Wood ash admixture to organic wastes improves compost and its performance

Abstract

Throughout Europe, increasing amounts of wood ash are produced from biomass incineration plants. Most of these ashes are currently landfilled, despite their nutrient and micronutrient contents. The aim of this research was to find a way to return wood ash from biomass incineration plants into the natural cycle of matter.Three composts from source separated organic waste were produced with 0%, 8% and 16% ash admixture. The composting process was monitored by in situ measurements of temperature and CO2 concentration in the windrows. Maturation of the composts was observed through the parameters basal respiration, microbial biomass, metabolic quotient, Corg, Ntot, C/N-ratio and plant growth tests with cress. Mature composts were further analysed for potential pH, electrical conductivity as well as for nutrient (Mg, K, P) and heavy metal contents. The process indicators showed that ash admixture had no adverse effects and all legal standards were met. All produced composts met the requirements of the Austrian Compost Ordinance (Compost Quality A or even A+).In a field experiment – a recultivation trial on an alpine ski-run – we compared the effects of the three composts with an organic fertilizer and a mineral fertilizer. Best plant growth was found on the compost amended plots, followed by the organic fertilizer. Soil respiration measurements indicated a better performance of composts amended with 8% or 16% ash as compared to compost that did not contain ash.Concluding it may be stated that up to 16% ash admixture to organic wastes does not impair the composting process but is even able to improve the product quality. However, it has to be made sure that only bottom ashes of low heavy metal contents are being used and strict quality control is implemented.

Full text

Wood ash admixture to organic wastes improves compost and its
performance
T. Kuba
a
, A. Tscho
¨ll
b
, C. Partl
b
, K. Meyer
c
, H. Insam
a,
*
a
University of Innsbruck, Institute of Microbiology, Technikerstraße 25 d, 6020 Innsbruck, Austria
b
Amt der Tiroler Landesregierung, Abteilung LWSJF, FB Landwirtschaftliches Versuchswesen, Boden- und Pflanzenschutz, Heiliggeiststraße 7-9, 6020 Innsbruck, Austria
c
Studio BioTreaT, Technikerstraße 25d, 6020 Innsbruck, Austria
1. Introduction
According to the EU-Commission White Paper (KOM/97/0599)
one of the goals of the EU is to raise the proportion of renewable
energy to 12% of the total energy consumption. Among other
measures, the construction of biomass combustion plants has been
encouraged, and therefore increasing amounts of wood ash are
starting to accumulate. Alone in Austria, around 100,000 t of ash
are being produced each year. Most of this ash is landfilled at high
cost, and alternatives are demanded.
Major nutrients found in ashes are K, Mg and Ca. P and
microelements are present in smaller and variable amounts
(Table 1). Nitrogen, which is essential for plant growth, is lacking.
Beside the nutrients, pollutants are also found in the ashes; the
concentration of heavy metals may vary greatly, depending on type
of incineration, type of ash (bottom or cyclone ash), and source of
fuel wood. However, bottom (or grate) ashes in many cases have
low heavy metals and xenobiotic contaminations as has been
shown in a review by Stockinger et al. (2006). Another problem for
the use of ashes is the high pH (ranging from 12 to 13) and the
salinity that may harm plant growth if ashes are not stabilized
(Lundborg, 1998).
The nutrient and micronutrient contents suggest a use of ash as
fertilizer, either pure, pelleted or in combination with other
materials (in particular, nitrogen containing compounds). One
option is to mix it with organic wastes and produce compost. As
background information, maximum permissible values according
to the Austrian Compost Ordinance – that currently allows a
maximum of 2% admixture – are given in Table 3. The present
investigation aims at challenging this limit value.
Finnish authors found that fertilization with ashes caused a
long-term elevation of pH (Perkio
¨ma
¨ki and Fritze, 2002; Saarsalmi
ARTICLE INFO
Article history:
Received 18 October 2007
Received in revised form 22 February 2008
Accepted 26 February 2008
Available online 22 April 2008
Keywords:
Wood ash
Compost
Organic wastes
Soil reclamation
Lysimeter
Nitrate leaching microbial biomass
Cress test
ABSTRACT
Throughout Europe, increasing amounts of wood ash are produced from biomass incineration plants.
Most of these ashes are currently landfilled, despite their nutrient and micronutrient conten ts.The aim of
this research was to find a way to return wood ash from biomass incineration plants into the natural cyc le
of matter.
Three composts from source separated organic waste were produced with 0%, 8% and 16% ash
admixture. The composting process was monitored by in situ measurements of temperature and CO
2
concentration in the windrows. Maturation of the composts was observed through the parameters basal
respiration, microbial biomass, metabolic quotient, C
org
,N
tot
, C/N-ratio and plant growth tests with cress.
