Differences in the cadmium content of some common Western Australian pasture plants grown in a soil amended with cadmium - describing the effects of level of cadmium supply

The uptake of cadmium (Cd) by capeweed (Arctotheca calendula), subterranean clover (Trifolium subterraneum), santiago medic (Medicago santiago), wimmera ryegrass (Lolium rigidum) and kikuyu grass (Pennisetum clandestinum) was measured over an eight week period following seedling emergence from a loamy sand amended with nine concentrations of Cd (0–50µg g^–1). The uptake of Cd from soil amended with either 0 or 1µg Cd g^–1 was also measured at 7 day intervals over the eight week growing period.With the exception of wimmera ryegrass, yields were reduced by addition of Cd, and this reduction could be described by simple linear or quadratic equations. Addition of Cd increased the concentration of Cd in plants and the increase could be described using a rescaled Mitscherlich function. However, the accumulation of Cd at high levels of addition was depressed due to the effect of Cd supply on yield and a modified function was used to describe this effect.The concentration of Cd in tops (µg g^–1) did not vary markedly with plant age. For Cd additions corresponding to typical levels of plant-available Cd in Western Australian (WA) pasture soils, the concentration of Cd in tops harvested six or eight weeks after emergence was about four times greater in capeweed than in subterranean clover or kikuyu, and about eight times greater than in wimmera ryegrass or santiago medic. However, because of differences in the moisture content of tops, there was only a threefold difference in the potential contribution to the Cd burden of grazing sheep between capeweed or subterranean clover at typical levels of soil Cd. For most plants, Cd concentrations in roots were about ten times greater than in tops, except in capeweed which translocated more of the Cd taken up to tops. A reduction in the Cd burden of grazing animals in WA would most likely be achieved by the production of pastures that are low in capeweed and dominated by species which can survive the drier periods of the grazing season.     

Response of subterranean clover (Trifolium subterraneum) to lime,magnesium, and boron on acid infertile soil in subtropical China

Severe symptoms observed in clover pastures sown in north Guangdong Province, China, warranted study of the response to Mg fertilizer, and possible interactions with lime and B fertilizer. Subterranean clover (Trifolium subterraneum) was sown in a 2 × 2 × 2 factorial pot experiment with Mg, lime, and B treatments and four replications. The highly acid soil collected from Lechang Model Cattle Farm was low in exchangeable Mg (0.06 meq 100 g^–1). Mg fertilizer doubled exchangeable Mg, increased Mg saturation to > 3% and raised the Mg/K ratio to > 1.0, but A1 saturation remained above 75%. Lime application doubled exchangeable Ca and reduced A1 saturation to < 1%, but did not affect exchangeable Mg or K levels.Clover yield increased (P < 0.01) with Mg application at 100 kg Mg ha^–1, but was not affected by lime or B fertilizer. Regression analyses showed that exchangeable Mg, soil Mg/K ratio, and Mg concentration in tops each accounted for > 70% of yield variation. Yield decreased markedly when exchangeable Mg was < 0.22 meq, soil Mg/K ratio was < 1.0, and when Mg in top growth fell below 0.15%. Symptom scores for Mg deficiency (including reddening and necrosis on older leaves) were correlated with yield (R² = 0.88) and tissue Mg (R² = 0.92). Plants without symptoms were present only where tissue Mg was > 0.26%. Liming to amend soil acidity did not increase tissue Mg or correct deficiency symptoms in clover plants without added Mg, but did reduce P and B to below critical levels. B deficiency did not limit pasture growth and application of 4 kg B ha^–1 was sufficient to raise B level in clover tops to > 25 mg kg^–1 on lime amended soil.The implications of correction of this acute Mg deficiency in relation to future fertilizer programs (especially K fertilizer) for crops and pastures grown on China's weathered red soils is discussed, as are the problems associated with grazing livestock on Mg deficient pastures.     

