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.     

Impacts of fertilizer practices on environmental risk of nitrate in semiarid farmlands in the Loess Plateau of China

The lack of understanding of nitrate dynamics in soil profiles of semiarid regions hampers the assessment of the environmental risks associated with nitrate. A long-term field experiment established in the Loess Plateau of Northwest China in 1984 was used to investigate the seasonal dynamics of water and nitrate contents in the soil profile (0–300 cm) under bare fallow and continuous winter wheat (Triticum aestivum L.) with various fertilizer treatments. For treatments without mineral N input (i.e., no fertilizer, farmyard manure alone, and with P fertilizer), the amount of nitrate accumulated in the soil profile (52–120 kg N ha^?1, the average for June, August, February and April) was significantly lower than that (292 kg N ha^?1) accumulated in the bare fallow treatment. A large amount of nitrate (1,065 kg N ha^?1) was found accumulated in the soil profile with the treatment applied with mineral N at a rate of 120 kg N ha^?1 year^?1 for 17 years (1984–2001) and this nitrate moved downward during the wet season (from August to February). Clearly, the amount of nitrate accumulated in the soil profiles, and its tendency of downward movement, appears to potentially be an environmental risk as it may reach groundwater. Fertilization as mineral N fertilizers coupled with FYM or P resulted in 50–70% less nitrate accumulation in the soil profiles than that using mineral N fertilizer alone, and therefore the environmental risk was reduced. It is proposed that a “break point” of nitrate distribution existed in the soil profiles, providing an indication of soil depth to which nitrate can transfer.     

Nitrous oxide fluxes from corn fields: on-farm assessment of the amount and timing of nitrogen fertilizer

Nitrogen fertilization is considered as an important source of atmospheric N₂O emission. A seven site‐year on‐farm field experiment was conducted at Ottawa and Guelph, ON and Saint‐Valentin, QC, Canada to characterize the affect of the amount and timing of N fertilizer on N₂O emission in corn (Zea mays L.) production. Using the static chamber method, gas samples were collected for 28‐days after preplant and 28‐days after sidedress fertilization at the seven site‐year, resulting in 14 monitoring periods. For both methods of fertilization, peak N₂O flux and cumulative emission increased with the amount of N applied, with rates ranging from 30 to 900 μg N m^?2 h^?1. Depending on N amount and time of application, cumulative emission varied from 0.05 to 2.42 kg N ha^?1, equivalent to 0.03% to 1.45% of the N fertilizer applied. Differences in N₂O emission peaks among fertilizer treatments were clearly separated in 13 out of 14 monitoring periods. Total N₂O emissions may have been underestimated compared with annual monitoring in 10 out of the 49 cases because the monitoring period ended before N₂O efflux returned to the baseline level. The flux of N₂O was negligible when soil mineral N in the 0–15 cm layer was < 20 mg N kg^?1. While rainfall stimulated emission, soil temperature > 15 °C was likely the driving force responsible for the higher levels of N₂O found for sidedress than preplant application methods. However, caution must be taken when interpreting these later results as preplant fertilization may have continuously stimulated N₂O emissions after the 28‐days monitoring period, especially in situations where N₂O effluxes have not fallen back to their baseline levels. Increasing fertilizer rates from 90 to 150 kg N ha^?1 resulted in slight increases in yields, but doubled cumulative N₂O emissions.     

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.     

Drip nitrogen fertigation of ‘Starking Delicious’ apple trees

The response of ‘Starking Delicious’ (Malus domestica Borkh.) apple trees to four N fertigation treatments in drip irrigation (50, 150, 250 and 400 kg N ha⁻¹, supplemented with a uniform dose of 400 kg K ha⁻¹) was investigated in a field experiment during 6 years. Nitrate nitrogen in the soil was proportional (7–58 mg kg⁻¹) to the applied N dose at the end of a 6 week fertigation period in the spring. At other times, the nitrate concentration in the soil was very low. Soil K decreased with depth and in the 0–30 cm soil layer it was negatively correlated with the applied N dose, before fertigation started. At the end of the spring fertigation period, higher K was found in all soil layers analyzed. Vegetative growth was correlated positively with the amount of N applied. Leaf chlorophyll and N were significantly lower only in the lowest N treatment, while increased fruit size and decreased fruit color were measured only in the higher N treatments. Yield was severely biennial. In the ‘On’ years crop load was heavier as less N was applied. An extremely high crop load in the lowest N treatment was followed by a reduced flower density and yield in the following season. In the ‘Off’ years, a significantly higher yield was obtained with the 150 kg ha⁻¹ dose. The dry weights, N, P and K contents of the above ground parts of mature apple trees were determined. Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel, No. 2342-E, 1988 Series.     

