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Agronomy Journal 94:373-380 (2002)
© 2002 American Society of Agronomy

NITROGEN MANAGEMENT

On-Farm Monitoring of Soil Nitrate-Nitrogen in Irrigated Cornfields in the Ebro Valley (Northeast Spain)

Josep M. Villar-Mir*,a, Pere Villar-Mira, Claudio O. Stockleb, Francesc Ferrera and Miquel Aranc

a Dep. of Environ. and Soil Sci., Univ. of Lleida, Rovira Roure 177, 25198 Lleida, Spain
b Dep. of Biol. Syst. Eng., Washington State Univ., Pullman, WA 99164-6420
c Soil Fertil. and Analysis Lab., Sidamon, Lleida, Spain

* Corresponding author (villar{at}rectorat.udl.es)



   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The irrigated area served by the Canal d'Urgell, a semiarid region in the Ebro Valley (northeast Spain), presents problems of ground water pollution by nitrates. Corn (Zea mays L.) is widely grown in this area, and N fertilization in corn production is the major source of N input in the area, with application rates in excess of crop requirements. In this study, we monitored several commercial cornfields for soil NO3–N levels, crop N uptake, and crop productivity over a 2-yr period to quantify the relationship among soil NO3–N, N fertilizer rates, crop N use, and N loss through leaching. Monitoring soil NO3–N profiles showed that in some fields, soil NO3–N was transported to deeper layers in the soil during the growing season. In many cases, important accumulation of NO3–N was observed at the bottom of the soil at physiological maturity, increasing the risk of winter leaching. Soil N availability, calculated as preplanting soil nitrate test + N fertilizer, was neither related to plant N uptake nor final biomass and grain yield. In some plots, the occurrence of drought during the growing season was more decisive than soil-available N in explaining crop N uptake and grain yield differences. Available N levels found in the soil were above 370 kg ha-1 in all cases. Apparently, these levels were enough to satisfy the crop N requirements to achieve yields above 11 to 12 Mg ha-1. Overall data showed that there is an excess of N in the system.

Abbreviations: EU, European Union • PMNT, physiological maturity soil nitrate test • PPNT, preplanting soil nitrate test • PSNT, presidedressing soil nitrate test


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
IN MANY AREAS OF THE WORLD, ground water pollution by nitrates has been attributed to excessive N fertilizer applications (Kessebalou et al., 1996; Meisinger and Randall, 1991; Moreno et al., 1996; Schepers et al., 1991; Weil et al., 1990). Soil and ground water NO-3 monitoring have been used in some regions to determine the agricultural contribution to this pollution (Davis et al., 1997; De la Rosa et al., 1993; Prunty and Greenland, 1997; Weil et al., 1990). Determination of soil NO3–N is important for characterizing NO-3 leaching and assessing the vulnerability of soils and waters to agricultural contamination (Liang et al., 1991; Liang and MacKenzie, 1994; Dou et al., 1995). Soil NO3–N distribution profiles and seasonal variation can be used as a diagnostic tool for evaluating the impact of N fertilization on the accumulation of NO3–N in soil and the risk of NO-3 leaching.

Throughout the world, irrigated areas with shallow water tables where intensive agriculture is practiced tend to present problems of ground water pollution by nitrates. Here we analyze the case of the irrigated area served by Canal d'Urgell, a semiarid region with calcareous soils located in the Ebro Valley (northeast Spain) where 30% of a total irrigated area of 72000 ha is dedicated to corn. A typical rotation comprises alfalfa (Medicago sativa L.), wheat (Triticum aestivum L.), and corn though some farmers crop continuous corn. Other important crops grown in the area include onion (Allium cepa L.) and orchard fruit, mostly apple (Malus sylvestris Mill.). Average corn yields range between 9 and 14 Mg ha-1 (14 g kg-1 moisture content). The corn hybrids that farmers grow are FAO Cycles 600 to 800. Nitrogen fertilization in corn production is the major source of N input in the area (Ferrer, 1999), with application rates averaging more than 300 kg N ha-1 in excess of crop requirements (Ballesta and Lloveras, 1996; Domingo et al., 1996; Villar et al., 1994). Nitrate concentrations found in drainage waters and ground water during the irrigation period (March to September) usually exceed 11.3 mg L-1 NO3–N (Ferrer et al., 1997), the maximum level recommended by the European Union (EU) for drinking water (EU Nitrates Directive 91/676/EEC), and reflect an excess of N in the system. Other studies involving irrigated corn around the world show a similar situation (CLRSWC, 1993; Plénet, 1995, Zhang et al., 1996; Ramos, 1996; Roman et al., 1996).

