Simulation of nitrate leaching for different nitrogen fertilization rates in a region of Valencia (Spain) using a GIS–GLEAMS system

Agriculture, Ecosystems and Environment 103 (2004) 59–73

Simulation of nitrate leaching for different nitrogen fertilization rates in a region of Valencia (Spain) using a GIS–GLEAMS system J.M. de Paz a,∗ , C. Ramos b,1 a

b

Centro de Investigaciones sobre Desertificación, Cam´ı de la marjal s/n, Apdo. Oficial, 46.470 Albal-Valencia, Spain Instituto Valenciano de Investigaciones Agrarias, Carretera Moncada-Náquera km 4,5, Apdo. Oficial, 46113 Moncada, Valencia, Spain Received 14 January 2003; received in revised form 10 October 2003; accepted 27 October 2003

Abstract The groundwater loading effects of agricultural management systems (GLEAMS) model coupled to a GIS was used to evaluate the effect of different fertilization treatments on the total N leaching in a selected area of eastern Spain with intensive agriculture. Four nitrogen fertilization rates (traditional or base, base rate reduced by 20%, reduced by 50%, and the rate calculated by the Nmin recommendation system for vegetables, and reduced by 70% for citrus) were evaluated at a regional scale to find the rate that minimized N leaching without reducing crop N uptake. Nitrate leaching maps were obtained for the different nitrogen rates studied. A great reduction of N leaching (up to 68% for vegetables, and 75% for citrus) was observed under the most reduced fertilization rates and this reduction was greater in areas irrigated with surface water in comparison to those irrigated with groundwater. The GIS–GLEAMS system was a useful tool to assess the N leaching at a regional scale for the different N management considered. For example, it was shown that the Nmin recommendation system was the most efficient for vegetables, and for citrus the most efficient fertilization rate was the reduced 50%, that is similar to that recommended by the Code of Good Agricultural Practices. The areas irrigated with groundwater with high nitrate content had a high leaching rate, and the nitrate applied in irrigation water should be considered when planning the crop fertilization. A temporal analysis of the NO3 -N in soil, N leaching, crop evapotranspiration and rainfall allowed to identify the influence of the soil NO3 -N and the rainfall on nitrate leaching. © 2003 Elsevier B.V. All rights reserved. Keywords: GIS; GLEAMS; Nitrate leaching; Nitrogen fertilization

1. Introduction During the last decade concern over the groundwater pollution associated to the excessive use of nitrogen ∗ Corresponding author. Tel.: +34-961-220540; fax: +34-961-270967. E-mail addresses: [email protected] (J.M. de Paz), [email protected] (C. Ramos). 1 Tel.: +34-963-424094; fax: +34-963-424001.

fertilizer in intensive agriculture areas has increased. Therefore, the European Union and some institutions from Spain have performed several studies (Meinardi et al., 1995; ITGME, 1998) and enacted directives to reduce this pollution (CEC, 1991; BOE, 1996). In Spain, the Valencian Community is one of the Spanish regions with many intensive agriculture areas where groundwater pollution by nitrate is important. This is mainly due to an excessive use of N fertilizer since most of the farmers traditionally do not follow

