Immobilisation, remineralisation and residual effects in subsequent crops of dairy cattle slurry nitrogen compared to mineral fertiliser nitrogen

About 50–60% of dairy cattle slurry nitrogen is ammonium N. Part of the ammonium N in cattle slurry is immobilised due to microbial decomposition of organic matter in the slurry after application to soil. The immobilisation and the remineralisation influence the fertiliser value of slurry N and the amount of organic N that is retained in soil. The immobilisation and the remineralisation of 15 N-labelled dairy cattle slurry NH₄-N were studied through three growing seasons after spring application under temperate conditions. Effects of slurry distribution (mixing, layer incorporation, injection, surface-banding) and extra litter straw in the slurry on the plant utilisation of labelled NH₄-N from slurry were studied and compared to the utilisation of ¹⁵N-labelled mineral fertiliser. The initial immobilisation of slurry N was influenced by the slurry distribution in soil. More N was immobilised when the slurry was mixed with soil. Surface-banding of slurry resulted in significant volatilisation losses and less residual ¹⁵N in soil. Much more N was immobilised after slurry incorporation than after mineral fertiliser application. After 2.5 years the recovery of labelled N in soil (0–25 cm) was 46% for slurry mixed with soil, 42% for injected slurry, 22% for surface-banded slurry and 24% for mineral fertiliser N. The total N uptake in a ryegrass cover crop was 5–10 kg N/ha higher in the autumn after spring-application of cattle slurry (100–120 kg NH₄-N/ha) compared to the mineral fertiliser N reference, but the immobilised slurry N (labelled N) only contributed little to the extra N uptake in the autumn. Even in the second autumn after slurry application there was an extra N uptake in the cover crop (0–10 kg N/ha). The residual effect of the cattle slurry on spring barley N uptake was insignificant in the year after slurry application (equivalent to 3% of total slurry N). Eighteen months after application, 13% of the residual ¹⁵N in soil was found in microbial biomass whether it derived from slurry or mineral fertiliser, but the remineralisation rate (% crop removal of residual ¹⁵N) was higher for fertiliser- than for slurry-derived N, except after surface-banding. Extra litter straw in the slurry had a negligible influence on the residual N effects in the year after application. It is concluded that a significant part of the organic N retained in soil after cattle slurry application is derived from immobilised ammonium N, but already a few months after application immobilised N is stabilised and only slowly released. The immobilised N has negligible influence on the residual N effect of cattle slurry in the first years after slurry application, and mainly contributes to the long-term accumulation of organic N in soil together with part of the organic slurry N. Under humid temperate conditions the residual N effects of the manure can only be optimally utilised when soil is also covered by plants in the autumn, because a significant part of the residual N is released in the autumn, and there is a higher risk of N leaching losses on soils that receive cattle slurry regularly compared to soils receiving only mineral N fertilisers.     

Nitrogen release from eucalypt leaves and legume residues as influenced by their biochemical quality and degree of contact with soil

Large areas of short-rotation eucalypt plantations are being established in south-western Australia on land previously used for agriculture. Options for maintaining soil N supply include retention of harvest residues and legume inter-cropping. We evaluated the effects of adding the residues of five legume species and Eucalyptus globulusleaves on inorganic N dynamics in two soils (a Rhodic Ferralsol or red earth and a Haplic Podzol or grey sand) using two modes of residue application in a laboratory incubation experiment (519 days). The time course of net N immobilisation and mineralisation in both soils was strongly influenced by the type and mode of application. Eucalypt leaves caused strong N immobilisation (–7 mg N g^–1 residue-C) over the entire 519-day incubation, whereas for the legume species, N that was eventually immobilised at the start of the incubation, remineralised later to different degrees. Amongst the legumes, largest amounts of N were released from lupin residues (18 mg N g^–1 residue-C) and lowest amounts from field pea (2 mg N g^–1 residue-C). However, initial residue quality parameters were not significantly (P > 0.05) correlated with N release from the residues. Grinding and incorporating of the residues caused a much greater immobilisation of N than when residues were cut and surface applied. When ground residues were incorporated, immobilisation of N was more severe and endured for longer in the finer textured red earth than in the coarse textured grey sand. Where residues were surface applied, N dynamics were similar for both soil types. The results of this study suggest that legumes used as a mulch in eucalypt plantations are a readily available source of N for trees, and that the benefits from retention of harvest residues are more likely in maintaining soil N fertility on the long-term.     

