Introduction
Challenges cultivating Andosols
Andosols occupy just 1–2 % of the land area worldwide. They are common in
high-altitude tropical environments, such as in the East African Rift Valley
(Chesworth, 2008; Perret and Dorel, 1999). Their high inherent fertility
makes them especially well-suited for the cultivation of high-value crops such as
coffee, tobacco and banana. Andosols feature a low bulk density, variable
charge characteristic (strongly dependent on the soil's pH), a low base
saturation (BS), thixotropy, a strong capacity to retain phosphorus (P), a
high pore volume, a high level of available water, a tendency to form microaggregates and a pronounced shrinking (Chesworth, 2008; Dörner et al., 2011;
Driessen et al., 2000; Zech et al., 2014). The dominant minerals in these soils are
allophanes, imogolites, ferrihydrites and halloysites, and the
concentrations of aluminium (Al), iron (Fe) and silicon (Si) are all high
(Chesworth, 2008). Metal–humus complexes are frequently formed when the pH
exceeds 5, while under more acid conditions Al–humus complexes in
combination with silica predominate (Chesworth, 2008;
Driessen et al., 2000). These structures serve to protect soil organic
matter from degradation, thus fostering C sequestration
(Driessen et al., 2000; Chesworth, 2008; Abera and Wolde-Meskel, 2013). The
total carbon concentration of these soils is often > 6 %
throughout their profile (Chesworth, 2008).
Andosols are rather sensitive to land use management (Dörner et al., 2011). For
example, shifting cultivation practices tend to deplete soil fertility
unless organic matter is deliberately added, while intensive mechanized
cultivation risks compacting the soil, with the hydraulic properties of the
soil being readily compromised (Perret and Dorel, 1999; Dorel et al., 2000).
Plants on Andosols typically suffer from P deficiency
(Buresh et al., 1997), as the soils have a high P fixation potential
(Batjes, 2011). Thus, crop productivity and sustainable land use require
consistent P replenishment, which generates a strong demand in sub-Saharan
Africa for appropriate soil amenders. Fertility amelioration measures have
included both liming to increase P availability and applying either manure
and/or other organic matter or synthetic P fertilizer
(Driessen et al., 2000; Tonfack et al., 2009).
Organic waste materials as soil amenders on Andosols in Karagwe, Tanzania
Andosols with strong P retention potential are also present in Karagwe
(Kagera region, NW Tanzania), which is located
nearby volcanic areas in the East African Rift Zone (Batjes, 2011). Soil
constraints for farmers in this region are the low soil pH (3.8–4.2), the
low availability of nutrients (especially P) and widespread soil erosion
(Krause et al., 2015). Small-scale farmers often have financially or
logistically restricted access to rock phosphates or synthetic fertilizers
and a lack of sufficient amounts of organic matter to replenish Andosols
(Buresh et al., 1997).
However, practices like ecological sanitation (EcoSan) and bioenergy
production can contribute to local matter and nutrient cycling with Andosols
receiving organic waste products (Krause et al., 2015). Human excreta
constitute a valuable source of plant nutrients, available in every human
settlement. EcoSan technologies can be implemented for the collection and
sanitization of toilet waste (Esrey et al., 2001), for example with urine-diverting
dry toilets (UDDT), composting toilets, and pasteurization of faeces to
ensure human health (Schönning and Stenström, 2004). The last point was
recently tested in Karagwe in an EcoSan pilot project named “Carbonization
and Sanitation” (CaSa) (Krause et al., 2015). In the CaSa approach,
so-called microgasifier stoves (Mukunda et al., 2010) provide the heat for
thermal sanitation of human faeces. In addition, further projects have been
locally initiated to implement bioenergy technologies for cooking such as
small-scale biogas digesters (Becker and Krause, 2011) and microgasifier
stoves (Ndibalema and Berten, 2015). Hence, increasing dissemination of
these technologies will supply waste matter such as biogas slurry from
anaerobic digestion, powdery charcoal residues from gasification and ashes
(Krause et al., 2015).
These locally available resources can be directly applied to the soil or
they can be processed as compost. The benefit of charcoal as a soil amender
(“biochar”) has been deduced from the fertility of Terra Preta soils
(Sombroek, 1966; Lehmann and Joseph, 2009). CaSa compost is a product
following this ancient example of co-composting (pasteurized) human faeces,
kitchen waste, harvest residues, terracotta particles, ashes and urine
mixed with char residues from gasification (Krause et al., 2015).
However, there is also reasonable doubt that application of biochar is
recommendable in all situations and on all soils. Mukherjee and Lal (2014) pointed out that data gaps exist, in particular, concerning field-scale information on crop response and soil quality for various soil–biochar combinations. From past experiments
using biochar as a soil amendment (Herath et al., 2013; Kammann et al., 2011;
Kimetu et al., 2008; Liu et al., 2012; Major et al., 2010; Nehls, 2002;
Petter et al., 2012; Schulz et al., 2013) and from meta-analysis by
Biederman and Harpole (2013), Jefferey et al. (2011) and
Liu et al. (2013), the following lessons can be learned for future
experiments: (i) pot experiments lead to overestimations of possible
positive impacts on biomass growth compared to field experiments; (ii) soil
chemical and soil hydraulic properties should be examined at the same time to
be able to distinguish between the observed effects; (iii) the assessment of biomass
growth should be combined with the assessment of crop yield and the
evaluation of plant nutrition; (iv) locally typical and economically
relevant plants should be selected and cultivated according to local
practice to assess the practical relevance of biochar application in the local
agroecosystem; and (v) long-term as well as short-term experiments are
needed. Although the latter are often criticized for not enhancing knowledge on
changes in soil hydraulic properties as well as on soil organic matter and
C sequestration, they are of high practical relevance to farmers who rely on
their harvests immediately.
