Nitrogen losses in puddled soils as affected by timing of water deficit and nitrogen fertilization

Erratic rainfall in rainfed lowlands and inadequate water supply in irrigated lowlands can results in alternate soil drying and flooding during a rice (Oryza sativa L.) cropping period. Effects of alternate soil drying and flooding on N loss by nitrification-denitrification have been inconsistent in previous field research. To determine the effects of water deficit and urea timing on soil NO₃ and NH₄, floodwater NO₃, and N loss from added ¹⁵N-labeled urea, a field experiment was conducted for 2 yr on an Andaqueptic Haplaquoll in the Philippines. Water regimes were continuously flooded, not irrigated from 15 to 35 d after transplanting (DT), or not irrigated from 41 to 63 DT. The nitrogen treatments in factorial combination with water regimes were no applied N and 80 kg urea-N ha^–1, either applied half basally and half at 37 DT or half at 11 DT and half at 65 DT. Water deficit at 15 to 35 DT and 41 to 63 DT, compared with continuous soil flooding, significantly reduced extractable NH₄ in the top 30-cm soil layer and resulted in significant but small (<1.0 kg N ha^–1) soil NO₃ accumulations. Soil NO₃, which accumulated during the water deficit, rapidly disappeared after reflooding. Water deficit at 15 to 35 DT, unlike that at 41 to 63 DT, increased the gaseous loss of added urea N as determined from unrecovered ¹⁵N in ¹⁵N balances. The results indicate that application of urea to young rice in saturated or flooded soil results in large, rapid losses of N (mean = 35% of applied N), presumably by NH₃ volatilization. Subsequent soil drying and flooding during the vegetative growth phase can result in additional N loss (mean = 14% of applied N), presumably by nitrification-denitrification. This additional N loss due to soil drying and flooding decreases with increasing crop age, apparently because of increased competition by rice with soil microorganisms for NH₄ and NO₃.     

Absorption of molecular urea by rice under flooded and non-flooded soil conditions

A pot experiment was conducted with rice to study the relative absorption of urea in molecular form compared to the other forms of N produced in soil from the applied urea. A method involving application of ¹⁴C-labelled urea and ¹⁵N-labelled urea alternately in two splits was used to quantify the absorption of molecular urea and other forms of N formed from it. Biomass production and N uptake were greater in plants grown under flooded soil conditions than in plants grown under non-flooded (upland) conditions. Absorption of N by rice increased with increasing rate of urea application up to 250 mg pot⁻¹ and declined thereafter. The absorption of urea from the flooded soil constituted 9.4% of total N uptake from applied N compared to only 0.2% from the non-flooded. Under submerged conditions, absorption of urea from topdressing was about twice that from basal application at planting. High water solubility of the fertilizer and better developed rice root system might have enhanced the absorption of molecular urea by flooded rice, especially from topdressing. Thus, in the flooded rice system, the direct absorption of molecular urea from topdressing accounted for 6.3% of the total N uptake from added urea. Under upland condition, it was 0.12%. This revised version was published online in June 2006 with corrections to the Cover Date.     

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.     

Design and development of a simple laboratory model to detect <Superscript>15</Superscript>N enrichment in cyanobacterial biomass and extra cellular ammonia using <Superscript>15</Superscript>N gas

A laboratory scale working model that could detect the ¹⁵N enrichment in cyanobacterial biomass and extracellular ammonia, using ¹⁵N gas under in vitro conditions was designed and fabricated. Using the model, ¹⁵N enrichment of 0.48% atom excess was detected in the cyanobacterial biomass on the 30 d after inoculation. The ¹⁵N enrichment increased linearly in the extracellular ammoniacal fraction from the 20 d onward. The model would prove to be a useful tool to quantify the extent of ¹⁵N enrichment under in vitro conditions using ¹⁵N gas.     