Despite of this evidence, there is a general lack of data with which to quantify the relationship among soil NO3–N levels, N fertilizer rates, crop N use, and N loss through leaching. Without this information, any attempt to improve N fertilization practices remains difficult. The present on-farm study is the first part of a broader project aimed at using soil NO3–N tests to adjust N fertilizer rates, and thereby minimize N loads to the system, while maintaining acceptable yields for the farmer (Ferrer, 1999; Carrasco et al., 2000). This is a priority objective for the reduction of ground water pollution and the improvement of N management within any cropping system (McCracken et al., 1994; Sexton et al., 1996). According to Magdoff (1991), the use of soil NO3–N tests at preplanting [preplanting soil nitrate test (PPNT)] or presidedressing [presidedressing soil nitrate test (PSNT)] allows farmers to adjust N fertilization rates in line with yield goals while taking into account factors such as soil type, weather conditions, and management needs, all of which affect N available to crops.

The objective of this experiment was to study and interpret the relationship, under the habitual range of conditions and farmers' practices in the area, among N fertilizer application rates, NO3–N accumulation in the soil profile, crop N use and performance, and the potential for N loss through leaching.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A 2-yr study was conducted in 10 commercial cornfields that included five soil series (Table 1). Seven fields were surveyed (Fields A through G) in 1993; in 1994, the survey was repeated in Fields C, E, F, and G, and three new fields were added (Fields H, J, and K). All fields were located within 10 km of Linyola (41°42' N, 0°52' E), at an altitude of between 200 and 250 m. The fields selected for the survey had been managed with continuous corn, but no manure had been applied for at least 3 yr previous to the study. The same corn cultivar was grown in all fields throughout the study period. The size of the surveyed fields ranged from 0.2 to 0.25 ha. Each field was taken as an experimental unit or plot, falling more inside the category of an observational, on-farm type of study, rather than a large-plot experiment with true replicates (Ecologistics Limited, 1994). For sampling purposes, each individual field was divided into three adjacent subfields or sampling units, each about 800 m2 (20 by 40 m). Soil profile, drainage, and management practices were uniform within each field. Even though subsamples from a uniformly treated field cannot be considered true replicates (Nelson and Buol, 1990), we thought that having an estimation of the experimental error within each plot (for the measured parameters) could help support data evaluation, giving more validity to the conclusions.


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  		Table 1. Soil classification information for fields surveyed.

 
Soils and Climatic Conditions
Fields were located on sites representative of five benchmark soils found in the area. There was only one soil type in each field. Percolation class was the main criteria for soil type selection (Table 1). Maximum percolation occurs in the Tornabous series (fine-loamy texture over sandy skeletal, mixed, mesic Calcixerollic Xerochrepts) and minimum percolation in soils with fine textures [(Castellsera (fine-loamy, mixed, mesic Gypsic Xerochrepts) and Linyola (fine-silty, mixed, mesic, Gypsic Xerochrepts) series]. More detailed soil information was published by Herrero et al. (1993).

The climate of the region is semiarid, with an annual precipitation/evapotranspiration ratio of 0.33. This typical Mediterranean, continental climate is dry and warm in summer and dry and cold in winter. Average temperature is 4.7°C in January and 23.4°C in August. Average annual temperature is 13.9°C. Absolute average maximum temperature is 37°C for July. A risk of frost exists in May and October. Spring is the season with the highest precipitation (120 mm) while winter has the lowest (75 mm). Average annual precipitation is 400 mm. The soil moisture regime is xeric, and the soil temperature regime is mesic (Soil Survey Staff, 1992). Weather conditions during the survey were recorded by an automated station (Campbell Sci., Logan, UT) located at Palau d'Anglesola (UTM31T CG 237133).