Mature composts were further analysed for potential pH, electrical conductivity as well as for nutrient
(Mg, K, P) and heavy metal contents. The process indicators showed that ash admixture had no adverse
effects and all legal standards were met. All produced composts met the requirements of the Austrian
Compost Ordinance (Compost Quality A or even A+).
In a field experiment – a recultivation trial on an alpine ski-run – we compared the effects of the three
composts with an organic fertilizer and a mineral fertilizer. Best plant growth was found on the compost
amended plots, followed by the organic fertilizer. Soil respiration measurements indicated a better
performance of composts amended with 8% or 16% ash as compared to compost that did not contain ash.
Concluding it may be stated that up to 16% ash admixture to organic wastes does not impair the
composting process but is even able to improve the product quality. However, it has to be made sure that
only bottom ashes of low heavy metal contents are being used and strict quality control is implemented.
ß2008 Elsevier B.V. All rights reserved.
* Corresponding author.
E-mail address: Heribert.Insam@uibk.ac.at (H. Insam).
0167-8809/$ – see front matter ß2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.agee.2008.02.012

et al., 2004), with a major role of carbonates, oxides and hydroxides
of Ca, Mg and K (Vance, 1996; Perkio
¨ma
¨ki and Fritze, 2002).
However, due to the mobility of these basic ions, in particular of K
(Khana et al., 1994), pH and cation concentration declined several
years after application. Saarsalmi et al. (2006) found that 23 years
after fertilization with ash, a significantly elevated pH was found
only for the highest dose (5 t ha
1
). Ca, Mg, as well as P, B, Cr, Cu,
Mn and Zn were also elevated. Another effect of fertilization with
ash is the reduced solubility of Al (Perkio
¨ma
¨ki and Fritze, 2002),
Mn, Zn, Fe and Cu (Clapham and Zibilske, 1992).
Jokinen et al. (2006) found not only an increase in microbial
activity (thymidine incorporation) due to ash amendment, but also
a change in the microbial community composition (PCR-denatur-
ing gradient gel electrophoresis). Wood ash increased the amount
of hydrophilic neutrals, while the amount of hydrophilic and
hydrophobic acids decreased. Perkio
¨ma
¨ki and Fritze (2002)
demonstrated that the effect of ashes on microorganisms is
persistent and depends to a high degree on the dose and pre-
treatment of the ashes. Hardening of ashes reduced the effects on
microorganisms. Besides the stimulation of microorganisms, ashes
are also known to promote plant growth (Voundi Nkana et al.,
1997; Moilanen et al., 2002). Experiments have shown that ashes
act in a similar way as mineral fertilizers (Holzner, 1999). On the
other hand, Demeyer et al. (2001) report negligible or negative
effects of ashes in case of N-limitations in soils, therefore
suggesting to balance ash amendments with N fertilization. This
is also supported by Saarsalmi et al. (2006), who found a long-term
effect only for combination treatments with N fertilizer.
According to Narodoslawsky and Obernberger (1994), the
admixture of ash to organic wastes improves the composting
process. The combination of ash with bio-organic residues from
agricultural or domestic sources reduces the nitrogen losses and
accelerates the degradation of organic compounds. Ash further
enhances K, Ca and Mg in the market-ready compost. Conversely,
N, P and humic matter contents are reduced due to dilution with
the ashes. Koivula et al. (2004) found an improved oxygenation
during the composting process, a reduced emission of odors and an
increased mineralisation and humification by admixing ash.
Electrical conductivity and pH were not altered.
Compost as fertilizer or soil conditioner may contribute to soil
quality by improving aeration, water status, and aggregate stability
and as a consequence erosion stability. Macro- and micronutrients
improve plant growth, and the cation exchange capacity is
improved (Amlinger et al., 2007). In addition, the soil microbiota
is activated and its biomass is enhanced, the extent of the effects
much depending on the quality and dose of the organic matter
(Insam, 1990; Leita et al., 1999). Concerning the risk of nitrate
leaching, composted organic wastes can be considered uncritical
(Insam and Merschak, 1997).
The aim of this paper was to investigate the suitability of using
ashes as a compost additive by studying both the composting
process itself, as well as the suitability of the products to reclaim
disturbed soils. For comparative purposes, we also used a
commercial organic fertilizer, Agrobiosol, and a mineral fertilizer.
