Nitrogen cycling in leucaena (Leucaena leucocephala) alley cropping in semi-arid tropics

In an alley cropping system, prunings from the hedgerow legume are expected to supply nitrogen (N) to the associated cereal. However, this may not be sufficient to achieve maximum crop yield. Three field experiments with alley-cropped maize were conducted in a semi-arid environment in northern Australia to determine: (1) the effect of N fertilizer on maize growth in the presence of fresh leucaena prunings; (2) the effect of incorporation of leucaena and maize residues on maize yield and the fate of plant residue¹⁵N in the alley cropping system; and (3) the¹⁵N recovery by maize from¹⁵N-labelled leucaena, maize residues and ammonium sulphate fertilizer.Leucaena residues increased maize crop yield and N uptake although they did not entirely satisfy the N requirement of the alley crop. Additional N fertilizer further increased the maize yield and N uptake in the presence of leucaena residues. Placement of leucaena residues had little effect on the availability of N to maize plants over a 2 month period. The incorporation of leucaena residues in the soil did not increase the recovery of leucaena¹⁵N by maize compared with placement of the residues on the soil surface. After 2 months, similar proportions of the residue¹⁵N were recovered by maize from mulched leucaena (6.3%), incorporated leucaena (6.1%) and incorporated maize (7.6%). By the end of one cropping season (3 months after application) about 9% of the added¹⁵N was taken up by maize from either¹⁵N-labelled leucaena as mulch or¹⁵N-labelled maize residues applied together with unlabelled fresh leucaena prunings as mulch. The recovery of the added¹⁵N was much higher (42.7%) from the¹⁵N-labelled ammonium sulphate fertilizer at 40 kg N ha^-1 in the presence of unlabelled leucaena prunings. Most of the added¹⁵N recovered in the 200 cm soil profile was distributed in the top 25 cm soil with little leached below that. About 27–41% of the leucaena¹⁵N was apparently lost, largely through denitrification from the soil and plant system, in one cropping season. This compared with 35% of the fertilizer¹⁵N lost when the N fertilizer was applied in the presence of prunings. ei]H Lambers     

Persistence and competitiveness of threeBradyrhizobium japonicum inoculant strains in clay loam Nile valley soil

The nitrogen contribution from the shoot and root system of symbiotically grown leucaena was evaluated in a field experiment on an Alfisol at IITA in Southern Nigeria. Maize in plots that received prunings from inoculated leucaena contained more N and grain yield was increased by 1.9 t.ha.^–1. Large quantities of nitrogen were harvested with leucaena prunings (300 kg N ha^–1 in six months) but the efficiency of utilization of this nitrogen by maize was low compared to inorganic N fertilizer (ammonium sulphate) at 80 kg N ha^–1. Maize yield data indicated that nitrogen in leucaena prunigs was 34 and 45% as efficient as 80 kg N ha^–1 of (NH₄)₂SO₄ for uninoculated and inoculated plants with Rhizobium IRc 1045, respectively. In plots where the prunings were removed, the leaf litter and decaying roots and nodules contributed N equivalent of 32 kg ha^–1. Twenty-five kg ha^–1 was the inorganic N equivalent from nitrogen fixed symbiotically by leucaena when inoculated with Rhizobium strain IRc 1045. Application of prunings from inoculated leucaena resulted in higher soil ogranic C, total N, pH and available NO₃.     