Denitrification losses from a natural grassland in the Basque Country under organic and inorganic fertilization

Denitrification losses from a poorly drained clayey loamy soil under natural pasture were measured over a two-year period using the acetylene inhibition technique. Plots received two different applications of fertilizer as calcium ammonium nitrate or cow slurry (a total of 145–290 kg N ha^–1 in 1991 and 120–240 kg in 1992). In the first year, N losses in the mineral treatments were about 4 times greater than losses in the slurry treatments. In the second year losses in the slurry treatments increased in such a way that losses in the higher slurry application became similar to those for the two mineral treatments. Soil nitrate was the factor producing differences between treatments. In this way, N mineralization in periods between fertilizations coinciding with high soil water contents was responsible in the second year for the increase in N losses in the slurry treatments. Denitrification rates greater than 0.1 kg N ha^–1 day^–1 occurred at soil water contents > 33 % (air filled porosity < 26 %) and soil nitrate contents > 1 mg N kg^–1 dry soil. Spring and autumn were the seasons of highest risk of denitrification because of N fertilizations coinciding with periods of soil saturation with water. Winter losses were low, but this is a period when there is a risk of denitrification in wetter seasons, particularly for a slurry application management.     

Assessment of Nutrient Leaching Losses and Crop Uptake with Organic Fertilization,Water Saving Practices and Reduced Inorganic Fertilizer

The increasing world population has forced excessive chemical fertilizer and irrigation to complete the global food demand, deteriorating the water quality and nutrient losses. Short-term studies do not compile the evidences; therefore, the study aimed to identify the effectiveness of reduced doses of inorganic fertilizer and water-saving practices, hence, a six-year experiment (2015–2020) was conducted in China to address the knowledge gap. The experimental treatments were: farmer accustomed fertilization used as control (525:180:30 kg NPK ha⁻¹), fertilizer decrement (450:150:15 kg NPK ha⁻¹), fertilizer decrement + water-saving irrigation (450:150:15 kg NPK ha⁻¹), application of organic and inorganic fertilizer + water-saving irrigation (375:120:0 kg NPK ha⁻¹ + 4.5 tones organic fertilizer ha⁻¹), and application of controlled-release fertilizer (80:120:15 kg NPK ha⁻¹). Each treatment was replicated thrice following a randomized complete block design. The results achieved herein showed that control has the highest losses in the six-year study for total nitrogen (225.97 mg L⁻¹), total soluble nitrogen (121.58 mg L⁻¹), nitrate nitrogen (0.93 mg L⁻¹), total phosphorus (0.57 mg L⁻¹), and total soluble phosphorus (0.57 mg L⁻¹) respectively. Reduced fertilizer and water application improved crop nutrient uptake, nitrogen concentration was significantly enhanced with organic and inorganic fertilizer + water-saving irrigation, P concentration was increased with fertilizer decrement + water-saving irrigation, and K concentration was improved with fertilizer decrement + water-saving irrigation. Hence, this study concludes that reduced inorganic fertilizer dose combined with water-saving practices is significantly helpful in reducing nutrient leaching losses and improving nutrient uptake and water pollution. Further studies are needed to explore the impacts of reduced fertilization and water-saving irrigation on leaching losses. The benefits at different climatic conditions, soil types, and fertilizer types with application methods are also a research gap.

     

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.     