0167-8809/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2003.10.006

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any fertilizer recommendation system (Ramos et al., 2002). In year 2000, the regional government, following an EU directive (CEC, 1991), published a list of those areas considered “vulnerable” (Generalitat Valenciana, 2000a). More than 2 million of inhabitants live in areas declared as vulnerable to nitrate pollution. In these areas the regional government recommends the application of good agricultural practices to reduce the risk of pollution. Groundwater pollution is related to excessive use of nitrogen fertilizer (Jüergens-Gschwind, 1989). This is in agreement with results obtained in a national study that identified many areas of Valencia contaminated by nitrate from intensive agricultural practices (ITGME, 1998). Recently, the Code of Good Management Practices, where guidelines for N fertilization of crops are given, has been published (Generalitat Valenciana, 2000b). In this code, the N fertilizer recommendation takes into account the N requirement by crops, the N in irrigation water and the mineral N in soil, in a way similar to the Nmin system (Neeteson, 1995). This system has been adopted in several European countries (Holland, Germany, etc.). A more complex approach to fertilizer rate recommendations is the use of simulation models (Neeteson, 1995; Ramos, 1996). Several simulation models have been developed to describe crop growth and N uptake, and to recommend N fertilizer rates (Neeteson, 1995). Others simulation models are more focused on N leaching from the root zone to groundwater. There are some reviews assessing nitrate leaching models (Addiscott and Wagenet, 1985; Willigen, 1991; National Research Council, 1993; Hansen et al., 1995; Wu and McGechan, 1998; Tabachow et al., 2001). The application of field models can be extended to a regional scale when they are combined with a geographical information system (GIS). Simulation models and GIS are powerful tools by themselves, but coupled together allow N modeling at a regional scale. A GIS-model system facilitates the evaluation of the risk and vulnerability of aquifers to nitrate pollution. Several authors have used a combination of simulation models and GIS to assess the risk of groundwater pollution by nitrate (Srinivasan and Arnold, 1994; Tim et al., 1996; Trabada-Crende and Vinten, 1998; Navulur and Engel, 1998; Lasserre et al., 1999). The GLEAMS model (groundwater loading effects of agricultural management systems) (Knisel, 1993),

selected in this study, has been used to predict or assess NO3 -N leaching in agricultural soils (Yoon et al., 1994; Minkara et al., 1995; Stone et al., 1998; Shirmohammadi et al., 1998; Bakhsh et al., 2000). This paper describes the linkage of the GLEAMS model to a GIS to evaluate nitrate leaching, at a regional scale, under various nitrogen fertilizer rates. The study was performed in an intensive agriculture zone in Eastern Spain.

2. Study area description The study area is located about 20 km North of Valencia city and has a surface area of 230 km2 (Fig. 1). This area was considered a vulnerable zone by the regional authorities following an EU directive (CEC, 1991). Main crops are vegetables (potato: Solanum tuberosum L., onion: Allium cepa L., lettuce: Lactuca sativa L., cauliflower: Brassica oleracea L., watermelon: Citrullus sativus L., artichoke: Cynara scolymus L., melon: Cucumis mello L., etc.) and citrus: Citrus spp. (orange and mandarine trees). These crops are irrigated mainly by furrow or basin irrigation systems with low efficiencies (ca. 65%). Over 490 ha of citrus orchards are irrigated with groundwater with high nitrate contents (40–350 mg l−1 ), and 920 ha are irrigated with surface water with low nitrate content. More than 1 million of inhabitants live in the study area where the groundwater nitrate levels are higher than the allowable maximum (50 mg l−1 ). The aquifer is multi-layered, with the shallowest layer made up of quaternary detritic material that

Fig. 1. Study area location.

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acts as a free aquifer and is highly polluted by nitrate. A deeper strata of sandstone and limestone forms the semi-confined aquifer that has low nitrate concentrations. The hydrological characteristics of the soils range between those with low infiltration rates (gleysols) to high infiltration rates and permeability (arenosols). The main soil type in the area is the fluvisol calcareous (classified according to FAO-UNESCO, 1988), which is a young soil formed from quaternary material with sandy loam texture in the surface to clayey loam in the deeper layers. These soils were developed in a flat topography, and generally are used to grow vegetable crops. The annual average rainfall ranges between 400 and 600 mm, but is distributed irregularly along the year. Generally, the maximum monthly precipitation occurs in October (70–100 mm), and the minimum in July (8–14 mm).

3. Modeling approach The study area was divided in homogeneous units with respect to soil characteristics, crop rotations, climate and nitrogen concentration in irrigation water using a GIS (de Paz and Ramos, 2002). On each homogeneous unit the GLEAMS model was applied. The GLEAMS model version used in this study was V2.10. This model is composed of four sub-modules dealing with hydrology, erosion, pesticides, and nutrients. For our purposes, only the modules that consider the hydrological and nitrogen cycle processes were used. The model takes into account the main inputs of the N cycle (mineral and organic fertilization, N in rainfall and irrigation water, symbiotic N fixation), and outputs (leaching, crop uptake, volatilization and denitrification), and the main transformations (mineralization, nitrification, and immobilization). A detailed description of the model can be found in the GLEAMS manual (Knisel, 1993). After a sensitivity analysis to find the main variables affecting nitrate leaching (de Paz, 1999), the model was calibrated and validated (de Paz and Ramos, 2002) using data from two experimental agricultural plots located within the study area, with two crops: potato (Rodrigo, 1995) and citrus (Lidón, 1994). The soil, climate and crop management data required by GLEAMS were obtained from several information

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sources, and stored in a vectorial GIS (de Paz and Ramos, 2002).