Effect of urea placement on leaching losses of nitrogen from flooded rice soils

In an effort to provide an explanation for the reported variability in fertilizer N efficiency from deepplaced urea on flooded rice, a set of controlled experiments was conducted to evaluate the effect of water percolation on fertilizer loss and plant uptake from¹⁵N labeled urea supergranules. Three soils of different texture (silt loam-clay) were subjected to various percolation rates (0–20 mm/day) while planted to rice which was harvested after approximately 40 days.The results indicate that moderate to high percolation through silt loam soil will lead to significant fertilizer N losses and drastically decrease the fertilizer uptake by plants. The permeability of the clay soil was too low for any leaching to take place. It is therefore concluded that deep placement of urea supergranules not be recommended in soils where percolation rates may exceed 5 mm/day, particularly if the cation exchange capacity of the soil is low. This experiment points to the need of evaluating and reporting the percolation rates in soils where experiments with supergranular urea are conducted.Contribution from the Agro-Economic Division of the International Fertilizer Development Center (IFDC), Muscle Shoals, Alabama 35660.     

Effects of land use on 15N natural abundance of soils in Ethiopian highlands

We used the natural abundance of ¹⁵N in soils in forests, pastures and cultivated lands in the Menagesha and Wendo-Genet areas of Ethiopia to make inferences about the N cycles in these ecosystems. Since we have described the history of these sites based on variations in ¹³C natural abundance, patterns of δ¹⁵N and δ¹³C values were compared to determine if shifts of ¹⁵N correlate with shifts of vegetation. At Menagesha, a > 500-yr-old planted forest, we found δ¹⁵N values from −8.8 to +3.5‰ in litter, from −3.5 to +4.5‰ in 0–10 cm soil layer, and from −1.5 to +6.8‰ at >20 cm soil depth. The low δ¹⁵N in litter and surface mineral soils suggests that a closed N cycle has operated for a long time. At this site, the low δ¹³C of the surface horizon and the high δ¹³C of the lower soil horizons is clear evidence of a long phase of C₄ grass dominance or cultivation of C₄ crops before the establishment of the forest >500 years ago. In contrast, at Wendo-Genet, high δ¹³C of soils reveals that most of the land has been uncovered by forests until recently. Soil δ¹⁵N was high throughout (3.4–9.8‰), and there were no major differences between forested, cultivated and pasture soils in δ¹⁵N values of surface mineral soils. The high δ¹⁵N values suggest that open N cycles operate in the Wendo-Genet area. From the points of view of soil fertility management, it is interesting that tall forest ecosystems with relatively closed N cycling could be established on the fairly steep slopes at Menagesha after a long period of grass vegetation cover or cultivation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Lysimeter studies on recovery of15N-labeled urea in wetland rice

Summary Results of a two year study on the fate on¹⁵N-labelled urea (9.95 atoms percent excess¹⁵N) applied @ 180 kg N/ha to flooded rice in monolith lysimeters at the Punjab Agricultural University Farm, Ludhiana are reported. The soil of the experimental field was sandy clay loam in texture (Typic Ustochrept), had pH 7.9, organic carbon 0.36 percent, available N 187 kg/ha and total N 0.08 percent. The results revealed that 18.1 to 53.0 per cent of the fertilizer N was utilized by the rice plant, 25.1 to 41.1 percent was immobilized in the soil and 4.8 to 7.2 percent was lost by denitrification. The losses due to ammonia volatilization and leaching were negligible. The data on vertical distribution of labelled N in the soil profile reflected a higher concentration (38.3 to 39.5 per cent) in the surface (0–30 cm) soil. The content sharply decreased (1.8 to 2.4, percent) in lower soil layers (30–150 cm). A balance sheet of the various pathways of applied N showed that 58.8 to 72.2 and 66.2 to 83.0 percent N was recovered in 1976 and 1977, respectively and 17 to 41.2 per cent of labelled N still remained unaccounted for. Utilization of fertilizer N by rice was increased and losses decreased when N was applied in three equal splits as compared to the single N application at transplanting.Availability of fertilizer N immobilized in the soil was investigated in the succeeding crops of wheat and rice. The results showed that 2.1 tot 3.4 per cent of the N applied to the preceding rice was utilized by the second rice crop grown in succession. This may look small but cannot be neglected on a long term basis. But there is need to initiate long term studies to investigate the, turnover of residual N and to determine the fate of applied N in varying soil and cropping systems by using improved techniques.     