Recovery of 15N-labelled fertilizer applied to winter wheat and perennial ryegrass crops and residual 15N recovery by succeeding wheat crops under different crop residue management practices

The recovery of ¹⁵N-labelled fertilizer applied to a winter wheat (120 kg N ha^–1) and also a perennial ryegrass (60 kg N ha^–1) crop grown for seed for 1 year in the Canterbury region of New Zealand in the 1993/94 season was studied in the field. After harvests, ryegrass and wheat residues were subjected to four different residue management practices (i.e. ploughed, rotary hoed, mulched and burned) and three subsequent wheat crops were grown, the first succeeding wheat crop sown in 1994/95 to examine the effects of different crop residue management practices on the residual ¹⁵N recovery by succeeding wheat crops. Total ¹⁵N recoveries by the winter wheat and ryegrass (seed, roots and tops) were 52% and 41%, respectively. Corresponding losses of ¹⁵N from the crop-soil systems represented by un-recovered ¹⁵N in crop and soil were 12% and 35%, respectively. These losses were attributed to leaching and denitrification. The proportions of ¹⁵N retained in the soil (0-400 mm depth) at the time of harvest of winter wheat and ryegrass were 36% and 24%, respectively. Although the soil functioned as a substantial sink for fertilizer N, the recovery of this residual fertilizer by subsequent three winter wheat crops was low (1-5%) and this was not affected by different crop residue management practices.     

Fertilizer nitrogen balance study on sandy loam with winter wheat

The uptake of labelled nitrate was studied under field conditions using a sandy loam soil and winter wheat as the crop. Thirty per cent of the added labelled nitrate was found in the grain, 13% in the straw plus chaff and only 2% in the harvested roots. The labelled nitrogen in the plant comprised about 6% of labelled fertilizer origin, and about 94% from the soil. By analysing the soil after harvest, it was shown that 16% of the fertilizer nitrogen was in the 0–6 cm layer, 19% in the 6–12 cm layer, 6% in the 12–25 cm layer and 5% in the 25–50 cm layer. This means a total residual fertilizer nitrogen of 46%. A study was made of the border effect by analysing the plants of the first and second row around the fertilized plot. The results show that the plants of the first row took up 1.4% of the added fertilizer and those of the second row only 0.4%. The total recovery of the added fertilizer in soil and plant was 93.1%, indicating that 6.9% was lost under conventional cultivation practice.     

Response of intensively grazed ryegrass dairy pastures to fertiliser phosphorus and potassium

Intensively grazed, rain-fed dairy pastures on the predominantly sandy soils in the high rainfall (>800 mm annual average) Mediterranean-type climate of south-western Australia comprise >90% ryegrass (annual ryegrass, Lolium rigidum Gaud. and Italian ryegrass, L. multiflorum Lam.). To maximise pasture use for milk production, the pastures are rotationally grazed by starting grazing when ryegrass plants have 3 leaves per tiller, and fertiliser nitrogen (N) and sulfur (S), in the ratio of 3–4 N and 1S, need to be applied after each grazing for profitable pasture dry matter (DM) production. In addition, farmers usually also apply low levels of phosphorus (P) and potassium (K) fertiliser to these pastures after each grazing, despite Colwell soil test P usually being well above critical values for pasture production, and fertilizer K being only required for clover in the traditional clover (Trifolium subterraneum L.) ryegrass pastures of the region. In field experiments undertaken May 2006–June 2010 on intensively grazed ryegrass dairy pastures in the region, no significant ryegrass DM responses to applied fertiliser P or K were obtained, regardless of level or method of P or K application. When no P was applied, soil test P declined gradually, by between 4.4 and 7.1 mg/kg per year, and remained above the critical value for the soils at 2 sites, but declined below the critical value for soil at a third site. Critical soil test P is located near the maximum yield plateau in the flat part of the relationship between yield and soil test P, particularly when, as appropriate for dairy production, the critical value is for 95% of the maximum pasture DM yield. Consequently, when no P is applied and soil test P decreases, significant pasture DM yield decreases will only occur when soil test P approaches the steeper part of the relationship, which can take some time. In addition, as occurs on farms, faeces deposited by cows while grazing supplied P to pasture even when no fertiliser P was applied. Soil K testing proved unreliable for indicating the need for fertiliser K applications to pasture in the next growing season because many soil samples collected within and between urine patches contained elevated levels of K deposited by cows while grazing. We conclude fertiliser P should only be applied to intensively grazed ryegrass dairy pastures when soil testing indicates it is required. Further research is required to assess if plant K testing is an alternative, but urine patches may also pose a problem for plant testing.     