Alleu cropping sequentially cropped maize and cowpea with Leucaena on a sandy soil in Southern Nigeria

Summary The potential of alley cropping maize and cowpea with the giant Leucaena (Leucaena leucocephala (Lam) de Wit) cultivar K-28 was studied on an Entisol (Psammentic Ustorthent) in Southern Nigeria. In this trial the crops were grown in 4 m wide alleys formed by periodically pruned leucaena hedgerows. The effect of application of leucaena prunings, nitrogen fertilizer and tillage was studied.Despite the very intensive pruning regime (five prunings/year) for a six-year period, the leucaena hedgerows continue to produce substantial amounts of prunings, nitrogen yield and stakes. Application of nitrogen to the maize crop increased dry matter and nitrogen yield from the leucaena prunings. Although high nitrogen yield was obtained from the prunings, the application of low nitrogen rates was still needed for obtaining a high maize yield. Maize grain yield can be sustained at about 2.0 t/ha with continuous application of leucacna prunings only. Without application of leucaena prunings and nitrogen maize yield continued to decline with subsequent croppings. Cowpea grain yield was not affected either by leucaena prunings or by residual nitrogen. Tillage (rototilling) resulted in either higher or the same maize and cowpea yields as compared with no-tillage.Application of leucaena prunings resulted in higher soil moisture retention, organic matter, exchangeable K, Ca, Mg, and also nitrate levels in the soil solution. Leucaena and maize appear to extract soil moisture from different zones in the soil. Timely pruning of hedgerows is necessary to minimize shading.     

Cultivar variation in the effect of chlorsulfuron in depressing the uptake of copper in wheat

The application of herbicides has induced symptoms of nutrient deficiencies under some circumstances. This glasshouse study examined the effect of chlorsulfuron on the uptake and utilization of copper (Cu) in four cultivars of wheat plants (Triticum aestivum L. cvs. Kulin, Cranbrook, Gamenya and Bodallin) on a Cu-responsive soil. Application of chlorsulfuron depressed the concentration of Cu in wheat plants receiving either inadequate or adequate Cu. In plants with inadequate Cu supply, chlorsulfuron increased the severity of Cu deficiency. Shoot weight was markedly decreased by chlorsulfuron at all levels of Cu, through decreasing the number of tillers and the elongation of leaves. This decreased growth of shoots occurred prior to the effect on Cu concentration in tissues. The retranslocation of Cu in old tissues over time was unaffected by chlorsulfuron. In all wheat cultivars, the decreased growth of shoots were correlated with the concentration of Cu in the youngest fully emerged leaf blade with critical levels of 1.6−1.7 at day 25 and 0.9−1.0 μg g⁻¹ d. wt. at day 60. The application of chlorsulfuron tended to increase the critical level at day 25 but not at day 60. In addition, Kulin seems to be most, and Cranbrook least, sensitive to chlorsulfuron. This sensitivity was associated with the sensitivity of the cultivars to Cu deficiency. It is suggested that chlorsulfuron application induces Cu deficiency in wheat plants mainly due to effects on the uptake of Cu. This revised version was published online in June 2006 with corrections to the Cover Date.     

Estimates of the residual nitrogen benefit of groundnut to maize in Northeast Thailand

Four cultivars of groundnut were grown in upland soil in Northeast Thailand to study the residual benefit of the stover to a subsequent maize crop. An N-balance estimate of the total residual N in the maize supplied by the groundnut was made. In addition three independent estimates were made of the residual benefits to maize when the groundnut stover was returned to the land and incorporated. The first estimate (Estimate 1) was an N-balance estimate. A dual labelling approach was used where ¹⁵N-labelled stover was added to unlabelled microplots (Estimate 2) or unlabelled stover was added to ¹⁵N-labelled soil microplots (Estimate 3). The nodulating groundnut cultivars fixed between 59–64% of their nitrogen (as estimated by the ¹⁵N isotope dilution method using non-nodulating groundnut as a non-fixing reference) producing between 100 and 130 kg N ha^-1 in their stover. Although the following maize crop suffered from drought stress, maize grain N and dry weights were up to 80% and 65% greater respectively in the plots where the stover was returned as compared with the plots where the stover was removed. These benefits were comparable with applications of 75 kg N ha^-1 nitrogen in the form of urea. The total residual N estimates of the contribution of the nodulated groundnut to the maize ranged from 16.4–27.5 kg N ha^-1. Estimates of the residual N supplied by the stover and fallen leaves ranged from 11.9–21.3 kg N ha^-1 using the N-balance method (Estimate 1), from 6.3–9.6 kg N ha^-1 with the labelled stover method (Estimate 2) and from 0–11.4 kg N ha^-1 with the labelled soil method. There was closest agreement between the two ¹⁵N based estimates suggesting that apparent added nitrogen interactions in these soils may not be important and that N balance estimates can overestimate the residual N in crops following legumes, even in very poor soils. This work also indicates the considerable ability of local groundnut cultivars to fix atmospheric nitrogen and the potential benefits from returning and incorporating legume residues to the soil in the upland cropping systems of Northeast Thailand. The applicability of the ¹⁵N methodology used here and possible reasons for the discrepancies between estimates 1, 2 and 3 are discussed.     