4. Model—GIS linkage The GLEAMS model was linked to a vectorial GIS (PC-ArcCad) to perform the evaluation of NO3 -N leaching, from different N fertilizer rates, at a regional scale. A graphical user interface (GUI) was developed to facilitate the use of the GIS–GLEAMS system, and to allow the input of crop management practices to the model. Several programming routines were developed to automate data input and running of the model, and also to visualize results as a table or map within the GIS. Fig. 2 shows the conceptual scheme of the linkage. This GIS–GLEAMS system was used to simulate different crop rotations and management practices, and to analyze at a regional scale the effect of the management practices on the N cycle. This would help to select the best management practices that minimize the risk of groundwater pollution by nitrate without a reduction in crop yields.

Fig. 2. Conceptual scheme of the GIS–GLEAMS model linkage.

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5. Comparison of N management practices

Table 2 Parameter A values of the Nmin fertilizer recommendation system (Eq. (1)) for the vegetables crops used in the simulation

For the vegetable growing area, two crop rotations were selected, including the most frequent crops in the study area—rotation A: early potato (S. tuberosum L.), lettuce (L. sativa L.), onion (A. cepa L.), cauliflower (B. oleracea L.) and rotation B: artichoke (C. scolymus L.). Four N fertilization rates were considered: (1) a traditional or “base” rate representing that used by the farmer, (2) a 20% reduction of “base” fertilization, (3) a 50% of “base”, and (4) a rate calculated by the Nmin recommendation system (Neeteson, 1995). Table 1 shows the crop management practices used in the simulation. The recommended amount of fertilizer (Nrec ) used by the Nmin system was calculated using the equation: Nrec = A − BNmin-soil

Crop

Parameter A

Potato Lettuce Onion Cauliflower Artichoke (first year) Artichoke (second year)

325 130 250 200 350 400

For the citrus growing area the same N fertilization rates were studied except for the Nmin fertilization rate because there are not data available for the Nmin recommendation system in citrus. In this case, a 70% reduction of the base rate was selected. These crop rotations were repeated during 8 years (1980–1987). Several assumptions were adopted in the simulation process: (a) maximum root depth was 60 cm for the vegetable crops (Greenwood et al., 1982), and 80 cm for citrus (Castle, 1978; Cahoon et al., 1961), (b) soil N mineral at the first day of simulation was taken as 115 kg N ha−1 for vegetables area, and 60 kg N ha−1

(1)

where A is a parameter that depend on the crop, B is usually one, and Nmin is the mineral N in the soil (down to 60 cm depth) at sowing or planting time. Table 2 shows the A values for the crops used in the simulation.

Table 1 Crop management practices used in the simulation with the GIS–GLEAMS system Rotation A

Rotation B

Citrus

Potato

Lettuce

Onion

Cauliflower

Artichokea

Orange

Planting date Harvest date

6 January 22 May

1 August 1 November

20 November 20 May

15 August 5 November

1 January 31 December

Organic manure (t ha−1 ) Yield (t ha−1 ) Irrigation (mm) Irrigation time

26b 46 330 18 February– 15 May

0 35 120 7 August– 20 September

18b 60 230 11 December– 15 May

0 35 120 20 August– 1 October

Rooting depth (cm) Mineral fertilizer “base” (kg N ha−1 ) Mineral fertilizer “20%” (kg N ha−1 ) Mineral fertilizer “50%” (kg N/ha) Mineral fertilizer “Nmin ”, “70%” (kg N ha−1 )c

60 445

60 0

60 338

60 200

1 August 31 December, 30 May 0 15/15 523/326 3 August– 1 November, 1 February– 1 November 60 646/406

356

0

270

160

517/324

298

223

0

169

100

323/203

186

102

72

53

161

253/377

111

a

0 48 452 1 April– 1 October

80 372

When two dates appear, the first one refers to the first year of rotation and the second one to the second year. Poultry litter with a 3.8% N (dry matter basis) and 20% water content. c The fertilizer recommendation system N min was used only in vegetables; in citrus this treatment was replaced by a 70% reduction of the “base” rate. b

