⁵N in the urine or in the faeces fraction, were produced by feeding a sheep with unlabelled and ¹⁵N-labelled hay and collecting faeces and urine separately. The slurries were applied (12 g total N ^-2) to a coarse sand and a sandy loam soil confined in lysimeters and growing spring barley (Hordeum vulgare L). Reference lysimeters without slurry were supplied with¹⁵ NH₄ ¹⁵NO³ corresponding to the inorganic N applied with the slurries (6 g N m^-2). In the second year, all lysimeters received unlabelled mineral fertilizer (6 g N m^-2) and grew spring barley. N harvested in the two crops (grain + straw) and the loss of nitrate by leaching were determined. ¹⁵N in the urine fraction was less available for crop uptake than mineral fertilizer ¹⁵N. The first barley crop on the sandy loam removed 49% of the ¹⁵N applied in mineral fertilizer and 36% of that applied with urine. The availability of fertilizer ¹⁵N (36%) and urine¹⁵ N (32%) differed less on the coarse sand. Of the¹⁵ N added with the faeces fraction, 12–14% was taken up by the barley crop on the two soils. N mineralized from faeces compensated for the reduced availability of urine N providing a similar or higher crop N uptake in manured lysimeters compared with mineral fertilized ones.About half of the total N uptake in the first crop originated from the N applied either as slurry or mineral fertilizer. The remaining N was derived from the soil N pool. Substantially smaller but similar proportions of¹⁵ N from faeces, urine and fertilizer were found in the second crop. The similar recoveries indicated a slow mineralization rate of the residual faeces N since more faeces was left in the soil after the first crop.More N was lost by leaching from manured lysimeters but as a percentage of N applied, losses were similar to those from mineral fertilizer. During the first and second winter, 3–5% and 1–3%, respectively, of the ¹⁵N in slurry and mineral fertilizer was leached as nitrate. Thus slurry N applied in spring just before sowing did not appear to be more prone to loss by nitrate leaching than N given in mineral fertilizer. Slurry N accounted for a higher proportion of the N leached, however, because more N was added in this treatment.     

Nitrogen incorporation and remobilization in different shoot components of field-grown winter oilseed rape (Brassica napus L.) as affected by rate of nitrogen application and irrigation

The seasonal course of nitrogen uptake, incorporation and remobilization in different shoot components of winter oilseed rape (Brassica napus L.) was studied under field conditions including three rates of ¹⁵N labelled nitrogen application (0, 100 or 200 kg N ha^-1) and two irrigation treatments (rainfed or watered at a deficit of 20 mm). The total amount of irrigation water applied was 260 mm, split over 13 occasions in a 7-week-period ranging from 1 week before onset of flowering until 4 weeks after flowering.Nitrogen application and irrigation increased plant growth and nitrogen accumulation. Irrespective of N and irrigation treatment more than 50% of total shoot N was present in the stem when flowering started. At the end of flowering, pod walls were the main N store containing about 30–40% of shoot N. The quantities of N remobilized from stems and pod walls amounted in all treatments to about 70% of the N present in these organs at mid-flowering. At harvest, stem and pod walls each contained about 10% of total shoot N, the remaining 80% being incorporated into seeds. The main component contributing to the response of seed N accumulation to nitrogen application and irrigation was pods in axillary racemes. Up to 20 kg N ha^-1, corresponding to about 10% of final shoot N content, was lost from the plants by leaf drop.Irrigation increased the recovery at harvest of applied N from 30% to about 50%, while the level of N application did not affect the N recovery. ¹⁵N labelled (fertilizer derived) nitrogen constituted a greater proportion of the N content in old leaves than in young leaves and increased with age in the former, but not in the latter. Relative to soil N, fertilizer derived N also contributed more to the N content of vegetative than to that of reproductive shoot components. Small net changes in shoot N content after flowering reflected a balance between N import and export, leading to continuous dilution of ¹⁵N labelled N with unlabelled N.     

