氮肥施用对冬小麦籽粒产量和氮素表观损失的影响

Excessive nitrogen （N） fertilizer application to winter wheat is a common problem on the North China Plain. To determine the optimum fertilizer N rate for winter wheat production while minimizing N losses, field experiments were conducted for two growing seasons at eight sites, in Huimin County, Shandong Province, from 2001 to 2003. The optimum N rate for maximum grain yield was inversely related to the initial soil mineral N content （Nmin） in the top 90 cm of the soil profile before sowing. There was no yield response to the applied N at the three sites with high initial soil mineral N levels （average 212 kg N ha^-1）. The average optimum N rate was 96 kg N ha^-1 for the five sites with low initial soil Nmin （average 155 kg N ha^-1） before sowing. Residual nitrate N in the top 90 cm of the soil profile after harvest increased with increasing fertilizer N application rate. The apparent N losses during the wheat-growing season also increased with increasing N application rate. The average apparent N losses with the optimum N rates were less than 15 kg N ha^-1, whereas the farmers＇ conventional N application rate resulted in losses of more than 100 kg N ha^-1. Therefore, optimizing N use for winter wheat considerably reduced N losses to the environment without compromising crop yields.     

Yield and uptake of fertilizer nitrogen by direct-drilled winter barley growing on a chalk soil

Abstract. This paper reports the growth and yield of grain and the utilization of fertilizer nitrogen applied on either one or two occasions in spring to a crop of winter barley established by direct drilling on a chalk soil in southern England. Nitrogen, as ammonium nitrate, was applied at rates of 0 to 140 kg N ha⁻¹ in a range of proportions on two occasions (March and April 1981); nitrogen-15 was used to facilitate study of the nitrogen utilization by the crop.
When sampled before the second top-dressing in April, the greatest number of tillers were found on plants treated with 70 and 100 kg N ha⁻¹ in March. The total above ground dry matter production at harvest was greatest when the split nitrogen dressing totalled more than 100 kg N ha⁻¹, although the apparent efficiency of nitrogen usage (kg DM per kg N applied) was greatest when 60 kg N ha⁻¹ was divided equally between the two application dates. Grain yield was heaviest (6.471 ha⁻¹) at the largest rate of nitrogen applied (140 kg N ha⁻¹); the lightest yield from the nitrogen treated crops was recorded from 100 kg N ha⁻¹ applied as a single dressing in April that stimulated shoot production and decreased individual grain weight. The recovery in grain and straw of labelled fertilizer nitrogen applied only in March averaged 42.2% and was 49.8% when the nitrogen was applied only in April. The recovery of nitrogen applied in both March and April at the total rate of 100 kg N ha⁻¹ but split 30/70 or 70/30 was 44.5% and 42.5% respectively. Non-fertilizer sources of nitrogen contributed 60.7–71.7% of the total nitrogen uptake by the crop at harvest.     

Nitrogen inputs and outputs in a small agricultural catchment in the eastern part of the United Kingdom

Abstract. Nitrate concentrations measured in an ephemeral stream draining a 170 ha clay catchment in eastern England, with about 23% arable land, were greater than 11.3 mg N 1–¹ on the resumption of flow each autumn but then declined. There was also a spring peak in two years out of seven, 1978–1984, which depend on the length of time soils was at field capacity in the preceding winter. Mean annual load measured in rain was 19 kg N ha^-1 and loss of nitrate in the stream 34 kg N ha^-1. A catchment nitrogen balance suggested that inputs, which averaged 130 kg N ha yr^-1, were generally more than outputs, average 108 kg N ha yr^-1', but gaseous losses were not taken into account.     