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Fig. 3. Dates and application rates of mineral nitrogen fertilizer (base rate, the different reductions in percentage of the base treatment, and the Nmin system) in each crop.

for citrus area, (c) a tillage operation was simulated 1 day before planting day to incorporate previous crop residues, (d) crop management practices for each homogeneous unit were the same. Nitrogen fertilizer application rates and dates for each crop rotation are shown in Fig. 3. These data were adapted from Pomares (1997) (cauliflower), Lidón (1994) (citrus), Bonet (1988) (artichoke), Maroto (1994) (potato, lettuce, onion), and agricultural extension personnel guidelines, and are supposed to represent the general management practices in the study area. There was no fertilizer application for lettuce except for the Nmin system, because farmers traditionally do not fertilize lettuce when it comes after potato.

6. Results and discussion 6.1. Leaching maps and analysis of N balance Leaching maps simulated for the four N fertilizer rates considered in this study are shown in Fig. 4. In

these maps we can observe that the citrus irrigated with groundwater with high nitrate contents, and the vegetable zones are the areas where the GLEAMS model predicts a higher N leaching. From these maps, it is apparent the reduction in nitrate leaching with the decrease in N fertilizer rates. This reduction is less pronounced in zones irrigated with groundwater with high nitrate content (>100 mg l−1 ), where even when the N rates are reduced 50%, 70, and Nmin , a high amount of nitrate is leached (from 150 to 225 kg N ha−1 per year) (Fig. 4). 6.2. Effect of fertilizer rate on the N balance terms The main terms of the N balance for the different fertilizer rates in the vegetables and citrus areas are shown in Table 3. It can be observed that applying the Nmin method to vegetables resulted in a 66% reduction in nitrate leaching in comparison to that of the “base” N application rate. This recommendation system was the most efficient because it used less nitrogen (211 kg N ha−1 , i.e. 57% less than the traditional), and N crop uptake was barely reduced (5% less than

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Fig. 4. N leaching result maps for the different N application rates considered (base rate reduced by 20, 50 and 70%, and following the Nmin method).

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Table 3 N balance terms in kg N/ha for the different N fertilizer rates in the vegetables and citrus areas (kg N ha−1 ) Fertilization

Crop

N inputs

NO3 -N in soil

N outputs

Mineral fertilization

Irrigation water

Leaching

Crop uptake

Volatilization

Denitrification

Base

Vegetables Citrus

496 372

11 89

294 210

321 199

81 0

99 49

7 25

20% reduction

Vegetables Citrus

396 297

11 89

227 155

320 198

81 0

78 38

0 17

50% reduction

Vegetables Citrus

248 186

11 89

138 74

311 192

81 0

50 21

−7 6

Nmin 70% reduction

Vegetables Citrus

211 112

11 89

99 43

304 156

81 0

40 14

−7 −3

in the traditional fertilization). Denitrification in the Nmin system was also reduced by 59% in comparison to the base fertilization rate. However, volatilization was the same for the four fertilizer rates considered, because the GLEAMS model includes volatilization from organic wastes and manures but not from mineral fertilizers. This approach is a simplification, because NH3 volatilization can also occur with mineral fertilization (Jarvis and Pain, 1990). In the citrus area, a 70% reduction in N fertilizer application rate resulted in a 79% reduction in nitrate leaching. However, this lower fertilizer rate reduced N uptake by 21% (from 198 to 156 kg N ha−1 ), and the crop requirements, established as an input

to the model, were not fully covered. The 50% rate (186 kg N ha−1 ) was the most efficient, because it reduced N leaching by 65% (from 210 to 74 kg N ha−1 ), and N crop uptake was hardly reduced (3%). Denitrification in the 50% rate was reduced by 57% with respect to the traditional fertilization rate. In the other fertilization rates, N leaching was reduced with N fertilization rates, and the N uptake by the crop was almost not changed. Fig. 5 shows the relationship between N leaching and N application rate observed by several authors for citrus and vegetable crops (Lidón, 1994; Dasberg et al., 1984; Rodrigo, 1995; Samaleh et al., 1997; Lamb et al., 1999), and also those simulated with the

