Nitrate leaching as influenced by soil tillage and catch crop

Because of public and political concern for the quality of surface and ground water, leaching of nitrate is of special concern in many countries. To evaluate the effects of tillage and growth of a catch crop on nitrate leaching, two field trials were conducted in spring barley (Hordeum vulgare L.) under temperate coastal climate conditions. On a coarse sand (1987–1992), ploughing in autumn or in spring in combination with perennial ryegrass (Lolium perenne L.) as a catch crop was evaluated. Furthermore, rotovating and direct drilling were included. The experiment was conducted on a 19-year-old field trial with continuous production of spring barley. On a sandy loam (1988–1992), ploughing in autumn or in spring in combination with stubble cultivation and perennial ryegrass, in addition to minimum tillage, was evaluated in a newly established field trial. For calculation of nitrate leaching, soil water isolates from depths of 0.8 or 1.0 m were taken using ceramic cups. No significant effect of tillage was found on the coarse sand; however, a significant effect of tillage was found on the sandy loam, where leaching from autumn ploughed plots without stubble cultivation was 16 kg N ha⁻¹ year⁻¹ higher than leaching from spring ploughed plots. Leaching was significantly less when stubble cultivation in autumn was omitted. Leaching on both soil types was significantly reduced by the growth of a catch crop which was ploughed under in autumn or in spring. It was concluded that soil cultivation increased leaching on the sandy loam but not on the coarse sand, and that the growth of perennial ryegrass as a catch crop reduced leaching on both soil types, particularly when ryegrass was ploughed under in spring.     

Effects of cover crops sown in autumn on N and P leaching

A field experiment with separately tile-drained plots was used to study the ability of oilseed radish (Rhaphanus sativus L.), as a cover crop sown after harvest of a main crop of cereals or peas, to reduce nitrogen (N) and phosphorus (P) leaching losses from a clay loam in southern Sweden over 6 years. In addition to oilseed radish in pure stand, two cover crop mixtures (hairy vetch (Vicia villosa) and rye (Secale cereale) for 3 years and oilseed radish in mixture with buckwheat (Fagopyrum esculentum) for 2 years) were tested. The cover crop plots (three replicates per treatment) were compared with unplanted plots as a control. Plots cropped with oilseed radish during autumn (August–November) had significantly smaller yearly mean N concentration in drainage water over 5 of 6 years compared with unplanted controls. Mineral N content in the soil profile in autumn was significantly less in oilseed radish plots than for control plots in all years. The cover crop mixtures of hairy vetch and rye or buckwheat and oilseed radish also showed the potential to reduce soil mineral N in autumn and N concentration in drainage water, compared with unplanted controls. The cover crops had no impact on P leaching. In conclusion, oilseed radish has the ability to reduce leaching losses of N, without increasing the risk of P leaching.     

Changes in N leaching and crop production as a result of measures to reduce N losses to water in a 6‐yr crop rotation

The effects of various measures introduced to increase nitrogen (N)‐use efficiency and reduce N losses to water in a 6‐yr crop rotation (winter wheat, spring barley, green manure, winter wheat, spring barley, spring oilseed rape) were examined with respect to N leaching, soil mineral N (SMN) accumulation and grain yield. An N‐use efficient system (NUE) with delayed tillage until late autumn and spring, direct drilling of winter wheat, earlier sowing of winter and spring crops and use of a catch crop in winter wheat was compared with a conventional system (CON) in a field experiment with six separately tile‐drained plots in south‐western Sweden during the period 1999–2011 (two crop rotation cycles). Total leaching of NO₃‐N from the NUE system was significantly 46 and 33% lower than in the CON system during the first and second crop rotation cycle, respectively, with the most pronounced differences apparently related to management strategies for winter wheat. Differences in NO₃‐N leaching largely reflected differences in SMN during autumn and winter. There was a tendency for lower yields in the NUE system, probably due to problems with couch grass. Overall, the measures for conserving N, when frequently used within a crop rotation, effectively reduced NO₃ concentrations in drainage water and NO₃‐N leaching losses, without severely affecting yield.     