2. Material and methods
2.1. Compost preparation
Three different windrows of composts were set up in the
composting plant Weer (Tyrol, Austria) on the 31st of January
2006. In all three cases a mixture of communal biowaste and tree-
bush-cuttings in the ratio of 53:47 (w/w) served as the organic
source. The variants differed in the amount of wood ash added, 0%
(K0), 8% (K8) and 16% (K16) (w/w). The wood ash was provided
from the incineration plant Kufstein (Tyrol, Austria) where bark,
sawdust and wood chips are used as input materials. The ash was
mixed with the compost substrates with a caterpillar. The pH and
electrical conductivity of the ash were 12.3 and 6.7 mS cm
1
,
respectively; the heavy metal contents are shown in Table 3. The
composts were force-aerated for 4 weeks and turned two times a
week during the 2nd month, and then sieved with a 1-cm sieve and
left in an open pile until the 21st of June (21 weeks).
Temperature and CO
2
evolution were measured weekly for the
whole composting period. The maturation was observed through
C/N, basal respiration and microbial biomass measurements as
well as a cress test for samples taken in April–June. Mature
composts were analysed for pH and electrical conductivity, heavy
metal (Pb, Cd, Cr, Cu, Ni, Hg, Zn) and nutrient contents (P, Ca, Mg,
N). Compost analyses were done in triplicate, where each sample
was a pooled sample out of 10 sub-samples taken from different
sides of the composting pile (surface material excluded).
2.2. Field trial
The experimental field, a reclamation trial on a ski-slope in the
Austrian Alps, was situated 1700 m above sea level, exposed to the
NE and having a slope of 27%. The soil type was an Andosol of
disturbed rendzina on a silica-based material, the soil texture was a
silty clay. The pH of the soil was 5.5 (1:2.5 in 0.1 mol KCl), and the
total contentsof K, P and Mg (Aqua regia, Kompostverordnung 2001)
were <50, <22 and 78 mg kg
1
d.m., respectively. Total carbon and
nitrogen were 2.65% and 1.37% of soil d.m. The trial was set up in a
randomized block with four replicates. The treatments were

AB: organic fertilizer (AgroBiosol
1
, a product containing fungal
mycelium, N content 7%, P 1%, K 1%) (1143 kg d.m. ha
1
).

MD: mineral fertilizer (Nitrophoska spezial, 12% N, 12% P
2
O
5
, 17%
K
2
O) (333 kg d.m. ha
1
).

K0: compost without ash (23.7 t d.m. ha
1
).

K8: compost with 8% wood ash (25.6 t d.m. ha
1
).

K16: compost with 16% wood ash (29.3 t d.m. ha
1
).
The composts and fertilizers were applied at N-equivalent rates
of 40 kg ha
1
available N (according to Amlinger, 2003, 10% of the
total N) per year on 28 June 2006. After the application of the
Table 1
Average composition of wood ashes (mixtures of bottom and cyclon-fly ashes)
produced in Austria (Obernberger, 1994, 1997)
Bark ash
(n= 12)
Ash from wood
chips (n= 12)
pH 12.7 12.8
Electrical conductivity (mS cm
1
) 9.2 9.8
C
org
(% d.m.) 0.8 1.3
P (% d.m.) 0.74 1.57
K (% d.m.) 4.23 5.56
Ca (% d.m.) 30.2 32.0
Mg (% d.m.) 3.92 2.89
Na (% d.m.) 0.59 0.45
Al (% d.m.) 3.76 2.43
Fe (% d.m.) 2.45 1.61
Mn (% d.m.) 1.16 1.32
Cu (mg kg
1
) 87.8 127
Zn (mg kg
1
) 619 376
Co (mg kg
1
) 23.9 15.3
Mo (mg kg
1
) 4.8 1.7
As (mg kg
1
) 11.4 8.2
Ni (mg kg
1
) 94.1 61.5
Cr (mg kg
1
) 133 54.1
Pb (mg kg
1
) 25.3 25.4
Cd (mg kg
1
) 3.9 4.8
V(mgkg
1
) 58.4 42

fertilizers a seed mixture containing Festuca nigrescens,Poa alpina,
Phleum rhaeticum,Phleum hirsutum,Trifolium pratense spp. nivale,
Anthyllis vulneraria spp. alpestris was sowed at a rate of
100 kg ha
1
(HBLFA Raumberg-Gumpenstein, Austria).
Soil sampling and evaluation of plant growth were done on 26
September, 3 months after setup of the trial. Overall impression
and green aspect were assessed on a scale from 1 (very good) to 9
(very bad). Total plant cover as well as soil cover by grasses,
leguminous plants, herbs and mosses were estimated as a
percentage of the total covered area. Soil samples were taken
from the top 10 cm with a Pu
¨rkhauer auger (6 cm diameter). For
each plot a pooled sample was created out of 15 sub-samples. The
soil was sieved with a 2-mm sieve and stored at room temperature
until analysis. Biological parameters were measured within 5 days.
