Estimation of nitrogen leaching load from agricultural fields in the Puck Commune with an interactive calculator

Factor A—soil type and the impact of the climate

In soils with high cation-exchange capacity, the nutrients supplied with fertilizers (e.g., ammonium, potassium, magnesium) are not leached into the soil profile and groundwater but are exchanged from the sorption complex during plant development. The sorption capacity is also of key importance for limiting the migration and bioavailability of trace metals. In soils with excessive metal contamination (e.g., cadmium or lead), a high sorption capacity reduces the leaching and transfer of metals to the food chain.

The total N content of the soil is most dependent on humus content, mineralization conditions shaped by water conditions of the soil and climate, the type of bedrock, the direction and degree of advancement of the soil-forming process. In soils used for agricultural purposes, an important factor shaping the N content is the level of organic and mineral fertilization and crop rotation, especially the share of legumes binding free N from the air (Lityński & Jurkowska, 1982). The vast majority of the N in the soil is incorporated into the organic part of the solid phase of the soil. N occurs in soil in the form of mineral and organic compounds and as molecular N in soil air. It comes either from fertilization or from microbiological processes—ammonification and nitrification. The average mineral nitrogen content in soils in Poland ranges from about 6 to 11 mg N kg⁻¹ depending on the soil type. The most easily available form of N for plants is nitrate nitrogen. It varies considerably during the year depending on the weather conditions, the intensity of uptake by the plants and the amount of fertilizer applied (Fotyma, Kęsik & Pietruch, 2010).

The majority of N transformations are determined by the activity of soil microflora. The transformations of N compounds in the soil have a significant influence on the overall natural N cycle. The balance of these transformations determines the conditions of N nutrition of plants and also determines the extent to which they use N fertilization. N mineralization consists of a set of processes leading to the formation of ammonia or ammonium N. This is essential for plants, as ammoniacal N is a form directly absorbed by their root system and is easily converted into nitrates, which are even more easily used by plants. N losses in the soil are caused by crop cultivation, water and wind erosion and denitrification processes. N in nitrate form can be denitrified or leached if it is not taken up by the plants. Because nitrate ions are highly mobile in soil, they move like water, that is, both upward (if evapotranspiration is higher than precipitation) and downward (otherwise). Therefore, a real threat of nitrate leaching occurs only during the winter half-year, because in the summer half-year, that is, when the temperature exceeds 5 °C, evapotranspiration dominates and water moves from deeper layers to the surface. Therefore, in the summer half-year nitrate leaching is recorded only after significant rain event. Nevertheless, with high nitrate content in the soil, there is a risk of eutrophication of surface water (especially the first layer) and therefore rational fertilizer management should be applied in accordance with the guidelines of the Code of Good Agricultural Practices (Department for Environment, Food & Rural Affairs, 2009) or the Nitrate Directive (The Council of the European Communities, 1991).

The method used in this article defines the concept of so-called basic leaching as the equivalent of N leaching losses in conventional cereal cultivation, under conditions of sustainable mineral fertilization and mid-autumn plowing, but without the use of organic fertilizers (Aronsson & Ulén, 2013). When determining the basic leaching value, the soil type and average precipitation in the region have been taken into account (Table 1).

It should be emphasized that basic leaching does not determine the exact quantity of N leached from a given field, because it does not take into account variations of temperature, amount of precipitation and other quantities influencing N leaching from a specific measurement year. Despite these simplifications, basic leaching calculations can help farmers better understand what factors affect N leaching and what actions they can take to reduce it.

Factor B—type of crop grown in the previous season

The highest N leaching occurs in autumn and winter, that is, at the beginning of each crop year. It is mostly determined by the way in which the field was used in the previous crop year. Thus, crops grown in the previous crop rotation also influence the level of N leaching in the current crop cycle (Table 2).

So if new crops are sown in the autumn, N leaching will decrease, which must be taken into account when estimating the losses. Where temporary grassland is plowed in spring before a new crop is introduced, particular attention should be paid and the relevant coefficient in Table 2 should be multiplied by 1.5. Data from Table 2 cannot be treated only as crop-specific leaching values. For example, N leaching rates in cases such as fodder crops, fallow land, sugar beet and postharvest crops include corrections (adjustments) related to other factors contributing to the reduction of N leaching, for example, late plowing, plow-less tillage.

