One of the definitions of sustainable is "capable of being maintained at a steady level without exhausting natural resources or causing severe ecological damage" (Anonymous 1992). Many current intensive systems of vegetable production are not sustainable in this way, because they cause severe ecological damage. Growers usually apply large amounts of nitrogen fertilizer to obtain high yields of good quality. This may be sound from an economic perspective, but may not be from the environmental perspective. Often, large amounts of nitrogen remain in the soil after a vegetable crop is harvested. This nitrogen includes residual soil mineral nitrogen, and nitrogen present in crop residues. Both sources of nitrogen may have a harmful effect on the environment. They affect groundwater quality through nitrate leaching, and air quality through nitrous oxide emission.
In this paper, quantitative data are presented of residual soil mineral nitrogen and nitrogen present in crop residues when the current nitrogen fertilizer recommendations for field vegetables are followed. Next, estimates are given of nitrate leaching and denitrification during the subsequent winter and early spring. Finally, strategies for sustainable nitrogen management in intensive field vegetable production systems are discussed. Sustainable nitrogen management should aim at supplying sufficient nitrogen for optimum crop growth and development, while keeping losses to the environment to a minimum.
The results presented are derived from research conducted in Western Europe, particularly in the Netherlands. The management strategies proposed are likely to be successful in other parts of the world as well.
Residual Soil Mineral Nitrogen
The extent to which mineral nitrogen accumulates in soil when current nitrogen fertilizer recommendations for field vegetables are followed is summarized in Table 1. Residual soil mineral nitrogen levels after application of the recommended nitrogen fertilizer rates to Brussels sprouts, white cabbage and onion (grown from seed) are low to moderate: 20-75 kg N/ha. After application of the recommended rates to other field vegetables, however, there may be large amounts of residual soil mineral nitrogen. This is especially the case after crops which are harvested before maturing, such as spinach. Residual soil mineral nitrogen after such crops may reach levels of more than 200 kg N/ha. Obviously, there will then be a risk that large amounts of nitrate will be leached during the following winter.
It should be emphasized that the values for residual soil mineral nitrogen given in Table 1 refer only to crops fertilized according to the current recommendations, and in a situation where there are no constraints to crop growth. However, when more nitrogen fertilizer is applied than is recommended, the level of residual soil mineral nitrogen can rise substantially. For example, on commercial holdings in Eastern England, mineral nitrogen in the soil after a spring crop of cabbage was found to range from 200 to 600 kg N/ha (Greenwood et al. 1996). This was apparently caused by far too high nitrogen fertilizer application rates. High values for residual soil mineral nitrogen can also be found in fields where the nitrogen mineralization rate of the soil is higher than average, or where uptake of nitrogen by the crop ceases early due to drought or disease.
The values of residual soil nitrate presented in Table 1 may even give too optimistic a picture, because they all originate from experimental fields. It is likely that the soils of experimental sites are more homogenous than those fields of ordinary farms. Calculations have shown that increased heterogeneity results in a need for higher nitrogen fertilizer rates to obtain optimum yield, giving higher values for residual nitrate (Van Noordwijk and Wadman 1992).
Nitrogen Present in Crop Residues
Not all nitrogen taken up by crops ends up in harvestable produce. Of the total amount of nitrogen taken up by, e.g., cauliflower only about 50% is removed from the field with the product (Everaarts et al. 1996). The other 50% remains on the field in the crop residues. Crop residues of spinach and celeriac contain 25-60 kg N/ha (Wehrmann and Scharpf 1989), and cauliflower 80-120 kg N/ha (Wehrmann and Scharpf 1989; Everaarts et al. 1996). White cabbage and Brussels sprouts contain as much as 150-250 kg N/ha (Rahn et al. 1992; Neeteson 1994). If the residues are (partly) decomposed before winter,nitrogen from the decomposed plant material may leach during the subsequent winter period.
Decomposition of the residues is dependent on the C:N ratio of the material. Whitmore and Groot (1994) followed residue composition in experiments where leaves of spinach (C:N = 6), leaves of cabbage (C:N = 18) and a mixture of sugar beet leaves and crowns (C:N = 42) were mixed with soil. Spinach released the most nitrogen within the shortest period of time. Cabbage leaves released nitrogen more slowly, while the sugar beet residues even immobilized nitrogen at the start of the experiment.