Decomposition and nitrogen dynamics of wheat residues and impact on wheat growth stages

N mineralisation and immobilisation were quantified in field conditions in the presence or in the absence of wheat residues. The incubation study was conducted in cylinders placed in microplots (no plants were grown in cylinders), and the rest of each microplot was sowed with the wheat crop (Triticum durum var. Massa). N mineralisation and immobilisation depend on the presence or the absence of wheat residues. In absence of residues, a linear model of regression was developed to follow the clear nitrogen mineralisation at different soil levels. Nitrogen mineralisation (mg kg-1), during the five months of wheat development, showed the following decreasing order: 0-15 cm (132.6) > 15-30 cm (120.6) > 30-45 cm (91.3). The mineralisation rate was 24.1, 22.9 and 18.9 mg kg-1 d-1 for 0-15, 15-30 and 30-45 cm levels, respectively. The supply of wheat residues resulted in a five months N immobilisation process. At level 0-15 cm the immobilisation (mg kg-1) showed the following decreasing order: (61.6) > (46.4) > (30.0) for the supply of wheat residues at seeding time, and 15 and 30 d before seeding respectively. At the other levels, the same decreasing order was recorded. The supply of 8 t ha-1 of wheat residues at seeding time, and 15 or 30 d before seeding, decreased the dry matter yield and N accumulation in wheat crop. In consequence, there was no synchronism between the nitrogen liberated by wheat residues decomposition and the wheat growth.     

Influence of biochemical quality on C and N mineralisation from a broad variety of plant materials in soil

We studied C and N mineralisation patterns from a large number of plant materials (76 samples, covering 37 species and several plant parts), and quantified how these patterns related to biological origin and selected indicators of chemical composition. We determined C and N contents of whole plant material, in water soluble material and in fractions (neutral detergent soluble material, cellulose, hemicellulose and lignin) obtained by stepwise chemical digestion (modified van Soest method). Plant materials were incubated in a sandy soil under standardised conditions (15 °C, optimal water content, no N limitation) for 217days, and CO₂ evolution and soil mineral N contents were monitored regularly. The chemical composition of the plant materials was very diverse, as indicated by total N ranging from 2 to 59 mg N g^–1, (i.e. C/N-ratios between 7 and 227). Few materials were lignified (median lignin=4% of total C). A large proportion of plant N was found in the neutral detergent soluble (NDS) fraction (average 84%) but less of the plant C (average 46%). Over the entire incubation period, holocellulose C content was the single factor that best explained the variability of C mineralisation (r=–0.73 to –0.82). Overall, lignin C explained only a small proportion of the variability in C mineralisation (r=–0.44 to –0.51), but the higher the lignin content, the narrower the range of cumulative C mineralisation. Initial net N mineralisation rate was most closely correlated (r=0.76) to water soluble N content of the plant materials, but from Day 22, net N mineralisation was most closely correlated to total plant N and NDS-N contents (r varied between 0.90 and 0.94). The NDS-N content could thus be used to roughly categorise the net N mineralisation patterns into (i) sustained net N immobilisation for several months; (ii) initial net N immobilisation, followed by some re-mineralisation; and (iii) initially rapid and substantial net N mineralisation. Contrary to other studies, we did not find plant residue C/N or lignin/N-ratio to be closely correlated to decomposition and N mineralisation.     

