Need for Savings in Environmentally Relevant Resources 551 Fertilisers

Fertilisers are an essential prerequisite for obtaining acceptable biomass yields. The average contribution of fertilisers to yields ranges from 40% to 60% and tends to be higher in the tropics (Stewart et al. 2005). However, most fertilisers are produced on base of exhaustible raw materials and/or are sources of environmental

Fig. 5.3 Statistical distribution of measured poplar yields in Germany divided into subspecies. Result of a survey of a total of n = 357 yield data of various poplar stands of 2-18 years on 25 different sites in Germany. The high frequency of the lower yields is caused by the high share of young stands, as the yield of poplar grows up to an age of 5-10 years

Fig. 5.3 Statistical distribution of measured poplar yields in Germany divided into subspecies. Result of a survey of a total of n = 357 yield data of various poplar stands of 2-18 years on 25 different sites in Germany. The high frequency of the lower yields is caused by the high share of young stands, as the yield of poplar grows up to an age of 5-10 years pollution. Fertilisers cause not only eutrophication and an increase of environmentally harmful substances in biofuels but also emissions during their production as well as during the crop production. Thus, the demand for fertiliser is an evident indicator of the environmental sustainability of the crop production.

Of the relevant nutrient fertilisers (N, P2O5, K2O, Ca, Mg, S), nitrogen (N), phosphate (P2O5) and potassium oxide (K2O) have the highest efficiencies, i.e. 33%, 20% and 60%, respectively (Engelstad 1968; Raun and Johnson 1999). However, these nutrients cause various environmental problems (Scholz and Ellerbrock 2002). Nitrogen is the most problematic nutrient. Its production requires a great deal of energy (Patyk and Reinhardt 1997) and its utilisation results in relevant emissions into air and water (Kaltschmitt and Reinhardt 1997). Phosphate is a globally limited raw material (Pradt 2003), and potassium is often used in the form of potassium chloride (KCl), which contains harmful chlorine (Cl). Therefore, minimising the application of these fertilisers improves the environmental compatibility of biofuel production.

For conventional food crops there are several fertilising rate recommendations, based on soil type, intended yield and nutrient content in the harvested crops. The recommended mean application rates in Germany for N range in

Conventional Fertilisers Soil

Fig. 5.4 Long-term impact of reduced nitrogen fertilisation on the yield of whole crop cereals and SRCs on a sandy soil in Germany (relative yield related to the yield of an application rate of 150 kg N ha-1). The reduction of the N application rate by 50% results in a mean relative yield loss of ~10% after 15 years for rye and triticale. Non-fertilisation cause significantly higher losses. By contrast, the relative yields of poplar and willow on reduced and even non-fertilised stands do not decrease, but instead increase, although the absolute yields grow over time. One of the reasons for this phenomenon seems to be mycorrhica

Fig. 5.4 Long-term impact of reduced nitrogen fertilisation on the yield of whole crop cereals and SRCs on a sandy soil in Germany (relative yield related to the yield of an application rate of 150 kg N ha-1). The reduction of the N application rate by 50% results in a mean relative yield loss of ~10% after 15 years for rye and triticale. Non-fertilisation cause significantly higher losses. By contrast, the relative yields of poplar and willow on reduced and even non-fertilised stands do not decrease, but instead increase, although the absolute yields grow over time. One of the reasons for this phenomenon seems to be mycorrhica general from 100 to 200 kg ha-1 year-1, for P2O5 from 50 to 110 kg ha-1 year-1, and for K2O from 90 to 380 kg ha-1 year-1 (KTBL 2005a). For energy crop species these recommendations are only partially correct, because (e.g. in the case of whole crop cereals) not only the grain (with a high N demand) but the whole plant is also used, because the energy efficiency of the cultivation may be higher with lower fertilising rates, and because some unconventional species such as SRCs need less or even no fertiliser (Fig. 5.4). Thus, the efficient use of fertilisers in energy crop production is an ongoing object of agricultural research.

