Losses of nitrate-nitrogen in water draining from under autumn-sown crops established by direct drilling or mouldboard ploughing

The leaching of nitrate-N under autumn-sown arable crops was measured using hydro-logically isolated plots, about 0.24 ha in area, from 1984–1988. Fluxes of water and nitrate moving over the soil surface (surface runoff), at the interface between topsoil and subsoil (interflow), and in the subsoil (drainflow) were monitored in plots with mole-and-pipe drain systems (drained plots); surface runoff and interflow only were monitored in ‘undrained’ plots. Half the drained and undrained plots were direct-drilled, and on the other half seedbeds were prepared by tillage to 200 mm. Tillage increased the total leaching loss of nitrate by 21 % compared with direct drilling in drained plots. About 95% or the nitrate moving from the soil was present in the water intercepted by the subsoil drains in these plots. In undrained plots less water and nitrate were collected in total; more of the nitrate was present in interflow on ploughed plots and in surface runoff in direct-drilled land. Losses of nitrate for the whole experiment from 1978-1988 were analysed. This showed that, between the harvest of one crop and the spring application of fertilizer to the next, loss of nitrate-N from ploughed land (L_p) was approximated by L_p=22+Fkg N ha^?1, where F was the autumn fertilizer-N applied. After fertilizer was applied in spring, loss of nitrate-N depended on rainfall such that for 100 mm rainfall about 30% of the fertilizer-N was lost by leaching. About 18% more nitrate-N was lost from direct-drilled land than from ploughed land in spring, but the total loss was generally small compared to that over winter. The apparent net mineralization of organic-N was measured in 1988. In autumn and winter there was little effect of tillage treatment (26 and 31 kg N ha^?1 on direct drilled and tilled plots respectively). However, over the year 83 kg N ha^?1 were mineralized in tilled plots, and 67 kg N ha^?1 in direct-drilled plots. Five factors governing the leaching of nitrate are assessed and this identified that fertilizer nitrogen application to the seedbed of winter sown crops and the mineralization of nitrogen from the residues of the previous crop are the most significant factors for nitrogen leaching in the UK.     

Fate of fertilizer nitrogen applied to grassland. I. Field leaching results

Results are presented from a 3 year investigation into nitrate leaching from isolated 0.4 ha grassland plots fertilized with 250, 500 and 900 kg N ha^?1 a^?1. Cumulative nitrate leaching over the 3 years was equivalent to 1.5%, 5.4% and 16.7% of the fertilizer applied at 250, 500 and 900 kg N ha^?1 rates respectively. Over a whole drainage season, mean nitrate leachate concentrations at 250 kg N ha^?1 did not exceed 4 mgl^?1, although maximum values of 13.3 mgl^?1 were observed. In contrast, at 900 kg N ha^?1, the mean nitrate leachate concentration in two of the years exceeded 90 mgl^?1. Mineral nitrogen balances constructed for the 1979 growing season indicated that leaching at 250 kg N ha^?1 was low because net mineralization of soil organic nitrogen was small, and crop nitrogen uptake almost balanced fertilizer application. Although the pattern of nitrate leaching suggested that by-passing occurred in the movement of water down the soil profile, it was not possible to confirm this using simulation models of leaching. Possible reasons for this, including the occurrence of rapid water flow down gravitationally drained macropores, are discussed.     

Variables controlling denitrification from earthworm casts and soil in permanent pastures

Summary Denitrification (using the acetylene block method) was determined in earthworm casts and soils from permanent, drained or undrained pasture plots fertilized with 0 or 200 kg N ha^-1 year^-1 as ammonium nitrate. Rates of N₂O production from soil cores were about three times higher from the fertilized than from the unfertilized plots while drainage had a relatively small effect. Denitrification rates from casts were 3–5 times higher than those from soil irrespective of the drainage treatment. Casts generally had higher NO _inf3 ^sup- , NH _inf4 ^sup+ , and moisture contents, and higher microbial respiration rates than soil. Rates of N₂O production were determined primarily by NO _inf3 ^sup- supply, secondarily by moisture; available C did not appear to limit denitrification in these pastures. Estimates of the potential contribution of casts to denitrification ranges from 10.1% of 29.3 kg ha^-1 year^-1 from the unfertilized, drained plot to 22% of 82.5 kg ha^-1 year^-1 from the fertilized undrained plot.     