The effect of crop, N-level, soil type and drainage on nitrate leaching from Danish soil

Abstract. Nitrate leaching measurements in Denmark were analysed to examine the effects of husbandry factors. The data comprised weekly measurements of drainage and nitrate concentration from pipe drains in six fields from 1971 to 1991, and weekly measurements of nitrate concentration in soil water, extracted by suction cups at a depth of 1 m, from 16 fields in 1988 to 1993. The soils varied from coarse sand to sandy clay loam.
The model used for analysing the data was: Y = exp (1.136–0.0628 clay + 0.00565N + crop ) D^0.416, with R²= 0.54, where Y is the nitrate leaching (kg N/ha per y), clay is the % clay in 0-25 cm depth (%), N is the average N-application in the rotation (kg/ha/y) and D is drainage (mm/y). The most important factor influencing leaching was the crop type. Grass and barley undersown with grass showed low rates of leaching (17-24 kg/ha/y). Winter cereal following a grass crop, beets, winter cereals following cereals and an autumn sown catch crop following cereals showed medium rates of leaching (36-46 kg/ha/y). High rates of leaching were estimated from winter cereals following rape/peas, bare soil following cereals and from autumn applications of animal manure on bare soil (71-78 kg/ha/y). Estimates of leaching from soil of 5, 12 and 20% clay were 68, 44 and 26 kg/ha/y, respectively. Leaching was estimated to rise significantly with increasing amounts of applied N.
The model is suitable for general calculations of the effects of crop rotation, soil type and N-application on nitrate leaching from sandy soil to sandy clay loarns in a temperate coastal climate.     

Cover crop growth and impact on N leaching as affected by pre‐ and postharvest sowing and time of incorporation

In Northern Europe, cover crops are traditionally established before spring crops by undersowing, but some cover crops might also have an effect if preharvest sown before spring crops and even winter crops. The effects of cover crop sowing date, sowing technique and succeeding main crop on biomass production, N uptake, nitrate leaching and soil inorganic N were tested in lysimeters and in the field. Cruciferous cover crops (oil radish, white mustard) were sown preharvest by broadcasting into winter wheat in July and were allowed to grow until a following winter wheat was established in September. Other preharvest cover crops were left in place until late autumn. For comparison, the same cruciferous cover crops were established postharvest after light harrowing. Perennial ryegrass undersown in spring barley was also included. Aboveground N uptake in preharvest cover crops amounted to a maximum of 24 kg N/ha in September before sowing winter wheat. When left until late autumn, preharvest oil radish took up a maximum of 66 kg N/ha, and ryegrass and postharvest cover crops 35 kg N/ha. Preharvest establishment of cruciferous cover crops before a spring‐sown crop thus seems promising. The soil was depleted of inorganic N to the same extent in late autumn irrespective of cover crop type, sowing time and technique within winter wheat or spring barley. However, the reduction in nitrate leaching of preharvest cover crops incorporated after 2 months and followed by winter wheat was only half of that achieved by cover crops left until late autumn or spring.     

Measured and simulated availability and leaching of nitrogen associated with frequent use of catch crops

Abstract. Results are presented from three years (1992-1995) of a field leaching experiment on a sandy soil in south-west Sweden. Plots of spring cereals, either with or without an undersown perennial ryegrass catch crop, were compared for nitrogen leaching and nitrogen status in soil. Both treatments were ploughed in spring, and other tillage regimes were also identical. Measurements of nitrogen leaching from drains, nitrogen uptake in crops and mineral nitrogen in the soil were made. Two coupled, simulation models, which describe water flow and nitrogen transformations and transport in soil, were used to interpret data and to calculate the nitrogen budget and nitrogen mineralization in the soil.
Nitrogen leaching was 40 50% less in the catch crop treatment compared with the control during years when the establishment of the catch crop succeeded. In the third year of the experiment nitrogen leaching was actually greater in the catch crop treatment (7 kg N/ha). This increase was caused by a poorly established catch crop coinciding with enhanced mineralization of previous catch crop residues. There was no simulated change in soil organic nitrogen in either of the treatments. Simulations showed increased nitrogen mineralization during April-July after incorporation of plant material in spring, especially in the catch crop treatment. However, the increased nitrogen mineralization probably occurred too late for the released nitrogen to be fully available to the main crop.     