Nitrogen Leaching and Denitrification after Harvest of Field Vegetables
In Western Europe, nitrate leaching from the root zone to groundwater occurs mainly in the period from late autumn to early spring, when precipitation exceeds evapotranspiration. There is considerable evidence that in the past two decades, the nitrate content of water in West European groundwater and surface water has increased, as a result of agricultural practices (Goulding et al. 1990; Owen and JÃ¼rgens-Gschwind 1986; Vinten et al. 1992). This is undesirable, because groundwater is an important source of drinking water. The European Union (EU) has set a maximum permissible concentration of 50 mg NO 3/L (= 11.3 mg N/L) for drinking water (Anonymous 1980). If we assume that no nitrate is lost from the groundwater through denitrification, that the drinking water is extracted from shallow groundwater only, and that the annual precipitation surplus is 300 mm (the average value for the Netherlands), the EU criterion would already be exceeded after leaching of 34 kg N/ha (3 x 10 6L precipitation surplus/ha x 11.3 mg N/L = 34.10 6 mg N/ha = 34 kg N/ha). Obviously, denitrification does occur, especially in heavy textured soils with a shallow groundwater table. Therefore, it has been suggested that the maximum acceptable amount of nitrate present in soil at the onset of winter should be set at 70 kg N/ha (Goossensen and Meeuwissen 1990). This value is about twice as high as the amount of nitrate that can enter the groundwater before exceeding the EU criterion, because it was assumed that 50% of the nitrate would be denitrified in soil and groundwater.
The nitrate leached during the winter period originates from residual soil nitrate, from nitrogen present in crop residues remaining on the field, and from soil organic nitrogen mineralized during the autumn and winter period. The major sources of nitrate leaching in field production of vegetables are residual soil nitrate and nitrogen present in crop residues. The literature yielded no results of direct measurements of nitrate leaching from fields where vegetables were grown. Measurements from arable fields were found to be scarce, and almost all of them were from cereals and sugar beet (Neeteson 1995). In these fields, the amount of nitrate leached during winter ranged from 10 to 55 kg N/ha (Goss et al. 1988; Maag et al. 1990; Nielsen et al. 1990). These relatively low values are probably because of the generally low values of residual soil mineral nitrogen observed after cereals and sugar beet (Neeteson 1995).
Currently available computer simulation models make it possible to generate realistic estimates of nitrogen losses. Whitmore (1996) developed a computer simulation model for the release and loss of nitrogen after the harvest of vegetable crops. The model makes it possible to discriminate between the effect on nitrate leaching of residual soil mineral nitrogen (and denitrification), and the effect of nitrogen present in crop residues. The model consists of a water movement section, to calculate leaching and evaporation, and an organic matter turnover section. The water movement section is taken from Addiscott and Whitmore (1987). The organic matter turnover section is based on Whitmore (1995).
Carbon and nitrogen from plant residues and stubble, and ageing crops and roots, decompose in soil to produce microbial biomass and humus. A proportion ( a) of the carbon entering the soil becomes microbial biomass, while a further proportion ( b) becomes humus. The remainder (1 - a - b) is lost from the soil as CO 2. The values of a and b depend on the clay content of the soil.
Nitrogen flow follows carbon, but where there is insufficient nitrogen in the residues to be decomposed, mineral nitrogen already present in the soil is immobilized. If there is insufficient mineral nitrogen in the soil, the rate of decomposition falls until enough is available. The model allows the C:N ratio of the products to vary, and the efficiency with which the biomass uses carbon ( a + b) to change. In this way, more carbon can be decomposed per unit of nitrogen where necessary. The critical range of the C:N ratio of the crop residues, at the point where mineralization switches to immobilization, is calculated to be between 20 and 25 (Whitmore 1996).
This model was used to simulate the fate of nitrogen after the harvest of field vegetables with crop residues which varied both in quality and quantity, and where there were also different amounts of residual soil mineral nitrogen ( Table 2). The simulations were performed for both a sandy and clay soil. The soil characteristics assumed are given in Table 3. Simulations began at the harvest of each vegetable crop, and ceased at the end of the following spring. The residues were assumed to have been incorporated in the top 25 cm of soil, and the soils were assumed to have been left bare over the winter.