Nitrate leaching from the Broadbalk Wheat Experiment, Rothamsted, UK, as influenced by fertilizer and manure inputs and the weather

Abstract. Nitrate leaching was measured over the eight drainage seasons spanning the nine years from 1990–1998 on the 157‐year old Broadbalk Experiment at Rothamsted, UK. The weather pattern of two dry, three wet and three dry years was the dominant factor controlling nitrogen (N) loss. Both the concentration of nitrate in the drainage waters and the amount of N leached increased with the amount of N applied, mostly because of long‐term, differential increases in soil organic matter and mineralization. On average, losses of N by leaching were 30 kg ha^?1yr^?1 when no more than the optimum N application was applied and were typical of amounts leached from arable land in the UK. Losses increased significantly in both amounts and as the percentage of N applied for supra‐optimal applications of N and from autumn‐applied farmyard manure (FYM). Extra spring‐applied fertilizer was very effective at increasing yields on plots given FYM in the autumn but at the expense of leaching losses three times those from optimum fertilizer N applications. Losses increased after potatoes because they left significant amounts of mineral N in the soil, and decreased after forage maize because it used applied N more effectively. Losses measured 120 years ago from identical treatments were 74% greater than current losses because of today's larger yields and more efficient varieties and management practices. Average concentrations of nitrate in drainage waters did not exceed the EU limit of 11.3 mg NO₃‐N l^?1 until supra‐optimal amounts of N fertilizer (>150–200 kg ha^?1yr^?1) were applied in spring or FYM was applied in autumn. However some drainage waters from all plots, even those that have not received fertilizer for >150 years, exceeded the limit when rain followed a dry summer and autumn. Nitrate leaching into waters will remain a problem for profitable arable farming in the drier parts of Eastern England and Europe despite increased N use efficiency.     

Effects of tillage and reseeding on phosphorus transfers from grassland

Pasture tillage and reseeding is part of the normal rotation cycle of grassland systems in the UK and a process that could increase the rate of phosphorus (P) transfer to water, thus potentially contributing to eutrophication. The effect of tillage and reseeding on P transfer was investigated at two scales: from drained and undrained 1 ha hydrologically isolated pasture plots within the Rowden Drainage Experiment in Devon, during a 6‐month winter drainage period in 1998–1999, and in replicated soil box experiments during simulated rainfall ‘events’. At the plot scale, total P exports of 3.75 kg P ha^?1 were determined over a 16‐day period, indicating that soil and P were most vulnerable to detachment and mobilization during rainfall and run‐off in this period. Once the sward had developed, and the vulnerability to soil detachment reduced, reseeded swards with pipe drainage transferred less P (approx. 0.3 kg P ha^?1 yr^?1) to water than is commonly measured on permanent grassland (approx. 1 kg P ha^?1 yr^?1). Soil box experiments showed that tilled soil transferred more P > 0.45 μm but P < 0.45 μm was retained. Sward cover is critical to reducing detachment and resulting P transfer from surface soil, and therefore careful consideration should be taken for the need to reseed. The effects of tillage and reseeding on phosphorus transfers from grassland can be potentially significant and ought to be mitigated against using low‐till practices to reduce potential contribution to water quality.     

Impact of grazed grassland management on total N accumulation in soil receiving different levels of N inputs

Nitrogen balances and total N and C accumulation in soil were studied in reseeded grazed grassland swards receiving different fertilizer N inputs (100–500 kg N ha^?1 year^?1) from March 1989 to February 1999, at an experimental site in Northern Ireland. Soil N and C accumulated linearly at rates of 102–152 kg N ha^?1 year^?1 and 1125–1454 kg C ha^?1 year^?1, respectively, in the top 15 cm soil during the 10 year period. Fertilizer N had a highly significant effect on the rate of N and C accumulation. In the sward receiving 500 kg fertilizer N ha^?1 year^?1 the input (wet deposition + fertilizer N applied) minus output (drainflow + animal product) averaged 417 kg N ha^?1 year^?1. Total N accumulation in the top 15 cm of soil was 152 kg N ha^?1 year^?1. The predicted range in NH₃ emission from this sward was 36–95 kg N ha^?1 year^?1. Evidence suggested that the remaining large imbalance was either caused by denitrification and/or other unknown loss processes. In the sward receiving 100 kg fertilizer N ha^?1 year^?1, it was apparent that N accumulation in the top 15 cm soil was greater than the input minus output balance, even before allowing for gaseous emissions. This suggested that there was an additional input source, possibly resulting from a redistribution of N from lower down the soil profile. This is an important factor to take into account in constructing N balances, as not all the N accumulating in the top 15 cm soil may be directly caused by N input. N redistribution within the soil profile would exacerbate the N deficit in budget studies.     