Sequential extraction methods for soil organic nitrogen

(pp. 825–831)
This study was carried out to clarify the effects of soil nitrate before cultivation and amounts of basal-dressed nitrogen on additional N application rate and yields of semi-forced tomato for three years from 1998 to 2000. The amounts and timing of additional N dressing were determined based on diagnosis of petiole sap nitrate. The top-dressing was carried out with a liquid fertilizer when the nitrate concentration of a leaflet's petiole sap of leaf beneath fruit which is 2–4 cm declined below 2000 mg L⁻¹.
For standard yield by the method of fertilizer application based on this condition, no basal-dressed nitrogen was required when soil nitrate before cultivation was 150 mg kg⁻¹ dry soil or higher in the 0–30 cm layer; 38 kg ha⁻¹ of basal-dressed nitrogen, which corresponds to 25% of the standard rate of fertilizer application of Chiba Prefecture, was optimum when soil nitrate before cultivation was 100150 mg kg⁻¹ dry soil; 75 kg ha⁻¹ of basal-dressed nitrogen, which corresponds to 50% of the standard, was optimum when soil nitrate before cultivation was under 100 mg kg⁻¹ dry soil. A standard yield was secured and the rate of nitrogen fertilizer application decreased by 49–76% of the standard by keeping the nitrate concentration of tomato petiole sap between 1000–2000 mg L⁻¹ from early harvest time to topping time under these conditions.     

Nitrate leaching losses under Miscanthus grass planted on a silty clay loam soil

Abstract. Nitrate leaching under newly planted Miscanthus grass was measured for three years. The crop received either no fertilizer-N or an annual spring application of 60 kg or 120 kg N ha^-1. During three winters soil water was collected from porous cup probes installed 90 cm deep. Nitrate leaching was calculated from the mean drain flow recorded in two drain gauges multiplied by the mean nitrate-N concentration in the soil water solutions collected. In the first year soil water nitrate concentrations were high on all treatments and N losses were 154, 187 and 228 kg ha^-1 respectively on the unfertilized treatment and those that received 60 or 120 kg N ha^-1. Leaching losses in the second and third years were, in turn, 8, 24 and 87 kg ha^-1 and 3, 11 and 30 kg ha^-1 for the unfertilized treatment and for the 60 and 120 kg N ha^-1 treatments respectively. Leaching losses were closer to those recorded under extensively managed grassland than arable land. The large losses in the first year were probably due to the previous agricultural management at the site and excessive inputs of N on the fertilized plots. In the second and third year, lower drainage volumes may also have influenced losses. The results show that Miscanthus , once established, can lead to low levels of nitrate leaching and improved groundwater quality compared with growing arable crops.     

Nitrate leaching from organic farms and conventional farms following best practice

Abstract. This paper compares nitrate leaching losses from organic farms, which depended on legumes for their nitrogen inputs (66 site years) with those from conventional farms using fertilizers under similar cropping and climatic conditions (188 site years). The conventional farms were within Nitrate Sensitive Areas in England, but sites following special practices associated with that scheme were excluded. Nitrate losses during the organic ley phase (including the winter of ploughing out) were similar (45 kg N ha^–1) to those from conventional long-term grass receiving fertilizer N inputs of less than 200 kg N ha^–1 (44 kg N ha^–1) and from the grass phase of conventional ley-arable rotations (50 kg N ha^–1). Losses from conventional grass receiving higher N inputs were greater than from organic or less intensive grass. Nitrate losses following arable crops averaged 47 and 58 kg N ha^–1 for the organic and conventional systems respectively, with part of the difference being due to the greater proportion of non-cereal break crops in the latter. Thus under similar cropping, losses from organic systems are similar to or slightly smaller than those from conventional farms following best practice.     