In this study, we assessed whether and how locally available organic waste
materials change the availability of nutrients and water in the soil and
improve the crop productivity in a one-season, practice-oriented field
experiment. In particular, our objectives were (i) to examine the effect of
CaSa compost, standard compost and biogas slurry on the physico-chemical
properties of the soil and (ii) to assess their impact on biomass growth,
crop yield and plant nutrition.
Materials and methods
Field site
The experimental site (see Figs. S2–S4 in the Supplement) is located in the Ihanda ward,
Karagwe district, Kagera region, NW Tanzania (1∘33.987′ S,
31∘07.160′ E; 1577 m a.s.l.), a hilly landscape characterized
by a semi-arid, tropical climate (Blösch, 2008). The annual rainfall
ranges from 1000 to 2100 mm and the mean annual potential evapotranspiration is
∼ 1200 mm (FAO Kagera, online http://www.fao.org/fileadmin/templates/nr/kagera/Documents/Suggested_readings/nr_info_kagera.pdf). The pattern of rainfall is
bimodal, featuring a long rainy season from March to May and a short one
from October to November (Tanzania, 2012). The predominant cropping system
comprises banana, intercropped with beans and coffee. Prior to the
experiment, the soil was surveyed by sampling the edges of the field
(Table 1 and Fig. S1). Stone and gravel concentrations increased with soil
depth. The bulk density (ρB) of the topsoil lay within the range
expected for an Andosol. The soil's total carbon (Ctot) and total
nitrogen (Ntot) concentrations were classified, respectively, as medium
and adequate, and its C / N ratio is suitable for cropping (Landon, 1991). The
soil pH was in the range of 3.6–3.8. The effective cation exchange
capacity (CECeff) of dry matter (DM) in the soil was only 8–17 cmol kg-1
compared to a typical range of 10–40 cmol kg-1 of DM (Chesworth, 2008).
The soil's BS was quite high (Ca saturation of up to 70 %). Comparable
levels of CECeff and BS have been recorded in both in Kenyan Ultisols
cultivated for about 35 years (Kimetu et al., 2008) and in an Ethiopian Andosol
(Abera and Wolde-Meskel, 2013). The quantity of available P in the topsoil
was 0.7 mg kg-1 (classified as “very low” according to KTBL, 2009),
whereas that of potassium (K) was “very high” (244.7 mg kg-1).
The characteristics of the investigated Vitric Andosol in Karagwe, Tanzania.
Aggregate size distribution
Depth
Colour
Clay
Silt
Sand
Structure
pH
ρB
FCfield
FClab
CECeff
BS
TOC
Ntot
C / N
cm
Munsell
%
%
%
KCl
kg dm-3
m3 m-3
m3 m-3
cmol kg1
%
%
%
Ap
20
2.5 YR 3/2
3.2
16.1
80.7
Very crumbly
3.8
0.94
0.38
0.35
16.7
99.6
3.5
0.3
12.9
Ah
37
2.5 YR 3/2
3.6
13.0
83.4
Blocky subangular to crumbly
3.8
0.88
B1
53
2.5 YR 2.5/3
2.2
16.3
81.5
Crumbly to blocky subangular
NA
1.08
0.36
NA
11.2
97.1
2.7
0.2
13.3
B2
74
2.5 YR 3/3
2.2
20.1
77.8
Macro: prismatic; micro: blocky subangular
NA
NA
NA
NA
8.0
94.5
2.0
0.2
12.5
C
100+
NA
NA
NA
NA
No aggregates, subangular gravel
NA
NA
NA
NA
NA
NA
NA
NA
NA
Water holding capacity (WHC) was determined in the field (FCfield) and
in the laboratory (FClab). ρB: bulk density; CEC: cation
exchange capacity; BS: base saturation; TOC: total organic carbon; NA: not analysed.
The experiment design: the plots were arranged as a Latin rectangle
with five columns and five rows (left panel) and each plot was
divided into two 4.5 m2 sections for the cultivation of seven selected crops
in an intercropping system (right panel); note that urine
treatment was a posteriori excluded from the analysis due to technical problems.
Plot preparation and soil amendments
We arranged a series of 3 m × 3 m plots in the form of a Latin rectangle
(Richter et al., 2009), with the five columns and five rows each separated from one
another by a 0.5 m deep trench. Each of the four treatments was applied to a
single row and a single column and thus studied with five replications
(Fig. 1). The treatments were as follows: (1) untreated (control), (2) biogas slurry in
a weekly application (from weeks 4 to 9 after sowing) of 1.7 dm3 m-2
on a cover of cut grass, (3) standard compost with a pre-sowing
application of 15.0 dm3 m-2, and (4) CaSa compost with a
pre-sowing application of 8.3 dm3 m-2, passed through a 20 mm
sieve. Macro- and micronutrients of the amendments were analysed according
to standard methods as described in Krause et al. (2015). Values are given
in dry matter (g kg-1) as well as in the practice-oriented fresh matter
concentrations (g dm-3) in Table 2.
The biogas slurry employed derived from anaerobic digestion of banana tree
stumps and cow dung (mixture 1 : 1 by volume). According to local practice,
biogas-slurry-amended plots were covered with cut grasses prior to sowing.
Therefore, the nutrient content of grass was analysed as well.
Standard compost was processed by local farmers during 3 months from
fresh and dried grasses (0.91 m3 m-3), kitchen waste (0.06 m3 m-3),
and ash (0.03 m3 m-3). The compost heap was regularly
watered and covered with soil and grasses to mitigate evaporation.