Nitrate-Nitrogen Concentration in the Water Table and Irrigation Water
Water table and irrigation water data were only collected as supplementary information and not intended for statistical analysis. As a result, sampling did not follow a consistent pattern in the two cases. A single 10-cm-diam. PVC tube (piezometer) was installed in each field. The length of the piezometers varied according to effective soil depth (Table 1). The level of the free water surface in the piezometer was monitored once every 2 wk throughout the growing season. At the same time, samples were taken from the water table to assess NO-3 concentrations. Irrigation water was sampled for NO-3 concentration once a week and at different points from that of water delivery. Nitrate concentrations at the water table and in irrigation water were estimated in the field using the Nitrachek meter quick test (Merck, Darmstadt, Germany), and they were expressed as NO3–N concentrations.

Crop Management Practices and Irrigation Management
Planting dates were 13 through 15 April (1993) and 6 and 7 April (1994). The Pioneer cultivar Juanita was planted in rows at all sites, with a spacing of 70 cm and a final population of 69000 plants ha-1. Physiological maturity was achieved around 16 September in 1993 and around 28 September in 1994. Crop management decisions, including when to apply N fertilizer, were made by farmers. In general, farmers used urea [(NH2)2CO] at preplanting, which was applied at the surface and incorporated into the first 20 cm of the soil. Sidedress applications were only made in 1994, and in just three fields, as broadcast ammonium nitrate (NH4NO3). It was applied between preplant sampling and the V6 growth stage. The total quantity of N applied ranged from 250 to 340 kg N ha-1.

All fields were surface (flood) irrigated, with the irrigation schedules depending on water availability. The irrigation system in the area has a rotatory delivery schedule, with turns every 10 to 15 d. On average, gross application depth is between 100 and 120 mm per irrigation (Ferrer et al., 1997). Although these irrigation depths refill the soil to field capacity, the soil water content often falls below maximum recommended depletion before the following irrigation (Solsona, 1998). In 1993, higher-than-normal precipitation occurred during the cropping season (Table 2), and all fields received six irrigation events, except for two that received seven (Table 3). In 1994, with lower-than-normal precipitation, all fields received seven irrigation events, except for one that received eight (Table 3).


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  		Table 2. Precipitation (P) during the survey and long-term average precipitation at Palau d'Anglesola.

 

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  		Table 3. Irrigation scheduling and simplified water balance.{dagger}

 
Soil Nitrate-Nitrogen Measurements
Each year, soil samples were taken for NO3–N from all fields before planting and any N fertilizer application and also at V6, tasseling, grain-filling, and physiological maturity stages. Three soil samples were taken each time that a field was sampled (one per subfield). Each sample at the 0- to 30-cm depth comprised five cores. Following Onken et al. (1985), two cores were taken per sample at 30-cm increments to a depth of 1.2 m.

Although determination of nitrites and NH4 may be of interest, only NO3–N was determined due to the conditions prevailing in the study area (well-aerated and calcareous soils) and the purpose of the study. Other experiments carried out in the same area (data not published) showed that NH4–N content was generally <15 kg ha-1 (0–30 cm) and accounted for <15% of the total mineral N (NO3–N + NH4–N) in the horizon. These data support studies by Moutonnet and Fardeau (1997) and Liang et al. (1991). All samples were extracted with water (1:5 soil/water ratio solution) and colorimetrically analyzed for NO3–N using a Technicon Autoanalyzer (Anasol 4P2S1BM2P, ICA Instruments, Tonbridge, Kent, UK). Soil NO3–N was expressed in kilograms per hectare at preplanting (PPNT), V6 growth stage (PSNT), and physiological maturity [physiological maturity nitrate test (PMNT)] considering a bulk density of 1.3 Mg m-3.

Crop Development, Biomass, Yield, and Nitrogen Uptake
The Hanway scale (Ritchie and Hanway, 1982) was used to monitor corn development. Crop yield and biomass were determined at physiological maturity. Each plant sample was taken by hand-harvesting 9.94 m2 (two rows by 10 m) within each of the three subfields or sampling areas defined in each plot. Grain, stems, leaves, and cobs were dried and weighed to determine moisture content, dry mass, and total Kjeldahl N, and grain yields were expressed on a 14 g kg-1 moisture basis. Aboveground N uptake was calculated by multiplying N concentration by total plant dry mass.