N recovery from legume prunings and priming effects are governed by the residue quality

Nitrogen recovery from 15N-labelled prunings of Gliricidia sepium, Peltophorum dasyrrachis, Calliandra calothyrsus and Leucaena leucocephala, each of two different chemical qualities, was followed over three cropping cycles in a growth room. Half of the pots of each treatment received a further addition of unlabelled pruning material, from the same species as that previously applied, before the second and third crop cycle. The cumulative maize total N accumulation revealed the largest benefit from N rich, low lignin and polyphenols Gliricidia prunings followed by Leucaena, Calliandra and Peltophorum. Cumulative N recovery measured using 15N over the three crop cycles ranged from 9% from Calliandra prunings to 44% from Gliricidia prunings. The vast majority of this N was recovered during the first crop cycle which agreed well with estimates using the N difference method. Recoveries in the second and third crops ranged from 0.4–5% (15N method) and 6–14% (N difference method) of the N initially applied. The protein binding capacity of polyphenols was the best predictor of N recovery at both initial and later crop cycles. Treatments which led to a large N recovery initially, continued to provide greater N benefits in subsequent cycles although with increasing harvest time this trend decreased. Thus, there was no compensation in initial N release from low quality prunings at later harvests and the agronomic implications of this are discussed. Addition of unlabelled Gliricidia prunings before the second and third cycle led to a positive apparent priming effect on previously applied 15N labelled prunings. By contrast, repeated additions of Peltophorum residues, rich in lignin and active polyphenols, resulted in a reduced recovery of initially applied pruning-15N. However, the maximum positive or negative effects on recovery of pruning N amounted to less than 2% recovery of the initial amount of N added over 14 weeks. Thus the scope for regulation of N release from tree prunings during these later stages of decomposition appears to be limited.     

Turnover of residual 15N-labelled fertilizer N in soil following harvest of oilseed rape (t Brassica napus L.)

We studied the fate of ¹⁵N-labelled fertilizer nitrogen in a sandy loam soil after harvest of winter oilseed rape (Brassica napus L. cv. Ceres) given 100 or 200 kg N ha^-1 in spring, with or without irrigation. Our main objective was to quantify the temporal variations of the soil mineral N, the extractable soil organic N and soil microbial biomass N, and fertilizer derived N in these pools during autumn and winter. Nitrogen use efficiency of the oilseed rape crop varied from 47% of applied N in the 100N, irrigated treatment to 34% in the 200N, non-irrigated treatment. However, only in the latter treatment did we find significantly higher fertilizer derived soil mineral N than in the three other treatments which all had low soil mineral N contents at the first sampling after harvest (8 days after stubble tillage). Between 31% and 42% of the applied N could not be accounted for in the harvested plants or 0-15 cm soil layer at this first sampling. Over the following autumn and winter none of the remaining fertilizer derived soil N was lost from the 0–5 cm depth, but from the 5–15 cm depth a marked proportion of N derived from fertilizer was lost, probably by leaching. Negligible amounts of fertilizer derived extractable soil organic and mineral N (<1 kg N ha^-1, 0-15 cm) were found in all treatments after the first sampling.Soil microbial biomass N was not significantly affected by treatments and showed only small temporal variability (±11% of the mean 76 kg N ha^-1, 0- 15 cm depth). Surprisingly, the average amount of soil microbial biomass N derived from fertilizer was significantly affected by the treatments, with the extremes being 5.5 and 3.1 kg N ha^-1 in the 200N, non-irrigated and 100N, irrigated treatments, respectively. Also, the estimated exponential decay rate of microbial biomass N derived from fertilizer, differed greatly (2 fold) between these two treatments, indicating highly different microbial turnover rates in spite of the similar total microbial biomass N values. In studies utilising ¹⁵N labelling to estimate turnover rates of different soil organic matter pools this finding is of great importance, because it may question the assumption that turnover rates are not affected by the insertion of the label.     