The fate of carbon and fertiliser nitrogen when dryland wheat is grown in monoliths of duplex soil

Triticum aestivum L. (cv. Gutha), a short-season wheat, was grown to maturity in large monoliths of duplex soil (sand over sandy-clay) in a daylight phytotron mimicking field conditions. Either ¹⁵N-labelled ammonium sulphate ((NH₄)₂SO₄) or urea was banded into the soil at a rate of 30 kg N ha^–1: even though roots were about 20% heavier when grown in the presence of (NH₄)₂SO₄ for 86 d (P<0.05), above-ground mass was not affected by the source of nitrogen. At four times through crop development up to grain-filling (50, 56, 70 and 86 d after sowing) shoots were labelled heavily with ¹⁴CO₂ with two purposes. First, to trace `instantaneous' assimilate movement over 24 h, revealing relative sink strengths throughout plants. This, in turn, allowed precise measurements of live root mass and the proportion of recent photoassimilates deposited in the rhizosphere. Although root systems were sparse, even in surface soil layers, they were strong sinks for photoassimilates early in development (0–50 d), supporting the conversion of inorganic applied nitrogen (N) to soil organic forms. In the presence of roots, up to 28% of ¹⁵N was immobilised, whereas only 12% of labelled ammonium sulphate was immobilised in unplanted plots in spite of a favourable moisture status in both treatments. The effect of plants on rates of ¹⁵N transformation is ascribed to recently imported photoassimilates sustaining rhizosphere metabolism. Not more than 15% of recently fixed carbon imported by roots was recovered from the rhizoplane, suggesting that a highly localised microbial biomass supported vigorous immobilisation of soil N. Thus, more than twice as much applied N was destined for soil organic fractions as for root material. By these processes, root- and soil-immobilised N become substantial stores of applied N and together with shoot N accounted for all the applied N under dryland conditions.     

Nitrogen dynamics in humus and soil beneath Sitka spruce (Picea sitchensis (Bong.) Carr.) planted in pure stands and in mixture with Scots pine (Pinus sylvestris L.)

Sitka spruce planted on nutrient-poor soils in mixture with pine or larch, unlike pure spruce, does not become N deficient and does not require N fertilizer. To test the hypothesis that N availability in the soil is enhanced beneath mixed species, the seasonal changes in different N forms were compared in humus (L+F+H) and soil beneath 15-year-old Sitka spruce (SS) and mixed Sitka spruce-Scots pine (SS and SP) planted on a gleyed heathland soil. Amounts of mineral and organic N extracted from humus in spring were significantly (p < 0.05) higher in SS and SP than in SS. Larger amounts were measured in the underlying soil, which favoured the deeper-rooting spruce and pine in SS and SP plots. Annual net N mineralization, measured by in-situ incubation, was 32 and 47 kg N ha^-1 in the surface 10 cm beneath SS and (SS and SP), respectively. In spring, readily mineralized organic N (waterlogged incubation at 30°C) was higher in humus and soil from (SS and SP) than from SS by 15 kg N ha^-1. The larger N pools beneath (SS and SP) were consistent with the higher total N content of the humus beneath (SS and SP), 446 compared with 255 kg N ha^-1 beneath SS. This indicated that beneath (SS and SP) N had been transferred from the underlying soil.     

Nitrogen mineralisation in podzol soils under boreal Scots pine and Norway spruce stands