Assessing Nitrification and Denitrification in the Seine River and Estuary Using Chemical and Isotopic Techniques

Downstream from metropolitan Paris (France), a large amount of ammonium is discharged into the Seine River by the effluents of the wastewater treatment plant at Achères. To assess the extent of nitrification and denitrification in the water column, concentrations and isotopic compositions of ammonium (δ¹⁵N–NH₄⁺) and nitrate (δ¹⁵N–NO₃⁻, δ¹⁸O–NO₃⁻) were measured during summer low-flow conditions along the lower Seine and its estuary. The results indicated that most of the ammonium released from the wastewater treatment plant is nitrified in the lower Seine River and its upper estuary, but there was no evidence for water-column denitrification. In the lower part of the estuary, however, concentration and isotopic data for nitrate were not consistent with simple mixing between riverine and marine nitrate. A significant departure of the nitrate isotopic composition from what would be expected from simple mixing of freshwater and marine nitrates suggested coupled nitrification and denitrification in the water, in spite of the apparent conservative behavior of nitrate. Denitrification rates of approximately 0.02 mg N/L/h were estimated for this part of the estuary.     

Nitrogen uptake in needles of Scots pine (Pinus sylvestris L.) when exposed to gaseous ammonia and ammonium fertilizer in the soil

Young saplings of Pinus sylvestris L. were exposed to gaseous NH₃ at 53 or 105 g m^–3 for one year in open-top chambers. Saplings received ¹⁵N-labelled (NH₄)₂SO₄ via the soil. To examine the importance of foliar N uptake, changes in the concentration of total and labelled N in the needles were followed. Increase in needle biomass and N concentration were found in trees exposed to NH₃, confirming that atmospheric NH₃ acted as a N fertilizer. NH₃ had a greater and quicker effect than (NH₄)₂SO₄: compared with the growth in ambient air, the N concentration in the needles exposed to NH₃ had increased by 49% in four months, while the increase after highest N-fertilization (200 kg N ha^–1 y^–1) was only 8%. The small contribution of NH₄ ⁺ fertilization to the total N concentration was not due to a deficient N uptake: the ¹⁵N concentration in the needles increased significantly with time. On the other hand, NH₃ uptake in shoots may have a negative effect on the NH₄ ⁺ root uptake. The relation between plant N and atmospheric NH₃ concentration was non-linear and possible reasons for this observation are discussed. Fumigation with NH₃ significantly decreased the ratios of K/N and P/N, showing that fumigation disrupted the nutrient balance.     

15N abundance of surface soils,roots and mycorrhizas in profiles of European forest soils

¹⁵N natural abundances of soil total N, roots and mycorrhizas were studied in surface soil profiles in coniferous and broadleaved forests along a transect from central to northern Europe. Under conditions of N limitation in Sweden, there was an increase in ¹⁵N of soil total N of up to 9% from the uppermost horizon of the organic mor layer down to the upper 0–5 cm of the mineral soil. The ¹⁵N of roots was only slightly lower than that of soil total N in the upper organic horizon, but further down roots were up to 5% depleted under such conditions. In experimentally N-enriched forest in Sweden, i.e. in plots which have received an average of c. 100 kg N ha^–1 year^–1 for 20 years and which retain less than 50% of this added N in the stand and the soil down to 20 cm depth, and in some forests in central Europe, the increase in ¹⁵N with depth in soil total N was smaller. An increase in ¹⁵N of the surface soil was even observed on experimentally N-enriched plots, although other data suggest that the N fertilizer added was depleted in¹⁵N. In such cases roots could be enriched in¹⁵N relative to soil total N, suggesting that labelling of the surface soil is via the pathway: — available pools of N-plant N-litter N. Under N-limiting conditions roots of different species sampled from the same soil horizon showed similar ¹⁵N. By contrast, in experimentally N-enriched forest ¹⁵N of roots increased in the sequence: ericaceous dwarf shrubs15N enriched compounds in fungal material, which could contribute to explain the observed ¹⁵N profiles if fungal material is enriched, because it is a precursor of stable organic matter and recalcitrant N. This could act in addition to the previous explanation of the isotopically lighter soil surface in forests: plant uptake of ¹⁵N-depleted N and its redeposition onto the soil surface by litter-fall.     