CaSa compost contained pasteurized human faeces (0.15 m3 m-3),
biochar from gasification (0.17 m3 m-3; eucalyptus sawdust,
pyrolysis at T > 500 ∘C, residence time ≥ 120 min),
kitchen waste and harvest residues (0.15 m3 m-3; bean straw,
banana peels), mineral material (0.31 m3 m-3; ash from eucalyptus
wood, brick particles, local soil to add minerals and soil microorganisms),
and lignin and cellulose sources (0.22 m3 m-3; sawdust, grasses).
Stored urine, mixed with sawdust or biochar, was added to the compost as
well (0.12 m3 m-3). Every week, 60–80 dm3 of the above-mentioned
matters were added to the shaded and grass-covered compost heap.
We adjusted the amendments so that each treatment delivered a comparable
quantity of mineral nitrogen (Nmin). The Nmin demand per cropping
season (Nmin,demand) was estimated as 17.5 g m-2, following
KTBL (2009). According to Horn et al. (2010), 33 % of organic nitrogen
contained in organic fertilizers (Norg,fertilizer) is mineralized
during the course of a cropping season. The amount of materials to be
amended to the soil, mfertilizer (kg m-2), was calculated based
on the quantity of Nmin present in the top 90 cm of the soil
(Nmin,soil with about 7.5 g m-2; see Table 3) and that provided
by the amendments as follows:
mfertilizer=Nmin,demand-Nmin,soilNmin,fertilizer+0.33⋅Norg,fertilizer.
Then, the addition of the other plant nutrients (Table 3) was calculated
according to Table 2.
The characteristics of the tested soil amendments according to
Krause et al. (2015).
Ctot
Ntot
Nmin
Stot
Ptot
Ktot
Mgtot
Catot
Altot
Fetot
Zntot
Mntot
in dry matter
g kg-1
g kg-1
g kg-1
g kg-1
g kg-1
g kg-1
g kg-1
g kg-1
g kg-1
g kg-1
mg kg-1
mg kg-1
Gras
426
1.9
ua.
1.7
1.0
13.8
2.8
8.6
4.9
4.0
24.1
172
Biogas slurry
348 ± 6
19.9 ± 0.1
16.0 ± 0.8
3.1 ± 0.02
7.6 ± 0.2
92.9 ± 8.4
12.2 ± 0.1
17.4 ± 0.9
4.0 ± 0.7
4.3 ± 0.1
115.3 ± 1.7
283 ± 9
Compost
91 ± 8
5.3 ± 0.2
0.12 ± 0.04
1.2 ± 0.1
1.2 ± 0.1
8.5 ± 1.2
3.2 ± 0.2
10.0 ± 1.2
77.5 ± 1.6
65 ± 10
59.5 ± 4.3
641 ± 106
CaSa compost
116 ± 11
6.0 ± 0.5
0.36 ± 0.07
1.3 ± 0.1
3.2 ± 0.2
14.6 ± 1.4
5.1 ± 0.5
29.6 ± 2.8
54.5 ± 1.4
84 ± 18
67.0 ± 4.7
480 ± 48
pH
in fresh matter
in KCl
g dm-3
g dm-3
g dm-3
g dm-3
g dm-3
g dm-3
g dm-3
g dm-3
g dm-3
g dm-3
mg dm-3
mg dm-3
Gras
25 ± 13
0.1 ± 0.1
ua.
0.1 ± 0.1
0.1 ± 0.03
0.8 ± 0.4
0.2 ± 0.1
0.5 ± 0.3
0.3 ± 0.2
0.2 ± 0.1
1.4 ± 0.7
10 ± 5
Biogas slurry
7.7
15 ± 1
0.9 ± 0.04
0.7 ± 0.05
0.1 ± 0.01
0.3 ± 0.02
4.1 ± 0.4
0.5 ± 0.03
0.8 ± 0.1
0.2 ± 0.03
0.2 ± 0.01
5.1 ± 0.3
12 ± 1
Compost
7.4
33 ± 6
1.9 ± 0.3
0.04 ± 0.02
0.4 ± 0.1
0.5 ± 0.1
3.1 ± 0.7
1.1 ± 0.2
3.6 ± 0.7
28.1 ± 4.5
24 ± 5
21.6 ± 3.7
233 ± 53
CaSa compost
7.5
60 ± 7
3.1 ± 0.3
0.2 ± 0.04
0.7 ± 0.1
1.7 ± 0.1
7.6 ± 0.9
2.7 ± 0.3
15.4 ± 1.7
28.3 ± 1.8
44 ± 10
34.9 ± 3.2
250 ± 29
Analyses as described in Krause et al. (2015): total concentrations of
nutrients, Ptot, Ktot, Catot, Mgtot, Zntot,
Mntot, Altot and Fetot, were determined using
HNO3 digestion under pressure (König, 2006) and an
iCAP 6000 inductively coupled plasma optical emission spectrometry (ICP-OES) device (Thermo Scientific, Waltham, USA). Total concentrations of C,
N and S were analysed according to ISO DIN 10694 (1995) for Ctot, ISO DIN 13878 (1998) for Ntot and DIN ISO15178 (HBU 3.4.1.54b) for
Stot, and using a Vario ELIII CNS analyser (Elementar, Hanau, Germany).
Mineral nitrogen (Nmin) was extracted with potassium chloride (KCl) and
analysed using test strips (AgroQuant 114602 Soil Laboratory, Merck,
Darmstadt, Germany). This method involved the suspension of 50 g material of
the amenders in 100 mL 0.1 M KCl. Within the same solution, pH was measured
by using a glass electrode (pH 330i, WTW, Weilheim, Germany). Values are
displayed with mean value and standard deviation with n = 1, 2 and 5,
respectively, for grasses, biogas slurry and compost as well as CaSa compost.