Factor C—soil tillage

Frequent tillage and the associated soil mixing stimulate the release of nitrate N form from the soil, especially if the tillage is carried out at the beginning of autumn. In case of delay or failure to carry out cultivation operations in autumn, nitrate leaching is reduced. Therefore, a coefficient from Table 3 must be used, taking into account the date of plowing in the previous year. If a perennial crop is grown in the field for fodder, the coefficient from the row “No plowing in the autumn” must be used. In the case of potatoes, beet and root crops, it should be assumed that harvesting means the same as soil tillage in late autumn.

Factor D—application of organic fertilizers

If manure is applied in autumn, some of its N content will be leached. Moreover, with fertilizer, both plant available (mineral) and unavailable (organic) N are introduced into the soil, and the release of mineral N from the latter is not always synchronized with the uptake cycle of the plants. This means that the risk of N leaching increases slightly even after spring application. As shown in Table 3, under the spring application of manure and liquid fertilizers, N leaching is only slightly higher than when only mineral fertilizers in balanced doses are applied. After the application of organic fertilizers in autumn, the leaching is greater than after the application of mineral fertilizers. Slurry (livestock urine with a possible small amount of feces and/or water; contains on average 1–3% of dry matter) consists mainly of plant-available ammonium N, so its fertilizing effect can be compared to that of mineral N fertilizers. Solid manure, on the other hand, contains almost exclusively N in organic form (Font-Palma, 2019). Therefore, the release of mineral N from solid manure can be slower than from liquid organic fertilizers (Antil et al., 2005). Probably the most favorable way to use manure for N leaching is in spring rather than in autumn. There are discrepancies in the permissible date of application of fertilizers, but the provisions in this respect should be strictly observed (organic fertilizers in liquid and solid form should be applied in the period from 1st March to 30 November, except for fertilizers used in crops under protection, i.e., in greenhouses).

Factor E—excess N leaching

When the field is fertilized with natural or mineral fertilizers at doses appropriate to the nutritional requirements of the crops grown, N leaching may be considered to be low. If too much fertilizer is applied, the leaching will increase, although an overdose of fertilizer is not intentional. Such a situation is possible during the summer drought when small plants cannot fully benefit from the N introduced with the fertilizers in spring and early summer. When estimating whether, and if so, too much N was applied on the field, it is appropriate to start by estimating the amount of crop-available N that remained from the previous growing season, that is, the total amount of mineral N supplied by mineral and/or natural fertilizers, and to add the amount of predicted additional N leaching losses due to exceeding the optimum fertilizer application rate for average yields on different soils (expressed in kg N·ha⁻¹). In this way, a sum of leaching is obtained. The amount of N applied should be compared with the recommended N dose needed to obtain planned yield of cultivated plants. A good source of information on nutrient requirements of plants is the Program of measures to reduce pollution of waters with nitrates from agricultural sources and to prevent further pollution (Ministry of Agriculture & Rural Development of Poland, 2018).

The N load applied is the sum of the amount of N from mineral fertilizer and the expected (approximately) amount of N contained in the natural fertilizers used for cultivation. If the actual amount of N is greater than the recommended amount, refer to Table 5 for the additional N leaching rate.

Calculations—total N leached from field

The first step in calculating the total N leached from the field (see Fig. 3) is to determine the extra N leaching from Table 5.