Data Evaluation
Based of the type of design that was used in this on-farm study, the tools for data analysis were mainly observational, aimed at defining possible relationships, trends, or interactions among the measured variables of interest. The observed parameters were related graphically and by calculating combined indexes and simple N and water balances in the system. The use of linear regressions helped check the influence of limiting factors on the measured parameters.

To support data interpretation and to characterize differences among fields, measurements within each field that were not true replicated were used to calculate the standard error of soil NO3–N levels and to conduct ANOVA tests for soil NO3–N and productivity variables. All data were analyzed separately for each year, with the field being the only source of variation. The effect of the field in a given year integrates the combined effect of soil type, NO3–N at preplanting (PPNT), N fertilizer rate, and irrigation management. A second ANOVA was conducted to analyze the effect of the year, only comparing the fields that were repeated in 1994 (C, E, F, and G). In both analyses, SAS (SAS Inst., 1996) was used to perform Duncan's multiple range test and evaluate whether the means were significantly different due to the effect of the source of variation.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Weather Conditions
Temperatures in July and August were higher in 1994 than in 1993 (Table 4). Precipitation for 1994 was higher than for 1993 although not during the growing period. Accumulated precipitation (May through August) was much lower in 1994 (23 mm) than in 1993 (101 mm). Reference evapotranspiration, computed using the Penman equation (Pruitt and Doorenbos, 1977), for the period of March through September was 707 mm for 1993 and 800 mm for 1994. Higher reference evapotranspiration and lower precipitation resulted in higher irrigation water requirements in 1994, which resulted in more irrigation events in 1994 than in 1993 (Table 3).


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  		Table 4. Weather data recorded at Palau d'Anglesola.

 
Nitrate-Nitrogen Concentration in the Water Table and Irrigation Water
Deep percolation from irrigation and precipitation caused a rise in the water table in some of the monitored fields (Table 5). The depth to the water table ranged from 70 to 150 cm, depending on the site and time of sampling (Table 5). The data show high NO3–N concentrations in the water, with values often above the recommended EU drinking-water level of 11.3 mg L-1 NO3–N (EU Nitrates Directive 91/676/EEC). In both years, mean NO3–N concentration in irrigation water was around 9 mg L-1 NO3–N, with fluctuations during the cropping season. Considering applications of 100 mm of water per irrigation event and an average of seven events during the growing season, irrigation throughout the growing season with water containing an average of 9 mg L-1 NO3–N represented the application of around 60 kg ha-1 NO3–N (Ferrer et al., 1997).


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  		Table 5. Depth and NO3–N concentration of the water table in selected fields. SE indicates the standard error of each data set.

 
Nitrate-Nitrogen in the Soil
Changes in total NO3–N in the 120-cm soil profile are shown in Tables 6 and 7, and some selected soil NO3–N profiles are displayed in Fig. 1 . Nitrate-N amounts in the soil at preplanting (PPNT) ranged between 123 and 310 kg ha-1 in 1993 and between 125 and 459 kg ha-1 in 1994 (Table 6). The distribution of NO3–N at the beginning of the growing season for three selected sites in 1993 (Sites F, B, and G) can be seen in Fig. 1. Proper interpretation of soil profile NO3–N contents and distribution at preplanting is not intended in this study because it would require more detailed information about residual NO3–N profiles at harvest, precipitation during the nongrowing period, soil texture, drainage, soil organic matter content, and depth to the water table. However, data indicate that at the beginning of the growing season, important amounts of NO3–N can be found in the soil profile and with different accumulation patterns, as seen in Fig. 1, such as surface accumulation (Site F and G) or surface and subsoil accumulation (Site B) among others.


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  		Table 6. Changes in soil NO3–N (0–120 cm).{dagger}

 

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  		Table 7. Interannual variation of soil NO3–N. Average for fields C, E, F, and G.

 


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  		Fig. 1. Changes in soil NO3–N profiles in some selected fields and years (•, preplanting; {circ}, V6 stage; {triangleup}, tasseling; {blacktriangleup}, physiological maturity). Error bars correspond to standard error values. Lines are not for statistical fit purposes and are only drawn for presentation.