Measurement of biological dinitrogen fixation in grassland: Comparison of the enriched 15N dilution and the natural 15N abundance methods at different nitrogen application rates and defoliation frequencies

A plant mixture of white clover (Trifolium repens L.), red clover (Trifolium pratense L.), and ryegrass (Lolium perenne L.) was established in the spring of 1991 under a cover-crop of barley. Treatments were two levels of nitrogen (400 and 20 kg N ha^-1) and two cutting intensities (3 and 6 cuts per season). Fixation of atmospheric derived nitrogen was estimated by two ¹⁵N dilution methods, one based on application of ¹⁵N to the soil, the other utilising small differences in natural abundance of ¹⁵N.Both methods showed that application of 400 kg N ha^-1 significantly reduced dinitrogen fixation, while cutting frequency had no effect. Atmospheric derived nitrogen constituted between 50 and 64% of harvested clover nitrogen in the high-N treatment, while between 73% and 96% of the harvested clover nitrogen was derived from the atmosphere in the low-N treatment. The amounts of fixed dinitrogen varied between 31–72 kg N ha^-1 and 118–161 kg N ha^-1 in the high-N and low-N treatment, respectively. The highest values for biological dinitrogen fixation were estimated by the enriched ¹⁵N dilution method.Estimates of transfer of atmospheric derived nitrogen from clover to grass obtained by the natural ¹⁵N abundance method were consistently higher than those obtained by the enriched ¹⁵N dilution method. Neither mineral nitrogen application nor defoliation frequency affected transfer of atmospheric derived nitrogen from clover to grass.Isotopic fractionation of ¹⁴N and ¹⁵N (B value) was estimated by comparing results for nitrogen fixation obtained by the enriched ¹⁵N dilution and the natural ¹⁵N abundance method, respectively. B was on average +1.20, which was in agreement with a B value determined by growing white clover in a nitrogen free media.     

Response to inoculation and N fertilization for increased yield and biological nitrogen fixation of common bean (Phaseolus vulgaris L.)

Common bean (Phaseolus vulgaris L.) is able to fix 20–60 kg N ha^–1 under tropical environments in Brazil, but these amounts are inadequate to meet the N requirement for economically attractive seed yields. When the plant is supplemented with N fertilizer, N₂ fixation by Rhizobium can be suppressed even at low rates of N. Using the ¹⁵N enriched method, two field experiments were conducted to compare the effect of foliar and soil applications of N-urea on N₂ fixation traits and seed yield. All treatments received a similar fertilization including 10 kg N ha^–1 at sowing. Increasing rates of N (10, 30 and 50 kg N ha^–1) were applied for both methods. Foliar application significantly enhanced nodulation, N₂ fixation (acetylene reduction activity) and yield at low N level (10 kg N ha^–1). Foliar nitrogen was less suppressive to nodulation, even at higher N levels, than soil N treatments. In the site where established Rhizobium was in low numbers, inoculation contributed substantially to increased N₂ fixation traits and yield. Both foliar and soil methods inhibited nodulation at high N rates and did not significantly increase bean yield, when comparing low (10 kg N ha^–1) and high (50 kg N ha^–1) rates applied after emergence. In both experiments, up to 30 kg N ha^–1 of biologically fixed N₂ were obtained when low rates of N were applied onto the leaves.     

Production,standing biomass and natural abundance of <Superscript>15</Superscript>N and <Superscript>13</Superscript>C in ectomycorrhizal mycelia collected at different soil depths in two forest types

Nutrient uptake by forest trees is dependent on ectomycorrhizal (EM) mycelia that grow out into the soil from the mycorrhizal root tips. We estimated the production of EM mycelia in root free samples of pure spruce and mixed spruce-oak stands in southern Sweden as mycelia grown into sand-filled mesh bags placed at three different soil depths (0–10, 10–20 and 20–30 cm). The mesh bags were collected after 12 months and we found that 590±70 kg ha^–1 year^–1 of pure mycelia was produced in spruce stands and 420±160 kg ha^–1 year^–1 in mixed stands. The production of EM mycelia in the mesh bags decreased with soil depth in both stand types but tended to be more concentrated in the top soil in the mixed stands compared to the spruce stands. The fungal biomass was also determined in soil samples taken from different depths by using phospholipid fatty acids as markers for fungal biomass. Subsamples were incubated at 20°C for 5 months and the amount of fungal biomass that degraded during the incubation period was used as an estimate of EM fungal biomass. The EM biomass in the soil profile decreased with soil depth and did not differ significantly between the two stand types. The total EM biomass in the pure spruce stands was estimated to be 4.8±0.9×10³ kg ha^–1 and in the mixed stands 5.8±1.1×10³ kg ha^–1 down to 70 cm depth. The biomass and production estimates of EM mycelia suggest a very long turnover time or that necromass has been included in the biomass estimates. The amount of N present in EM mycelia was estimated to be 121 kg N ha^–1 in spruce stands and 187 kg N ha^–1 in mixed stands. The ¹³C value for mycelia in mesh bags was not influenced by soil depth, indicating that the fungi obtained all their carbon from the tree roots. The ¹³C values in mycelia collected from mixed stands were intermediate to values from pure spruce and pure oak stands suggesting that the EM mycelia received carbon from both spruce and oak trees in the mixed stands. The ¹⁵N value for the EM mycelia and the surrounding soil increased with soil depth suggesting that they obtained their entire N from the surrounding soil.     

