ENVIRONMENTAL EFFECTS OF SPATIALLY VARIABLE NITROGEN APPLICATION

Maria Walczykova, Mirosław Zagórda
Department of Machinery Management, Ergonomics and Production Processes, Faculty of Production and Power Engineering, University of Agriculture in Krakow, Poland

ABSTRACT

The purpose of this study was to study the effects of N fertilizer topdress application with timing based on plant demands and using variable rates technology for spreading. The experiment was carried out on two commercially managed fields of 13 (F1) and 19 (F2) hectares. On both fields, phosphorus and potassium were applied uniformly. As far as nitrogen is concerned, half of the required dosage was spread uniformly on both fields in the early spring. On field F1 the second half was arbitrarily split into two rates and applied uniformly at the beginning of two growth stages – shooting and earing. On field F2 the dosage of N fertilizer, applied at the same time as stated above, was based on plant nutrient requirement and was spread using the spatially variable technique. After harvest, soil sampling on both fields for mineral nitrogen residues was carried out. It was found that the topdressing application of the N fertilizer, based on greenness measurements, resulted in 28.2 and 20.8% decrease of the required dosage at the shooting and earing growth stages accordingly, and in lower nitrogen residues, by 19.8–56.1%, in examined layers, and yield higher by ca 27%, in comparison with the arbitrary determined and uniformly spread rates.

Key words: nitrogen, residues, SPAD, topdress application, variable rates, yields, winter wheat.

INTRODUCTION

The efficient use of nitrogen fertilizer is important to because it affects economic production, the quality of grain as well as the ground and surface waters. Insufficient N application can result in reduced yield, whilst, on the other hand, excessive N can cause plant lodging and disease [1, 3, 10]. Nitrogen, in a form which is available for plant uptake, is highly soluble and easily lost to leaching as water moves through the soil profile and pollutes ground waters [2, 12]. Nitrate leaching from plant production systems is of environmental and also economic importance, as it presents a direct financial loss to the producer [8]. Welsh et al. [16] are of the same opinion and they suggest that, although the determination of specific rates of N is complex in comparison with P and K fertilization, it can be varied by the farmer, as the application of nitrogen fertilizer is one of the most significant factors that influence the economics of arable cropping. According to Jones and Olson-Rutz [8], annual soil testing and yield goals help producers to calculate N fertilizer rates, to avoid over-fertilization and to reduce nitrate leaching. They suggest that in cereal crop the first rate of N fertilizer, needed at the cultivation time, should be applied in uniform or spatially variable rates and then be followed by split topdress applications, taking into consideration plant nutrition requirements or growth stages. A similar view is presented by other authors [5, 7, 11, 15]. As nitrogen is closely associated with leaf chlorophyll, measurements of leaf greenness is widely used for obtaining information on plant requirements for nitrogen. Link at al. [9] emphasize that due to the importance of nitrogen in plant nutrition, its influence on crop yield and quality and its mobility in soils, it is expected that a variable rate of N application would improve the economic and environmental outcome of plant production. The same opinion is strongly expressed in the work by Stoorvogel and Bouma [14], who claim that precision agriculture not only improves the agricultural production process but also controls losses of agricultural chemicals to the environment. A better match in space and time between nutrient application to plants and their requirements is necessary to minimize losses of excess nutrients to the environment.

The scope of the presented research was looking for answers to the following questions:

• How big is the difference between the amount of nitrogen applied by uniform topdressing and a spatially variable one, which takes into account the plants requirements?
• Will the dosage of nitrogen, tailored to the requirements of plants and applied using variable rates, influence nitrogen residues at depths of 0.3, 0.6 and 0.9 m, in comparison with the conventional approach? 
• Will the dosage of nitrogen, matched to the requirements of plants and applied using variable rates, influence the yield?

MATERIALS AND METHODS

The experiment was carried out on two commercially managed fields of 13 and 19 hectares, located on a family farm. In this paper they will be designated as F1 and F2. On both fields, which were located to each other, winter wheat was grown. The soil type was a silty clay loam on both sites (Tab. 1), and also terrain conditions and technological processes in terms of operations and used machinery were identical. To replenish phosphorus and potassium, their amounts were calculated on the basis of soil tests, made after forecrop harvest and with 7.5 t·ha^-1 as the expected yield, typical for the farm where the experiment took place. The application of phosphorus (70 and 75  kg·ha^-1, on fields  F1 and F2 accordingly)  and potassium  (125 and 128 kg·ha^-1, on fields F1 and F2 accordingly) was made pre-planting. When calculating N fertilizer rates, the above mentioned yield was considered together with the nitrate-N content in soil at layers of 0.3 m, 0.6 m and 0.9 m. Sampling for soil N content was done in the spring time and overall rates of N fertilizer were determined according to recommendations of the extension service [6].