Fig. 5. Relationship between N applied (mineral fertilizer, N in irrigation water and in rainfall) and the N leached observed by several authors for citrus: Dasberg et al. (1984) (䊏), Dasberg (1978) (䊏), Lid´on (1994) (䉱), Lamb et al. (1999) (+), Pratt and Adriano (1973) (䊉), Embleton et al. (1981) (䉱), Avnimelech and Ravheh (1976) (䊉), Bingham et al. (1971) (䉬), and simulated with the GLEAMS model using our data (䉬), and for vegetables crops: Samaleh et al. (1997) (䊏) for chile and onion, Ramos et al. (2002) (+) for several crops, Rodrigo (1995) (䉱) for potato, Pratt and Adriano (1973) (䉬) for several crops, and simulated with the GLEAMS model using our data (䉬).

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GLEAMS model using our data and the N applied as mineral fertilizer and irrigation water. N leaching increases with the N application rate both for vegetables and citrus crops. The relationship between N-input and N leaching simulated by GLEAMS is within the range observed by these authors. There is a low correlation between N leaching and the N input to soil (R2 between 0.4 and 0.5), both in vegetables and citrus (Fig. 5), although for similar N inputs nitrate leaching in citrus is much lower than in vegetables, suggesting a lower N use efficiency for vegetables. This low correlation is probably due to the influence of other parameters like soil N-initial content, drainage, N uptake, fertilizer application timing, chemical form of fertilizer application, N applied in organic matter, etc., on nitrate leaching. But, we can conclude that the N input in mineral fertilization and with irrigation water is a very important parameter to assess N leaching, and reducing this input could decrease the risk of N leaching.

(Burkart et al., 1999). Fig. 6 shows the N leaching and drainage averaged for each soil type (soils classified according to FAO-UNESCO, 1988) in the study zone. Fine-textured soils, with lower permeability, like Gleyc Cambisols have low drainage and N leaching rates, but high denitrification rates, in agreement with Neeteson et al. (1999). Sandy soils with high infiltration and drainage rates, like Albic Arenosols, leach large amounts of nitrate, increasing the risk of groundwater pollution; furthermore, these soils are frequently in zones where the water table is shallow (<1 m). Other authors have also found that NO3 -N leaching on fine-textured soils was lower than in coarse-textured soils (Sogbedji et al., 2000). In the other soils, differences in hydrologic characteristics are not large enough to greatly affect the variability of N leaching among them, and the N added as mineral fertilizer and with irrigation water are the main factors that determine the variation in N leaching.

6.3. The influence of soil type on nitrate leaching

6.4. Nitrogen leaching and plant uptake for different crops

The soil characteristics most related to the nitrate content in groundwater are those that determine drainage (i.e. % clay, bulk density, hydrologic group)

One aspect that should be considered when studying the N leaching is the crop uptake efficiency. Fig. 7 summarizes the N leaching and crop uptake in

Fig. 6. Simulated N leaching, deep percolation, and denitrification when using the base fertilization rate, in the different soil types classified following FAO-UNESCO (1988) soil taxonomy.

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Fig. 7. N leaching (a) and crop uptake (b) in relation to N mineral fertilizer application.

relation to N application for each crop and fertilizer rate. Potato, artichoke (in the first year) and citrus are the crops that lose a higher amount of N by leaching. Artichoke and citrus leach greater amounts of nitrate because they are fertilized with high rates of mineral N, and also because they stay in the field during the whole year, and is in autumn when most of the N leaching occurs due to the intensive rains (Figs. 8 and 9). Potato is a crop that needs high amount of N (organic and inorganic), and its N use efficiency is low (Tyler et al., 1983). In general, for each crop, N leaching is reduced when the rates are reduced, without an appreciable reduction in the N uptake (Fig. 7b). Only in cases of a high reduction in N application like 70% in citrus, 50% in artichoke (second year) or when using the

Nmin system in onion, there is an important reduction in crop N uptake, probably because these crops grow during autumn, when high rainfall leaches the soil nitrate, and this reduces N uptake by the plant. Although the citrus crop takes up N efficiently, N leaching is high mainly due to a high nitrate input in irrigation water. The best nitrogen fertilizer rate is that recommended by the Nmin system, but in the case of a high rainfall in autumn, an extra N application should be added afterwards to compensate for the leaching this rainfall could produce. In general, the 50% reduced rate is the closest to the fertilization rate recommended by the Code of Good Agricultural Practices of the Valencian Community (Generalitat Valenciana, 2000b) (Table 4).