An empirical model for estimating nitrate leaching as affected by crop type and the long-term N fertilizer rate

Abstract. An empirical model was developed for prediction of annual average nitrate leaching as affected by the long-term rate of N fertilization and crop type. The effect of N fertilization was estimated from annual values of nitrate leaching obtained from two Danish investigations of drainage from pipe drains with four rates of N fertilization on a loamy sand and sandy clay loam from 1973-89. The effect of crop at normal N fertilization was estimated from 147 observations of annual nitrate leaching obtained from field measurements. The nitrate leaching model consists of a relative N fertilization submodel and an absolute submodel for specific combinations of crop, soil and drainage at the normal rate of N fertilization. The relative submodel is Y/Y _lN= exp[0.7l(N/ N₁– I)], where Y is the nitrate leaching (kg N/ha per year) at fertilization rate N , and Y _IN and N₁ are the corresponding values at the normal rate of N fertilization. The relative submodel is valid for cereals, root crops and grass leys fertilized with mineral fertilizer at N/N ₁ < 1.5, and on the prerequisite that the fertilization rate N has been constant for some years. To illustrate the use of the relative leaching submodel, estimated values of Y _IN corrected to mean annual drainage for 1970 to 1990 in Denmark for spring cereals and grass on sandy and loamy soils are given as input to the relative leaching submodel. The model can be used for sandy to loamy soils to estimate the mean nitrate leaching over a number of years.     

Yield and N uptake as affected by soil tillage and catch crop

Two field trials with spring barley (Hordeum vulgare L.) were conducted at two locations in Denmark in order to evaluate the effects of tillage and growth of a catch crop on yield parameters under temperate coastal climate conditions. Ploughing in autumn or spring in combination with perennial ryegrass (Lolium perenne L.) as a catch crop was evaluated on a coarse sand (Orthic Haplohumod) from 1987 to 1992 at three rates of N fertiliser application (60, 90 and 120 kg N ha⁻¹ year⁻¹). Rotovating and direct drilling were also included as additional tillage practices. The experiment was conducted on a 19-year-old field trial with continuous production of spring barley. Ploughing in autumn or spring in combination with stubble cultivation and a catch crop, in addition to minimum tillage, was evaluated in a newly established field trial on a sandy loam (Typic Agrudalf) from 1988 to 1992. Yield parameters and N concentrations in grain and straw were determined. On the coarse sand, N uptake in the grain in ploughed plots without a catch crop was significantly greater when spring ploughed as opposed to autumn ploughed, but grain and straw yields did not differ significantly. Grain yield, straw yield and total N uptake did not differ significantly between direct drilled and autumn ploughed plots, but the trend was for grain yield to be lower with direct drilling. After 19 years of catch crop use, yield parameters in ploughed plots were greater than in plots without catch crops. This was most pronounced in the autumn ploughed plots. Rotovating the catch crop in the spring decreased grain yield compared with underploughing the catch crop in autumn or spring. No significant interactions were found between tillage and N rates. On the sandy loam, grain as well as straw yield and total N uptake were not significantly affected by catch crop or time of ploughing. Grain yield was significantly lower with reduced tillage (stubble cultivation in autumn) than in all other treatments.     

Nitrate leaching from organic arable crop rotations: effects of location, manure and catch crop

Abstract. Nitrate leaching from crop rotations supporting organic grain production was investigated from 1997 to 2000 in a field experiment at three locations in Denmark on different soil types. Three experimental factors were included in the experiment in a factorial design: (1) proportion of N₂-fixing crops in the rotation (crop rotation), (2) catch crop (with and without), and (3) manure (with and without). Three, four-course rotations were compared, two at each location. The nitrate leaching was measured using ceramic suction cells. Leaching losses from the crop rotation with grass–clover green manure and without catch crops were 104, 54 and 35 kg N ha⁻¹ yr⁻¹ on the coarse sand, the loamy sand, and the sandy loam, respectively. There was no effect of manure application or time of ploughing-in the grass–clover green manure crop on the accumulated nitrate leaching from the entire rotation. Catch crops reduced nitrate leaching significantly, by 30–38%, on the sandy soils. At all locations catch crops reduced the annual averaged nitrate concentration to meet drinking water quality standards in the crop rotation with green manure. On the coarse sand there was a time lag between the onset of drainage and the start of N-uptake by the catch crop.     

Nitrogen effects of non-legume catch crops

Nitrate leaching during the winter period can be reduced and often prevented by growing catch crops after the harvest of a main crop. However, catch crops which effectively take up residual nitrogen do not necessarily show good nutrient effects on a succeeding main crop. The objective of this experiment was to investigate how the content of soil mineral nitrogen in spring was affected by the time of incorporation of non-legume catch crops and how the yield and nitrogen uptake of a succeeding main crop was influenced. The yield of spring sown onion and white cabbage was significantly increased by catch crop growing the previous autumn. The nitrogen effect of Italian ryegrass corresponded to 50–100 kg N per ha in the vegetables. However, the yield of spring barley was not significantly affected by the nitrogen released from decomposing catch crops. During decomposition of non-legume catch crops, grown at a high level of nitrogen fertility, nitrogen immobilization did not occur.     