The dominant form of available Nmin was NH4 for biogas slurry and
NO3 for compost as well as CaSa compost.
Before planting, we hoed the soil by hand, as it is common local practice.
We applied the composts by first spreading them evenly and then incorporating them with a
fork hoe. Planting was carried out at the beginning of the rainy season
(March 2014), and the plots were mulched in mid April (end of rainy
season) to minimize evaporative loss. We harvested the crops during June and
July. Precipitation was recorded on a daily basis, while the air temperature
and relative humidity prevailing 2 m above ground were measured every 15 min.
We divided each plot into two 4.5 m2 sections (Fig. 1), one used to
cultivate maize cv. Stuka, and the other planted with a mixture of common
bean cv. Lyamungu 90, carrot cv. Nantes, cabbage cv. Glory of Enkhuizen and
local landraces of onion. In addition, African eggplant (Solanum aethiopicum) and sweet pepper
were planted as important parts of the chosen local adjusted intercropping
practice. However, as these two plant species are perennial, biomass harvest
exceeded the experimental time frame, and therefore we excluded them from analysis.
The maize was sown on 4 March with two grains per dibbing hole and thinned
after germination. Carrot seed was directly sown on the plot on 6 March
and the beans were sown on 14 March; carrot was thinned after 40 days. The
other species were transplanted as seedlings in mid March. The maize and
beans were entirely rain-fed, while the other crops were irrigated as
required. The plots were hand-weeded once a week, and insects were
controlled by spraying with a mixture of ash and “moluku” (prepared from
the leaves of the Neem tree and the Fish Poison tree suspended in soapy water).
We sampled the soil (two samples per plot) using a 1 m Pürckhauer
universal gauge auger on three occasions during the experiment: the first
prior to sowing (t0, beginning of February), the second at the end of
the rainy season (t1, end of April) and the final one after harvest
(t2, beginning of July). The soil sample was divided into three
subsamples: 0–30, 30–60 and 60–90 cm. The two samples from each plot were
combined. For the t0 sample, 16 sampling sites were selected, from which
four mixed samples were
prepared for each soil layer to represent each quarter of the field. At
t1, all 25 plots were sampled, but at t2 the sampling involved
three of the five plots for each treatment.
Soil nutrient status before applying the amendments and the
nutrient loads of the amendments.
FM
FM
DM
Nmin
P
K
Mg
Ca
Al
Zn
Mn
dm3 m-2
kg m-2
kg m-2
g m-2
g m-2
g m-2
g m-2
g m-2
g m-2
g m-2
g m-2
Soil (0–90 cm)
900
1039
869
7.5
0.4
141
1107
2761
60
n.d.
NA
Biogas slurry
10.2
10.2
0.4
4.9
3.4
41.3
5.4
7.7
1.8
0.05
0.13
Gras
15.6
1.2
0.9
5.8
0.9
12.5
2.6
7.8
4.4
0.02
0.16
∑ Biogas*
25.8
11.4
1.3
10.7
4.3
53.8
8.0
15.5
6.2
0.07
0.29
Compost
15.0
8.2
5.4
10.4
6.8
46.5
17.2
54.4
421.5
0.32
3.49
CaSa compost
8.3
6.4
4.3
9.5
13.8
63.2
22.2
128.1
236.2
0.29
2.08
Concentrations in the dry soil were analysed as described in Sect. 2.3. Calculations of the content in fresh matter of the treatments derived
concentrations provided by Krause et al. (2015); see Table 2 for description
of methods. * For the biogas slurry treatment, the nutrient load was derived from both
grasses and slurry (∑ Biogas). Uncommon abbreviations: DM: dry matter; FM: fresh matter; NA: not
analysed; n.d.: not detectable.
Soil analyses
The water retention curve (WRC) and ρB were determined from
undisturbed soil samples taken using a 0.1 dm3 stainless steel
cylinder. In the field, we monitored the topsoils' volumetric water content
(θ) (m3 m-3) twice a week over the first 6 weeks after sowing at five points per plot, using a TDR
probe (Field Scout 100, 8′′ rods, Spectrum Technologies, Aurora, USA).
Furthermore, θ for each of the three soil layers was determined
gravimetrically at t0, t1 and t2. We performed double-ring
infiltration experiments to determine the infiltration rate (IR) as well as
the field capacity (FC) for the untreated soil at t0 and for the
treated soils at t2 following Landon (1991). The WRC was measured using
pressure plates as well as using the laboratory evaporation method (Hyprop,
UMS, Munich, Germany). The latter data were used to derive the general form
of the Andosol's WRC and to parameterize the Peters–Durner–Iden (PDI)
model (Peters et al., 2015) (Fig. 2). The available water capacity (AWC) was
calculated as θpF 1.8 - θpF 4.2. The
porosity (e) and pore volume (PV) were calculated from dry bulk density and particle
density (ρp) measured using a Multipycnometer (Quantchrome, Boynton Beach, USA).