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     

Intact amino acid uptake by northern hardwood and conifer trees

Empirical and modeling studies of the N cycle in temperate forests of eastern North America have focused on the mechanisms regulating the production of inorganic N, and assumed that only inorganic forms of N are available for plant growth. Recent isotope studies in field conditions suggest that amino acid capture is a widespread ecological phenomenon, although northern temperate forests have yet to be studied. We quantified fine root biomass and applied tracer-level quantities of U–¹³C₂–¹⁵N-glycine, ¹⁵NH₄ ⁺ and ¹⁵NO₃ ⁻ in two stands, one dominated by sugar maple and white ash, the other dominated by red oak, beech, and hemlock, to assess the importance of amino acids to the N nutrition of northeastern US forests. Significant enrichment of ¹³C in fine roots 2 and 5 h following tracer application indicated intact glycine uptake in both stands. Glycine accounted for up to 77% of total N uptake in the oak–beech–hemlock stand, a stand that produces recalcitrant litter, cycles N slowly and has a thick, amino acid-rich organic horizon. By contrast, glycine accounted for only 20% of total N uptake in the sugar maple and white ash stand, a stand characterized by labile litter and rapid rates of amino acid production and turnover resulting in high rates of mineralization and nitrification. This study shows that amino acid uptake is an important process occurring in two widespread, northeastern US temperate forest types with widely differing rates of N cycling.     

The fate of residual 15N-labelled fertilizer in arable soils: its availability to subsequent crops and retention in soil

An earlier paper (Macdonald et al., 1997; J. Agric. Sci. (Cambridge) 129, 125) presented data from a series of field experiments in which ¹⁵N-labelled fertilizers were applied in spring to winter wheat, winter oilseed rape, potatoes, sugar beet and spring beans grown on four different soils in SE England. Part of this N was retained in the soil and some remained in crop residues on the soil surface when the crop was harvested. In all cases the majority of this labelled N remained in organic form. In the present paper we describe experiments designed to follow the fate of this `residual' <sup>15</sup>N over the next 2 years (termed the first and second residual years) and measure its value to subsequent cereal crops. Averaging over all of the initial crops and soils, 6.3% of this `residual' ¹⁵N was taken up during the first residual year when the following crop was winter wheat and significantly less (5.5%) if it was spring barley. In the second year after the original application, a further 2.1% was recovered, this time by winter barley. Labelled N remaining after potatoes and sugar beet was more available to the first residual crop than that remaining after oilseed rape or winter wheat. By the second residual year, this difference had almost disappeared. The availability to subsequent crops of the labelled N remaining in or on the soil at harvest of the application year decreased in the order: silty clay loam>sandy loam>chalky loam>heavy clay. In most cases, only a small proportion of the residual fertilizer N available for plant uptake was recovered by the subsequent crop, indicating poor synchrony between the mineralization of ¹⁵N-labelled organic residues and crop N uptake. Averaging over all soils and crops, 22% of the labelled N applied as fertilizer was lost (i.e., unaccounted for in harvested crop and soil to a depth of 100 cm) by harvest in the year of application, rising to 34% at harvest of the first residual year and to 35% in the second residual year. In the first residual year, losses of labelled N were much greater after spring beans than after any of the other crops.     