Row spacing effects of N2-fixation,N-yield and soil N uptake of intercropped cowpea and maize

In the tropics, cowpea is often intercropped with maize. Little is known about the effect of the intercropped maize on N₂-fixation by cowpea or how intercropping affects nitrogen fertilizer use effiency or soil N-uptake of both crops. Cowpea and maize were grown as a monocrop at row spacings of 40, 50, 60, 80, and 120 cm and intercropped at row spacing of 40, 50, and 60 cm. Plots were fertilized with 50 kg N as (NH₄)₂SO₄; microplots within each plot received the same amount of¹⁵N-depleted (NH₄)₂SO₄. Using the¹⁵N-dilution method, the percentage of N derived from N₂-fixation by cowpea and the recovery of N-fertilizer and soil N-uptake was measured for both crops at 50 and 80 days after planting.Significant differences in yield and total N for cowpea and maize at both harvest periods were dependent on row spacing and cropping systems. Maize grown at the closer row spacing accumulated most of its N during the first 50 days after planting, whereas maize grown at the widest row spacing accumulated a significant portion of its N during the last 30 days before the final harvest, 80 days after planting.Overall, no significant differences in the percentage of N derived from N₂-fixation for monocropped or intercropped cowpea was observed and between 30 and 50% of its N was derived from N₂.At 50 DAP, fertilizer and soil N uptake was dependent on row spacing with maize grown at the narrowest row spacing having a higher fertilizer and soil N recovery than maize grown at wider spacings. At 50 and 80 DAP, intercropped maize/cowpea did not have a higher fertilizer and soil N uptake than monocropped cowpea or maize at the same row spacing. Monocropped maize and cowpea at the same row spacing took up about the same amount of fertilizer or soil N. When intercropped, maize took up twice as much soil and fertilizer N as cowpea. Apparently intercropped cowpea was not able to maintain its yield potential.Whereas significant differences in total N for maize was observed at 50 and 80 DAP, no significant differences in the atom %¹⁴N excess were observed. Therefore, in this study, the atom %¹⁴N excess of the reference crop was yield independent. Furthermore, the similarity in the atom %¹⁴N excess for intercropped and monocropped maize indicated that transfer of N from the legume to the non-legume was small or not detectable.     

Leaching and crop recovery of15N from ammonium sulphate and labelled maize (Zea mays) material in lysimeters at a site in Zimbabwe

The fate of¹⁵N-ammonium sulphate fertilizer that was applied to four lysimeters in the 1990/91 summer was studied over three consecutive growing seasons during which either maize or wheat was grown. Aboveground portions of¹⁵N-labelled maize plants from the first harvest were applied to four other lysimeters at 5 t ha^–1. Two lysimeters in each of the sets of four were assigned a low and a high moisture treatment using irrigation. In both moisture treatments, plant recovery of fertilizer-¹⁵N in the first season was 27% and a further 2% was recovered by plants during the next two seasons. During the second and third seasons, total recovery of¹⁵N by aboveground plant portions from lysimeters that received¹⁵N-labelled maize material was equivalent to 2.5% of applied fertilizer-¹⁵N. This corresponded to ca. 18% recovery of the¹⁵N added in maize material. Leaching of fertilizer-N over the three growing seasons did not exceed 0.3% in total. During the first season, a maximum of 0.25 kg N ha^–1, equivalent to 0.25% of the applied fertilizer-N, was leached in the high moisture treatment. This represented 1.8% of the nitrate load in leachates. Less than 0.002% of the applied fertilizer-N was leached in the low moisture treatment during the first season.     