We measured Nmin and the pH of the soil in situ at both t0 and
t1, while at t2 only the pH was taken; the method involved the
suspension of 50 g soil in 100 mL 0.1 M KCl, which was assayed using an
AgroQuant 114602 test strip (Merck, Darmstadt, Germany) and a pH 330i glass
electrode (WTW, Weilheim, Germany). Further chemical analyses were carried
out on air- or oven-dried t0 and t2 samples, which were first
passed through a 2 mm sieve. The oven-dried samples were used to determine
the concentration of Ctot, Ntot and total
sulfur (Stot), following ISO DIN 10694 (1995) and
ISO DIN 13878 (1998) protocols and using an Elementar Vario ELIII CNS
analyser (Elementar, Hanau, Germany). Concentrations of calcium acetate
lactate (CAL) soluble P (PCAL) and K (KCAL) were
determined with an iCAP 6000 inductively coupled plasma optical emission
spectrometry (ICP-OES) device (Thermo Scientific, Waltham, USA) from
air-dried soil suspended in CAL solution (0.05 M calcium
acetate–calcium lactate and 0.3 M acetic acid) following the
protocol given in chapter A 6.2.1.1 of VDLUFA (2012). Cations such as
Al3+, Ca2+, Mg2+, Fe2+, Mn2+ and Zn2+ were
exchanged with ammonium chloride (NH4Cl) and their concentration
measured using ICP-OES, following the protocol given in chapter A3.2.1.8 of
König (2006). We calculated CECeff from the sum of the ion
equivalents of K, Al, calcium (Ca), magnesium (Mg), manganese (Mn) and
hydrogen (H). The BS represented the ratio between the sum of the ion
equivalents of K, Ca and Mg and CECeff.
Water retention curve (WRC) of the untreated Andosol and of the
soil treated with biogas slurry, standard compost and CaSa compost. The
PDI model for the control Andosol was fitted to data measured using the
simplified evaporation method. Error indicators belong to “Andosol ceramic
plate”. Plot data are provided in Tables S1 and S2.
Biomass production
We harvested maize plants 14 weeks after the two-leaf stage, and the other
crops at maturity. For maize, bean, cabbage, carrot and onion, the
above-ground biomass was considered as the “harvest product” (weight of fresh
mass (FM) in g plant-1), while “market product” represented the
weight of maize grain, bean seed and onion bulb after a week's drying in the
sun (air-dried mass in g plant-1). For maize, we measured the stem
diameter and plant height, and for beans, we determined the pod number per
plant. In each case, a random sample of plants was used, avoiding plants at
the edge of the plot. The overall numbers of samples were as follows: onion
(10/20 plants), cabbage (all plants producing a head), bean (8/16 plants) and
maize (5/24 plants, excluding plants without cobs). For carrots, the
weight of the whole set of plants on a plot was determined. To estimate the
total production per plot (Fig. 3), we multiplied means of weight per plant
and the total number of harvested plants per plot. Total above-ground
biomass production was estimated for 19 maize, 16 bean, 6 cabbage and
20 onion plants per plot for all the treatments (except for the control, which did not include cabbage). Values for market products were estimated for developed maize
cops, onion bulbs, cabbage heads and carrots.
Plant nutritional status
Measurements of plant nutritional status were only made on maize; the plants
were divided into the shoot, the corncob and the grains. Five harvested
plants per treatment were bulked to give a single sample for each plant
fraction per plot. The water content of the biomass was determined
gravimetrically. Following oven drying, the material was ground, passed
through a 0.25 mm sieve and analysed for Ctot and Ntot as above.
We assessed concentration of Ptot, Ktot, Catot, Mgtot,
Zntot, Btot, Cutot, Fetot, Mntot and Motot
after microwave digestion with nitric acid (HNO3) and hydrogen
peroxide (H2O2) using an iCAP 6300 Duo MFC ICP-OES device (Thermo
Scientific, Waltham, USA), following the protocol given in chapter 2.1.1. of VDLUFA (2011).
In addition, we conducted a vector nutrient analysis on harvest product,
nutrient concentration and nutrient uptake following Imo (2012). Uptake and
concentrations of the various nutrient elements were plotted based on the
following scheme: the lower horizontal x axis represented the nutrient
uptake, the vertical y axis the nutrient concentration and the z axis the
biomass (Isaac and Kimaro, 2011). The control treatment's performance was normalized to 100, so that the levels of biomass production and nutrient
concentration reflected the effect of the various soil treatments
(Kimaro et al., 2009). Nutrient diagnosis was based on both the direction
(increase, decrease or no change) and the length of the vectors (strength of
response) following Isaac and Kimaro (2011).
Nutrient balance
For the section of the plots which were cultivated with maize we estimated
changes in the soil nutrient status (Δ Nut) for each treatment, according to
ΔNut=Nutapp-Nutup=ΔNutav+ΔNutnav,
where Nutapp represented nutrients supplied by the treatment
(nutrient application), Nutup nutrients taken up by the maize plants,
Δ Nutav the changes in the soil's available nutrient stock (where
“available” referred to the nutrients being extractable with CAL
solution), Δ Nutnav the change in the soil's nutrient stock,
which was “non-available” due to leaching, interflow, surface
run-off, soil erosion, P fixation, not yet being mineralized, etc. The balance was
calculated for P and K, firstly per plot and then per treatment as an
average of three plots.
Total above-ground biomass production and marketable yields of
food crops given as grammes per plot. Each plot comprised a 4.5 m2 area sown
with maize and a 4.5 m2 area intercropped with onions, beans, cabbage,
carrots, African eggplant and pepper. Different letters reflect means
differing significantly from one another (HSD, Tukey test, α = 0.05;
n = 4 for the untreated control plots; n = 5 for the amended plots). Plot
data are provided in Table S3.
Statistical analysis
Analyses of variance (ANOVA) were performed using the STATISTICA software
(StatSoft Inc., Tulsa, Oklahoma, USA). The main effect was considered to be
the soil treatment. Means were compared using the Tukey honest significant
difference (HSD) test, with α = 0.05.
According to the design of the experiment as a Latin rectangle, the number of
replications of the four treatments did not differ and was n = 5 for all
treatments. However, we had to eliminate one outlier in the control
treatment so that for statistical analyses n was 4. Hence, n = 5 (for biogas
slurry, compost and CaSa-compost treatment) was combined with n = 4 (for the
control treatment) for all parameters we collected during harvesting,
e.g. biomass growth and crop yields. Because of financial restrictions we had to
use a block design with n = 3 for all soil chemical and physical parameters
as well as examinations of nutrient content in the maize plants.
