Abstract

In the growing of winter oilseed rape nitrogen stands for the major part of fossil energy input, thus influencing the energy output decisively. Energy balance sheets as result of different N supply were analysed in a field experiment (1995-1998) in the Hercynian dry region of Central Germany. The N provision was modified by different precrops (winter barley, peas) and differentiated nitrogen nutrition (mineral, organic fertilizer; 0-240 kg N ha^-1). The energy efficiency was evaluated by use of the parameters "energy gain", "energy intensity" and "output/input-ratio".

The required nitrogen rate and thus the fossil energy input was influenced by the precrops. Rape grown after peas needed less N and gave higher yields than after barley. This led to higher "energy gain", lower "energy intensity" and a better "output/input-ratio". The effect of the fertilizer type was related to the N concentration in the fertilizer and to the energetic evaluation of the manure. Assuming that fossil energy was needed for the production of manure, the expenditure was lower for the provision of mineral fertilizer. Yield as well as "energy gain" were generally higher with mineral fertilizer than with manure. The "energy intensity" was lowest with mineral fertilizer and highest with energy expenditure for manure production. In contrast, the "output/input-ratio" increased inversely. The energy balance sheet was largely influenced by the N rate. Although the rising N level was accompanied by higher fossil energy input, "energy gain" as well as "energy intensity" were enhanced. The lowest "output/input-ratio" was found at the highest N level.

Introduction

Apart from its value in food production, winter oilseed rape is grown as energy crop. The energetic usefulness of winter rape depends on the fossil energy input for production and industrial processing as well as the involved energy recovery. An appropriate means for estimating the energy recovery of crops are energy balance sheets, the quantitative juxtaposition of energy input and output. They have been commonly recognized as tool to evaluate intensity and environmental acceptability of crop production (Eckert and Breitschuh 1994, SRU 1994, Hülsbergen and Kalk 1997). Energy balance sheets in crop farming are mainly influenced by the fossil energy input. The latter depends markedly on the input of nitrogen fertilizers (Diepenbrock et al. 1995). In contrast to mineral fertilizers, few information is available about the energetic evaluation of manure, because there is no common production technology of manure and, thus, no standard yardstick for their nutrient concentration as in the manufacturing of mineral fertilizers. Energy efficiency in plant production can be characterised by parameters like "energy gain", "energy intensity" and "output/input-ratio". The "energy gain" is the difference between the energetic yield (output) and the expenditure of fossil energy (input). Generally, the aim is to maximize the "energy gain". The "energy intensity" is a tool to determine minimum input of fossil energy per natural yield unit converted into grain equivalents. Within an energy-efficient system, the input per product unit output should be as low as possible. The parameter "output/input-ratio" ensues from the ratio of energetic yield (output) and fossil energy input. It should be as high as possible.

Material and methods

Experiments

The energetic calculations are based on results which were obtained from field experiments (1995-1998) carried out in the Hercynian dry region of Central Germany. The soil type is sandy loam (FAO-classification: Stagno-Luvic Gleysol). The long-term mean of annual rainfall is 551 mm, the mean temperature of the atmosphere 9.3 °C (1961-1990).

Winter oilseed rape has been grown since 1994 in the crop rotation winter barley - winter rape - winter wheat or peas - winter rape - winter wheat with different N fertilizers (mineral, organic) and distinct N rates (N0: 0, N1: 80, N2: 160, N3: 240 kg N ha^-1). The trials were arranged in a three factorial split plot design with four replications. Independent of precrop and fertilizer, the nitrogen application based on the same N target amount including the plant available N in the soil (N_min). For the N fertilization calcium-ammonium nitrate (27 % N_t) or cattle liquid manure (0.40 % N_{t fresh}) were used. For the calculation of the needed liquid manure amount, an effectiveness of 60 % compared to the mineral fertilizer was assumed (Görlitz et al. 1985). This means 1 kg calcium-ammonium nitrate-N is substitutable by 1.7 kg liquid manure-N. The N applications were carried out early in spring, and, in the ratio 60:40, at the level N2 and N3 additionally at the beginning of bud development.