N mineralisation was investigated in the mor humus layer of a podzol at a forested catchment area of Saarejärve Lake in Eastern Estonia. The investigated areas were pine (Rhodococcumunderstorey) and spruce (Vaccinium understorey) stands, which are permanent sample plots of an integrated monitoring network. The seasonal pattern of net N mineralisation was studied by incubating undisturbed cores of mor humus (0–8 cm) in buried polyethylene bags in situ. Samples were collected and incubated between July 1996 and April 1998. The period of incubation was approximately 1 month, except for wintertime when incubation lasted till thawing of ground (5 months). The amounts of mineral nitrogen formed during monthly incubations in vegetation period vary considerably (0.4–8.7 kg ha^–1). About 70% of the variation of net ammonification could be explained by environmental factors - temperature, initial moisture and pH. Ammonium was the dominant form of mineral nitrogen, which is typical for mor humus. The rate of nitrification was very low, and most of the annual net nitrification occurred during just one or two months (May–June, October) depending on site and year. Measured annual net N mineralisation was 29.2 kg ha^–1 for the spruce stand and 23.6 kg ha^–1 for the pine stand. These measures were found to be in good accordance with other N-fluxes in the ecosystem.     

Modelling the effect of active roots on soil organic matter turnover

The aim of this experiment was to study the effect of living roots on soil carbon metabolism at different decomposition stages during a long-term incubation. Plant material labelled with ¹⁴C and ¹⁵N was incubated in two contrasting soils under controlled laboratory conditions, over two years. Half the samples were cropped with wheat (Triticum aestivum) 11 times in succession. At earing time the wheat was harvested, the roots were extracted from the soil and a new crop was started. Thus the soils were continuously occupied by active root systems. The other half of the samples was maintained bare, without plants under the same conditions. Over the 2 years, pairs of cropped and bare soils were analysed at eight sampling occasions (total-, plant debris-, and microbial biomass-C and -¹⁴C). A five compartment (labile and recalcitrant plant residues, labile microbial metabolites, microbial biomass and stabilised humified compounds) decomposition model was fitted to the labelled and soil native organic matter data of the bare and cropped soils. Two different phases in the decomposition processes showed a different plant effect. (1) During the initial fast decomposition stage, labile ¹⁴C-material stimulated microbial activities and N immobilisation, increasing the ¹⁴C-microbial biomass. In the presence of living roots, competition between micro-organisms and plants for inorganic N weakly lowered the measured and predicted total-¹⁴C mineralisation and resulted in a lower plant productivity compared to subsequent growths. (2) In contrast, beyond 3–6 months, when the labile material was exhausted, during the slow decomposition stage, the presence of living roots stimulated the mineralisation of the recalcitrant plant residue-¹⁴C in the sandy soil and of the humified-¹⁴C in the clay soil. In the sandy soil, the presence of roots also substantially stimulated decomposition of old soil native humus compounds. During this slow decomposition stage, the measured and predicted plant induced decrease in total-¹⁴C and -C was essentially explained by the predicted decrease in humus-¹⁴C and -C. The ¹⁴C-microbial biomass (MB) partly decayed or became inactive in the bare soils, whereas in the rooted soils, the labelled MB turnover was accelerated: the MB-¹⁴C was replaced by unlabelled-C from C derived from living roots. At the end of experiment, the MB-C in the cropped soils was 2.5–3 times higher than in the bare soils. To sustain this biomass and activity, the model predicted a daily root derived C input (rhizodeposition), amounting to 5.4 and 3.2% of the plant biomass-C or estimated at 46 and 41% of the daily net assimilated C (shoot + root + rhizodeposition C) in the clay and sandy soil, respectively. This revised version was published online in June 2006 with corrections to the Cover Date.     

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

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

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

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

Short-term immobilisation and crop uptake of fertiliser nitrogen applied to winter wheat: effect of date of application in spring

Previous studies on the fate of fertiliser nitrogen applied to winter wheat in temperate climates have shown that nitrogen (N) applied early, at tillering for wheat, was less efficiently taken up than N applied later in the growth cycle. We examined the extent to which the soil microbial N immobilisation varied during the wheat spring growth cycle and how microbial immobilisation and plant uptake competed for nitrogen. We set up a pulse-15N labelled field experiment in which N was applied at eight development stages from tillering (beginning of March) to anthesis (mid-June). Each application was 50 kg N ha-1 as 15N labelled urea except for the first application which was 25 kg N ha-1. The distribution of fertiliser 15N in shoots, roots, mineral and organic soil N was examined by destructive sampling 7 and 14 days after each 15N pulse. The inorganic 15N pool was almost depleted by day 14. The N uptake efficiency increased with later applications from 45% at tillering to 65% at flowering. N immobilisation was rather constant at 13–16% of N applied, whatever the date of application. The increase in plant 15N uptake resulted in an increase in the total 15N recovery in the plant-soil system (15N in soil +15N in plant), suggesting that gaseous losses were lower at the later application dates.     