 
The profiles in Fig. 1 show an accumulation of NO3–N at the soil surface in all profiles (Fields F, B, and G in 1993) at the V6 stage, resulting from NO3–N present at preplanting, N fertilizer applications, and contributions from net N mineralization in early spring (Magdoff, 1991). During the same period, in Field B93 (Fig. 1b), the NO3–N that accumulated at the bottom of the profile at preplanting was probably lost below the rooting depth at the V6 stage due to low N uptake by the crop at this stage (Magdoff, 1991) and deep percolation from the large amounts of precipitation and irrigation during the same period (Tables 2 and 3). At tasseling, surface NO3–N showed a sharp decrease at all sites, probably due to active N uptake by the crop from the V6 stage until flowering. In field B93, soil NO3–N reduction was less apparent, and part of the unused N was transported to lower layers (Fig. 1) because crop N uptake was lower than in other fields (Table 8). The NO3–N profiles in Fig. 1 showed little variation after tasseling except for some NO3–N losses from the subsurface horizons, probably as a result of crop N uptake during grain filling, and some N losses from the deepest layers through leaching. Later, at physiological maturity, NO3–N accumulated at the bottom of the profile in Fields F and B (Fig. 1a and 1b), thereby increasing the risk of winter leaching.


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  		Table 8. Summary of corn productivity, N uptake, N at preplanting (PPNT), N applied, and residual N (PMNT).{dagger}

 
Residual N at harvest was large in 1993, with five out of seven fields presenting amounts of N >200 kg ha-1 (Table 6). In 1994, PMNT values were significantly lower, with five out of seven fields presenting PMNT levels <100 kg ha-1 (Table 6). The PMNT levels depended on the amount of N available in the soil that was not used by the crop and on the amount of N leached within the season. A simple N balance in each plot, for the whole growing season and the soil profile (preplanting to maturity, 0–120 cm), can be calculated in terms of kilograms of N per hectare as:

where Nfert is the N applied by the farmer, Nir is the N delivered by the irrigation water (set to 60 kg ha-1 NO3–N as an average value in all sites), Nup is the aboveground plant N uptake, and Nuncc is the net sum of the unaccounted N balance components (Ninput - Noutput). If the relation between PMNT and the unaccounted N (Nuncc) is plotted and a 1:1 line is drawn (Fig. 2) , the plots above the line mean that there was a net loss of N from the system during the growing season. Looking at Fig. 2, it can be seen that the conditions during 1994 resulted in higher N losses. If we assume that management practices where similar in all of the fields and that variations of total mineralized N and N loss through denitrification and volatilization were not that wide between the 2 yr, it seems that the N losses detected in 1994 could be attributed to leaching. The most likely explanation is that N leaching losses were more important in 1994 due to the greater number of irrigation events and high precipitation in September (Tables 2 and 3).



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  		Fig. 2. Soil NO3–N content at maturity (PMNT) and unaccounted N (Nuncc = PMNT + Nup - PPNT - Nfert - Nir, where Nup is the aboveground plant N uptake, PPNT is soil NO3–N content at preplant, Nfert is the N applied by the farmer, and Nir is the N delivered by the irrigation water.

 
Corn Productivity and Nitrogen Uptake
Total average values for corn grain yields, accumulated biomass, and N uptake were 11.3 Mg ha-1, 20.3 Mg ha-1, and 241 kg ha-1, respectively. For the same sites and cultivar, grain yields, biomass, and N uptake were about 25% greater in 1993 than in 1994 (Table 8). Poor performance in 1994 could be related to lower precipitation together with dryer atmospheric conditions during early growth (Tables 2 and 4) that may have restricted growth due to lower soil water availability. A simplified water balance (precipitation + irrigation - crop evapotraspiration) was calculated for each field on a monthly basis (Table 3). On average, the water deficit was twice as high in May 1994 than in May 1993, reaffirming the existence of a period of drought during the vegetative growth stages that affected final biomass and yield (Grant et al., 1989; NeSmith and Ritchie, 1992). Yield differences between fields in 1993 were probably more related to water stress than to low crop N availability. Longer intervals between irrigation events during July 1993 in Fields A, B, C, and D, combined with low rainfall during the same month, resulted in poor crop performance. In 1994, Fields F and C had the lowest yield, but their biomass and N uptake accumulations were not the lowest. This could be explained by irrigation delays in these fields during mid- to late August, which affected C and N translocation to grains and resulted in a lower harvest index. These periods of drought during grain filling were detected by calculating the previously mentioned simplified water balance (Table 3). In 1994, Fields K and G performed reasonably well, despite having shallower and coarser soils. This could be explained by the shallow water tables in these fields (Table 5), which mitigated the effects of drought.