The decomposition of sugar beet residues: mineralization versus immobilization in contrasting soil types

The leaves and crowns from ¹⁵N-labelled sugar beets were incubated in either a silty clay loam or sand soil for almost one year. Four additions of fresh, chopped residues mixed with soil were tested: ¹⁵N-labelled leaves alone, ¹⁵N-labelled leaves plus unlabelled crowns, unlabelled leaves plus ¹⁵N-labelled crowns, and ¹⁵N-labelled crowns alone; a control with no addition was also incubated. The C:N ratio of the leaves was 11 and that of the crowns 40. Incubations were carried out in pots kept at 20 °C and optimal moisture conditions. The leaves mineralized N from the start of the experiment but the addition of crowns to soil at first caused immobilization of nitrogen followed eventually by mineralization after 6 or 12 weeks depending on soil type. The extra amounts of mineral N found in soil at the end of the experiment where additions were made corresponded to the sum of the background mineralization and the addition; no priming effects were encountered. Very slight differences only were found between the initial rates of mineralization of C in all of the treatments. Although there was also little difference between the sand and silty clay loam soils in the direct mineralization of nitrogen from the sugar beet leaves, where N was first immobilized (i.e. from crowns or a mixture) re-release of N took place more quickly in the sand soil. The total recovery of¹⁵ N found in soils after 24 weeks incubation ranged from 70% to 90% with least being lost from the sugar-rich but N-deficient crowns. Where leaves plus crowns were incubated together both residues contributed to the microbial biomass N.In practice, immobilization of this magnitude and duration (expressed as a temperature sum) could exceed the growth period of a spring sown crop. The actual immobilization found in any one field is likely to depend on the C:N ratio of the residues and could account for much of the variation in the residual benefit of sugar beet residues reported in the literature.     

Nitrogen fixation by groundnut and soyabean and residual nitrogen benefits to rice in farmers' fields in Northeast Thailand

Nitrogen fixation in groundnut and soyabean and the residual benefits of incoporated legume stover to subsequent rice crops were estimated in farmers' fields using¹⁵N-isotope methods. Three field experiments were conducted, two which examined N₂-fixation in groundnut by¹⁵N-isotope dilution using a non-nodulating groundnut as a reference crop and one in which N₂-fixation in two soyabean genotypes was compared using maize as the non-fixing reference crop. Groundnut fixed 72–77% of its N amounting to 150–200 kg N ha^-1 in 106–119 days and soyabean derived 66–68% of its N from N₂-fixation which amounted to 108–152 kg N ha^-1 under similar conditions. When legume stover was returned to the soil, there was a net contribution of N from N₂-fixing varieties of groundnut in all cases ranging from 13–100 kg N ha^-1, whilst due to the high % N harvest index in soyabean (87–88%) there was a net removal of N of 37–46 kg N ha^-1. In all cases if the legume stover was removed there was a net removal of N in the legume crop which ranged between 54 and 74 kg N ha^-1 in N₂-fixing varieties of groundnut and from 58 to 73 kg N ha^-1 in soyabean, whilst maize removed 66 kg N ha^-1 if its stover was returned and 101 kg N ha^-1 when the stover was removed. Growth of rice was improved in all cases where groundnut stover was returned resulting in increases in grain yield of 12–26% and increases in total dry matter production of 26–31%. Soyabean residues gave no increases in rice grain yield but increased total dry matter production by 12–20%. Rice accumulated more N in all cases where legume stover was returned to the soil, and N yields were larger in all cases after the N₂-fixing legumes than after the non-fixing reference crops. N difference estimates of the total residual N benefits from the N₂-fixing legumes ranged from 11–19 kg N ha^-1 after groundnut and 15–16 kg N ha^-1 after soyabean. The amounts of N estimated directly by application of¹⁵N-labelled stover amounted to 7.2–20.5 kg N ha^-1 with groundnut which represented recovery of 8–22% of the N added in the stover. In soyabean only 3.0–5.8 kg N ha^-1 was estimated to be recovered by¹⁵N-labelling which was 15–23% of the added N, whilst only 1.3 kg N ha^-1 (4% of the N added) was recovered by rice from the maize stover. An indirect¹⁵N-method based on addition of unlabelled stover to microplots where the soil had previously been labelled with¹⁵N gave extremely variable and often negative estimates of residual N benefits. Estimates of residual N from the added stover made by N difference calculations did not correspond with the estimates by direct¹⁵N-labelling in all cases and possible reasons for this are discussed.     