Mineralisation of nitrogen from an incorporated catch crop at low temperatures: experiment and simulation

The release of nitrogen from incorporated catch crop material in winter is strongly influenced by soil temperatures. A laboratory experiment was carried out to investigate this influence in the range of 1-15 °C. Samples of sandy soil or a mixture of sandy soil with rye shoots were incubated at 1-5-10-15 °C, and samples of sandy soil with rye roots were incubated at 5-10-15 °C. Concentrations of N_min (NH₄ ⁺-N and NO₃ ^--N) were measured after 0-1-2-4-7-10 weeks for the sandy soil and the sandy soil:rye shoot mixture, and after 0-2-7-10 weeks for the sandy soil:rye root mixture. At 1 °C, 20% of total organic N in the crop material had been mineralised after ten weeks, indicating that mineralisation at low temperatures is not negligible. Maximum mineralisation occurred at 15 °C; after ten weeks, it was 39% of total applied organic nitrogen from shoot and 35% from root material. The time course of mineralisation was calculated using an exponential decay function. It was found that the influence of temperature in the range 1-15 °C could be described by the Arrhenius equation, stating a linear increase of ln(k) with T^-1, k being the relative mineralisation rate in day^-1 and T the temperature (°C). A simulation model was developed in which decomposition, mineralisation and nitrification were modelled as one step processes, following first order kinetics. The relative decomposition rate was influenced by soil temperature and soil moisture content, and the mineralisation of N was calculated from the decomposition of C, the C to N ratio of the catch crop material and the C to N ratio of the microbial biomass. The model was validated first with the results of the experiment. The model was further validated with the results of an independent field experiment, with temperatures fluctuating between 3 and 20 °C. The simulated time course of mineralisation differed significantly from the experimental values, due to an underestimation of the mineralisation during the first weeks of incubation.     

The fate of fresh and stored 15N-labelled sheep urine and urea applied to a sandy and a sandy loam soil using different application strategies

The fate of nitrogen from ¹⁵N-labelled sheep urine and urea applied to two soils was studied under field conditions. Labelled and stored urine equivalent to 204 kg N ha^–1 was either incorporated in soil or applied to the soil surface prior to sowing of Italian ryegrass (Lolium multiflorum L.), or it was applied to ryegrass one month after sowing. In a sandy loam soil, 62% of the incorporated urine N and 78% of the incorporated urea N was recovered in three cuts of herbage after 5 months. In a sandy soil, 51–53% of the labelled N was recovered in the herbage and the distribution of labelled N in plant and soil was not significantly different for incorporated urine and urea. Almost all the supplied labelled N was accounted for in soil and herbage in the sandy loam soil, whereas 33–34% of the labelled N was unaccounted for in the sandy soil. When the stored urine was applied to the soil surface, 20–24% less labelled N was recovered in herbage plus soil compared to the treatments where urine or urea were incorporated, irrespective of soil type. After a simulated urination on grass, 69% of the labelled urine N was recovered in herbage and 15% of the labelled N was unaccounted for. The labelled N unaccounted for was probably mainly lost by ammonia volatilization.Significantly more urine- than urea-derived N (36 and 19%, respectively) was immobilized in the sandy loam soil, whereas the immobilization of N from urea and urine was similar in the sandy soil (13–16%). The distribution of urine N, whether incorporated or applied to the soil surface prior to sowing, did not influence the immobilization of labelled urine N in soil. The immobilization of urine-derived N was also similar whether the urine was applied alone or in an animal slurry consisting of labelled urine and unlabelled faecal N. When urine was applied to growing ryegrass at the sandy loam soil, the immobilization of urine-derived N was significantly reduced compared to application prior to sowing. The results indicated that the net mineralization of urine N was similar to that of urea in the sandy soil, but only about 75% of the urine N was net mineralized in the sandy loam soil, when urine was applied prior to sowing. Thus, the fertilizer effect of urine N may be significantly lower than that of urea N on fine-textured soils, even when gaseous losses of urine N are negligible.     