On both fields, phosphorus and potassium were applied uniformly. As far as nitrogen is concerned, half of the calculated dosage was spread uniformly on both fields in early spring. On field F1 the second half was arbitrarily split into two rates and applied uniformly at the beginning of two growth stages – shooting and earing. On field F2 the dose of N fertilizer for two topdress applications was based on plant requirements and it was spread using the spatially variable technique. In order that topdress N fertilization matched plants requirements, the following was done:

1. a chlorophyll test was carried out using a chlorophyll SPAD meter at the beginning of the above mentioned two stages of growth, to determine the crop’s N status,
2. elaboration of digital maps showing the spatially variable leaf greenness at both growth stages, 
3. calculation of the required amount of nitrogen for second and third application, based on digital maps presenting local needs,
4. elaboration of maps of application for spatially variable nitrogen spreading.

The Minolta chlorophyll meter model SPAD502DL, which enables non-destructive measurements, was used to monitor the crop’s N status. On a digital map of the field, made by the GPS receiver running on FarmWorks SiteMate 9.3 software, a grid of 50x50 m was laid. From each square, measurements of SPAD were made on 10 plants conforming to the method described by Murdock et al. [10] and Shapiro et al. [13]. For each square the averaged result was attributed to its geographically determined centre [18]. Digital maps visualizing spatially variable greenness were prepared in ArcView GIS 3.3 software. For each polygon on those maps, depicting the variable SPAD values, the required rates of nitrogen were determined. To accomplish this task, calibration elaborated by Fotyma and Fotyma [4] for winter wheat was applied. On field F2, a reference strip without nitrogen fertilization was laid. Measurements of SPAD were made there at the same time as on the remaining parts of the field. The recommended rate of nitrogen depends on the SPAD value quotient obtained on the production area and the reference strip (Tab. 2).

Spatially variable application was carried out by Rauch Alpha 1131 spreader, controlled by a universal board computer LH 5000 GPS.

The combine harvester Claas Lexion 430 was equipped with an optical system to monitor yields in order to obtain information on the spatial distribution of yields. The raw data from both fields were processed using AgroMap 6.0 software, produced by Agrocom [17].

After harvest, soil sampling on both fields for mineral nitrogen residues (N-NO₃ and N-NH₄) was carried out. Samples were collected from each square of the grids at three depths: 0.3 m, 0.6 m and 0.9 m. Maps of nitrogen residues at different depths and the total for three examined depths were prepared in ArcView GIS 3.3.

Finally, the yield maps resulting from uniform and spatially variable nitrogen topdress application were accomplished.

RESULTS AND DISCUSSION

Plant demand on nitrogen
On field F1 (13 ha) three rates of nitrogen fertilizer were spread uniformly. The calculated first rate amounted to 75.00 kg·ha^-1, the second and third to 40.80 kg·ha^-1. So, for topdress application 91.60 kg·ha^-1 was used and the total nitrogen applied on this field was as much as 156.60 kg·ha^-1.

As far as the first rate of nitrogen is concerned, the same was applied to field F2. This was followed by a second rate at the beginning of shooting, using a variable rate application according to local needs, as is shown in Figure 1. The average value of the second rate, calculated as a weighted arithmetic mean from all polygons (Fig. 1), amounted to 29.34 kg·ha^-1 (Tab. 3). This was lower by 11.5 kg·ha^-1 than in the case of uniform application, which was, as quoted above, 40.80 kg·ha^-1, i.e. by 28.19%.

The average rate of the 3rd rate was calculated in the same way. Based on the map of application for nitrogen fertilization before earing (Fig. 2), the weighted arithmetic mean amounted to 32.29 kg·ha^-1 (Tab. 4) and it was lower by 8.5 kg·ha^-1 in comparison with the analogous rate applied conventionally, i.e. by 20.34%. Although this time the difference was lower than in the case of the 2nd rate, it still meant less N left in the soil than can be leached.