Figure 3 shows no relationship between soil-available N, calculated as PPNT + N fertilizer (Roth and Fox, 1990), and aboveground plant N uptake for the 2 yr. This lack of relation may reflect the effect of water stress on N crop demand and soil N availability. It may also be an indication that, even for the nonrestricted data plots, crop N requirements were satisfied by the sum of PPNT and N fertilizer. Reinforcing this last hypothesis, in all fields, available soil N (PPNT + N fertilizer) was >370 kg N ha-1 (Fig. 3). According to Dara et al. (1992), available soil N levels (NO3–N, 0–60 cm at preplanting plus N fertilizer applied) of 250 kg N ha-1 should support corn grain yields of 11.9 Mg ha-1, which are similar to the grain yields observed in the study.



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  		Fig. 3. Available soil N (PPNT + N fertilizer) and aboveground plant N uptake.

 
Nitrogen Concentration in the Grain
Average concentration of N in the grain (Table 9) fluctuated between 1.28 and 1.91 g kg-1, with significantly higher levels in 1993 than in 1994 (average of 1.67 vs. 1.44 g kg-1, respectively). Significant differences between fields were found for N concentration in the grain and in the ratio of N grain to total N uptake. Nitrogen concentrations were similar to values reported by other authors, 1.6 g kg-1 (Hagin and Tucker, 1982) and 1.55 g kg-1 (Fonnesbeck et al., 1984, as cited by Meisinger and Randall, 1991), with reported variability ranging between 1.35 and 1.75 g kg-1. In general, the grain N/total N uptake ratios fluctuated between the 0.64 and 0.78 interval reported by Schepers and Mosier (1991). In 1993, lower N concentrations in the grain were related to fields with lower N uptake and lower yields (Table 8). In 1994, Fields F and C showed the lowest grain yields, highest N concentrations in the grain, and lowest grain N/total N uptake ratios. This seems to reinforce the hypothesis that the crop was not deficient in N but rather that water stress during the grain-filling period affected the final grain yield. In all cases, N concentrations in the grain were above 1.20 g kg-1, the critical level for crop response reported by Ferrer (1999) for similar conditions. Overall, results suggested that the crop was able to satisfy N requirements for the achieved yields even though, as previously commented, crop growth may have been restricted by drought during the growing season.


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  		Table 9. Average concentration of N in the grain, and ratio of grain N to total N uptake.{dagger}

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
1. Monitoring soil NO3–N profiles showed that, in some fields, soil NO3–N was transported to deeper layers in the soil during the growing season. In many cases, important accumulation of NO3–N was observed at the bottom of the soil at physiological maturity, increasing the risk of winter leaching. A simple N balance suggested that N loss through leaching could have been more severe during the 1994 growing season where, in most of the fields, soil NO3–N was not used by the crop and did not accumulate at maturity.

2. Soil N availability, calculated as PPNT + N fertilizer, was neither related to plant N uptake nor to final biomass and grain yield. In some plots, the occurrence of drought during the growing season was more decisive than soil-available N in explaining crop N uptake and grain yield differences. Available N levels found in the soil were above 370 kg ha-1 in all cases. Apparently, these levels were high enough to satisfy the crop N requirements to achieve yields above 11 to 12 Mg ha-1.

3. To sum up the data from the study, it seems that there is an excess of N in the system. The following aspects corroborate this conclusion: high concentrations of NO3–N in the water table; the presence of large amounts of available N at the beginning of the growing season; important accumulation of residual NO3–N in some cases; and the evidence that, in some situations, a large pool of unaccounted N can be lost through leaching.


   ACKNOWLEDGMENTS
 
We thank farmers for their cooperation. Soil, plant, and water analyses were done by staff of the Soil Fertility and Analysis Laboratory (LAF, Sidamon, Lleida). This project was partially supported by Departament de Medi Ambient (Generalitat de Catalunya) and by CICYT-DCTI (Contract 95/0028).


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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