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.     

Nitrogen release from incorporated 15N-labelled Chinese milk vetch (Astragalus sinicus L.) residue and its dynamics in a double rice cropping system

Background and aims

Chinese milk vetch (Astragalus sinicus L. CMV), a leguminous cover crop, has been shown to provide N benefits to rice crops, but little is known about the pathway of incorporated CMV and its N dynamics. In this study, effects of CMV under different application treatments (incorporated alone, applied in conjunction with urea fertilizer and applied with ryegrass (Lolium multiflorum Lam.)) on N dynamics, rice yields and N uptake were investigated and compared with those of chemical fertilizer (CF) and no fertilizer (NF) in a double rice cropping system.

Methods

Nitrogen release from incorporated CMV residue was quantified by using a bag method. Nitrogen dynamics of CMV were evaluated by using ¹⁵N-labelled fresh CMV tops and compared with those of CF (¹⁵N-labelled urea).

Results

CMV residue decomposition pattern and its N release pattern followed a single exponential decay model, with 87.8–89.5 % of the applied CMV decomposed and 95.1–96.1 % of the original N released in the double rice season (177 days after fertilizer application). CMV treatments had higher rice N uptake efficiency than CF (39.2–51.3 % vs. 29.9 %) at the sum of early and late rice seasons. Rice yield, N accumulation and mineral fertilizer efficiency in CMV treated treatments were higher than those in CF. After two consecutive rice seasons the amounts of residual N remained in the soil were higher in the CMV treated fields than in CF (29.4–33.2 % vs. 14.1 %).

Conclusions

CMV can be considered an efficient N source alternative to chemical fertilizer in double rice cropping systems.     

Nitrogen recoveries from organic amendments in crop and soil assessed by isotope techniques under tropical field conditions

The integration of multipurpose legumes into low-input tropical agricultural systems is needed because they are a nitrogen (N) input through symbiotic fixation. The drought-tolerant cover legume canavalia (Canavalia brasiliensis) has been introduced for use either as forage or as a green manure into the crop-livestock system of the Nicaraguan hillsides. To evaluate its impact on the subsequent maize crop, an in-depth study on N dynamics in the soil-plant system was conducted. Microplots were installed in a 6-year old field experiment with maize-canavalia rotation. Direct and indirect ¹⁵N-labelling techniques were used to determine N uptake by maize from canavalia residues and canavalia-fed cows?? manure compared to mineral fertilizer. Litter bags were used to determine the N release from canavalia residues. The incorporation of N from the amendment into different soil N pools (total N, mineral N, microbial biomass) was followed during the maize cropping season. Maize took up an average of 13.3 g?N?m^?2, within which 1.0 g?N?m^?2 was from canavalia residues and 2.6 g?N?m^?2 was from mineral fertilizer, corresponding to an amendment N recovery of 12% and 32%, respectively. Recoveries in maize would probably be higher at a site with lower soil available N content. Most of the amendment N remained in the soil. Mineral N and microbial N were composed mainly of N derived from the soil. Combined total ¹⁵N recovery in maize and soil at harvest was highest for the canavalia residue treatment with 98% recovery, followed by the mineral fertilizer treatment with 83% recovery. Despite similar initial enrichment of soil microbial and mineral N pools, the indirect labelling technique failed to assess the N fertilizer value of mineral and organic amendments due to a high N mineralization from the soil organic matter.     