Nutrient distribution in a Norway spruce stand after long-term application of ammonium nitrate and superphosphate

The distribution of nutrients between soil layers and above-ground tree components was examined in a Norway spruce stand that had received ammonium nitrate (annually) and superphosphate (about every third year) for 22 years. Four treatments were included in the study; control (n = 4), N1P1, N2P2 and N3P2 (n = 2), which had received a total of 0, 730, 1700 and 2550 kg nitrogen (N) ha^-1, 0, 130, 300 and 300 kg phosphorus (P) ha^-1 and 0, 340, 784 and 784 kg calcium (Ca) ha^-1, respectively. Compared with the control, stem-wood growth had been three times higher in N1P1 and three and a half times higher in N2P2 and N3P2. Amounts of N, P, Ca, potassium (K) and magnesium (Mg) in the above-ground tree biomass increased (p<0.05) with the fertilizer dose, whereas manganese (Mn) did not. The recovery of fertilizer N and Ca in soil and above-ground tree biomass was negatively related to the fertilizer dose, although there had been a buildup of the N and Ca pools in the mor layer. This strongly indicates that at least the higher doses of N addition caused substantial nitrate leaching. Soil pools of K, Mg and Mn decreased as the fertilizer dose increased. However, the system total amounts (above-ground tree biomass plus soil) of K and Mg did not differ between treatments suggesting that no extra losses of these ions induced by nitrate leaching have occurred. Thus, in an aggrading forest ecosystem, N additions are likely to be followed by increased uptake of K, Mg and Ca. This may to some extent prevent extra leaching of these ions, which otherwise would be expected when there is an increase in nitrate leaching.     

Sulphur cycling in New Zealand hill country pastures. II. The fate of fertilizer sulphur

The fate of fertilizer sulphur (S) applied as single superphosphate (SSP) to grazed pasture was examined in a field experiment for a period of 18 months using ³⁵S-labelled SSP. Four sites were selected on the basis of contrasting fertilizer history and land slope. The fertilizer histories since 1981 for the sites were 125 (LF) and 375 (HF) kg ha^-1 a^-1 SSP and the slope gradients were low (LS, 0-12°) and medium (MS, 13–26°). The amount of fertilizer S taken up by pasture as a fraction of total applied was greater at the LF (12%) than the HF (6%) site, suggesting that pasture at the LF site depended more on fertilizer than pasture at the HF site. At the LF site, fertilizer application did not significantly increase leaching losses of S (13 and 8.6 kg S ha^-1 for fertilized and unfertilized plots, respectively). At the HF site, fertilizer application significantly increased leaching losses of S (38 and 21 kg S ha^-1 for fertilized and unfertilized plots, respectively). The amount of fertilizer S lost by leaching as a fraction of total applied was greater at the HF (20%) than the LF site (7.6%). Most fertilizer S remained as soil organic matter. Plant uptake and leaching losses of fertilizer S were greater in the first year after application. The amount of N lost by leaching was very small in terms of N cycled through soil-plant system (1 to 6 kg N ha^-1). The majority (> 80%) of the S and N taken up by pasture and lost by leaching was derived from the mineralization of soil organic matter and not from freshly applied fertilizer.     

A lysimeter study of the fate of fertilizer nitrogen in spring barley crops grown on shallow soil overlying Chalk: crop uptake and leaching losses