Nitrogen cycling in dense plantings of hybrid poplar and black alder

Summary Nitrogen cycling was studied during the third growing season in pure and mixed plantings (33×33 cm spacing) of hybrid poplar and black alder in southeastern Canada. After 3 years, hybrid poplar growth and N content of living tissues in a plot and of individual hybrid poplar plants increased with the proportion of black alder in a planting. No differences were detected among N contents of individual alder plants regardless of plot treatment. Black alder allocated a larger portion of its N to roots than hybrid poplar. Symbiotic nitrogen fixation was estimated to account for 80% of the nitrogen in aboveground alder tissues in the pure treatment using natural¹⁵N dilution. N return in leaf litter was estimated to be 70kg ha^–1 in the pure alder treatment and decreased to a minimum of 20 kg ha^–1 in the pure hybrid poplar plots. No difference was detected among treatments for throughfall N content. Nitrogen concentration in roots and leaf litterfall of black alder was higher than hybrid poplar. Significant soil N accretion occurred in mixed plantings containing two alders to one poplar and pure black alder plantings. Nitrogen availability (NO₃–N) increased with the amount of black alder in a plot. Results suggest that the early increase in nitrogen accumulation of hybrid poplar in mixed treatments can be attributed to an increase of total soil N availability resulting from the input of large amounts of N from easily mineralizable alder tissue.     

Differences among maize cultivars in the utilization of soil nitrate and the related losses of nitrate through leaching

In a 2-year field experiment conducted on a Gleyic Luvisol in Stuttgart-Hohenheim one experimental and nine commercial maize cultivars were compared for their ability to utilize soil nitrate and to reduce related losses of nitrate through leaching. Soil nitrate was monitored periodically in CaCl₂ extracts and in suction cup water. Nitrate concentrations in suction water were generally higher than in CaCl₂ extracts. Both methods revealed that all cultivars examined were able to extract nitrate down to a soil depth of at least 120 cm (1988 season) or 150 cm (1987 season). Significant differences among the cultivars existed in nitrate depletion particularly in the subsoil. At harvest, residual nitrate in the upper 150 cm of the profile ranged from 73–110 kg N ha^–1 in 1987 and from 59–119 kg N ha^–1 in 1988. Residual nitrate was closely correlated with nitrate losses by leaching because water infiltration at 120 cm soil depth started 4 weeks after harvest (1987) or immediately after harvest (1988) and continued until early summer of the following year. The calculated amount of nitrate lost by leaching was strongly influenced by the method of calculation. During the winter of 1987/88 nitrate leaching ranged from 57–84 kg N ha^–1 (suction cups) and 40–55 kg N ha^–1 (CaCl₂ extracts), respectively. The corresponding values for the winter of 1988/89 were 47–79 and 20–39 kg N ha^–1, respectively. ei]Section editor: B E Clothier     

Nitrogen inputs and losses from New Zealand dairy farmlets, as affected by nitrogen fertilizer application: year one

Inputs and losses of nitrogen (N) were determined in dairy cow farmlets receiving 0, 225 or 360 kg N ha^-1 (in split applications as urea) in the first year of a large grazing experiment near Hamilton, New Zealand. Cows grazed perennial ryegrass/white clover pastures all year round on a free-draining soil. N₂ fixation was estimated (using ¹⁵N dilution) to be 212, 165 and 74 kg N ha^-1 yr^-1 in the 0, 225 and 360 N treatments, respectively. The intermediate N rate had little effect on clover growth during spring but favoured more total pasture cover in summer and autumn, thereby reducing overgrazing and resulting in 140% more clover growth during the latter period.Removal of N in milk was 76,89 and 92 kg N ha^-1 in the 0, 225 and 360 N treatments, respectively. Denitrification losses were low (7–14 kg N ha^-1 yr^-1), increased with N application, and occurred predominantly during winter. Ammonia volatilization was estimated by micrometeorological mass balance at 15, 45 and 63 kg N ha^-1 yr^-1 in the 0, 225 and 360 N treatments, respectively. Most of the increase in ammonia loss was attributed to direct loss after application of the urea fertilizer.Leaching of nitrate was estimated (using ceramic cup samplers at 1 m soil depth, in conjunction with lysimeters) to be 13, 18 and 31 kg N ha^-1 yr^-1 in a year of relatively low rainfall (990 mm yr^-1) and drainage (170–210 mm yr^-1). Drainage was lower in the N fertilized treatments and this was attributed to enhanced evapotranspiration associated with increased grass growth.Nitrate-N concentrations in leachates increased gradually over time to 30 mg L^-1 in the 360 N treatment whereas there was little temporal variation evident in the 0 (mean 6.4 mg L^-1) and 225 (mean 10.1 mg L^-1) N treatments. Thus, the 360 N treatment had a major effect by greatly reducing N₂ fixation and increasing N losses, whereas the 225 N treatment had little effect on N₂ fixation or on nitrate leaching. However, these results refer to the first year of the experiment and further measurements over time will determine the longer-term effects of these treatments on N inputs, transformations and losses.     