2.3. Laboratory analyses
Nutrient contents in the mature composts were determinated
accordingto the Austrian Compost Ordinance(2001) with Aqua regia
digestion. Electrical conductivity (EC) and pH of composts were
measured in 1:6.25 (w/v) compost/0.01 mol KCl solution slurries,
whereas EC and pH ofsoils were determined in mixtures of 10 g soil
with 25 ml solution (deionized water/KCl). Carbon and total N were
measured from soil samples dried at 45 8C for 48 h (Leco TruSpec
Macro CHN). One gram of soil was muffled in a furnace (Carbolite,
CWF 1000) for 5 h at 550 8C to eliminate the organic carbon,and then
again analysed for C as above with a CHN analyser.
Soil basal respiration was measured as CO
2
evolution from moist
(60% WHC) soil samples at 22 8C, using continuous flow infrared gas
analysis (IRGA) (Heinemeyer et al., 1989). Microbial biomass carbon
(C
mic
) was determined by substrate-induced respiration (SIR) after
the addition of 1% glucose (Anderson and Domsch, 1978), using the
IRGA as above. From basal respiration and C
mic
the metabolic
quotient (qCO
2
,
m
gCO
2
-C g
1
C
mic
h
1
) was calculated.
A plant growth test was done according to the Austrian
Compost Ordinance. Glass dishes (diameter 12 cm, 6 cm high)
were filled with 100 ml glass sand, and on top of it 200 g of a
mixture composed of tennis sand, standard soil and 0%, 15% and
30% (w/w) of compost. The surface was seeded with 0.4 g of cress
seeds that were again covered with another 50 g of glass sand.
Substrates were water saturated and covered with a black foil until
germination. We determined the lag time of germination, as the
total number of germinated seeds after 9 days. The aboveground
dry weight of the cress was also determined after 9 days.
2.4. Statistical evaluation
Compost samples were taken in triplicates. The field experi-
ment was set up in randomized block design with four replicates.
Data sets were compared by ANOVA followed by a Tukey B-test.
3. Results
3.1. Compost production and quality
The temperature dynamics was similar in all three windrows,
and showed the typical peak about 2 weeks after the start of the
forced aeration (Fig. 1). In all three composts the legal hygiene
requirement of a minimum of 6 days above 65 8C were met. The
maximum temperatures reached were 73.4, 69.3 and 73.4 8C for
K0, K8 and K16, respectively. In the unaerated period after sieving,
the ash composts exhibited a slightly higher temperature than the
unamended compost. After 17 weeks, all windrows had reached
ambient temperature.
During the forced aeration phase in the macropores a mean CO
2
concentration of about 10% was measured. After sieving, the
compost piles remained unaerated, and we found up to 20% CO
2
one week after sieving, decreasing to approximately 8% at the end
of the process. No significant differences among the three
composts were found in the later stages of the composting
process, while the CO
2
content was significantly lower for the
compost with the highest ash dose in the first weeks of composting
(data not shown).
Organic C contents decreased during the rotting process; in the
mature composts we found 217, 192 and 169 mg g dm
1
for the
K0, K8 and K16 samples, respectively. Total N remained constant,
for K16 we found slightly lower values than for K0 and K8(Table 2).
The C/N ratios were similar in all three composts. Phosphate
contents were lower in the ash composts than in the control, while
K and Mg contents were enhanced.
All biological process parameters like basal respiration, micro-
bial biomass, metabolic quotient, C
org
,N
tot
, C/N-ratio and plant
growth tests with cress showed that ash admixture had no adverse
effects on process performance and all legal standards were met. At
the end of the composting process, microbial biomass and basal
respiration (data not shown) were slightly lower in the ash
composts than in the control. Increasing ash doses increased the
pH significantly from 6.9 to 7.7. Electrical conductivity decreased
from 1.6 to 1.1 mS cm
1
.
Heavy metal contents in the ash are limiting admixture in
composting; the ashes we used were below the limits according to
Austrian Compost Ordinance. Table 3 shows the heavy metal
contents in the mature composts and, for comparison, the limit
values according to the compost ordinance. In the case of Cr, Ni and
Fig. 1. Temperature during the composting of organic wastes without, and with 8%
and 16% wood ash (mean coefficient of variance = 17.3%).
