22 Phosphorus Recycling Using Residues from Bioenergy Production

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Health and Sustainable Agriculture

Editor: Christine Jakobsson

Sustainable Agriculture

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Bioenergy and Phosphorus Recycling

Bioenergy can be produced from agricultural and forest residues, industrial or residential organic wastes as well as from energy crops. Currently all countries significant- ly underuse their domestic potential of sustainable bioenergy (Fritsche et al., 2009). However, the use of biomass as a source of energy has strongly increased in the past decade and it is estimated to continue increasing in the European Union during coming decades (see Table 22.1 and Figure 22.1).

These developments in the bioenergy sector raise the question of how to use the residues from bioenergy processes. The reutilisation of these residues in agricultural and forestry systems can be an important factor in reduc-

Phosphorus Recycling Using Residues from Bioenergy Production

Bettina Eichler Löbermann

University of Rostock, Germany

Table 22.1. Predicted EU biomass production potential for bioenergy without harming the environment (Eurostat, 2003).

Mtoe* Biomass consumption 2003 Potential 2010 Potential 2020 Potential 2030 Wood direct from forest

(increment and residues)

67

43 39 - 45 39 – 72

Organic wastes, residues from wood industry, agriculture and food processing,

100 100 102

Energy crops from agriculture 2 43 - 46 76 - 94 102 - 142

Total 69 186 - 189 215 - 239 243 - 316

*Mtoe = Million tonnes oil equivalent

Figure 22.1. Changes in the number of biogas plants in Germany, 1995- 2008.

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Figure 22.2. Soil P cycle and P fractions.

ing the use of artificial fertilisers and in achieving nutrient recycling in agriculture. This has special importance for phosphorus (P), since the P resources world-wide are limited and the price of commercial P fertilisers will increase in the long run.

The application of the by-products of bioenergy production to soil affects the complex soil P cycle (Figure 22.2), not only due to P supply but also due to its influence on chemical, physical and biological soil parameters.

Since combustion of solid biomass and anaerobic biogas production are the most important bioenergy con- version processes in the Baltic Sea region, this chapter focuses on the fertiliser effects of biomass ash and biogas slurry.

Phosphorus Fertilisation Effect of Biomass Ashes

The residues of biomass combustion are the oldest mineral fertilisers in the world and they contain nearly all the nutrients plants require except nitrogen (N). Depending on the original biomass, the P content in ash varies widely (Figure 22.3). The P content in woody biomass ash is usually low, whereas the P content in ash based on cereals and oilseed crops or animal manure is higher.

In general, biomass ash has been shown to have positive effects on the dry matter yield of crop plants (Van

Reuler and Janssen, 1996; Krejsl and Scanlon, 1996;

Patterson et al., 2004). Besides being a source of nutrients, the application of biomass ash may also influence the form and availability of nutrients, for example by increasing the pH of the soil (Ohno and Erich, 1990; Muse and Mitchel, 1995; Odlare, 2005).

The experimental results concerning the specific effects of biomass ash on plant P nutrition and plant-available P in soil are inconsistent. Little or no effect of ash application on P uptake and plant-available P has been reported by Mozaffari et al. (2002). On the other hand, Codling et al. (2002) found a positive effect of poultry litter ash on plant P uptake and high available P contents in the soil.

In pot experiments with different crops (Eichler- Löbermann et al., 2008), the effect of ash on crop yield and P uptake was comparable or even higher than the effect of highly soluble P compounds (KH₂PO₄ or triple- super-P) (see Table 22.2 and Figure 22.4).

The total P content of the soil increases when P is supplied, but depending on the P source provided, increasing the P supply does not necessarily result in an increase in the bio-available P content in soil. For biomass ash, positive effects with high available P contents in soil are usually found. For example in a pot experiment (Eichler- Löbermann et al., 2008), poultry litter ash increased the bio-available P content (Pdl) in soil even more than highly soluble P (KH₂PO₄) (see Table 22.3).

The biomass ash effects on soil P content also de- pend on the crop grown (Table 22.2). In another study of Schiemenz and Eichler-Löbermann (2010) eight different crops were investigated in combination with supply of

Figure 22.3. Percentage P content in different types of biomass ash.