Methodics of energy balancing

Energy balancing requires the consideration of direct and indirect energy consumption and energy output. The energy balance sheets were based on the economic approach by Fluck and Baird (1980). The energetic evaluation of the direct energy input was made via the consumption of diesel. According to Reinhardt (1993) an energy equivalent of 35.65 MJ l^-1 diesel was assumed. Indirect energy input was supplied with seed material (140 MJ ha^-1), fertilizers (47.1 MJ kg^-1 mineral-N, 15.8 MJ kg^-1 P₂O₅ and 9.3 MJ kg^-1 K₂O), plant protection agents and capital goods. Since phosphorus and potassium were applied to the crop rotation in a single dose, only one third of this energy input was destined for winter rape. The nitrogen in the liquid manure was energetically evaluated in correspondence with the effectiveness of mineral fertilizer; this means 28.26 MJ kg^-1 N. According to Green (1987) the energetic evaluation of plant protection agents was made separately for the categories herbicide (264 MJ kg^-1 active ingredient, AI), insecticide (214 MJ kg^-1 AI) and fungicide (168 MJ kg^-1 AI). The energetic evaluation of the energy input via capital goods was calculated according to Kalk (1997). For energy recovery by winter oilseed rape, only the oil output was considered. The oil yield results from the yield and its oil content as determined by Nuclear Magnetic Resonance (Oxford 4000, Oxford Instruments, GB) at harvest, reduced by the loss during oil production. Finally, the energy output is the result of oil yield and energy density of the oil (3 900 MJ dt ^-1; Fluck 1992, Hansen 1994). To determine the energy intensity, 1 dt of rape oil was converted into 3.1 grain equivalents (GE). For further information see Rathke et al. (1998).

Mathematical statistics

The statistical handling was made with the MIXED-Procedure of the software package SAS. In the analysis of the whole trial series, the years were accidental and the location range was fixed. Based on variance analysis, tests of means (Newman-Keuls-test) were carried out with consideration of interactions (p = 5 %). Since no significant interactions were observed, only main effects are described below.

Results and Discussion

Energy input and output

Winter rape after peas required less direct and indirect energy via nitrogen fertilization than after barley due to higher amounts of N_min in the soil. Higher yields as well as slightly higher oil contents were also recorded. This caused a higher direct energy input for harvesting and oil production, and finally nearly the equal total energy input after both precrops (Table 1). Based on the smaller energy equivalent assumed for liquid manure-N, the indirect energy input for nitrogen fertilization was reduced vis-à-vis the mineral-N. Because of its larger volume, the direct energy input for storage and transport of liquid manure was generally high. As a result, the effect of both fertilizer types on the energy input was nearly the same (Table 1). In contrast to the precrops and fertilizer types, the energy input was significantly influenced by the N rate. The indirect energy input for N fertilizer as well as direct energy input for storage, transport and application rose with increasing N rates. Due to higher yields, the direct energy input for harvesting and oil production increased, too. Altogether, the lowest total energy input was found without N fertilization (N0), the highest on the high (N3) level (Table 1).

The energy output depended stronger on the different yields than on the various oil contents. Different types of N supply (Table 1) had also a great influence. The energy output after peas was significantly higher than after barley. Even the energy output was higher after mineral fertilizer than after liquid manure, the difference was not significant. Increasing the N quantity enhanced the energy output, although there was only little difference between levels N2 and N3.

Table 1: Energy input and output (GJ ha^-1)

Different letters within one row and form of N supply indicate significant differences (p = 5 %)

Energy efficiency

The energy efficiency was mainly dependent on yield performance because the energy recovery exceeded the input of fossil energy by several times. Parameters to describe the energy efficiency are "energy gain" (Fig. 1) and "energy intensity" (Fig. 2).

According to similar energy inputs and higher energy outputs compared to rape after precrop barley, it is obvious that the highest "energy gain" was yielded after peas. As a result of the higher output, the mineral fertilizer led to significantly higher "energy gain" and lower "energy intensity" compared to liquid manure. With increasing N rates the "energy gain" rose up to a maximum, then it declined. The highest "energy gain" was achieved with N2. The decline with additional N application (from N2 to N3) can be explained by the raising energy input, while there was nearly no increase in output (compare Table 1).