Table 4 shows the predicted nitrogen losses through leaching and denitrification of both soil nitrogen and the nitrogen derived from the vegetable crop residues. The absolute amounts of soil nitrogen leached were large after all crops, but the denitrification of soil nitrogen was low. The crop residues increased leaching only by a relatively small amount, and in some cases even reduced it. After spinach or leek, total leaching losses exceeded 200 kg N/ha. Spinach and leek are both crops that use applied nitrogen inefficiently, and leave little in the way of N residues behind after harvest. Losses after Brussels sprouts and cabbage were much lower, because both crops extract mineral nitrogen quite efficiently and leave little residual soil mineral nitrogen ( Table 2).
Both Brussels sprouts and cabbage do, however, leave a large mass of crop residues in the soil, in contrast to spinach and leek. However, the differences in C:N ratio led to different effects on the amount of nitrate leached. From the cabbage residues, with a C:N ratio of 15 ( Table 2), 30-40 kg N/ha was leached. The sprout residues, on the other hand, with a C:N ratio of 25 ( Table 2), did not contribute to leaching. In fact, they even immobilized some nitrogen ( Table 4).
Crops such as spinach and leek that use nitrogen inefficiently do so, at least in part, because their roots do not penetrate the subsoil. Leaching from these two crops may be even worse than predicted in Table 4, if the model calculations have assumed wrongly that the residual mineral nitrogen was higher-up in the soil profile than it actually was. In practice, more nitrogen in the subsoil would lead to earlier and more pronounced leaching. Furthermore, a good deal of nitrogen may leach during the growing season from crops with shallow roots or those that receive a large amount of irrigation.
Strategies for Sustainable Nitrogen Management
Current nitrogen recommendations for intensively grown field vegetables aim at predicting economically optimum application rates of nitrogen fertilizers. They take into account the cost of nitrogen fertilizers, and the expected price of the crops or crop products. The recommended rates of nitrogen fertilizer further depend on the type of crop and, in most cases, the amount of mineral nitrogen present in the rooting zone of the soil at the start of the growing season. The fertilizer rates at which optimum yields are obtained can be predicted satisfactorily on the basis of the current system of fertilizer recommendations. However, the environmental side-effects of nitrogen application, in particular the leaching of nitrate, are not explicitly taken into account in the current recommendation systems. Ideally, recommendations should provide farmers with accurate tools to obtain high crop yields of good quality with the minimum burden on the environment.
Everaarts et al. (1996) produced a relationship between the amount of nitrogen applied as fertilizer, the nitrogen remaining in soil, and the yield of cauliflower, shown in ( Fig. 1). It can be seen that with only a little extra applied nitrogen, the yield ceases to increase or can even fall, while the residual nitrogen in the soil begins to increase steeply. Chaney (1990) showed a similar flat response of residual nitrogen to applied nitrogen in the case of winter wheat. Similarly, results from other crops showed that as applied nitrogen increases, so does the residual soil nitrogen (potato and onion (Neeteson and Wadman 1991, Neeteson 1994), oilseed rape (Shepherd and Sylvester-Bradley 1996)).
Van Noordwijk and Wadman (1992), and Whitmore and Van Noordwijk (1995), examined the variability of the nitrogen supply in soil, and argued that the rise in residual nitrogen with increasing nitrogen applications is a consequence of field variability in the supply of nitrogen to plants. Under these circumstances, farmers who apply fertilizer uniformly to their fields are behaving quite rationally in applying large amounts of nitrogen. They have to do this, in order to overcome the variability of supply, and provide sufficient nitrogen to the parts of the field which need it most.
Vos (1995) has categorized three kinds of measures that may be taken to maximize nitrogen use efficiency while minimizing environmental losses. These are
- Matching supply and demand;
- Making effective use of crop residues, including residual soil mineral nitrogen; and
- Reducing losses outside the growing season.
All three categories demand more information than is available to most growers. Indeed, the only variables under human control in the field are the amount and timing of the nitrogen applied, the timing of field operations such as sowing, and selection of the crop and cropping system. Since the grower cannot be certain of the levels of residual soil mineral nitrogen, future weather conditions or the nitrogen demand of the crops at any time, the problem seems hopeless. However, it is not. The grower can do an enormous amount by following sensible farm practices, and with the addition of a little modern science any remaining shortfall in knowledge can be made up.