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.     

Fate of 15N-labelled nitrogen fertilizer applied to kiwifruit (Actinidia deliciosa) vines

The fate of ¹⁵N-labelled ammonium fertilizer applied once to six-year-old field-grown kiwifruit (Actinidia deliciosa Hayward) vines was measured over three years. The three main treatments were nitrogen (N) applied singularly at 100 or 200 kg N ha^–1 in early spring (two weeks before bud burst) or split with 100 kg N ha^–1 (unlabelled) in early spring and 100 kg N ha^–1 (¹⁵N-labelled) ten weeks later. All N treatments were applied to vines with a history of either 50 or 200 kg N ha^–1 yr^–1. For three years after ¹⁵n application, components of the vines and soil (0–600 mm depth) were sampled at harvest in late autumn and the N and ¹⁵N contents determined.By the first harvest, all plant uptake of ¹⁵N had occurred and this represented 48–53% of the ¹⁵N applied. There was no significant effect of current N fertilizer treatment or of N history on ¹⁵N recovery by vines. Removal of ¹⁵N in harvested fruit was small at 5–6% in the first year and 8% over 3 years. After 2–3 years, most plant ¹⁵N occurred in the roots and this component declined only slowly over time. In contrast, there was a large temporal decline in ¹⁵N in above-ground plant components due to the annual removal in leaf fall and pruning. An associated experiment showed that when ¹⁵N-labelled prunings and leaves were mulched and returned to the soil, only about 9% was recovered by plants within 2 years. Almost all remaining mulched material had been immobilised into the soil organic N.In all treatments, about 20% of the added ¹⁵N remained in soil at the first harvest. This was almost entirely in organic fractions (<0.4% in inorganic N) and mostly in the surface 150-mm layer. The ¹⁵N content in soil changed little over time (from 20 to 17% between the first and third harvests respectively) and indicated that most of the N had been immobilised into stable humus forms.     

Fate of nitrogen applied as Azolla and blue-green algae (Cyanobacteria) in waterlogged rice soils—a15N tracer study

Summary ¹⁵N tracer was used to detect the extent to which nitrogen of appliedAzolla caroliniana, Anabaena variabilis andNostoc muscorum was available for assimilation by the growing rice plants in pots under 4 cm flood water for 60 days. The rate of release of nitrogen from the above biofertilizers, the amount of nitrogen remaining in the soils and the amount that was lost from the soils during this period were also examined. Previously¹⁵N-labelled biomass of Azolla, Anabaena and Nostoc to provide 40 mg N was mixed thoroughly with 0.5 kg silt loam Bangladesh soil (Sonatola series) in each of three pots used for a single treatment. Each pot received four 16 days old IR8 rice seedlings. A parallet set of experiments was conducted without rice plants.It was found that nitrogen uptake in the rice plants was increased by 91, 176 and 215% on using Azolla, Anabaena and Nostoc which resulted in increased total dry matter yields (shoot plus root) of 74, 105 and 125%, respectively. Of the total¹⁵N applied at the start, 26, 49 and 53% was released from Azolla, Anabaena and Nostoc; about 7, 14 and 13% was lost by denitrification and 74, 51 and 47% remained in the soils as the undecomposed part of the biofertilizers, respeciively, after 60 days. Of 15.76, 22.72 and 25.92 mg N assimilated by the rice plants, 48, 61 and 62% was supplied by Azolla, Anabaena and Nostoc, respectively. The rest was obtained from the soil used.In the absence of the rice plants 30, 43 and 45% of applied¹⁵N of Azolla, Anabaena and Nostoc was released, respectively, in 60 days of which 93–96% was lost as N₂ through denitrification.     