Results and discussion
Between March and May, the mean air temperature was 21.6 ∘C
(maximum 48.9 ∘C, minimum 13.5 ∘C) (Fig. S8) and the
total rainfall was ∼ 360 mm, of which 85 % fell before the
end of April (Fig. S7).
The physico-chemical status of the soil
None of the amendments significantly affected the studied soil hydraulic properties IR (18–36 cm h-1) and FC (0.28 and
0.20 m3 m-3 in the topsoil and in
the subsoil respectively) as measured with the double-ring infiltration
experiments. Also, the WRCs were not significantly influenced by the
amendments and still showed the typical shape of an Andosol (Fig. 2). This
may be due to the low application dose of the amendments that did not
influence ρB of the Andosol (0.99 and
1.02 g cm-3). Nevertheless, we had the subjective
impression during fieldwork, that CaSa compost aided the workability of the soil
by making it more friable.
The topsoil's PV was estimated as being 0.59–0.63 m3 m-2 and may have been homogenized throughout the
treatments by tillage (i.e. with a hand hoe) and then compaction (e.g. by
walking on the plots when working). The calculated FC and AWC derived from
the studied WRC were, respectively, ∼ 0.35 and
0.13 m3 m-3 and exhibited a low site
heterogeneity with the coefficient of variance for θpF 1.8
between 1.3 % in the control and 2.8 % in plots treated with
CaSa compost. The θ did not vary significantly across the three soil
layers at neither t0 nor t1. At t2, θ was lower in
the topsoils of plots treated with the CaSa compost
(0.13 m3 m-3) and on biogas slurry
and standard compost treated plots (0.16 m3 m-3) compared to the control plots
(0.17 m3 m-3). These differences at
the end of the growing season may be caused by higher
evapotranspiration and interception losses due to higher biomass growth (see
below) rather than by different soil hydraulic properties.
Similar findings are reported for the application of uncomposted biochar
(10–17.3 t ha-1) to a New Zealand Andosol, which failed to influence
either ρB, PV or AWC (Herath et al., 2013). Biochar application
had also little effect on AWC either in a high clay content soil
(Asai et al., 2009) or in soils featuring a high carbon concentration or a
low ρB (Abel et al., 2013). Hence, our results imply that none of
the amendments altered the availability of moisture significantly, meaning
that the observed treatment effects on crop yield and plant nutrition were
most likely related to different nutrient availability.
Chemical analysis of the untreated Andosol in Karagwe, Tanzania, and
the amended topsoil (0–30 cm) horizons sampled at the end of the experiment.
Treatment
pH in KCl
PCAL in mg kg-1
Control Andosol
5.3
a
0.5
a
Biogas slurry
5.4
ab
0.7
a
Compost
5.5
ab
1.1
a
CaSa compost
5.9
b
4.4
b
Different letters reflect means differing significantly from one another
(HSD, Tukey test, α = 0.05; n = 3).
The chemical status of the soil prior at t0 is given in Tables 1 and 2.
There was a significant treatment effect on PCAL and pH in the
topsoil (Table 4). The CaSa-compost treatment improved PCAL at
t2 (4.4 vs. 0.5 mg kg-1 in soil DM), but the level of P remained
very low as in the remaining plots (classified based on KTBL, 2009).
According to Finck (2007), a level of 10–30 mg kg-1 in DM is needed
to ensure an adequate supply of P, while Landon (1991) has suggested that
13–22 mg kg-1 in DM should be adequate for most African soils.
Possible explanations for the observation that only the CaSa-compost
treatment altered PCAL are (i) that the treatment provided more P
(1.7 g P dm-3 in FM) than the others did (0.3 and
0.5 g P dm-3 in FM, respectively, in the biogas slurry and in the
standard compost treatment (Table 2)); (ii) that the provision of biochar
promoted nutrient capturing in the soil by the adsorption of P on the biochar
particles (Gronwald et al., 2015; Kammann et al., 2015); and (iii) that the
availability of the recycled P was promoted by liming (Batjes and Sombroek,
1997).
The last point can be supported by our findings, that the topsoil pH was higher
at t2 in the CaSa-compost treatment than in the control plots (5.9 vs. 5.3)
(Table 4). The optimal topsoil pH range for cropping is 5.5–6.5 according to
Horn et al. (2010). Glaser and Birk (2012) have shown that the
highly productive Central Amazonian Terra Preta soils have a pH between 5.2
and 6.4. Through influencing soil pH, the addition of biochar is
particularly effective in soils suffering from poor P availability
(Biedermann and Harpole, 2013). In an earlier publication,
Krause et al. (2015) derived estimates for the liming potential of the present
soil amendments and found 100 kg of DM of biogas slurry, standard compost
and CaSa compost to be equivalent to, respectively, 6.8, 1.4 and 4.7 kg of
CaO. The applied equivalents in this study were 0.03, 0.07 and
0.2 kg m-2 of CaO for biogas slurry, standard compost and
CaSa compost. We found, that the application of CaSa compost had an
immediate effect on soil pH. Finck (2007) recommended the application of lime
equivalent to 0.1–0.2 kg m-2 of CaO every 3 years to maintain the
soil pH. Thus, amending CaSa compost at the applied rate was in the range
for soil melioration if the application of the treatment is repeated every 3 years.
Neither concentration of total organic carbon (TOC) in the soil nor
CECeff was altered significantly by the amendments (Table 3).