Availability of nitrogen from 15N-labelled Azolla pinnata and urea to flooded rice

In view of the recently generated interest in Azolla and the high cost of N fertilizers, this field study was aimed at measuring the availability of Azolla-N applied in two split application in comparison to urea-N. Azolla was cultivated and labelled with ¹⁵N isotope in the field. A total of about 60 kg N ha^-1 was applied as Azolla, urea or Azolla and urea in combination, in two equal splits at transplanting and at maximum tillering, i.e. 30 days after transplanting (30 DAT).The recovery by the crop of Azolla-N applied at 30 DAT was significantly higher than that applied at transplanting, viz. 30.2% and 20.2%, respectively. The recoveries of urea-N applied at the same stages were similarly low, viz. 22.5% at transplanting and 38.6% at 30 DAT. Total recoveries of fertilizer N at the time of harvest were 26.8% from Azolla, 30.7% from urea applied in the same two splits and 49.1% from urea applied in locally recommended three splits. Recoveries of labelled Azolla-N in succeeding rice crop were twice higher than those of labelled urea-N. The recoveries ranged from 1.9 to 2.1% from urea-N and 4.0 to 4.9% from Azolla-N. There were no differences in residual ¹⁵N recovery in the succeeding crop between Azolla and urea either applied at transplanting or at 30 DAT.     

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.     

Mineralization-immobilization and plant uptake of nitrogen as influenced by the spatial distribution of cattle slurry in soils of different texture

The effect of incorporating cattle slurry in soil, either by mixing or by simulated injection into a hollow in soil, on the ryegrass uptake of total N and ¹⁵NH₄⁺-N was determined in three soils of different texture. The N accumulation in Italian ryegrass (Lolium multiflorum L.) from slurry N and from an equivalent amount of NH₄⁺-N in (¹⁵NH₄) SO₄ (control) was measured during 6 months of growth in pots. After this period the total recovery of labelled N in the top soil plus herbage was similar in the slurry and the control treatments. This indicated that gaseous losses from slurry NH₄⁺-N were insignificant. Consequently, the availability of slurry N to plants was mainly influenced by the mineralization-immobilization processes. The apparent utilization of slurry NH₄⁺-N mixed into soil was 7%, 14% and 24% lower than the utilization of (NH₄)₂SO₄-N in a sand soil, a sandy loam soil and a loam soil, respectively. Thus, the net immobilization of N due to slurry application increased with increasing soil clay content, whereas the recovery in plants of ¹⁵N-labelled NH₄⁺-N from slurry was similar on the three soils. A parallel incubation experiment showed that the immobilization of slurry N occurred within the first week after slurry application. The incorporation of slurry N by simulated injection increased the plant uptake of both total and labelled N compared to mixing the slurry into the soil. The apparent utilization of injected slurry NH₄⁺-N was 7% higher, 8% lower and 4% higher than the utilization of (NH₄)₂SO₄-N in the sand, the sandy loam and the loam soil, respectively. It is concluded that the spatial distribution of slurry in soil influenced the net mineralization of N to the same degree as did the soil type.     

Fertilization rates and clay fixed ammonium in two Quebec soils

Clay fixed NH₄ ⁺ can provide a significant sink for fertilizer N, as well as a source of N for plant uptake. Knowledge or soil NH₄ ⁺ fixing capacity and release for crops is necessary to develop long-term fertilizer programs. Field experiments with corn (Zea mays L.) were carried out to investigate soil NH₄ ⁺ fixing capacity and subsequent release as influenced by fertilizer rates using ¹⁵N in a Ste. Rosalie clay (fine, mixed, frigid, Typic Humaquept) and a Chicot sandy clay loam (fine-loamy, mixed, frigid, Typic Hapludalf). With high N rates increased NH₄ ⁺ fixation occurred only in the Ste. Rosalie soil. At the end of the first growing season, fertilizer N recovery as clay fixed NH₄ ⁺ for high and normal rates of fertilizer in the Ste. Rosalie soil was 17.8% and 28.7%, respectively and the recovery for the high and normal rates in the Chicot soil was 4.6 and 10.5%, respectively. Significant amounts of clay fixed NH₄ ⁺-N were released in the soil profile in the second year after ¹⁵N application on the Chicot soil. Recently clay fixed fertilizer NH₄ ⁺N was released more rapidly than that of the native fixed NH₄ ⁺, from the surface layer of the Ste. Rosalie soil. The fertilizer fixed NH₄ ⁺ seems to be in a more labile N pool than the native fixed NH₄ ⁺-N in the Chicot soil.     