Crop residues remaining on the field (straw, leaves and roots) as well as the recycling and refeeding of the residues and wastes of the crop products used for energy purposes such as ash and digested sludge contribute to minimising the demand for mineral fertiliser. Although the combustion and thermal gasification of biomass results in a major loss of nitrogen N (96%, ..., 100%) and sulphur S (70%, ..., 92%), the loss of P and K is lower and ranges between 30% and 100% (Hartmann and Strehler 1995; Heard et al. 2006). However, there are differences between the ash fractions (grate, fine and filter ash) concerning this matter. The grate ash used in practice is only 80-90% of the total ash content (percentage by weight) for cereals or grass and 60-90% for wood (Obernberger 1997), so that the actual nutrient recycling rate of solid biofuels is lower. Moreover, it must be considered that it is not the total percentages of these ash nutrients that are available to plants (Table 5.4).

Table 5.4 Selected nutrients in plant residues after combustion or digestion

Percent by

Content of nutrients in residue15 (% DM)

Residue

Crop species

weighta (%DM)

N

P2O5

K2O

Grateash

Cereals0

4.0 ± 1.5

0, ..., 2

7, ..., 10

5, ..., 18

Grass

7.0 ± 2.5

0, ..., 2

0.4, ., 1

11, ., 29

Wood

1.5 ± 1.0

0, ., 2

1, ., 5

4, ., 12

Digested sludge

Grain

25 ± 5

5.4

3.1

2.5

Cerealsc

24 ± 5

3.2

2.0

5.0

Grass

38 ± 5

3.9, ..., 4.7

1.6, ..., 2.6

7.2, ., 10.5

Maize

30 ± 5

2.1, ..., 3.1

1.4, ., 1.8

3.9, ., 7.2

Beets

25 ± 5

2.8

1.2

3.4

aAsh content according to Obernberger (1997) and FNR (2005). Percentage of sludge, stoichio-metrically calculated by Mähnert (2007) with moisture contents as shown in Table 5.3, a methane percentage of 55% and the biogas yields of Table 5.11

b According to Ruckenbauer et al. (1992), Vetter et al. (1995), Hasler and Nussbaumer (1996), Hartmann and Strehler (1995), Obernberger (1997), Frieß et al. (1998), KTBL (2005a,b), Holzner (2006), Heard et al. (2006) and Reinhold and Zorn (2007), converted by the mass equations P2O5 = 2.29 P and K2O = 1.20 K c Whole crops aAsh content according to Obernberger (1997) and FNR (2005). Percentage of sludge, stoichio-metrically calculated by Mähnert (2007) with moisture contents as shown in Table 5.3, a methane percentage of 55% and the biogas yields of Table 5.11

b According to Ruckenbauer et al. (1992), Vetter et al. (1995), Hasler and Nussbaumer (1996), Hartmann and Strehler (1995), Obernberger (1997), Frieß et al. (1998), KTBL (2005a,b), Holzner (2006), Heard et al. (2006) and Reinhold and Zorn (2007), converted by the mass equations P2O5 = 2.29 P and K2O = 1.20 K c Whole crops

During anaerobic digestion of energy crops in biogas reactors, the loss of N, P, K, S and other nutrient elements is theoretically zero, because only C, O and H in the form of methane (CH4) and carbon dioxide (CO2) are released. Trace gases, e.g. hydrosulphide (H2S) and ammonia (NH3), are insignificant in this connection and/ or can be limited by technical means (Amon et al. 2002; FNR 2004). Although a calculation by means of the denoted weight percentages and nutrient contents results in other figures, there are some practical results which confirm the zero-loss hypothesis (Herrmann and Taube 2006). A special advantage of biogas residues (digested output) is the high share of the vegetable valuable nitrogen constituents (NH4-N) and the high plant availability. Nearly 65% of the total N of maize sludge is NH4-N, and 75% of this is available for plants (Wendland and Offenberger 2007).

The plant availability of some nutrients may be limited in both types of residues. Moreover, in certain cases the contents of some heavy metals (Cd, Pb, Cu, Zn, Ni, Cr and Hg) may exceed the legal thresholds (BioAbfV 2002). Nevertheless, the use of energy crop residues as fertiliser significantly reduces the need for mineral fertiliser.

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