The objective of this work was to determine the fate of fertilizer nitrogen (labelled with nitrogen-15) applied to an undisturbed shallow soil overlying Chalk contained in 10 lysimeters (80 cm diameter, 135 cm deep). Measurements are reported of the nitrogen uptake by four spring barley crops and the rate and extent of leaching of nitrate beyond the roots. The crops were fertilized with 0, 80 or 120 kg N ha^?1 in each of four years, but only the first application in 1977 was labelled with nitrogen ?15. Rainfall and irrigation approximated to the long-term average, but in two treatments dry or wet spring conditions were imposed for the 10 weeks after sowing the first crop in 1977. The dry matter and grain yields of the spring barley crops varied from year to year in the ranges 8.7–14.0 t ha^?1 and 3.5–6.1 t ha^?1 respectively. The total nitrogen harvested in the crop approximated to the amount of nitrogen applied in each year with an apparent recovery of fertilizer in the range 38–76%. The recovery of nitrogen derived from fertilizer (labelled with nitrogen-15) was 46–54% in the first crop and after 2 years rapidly declined to below 1%. The total amount of nitrogen-15 labelled fertilizer recovered in four barley crops was 49–57% of that applied. Mean annual nitrate concentrations in water draining from the base of the lysimeters were in the range 11.8–26.7 mg N 1^?1 and did not differ significantly between nitrogen fertilizer treatments (0, 80 and 120 kg N ha^?1 a^?1). In all treatments nitrate concentrations varied considerably within each growing season, with a cycle of peaks and troughs. Annual losses of nitrate were in the range 39–128 kg N ha^?1, and the mean annual losses over the 4 years varied between lysimeters from 65 to 83 kg N ha^?1. Nitrogen-15 labelled nitrate was detected in the first drainage water collected in autumn following its spring application, 5 months earlier. Recovery of fertilizer-derived nitrogen in drainage water was greatest during the winter following the second barley crop, and was 3.4–3.7% of the nitrogen-15 applied. Over the 4 years of the experiment 6.3–6.6% of labelled fertilizer was accounted for in drainage water, representing 2–3% of the total nitrogen lost by leaching.     

上海郊区蔬菜田氮素流失的研

Nitrogen （N） leaching in vegetable fields from December 2002 to May 2003 with equal dressings of total N for a sequential rotation of Chinese flat cabbage （Brassica chinensis L. var. rosularis） and lettuce （Lactuca sativa L.） in a suburban major vegetable production base of Shanghai were examined using the lysimeter method to provide a scientific basis for rational utilization of nitrogen fertilizers so as to prevent nitrogen pollution of water resources. Results showed that leached N consisted mainly of nitrate N, which accounted for up to more than 90% of the total N loss and could contribute to groundwater pollution. Data also showed that by partly substituting chemical N （30%） in a basal dressing with equivalent N of refined organic fertilizer in the Chinese flat cabbage field, 64.5% of the leached nitrate N was reduced, while in the lettuce （Lactuca sativa L.） field, substituting 1/2 of the chemical N in a basal dressing and 1/3 of the chemical N in a top dressing with equivalent N of refined organic fertilizers reduced 46.6% of the leached nitrate N. In the twoyear sequential rotation system of Chinese flat cabbage and lettuce, nitrate-N leaching in the treatment with the highest amount of chemical fertilizer was up to 46.55 kg ha^-1, while treatment plots with the highest amount of organic fertilizer had only 17.58 kg ha^-1. Thus, partly substituting refined organic fertilizer for chemical nitrogen in the first two seasons has a great advantage of reducing nitrate-N leaching.     

Denitrification loss from a grassland soil in the field receiving different rates of nitrogen as ammonium nitrate

Denitrification loss from a loam under a cut ryegrass sward receiving 0, 250 and 500 kg N ha^?1 a^?1 in four equal amounts was measured during 14 months using the acetylene-inhibition technique. The rate of denitrification responded rapidly to changes in soil water content as affected by rain. Mean rates of denitrification exceeded 0.2 kg N ha^?1 day^?1 only when the soil water content was >20% (w/w) and nitrate was >5μ N g^?1 in the upper 20 cm of the profile and when soil temperature at 2 cm was >5–8°C. When the soil dried to a water content <20%, denitrification decreased to <0.05 kg N ha^?1 day^?1. Highest rates (up to 2.0 kg N ha^?1 day^?1) were observed following application of fertilizer to soil at a water content of about 30% (w/w) in early spring. Denitrification in the control plot during this period was generally about a hundredth of that in plots treated with ammonium nitrate. High rates of N₂O loss (up to 0.30 kg N ha^?1 day-1) were invariably associated with high rates of denitrification (> 0.2 kg N ha^?1 day^?1). However, within 2–3 weeks following application of fertilizer to the plot receiving 250 kg N ha^?1 a^?1 the soil acted as a sink for atmospheric N₂O when its water content was >20% and its temperature >5–8°C. Annual N losses arising from denitrification were 1.6, 11.1 and 29.1 kg N ha^?1 for the plots receiving 0, 250 and 500 kg N ha^?1 a^?1, respectively. More than 60% of the annual loss occurred during a period of 8 weeks when fertilizer was applied to soil with a water content >20%.     