水氮配合对绿洲沙地农田玉米产量、土壤硝态氮和氮平衡的影响

在黑河中游边缘绿洲沙地农田研究了不同的水氮配合对玉米产量、土壤硝态氮在剖面中的累积和氮平衡的影响.结果表明,施氮处理较不施氮处理产量增加48.22%～108.6%,施氮量超过225 kg hm-2,玉米产量不再显著增加.受土壤结构影响土壤硝态氮在土壤中呈"W"型分布,即土壤硝态氮含量在0～20 cm、140～160 cm和260～300 cm土层均出现峰值,并随施氮量增加,峰值增高.在常规高灌溉量处理硝态氮含量峰值最高值出现在260～300 cm土层,节水25%灌溉处理硝态氮含量峰值最高值出现在土壤表层0～20 cm土层.在常规高灌溉量处理0～300 cm土层中200～300土层硝态氮累积量所占比例最高,介于27.56%～51.86%之间;节水25%灌溉处理在0～300 cm土层中100～200土层硝态氮累积量所占比例最高,介于32.94%～38.07%之间;表明低灌溉处理下土壤硝态氮在土壤浅层累积较多,而高灌溉处理使更多的硝态氮淋溶至土壤深层.与2006年相比,2007年不施氮处理0～200 cm土层土壤硝态氮含量和积累量均明显减少;而施氮处理变化很小,在低灌溉处理甚至表现出硝态氮含量和积累量增加,表明施氮是土壤硝态氮累积的主要来源,而灌溉则使硝态氮向土壤深层淋溶.0～200 cm 土层土壤硝态氮累积量平均介于27.66～116.68 kg hm-2、氮素表观损失量平均介于77.35～260.96 kg hm-2,和施氮量均呈线性相关,即随施氮量增加,土壤硝态氮累积量和氮素表观损失量均增加,相关系数R2介于0.79～0.99之间,相关均显著.随施氮量增加,玉米总吸氮量和氮收获指数增加,氮的农学利用率降低,而灌溉的影响较小.施氮量超过225 kg hm-2时,地上部植株氮肥吸收利用率和籽粒氮肥吸收利用率开始有降低趋势.所以,在沙地农田,节水10%～25%的灌溉水平和225 kg hm-2的施氮水平可以在避免水肥过量投入的基础上减少土壤有机氮淋溶对地下水造成的污染威胁.     

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

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

Turfgrass (Cynodon dactylon L.) sod production on sandy soils: I. Effects of irrigation and fertiliser regimes on growth and quality

The effects on growth, quality and N uptake by turfgrass (Cynodon dactylon L.) during sod production of four fertiliser types applied at three application rates (100, 200 or 300 kg N ha⁻¹ per ‘crop’) under two irrigation treatments (70% and 140% daily replacement of pan evaporation) were investigated. The fertiliser types were: water-soluble (predominately NH₄NO₃), control-release, pelletised poultry manure, and pelletised biosolids; and the experiment was conducted on a sandy soil in a Mediterranean-type climate. Plots were established from rhizomes, with the turfgrass harvested as sod every 16–28 weeks depending upon the time of the year. Four crops were produced during the study. Applying water-soluble and control-release fertilisers doubled shoot growth and improved turfgrass greenness by up to 10% in comparison with plots receiving pelletised poultry manure and pelletised biosolids. Nitrogen uptake into the shoots after four crops (averaged across irrigation treatments and N rates) was 497 kg N ha⁻¹ for the water-soluble fertiliser, 402 kg N ha⁻¹ for the control-release, 188 kg N ha⁻¹ for the pelletised poultry manure and 237 kg N ha⁻¹ for the pelletised biosolids. Consequently, the agronomic nitrogen-use efficiency (NAE, kg DM kg⁻¹ N applied) of the inorganic fertilisers was approximately twice that of the organic fertilisers. Increasing irrigation from 70% to 140% replacement of pan evaporation was detrimental to turfgrass growth and N uptake for the first crop when supplied with the water-soluble fertiliser. Under the low irrigation treatment, inorganic N fertilisers applied at 200–300 kg N ha⁻¹ were adequate for production of turfgrass sod. Section Editor: P. J. Randall     