Table 4 A comparison of the different N fertilizer application rates for the crops considered in this study and those recommended by the Code of Good Agricultural Practice of the Valencian Community N application rates (kg N ha−1 )

Crop

Artichokea Potato Onion Lettuce Citrus a b

Nmin system

50% reduction

20% reduction

Base

253/377 102 53 72 111b

323/203 223 169 0 186

517/324 356 270 0 298

646/406 445 200 0 372

N fertilizer rate recommended by the Code of GAP (kg N ha−1 ) 250–300 250–300 200–250 150–220 200–240

First/second year. This amount corresponds to a 70% reduction of the base rate, and not to the Nmin recommendation system.

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Fig. 8. Temporal evolution of rainfall, percolation, evapotranspiration, NO3 -N in soil and cumulative N leaching simulated with GLEAMS from 1980 to 1987 in the vegetable crops area.

6.5. Temporal evolution of soil NO3 -N and leaching Using the GIS utilities, the GLEAMS simulation results were averaged monthly and grouped for vegetables (rotation A) and citrus. The temporal evolu-

tion of NO3 -N in soil, rainfall, evapotranspiration, drainage, and N leaching during the 8-year period, for the four mineral fertilization rates considered is shown in Figs. 8 and 9. Generally, the most important N leaching events occurred after high rainfall periods, and this resulted in a decrease of soil mineral N.

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Fig. 9. Temporal evolution of rainfall, percolation, evapotranspiration, NO3 -N in soil and cumulative N simulated with GLEAMS from 1980 to 1987 in the citrus area.

In the vegetable crops area the rainfall of the first three months of simulation did not leach much nitrate because soil NO3 -N was mainly in the first 30 cm of the soil profile. During this time for the traditional fertilization rate the soil NO3 -N was reduced mainly due to crop uptake, and the increments were due to the fertilizer applications. Soil nitrate increased

up to 500 kg N ha−1 during the potato crop as a result of the high application rate of mineral fertilizer (445 kg N ha−1 per year), and the mineralization of the poultry manure applied (26 t ha−1 ). This high content of soil NO3 -N is within the range found by other authors. Greenwood et al. (1996) measured 200–600 kg NO3 -N ha−1 in a depth of 60 cm after a

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brassica crop, and Salomez et al. (1999) found up to 900 kg N ha−1 of soil NO3 -N in a soil layer between 30 and 120 cm in a greenhouse lettuce crop. Because of this high Nmin content in soil, two important N leaching events (140 and 147 kg N ha−1 ) occurred during two rainy months in 1982 (106 mm in March, and 114 mm in April). No more important leaching events are observed in the vegetable area until October 1986 when a 200 mm rainfall coincides with a decreasing crop evapotranspiration period, and this resulted in a increase in drainage and N leaching (237 kg N ha−1 for the traditional fertilization). In this event, N leaching under the N application rate recommended by the Nmin system was much lower than under the base or traditional N application rate. In the 8-year simulation period, a difference of 1600 kg ha−1 in N leaching is observed between the traditional and Nmin fertilization rates. In the citrus area, the traditional N application rate is quite high (372 kg N ha−1 per year), and this, together with the important N input with the irrigation water (86 kg N ha−1 per year), resulted in high soil nitrate levels (Fig. 9). The simulated values for NO3 -N in the 0–80 cm soil layer during the study period are quite high (in the range of 400–700 kg ha−1 ) for the “base” N application rate. There are not many available data in the literature to compare with. Lidón (1994) in two citrus orchards in the Valencia region (Spain) found values in the range 50–350 kg NO3 -N in the 0–80 cm soil layer for N application rates of 60–180 kg N ha−1 per year. Embleton and Jones (1978) give data on a N citrus fertilization experiment that allow an estimation of NO3 -N in the 0–80 cm soil layer. They found that for the fertilizer treatment with an annual application rate of 368 kg N ha−1 , the NO3 -N in that layer was up