Potassium leaching from cut grassland and from urine patches

In grassland farming, especially on coarse‐textured soils, K can be a critical element. On these soils, the actual K management as well as fertilizer history to a large extent determine the leaching of K. The effects of four fertilizer regimes on the nutrient balances and leaching of K from grassland grown on a sandy soil were investigated. The swards differed in the source and level of N input and K fertilizer: no fertilizer N + 166 kg K ha^?1 year^?1 (Control), 320 kg inorganic N ha^?1 + 300 kg K ha^?1 year^?1 (MIN 320), 320 kg N + 425 kg K ha^?1 year^?1 in form of cattle slurry (SLR 320) and a grass–clover sward + 166 kg K ha^?1 year^?1 (WCL 0) without any inorganic N input. In a second experimental phase, cores from these swards were used in a mini‐lysimeter study on the fate of K from urine patches. On cut grassland after 6 years K input minus removal in herbage resulted in average K surpluses per year of 47, 39, 56 and 159 kg K ha^?1 for the Control, MIN 320, WCL 0 and SLR 320, respectively. Related leaching losses per year averaged 7.5, 5, 15 and 25 kg K ha^?1. Losses of urinary‐K through leaching were 2.2–4.5 and 5.7–8.4% of the K supplied in summer and autumn applications, respectively. Plant and soil were the major sinks for K from fertilizer or urine. High levels of exchangeable K in the soil and/or large and late fertilizer or urine applications stimulated leaching of K.     

A 10‐year study of phosphorus balances and the impact of grazed grassland on total P redistribution within the soil profile

Phosphorus (P) inputs (wet deposition and fertilizer P) and outputs (animal product and drainflow) were studied on reseeded grazed grassland swards receiving different nitrogen (N) inputs (100–500 kg N ha^?1 year^?1) for 10 years (March 1989–February 1999), at an experimental site in Northern Ireland. All plots received the same maintenance application of P fertilizer (8.5 kg P ha^?1 year^?1) to meet grass requirements, to minimize the P surplus and to quantify the impact on P losses to land drainage water. The annual flow weighted mean total P concentrations in drainflow ranged from 187 to 273 μg P litre^?1 and were well above the concentrations believed to trigger eutrophication. Annual total P lost to drainage water ranged from 0.28 to 1.73 kg P ha^?1, but was unaffected by N input. As the average annual P balance was zero, there was no significant change in total P in the top 15 cm of soil. However, there was a highly significant redistribution of P to the soil surface from the 10–15 cm depth, possibly as a result of root acquisition and earthworm activity. Total P in the top 5 cm of soil increased from 0.85 g kg^?1 to 1.04 g kg^?1, over the 10 years of the study, despite there being no net P input. This P accumulation in the top few cm of soil is likely to exacerbate P losses in overland flow and make improvements in water quality difficult to achieve.     

Potassium budgets in grassland systems as affected by nitrogen and drainage

Abstract. The main inputs, outputs and transfers of potassium (K) in soils and swards under typical south west England conditions were determined during 1999/00 and 2000/01 to establish soil and field gate K budgets under different fertilizer nitrogen (N) (0 and 280 kg ha⁻¹ yr⁻¹) and drainage (undrained and drained) treatments. Plots receiving fertilizer N also received farmyard manure (FYM). Potassium soil budgets ranged, on average for the two years, from −5 (+N, drained) to +9 (no N and undrained) kg K ha⁻¹ yr⁻¹ and field gate budgets from +23 (+N, drained) to +89 (+N, undrained). The main inputs and outputs to the soil K budgets were fertilizer application (65%) and plant uptake (93%). Animals had a minor effect on K export but a major impact on K recycling. Nitrogen fertilizer application and drainage increased K uptake by the grass and, with it, the efficiency of K used. It also depleted easily available soil K, which could be associated with smaller K losses by leaching.     