Original Articles: Dynamic Redistribution of Isotopically Labeled Cohorts of Nitrogen Inputs in Two Temperate Forests

We compared simulated time series of nitrogen-15 (¹⁵N) redistribution following a large-scale labeling experiment against field recoveries of ¹⁵NH₄ ⁺ and ¹⁵NO₃ ⁻ in vegetation tissues. We sought to gain insight into the altered modes of N cycling under long-term, experimentally elevated N inputs. The study took place in two contrasting forests: a red pine stand and a mixed deciduous stand (predominantly oak) at the Harvard Forest, Massachusetts, USA. We used TRACE, a dynamic simulation model of ecosystem biogeochemistry that includes ¹⁵N/¹⁴N ratios in N pools and fluxes. We simulated input–output and internal fluxes of N, tracing the labeled cohorts of N inputs through ecosystem pools for one decade. TRACE simulated the peaks and timing of ¹⁵N recovery in foliage well, providing a key link between modeling and field studies. Recovery of tracers in fine roots was captured less well. The model was structured to provide rapid, initial sinks for ¹⁵NO₃ ⁻ and ¹⁵NH₄ ⁺ in both forests, as indicated by field data. In simulations, N in litter turned over rapidly, even as humus provided a long-term sink for rapidly cycling N. This sink was greater in the oak forest. Plant uptake fluxes of N in these fertilized plots were on the same order of magnitude as net assimilation fluxes in forest-floor humus. A striking result was the small rate of incorporation of N in humus resulting from the transfer of litter material to humus, compared with large fluxes of N into humus and its associated microorganisms through direct transfers from pools of inorganic N in soils. Received 19 May 1998; accepted 30 September 1998     

Estimation of biological nitrogen fixation associated withBrachiaria andPaspalum grasses using15N labelled organic matter and fertilizer

Summary Six pasture grasses,Paspalum notatum cv batatais,P. notatum cv pensacola,Brachiaria radicans, B. ruziziensis, B. decumbens andB. humidicola, were grown in concrete cylinders (60 cm diameter) in the field for 31 months. The soil was amended with either a single addition of¹⁵N labelled organic matter or frequent small (2 kg N. ha^–1) additions of¹⁵N enriched (NH₄)₂SO₄. In the labelled fertilizer treatment soil analysis revealed that there was a very drastic change in¹⁵N enrichment in plant-available nitrogen (NO ₃ ^– +NH ₄ ⁺ ) with depth. The different grass cultivars recovered different quantities of applied labelled N, and evidence was obtained to suggest that the roots exploited the soil to different depths thus obtaining different¹⁵N enrichments in soil derived N. This invalidated the application of the isotope dilution technique to estimate the contribution of nitrogen fixation to the grass cultivars in this treatment. In the labelled organic matter treatment the¹⁵N label in the plant-available N declined at a decreasing rate during the experiment until in the last 12 months the decrease was only from 0.274 to 0.222 atom % excess. There was little change in¹⁵N enrichment of available N with depth, hence it was concluded that although the grasses recovered different quantities of labelled N, they all obtained virtually the same¹⁵N enrichment in soil derived N. Data from the final harvests of this treatment indicated thatB. humidicola andB. decumbens obtained 30 and 40% respectively of their nitrogen from N₂ fixation amounting to an input of 30 and 45 kg N.ha^–1 year^–1 respectively.     