to 620 kg N ha−1 , that is within the range found in the present GLEAMS simulation. In the traditional fertilizer application the total applied N is much higher than that recommended by the Code of Good Agricultural practices of the Valencian Community (Table 4). This code recommends a total N application to citrus of 200–240 kg N ha−1 , including the N added in the irrigation water. In citrus orchards, nitrogen uptake is from soil layers deeper than in vegetables fields, therefore the risk of N leaching is reduced. But, in the base treatment, and during the simulation time, two important N leaching events occurred, one in September 1985 (240 kg N ha−1 ) and another in November 1986 (432 kg N ha−1 ). These nitrate leaching were caused mainly by the high rainfall during these months (100 and 200 mm, respectively) that coincided with a period of low N uptake by the crop, and the soil NO3 -N was high and located mostly at the bottom of the soil profile. In the case of fertilizer rates reduced by 50 and 70%, the soil nitrate content at the end of the simulation period was reduced to 150 and 100 kg N ha−1 , respectively, and the N leaching was also reduced to 540 and 350 kg N ha−1 respectively, for the 8-year period, in comparison to the 1700 kg N ha−1 in the traditional rate (Fig. 9). Observing the N leaching for the different seasons of the year (Fig. 10), it can be concluded that in the citrus growing area the autumn rainfall is one of the main factors causing nitrate leaching. Adding fertilizer N in or just before autumn should be avoided to reduce the N mineral content in soil. In Holland, Goossensen and Meeuwissen (1990) recommend that the maximum amount of soil nitrate at the onset of winter should be about 70 kg N ha−1 . However, in the vegetable growing area, the season with the highest

Fig. 10. N leaching for different N mineral fertilization rates, and rainfall in the year seasons for the vegetables and citrus areas.

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risk of N leaching is spring, because is then when the farmer applies high rates of fertilizer and manure. For that reason, a reduction in the application of N in spring would be effective to reduce the nitrate leaching (Fig. 10). Several uncertainties affect the modeling process of N leaching due to the spatial and time variability of soil, climate and management practices, and to assumptions made in the modeling processes. These uncertainties also come from the lack of good data at the regional scale, and from the assumption that the heterogeneous media behaves like and homogeneous one depending on scale. As mentioned in the modeling approach section, the GLEAMS model in this study was calibrated and validated for potato and citrus using data from a few experimental plots. The GLEAMS–GIS system would improve its results if data for calibrating it with the other crops included in the rotation, and with more soil types were available. Despite these uncertainties the GLEAMS–GIS system was an effective tool to evaluate at regional scale the different mineral fertilizer management options.

7. Summary and conclusions The GIS–GLEAMS system allowed to show maps of annual N leaching rate for the different fertilizer management practices considered in the study area, and to analyze the main factors affecting nitrate leaching. Therefore, this tool could be used to find the best N fertilization rates that minimize the nitrate losses by leaching without reducing yield. The most efficient N management system for the vegetable crops was the Nmin fertilizer recommendation system because the N leaching was reduced by more than 1000 Mg N per year. In comparison to traditional management, with a non-significant reduction in N uptake. Using this fertilizer recommendation system would minimize the risk of nitrate pollution and reduce the use of N fertilizer, therefore increasing the economic benefit to farmers. In the citrus area the most efficient fertilization rate was a 50% reduction, that is slightly lower than the one recommended by the Valencian Code of Good Agricultural Practices (200–240 kg N ha−1 ). With this fertilization rate nitrate leaching was reduced in 1200 Mg N per year respect to the traditional man-

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agement. In this area, autumn was the season when N leaching was higher due to the heavy rains, and a reduction in the N mineral fertilization before autumn would reduce the N leaching risk. When irrigating with groundwater with high nitrate concentration, this N input should be taken into account in the N fertilization planning. Sandy texture soils had the highest nitrate leaching and the lowest denitrification rates and, in these soils, using an efficient irrigation system and splitting N fertilizer applications would reduce nitrate leaching.

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