The mineralisation and fate of nitrogen following ploughing of grass and grass-clover swards

This project aimed to investigate the release of mineral N following the ploughing of clover-rich and grass-dominated swards, previously subject either to cutting or grazing regimes. The hypotheses tested were firstly that N mineralisation and losses following incorporation of grass-clover swards are greater than from grass swards, and secondly that N mineralisation and losses following incorporation of previously grazed swards are greater than from previously cut swards. Following ploughing of previously grazed swards in 1992 and swards that had been subjected to an unfertilised, ungrazed regime in 1993, N uptake, N leaching losses (measured by soil solution samplers with drainage estimation from a nearby experiment) and N₂O losses (measured by the closed chamber method) were determined on both resown and fallow plots. Results showed: (1) higher N release after ploughing from the grass-fallow treatment (449 kg N ha^-1) than from the grass-clover fallow treatment (244 kg N ha^-1) over 18 months; (2) the net release of N after ploughing and reseeding, compared with a continued unfertilised sward, was about 85 kg ha^-1 for the grass-clover plots and 140 kg ha^-1 for the grass-only plots, over the following 18 months. Of this, the net releases in the second cropping season after incorporation were 19 and 25 kg N ha^-1 on the resown grass-clover and grass-only plots, respectively; (3) the net release of mineral N after ploughing in 1993/1994, when swards had not been grazed for over a year, was only about 40 kg ha^-1 and no effect of the previous sward was evident; (4) in the 7 weeks after the 1992 ploughing, there was a considerable short-term input of N₂O to the atmosphere (1.5-3.7 kg N ha^-1), due to the supply of readily available C. Leaving swards ungrazed and unfertilised over winter before ploughing in spring has the potential to reduce such emissions considerably. We conclude that N release following cultivation of grazed swards is more a function of grazing intensity and history prior to ploughing rather than of sward composition.     

A LYSIMETER STUDY USING NITROGEN-15 ON THE UPTAKE OF FERTILIZER NITROGEN BY PERENNIAL RYEGRASS SWARDS AND LOSSES BY LEACHING

Perennial ryegrass growing in monolith lysimeters and treated with 400 kg N ha^-1 as calcium nitrate labelled with nitrogen-15 (10.5 atoms per cent), during one growing season recovered between 43 and 54 per cent of the fertilizer nitrogen. In the following year without further nitrogen additions 4.6–9.5 per cent was taken up, whilst in the fifth year the recovery was less than 1 per cent. The contribution of non-fertilizer sources of nitrogen to the total nitrogen taken up by the plants during the season that nitrogen was applied was estimated using tracer methods to be about 13–14 g N m^-2 year^-1. The estimate from measuring the nitrogen content of an unfertilized sward was 7 g N m^-2 year^-1. The residual effects of a fertilizer application are likely to be detectable for a period of between 6 and 9 years. Losses of nitrogen to drainage in the winter after application represented 2–5 per cent of the fertilizer applied, whilst in subsequent years the amounts did not exceed 0.1 per cent. Mean concentrations of nitrate ranged between 4 and 16 mg N I^-1. Fertilizer contributed about 60–70 per cent of the total nitrogen lost in the first winter after nitrogen application and 45–60 per cent averaged over three winters.     

INFLUENCE OF ORGANIC AND MINERAL FERTILIZATION ON GERMINATION,LEAF NITROGEN,NITRATE ACCUMULATION AND YIELD OF VEGETABLE AMARANTH

The influence of manure and diammonium phosphate (DAP) mineral fertilizer on germination, leaf nitrogen content, nitrate accumulation and yield of vegetable amaranth (Amaranthus hypochondriacus) was investigated. Field trials were set up at the University of Nairobi Field Station at the Upper Kabete Campus during the long rains of March–May in 2007 and 2008. Trials were laid out as complete randomized block design with four fertilization treatments: 20, 40, and 60 kg nitrogen (N) ha^?1 supplied by DAP (18:46:0), 40 kg N ha^?1 supplied by cattle manure and an unfertilized control variant. The vegetables were harvested at three maturity stages at 6, 7, and 8 weeks after planting. Results indicated that there were significant differences between treatments in germination percentage, leaf nitrogen content, nitrate accumulation and vegetable yield. Plants that received manure had a higher germination percentage than those that received the same amount of N supplied by the chemical fertilizer DAP. The yields generally increased from week 6 to week 8. The highest yield was recorded in plots receiving 40 kg N ha^?1 from DAP at eight weeks after planting. Plots that were supplied with manure recorded the lowest yield when compared to the fertilizer treated plots at all rates. Leaf nitrogen content increased with increasing rate of N but only when N was supplied by DAP fertilizer. The leaf nitrogen content decreased with increasing age of the plants. The leaf nitrate content increased with increase in DAP application rate. Results indicate that manure application produced quality vegetables in terms of low nitrate levels, but leaf nitrogen and vegetable yields were low. DAP application effected higher yields, but the vegetables had high though acceptable nitrate levels.     