Nitrogen fertilizer requirements of cereals following grass

Abstract. The effect of increasing rates of nitrogen (N) fertilizer on the yield response of 3 or 4 consecutive winter cereal crops after ploughing out grass was investigated at six field sites on commercial farms in England and Wales. Amounts of N required for an economically optimum yield (>3 kg of grain for each kg of fertilizer N applied) ranged from 0 to 265 kg ha⁻¹ and were dependent on soil N supply, but not on crop yield. Optimum N rates were large (mean 197 kg N ha⁻¹) at three sites: two sites where cereals followed 2-year grass leys receiving low N inputs (<200 kg N ha⁻¹), and at one site where a cut and grazed 4-year ley had received c . 315 kg N ha⁻¹ of fertilizer N annually. At the other three sites where 4 and 5-year grass leys had received large regular amounts of organic manures (20–30 t or m³ ha⁻¹) plus fertilizer N ( c . 300 kg ha⁻¹ each year), optimum N rates were low (mean 93 kg N ha⁻¹) and consistently over-estimated by the farmer by an average of 107 kg N ha⁻¹. Optimum N rates generally increased in successive years after ploughing as the N supply from the soil declined. Determination of soil C:N ratio and mineral N (NO₃N+NH₄N) to 90 cm depth in autumn were helpful in assessing fertilizer N need. The results suggest there is scope to improve current fertilizer recommendations for cereals after grass by removing crop yield as a determinant and including an assessment of soil mineralizable N during the growing season.     

Nitrogen leaching losses under a less intensive farming and environment (LIFE) integrated system

Abstract. Less Intensive Farming and Environment (LIFE) management is a form of integrated farming which aims to meet farming's economic and environmental requirements. We used a farm-scale LIFE demonstration to measure nitrogen (N) leaching losses over a 6 year period (1995–2001) using ceramic suction cups and a meteorological model to give estimates of drainage volumes. Losses from the system averaged 49 kg N ha⁻¹, with an average drainage nitrate concentration of 15.5 mg N L⁻¹. Rainfall and its distribution strongly influenced the loss, and drainage N concentration only fell below the nominal target of 11.3 mg N L⁻¹ (the EU limit for potable water) in the two wettest seasons. Crop type did not have a significant effect on either postharvest mineral N (PHMN) in soil or the leaching loss in the subsequent winter. However PHMN and overwinter N leaching declined with increasing crop yield. Overwinter crop N uptake increased with early sowing: leaching loss was only 5 kg N ha⁻¹ under grass sown in early September. Measurements of PHMN, crop sowing date and drainage data were used to construct simple equations to predict average drainage N concentration under various scenarios. The large N loss from our site is partially attributable to soil type (shallow over limestone), indeed on similar soil the loss from a conventional farm nearby was greater. The LIFE practices of postharvest harrowing and late cereal sowing will minimize the need for agrochemical use but they stimulate mineralization and reduce plant N uptake in autumn, leaving more N at risk to leaching. Some assessment of all environmental impacts is needed if the benefits of integrated practices such as those used in LIFE are to be quantified.     

The estimation of mineralization, immobilization and nitrification in nitrogen-15 field experiments using computer simulation

A combination of mathematical analysis and computer simulation, using parameters readily measured in a nitrogen-15 field experiment, is employed to determine rates of mineralization, immobilization and nitrification under a growing crop. The procedure also yields the proportion of crop nitrogen uptake occurring as ammonium and nitrate.
When applied to -results from grass lysimeters receiving 250 or 900 kg N ha^–1 a^–1 as ammonium nitrate, the analysis suggested that at 250 kgN ha^–1 a^–1 64–66% of crop nitrogen uptake was as ammonium; at 900 kg N ha^–1 a^–1 the figure was 43–49%. Nitrification at 250kgNha^–1 was only 13–19kgN ha^–1 over 160d while at 900 kg N ha^–1 between 191 and 232 kg N ha^–1 were nitrified.
The results suggested that the apparent inhibition of nitrification in grassland soils may simply reflect poor substrate competition by nitrifying bacteria. Finally, there was a suggestion that mineralization/immobilization was lower at the high fertilizer rate.     