Granite powder as a source of potassium for plants: a glasshouse bioassay comparing two pasture species

The effect of granite powder (<70µm) as a K fertilizer was investigated in a glasshouse pot experiment conducted with three acid, sandy topsoils from podzols of South Western Australia and with three fertilizer treatments: a control without K application, a KCl treatment (90 mg K kg^–1 soil) and a granite treatment (20 g granite kg^–1 soil, yielding 640 mg K kg^–1 soil). Subterranean clover (Trifolium subterraneum) and ryegrass (Lolium rigidum) were cropped in triplicated pots for 7 weeks, harvested and allowed to regrow for another 13 weeks. Clover growth at 7 weeks was in the following order: control < granite < KCl. The growth of ryegrass after 7 weeks was not significantly affected by granite as compared to the control treatment. After another 13 weeks, both species showed a significant growth response to granite application for two of the three soils studied. For both species and all three soils K concentrations in the plant tissue were systematically and significantly higher for KCl relative to granite and for granite relative to control treatment. Minor dissolution of granite occurred during the short duration of the experiment as indicated by changes in soil exchangeable K in uncropped pots (about 1-2% of K applied) and resulted in the increased K concentration in plants and the growth response of subterranean clover after 7 and 20 weeks and ryegrass after 20 weeks of cropping. The possible use of granite powder as a slow-release K fertilizer is discussed.     

Nitrogen uptake by autumn sown sugar beet

The influence of N fertilizer rate on uptake and distribution of N in the plant,¹⁵N labelled fertilizer uptake and sugar yield were studied for 3 years on autumn sown sugar beet (Beta vulgaris L.) under Mediterranean (Southern Spain) rain-fed and irrigated conditions. Available average soil N prior to sowing was 69 kg N ha^–1, and net mineralisation in the soil during the growth period was 130 kg N ha^–1. Maximum N uptake occurred in the spring and increased with increasing fertilizer rates in the irrigated crop. There was no increase in N uptake in the sugar beet cropped under rain-fed conditions because of water shortage. Maximum average N uptake both by roots and tops was between 200 and 250 kg N ha^–1. When N fertilizer was not applied, average uptake from the soil was between 130 and 140 kg N ha^–1. At the end of the growth period there was a marked translocation of N from the leaves to the root which increased with the N fertilizer rate. The N ratio top/roots at harvest was 0.45–0.5 and 0.8- - 1 in the irrigated and rain-fed sugar beet, respectively. Maximum¹⁵N labelled fertilizer uptake took place in May-June, being larger in irrigated sugar beet or when spring rainfall was more abundant. Fertilizer use efficiency varied between 30% and 68%. Sugar yield response to N fertilizer rates depended on the N available in the soil and on the total water input to the crop, particularly in spring. The response was more constant in the irrigated crop, where optimum yield was obtained with a fertilizer rate of 160 kg N ha^–1. In the rain-fed crop, the optimum dose proved more erratic, with an estimated mean of 100 kg N ha^–1. The amount of N required to produce 1 t of root and of sugar ranged between 1.5 and 3.8 kg N and between 11.1 and 22.4 kg N respectively, and varied according to the N fertilizer rates applied.     