Nitrate leaching as affected by long-term N fertilization on a coarse sand

Abstract. A field experiment on a coarse sand (1987–92) was conducted with spring barley ( Hordeum vulgare L.), in order to evaluate the effects of increasing N fertilization on nitrate leaching under temperate coastal climate conditions. The N fertilizer levels were 60 and 120 kg N/ha. The experiment was conducted on a 19-year old permanent field trial with continuous spring barley, initiated in 1968, and included treatments with ploughing in autumn or spring, with or without perennial ryegrass ( Lolium perenne L.) as a catch crop undersown in spring. Prior to 1987, the low and high levels of N fertilizer were 70 and 150 kg N/ha, respectively. To calculate nitrate leaching, soil water samples were taken from a depth of 0.8 m using ceramic cups. The average annual nitrate leaching from plots with 60 and 120 kg N/ha was 38 and 52 kg N/ha/y, respectively. The increased leaching associated with increasing fertilizer application was not caused by inorganic N in the soil at harvest, but rather by greater mineralization, mainly in autumn. Growing of a catch crop was relatively more efficient for reducing nitrate leaching than a long-term low fertilizer application. A 50% reduction in N application decreased average yield by 26%, while nitrate leaching decreased by 27%.     

Incorporation of catch crop residues does not increase phosphorus leaching: a soil column experiment in unsaturated conditions

Some studies suggest that incorporation of catch crop residues leads to increased availability of P to plants. However, little information is available on how this affects P leaching in soils with a high P load. We tested the effect of catch‐crop residue incorporation at the end of winter on the P leaching potential in a soil column experiment under unsaturated conditions using a typical sandy loam soil of NW Europe characterized by a high P load. We sampled the catch crops white mustard (Sinapis alba L.), Italian ryegrass (Lolium multiflorum L.), black oats (Avena strigosa L.) and a perennial ryegrass‐white clover mix (Lolium perenne L.‐Trifolium repens L.) from a field trial on catch crops and soil from the plots where they were grown. Plant biomass was incorporated taking account of the differences in conditions of the plant material at the end of winter and the biomass yield of each catch crop. Incorporation of catch‐crop residues decreased P leaching compared to the fallow treatment probably through immobilization of soil P during catch crop residue decomposition. The exception was black oats, where the leaching of P was the same as for fallow soil. We observed clear differences in C/N, C/P, water soluble and total P concentration, and biodegradability between the tested catch crops, which seemed to affect the P leaching. We conclude that the incorporation of catch crop residues under typical soil and weather conditions and agricultural practices of NW Europe does not increase the potential P leaching losses.     

Incorporation time of nitrogen catch crops influences the N effect for the succeeding crop

The significance of incorporation date of a catch crop on the nitrogen supply for the subsequent crop, the N effect (N_eff), was examined. Winter rye was grown as a catch crop for 3 years during the autumn, and incorporated on five dates, two in the autumn and three in the spring. Two of the winters had high precipitation, and the N_eff was small at the early autumn incorporation date, but increased when incorporation was delayed into late autumn and further increased by early spring incorporation. In the third winter, which was very dry, the N_eff was negative at all incorporation dates, with the negative effect gradually increasing in value the later the incorporation date. In all 3 years the N_eff was reduced when incorporation was delayed from early spring until later in the spring. The main processes determining this pattern were found to be (1) the risk of leaching of N mineralized after catch crop incorporation, which can reduce the N_eff at early incorporation under wet conditions, (2) pre-emptive competition which can reduce the N_eff when incorporation is delayed until later in the spring, and in dry conditions is already apparent during the autumn, and (3) catch crop growth leading to carbon gain and increased C/N ratio which decreases mineralization and thus the N_eff after delayed incorporation in the spring. Lack of time for catch crop N uptake prior to early incorporation, or lack of time for N mineralization after late incorporation which might also reduce the N_eff did not appear to be important in our experiment. The results show that catch crops grown in high rainfall areas on sandy soils should be incorporated later than those in low rainfall areas on nitrate retentive soils.     