Finally, application maps of spatially variable rates for 1st (not included, as it presents uniform spreading), 2nd (Fig. 1) and 3rd (Fig. 3) fertilization were overlapped and the resulting map is shown on Figure 3. The weighted arithmetic mean from the summed-up maps turned out to be as much as 139.49 kg·ha^-1 (Tab. 5), which means less by 17.11 kg·ha^-1 in comparison with the amount resulting from the conventional way of fixing the topdressing rates and uniform spreading, that was 156.60 kg·ha^-1.

Mineral nitrogen residues in soil
The maps of nitrogen residues spatial distribution (Fig. 4) clearly show that the N use efficiency is much better on field F2, where rates for topdressing were adjusted to the plants requirements and the specific site management was applied for fertilizer spreading. On field F1, with traditional application, 66% of the field area contained 9–12 mg N·kg^-1 N in fresh soil (Tab. 6). The lower nitrogen content, from a range of 6–9 mg N·kg^-1 of fresh soil, prevailed on the variably treated field F2 and it occurred on 73% of the field area (Tab. 6). The difference between the average amount of nitrogen residues, found on uniformly and variably treated field, was 19.8% (Tab. 6).

A similar tendency was observed at a depth of 0.6 m. As shown in Figure 5, on the uniformly fertilized field F1, areas with higher mineral nitrogen content predominate than on field F2, where variable rates were applied. On the first one, on 64% of the total area, 6–9 mg N·kg^-1 of fresh soil was found (Tab. 7). On the second one area amounted to 89%, with lower nitrogen content from the range 3–6 mg N·kg^-1 of fresh soil, definitely prevails (Tab. 7). At this depth the difference between the amount of residues, in favour of the variable mode of application, was higher than at a 0.3 m depth (Tab. 6).

Examination of the last layer, at a depth of 0.9, m proved there was a clear-cut superiority of specific site management over uniform fertilization, in relation to environmental protection (Fig. 6). The variable rate application of nitrogen resulted in 0–3 mg N·kg^-1 of fresh soil residues on 96% of the F2 field area, while in the case of conventional treatment it amounted to 59% (Tab. 8). The difference amounted to 56.1% (Tab. 8).

Based on the calculation of nitrogen residues, from maps from Figure 4–6 and Table 6–8, it can be stated that in general, the variable rate of topdress N fertilization, adapted to plants requirements, resulted in lower nitrogen residues by 19.8–56.1% in the examined layers, when compared with the arbitrary determined and uniformly spread rates.

The final illustration of differences between uniform and variable rates, in terms of nitrogen left in the soil, is shown in Figure 7. Here, the spatial distribution of the overall residues at the examined depths is presented. On the conventionally treated field F1, the amount of residues falls into two ranges: 15–20 and 20–25 mg N·kg^-1 of fresh soil and this occurred on 87% of the total field area (Tab. 9). As far as the variable technique is concerned, the nitrogen residues on 81% of the total field area fall into ranges 5–10 and 10–15 mg N·kg^-1 of fresh soil (Tab. 9). The difference between considered application techniques, regarding the total amount of nitrogen residues, was 32.5%.

The above quoted results clearly indicate that the environmental outcome of plant production, when site specific management is applied, is much more favourable and decreases the risk of overfertilizing.

Level of yields on the experimental sites
Yield monitoring made it possible to visualize the distribution of its different values over the area of fields (Fig. 8, 9). The quoted histograms, made on the basis of yield maps, show that on the uniformly fertilized field (Fig. 8) the highest yield amounted to 6.3–7.5 t·ha^-1 and occurred on 49% of the field area. Specific site management turned out to be more favourable also with regard to the yield level. On the field with variable rate application (Fig. 9), 73.5% of the field yields were situated in ranges 7.1–8.3 t·ha^-1 (35.3%) and 8.3–9.6 t·ha^-1 (37.65%). The average yield from this field was higher in comparison with the uniformly fertilized one by ca 27%. 

CONCLUSION

1. The topdressing application of nitrogen fertilizer, matched to plant requirements and spread in variable rates, resulted in a 28.19 and 20.83% decrease of the required dosage at shooting and earing growth stages accordingly, in comparison with the conventional technique, i.e. arbitrary determined and uniformly spread rates.
2. In general, variable rates of topdress N fertilization, adapted to plants requirements, resulted in lower nitrogen residues by 19.8–56.1% in the examined layers, when compared with uniform rates.
3. When adjusted to the plants requirements and variably spread nitrogen fertilizer, the yield was higher by ca 27% in comparison with the conventional application.

ACKNOWLEDGEMENTS

This Research was financed by the Ministry of Science and Higher Education of the Republic of Poland.

REFERENCES

Accepted for print: 01.03.2016

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