Total P content (extractable in aqua regia) and bio-available P content (extractable in citric acid).

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Table 22.2. Yield (g DM pot-1) and P uptake (mg pot-1) (shoot) of different crops with different fertiliser treatments in a pot experiment (6 kg of P poor loamy sand, 5 weeks vegetation time) (Eichler-Löbermann et al., 2008).

Treatment Phacelia Buckwheat Ryegrass Oil radish

Yield

Without P 13.8 16.5 11.4 a 18.8 a

KH₂PO₄ 14.9 16.2 13.0 b 23.1 b

PL ash¹ 16.3 16.1 13.3 b 23.8 b

p 0.540 0.800 0.001*** 0.001***

P uptake Without P 56.0 a 51.6 a 42.0 a 86.1 a

KH₂PO₄ 68.7 ab 64.7 b 63.7 b 129.4 b

PL ash¹ 87.1 b 68.7 b 60.8 b 149.1 c

p 0.035* 0.002*** 0.001*** 0.001***

Figure 22.4. Mean crop P uptake with different fertiliser treatments (different types pf biomass ash, triple-super-P, potassium chloride) in a pot experiment (6 kg of P poor loamy sand, 8 weeks vegetation time), relative values, control = 100%.

Table 22.3. Soil pH, Pdl content in soil (mg kg-1) and Pw content in soil (mg kg-1) with different fertiliser treatments applied to four different crops in a pot experiment (6 kg of P poor loamy sand, 5 weeks vegetation time) (Eichler-Löbermann et al., 2008).

Treatment Phacelia Buckwheat Ryegrass Oil radish

pH

Without P 5.1 a 5.7 a 5.8 a 5.7 a

KH₂PO₄ 5.2 a 5.7 a 5.7 a 5.8 a

PL ash¹ 6.3 b 6.5 b 6.5 b 6.6 b

p 0.001*** 0.001*** 0.001*** 0.001***

Pdl Without P 35 a 36 a 38 a 33 a

KH₂PO₄ 44 b 50 b 53 b 46 b

PL ash¹ 64 c 74 c 74 c 56 c

p 0.001*** 0.001*** 0.001*** 0.001***

Without P 3.2 a 2.8 a 2.9 a 2.4 a

Pw KH₂PO₄ 5.1 b 5.4 c 5.2 b 5.1 c

PL ash¹ 4.9 b 4.8 b 4.9 b 3.8 b

p 0.005*** 0.001*** 0.002*** 0.001***

1PL = poultry litter. Different letters indicate significant differences between means, p<0.05, Pw = water soluble P, Pdl = double lactate soluble P

1 Poultry litter ash. Different letters indicate significant differences between means for fertiliser treatments, p<0.05

KCl Cereal ash Straw ash Rapeseed meal ash TSP Control

P Uptake

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three different ashes. These investigations also confirmed the different potential of crops to utilize P from ashes.

The exudation of ions, organic acids or enzymes into the rhizosphere enables crops to acquire P from less available fractions. These interactions between ash and crop cultivation may have an additional effect on utilisation of P from ashes. In particular, P-efficient catch crops, which are used as green manures, can be important in this re- gard. High uptake of P from biomass ash by green manure crops ensures high P release after decomposition of the green manure. Furthermore, the incorporation of plant material into the soil increases the soil organic matter content. Therefore, a combination of ash fertilisation with a P mobilising catch crop seems to be very promising.

Due to the varying nutrient content of biomass ash, general recommendations regarding the fertilisation ef-

ficiency cannot be given. However, a positive effect on plant P nutrition can usually be expected.

Phosphorus Fertiliser Effect of Biogas Slurry

Biogas slurry is usually applied on agricultural land.

While there are data available regarding the effect of biogas slurries on nitrogen (N) and organic matter cycles in soil, the effect of biogas slurries on the soil P cycle has been much less well investigated.

To evaluate the benefits of biogas slurry, more knowl- edge is required about its effect on nutrient availability.