The combined effect of fertiliser nitrogen and phosphorus on herbage yield and changes in soil nutrients of a grass/clover and grass-only sward

The combined effect of reduced nitrogen (N) and phosphorus (P) application on the production of grass-only and grass/clover swards was studied in a five-year cutting experiment on a marine clay soil, established on newly sown swards. Furthermore, changes in soil N, P and carbon (C) were measured. Treatments included four P (0, 35, 70 and 105 kg P ha^–1 year^–1) and three N levels (0, 190 and 380 N kg ha^–1 year^–1) and two sward types (grass-only and grass/clover). Nitrogen was the main factor determining the yield and quality of the harvested herbage. On the grass-only swards, N application increased the DM yield with 28 or 22 kg DM kg N^–1, at 190 or 380 kg N ha^–1 year^–1, respectively. The average apparent N recovery was 0.78 kg kg^–1. On the grass/clover swards, N application of 190 ha^–1 year^–1 increased grass production at the cost of white clover, which decreased from 41 to 16%. Phosphorus application increased grass yields, but did not increase clover yields. A positive interaction between N and P applications was observed. However, the consequences of this interaction for the optimal N application were only minor, and of little practical relevance. Both the P-AL-value and total soil P showed a positive response to P application and a negative response to N application. Furthermore, the positive effect of P application decreased with increasing N application. The annual changes in P-AL-value and total soil P were closely related to the soil surface surplus, which in turn was determined by the level of N and P application and their interaction. The accumulation of soil N was similar on both sward types, but within the grass-only swards soil N was positively affected by N application. The accumulation of organic C was unaffected by N or P application, but was lower under grass/clover than under grass-only.     

Recovery of 15N-labelled fertiliser by a perennial ryegrass seed crop and a subsequent wheat crop

Large amounts of nitrogen (N) fertiliser (150–200 kg N/ha) are currently being applied to perennial ryegrass (Lolium perenneL.) seed crops in New Zealand. Due to increasing requirements for efficient use of N fertilisers and minimising nitrate contamination of the environment, a field experiment was established using ¹⁵N-labelled fertiliser to follow the fate of applied N. Urea-¹⁵N was applied to a perennial ryegrass seed crop in April (30 kg N/ha), August (30 kg N/ha), September (60 kg N/ha) and October (60 kg N/ha). The urea-¹⁵N was applied in solution and watered in to minimise volatilisation loss. At the time of harvest (December), 9% of the applied ¹⁵N was in the seed, 29% in the straw, 19% in the roots and 39% in the soil organic matter. Losses of ¹⁵N were minimal as the N was applied in several applications, each one at a relatively low rate, and at times when leaching was unlikely to occur. Ryegrass plants used a greater proportion of the N applied in September and October (61–65%) compared with that applied in April (44%). Consequently more N was recovered from the soil in the autumn application (57%) than from the September and October applications (28–44%). The availability of the residual fertiliser N to a subsequent wheat (Triticum aestivum L.) crop was studied in a glasshouse experiment. The residual fertiliser N was present in the soil and ryegrass roots and stubble. The wheat plants only recovered 7–9% of this residual N. Most of the N taken up by the wheat came from the soil organic N pool. Overall, applying a total of 180 kg N/ha to the ryegrass appeared to have minimal direct impact on the environment. In the short term N not used by the ryegrass plants contributed to the soil organic N pool.     

Nutrient release during the decomposition of mowed perennial ryegrass and white clover and its contribution to nitrogen nutrition of grapevine