Crop uptake and leaching losses of15N labelled fertilizer nitrogen in relation to waterlogging of clay and sandy loam soils

Summary Ammonium nitrate fertilizer, labelled with¹⁵N, was applied in spring to winter wheat growing in undisturbed monoliths of clay and sandy loam soil in lysimeters; the rates of application were respectively 95 and 102 kg N ha⁻¹ in the spring of 1976 and 1975. Crops of winter wheat, oilseed rape, peas and barley grown in the following 5 or 6 years were treated with unlabelled nitrogen fertilizer at rates recommended for maximum yields. During each year of the experiments the lysimeters were divided into treatments which were either freelydrained or subjected to periods of waterlogging. Another labelled nitrogen application was made in 1980 to a separate group of lysimeters with a clay soil and a winter wheat crop to study further the uptake of nitrogen fertilizer in relation to waterlogging. In the first growing season, shoots of the winter wheat at harvest contained 46 and 58% of the fertilizer nitrogen applied to the clay and sandy loam soils respectively. In the following year the crops contained a further 1–2% of the labelled fertilizer, and after 5 and 6 years the total recoveries of labelled fertilizer in the crops were 49 and 62% on the clay and sandy loam soils respectively. In the first winter after the labelled fertilizer was applied, less than 1% of the fertilizer was lost in the drainage water, and only about 2% of the total nitrogen (mainly nitrate) in the drainage water from both soils was derived from the fertilizer. Maximum annual loss occurred the following year but the proportion of tracer nitrogen in drainage was nevertheless smaller. Leaching losses over the 5 and 6 years from the clay and sandy loam soil were respectively 1.3 and 3.9% of the original application. On both soils the percentage of labelled nitrogen to the total crop nitrogen content was greater after a period of winter waterlogging than for freely-drained treatments. This was most marked on the clay soil; evidence points to winter waterlogging promoting denitrification and the consequent loss of soil nitrogen making the crop more dependent on spring fertilizer applications.     

Fertilizer nitrogen budgets of two doses of Na15NO3 dressings split-applied to winter wheat in microplots on a loam soil

In a field experiment performed in microplots, winter wheat was fertilized at two different total N dressings (135 and 180 kg ha^–1) split-applied as Na¹⁵NO₃ in three equal applications at tillering, stem elongation, and flag leaf.No significant differences were found in the percentage recovery values for the entire plant at the three split applications between the two N dressings. The total percentage recovery of fertilizer N by the plant was high and practically equal at both fertilization levels (76.65% and 75.84% for 135 and 180 kg N ha^–1, respectively); crop yields were also similar. In contrast, gaseous losses calculated after drawing up the balance sheet were, in absolute values, higher for the tillering and stem elongation split applications when using the 180 kg N ha^–1 dressing (7.67 and 4.84 kg N ha^–1, respectively) than for the 135 kg N ha^–1 dressing (3.45 and 1.26 kg N ha^–1, respectively). They were found to be zero at flag leaf at both fertilization levels. The amount of applied fertilizer N did not influence the amount of N taken up from the soil which was about 143 kg ha^–1.     

Fertilizer and soil nitrogen accumulation by irrigated barley grown on a red-brown earth in south-eastern Australia

¹⁵N labelled (NH₄)₂SO₄ was applied to barley at 5 g N m⁻² (50 kg N ha⁻¹) in microplots at sowing to study the timing of the N losses and the contribution of soil and fertilizer N to the plant. Water treatments included rainfed and irrigation at 45–50 mm deficit beginning in the spring. Recovery of¹⁵N in the plant increased to a maximum of about 20% within 91 days after sowing (DAS 91) and then remained constant. Approximately 16% (0.8 g N m⁻²) of the fertilizer was in the stem and leaves at DAS 91 and this N was subsequently redistributed to the head. At maturity, approximately 75% of the¹⁵N assimilated by the tops was recovered in the grain. Soil N contributed 3.6 g N m⁻² to the head; 2.2 g N m⁻² was remobilized from the stem and leaves, and the balance, approximately 1.4 g N m⁻², was taken up from the soil between DAS 69 to 91. Effects of irrigation treatments on N accumulation were not significant. Residual¹⁵N fertilizer in the soil decreased with time from sowing, and at maturity 40% of the applied N was recovered in the surface 0.15 m.¹⁵N movement to depth was limited and less than 5% of the fertilizer was recovered below 0.15 m. Irrigation had no effect on the¹⁵N recovery at depth. Total recovery of the¹⁵N varied between 60 and 67% and implies that 33–40% was lost from the soil-plant system. The total recovery in the soil and plant was not affected by time or irrigation in the interval DAS 39 to 134. Losses occurred before DAS 39 when crop uptake of N was small and soil mineral N content was high. There was an apparent loss of 1.9 g fertilizer N m⁻² (i.e. 38% of that applied) between DAS 1 and 15. This loss occurred before crop emergence when rainfall provided conditions suitable for denitrification.     