This is complicated, since the transformation of organic compounds and nutrient release is a complex process and depends on many factors, such as the stability of or-

Table 22.4. Nutrient content in non-digested dairy slurry and in different types of biogas slurry (% of fresh matter) (Bachmann and Eichler- Löbermann, 2009).

Table 22.5. Crop P uptake (mg pot-1) and pH, Pdl and Kdl content in soil (mg kg-1) with different cultivated crops and fertiliser treatments in an 8-week pot experiment.

DM = dry matter, OM = organic matter

Parameter Fertiliser Maize Amaranth

P uptake Without P 90.3 a 103 a

TSP 97.7 a 161 b

Dairy slurry 108 a 137 ab

Biogas dairy

slurry 109 a 167 b

Pdl Without P 26.6 a 25.6 a

TSP 36.4 c 30.3 b

Dairy slurry 33.0 b 31.0 b

Biogas dairy

slurry 34.0 b 28.9 b

Kdl Without P 49.7 a 55.2 ab

TSP 52.0 a 52.0 a

Dairy slurry 50.7 a 66.5 c

Biogas dairy

slurry 49.6 a 58.4 b

pH Without P 5.63 a 5.43 a

TSP 5.72 a 5.44 a

Dairy slurry 5.87 b 6.03 b

Biogas dairy

slurry 6.01 b 6.01 b

Mean values followed by different letters in the same column indicate significant differences between the fertiliser treatments (Duncan, p<0.05). TSP = Triple-Super-P.

Figure 22.5. Microbial activity in soil (measured as activity of dehydro- genase (DHA, µg TPF g-1 24 h) with different cultivated crops and fertiliser treatments in an 8-week pot experiment.

Substate DM OM N NH₁-N P K Mg

Dairy slurry 9.3 7.5 0.46 0.23 0.08 0.32 0.07 Biogas dairy

slurry 8.1 6.3 0.50 0.25 0.08 0.33 0.07 Biogas pig

slurry 4.2 3.1 0.46 0.35 0.07 0.20 0.05 Biogas maize

slurry 11.3 8.5 0.64 0.30 0.16 0.47 0.09

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ganic substances (Gutser et al., 2005), climatic conditions (Dorado et al., 2003), soil properties (Huffman et al., 1996), type of cropping system (Van den Bossche et al., 2005) and interaction with mineral fertilisers (Kaur et al., 2005).

Biogas slurry contains P at concentrations between 0.4 and 0.8 kg/m³. Regarding the solubility of the P in biogas slurry, different investigations have produced different results. High availability of such P was reported by Roschke (2003), whereas Umetsu et al. (2001) and Loria and Sawyer (2005) showed only delayed P release from biogas slurry.

Furthermore, biogas slurry contains N and C compounds, which have a decisive effect on the soil micro- flora. During the biodigestion process, the readily de- composable organic compounds become degraded and compounds such as lignin remain. However, lignin is not an adequate carbon and energy source for micro-organ- isms (Mokry and Bockholt, 2008). Therefore, a general effect of biogas slurry on the microbial soil P cycle can be expected.

A direct comparison between digested (biogas) slurry and non-digested (animal manure) slurry can help to evaluate the P fertiliser effect of the former. Chemical analysis of non-digested dairy slurry and biogas dairy slurry showed that these products had almost the same nutrient composition (Table 22.4).

As can be seen in Table 22.4, only the organic matter content decreased during the digestion process. In pot experiments with different crops, the effect of biogas dairy slurry on plant and soil parameters was similar to that of non-digested slurry (see Table 22.5 and Figure 22.5).

However, investigation of the microbial activity showed a lower activity after biogas dairy slurry application.

Since the nutrients supplied with the two types of slurry were similar, the different results can only be explained by the different quality of organic compounds in these two substrates.

Using biogas slurry as a fertiliser is an important way to close nutrient cycles in agriculture and to conserve nutrient resources. Based on preliminary results, good effects of biogas slurry can be expected regarding the P supply of crops. Some negative influences regarding the organic matter content in soil and the microbial activity may occur if biogas residues are the sole fertiliser used.

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