Table 2
Organic C and nutrient contents (
standard deviation) in the three produced composts
C
org
(% d.m.) (n= 3) N (% d.m.) (n= 3) C/N (n= 3) P (% d.m.) (n= 3) K (% d.m.) (n= 3) Mg (% d.m.) (n=3)
K0 21.7 (
4.6) a
1.69 (
0.45) a
13.1 (
1.1) a
0.42 (
0.00) a
1.18 (
0.02) a
1.2 (
0.01) a
K8 19.2 (
1.0) a
1.56 (
0.1) a
12.4 (
1.2) a
0.39 (
0.00) b
1.33 (
0.01) b
1.57 (
0.02) b
K16 16.9 (
0.2) a
1.37 (
0.02) a
12.3 (
0.1) a
0.36 (
0.00) c
1.28 (
0.01) b
1.58 (
0.01) b
Dissimilar letters in a column indicate statistically significant differences among the composts (n= 3; Tukey B-test).

Zn we found higher contents in the ash composts, while for Pb, Cd,
Cu and Hg the values were lower when ash was added. Heavy
metal contents did not exceed the limits for quality A+ in any of the
ash-amended composts.
3.2. Field trial
3.2.1. Soil chemical and microbiological analysis
Fertilizers, and in particular organic fertilizers and composts are
known to enhance the availability of organic matter for microbial
degradation either indirectly (through stimulating plant growth) or
directly (through carbon input),a phenomenon also termed priming
effect. In this case, we found a pronounced stimulation of microbial
basal respiration by all organic amendments; compared to the
mineral fertilization treatment however, this effect was statistically
significant only for the 8% ash-amended compost (Table 4).
Microbial biomass was highest on the plots that had received
compost with 8% ash. The effects of all other treatments were,
compared to the mineral fertilizer and organic fertilizer, not
statistically significant.
3.2.2. Classification of plant growth
Already 3 months after setup of the trial, soil plant cover was
significantly different among the treatments (Table 5). Total soil
cover was 65% for K16 and exceeded 70% for K0 and K8; for AB and
mineral fertilizer, 53% and 38%, were estimated, respectively.
Composts enhanced coverage of both grasses and leguminous
plants to a similar degree. No difference was found between the
three different kinds of composts (0, 8 and 16% ash).
The lushest green was obtained with mineral fertilizer, pointing
towards luxurious supply with nitrogen. The best overall
impression was achieved with K8 (mark 2.3), followed by K0
and K16 with marks 2.4 and 2.9, respectively (Table 5).
4. Discussion
4.1. Composting process
In general, the observed temperature dynamics is typical for
composting processes with sequential mesophilic, thermophilic
and cooling phases (Insam and de Bertoldi, 2007). The sudden
temperature changes between week 3 and 4, and week 7 and 8,
may be explained by the onset of forced aeration in week 3 and the
sieving event at week 7, respectively (Fig. 1). Our results confirm
earlier observations (Koivula et al., 2004; Narodoslawsky and
Obernberger, 1994) that ash admixture to organic wastes may
enhance heat production and may thus indicate accelerated
microbial activity. However, when wood ash is being composted,
Table 3
Heavy metal contents of different ashes from the Kufstein biomass incineration plant and of composts produced without, and with 8% and 16% ash (mean
S.D.) and the limit
values for quality composts A+, A and B according to the Austrian Compost Ordinance (2002)
Heavy metal contents in ashes (n= 3) Heavy metal contents in mature compost (mg kg
1
d.m.) Limits for different compost
qualities
Min Mean Max K0% K8% K16% A
+
AB
Pb 2.5 12 31 58.0 (
2.9) a
39.3 (
2.4) b
34.6 (
4.2) c
45 120 200
Cd 0.1 1.4 4.4 0.8 (
0.1) a
0.6 (
0.1) b
0.7 (
0.1) b
0.7 1 3
Cr 25 112 250 25.2 (
13.7) a
27.1 (
2.8) a
27.9 (
0.2) a
70 170 250
Cu 40 92 320 67.3 (
6.7) a
66.6 (
11.3) a
55.4 (
1.9) b
70 150 500
Ni 9 46 56 16.4 (
1.4) a
22.1 (
10.7) b
20.6 (
2.0) ab
25 60 100
Hg 0.05 0.07 0.5 0.4 (
0.1) a
0.2 (
0.1) b
0.2 (
0.1) b
0.4 0.7 3
Zn 25 344 1100 181 (
19) a
183 (
24) a
189 (
7) a
200 500 1800
Quality A
+
may be used in organic (biological) agriculture, quality A conforms for general use in agriculture, quality B composts may only be used for landscaping. For K0, K8
and K16 dissimilar letters in a row indicate statistically significant differences among the variants (Tukey B-test).