Decomposition of <Superscript>15</Superscript>N-labelled beech litter and fate of nitrogen derived from litter in a beech forest

The decomposition and the fate of ¹⁵N- labelled beech litter was monitored in a beech forest (Vosges mountains, France) over 3 years. Circular plots around beech trees were isolated from neighbouring tree roots by soil trenching. After removal of the litter layer, ¹⁵N-labelled litter was distributed on the soil. Samples [labelled litter, soil (0–15 cm depths], fine roots, mycorrhizal root tips, leaves) were collected during the subsequent vegetation periods and analysed for total N and ¹⁵N concentration. Mass loss of the ¹⁵N-labelled litter was estimated using mass loss data from a litterbag experiment set up at the field site. An initial and rapid release of soluble N from the decomposing litter was balanced by the incorporation of exogenous N into the litter. Fungal N accounted for approximately 35% of the N incorporation. Over 2 years, litter N was continuously released and rates of N and mass loss were equivalent, while litter N was preferentially lost during the 3rd year. Released ¹⁵N accumulated essentially at the soil surface. ¹⁵N from the decomposing litter was rapidly (i.e. in 6 months) detected in roots and beech leaves and its level increased regularly and linearly over the course of the labelling experiment. After 3 years, about 2% of the original litter N had accumulated in the trees. ¹⁵N budgets indicated that soluble N was the main source for soil microbial biomass. Nitrogen accumulated in storage compounds was the main source of leaf N, while soil organic N was the main source of mycorrhizal N. Use of ¹⁵N-labelled beech litter as decomposing substrate allowed assessment of the fate of litter N in the soil and tree N pools in a beech forest on different time scales. Received: 3 May 1999 / Accepted: 3 January 2000     

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.     

Soil nitrogen heterogeneity in a Dehesa ecosystem

The C mineralization and N transformations during the decomposition of sunflower stalks (Helianthus annuus L.) and wheat straw (Triticum aestivum L.) with and without addition of (NH₄)₂SO₄ (27.53 atom% ¹⁵N) were studied in a Vertisol. Soil samples were incubated under aerobic conditions for 224 days at 22 °C. The plant residues were added at a rate of 5.2 g kg^-1 soil. Nitrogen was applied at a rate of 50.7 mg N kg^-1 soil. Carbon dioxide emission and inorganic N content in soil were periodically determined. Gross N immobilization and remineralization were calculated on the basis of the isotopic dilution technique. At the end of the incubation period a ¹⁵N balance was established. Respectively, 68 and 45% of the applied residue-C mineralized from the sunflower stalks and wheat straw after 224 days. Both crop residues caused losses of up to 25% of added ¹⁵N after 224 days of incubation. These ¹⁵N losses were about three times larger than in the control soil, and were probably due to denitrification. The net immobilization of soil derived N following residue incorporation was largest in the case of wheat straw, depleting all soil inorganic N. In the wheat straw treatment with added (NH₄)₂SO₄ soil inorganic N remained available, resulting in an enhanced initial C mineralization and N immobilization compared to the treatment without added N. In the case of the sunflower stalks, the high inorganic N content of the stalks suppressed the effects of N addition on C mineralization and N immobilization/mineralization. Gross N immobilization amounted to 31.9 and 28.2 mg N g^-1 added C after 14 days for wheat straw and sunflower stalks, respectively. At the end of the incubation, about 35% of the newly immobilized N was remineralized in both plant residue treatments. Gross N immobilization plotted against decomposed C suggests that fairly uniform C-N relationships exist during the decomposition of divers C substrates. The results demonstrate that low fertilizer N use efficiencies may be expected in a wheat-sunflower cropping system with incorporation of crop residues, as the fertilizer N applied becomes largely immobilized in the soil organic fraction. This revised version was published online in August 2006 with corrections to the Cover Date.