Denitrification in drained and undrained arable clay soil

Emissions of nitrous oxide (N₂O) and nitrogen gas (N₂) from denitrification were measured using the acetylene inhibition method on drained and undrained clay soil during November 1980-June 1981. Drainage limited denitrification to about 65% of losses from undrained soil. Emissions from the undrained soil were in the range 1 to 12 g N ha^–1 h^–1 while those from the drained soil ranged from 0.5 to 6 g N ha^–1 h^–1 giving estimated total losses (N₂O + N₂) of 14 and 9 kgN ha^–1.
Drainage also changed the fraction of nitrous oxide in the total denitrification product. During December, emissions from the drained soil (1.8±0.6 gN ha^–1 h^–1) were composed entirely of nitrous oxide, but losses from the undrained soil (2.7 ± 1.1 g N ha^–1 h^–1) were almost entirely in the form of nitrogen gas (the fraction of N₂O in the total loss was 0.02). In February denitrification declined in colder conditions and the emission of nitrous oxide from drained soil declined relative to nitrogen gas so that the fraction of N₂O was 0.03 on both drainage treatments. The delayed onset of N₂O reduction in the drained soil was related to oxygen and nitrate concentrations. Fertilizer applications in the spring gave rise to maximum rates of emission (5–12g N ha^–1 h^–1) with the balance shifting towards nitrous oxide production, so that the fraction of N₂O was 0.2–0.8 in April and May.     

Fate of nitrogen in cattle slurry following surface application or injection to grassland

Two field experiments commencing in winter (December) and spring (April) were conducted to determine the fate of nitrogen (N) in cattle slurry following application to grassland. In each experiment three methods of application were used: surface application, and injection ± the nitrification inhibitor, nitrapyrin. Slurry was applied at 80t ha⁻¹, (≡248 kg total N ha⁻¹ in the winter experiment, and 262 kg N ha⁻¹ in the spring experiment). From slurry applied to the surface, total losses of N through NH₃ volatilization, measured using a system of wind tunnels, were 77 and 53 kg N ha⁻¹ respectively for the winter and spring experiments. Injection reduced the total NH₃ volatilization loss to ∼2 kg N ha ⁻¹. Following surface application, loss by denitrification, measured using an adaptation of the acetylene-inhibition technique, was 30 and 5 kg N ha⁻¹ for the two experiments. Larger denitrification losses were observed for the injected treatments; in the winter experiment the loss from the injected slurry without nitrapyrin was 53 kgN ha ⁻¹, and with nitrapyrin 23 kgN ha⁻¹. Total denitrification losses for the corresponding injected treatments in the spring experiment were 18 and 14 kg N ha ⁻¹. Apparent recoveries of N in grass herbage in both experiments broadly reflected the differences between treatments in total gaseous loss.     

Nitrate leaching from reseeded pasture

Abstract. Nitrate leaching and soil mineral N status under grassland were measured on three contrasting soils, spanning winters 1995/96, 1996/97 and 1997/98, in Western England. The soils investigated were a freely draining silty clay loam (Rosemaund), a well drained loam (IGER 1) and a poorly drained clay loam (IGER 2). The effects of reseeding (ploughing and resowing grass) at IGER 1 and IGER 2 in autumn 1995 or 1996 were compared with undisturbed pasture. Reseeding at Rosemaund, in autumns 1995 or 1996, or spring 1996 was compared with undisturbed pasture of 3 sward ages (2, 5, >50 years).
Nitrate-N leaching losses during the winter immediately following autumn reseeding ranged between 60 and 350 kg N ha^–1 in 1995/96, depending on soil type, sward management history and rainfall. Losses were much less in the following winter when treatments were repeated (10–107 kg N ha^–1).
Reseeding in spring had little effect on soil mineral N content or leaching losses in the following autumn, compared with undisturbed pasture. Similarly, leaching losses from autumn reseeds in the second winter after cultivation were the same as undisturbed pasture (1-19 kg N ha^–1). The effect of ploughing grassland for reseeding was relatively short-term, in contrast to the effect of repeated annual cultivation associated with arable rotations.     