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.     

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.     

Nitrogen metabolism of young barley plants as affected by NaCl-salinity and potassium

Summary In a solution culture experiment with 31 days old barley plants (var. Miura) the influence of NaCl-salinization (80 mM) and KCl addition (5 and 10 mM) on the uptake and turnover of labelled nitrogen (¹⁵NH₄ ¹⁵NO₃) was studied. Labelled N was applied for 24 h at the end of a 20 days' salinization period. Salinization impaired growth and incorporation of labelled N into the protein fraction paralleled by accumulation of labelle dinorganic N. All salt effects were much more pronounced in the shoots than in the roots.Potassium addition enhanced N uptake (total¹⁵N-content) and incorporation into protien, reduced the accumulation of inorganic N and improved the growth of salinized plants.The presented data support the point of view that impairment of protein (enzyme) metabolism is an important aspect of salt stress which is probably induced by the disturbance of the K/Na balance of the tissues under saline conditions.This work was supported by a grant from the Alexander von Humboldt foundation.     

Relationship between rate of crop growth at date of fertiliser N application and fate of fertiliser N applied to winter wheat

To investigate the relationship between the timing of fertiliser N applications and the N use efficiency of wheat, three field experiments with ¹⁵N were set up on winter wheat, on three different soils in France. Different crop N demands on the day of fertiliser application were obtained by varying either crop densities or date of fertiliser application. Labelled ¹⁵NH₄ ¹⁵NO₃ was applied at tillering and during stem elongation. The ¹⁵N recovered from plant and soil at different dates after ¹⁵N addition and at maturity of wheat was measured. The fate of fertiliser N was rapidly determined, most of the fertiliser N accumulated in the wheat at maturity having been taken up within a few days of application. ¹⁵N recovery by the crop at final harvest (%) varied greatly (19–55% N applied) according to crop density, soil type and date of application. It was linearly related to the instantaneous crop growth rate calculated at the day of ¹⁵N application. The amount of fertiliser N immobilised in the soil was constant at 20 kg N ha⁻¹, for all soil types and crop densities. Because residual mineral ¹⁵N in the soil at harvest was negligible and immobilisation was constant, the level of total ¹⁵N measured in the different N pools (soil+plant) reflected the% ¹⁵N uptake by the plant. There was consequently a negative linear relationship between the percentage of ¹⁵N not recovered for measurement, and crop growth rate (i.e. crop N demand) at date of fertiliser application. These results suggest that crop N demand at the time of N application determines the ability of the crop to compete for N with other processes, and may be a major factor determining the division of N between soil and crop. This revised version was published online in June 2006 with corrections to the Cover Date.     

Role of soil animals in C and N mineralisation

Addition of single species of soil animals to animal-free microcosms often increases total heterotrophic respiration, but sometimes additions of microarthropods have been reported not to increase or even decrease CO₂ evolution rates. Most studies indicate that addition of soil animals increases net N mineralisation. In a study with F/H layer materials from a spruce stand in central Sweden kept at two temperatures (5 and 15°C) and three moisture levels (15, 30 and 60% of WHC), addition of a mixed fauna of soil arthropods, mainly microarthropods, could not be shown to change the CO₂ evolution rates in comparison with materials where arthropods were absent. However, addition of the arthropods significantly increased net N mineralisation for each of the temperature and moisture combinations. The increase due to the arthropods was dependent on soil temperature but not on soil moisture. Because the total net N mineralisation decreased with decreasing soil moisture, the soil arthropods had a much larger relative effect on net N mineralisation under dry than under moist conditions. It is concluded that soil arthropods are important in maintaining net N mineralisation under dry conditions when the microflora is largely inactive. The microbial/faunal release of mineral N is discussed in relation to the CN of the substrate.