Sustainable management of mineral nutrition in vineyards, as well as in other fruit plantations, should aim at exploiting the use of internal sources of nutrients, in order to reduce the need for external nutrient inputs. In this paper we explore the potential of the grassed alleys to provide nutrients to the vines. We followed for one vegetative season the decomposition of ryegrass and clover, frequently present as floor vegetation in vineyards, using litter bags filled with ¹⁵N-enriched grass material. In addition, we quantified the amount of nitrogen (N) transferred from the decomposing litter to field-grown grapevines. Ryegrass and clover had a relatively rapid decomposition rate, with a loss of C approaching 80% in only 16 weeks. The release of nutrients was particularly fast for potassium (95% in 16 weeks) followed by nitrogen (80%), calcium (70–80%), phosphorous (65–85%), magnesium (70–75%), and sulfur (60–70%). In spite of the rapid release of N from decomposing material, the N uptake by grapevines was on average less than 4% of the initial amount of N present in the litter of ryegrass and clover. Even if N release during the decomposition of mowed perennial ryegrass and white clover little contributed to the N nutrition of grapevine in the same growing season, most N from mowed grassed was still recovered in the soil.     

A microplot study of the fate of15N-labelled ammonium nitrate and urea applied at two rates to ryegrass in spring

Confined microplots were used to study the fate of¹⁵N-labelled ammonium nitrate and urea when applied to ryegrass in spring at 3 lowland sites (S1, S2 and S3). Urea and differentially and doubly labelled ammonium nitrate were applied at 50 and 100 kg N ha^–1. The % utilization of the¹⁵N-labelled fertilizer was measured in 3 cuts of herbage and in soil to a depth of 15 cm (soil_0–15).Over all rates, forms and sites, the % utilization values for cuts 1, 2, 3 and soil_0–15 were 52.4, 5.3, 2.4 and 16.0% respectively. The % utilization of¹⁵N in herbage varied little as the rate of application increased but the % utilization in the soil_0–15 decreased as the rate of application increased. The total % utilization values in herbage plus soil_0–15 indicated that losses of N increased from 12 to 25 kg N ha^–1 as the rate of N application was increased from 50 to 100 kg N ha^–1.The total % utilization values in herbage plus soil_0–15 over both rates of fertilizer N application were 84.1, 80.8 and 81.0% for urea compared with 74.9, 72.5 and 74.4% for all ammonium nitrate forms at S1, S2 and S3 respectively. Within ammonium nitrate forms, the total % utilization values in herbage plus soil_0–15 over both rates and all sites were 76.7, 69.4 and 75.7% for¹⁵NH₄NO₃, NH₄ ¹⁵NO₃ and¹⁵NH₄ ¹⁵NO₃ respectively. The utilization of the nitrate moiety of ammonium nitrate was lower than the utilization of the ammonium moiety.The distribution of labelled fertilizer between herbage and soil_0–15 varied with soil type. As the total utilization of labelled fertilizer was similar at all sites the cumulative losses due to denitrification and downward movement appeared to account for approximately equal amounts of N at each site.     

Recovery of15N-labelled fertilizer by sugar beet—spring wheat and winter rye—sugar beet cropping sequences

The recovery of¹⁵N labelled ammonium fertilizer was studied during two cropping sequences: sugar beet—spring wheat and winter rye—sugar beet with the labelled N applied to the first crop of each sequence. The difference between fall and spring application was also investigated. For the first cropping sequence 100 kg N ha^–1 labelled with 11.4%¹⁵N atom excess (a.e.) was applied to the sugar beets. This labelled N was followed in the sugar beets, in the soil profile at harvesting time as well as in the spring wheat of the following year. The first crop of sugar beet recovered 43–46% of the applied N, with 26–29% remaining in the soil at harvesting time and 25–31% could not be accounted for. Of the residual N, less than 1% could be recovered by the next crop of spring wheat. For the second cropping sequence 50 kg N ha^–1 labelled with 11.5%¹⁵N a.e. was applied to the winter rye and followed in the winter rye and in the sugar beets of the following year. The recovery of the labelled fertilizer N applied to the winter rye of the second sequence was 20–27% and the sugar beets of the next year could only recover 2%. With respect to time of application, no difference in fertilizer N recovery was found between fall or spring application for the two sequences.     