Uptake and use patterns of nitrogen from urea,oxamide, and isobutylidene diurea by rice plants

Summary Two¹⁵N-labelled slow-release nitrogen (N) sources, oxamide and isobutylidene diurea (IBDU), each at two particle sizes, and¹⁵N-labelled urea were compared at two rates as sources of N for rice (Oryza sativa) under two watering regimes which simulated a transplant (continuous flood, CF) and a direct-seeded (A/F) system of paddy rice culture. Highest grain yields were obtained from −8+10-mesh oxamide particles applied at the rate of 2,000 mg of N/5 kg of soil, CF series; this yield was slightly higher than that obtained from −3+4-mesh oxamide, A/F series. Incubating the N fertilizers in moist (22% water) soil for 21 days immediately before flooding and transplanting rice greatly reduced N supply because of nitrification during the preflood period, followed by denitrification after flooding. This resulted in less plant uptake of N and less grain yield from urea, fine oxamide and IBDU, A/F series. For coarse oxamide, N release during the preflood period resulted in higher N uptake and grain yield in the A/F rather than in the corresponding CF series. The pattern of fertilizer N uptake by rice plants was affected by kind of fertilizer, particle size of oxamide and IBDU, and watering regime. Uptake of fertilizer N generally paralleled uptake of soil N throughout the growth period. Plant tops continued to accumulate some N during the period of grain filling, but much of the N in plant tops was translocated to the grain after heading. There was a large decrease in dry weight, N content, and¹⁵N content of tops after heading. Root weight and N content increased rapidly at first, and then at a diminishing rate until maturity. Unexplained N deficits occurred in the CF series (14–23% of the N applied, depending on N rate and source), and in the A/F series for IBDU (37–43% of the N applied).     

Influence of urea on biological N2 fixation and N transfer from Azolla intercropped with rice

The N₂ fixed by Azolla before and after urea application during the rice cycle, the mineralisation of Azolla-N as well as its availability to rice was studied in two greenhouse experiments conducted in 1996 and 1997 and in June 1998 in Goettingen (Germany). Dry matter production of the various rice parts of experiment 1 showed a clear positive synergism between treatment with Azolla and urea with a resulting apparent N recovery by rice increasing from 40% (without Azolla) to 57% in the presence of Azolla. Part of this increase may be due to N fixed biologically by Azolla and transferred to the rice. The second experiment shed some light on the role of BNF. Using an iterative method of estimation, the daily rate of N fixation was estimated at 0.6 – 0.7 kg N ha^–1. The rate was not so much affected by the age of the Azolla crop. At this rate, the BNF would amount to up to 100 kg N ha^–1 over a 130-day season. Assuming that BNF may be inhibited for a period of 5 – 10 days following urea application due to high levels of N in the floodwater, this might reduce the BNF by between 6 and 14 kg N ha over the season. Using the mean-pool-abundance concept, it was estimated that around 75 – 80% of the Azolla-N mineralized during the growth period was actually absorbed by the rice plants. Of the N taken up by rice around 28% was derived from the biologically fixed Azolla N, the remainder was urea N cycled through the Azolla. Azolla also seems to help sustain the soil N supply by returning N to the soil in quantities roughly equal to those extracted from the soil by the rice plant.     

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.