Soil nitrate accumulation, leaching and crop nitrogen use as influenced by fertilization and irrigation in an intensive wheat–maize double cropping system in the North China Plain

There is a growing concern about excessive nitrogen (N) and water use in agricultural systems in North China due to the reduced resource use efficiency and increased groundwater pollution. A two-year experiment with two soil moisture by four N treatments was conducted to investigate the effects of N application rates and soil moisture on soil N dynamics, crop yield, N uptake and use efficiency in an intensive wheat–maize double cropping system (wheat–maize rotation) in the North China Plain. Under the experimental conditions, crop yield of both wheat and maize did␣not␣increase significantly at N rates above 200 kg N ha⁻¹. Nitrogen application rates affected little on ammonium-N (NH₄-N) content in the 0–100 cm soil profiles. Excess nitrate-N (NO₃-N), ranging from 221 kg N ha⁻¹ to 620 kg N ha⁻¹, accumulated in the 0–100 cm soil profile at the end of second rotation in the treatments with N rates of 200 kg N ha⁻¹ and 300 kg N ha⁻¹. In general, maize crop has higher N use efficiency than wheat crop. Higher NO₃-N leaching occurred in maize season than in wheat season due to more water leakage caused by the concentrated summer rainfall. The results of this study indicate that the optimum N rate may be much lower than that used in many areas in the North China Plain given the high level of N already in the soil, and there is great potential for reducing N inputs to increase N use efficiency and to mitigate N leaching into the groundwater. Avoiding excess water leakage through controlled irrigation and matching N application to crop N demand is the key to reduce NO₃-N leaching and maintain crop yield. Such management requires knowledge of crop water and N demand and soil N dynamics as they change with variable climate temporally and spatially. Simulation modeling can capture those interactions and is considered as a powerful tool to assist in␣the␣future optimization of N and irrigation managements. Section Editor: L. Wade     

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

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

Interactions between white clover and ryegrass under contrasting nitrogen availability: N2 fixation,N fertilizer recovery,N transfer and water use efficiency

Seasonal variation in N₂ fixation, N transfer from clover to ryegrass, and soil N absorption in white clover–ryegrass swards were investigated under field conditions over three consecutive years. The plots were established with different seeding ratios of clover and ryegrass and contrasting fertilizer N ranging from 3 to 72 kg ha^-1 year^-1.An initially poor clover population needed at least one growing season to reach the same yield output as an initially well established clover population. The clover content of the sward decreased by the annual application of 72 kg N ha^-1 but not by smaller N dressings.The total amount of atmospherically derived N in clover growing in mixture with ryegrass was, on average over the three years equal to 83, 71, 68 and 60 kg N ha^-1 for the treatments of 3, 24, 48 and 72 kg N ha^-1, respectively. The proportion of atmospherically derived N declined with increasing N application, but never became smaller than 80% of total clover N. The proportion of atmospherically derived N in a pure stand white clover amounted to 60–80% of the total N content, equivalent to 109, 110, 103 and 90 kg N ha^-1 for the treatments of 3, 24, 48 and 72 kg N ha^-1, respectively.Only small amounts of atmospherically derived N was transferred to the associated ryegrass during the first production year, while in each of the following years up to 21 kg ha^-1 was transferred. The average amount of N transferred from clover to ryegrass was equivalent to 3, 16 and 31% of the N accumulated in ryegrass in the first, second and third production year, respectively. Expressed relative to the total amount of fixed N₂ in the clover–ryegrass mixture, the transfer amounted to 3, 17 and 22% in the first, second and third production year, respectively. Thus transfer of atmospherically derived N from clover contributed significantly to the N economy of the associated ryegrass.The clover–ryegrass mixture absorbed constantly higher amount of soil derived N than the pure stands of the two species. Only 11% of the total accumulated fertilizer N and soil derived N in the mixture was contained within the clover component. Lower water use efficiencies for the plants grown in mixture compared to pure stands were mainly related to the increased N uptake in the mixture, with the subsequent increase in growth compared to the pure stands.It is concluded that positive interactions between clover and ryegrass growing in mixture ensure a more efficient fixation of atmospheric N₂ and absorption of fertilizer N and soil derived N than pure stands of the same species.     