Figure 1: Energy gain (GJ ha^-1)

In contrast to the "energy gain" there was no significant difference in the "energy intensity" related to the precrop. After peas, the "energy intensity" was lower, yet only in tendency. The use of liquid manure resulted in a higher "energy intensity" compared to mineral fertilizer. The "energy intensity" increased up to the highest N level. Due to the low productivity, the smallest "energy intensity" was achieved without nitrogen fertilization (N0). The N demand that involved a maximum "energy gain" was higher than the N rate that furnished a minimum "energy intensity". The course of the "output/input-ratio" (not indicated here) was exactly the opposite of the "energy intensity".

Figure 2: Energy intensity (MJ GE^-1)

Conclusion

The energetic usefulness of winter rape cropping depends on the purpose as well as the parameter chosen for energy efficiency. While the aim of energy cropping is to maximize the "energy gain", "energy intensity" and "output/input-ratio" are used to quantify the intensity and environmental acceptability of a cropping system. Within the framework of the described field experiment, the energy efficiency was primarily influenced by different N rates, secondarily by the type of fertilizer. However, there was only little influence by the precrops. On the one hand, the highest "energy gain" was produced at the medium N rate (N2). On the other hand, the lowest "energy intensity" and the most favourable "output/input-ratio" were achieved without N fertilization (N0).

Acknowledgements

This work was supported by the German Research Foundation (DFG).

References

1. Diepenbrock, W., Pelzer, B. and Radtke, J. (1995): Energiebilanz im Ackerbau. KTBL Arbeitspapier 211. Landwirtschaftsverlag Münster-Hiltrup.

2. Eckert, H. and Breitschuh, G. (1994): Kritische Umweltbelastung Landwirtschaft (KUL) - Ermittlung und Bewertung der Energiebilanz. Arch. Acker- Pfl. Bodenkd. 38, 337-348.

3. Fluck, R.C. (1992): Energy in agricultural products. In: Fluck, R.C. (ed.): Energy in farm production. Elsevier, Amsterdam, 39-43.

4. Fluck, R.C. and Baird, D.C. (1980): Agricultural energetics. AVI Publ. Comp., Westport, Connecticut.

5. Görlitz, H., Asmus, F. and Breternitz, R. (1985): Kennzahlen und Richtlinien für den Gülleeinsatz. Feldwirtschaft 26 (10), 454-457.

6. Green, M.B. (1987): Energy in pesticide manufacture, distribution and use. In. Stout, B.A. (ed.): Energy in world agriculture. Volume 2.

7. Hansen, F. (1994): Die energetische Bewertung von Ertrag und Ertragsbildung verschiedener Kulturarten in einer Getreide/Ölfrucht-Rotation auf der Basis von Strahlungs- und Stickstoffnutzung im Bestand. Thesis, University of Kiel.

8. Hülsbergen, H.-J. and Kalk, W.-D. (1997): Stoff- und Energiebilanzen im Dauerfeldversuch. Wiss. Beiträge der 5. Hochschultagung der Landwirt. Fakultät der Universität Halle, 192-200.

9. Kalk, W.-D. (1997): Calculation of the direct energy input via capital goods. Unpublished information.

10. Rathke, G.-W., Schuster, C. and Diepenbrock, W. (1998): Auswirkung differenzierter Stickstoffversorgung auf die Energiebilanz im Winterrapsanbau (Brassica napus L.). Pflanzenbauwissenschaften 2 (2), 76-83.

11. Reinhardt, G.A. (1993): Energie- und CO₂-Bilanzierung Nachwachsender Rohstoffe. Vieweg-Verlag, Braunschweig, Wiesbaden.

12. Sachverständigenrat für Umweltfragen, SRU (1994): Umweltgutachten 1994. Verlag Metzler-Poeschel, Braunschweig, Wiesbaden.