Mineral nitrogen dynamics in poorly drained blanket peat

Summary Mineral-N dynamics have been measured over a period of 3 years in PK- and NPK-treated plots (4 m²) laid out on an area of poorly drained, reseeded, blanket peat in the north of Scotland. Mineral-N, present in the peat almost entirely as NH _in4 ^sup+ , accumulated in winter, reaching 42 kg N ha^–1 in the surface 10 cm in April before the application of 112.5 kg N ha^–1 as NH₄NO₃ or urea. In situ incubation of peat cores isolated to prevent leaching, and with grass tops removed, confirmed that net mineralization occurred between November and April, with the greatest rate, 1.2 kg N ha^–1 day^–1, recorded between March and April. During the period May to early June, immobilization of N predominated and rates of net immobilization ranged between 0.2 and 0.8 kg N ha^–1 day^–1. This coincided with a poor uptake into herbage, less than 16% of soil mineral N and fertilizer NH₄NO₃ in June of the first 2 years. The largest counts (most probable number) of ammonifying bacteria in the surface 5 cm were recorded in July for aerobes (27.1×10⁹ litre^–1) and August for anaerorbes (7.1×10⁹ litre^–1). N fertilizer increased these counts significantly (P<0.05) to 56×10⁹ aerobes and 13×10⁹ anaerobes. During July and August, in 2 out of the 3 years, mineralization predominated over immobilization and mean net rates of up to 0.9 kg N ha^–1 were recorded.     

Nitrate leaching in an organic dairy/crop rotationas affected by organic manure type, livestock densityand crop

Abstract. In dairy farming systems the risk of nitrate leaching is increased by mixed rotations (pasture/arable) and the use of organic manure. We investigated the effect of four organic farming systems with different livestock densities and different types of organic manure on crop yields, nitrate leaching and N balance in an organic dairy/crop rotation (barley–grass-clover–grass-clover–barley/pea–winter wheat–fodder beet) from 1994 to 1998. Nitrate concentrations in soil water extracted by ceramic suction cups ranged from below 1 mg NO₃-N l^?1 in 1^st year grass-clover to 20–50 mg NO₃-N l^?1 in the winter following barley/pea and winter wheat. Peaks of high nitrate concentrations were observed in 2^nd year grass-clover, probably due to urination by grazing cattle. Nitrate leaching was affected by climatic conditions (drainage volume), livestock density and time since ploughing in of grass-clover. No difference in nitrate leaching was observed between the use of slurry alone and farmyard manure from deep litter housing in combination with slurry. Increasing the total-N input to the rotation by 40 kg N ha^?1 year^?1 (from 0.9 to 1.4 livestock units ha^?1) only increased leaching by 6 kg NO₃-N ha^?1. Nitrate leaching was highest in the second winter (after winter wheat) following ploughing in of the grass-clover (61 kg NO₃-N ha^?1). Leaching losses were lowest in 1^st year grass-clover (20 kg NO₃-N ha^?1). Averaged over the four years, nitrate concentration in drainage water was 57 mg l^?1. Minimizing leaching losses requires improved utilization of organic N accumulated in grazed grass-clover pastures. The N balance for the crop rotation as a whole indicated that accumulation of N in soil organic matter in the fields of these systems was small.     

Fate of fertilizer nitrogen applied to grassland. II. Nitrogen-15 leaching results

Results are presented from a 3-year investigation into nitrate leaching from grassed monolith lysimeters treated with double (¹⁵NH₄¹⁵NO₃) or single (¹⁵NH₄NO₃) labelled ammonium nitrate at three rates, 250, 500 and 900 kg N ha^?1 a^?1. Over the 3 years of the experiment, 0.14%, 3.1% and 18.1% of the applied fertilizer was recovered in the leachate at 250, 500 and 900kg N ha^?1 respectively. This represented 9%, 39% and 75% of the overall nitrate leaching at the three application rates. A significant proportion of the fertilizer leached as nitrate at the three application rates was derived, via nitrification, from the fertilizer ammonium. Increasing fertilizer applications caused a rise in the leaching of both soil and fertilizer derived nitrogen, although whether the increase reflected a true priming effect was not clear.