Soil nitrous oxide emissions from a typical semiarid temperate steppe in inner Mongolia: effects of mineral nitrogen fertilizer levels and forms

Nitrous oxide (N₂O) emissions can be significantly affected by the amounts and forms of nitrogen (N) available in soils, but the effect is highly dependent on local climate and soil conditions in specific ecosystem. To improve our understanding of the response of N₂O emissions to different N sources of fertilizer in a typical semiarid temperate steppe in Inner Mongolia, a 2-year field experiment was conducted to investigate the effects of high, medium and low N fertilizer levels (HN: 200 kg N?ha^-1y^-1, MN: 100 kg N ha^-1y^-1, and LN: 50 kg N ha^-1y^-1) respectively and N fertilizer forms (CAN: calcium ammonium nitrate, AS: ammonium sulphate and NS: sodium nitrate) on N₂O emissions using static closed chamber method. Our data showed that peak N₂O fluxes induced by N treatments were concentrated in short periods (2 to 3 weeks) after fertilization in summer and in soil thawing periods in early spring; there were similarly low N₂O fluxes from all treatments in the remaining seasons of the year. The three N levels increased annual N₂O emissions significantly (P?<?0.05) in the order of MN > HN > LN compared with the CK (control) treatment in year 1; in year 2, the elevation of annual N₂O emissions was significant (P?<?0.05) by HN and MN treatments but was insignificant by LN treatments (P?>?0.05). The three N forms also had strong effects on N₂O emissions. Significantly (P?<?0.05) higher annual N₂O emissions were observed in the soils of CAN and AS fertilizer treatments than in the soils of NS fertilizer treatments in both measured years, but the difference between CAN and AS was not significant (P?>?0.05). Annual N₂O emission factors (EF) ranged from 0.060 to 0.298% for different N fertilizer treatments in the two observed years, with an overall EF value of 0.125%. The EF values were by far less than the mean default EF proposed by the Intergovernmental Panel on Climate Change (IPCC).     

Mineralization of nitrogen in long-term pasture soils: effects of management

A field incubation technique with acetylene to inhibit nitrification was used to estimate net N mineralization rates in some grassland soils through an annual cycle. Measurements were made on previously long-term grazed pastures on a silty clay loam soil in S.W. England which had background managements of +/– drainage and +/– fertilizer (200 kg N ha^–1 yr^–1). The effect of fertilizer addition on mineralization during the year of measurement was also determined. Small plots with animals excluded, and with herbage clipped and removed were used as treatment areas and measurements were made using an incubation period of 7 days at intervals of 7 or 14 days through the year. Soil temperature, moisture and mineral N contents were also determined. Mineralization rates fluctuated considerably in each treatment. Maximum daily rates ranged from 1.01 to 3.19 kg N ha^–1, and there was substantial net release of N through the winter period (representing, on average, 27% of the annual release). Changes in temperature accounted for 35% of the variability but there was little significant effect of soil moisture. Annual net release of N ranged from 135 kg ha^–1 (undrained soil, no previous or current fertilizer) to 376 (drained soil, +200 kg N ha^–1 yr^–1 previous and current fertilizer addition). Addition of fertilizer N to a previously unfertilized sward significantly increased the net release of N but there was no immediate effect of withholding fertilizer on mineralization during the year in which measurements were made.     

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

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