It is necessary to calculate the fertilizer intensity first as: $$I=\frac{\mathrm{T}\mathrm{N}}{A},\mathrm{T}\mathrm{N}=\sum _f{m}_{\mathrm{f}}\cdot c_{\mathrm{f}},$$where I is the fertilizer intensity (kg N·ha⁻¹), TN is the total N load applied to the field (kg N), A is the area of the field (ha), m_f and c_f are mass of fertilizer (kg) and N content in specific fertilizer respectively, f indexes the fertilizers used in the field. It should be stressed that in the presented method the forms of nitrogen applied with mineral fertilizers are not differentiated and the exact date of application is not taken into account. This means that the method is susceptible to further improvements as the precise determination of the impact of these factors can significantly improve the values of the estimated quantities. In the next step, the excess over the recommended fertilizer intensity should be calculate as: $$\mathrm{E}\mathrm{x}\mathrm{c}=I-R\cdot C,$$where Exc is the excess over the recommended fertilizer intensity (kg N·ha⁻¹), R is the recommended N load per tonne of product (kg N·tonne⁻¹), C is the expected crop (tonnes ha⁻¹). Depending on the value of Exc, for a given soil type, the appropriate value of estimated extra N leaching E is now selected from Table 5. Finally, the total N leaching from the field is calculated as: $$\mathrm{T}\mathrm{N}\mathrm{L}=A\cdot B\cdot C\cdot D+E,$$where TNL is the total N leaching from field (kg N·ha⁻¹), A is basic leaching (kg N·ha⁻¹) from Table 1, B is the factor affecting basic leaching depending on the crop yield in the previous year from Table 2, C is the factor estimating the effect of soil tillage on N basic leaching from Table 3 and D is the factor for additional N leaching losses compared with basic leaching depending on manure type from Table 4.

Opinion poll

An opinion poll was conducted on 31 farms within the Puck Commune, which is approximately 3.6% of all farms in this Commune. Field experiments were approved by the Head of the Puck Commune. Farmers provided the following data for all their fields in the survey:

• soil type (determination of factor A)

• type of crop (determination of factor B)

• date of plowing (determination of factor C)

• information on manure (determination of factor D)

• mass of the product (determination of factor E)

• field area (determination of factor E)

• types and amounts of mineral fertilizers applied on the field (determination of factor E)

N leaching calculator

Within the Water PUCK project, a website in the form of an interactive calculator to assist farmers in determining the quantity of N leaching from the field was developed. Access to the calculator is through the main website of the project www.waterpuck.pl through the “Services” tab.

The method of calculating N leaching from an agricultural field described in this paper has been implemented as a website’s back-end. After entering the correct input data, the result is refreshed immediately.

The user can easily enter the same information as collected in opinion polls into the leaching calculator (see Fig. 4). Entering data is very intuitive and the result is refreshed on the fly. As a result, the farmer, agricultural adviser or other interested parties can quickly and easily obtain information about:

• basic N leaching (kg N·ha⁻¹),

• total mass of N applied (kg N),

• modified N leaching (kg N·ha⁻¹),

• crop yield (tonnes ha⁻¹),

• fertilization intensity (kg N·ha⁻¹),

• extra N leaching (kg N·ha⁻¹),

• total N leaching (kg N·ha⁻¹),

• total N leached (kg N).

Using the N leaching calculator described here should help farmers to choose the right dosage of N-containing fertilizers to be applied on the field. In addition, the user of the calculator can check what effect the use of natural fertilizers will have on the N leaching. It also informs which fertilization practices increase the risk of excessive leaching of N.

Surface area of the studied fields

The Puck Commune has the area of 24,266 ha (242.6 km²), which is 1.33% of the area of Pomeranian Voivodeship. Agricultural land is 61% of the Commune’s area, including 72.7% of arable land, 19.2% of meadows, 0.2% of orchards and 4.4% of pastures. Forests are 31.2% of the Puck Commune’s area. The area of 291 studied fields varies from 0.1 to 25 ha with a median of 2.3 ha. The distribution of the size of the fields according to the type of crop is shown in a box diagram (see Fig. 5). On the vast majority of agricultural fields (n = 182) cereals (wheat, rye, oats, barley, triticale, grain mixtures) are grown and a median area of these fields is equal to 2.25 ha. The second crops with the highest number of fields are fodder crops (silage maize, grass mixtures on arable land) (n = 55) with a median area equal to 2.50 ha. Oilseeds (colza) are grown on 30 fields with a median area of 2.32 ha, root crops (potatoes) on 19 with a median area of 0.60 ha, legumes (field bean, lupin, field pea) on 4 with a median area of 4.59 ha and textile crops (linum) are grown on only one field of 5.00 ha.