Table 4
Soil organic C, total nitrogen, C/N ratio, pH, electrical conductivity (EC) and biological properties (basal respiration R
mic
, microbial biomass C
mic
and metabolic quotient qCO
2
)
on revegetation plots on the Mutterer Alm skiing-area near Innsbruck, Austria, fertilized with composts without (K0), with an admixture of 8 (K8) and 16% (K16) wood ash,
AgroBiosol (AB) and a mineral fertilizer (MD) (mean
standard deviation)
C
org
(% d.m.) N (% d.m.) C/N pH EC (
m
Scm
1
)R
mic
(
m
gCg
1
C
org
h
1
)C
mic
(
m
gCO
2
-C g
1
d.m.) qCO
2
(mg CO
2
-C g
1
h
1
)
K0 3.1 (
0.6) ab
0.19 (
0.04) a
16.5 (
1.0) a
6.4 (
0.1) a
130 (
21) ab
5.74 (
1.57) abc
370 (
136) ab
14.0 (
0.7) a
K8 3.3 (
0.5) a
0.20 (
0.04) a
16.9 (
1.3) a
6.6 (
0.1) a
196 (
54) a
8.48 (
2.29) a
597 (
183) a
14.5 (
0.9) a
K16 2.6 (
0.5) abc
0.15 (
0.04) ab
17.1 (
0.4) a
6.6 (
0.1) a
186 (
50) a
6.61 (
3.2) ab
425 (
67) ab
14.2 (
1.6) a
AB 1.8 (
0.4) c
0.09 (
0.03) b
21.6 (
4.3) a
6.6 (
0.1) a
98 (
23) b
4.59 (
0.81) bc
297 (
23) ab
15.6 (
3.2) a
MD 2.0 (
0.2) bc
0.11 (
0.02) b
18.9 (
18.9) a
5.6 (
0.3) b
80 (
22) b
3.20 (
0.71) c
255 (
43) b
12.5 (
1.3) a
Dissimilar letters in a column indicate statistically significant differences among the treatments (Tukey B-test).
Table 5
Comparison of plant growth in the field experiment
K0K8K16 AB MD
Total plant cover (%) 70.5(
4.2) a
71.5(
5.0) a
65.0(
3.1) a
52.5(
9.5) b
38.0(
2.7) c
Grass cover (%) 39.0(
2.6) a
40.0(
5.1) a
37.0(
2.0) a
31.8(
7.2) b
26.0(
2.6) b
Leguminous plant cover (%) 29.0(
2.3) a
30(
2.9) a
26.8(
2.1) a
19.3(
2.6) b
10.5(
5.0) c
Forb cover (%) 2(
1.7) a
1.5(
0.5) a
1.3(
1.4) a
1.5(
0.5) a
1.5(
0.5) a
Moss cover (%) 1.1(
0.5) a
0.9(
0.2) a
1.3(
0.4) a
1.5(
0.5) a
1.5(
0.9) a
Green aspect (1–9) 4.9(
0.2) a
4.8(
0.3) a
5.0(
0.4) a
4.6(
0.8) a
3.4(
1.1) a
Overall impression (1–9) 2.4(
0.7) a
2.3(
0.9) a
2.9(
0.6) a
5.1(
1.4) b
7.1(
0.41) c
Mean (
S.D.) of total soil cover (% of the area), and coverage by grasses, leguminous plants, forbs and mosses, as well as green aspect and overall impression (1–9, 1 = very good,
9 = very poor). Dissimilar letters in a row indicate statistically significant differences among the treatments (Tukey B-test).
T. Kuba et al. / Agriculture, Ecosystems and Environment 127 (2008) 43–49
46

the onset of an exothermic carbonisation process of the oxidic
compounds is to be expected (Narodoslawsky and Obernberger,
1994). This might result in abiotic heat production, obscuring the
effects of microbial activity. Koivula et al. (2004) argue that
prolonged high temperatures (as were also found in this study)
might result from a higher heat capacity of the material due to its
ash contents. The lower microbial biomass of ash composts in the
end of the composting process may be due to exhaustion of
substrates, or to a lower availability of nutrients.
The low temperature of the material at the end of the process
indicates that the compost has achieved a state of considerable
maturity. It is also important to note that with all three composts
the current legal requirements concerning hygiene were met with
at least 6 days at a temperature exceeding 658(Kompostver-
ordnung, 2002).
Alike heat production, CO
2
production is an indicator of
microbial activity in composts (Itavaara et al., 2002); however,
the above-mentioned carbonisation reactions result in the uptake
of produced CO
2
and may thus obscure activity measurements
based on CO
2
(Narodoslawsky and Obernberger, 1994). Thus we
cannot tell with certainty to which degree ash admixture
influenced the biological activity. The in situ measurements of
CO
2
indicate clearly the effect of the forced aeration (during this
period, however, the data variability was quite high), as well as the
decreasing microbial activity towards the end of the process.