Dinitrogen fixation in white clover grown in pure stand and mixture with ryegrass estimated by the immobilized 15N isotope dilution method

Dinitrogen fixation in white clover (Trifolium repens L.) grown in pure stand and mixture with perennial ryegrass (Lolium perenne L.) was determined in the field using ¹⁵N isotope dilution and harvest of the shoots. The apparent transfer of clover N to perennial ryegrass was simultaneously assessed. The soil was labelled either by immobilizing ¹⁵N in organic matter prior to establishment of the sward or by using the conventional labelling procedure in which ¹⁵N fertilizer is added after sward establishment. Immobilization of ¹⁵N in the soil organic matter has not previously been used in studies of N₂ fixation in grass/clover pastures. However, this approach was a successful means of labelling, since the ¹⁵N enrichment only declined at a very slow rate during the experiment. After the second production year only 10–16% of the applied ¹⁵N was recovered in the harvested herbage. The two labelling methods gave, nonetheless, a similar estimate of the percentage of clover N derived from N₂ fixation. In pure stand clover, 75–94% of the N was derived from N₂ fixation and in the mixture 85–97%. The dry matter yield of the clover in mixture as percentage of total dry matter yield was relatively high and increased from 59% in the first to 65% in the second production year. The average daily N₂ fixation rate in the mixture-grown clover varied from less than 0.5 kg N ha⁻¹ day⁻¹ in autumn to more than 2.6 kg N ha⁻¹ day⁻¹ in June. For clover in pure stand the average N₂ fixation rate was greater and varied between 0.5 and 3.3 kg N ha⁻¹ day⁻¹, but with the same seasonal pattern as for clover in mixture. The amount of N fixed in the mixture was 23, 187 and 177 kg N ha⁻¹ in the seeding, first and second production year, respectively, whereas pure stand clover fixed 28, 262 and 211 kg N ha⁻¹ in the three years. The apparent transfer of clover N to grass was negligible in the seeding year, but clover N deposited in the rhizosphere or released by turnover of stolons, roots and nodules, contributed 19 and 28 kg N ha⁻¹ to the grass in the first and second production year, respectively. This revised version was published online in June 2006 with corrections to the Cover Date.     

Fertilizer vs. organic matter contributions to nitrogen leaching in cropping systems of the Pampas: 15N application in field lysimeters

Nitrogen (N) export from soils to streams and groundwater under the intensifying cropping schemes of the Pampas is modest compared to intensively cultivated basins of Europe and North America; however, a slow N enrichment of water resources has been suggested. We (1) analyzed the fate of fertilizer N and (2) evaluated the contribution of fertilizer and soil organic matter (SOM) to N leaching under the typical cropping conditions of the Pampas. Fertilizer N was applied as ¹⁵N-labeled ammonium sulfate to corn (in a corn/soybean rotation) sown under zero tillage in filled-in lysimeters containing two soils of different texture representative of the Pampean region (52 and 78 kg N ha^-1, added to the silt loam and sandy loam soil, respectively). Total fertilizer recovery at corn harvest averaged 84 and 64% for the silt loam and sandy loam lysimeters, respectively. Most fertilizer N was removed with plant biomass (39%) or remained immobilized in the soil (29 and 15%, for the silt loam and sandy loam soil, respectively) whereas its loss through drainage was negligible (<0.01%). We presume that the unaccounted fertilizer N losses were related to volatilization and denitrification. Throughout the corn growing season, subsequent fallow and soybean crop, which took place during an exceptionally dry period, the fertilizer N immobilized in the organic pool remained stable, and N leaching was scarce (7.5 kg N ha^-1), similar at both soils, and had a low contribution of fertilizer N (0–3.5%), implying that >96% of the leached N was derived from SOM mineralization. The inherent high SOM of Pampean soils and the favorable climatic conditions are likely to propitiate year-round production of nitrate, favoring its participation in crop nutrition and leaching. The presence of ¹⁵N in drainage water, however, suggests that fertilizer N leaching could become significant in situations with higher fertilization rates or more rainy seasons.