The effectiveness of cover crops during eight years of a UK sandland rotation

Abstract. Growing cover crops during the winter before spring-planted crops is often suggested as an effective method to decrease nitrate leaching. A four-course crop rotation (potatoes-cereal-sugarbeet-cereal) was followed through two rotations on a sandy soil in the English Midlands. Three management systems were imposed on the rotation to test their effects on nitrate loss. The effects of cover crops on nitrate leaching and crop yields were compared with the more conventional practice of over-winter bare fallow before potatoes and sugarbeet.
Cover crop N uptake was variable between years, averaging 25 kg ha⁻¹, which is typical of their performance on sandy soils in the UK. The cover crops usually decreased nitrate leaching but their effectiveness depended on good establishment before the start of drainage. Over 7 years, cover crops decreased the average N concentration in the drainage from 24 to 11 mg l⁻¹. Potato yield and tuber N offtake increased after cover crops. Ware tuber yield increased by an average of c . 8%; this was unlikely to be due to additional N mineralization from the cover crop because the potatoes received 220–250 kg fertilizer N ha⁻¹, and non-N effects are therefore implicated. Sugar yield was not increased following a cover crop.
After 8 years of nitrate-retentive practices, there were no measurable differences in soil organic matter. However, plots that had received only half of the N fertilizer each year contained, on average, 0.14% less organic matter at the end of the experiment.     

Simulating nitrate retention in soils and the effect of catch crop use and rooting pattern under the climatic conditions of Northern Europe

This model analysis of catch crop effects on nitrate retention covered three soil texture classes (sand, loamy sand, sandy loam) and three precipitation regimes in a temperate climate representative of northern Europe (annual precipitation 709–1026 mm) for a period of 43 years. Simulations were made with two catch crops (ryegrass and Brassica) with different rooting depths, and soil N effects in the next spring were analysed to 0.25, 0.75 and 2.0 m depth to represent the catch crop effect on following crops with different rooting depths. Nitrate retained without a catch crop was generally located in deeper soil layers. In the low precipitation regime the overall fraction of nitrate retained in the 0–2.0 m soil profile was 0.23 for the sandy soil, 0.69 for the loamy sand and 0.81 for the sandy loam. Ryegrass reduced leaching losses much less efficiently than Brassica, which depleted nitrate in the 0–0.75 m soil layer more completely, but also in the deeper soil layer, which the ryegrass could not reach. A positive N effect (N_eff, spring mineral N availability after catch crop compared with bare soil) was found in the 0–0.25 m layer (that is shallow rooting depth of a subsequent main crop) in all three soil texture classes, with on average 10 kg N/ha for ryegrass and 34 kg N/ha for Brassica. Considering the whole soil profile (0–2.0 m deep rooting of next crop), a positive N_eff was found in the sand whereas generally a negative N_eff was found in the loamy sand and especially the sandy loam. The simulations showed that for shallow‐rooted crops, catch crop N_eff values were always positive, whereas N_eff for deeper‐rooted crops depended strongly on soil type and annual variations in precipitations. These results are crucial both for farmers crop rotation planning and for design of appropriate catch crop strategies with the aim of protecting the aquatic environment.     

Ergebnisse von Simulationsrechnungen mit einem Bodenstickstoffmodell zur Düngung und zum Zwischenfruchtbau in Trinkwasserschutzgebieten

Results of computer simulations on fertilization and catch cropping problems in water protection areas by means of a soil nitrogen model A simple model of the nitrogen turnover in soil is presented. The model was validated by field experiment time series. The simulation results showed that dividing of the mineral nitrogen fertilization during spring for root crops or maize as well as shortening the first spring nitrogen fertilization for winter cereals diminished the leaching of nitrate only in extremely wet springs on sandy soils. The great importance of winter catch cropping in a cereal-root crop or a cereal-maize rotation on all soils and the necessity to avoid liquid manuring during late summer and early autumn, especially on sandy soils without catch cropping, are demonstrated. The results underline the predominant influence of the weather conditions on nitrate leaching.     

The influence of sub-optimal irrigation and drought on crop yield, N uptake and risk of N leaching from sugarbeet

Abstract. Variously timed sub-optimal irrigation strategies were applied to sugarbeet grown on a light soil (loamy sand or sandy loam) over four seasons (1991 to 1994) to investigate the effect on crop growth and nitrate leaching risk. Data from the two dry seasons, (1991 and 1994) are reported here. In the driest year (1991) soil mineral N levels after autumn harvest were negatively related to crop water use ( P < 0.05). In this season, there was little drainage from the soil profile, and full irrigation reduced residual soil N by 31 kg N/ha (0-90 cm) compared with the unirrigated treatment (79 kg N/ha). The potential for N leaching during the ensuing winter was consequently more than halved. In 1991 and 1994 there was a strong positive linear relationship between dry matter yield, N uptake and water use, but a negative relationship between plant N concentration and water use. These relationships were a function of the severity and not the timing of drought. The additional N uptake associated with increased irrigation and crop water use was biased towards a large concentration in the aboveground crop (tops), which are normally returned to the soil. The C:N ratio of sugarbeet tops was affected by crop water supply with droughted crops having lower values. This would also influence N release and subsequent leaching risk. However, the effects of drought on N leaching risk were relatively small when compared with other root crops such as potatoes.     