Agronomic and nutritional effects of Linz-Donawitz slag application to two pastures in Northern Spain

Most grassland soils in western European countries are acidic in their natural state and require a liming material to bring them to their optimum pH. A study was conducted to determine whether Linz-Donawitz (LD) slag, a by-product of the iron and steelmaking industry, could be used as a dolomitic agent for pastures. Six rates of slag (0, 1, 1.5, 3, 5, and 7.5 t ha^–1), with and without fertilizer, were investigated for their effects on soil properties, pasture yield and botanical composition, and herbage mineral concentrations. The three-year study was conducted on a newly established pasture of perennial ryegrass (Lolium perenne L.), cocksfoot (Dactylis glomerata L.), and white clover (Trifolium repens L.), and on a resident pasture dominated by Yorkshire fog (Holcus lanatus L.) and browntop (Agrostis tenuis Sibth.). Application of slag increased soil pH (0.15 and 0.11 units per ton of slag applied at Derio and Abadiano, respectively) and decreased Al percentage of the soil complex to levels not considered harmful to plant growth. Exchangeable Ca increased markedly and exchangeable Mg slightly. In general, herbage Ca and Mg concentrations increased accordingly to their increase in the soil, while Fe, Mn, Cu, and Zn decreased with increasing rates of slag. LD slag appears to be a useful liming material for correcting soil acidity in pasture scils, and for increasing Ca and Mg, and decreasing Mn concentrations, in herbage.     

Fractions of organic carbon in soils under different crop rotations, cover crops and fertilization practices

We investigated the long-term effects (13–48 years) of crop rotations, cover crops and fertilization practices on soil organic carbon fractions. Two long-term experiments conducted on a clay loam soil in southeastern Norway were used. From the crop rotation experiment, two rotations, one with two years grain + four years grass and the second with grain alone (both for 6 years), were selected. Each rotation was divided into moderate fertilizer rate (30–40 kg N ha^–1), normal fertilizer rate (80–120 kg N ha^–1) and farmyard manure (FYM 60 Mg ha^–1 + inorganic N at normal rate). Farmyard manure was applied only once in a 6-year rotation, while NPK was applied to every crop. The cover crop experiment with principal cereal crops consisted of three treatments: no cover, rye grass and clover as cover crops. Each cover crop was fertilized with 0 and 120 kg ha^–1 N rates. Soil samples from both experiments were taken from 0–10 cm and 10–25 cm depths in the autumn of 2001. The classical extraction procedure with alkali and acid solution was used to separate humic acid (HA), fulvic acid (FA) and humin fractions, while H₂O₂ was used to separate black carbon (BC) from the humin fraction. The rotation of grain + grass showed a significantly higher soil organic carbon (SOC) compared with grain alone at both depths. Farmyard manure application resulted in significantly higher SOC than that of mineral fertilizer only. However, cover crops and N rates did not affect SOC significantly. Organic carbon content of FA, HA and humin fractions accounted for about 29%, 25% and 44% of SOC, respectively. The rotation of grain+grass gave a higher C content in HA and humin fractions, and a lower C in the FA fraction as compared with the rotation with grain alone. Farmyard manure increased HA and humin fractions more than did chemical fertilizers. Clover cover crop increased the C proportion of humin more than rye grass and no cover crop. No significant differences in C contents of FA, HA and humin fractions were observed between N rates. Effects of cover crop and N rates as well as fertilization with NPK on black carbon (BC) content were significant only at 10–25 cm depths. Farmyard manure increased the BC fraction compared with chemical fertilizers. Clover crop also enhanced the accumulation of the BC fraction. Application of 120 kg N ha^–1 resulted in a significant increase of the BC fraction.     