Nitrate leaching from a shallow limestone soil growing a five course combinable crop rotation: the effects of crop husbandry and nitrogen fertilizer rate on losses from the second complete rotation

Abstract. The field experiment tested the effects of three management systems on nitrate leaching losses from a five crop rotation on the Lincolnshire Limestone in Eastern England. The Standard system was similar to farming practice in the area. The Protective system integrated individual practices which were expected to decrease nitrate losses (e.g. cover crops, cultivation delay in autumn and reduced intensity, manipulation of drilling dates and, during the first few years of the first rotation, straw incorporation). The Intermediate system was a compromise between the two extremes. All crops were grown at full and half recommended nitrogen rates. This paper reports data from the second full rotation (years 6–10), thus enabling the medium-term effects of continued management practices to be investigated. Average annual nitrogen leaching losses at 49, 35 and 25 kg N ha^–1 for Standard, Intermediate and Protective systems, respectively, were significantly different. The respective flow-weighted average NO₃ concentrations were 167, 131 and 96 mg l^–1. Thus, adopting nitrate retentive practices through the rotation was able to substantially decrease losses. The Protective system was as effective as in the first full rotation, demonstrating that 10 years of such practices had not failed in the medium-term. However, continued minimal cultivation caused serious problems of weed build-up. The cost of weed control and yield loss caused by grass weeds made cereal production uneconomic in some years. Thus, rules for nitrate leaching control need to be tempered with practical and agronomic considerations. Also, few (if any) management techniques tested guaranteed that nitrate losses would be small in all years, as the interaction with winter weather, particularly rainfall, was of vital importance.     

Phosphorus and nitrogen turnover and risk of waterborne phosphorus emissions in crop rotations on a clay soil in southwest Sweden

Abstract. Nutrient losses from arable land are important contributors to eutrophication of surface waters, and phosphorus (P) and nitrogen (N) usually act together to regulate production of Cyanobacteria. Concentrations and losses of both nutrients in drainage water from pipe drains were studied and compared in 15 crop rotations on a clay soil in southwest Sweden. Special emphasis was placed on P and it was possible to evaluate critical components of the crop rotations by flow-proportional water sampling. Total P concentrations in drainage water were generally small (0.04–0.18 mg L⁻¹), but during two wetter years out of six, high P concentrations were measured following certain management practices, including ploughing-in lucerne ( Medicago sativa L.) and fertilizing in advance without incorporation into the soil to meet the needs of several subsequent crops. This resulted in average flow-weighted concentrations of total P between 0.3 and 0.7 mg L⁻¹. In crop rotations containing green manures, green fallow or leguminous leys, there was also a risk for increased P losses after these crops were ploughed in. The losses increased in the order: cash crops < dairy with grass < dairy with lucerne < monoculture with barley < organic farming with cattle slurry < stockless organic farming with green manure. P balances varied between −9 and +8 kg P ha⁻¹ and N balances between +4 and +35 kg N ha⁻¹. The balances were not related to actual leaching losses. Phosphorus losses in drainage from set-aside were 67–82% of those from cash crops grown in ploughed and P-fertilized soil at the same site, indicating a high background P loss from this clay soil.     

Inhibition of nitrate accumulation in tropical grassland soils: effect of nitrogen fertilization and soil disturbance

In acid soils in the Eastern Plains of Colombia, forage grasses planted on land prepared before the previous dry season produced 40–50% more dry matter than when land was prepared immediately before planting. Virtually no NO₃⁻ accumulated in surface (0–10 cm) soil from three native undisturbed savanna sites. Where land was ploughed before the dry season, NO₃⁻ levels increased gradually after a 2–3 month lag, and dropped at the beginning of the rains. In samples incubated for 4 weeks, more NO₃⁻ accumulated in the wet than the dry season. A similar 2–3-month lag occurred when land was ploughed after the dry season. NH₄⁺ levels were higher in ploughed than savanna soils, and rose in all soils at the beginning of the rains. More NO₃⁻ and NH₄⁺ accumulated on incubation in pots than in soil cores. Forage grasses inhibited NO₃⁻ accumulation in the soil, relative to plant-free plots, and legumes stimulated it. N fertilization overcame this inhibition except in the case of Brachiaria humidicola .     