Similarly, Liu et al. (2012) reported that the CECeff is hardly
disturbed by a single dose of biochar. From the volume of CaSa compost
applied (8.3 dm3 m-2) and its composition
(Sect. 2.2), we estimated the quantity of dry biochar supplied by
∼ 2.2 kg m-2, equivalent to a Ctot supplement of
∼ 1.3–1.6 kg m-2, a level which was modest compared to
common applications of biochar ranging from 5 to 20 kg m-2
(Kammann et al., 2011; Herath et al., 2013). Liu et al. (2012) have
suggested a rate of 5 kg m-2 as the minimum necessary to
significantly and sustainably increase TOC in the soil. Nevertheless,
Kimetu et al. (2008) were able to show that treating a highly degraded soil
in the highlands of western Kenya with just 0.6 kg C m-2 for three
consecutive seasons, was effective in increasing the quantity of organic
matter in the soil by 45 %.
For an acid soil, the concentration of exchangeable Al was unexpectedly low.
The slope of a linear regression of the concentration of exchangeable Al
against the pH is two and not three (Fig. S6), as predicted if the dominant
form of Al in the soil is Al3+ (reflecting the reaction
equilibrium Al(OH)3 + 3H+ = Al3+ + 3H2O).
Andosols are known to accumulate organic matter through the formation of
metal–humus and allophane-organo complexes. At pHs above 5, the latter
structures dominate (Chesworth, 2008). Thus, most likely the observed low
concentration of exchangeable Al reflected the presence of complexes
involving Al and organic matter.
Biomass production
Amending compost significantly increased the harvested biomass of onion. The
mass of the bulbs produced in plots provided with standard compost or
CaSa compost was, respectively, 52.8 and 54.4 g plant-1, compared to
only 22.2 g plant-1 for the untreated plots (Fig. 3; further see Fig. S5
for visual impressions). In contrast, the soil amendments had no effect
on the yield of carrots. Cabbage plants grown on the untreated soil remained
small and did not develop any heads. In contrast, amending CaSa compost,
standard compost or biogas slurry delivered average yields of heads of,
respectively, 1020, 825 and 720 g plant-1.
Significantly, the above-ground biomass of the bean plants was highest from
those plots amended with CaSa compost with 78 g plant-1, compared
to 32, 22 and 12 g plant-1 grown on plots containing, respectively,
standard compost, biogas slurry and no amendment. There were also
significant differences between the treatments with respect to the average
pod number per plant, ranging from 18.8 for plants grown on CaSa compost to
only 4.7 for those grown in the control soil.
The CaSa compost also promoted a greater stem diameter and height of the
maize plants (respectively 22.8 and 1950 mm), compared to the 16.1 and
1423 mm achieved by the plants grown on unamended soil. The treatment with
biogas slurry, standard compost and CaSa compost increased the per unit area
above-ground biomass accumulated by maize to, respectively, 140, 154 and
211 % compared to plants in the control treatment (Table 5). The
amendments led to grain yields of 263 (biogas slurry), 318 (standard
compost) and 440 g m-2 (CaSa compost) compared to 110 g m-2 from
the control plots.
Harvest and market products of maize and in relation to the
untreated control (100 %).
Harvest product
total above-ground
Market product
biomass, FM
maize grains, air-dry
g m-2
%
g m-2
%
Control Andosol
1595
100
a
110
100
a
Biogas slurry
2229
140
a
263
238
ab
Compost
2464
154
ab
318
288
bc
CaSa compost
3372
211
b
438
397
c
Different letters reflect means differing significantly from one another
(HSD, Tukey test, α = 0.05) with n = 4 for control and n = 5 for
other treatments.
The grain yield from the control plots was below both the average national
Tanzanian yield (2012: 124 g m-2) and that for eastern Africa
(180 g m-2), while the yield from the CaSa-compost-treated plots
matched those obtained in Croatia (434 g m-2) and Cambodia
(441 g m-2) (FAOSTAT, 2012). A field experiment in the Dodoma region of
Tanzania produced a grain yield of about 100 g m-2 from unfertilized
plots and 380–430 g m-2 from mineral-fertilized plots
(Kimaro et al., 2009), while in the Morogoro region the same maize cultivar
yielded 117, 257 and 445 g m-2 from plots supplemented with,
respectively, 0, 15 and 80 g N m-2 (Mourice et al., 2014). Thus, the
benefit of providing CaSa compost matched that of a higher (i.e. extremely
high) input of synthetic N fertilizer, however, provided by locally
available nutrients.
The observed benefits of CaSa compost were largely in line with the known
effects of biochar amendments to soils. Two meta-analyses have suggested
that for various crops, the addition of 2 ± 0.5 kg m-2 biochar
induces a -3 to +23 % crop yield response compared to unamended
control plots (Jeffery et al., 2011; Liu et al., 2013). Maize responds to the
supplement by increasing its grain yield by 16 % and its biomass by
14 %. On acidic soils (pH of < 5.0), the positive effect of
biochar is between 25 and 35 %. The positive effect of the CaSa compost
on the soil and on biomass growth was most probably due to its liming
effect, which improved the availability of various nutrients, in particular
that of P. The positive effects of applying CaSa compost may last for
several cropping seasons, as shown by Major et al. (2010) in a 4-year study.
Nutrient concentration in dry matter of maize grains compared to
levels reported in the literature. The italic writing indicates the statistical
p values, which belong to the nutrient concentrations in the respective column.