Leaching of nitrate and phosphorus after autumn and spring application of separated solid animal manures to winter wheat

Animal slurry can be separated into solid and liquid manure fractions to facilitate the transport of nutrients from livestock farms. In Denmark, untreated slurry is normally applied in spring whereas the solid fraction may be applied in autumn, causing increased risk of nitrate and phosphorus (P) leaching. We studied the leaching of nitrate and P in lysimeters with winter wheat crops (Triticum aestivum L.) after autumn incorporation versus spring surface application of solid manure fractions, and we compared also spring applications of mineral N fertilizer and pig slurry. Leaching was compared on a loamy sand and a sandy loam soil. The leaching experiment lasted for 2 yr, and the whole experiment was replicated twice. Nitrate leaching was generally low (19–34 kg N/ha) after spring applications of mineral fertilizer and manures. Nitrate leaching increased significantly after autumn application of the solid manures, and the extra nitrate leached was equivalent to 23–35% of total manure N and corresponded to the ammonium content of the manures. After spring application of solid manures and pig slurry, only a slight rise in N leaching was observed during the following autumn/winter (<5% of total manure N). Total P leaching was 40–165 g P/ha/yr, and the application of solid manure in autumn did not increase P leaching. The nitrogen fertilizer replacement value of solid manure N was similar after autumn and spring application (17–32% of total N). We conclude that from an environmental perspective, solid manure fractions should not be applied to winter wheat on sandy and sandy loam soils under humid North European conditions.     

Nitrate leaching after cut grass/clover leys as affected by time of ploughing

Abstract. Nitrate leaching after one year of a cut grass/clover ley was measured in two succeeding years to investigate how the postponing of ploughing leys from early to late autumn or spring, in combination with spring or winter cereals affected leaching of nitrate. The experiment was conducted as three field trials, two on a coarse sandy soil and one on a sandy loam soil. For calculation of nitrate leaching, soil water samples were taken using ceramic suction cups. The experiments started in spring in a first year ley and ended in spring three years later. Total nitrate leaching for the three year periods for each trial ranged between 160–254 and 189–254 kg N/ha on the coarse sand and 129–233 kg N/ha on the sandy loam. The results showed that winter wheat ( Triticum aestivum L.) did not have the potential for taking up the mineralized N in autumn after early autumn ploughing of grass/clover leys, and that the least leaching was generally found when ploughing was postponed until spring, and when winter rye ( Secale cereale L.) was grown as the second crop rather than spring barley ( Hordeum vulgare L.). Nevertheless, leaching was generally high in the winter period even when winter rye was grown. On these soil types ploughing out should be postponed, whenever possible, to spring. Crop systems that maximize the utilization of mineralized N and thereby minimize nitrate leaching need to be further developed. Based on N balances, the data were further used to estimate the biological N fixation by the clover.     

Simulation of nitrogen dynamics in farmland areas of Denmark (1989–1993)

Abstract. Each year since 1986 information has been collected about the farming systems at intersections of a nationwide 7 km square grid in Denmark. These management data and corresponding soil analyses were used in the model DAISY to simulate water and nitrogen dynamics. The model was validated with respect to harvested dry matter yield and nitrogen content in the soil. Simulated nitrate leaching from farmland areas from 1 April 1989 to 31 March 1993 was related to precipitation zones, soil type, fertilizer strategies and cropping systems. The mean simulated nitrate leaching for the whole of Denmark was 74 kg N/ha/yr, with a large yearly variation in the period considered. The simulated nitrate leached from soils with a sandy subsoil corresponded to 51% of the applied fertilizer, twice that leached from soils with a loamy subsoil. The application of pig manure resulted in average leaching losses of 105 kg N/ha/yr. The simulated nitrate leaching losses at sites where only artificial fertilizer was applied were in the following order: cereal with undersown grass < crop followed by winter cereal or winter rape < cereal or rape without a catch crop < root crops without a catch crop. Where only artificial fertilizers were applied, the simulated mean annual leaching was 59 kg N/ha from spring barley and 40 kg N/ha from winter wheat. A map of simulated nitrate leaching in Denmark was produced using a Geographical Information System.