Effects of ridging on crop performance and symbiotic N2 fixation of fababean (Vicia faba L.)

Abstract. The success of organic cropping systems depends on symbiotic N₂ fixation by leguminous crops, and it is important to explore new management systems to improve the nitrogen input through N₂ fixation. During two growing seasons the possible advantage of growing fababean ( Vicia faba L.) in ridges was studied in comparison to the traditional method on flat soil. Differences in soil physical parameters resulted in a significantly greater microbial activity and a deeper root system at the flowering stage when grown in the ridge than on the flat. Consequently, the amount of fixed N at flowering was significantly greater in ridges than in flat soil. However, during the period from flowering until harvest, when the major part of the N uptake and N₂ fixation took place, the differences between the treatments disappeared. Average values for the growing season of fluorescein diacetate hydrolysis, arylamidase activity and arylsulphatase activity were significantly greater in the ridge than on the flat, and the microbial biomass-C, derived from substrate induced respiration (SIR), was on average 232 and 223 μg C g⁻¹ soil in the ridge and on the flat, respectively. Measured total-N uptake, including root N (0–30 cm depth), ranged from 206 to 247 kg N ha⁻¹, of which 182–201 kg N ha⁻¹ was fixed N. From 154 to 173 kg N ha⁻¹ was removed in grain resulting in a soil-N balance of +28 kg N ha⁻¹ in both years. However, by including estimates of total root N and rhizodeposition-N the soil-N balance ranged from +52 to +62 kg N ha⁻¹.     

Leaching and balances of phosphorus and other nutrients in lysimeters after application of organic manures or fertilizers

Abstract. A long-term lysimeter experiment with undisturbed monoliths studied leaching behaviour and balances of phosphorus (P), potassium (K) and nitrogen (N) during a seven year crop rotation on four types of soil receiving inorganic fertilizers, manure and grass compost respectively. It was shown that application of manure did not lead to any direct change in nutrient leaching, unlike the application of fertilizers to soils of normal fertility. However, soil type considerably affected the nutrient concentrations in the drainage water.
Manure applied in amounts equal to the maximum animal density allowed by Swedish legislation slightly oversupplied P and N (0.5–3.5 and 18–38 kg ha⁻¹ y⁻¹ respectively) compared to the crop requirement and leaching losses for most of the soils. The relationship between lactate-soluble P in the topsoil and the concentrations of dissolved P in the drainage water was very strong. However the strength of this relationship was dependent on just one or two soils. P losses from a fertile sandy soil were large (1–11 kg ha⁻¹ y⁻¹) throughout the crop rotation and average crop removal (13 kg ha⁻¹ y⁻¹) plus the leaching losses were not balanced (average deficit 3–6 kg ha⁻¹ y⁻¹) by the addition of fertilizer, manure or grass compost. No decreasing trend was found in the P losses during seven years. However, the K deficit (average 26 kg ha⁻¹ y⁻¹) led to a significant reduction in the leaching trend from this soil. The other soils that had a smaller K deficit showed no significant reduction in the leaching of K.     