The lower organic C content in the ash composts may be
explained by the dilution through ash admixture, and by an
enhanced mineralisation. Carbon contents of our composts are
well within the range that is usually demanded for mature
composts (Pfundtner, 1998).
Nitrogen may be lost during composting by ammonia
evaporation which is particularly high during the thermophilic
phase and when the pH is high. High microbial biomass, on the
other hand, ensures proper binding of the nitrogen. According to de
Bertoldi et al. (1983) even microbial N fixation is possible. The
addition of ash resulted in a dilution of N, however, N contents
were still in the usual range (0.7–1.7% d.m.).
To make sure that enough N is available to the plants, a C/N ratio
<20 is required (Hue and Sobiesczyk, 1999) which is met by all our
composts. K0 and K8 have reached values <15 already after 2
months, and K16 after 3 months; however, temperature, CO
2
concentration and cress tests indicated that at that time the
maturation had not been finished (data not shown). This supports
the studies by Zmora-Nahum et al. (2005) and Jimenez and Garcia
(1991) who do not regard the C/N ratios in solid phase as a reliable
parameter to indicate maturity. Similar to our study, also
Narodoslawsky and Obernberger (1994) found C/N ratios between
12 and 13 for ash composts. On the other hand, if the C/N ratio is
narrow (<12) there is a certain risk of N leaching which could cause
groundwater problems. This was tested in a mini-lysimeter
experiment (as in Insam and Merschak, 1997; data not shown)
where we did not find nitrate concentrations in the leachate
exceeding 35 mg ml
1
. The observations corresponded with the
study of Insam and Merschak (1997). The total sum of nitrate that
was leached from the compost amended plots was lower when the
composts had received ash, an observation that has also been made
by Plank (2007) and Niederkofler et al. (2007) who studied
combination effects of ash with anaerobic sludges.
Heavy metals may harm the environment, and therefore the
Austrian compost ordinance sets limits not only for the product,
but also for the input materials (Kompostverordnung, 2002). Since
heavy metal contents of wood ashes often exceed those of organic
wastes, the admixture of ash might decrease the quality of the end
product compost (Koivula et al., 2004; Narodoslawsky and
Obernberger, 1994). With the exception of Ni, Cr and Zn this
was not the case here since in most cases the heavy metal contents
in the ash-amended composts were lower than in the unamended
one (Table 3). In terms of end product quality, the ash composts
qualified for A+ while the unamended compost did not. This may
be attributed to the very low heavy metal contents in the ash,
which was in the lower range of the ashes usually found. In
addition, it is possible that the extractability of heavy metals from
ash-amended composts with Aqua regia is lower than that of
unamended composts. However, calculations showed that if ash of
the poorest quality ever found in the Kufstein biomass incineration
plant (see Table 3) would have been used, a downgrading to
compost class B might have occurred with 16% ash amendment
(data not shown). It is thus indispensable that ashes are being
analysed before admixture. Magnesium and potassium contents in
the composts were increased by ash which increased their
nutritional value; P contents, on the other hand, were decreased
which may be attributed to a dilution effect (Narodoslawsky and
Obernberger, 1994) or to a less efficient extraction when ash is
contained in the matrix. In summary, in all cases Mg, K and P
contents were in the typical range of composts produced in Austria
(Bundesministerium fu
¨r Land- und Forstwirtschaft, Umwelt und
Wasserwirtschaft, 1998).
Through carbonates, oxides and hydroxides of its Mg, Ca and K,
ash increases the pH of acid soils (Vance, 1996). While Koivula et al.
(2004) – probably due to the high buffering capacity of the organic
matter – did not find any pH effect of ash, we found that ash
addition increased the pH in the early stages of composting. The
final pH of both the control and ash composts was in the range
usually found in composts (pH 7–8) (Jimenez and Garcia, 1991).
The electrical conductivity is a measure of salinity; while wood
ash is characterized by high salinity, carbonatisation reduces
solubility, and thus a large increase in conductivity is not to be
anticipated (Obernberger, 1997). We even found a reduction of
conductivity compared to the control. For reasons of plant health
the conductivity should not exceed <2mScm
1
, a limit that was
never reached. From this point of view, all the composts may be
regarded harmless for plants.
Immature compost may contain phytotoxic compounds (Zuc-
coni et al., 1983), which may explain that our 2- and 3-months-old
composts showed a decreased cress biomass production (data not
shown). The final product, however, did not show any difference to
the control. Ash amendment had no adverse effect, but also did not
enhance germination and cress biomass production.