Matching Supply and Demand
At first sight, the only action a grower seems able to take here is to apply more nitrogen or less. Information, however, is the key. Smit et al. (1996) showed that while a crop of Brussels sprouts took up most of its nitrogen requirement early in growth and sent roots deep into soil, leek took up small amounts of nitrogen throughout growth using a rather poor root system. Brussels sprouts can be given nitrogen in a single dressing early on, but leek and crops like it should be given small amounts of nitrogen regularly throughout the growing period. Growers need to know how much growth there has been and how much nitrogen is now needed for a particular crop of leek. Booij et al. (1996) showed that growth, leaf area and nitrogen uptake of vegetable crops in early growth was closely related to the amount of solar radiation they had received. Simple formulae allowed them to calculate quite accurately how much nitrogen was being used by various crops. Where the soil nitrogen supply from native organic matter is very variable, more fertilizer could be applied to the deficient parts of the field, and less to those naturally rich in nitrogen.
Making Effective Use of Crop Residues
Current recommendations for field vegetables take no account of the residues (mineral or organic) left in the field from the preceding crop. However, the quantity of these may be very large, and most of the nitrogen they contain may be lost unless they are properly used ( Table 4). The computer model used to produce Table 4 requires only the daily rainfall, evaporation and temperature, and details about farm operations and soil texture. It can be used to calculate the fate of crop residues between the harvest of one crop and the sowing of the next. Clearly, computer models have a lot to offer growers, both during and immediately after the growing season.
Reducing Losses Outside the Growing Season
Nitrogen losses outside the growing season might appear to be completely outside the grower's control, but this is not the case. It is possible to recover the residual soil mineral nitrogen from earlier crops, by lengthening the growing season by a second crop or even a third, preferably a deep-rooted one. If a commercial crop cannot be grown, a catch crop could be scheduled which is ploughed into the soil the following year, well before sowing. If the remaining growing season is too short even for a fast-growing catch crop, Everaarts et al. (1996) suggest that growers should leave residues unchopped on the surface of the soil and delay ploughing until spring.
Clearly, in treating residues there may be considerations other than nitrogen to take into account, such as access by farm machinery and the persistence of disease, but the point is that field operations also have an effect on nitrogen and can be scheduled to minimize losses. For example, crop residues might be removed from the field altogether and composted, or the remaining mineral nitrogen in soil might be immobilized in soil organic matter with the help of another nitrogen-deficient crop residue from elsewhere, such as wheat straw. Because the amount of residual soil mineral nitrogen is generally greater than the amount of mineral nitrogen in crop residues, measures designed to reduce losses from soil will generally have a greater benefit.
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During the paper discussion, it was suggested that simulation models might be used to follow the fate of N after the harvest of vegetables. One participant pointed out that modeling is increasingly used to study soil processes, but when it is used for tropical soils, the results are inappropriate. For example, water movement in tropical soils where the subsoil has a positive charge is quite different from water movement in a temperate climate, where most models were developed.
Dr. Neeteson agreed, and said that while the models work well for Dutch conditions, further work is needed to adapt them to the tropics. He himself is now working on two projects to modify the models for tropical soils, in Thailand and India.
Dr. Neeteson was also asked about the main parameter of sustainability in nitrogen use. Dr. Neeteson answered that N leaching was the main parameter he had used in this paper, but other criteria also exist. It was agreed that recommendations should be developed to obtain high crop yields, while taking into account environmental safety. This means matching the nutrient supply with the crop requirements, making efficient use of cop residues, and reducing nitrogen losses outside the growing season by growing deep-rooted crops and catch crops.
Index of Images
Figure 1 Nitrogen Fertilizer Application Rate (KG N/Ha)
Table 1 Residual Soil Mineral Nitrogen after Fertilizing Field Vegetables with the Recommended Rates of Nitrogen Fertilizer
Table 2 Characteristics of the Crops Used in the Simulations
Table 3 Characteristics of the Soils Used in the Simulations
Table 4 Nitrogen Losses during Winter after Field Vegetables. Losses from Soil Originate from Residual Soil Mineral Nitrogen and from Mineralization of Native Soil Organic Matter
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