Nitrate leaching from a felled Sitka spruce plantation in Beddgelert Forest, North Wales

Abstract. Soil water samples from five horizons in a stagnopodzol were collected regularly over a five-year period in a Sitka spruce plantation at Beddgelert Forest, North Wales. Samples were analysed for nitrate-N and ammonium-N. After felling, inorganic-N concentrations increased markedly in the C horizon, generally decreased in the surface horizons and showed little change in the E and Bs horizons. Fluxes through the C horizon increased after felling from 10 to 70 kg N ha^-1 a^-1, the latter being equivalent to leaching losses in intensive lowland agricultural systems. Trends in concentration and flux were attributed to seasonal temperature and rainfall variations.
Nitrate-N dominated the dissolved inorganic-N, especially in the lower horizons. Nitrification was obviously active, despite the acid soil. Nitrate leaching losses occurred, even beneath the standing crop. On felling, cessation of nitrogen uptake allowed substantially more nitrate to be leached as no alternative sink was immediately available.     

Simulation of nitrogen dynamics in an alluvial sandy soil with drip fertigation of apple trees

Abstract. In order to optimize the management of the N-fertilizer inputs with drip fertigation on sandy-silt soil under apple tree orchard cultivation, we observed in situ: (i) the N and water soil transfers, (ii) the N levels in all leaves, fruits and annual shoots, and (iii) the root distribution. Then we used a mechanistic one-dimensional model (WAVE, Vanclooster et al. , 1994) to quantify the annual parameters of the water and nitrogen balance on a daily basis. The horizontal heterogeneity along the row of the tree-soil-dripper system has been treated with two adjacent compartments: one under the dripper and receiving fertigation and the other outside this zone. N transfers in the tree make it impossible to estimate directly N uptake by roots over time.
The simulated N losses were due to equal amounts of N leaching below 0.9 m deep (9 g N tree⁻¹year⁻¹ and denitrification (7 g N tree⁻¹year⁻¹. The simulated losses of gaseous N were localized predominantly in the compartment under the dripper and showed a higher rate of leaching during the period of N input when the wet conditions and the high NO₃⁻ concentrations were favourable to denitrification. The N-leaching at 0.9 m depth was greatest outside the growing season and was caused by the extension of the N-inputs after the harvest date. This practice, based on the objective to store nitrogen before the period of dormancy does not seem to be justified.     

Nitrogen conserving potential of successive ryegrass catch crops in continuous spring barley

Abstract. The nitrogen (N) conserving effects of Italian ryegrass ( Lolium multiflorum L.) undersown as a nitrate catch crop in spring barley ( Hordeum vulgare L.) were evaluated over a ten-year period in outdoor lysimeters (1.5 m deep, diam. 1 m) with sandy loam soil. Spring barley grown every year received 11.0 or 16.5 g N m⁻² before planting or was kept unfertilized. The N was given either as calcium ammonium nitrate or as ammoniacal N in pig slurry. From 1985 to 1989, ryegrass was undersown in the barley in half of the lysimeters while barley was grown alone in the remaining lysimeters. The grass sward was left uncut after barley harvest and incorporated in late winter/early spring. From 1990 to 1994 all lysimeters were in barley only.
Barley dry matter yields and crop N offtakes were not affected by the presence of undersown ryegrass, although grain yields appeared to be slightly reduced. After termination of ryegrass growing, N offtake in barley (grain+straw) was higher in lysimeters in which catch crops had been grown previously.
The loss of nitrate by leaching increased with N addition rate. Regardless of N dressing, ryegrass catch crops halved the total nitrate loss during 1985–1989, corresponding to a mean annual reduction in nitrate leaching of 2.0–3.5 g N m⁻². From 1990 to 1994, lysimeters previously undersown with ryegrass lost more nitrate than lysimeters with no history of ryegrass. The extra loss of nitrate accounted for 30% of the N retained by ryegrass catch crops during 1985–1989.
It is concluded that a substantial proportion of the N saved from leaching by ryegrass catch crops is readily mineralized and available for crop offtake as well as leaching as nitrate. To maximize benefits from ryegrass catch crops, the cropping system must be adjusted to exploit the extra N mineralization derived from the turnover of N incorporated in ryegrass biomass.