4.2. Field trial
4.2.1. Organic C (C
org
), nitrogen (N
tot
) and C
org
/N
tot
ratio
Organic matter has many beneficial properties for the
reclamation of unvegetated slopes ranging from improved soil
aeration, supply of nutrients and erosion stability (Amlinger et al.,
2007). Increased soil organic matter contents due to compost
application were also found here; besides C
org
, also the soil N
contents of the compost variants were higher than those of the
Agrobiosol and mineral fertilizer variants. The lower C/N ratio in
the ash composts indicates a more advanced maturation.
4.2.2. Other nutrients and heavy metals
Plant available (acetate extraction) K (50 mg kg
1
d.m.) and P
(22 mg kg
1
d.m.) contents on the trial site were low while Mg
contents (130 mg kg
1
d.m.) were considerable. An amelioration
measure that brings additional K and P would thus be desirable to
support long-term establishment of a vegetation cover. It may be
assumed that the compost fertilization will cover nutrient
demands for a few years, while the other fertilizers will have to
be applied repeatedly.

4.2.3. Potential pH and electrical conductivity (EC)
Compost fertilization may increase the pH of acid grassland
soils (Amlinger et al., 2007); this was also found here, but no
additional effect was found by the admixture of ash. This may be
attributed to the fact that the ash had already been neutralized to a
large degree during the composting process. The high application
rate resulted in an increased EC. This is in contrast to the
observations with the composts themselves where a lower EC was
found for the ash-amended products. In any case, negative effects
for plant health are only expected at an EC >1.5 mS cm
1
(Pfundtner, 1998).
4.2.4. Basal respiration (R
mic
), microbial biomass (C
mic
) and metabolic
quotient (qCO
2
)
It is commonly known that composts and organic fertilizers are
able to enhance microbial activity and biomass (Insam, 1990).
These parameters were slightly enhanced in the field trial (and
were also significant in a mini-lysimeter experiment—data not
shown). Similar results were obtained with the application of pure
ash (Plank, 2007; Perkio
¨ma
¨ki and Fritze, 2002; Jokinen et al., 2006).
Calculating the results on basis of C
org
rather than on soil dry
matter suggests that the ash improves degradability of organic
matter (Jokinen et al., 2006). This may be regarded as a positive
side effect, since more nutrients are released this way for the
vegetation.
The metabolic quotient (qCO
2
) is often used as a stress indicator
(Liao and Xie, 2007; Moreno and Hernandez, 1999). None of the
treatments resulted in a significant change of the qCO
2
; adverse
effects of the ash on the microbial metabolic efficiency may thus be
excluded.
4.2.5. Classification of plant growth
Ultimate aim of any high-alpine reclamation effort is to obtain a
plant cover of 70% (which is considered a threshold for effective
erosion prevention) as soon as possible, which was best obtained
with the composts, supporting earlier findings by Insam (1990)
and Amlinger, personal communication. The success is related to
slow but sufficient nutrient release, in combination with improved
aggregate formation (de Bertoldi et al., 1983). For pure ash
fertilization it is known that plant growth is improved as long as N
is not a limiting factor (Holzner, 1999; Voundi Nkana et al., 1997;
Moilanen et al., 2002) which was also supported by the present
data.
Concerning the green aspect the best score was obtained with
the mineral fertilizer. This may be attributed to the high amount of
plant available N. The composition of the plant community was
changed by the composts in favour of grasses and leguminous
plants (Table 5); a similar effect was found on the Gamlitz Alm,
another alpine pasture (Amlinger, personal communication). In
particular the enhancement is desirable, since this way biological
nitrogen fixation is anticipated to contribute to a considerable
extent to the long-term N supply.
5. Conclusion
This study demonstrated that wood ash may safely be added
to organic wastes up to an amount of 16% before the composting
process starts. Ash does neither negatively affect the composting
process (temperature dynamics, microbial activity) nor the
product (no increased heavy metal contents, improved nutrient
balance). Trials on re-vegetated ski-slopes demonstrated that all
the investigated composts, and in particular the ash-amended
composts, showed a better performance than mineral and
organic fertilizers in terms of plant cover and soil microbiological
properties. Great potential is seen in the utilization of high-
quality ashes when it comes to the development of site- and
crop-specific composts and fertilizers. Prerequisite, however, is
the development of a quality standard for ashes, and regular
tests.
Acknowledgements
The support by Amt der Tiroler Landesregierung, Abteilung
LWSJF, as well as by Wolfgang Enzenberg (compost plant Weer) is
gratefully acknowledged.
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