Effects of nitrogen on growth,and nitrogen and phosphorus uptake in tops and roots of sorghum grown in an Alfisol and a Vertisol

A greenhouse pot experiment was conducted to study the effects of added nitrogen (0, 10, 25, 50 and 100 mg N kg^–1 soil) on dry matter production, and N and P uptake in tops and roots of sorghum (cv CSH6) grown in a Vertisol and an Alfisol for 42 days at field capacity soil moisture content. More dry matter accumulated in the tops and roots of sorghum growing in the Alfisol than in the Vertisol. This resulted in higher N and P uptake. Top dry weight responded to N application up to 50 mg N kg^–1 soil, whereas the root weight increased at N application up to 25 mg N kg^–1. Ratios of root dry weight to total plant dry weight and N uptake in roots/total N uptake were similar in the two soils. Ratio of P uptake in roots to total P uptake was higher in Alfisol than in Vertisol. This result was attributed mainly to higher ratio of P content in roots compared to tops in the Alfisol.Approved for publication as ICRISAT Journal Article No. 709.     

Biologically Fixed N2 as a Source for N2O Production in a Grass–clover Mixture, Measured by 15N2

The contribution of biologically fixed dinitrogen (N₂) to the nitrous oxide (N₂O) production in grasslands is unknown. To assess the contribution of recently fixed N₂ as a source of N₂O and the transfer of fixed N from clover to companion grass, mixtures of white clover and perennial ryegrass were incubated for 14 days in a growth cabinet with a ¹⁵N₂-enriched atmosphere (0.4 atom% excess). Immediately after labelling, half of the grass–clover pots were sampled for N₂ fixation determination, whereas the remaining half were examined for emission of ¹⁵N labelled N₂O for another 8 days using a static chamber method. Biological N₂ fixation measured in grass–clover shoots and roots as well as in soil constituted 342, 38 and 67 mg N m⁻² d⁻¹ at 16, 26 and 36 weeks after emergence, respectively. The drop in N₂ fixation was most likely due to a severe aphid attack on the clover component. Transfer of recently fixed N from clover to companion grass was detected at 26 and 36 weeks after emergence and amounted to 0.7 ± 0.1 mg N m⁻² d⁻¹, which represented 1.7 ± 0.3% of the N accumulated in grass shoots during the labelling period. Total N₂O emission was 91, 416 and 259 μg N₂O–N m⁻² d⁻¹ at 16, 26 and 36 weeks after emergence, respectively. Only 3.2 ± 0.5 ppm of the recently fixed N₂ was emitted as N₂O on a daily basis, which accounted for 2.1 ± 0.5% of the total N₂O–N emission. Thus, recently fixed N released via easily degradable clover residues appears to be a minor source of N₂O. An erratum to this article is available at .     

N application to winter wheat at tillering and shooting: N balance at different growth stages

At two sites, microplots under winter wheat were given 140 kg N ha^–1 as labelled ammonium nitrate split in 80 kg N ha^–1 at tillering and 60 kg N ha^–1 at shooting. Soil and plant samples were analyzed at shooting, after anthesis and at grain harvest and a¹⁵N balance was established. The average recovery rate of 95% indicates that there were no marked N losses due to leaching and denitrification, which is attributed to the low rainfall in the two months after fertilizer application. Between 19 and 23% of the fertilizer N remained in the 0–30 cm soil layer as organically bound soil N. Up to 64% was taken up by the above-ground crop. On the loamy sand, 4% of the fertilizer N at harvest remained in the roots in the 0–30 cm layer and only 3% was found as inorganic N in the 0–90 cm soil layer. The fertilizer N applied diminished plant uptake of soil N in the period between fertilizer application and harvest. As compared with the control, the fertilized plants extracted 25 and 28% less soil N from loamy sand and loess soil, respectively. The results show that application of mineral N fertilizer helps to maintain the mineralizable N content of the soil, which has been accumulated in the course of long-term intensive crop production, by adding N to the soil organic pool and simultaneously reducing the supply of soil N to the plants.     