The total area of all studied fields is equal to 956.74 ha which is about 6.5% of total agricultural land of the Puck Commune. Share of individual crops in the total studied area is presented in Fig. 6. Cereals are grown on more than 60% of the studied area, fodder crops on 22.5%, oilseeds on 10.5%, root crops on 3.4%, legumes on 3% and textile crops on 0.5%.

Basic leaching and its modifications

Clay soils with 15 kg N·ha⁻¹ of basic leaching (Table 1) are 47.5% (n = 140) of the surface area of all studied fields, loamy soils with 20 kg N·ha⁻¹ of basic leaching are 45.7% (n = 134) and sandy together with organic soils (15 kg N·ha⁻¹ of basic leaching) are 6.8% (n = 17) of the surface area of all studied fields. Table 2 shows that the type of crop cultivated in the previous year may have the greatest influence on the change in basic leaching and its modifications may range from −40% to 100% of the original value. The number of fields with a specific modification of base leaching is presented in Table 6.

Another factor that may influence the basic leaching is the soil tillage time. According to Table 3, the plowing time can change the basic leaching even up to −30% (if no plowing is done at all). Table 7 shows the number of fields depending on the plowing time.

The third and last factor influencing tfhe basic leaching rate is the application of natural fertilizers. In the case of spring natural fertilizer application, basic leaching is modified by +10% regardless of the type of fertilizer. In the case of natural fertilization in autumn, the use of solid manure increases the basic leaching by 15%, while the use of slurry increases the basic leaching by 30%. The categorization of fields by natural fertilization type is shown in Table 8.

It should be emphasized that the change in basic leaching is the product of all three factors analyzed above. Lack of autumn plowing or late autumn plowing can only reduce the amount of basic leaching. However, both the type of crop cultivated in the previous year and the use of manure can potentially increase this value. Thus, the total change in basic leaching due to these factors can range from −51% to even +160% of its initial value resulting from soil type and average annual precipitation in a given region.

Fertilization intensity

The average value of mineral fertilization intensity calculated as the sum of the total load of N applied to the fields divided by the total area of all fields is equal to 110.94 kg N·ha⁻¹. The mineral fertilization intensity for each type of crop is shown in the box plot (see Fig. 7). The highest average intensity of mineral fertilization was applied to oilseeds fields (140.87 kg N·ha⁻¹) and the lowest to legumes and textile crops fields (32 and 34 kg N·ha⁻¹ respectively). The most intensively fertilized fields (about 340 kg N·ha⁻¹) were cultivated with fodder crops.

It should be noted that a large variation in the intensity of fertilization within a given type of crop does not necessarily mean that the intensity of fertilization deviates strongly from the recommended dose, but may result from the different N demand of plants included in a particular crop group.

Extra N leaching from field

For all 291 studied fields, on the basis of calculations of exceeding the recommended fertilization intensity and data from Table 4, an estimated value of the extra N leaching was determined. For almost half of all fields (49.8%) the extra N leaching is equal to 0 kg N·ha⁻¹. For 37.8% of the fields, the extra N leaching value is between 2 and 7 kg N·ha⁻¹. In the remaining 12.4% of the fields, the value of the extra N leaching exceeds 7 kg N·ha⁻¹. The amount of extra leaching due to the type of plant was presented as a bar chart (see Fig. 8).

The extra leaching of N depends on the excess over recommended fertilization intensity and the soil type on which the plant is cultivated. The higher the excess of the actual fertilizer intensity over the recommended fertilizer intensity, the greater the extra N leaching from the field is (Table 5). It is also worth comparing how the extra N leaching from the field varies due to the soil type (i.e., whether farmers apply higher than recommended doses on specific soil types). This comparison is presented in Table 9 and shows that such a relationship does not exist (i.e., the distribution of extra N leaching depending on the soil type is similar to the collective distribution for all fields).

Total N leaching from field

The total estimated N leaching from studied fields varies from 4.0 to 68.2 kg N·ha⁻¹ with a median of 19.8 kg N·ha⁻¹. The distribution of the total N leaching from field according to the type of crop is shown in a box diagram (see Fig. 9).

The highest average total leaching of N (weighted by fields’ surface areas) is for fields cultivated with root crops (about 33 kg N·ha⁻¹) and the lowest for the field cultivated with textile crop (12 kg N·ha⁻¹).