Evaluation of iron pyrites as sulphur fertilizer

A field experiment was conducted for three consecutive winter crop seasons commencing in 1979–80 on the Typic Ustochrept of Pura to evaluate iron pyrites as S fertilizer. Four crops viz, wheat, chickpea, mustard and Egyptian clover were tested for their responsiveness to added pyrites. All the crops responded significantly to added pyrites. Mustard proved most sensitive to S deficiency in soil and wheat the least. Between the two legumes, Egyptian clover was more sensitive to S stress than chickpea. Average biomass production by Egyptian clover was highest followed by wheat, mustard and chickpea. Mustard and Egyptian clover required more S to achieve maximum biomass production compared with wheat and chickpea but they also recovered from the soil a large proportion of added S than wheat and chickpea. Addition of pyrites increased availability of S in soil. Pyrites enhanced mobilization of soil P and its utilization by the crops.     

Fate of nitrogen (15N) from oxamide and urea applied to turf grass: A lysimeter study

Slow release N fertilizers are receiving increasing attention for use on turf grass, but their fate in the plant-soil system is still poorly understood. We aimed to quantify the uptake and recovery of N by a mixture of grasses when applied as either urea or oxamide in different diameter granules using a tracer technique (¹⁵N). The effects of the N source on soil biomass, root density and amount of readily available organic C in soil were also evaluated.In a first experiment oxamide in 4–5 mm diameter granules was compared with urea. The initial N absorption, 40 days after fertilization (d.a.f.), was higher for urea (23.5%) than for oxamide (12.1%), but after 64 days absorption efficiencies were about the same (11%) for both fertilizers. Fertilizer-derived N lost by leaching was much greater from the urea-fertilized soil (1.57 g), compared with losses from oxamide-fertilized soil (0.05 g). The total residual fertilizer N remaining in the system at the end of the experiment was 26.7% of applied urea N and 39.6% of applied oxamide N. Cumulated absorption efficiencies, calculated after dismantling the lysimeters, were 43.1% for urea and 54.8% for oxamide (roots included). A priming effect caused by a larger uptake of soil N because of the better root development was found in the oxamide-treated lysimeter. Fertilization with oxamide also caused an increase in the amount of soil microbial biomass.In a second experiment, the efficiencies and fertilizer N uptake rates from oxamide applied at two different granule sizes (1–2 mm and 5–10 mm) were evaluated. The amount of soil N taken up by the grass was linearly related to root density (r = 0.92).     

Uptake of fertilizer and soil nitrogen by ryegrass swards during spring and mid-season

Double-labelled¹⁵N ammonium nitrate was used to determine the uptake of fertilizer and soil N by ryegrass swards during spring and mid-season. The effects of water stress (40% of mean rainfall v 25 mm irrigation per 25 mm soil water deficit) and the rate of application of N in the spring (40 v 130 kg ha^–1) on the recovery of 130 kg N ha^–1 applied in mid-season were also evaluated. Apparent recovery of fertilizer N (uptake of N in the fertilized plot minus that in the control expressed as a percentage of the N applied) was 95 and 79% for fertilizer N applied in the spring at rates of 40 and 130 kg ha^–1, respectively. Actual recovery of the fertilizer N assessed from the uptake of¹⁵N was only 31 and 48%, respectively. The uptake of soil N by the fertilized swards was substantially greater than that by the control. However, the increased uptake of soil N was always less than the amount of fertilizer N retained in or lost from the soil. Broadly similar patterns for the uptake of fertilizer and soil N were observed during mid-season. Uptake of N in mid-season was highest for swards which received 40 kg N ha^–1 in the spring and suffered minimal water stress during this period. Application of 130 kg N ha^–1 in spring reduced the uptake of N in mid-season to an extent similar to that arising from water stress. Only 1.8 to 4.2 kg ha^–1 (3 to 10%) of the N residual from fertilizer applied in the spring was recovered during mid-season. Laboratory incubation studies suggested that only a small part of the increased uptake of soil N by fertilized swards could be attributed to increased mineralisation of soil N induced by addition of fertilizer. It is considered that the increased uptake of soil N is partly real but mostly apparent, the latter arising from microbially mediated exchange of inorganic¹⁵N in the soil.