Ntot
Ptot
Ktot
Catot
Mgtot
g kg-1
g kg-1
g kg-1
g kg-1
g kg-1
Control Andosol
15.9
2.3
4.4
0.1
1.0
Biogas slurry
16.5
2.6
4.0
0.1
1.0
Compost
15.6
2.5
3.6
0.1
1.0
CaSa compost
16.8
3.0
3.9
0.1
1.1
p (n = 3)
0.58
0.08
0.03
0.71
0.34
Finck (2007)
17.5
4.0
4.9
2.1
1.4
Kimetu et al. (2008) (Kenya)
Control
11.8
2.3
2.7
0.03
0.9
Biochar
12.5
2.2
2.6
0.1
0.8
Furthermore, we experienced that biogas slurry may not be suitable as a soil
amender for bean crops, since the plants did not appear to respond well
compared to compost or CaSa compost. Although most recent work using
biogas slurry as a soil amender observed a positive plant response in terms of
productivity (Baba et al., 2013; Clements et al., 2012; Garfí et al., 2011;
Komakech et al., 2015), others also revealed decreasing yields (e.g. Sieling
et al., 2013). Salminen et al. (2001) attributed observed a negative plant
response to organic acids and ammonia contained in biogas slurry, which can
be phytotoxic for plants if not applied in moderate quantities.
Nevertheless, composting could reduce the aforementioned substances as
shown by Abdullahi et al. (2008). Therefore, this material should be
combined with other organic matter.
Analysis of plant nutritional responses
The shoot, grain and corncob biomass produced by the maize crop was
responsive to the soil amendments, whereas its water content was not
significantly affected. According to Finck (2007), the concentrations of
each of the nutrients were below recommended levels. However, compared to
the outcomes of the experiment in Kenya reported by
Kimetu et al. (2008), the grain concentrations of both N and K were slightly
higher, while those of P, Ca and Mg were similar. In our experiment, the dry
shoot material was deficient with respect to both P (0.7–0.9 g kg-1,
instead of recommended concentrations of 2.0–3.5 g kg-1) and N
(8–11 g kg-1, compared to a recommended range of 15–32 g kg-1)
(Bergmann, 1999; Marschner, 2011). Only the nutrient concentrations in the
maize grains responded significantly to the treatments, especially for K
(p = 0.03) and P (p = 0.08) (Table 6). Here, we observed a dilution effect
for K, while the concentration of P was slightly increased in maize grains grown
on plots amended with CaSa compost. With respect to the N concentration,
there was no significant treatment effect, since the N inputs had been
adjusted a priori so that each treatment offered the same amount of N.
The vector nutrient analysis illustrated primarily the response of maize to
mitigated P deficiency, with the longest arrow indicating the largest
response (Fig. 4). Here, an increase in each of the three parameters
(biomass growth, nutrient concentration, nutrient uptake) was generated by
an increased supply of the limiting nutrient P. This is because (i) more P
was supplied with CaSa compost (see Sect. 3.1) and (ii) its availability was
increased due to the raised soil pH (Batjes, 2011). Furthermore, nutrient
uptake by maize was proportional to biomass growth. Hence, plants grown on
plots amended with CaSa compost were able to take up significantly greater
amounts of N, P, K, Ca, Mg and Zn in their grains than those grown on the
other plots (Fig. 4).
Vector nutrient analysis for maize, showing the responses of air-dry
grain yield (g plant-1), relative nutrient concentration in DM (with
the untreated Andosol: 100 %) and relative nutrient uptake (with the
untreated Andosol: 100 %). Different letters reflect means differing
significantly from one another (HSD, Tukey test, α = 0.05; n = 3).
The arrow indicates the largest response and depicts a primary response of
maize plants to mitigated P deficiency. Plot data are provided in Table S4.
As the native soil's KCAL was already very high and further K was
provided by the amendments (Table 3), an antagonistic effect on nutrient
uptake between K and Ca as well Mg would have been possible (Finck, 2007).
However, observed changes in concentrations of Ca and Mg were not
significant, but there was a significant decrease in K concentration in
maize grains. However, this may possibly be due to the dilution imposed by
growth stimulation.
Nutrient balancing
On the plots treated with biogas slurry, standard compost and CaSa compost,
Nutapp of P varied with, respectively, 4.2, 6.8 and 13.8 g m-2.
This can be considered a low to high application compared to a recommended
fertilizer rate of 7.0–8.4 g m-2 yr-1 for maize on P-deficient
soils (KTBL, 2009; Finck, 2007). By contrast, Nutapp of K was very
high with 53.8, 46.5 and 63.2 g m-2, compared to a
recommended dose of 9.3–12.4 g m-2 yr-1 for maize on soils
with high K content (KTBL, 2009; Finck, 2007). On the plots treated with biogas slurry, plants took
up ∼ 19 % of the total applied P; the equivalents for the
standard compost and CaSa-compost treatments were ∼ 16 and
∼ 12 %, respectively. These rates are consistent with
the ∼ 15 % reported by Finck (2007) as being available in
the first year after fertilizer application. With respect to K, Nutup
was about ∼ 10 % of Nutapp in the biogas slurry
treatment, ∼ 18 % in the standard compost treatment and
∼ 17 % in the CaSa-compost treatment. These rates differ
greatly from the ∼ 60 % figure suggested by Finck (2007).
The disparity relates most likely to the soil's high level of KCAL.
We estimate that soil Ptot and Ktot were both depleted
(Δ Nut < 0) on the control plots (Table 7). In the biogas slurry,
standard compost and CaSa-compost-treated plots, Δ Nut was positive
for both P and K. However, the only significant change to the topsoil's
PCAL was recorded in the CaSa-compost treatment (Sect. 3.1.). Hence,
about 1.1 g P m-2 was assignable to Δ Nutav
in the plots supplied with CaSa compost, with the rest being
“non-available”. Some of the latter may include P that had not been
released through mineralization of the organic matter, while some may have
been immobilized in the form of metal–humus complexes, which are
characteristic for Andosols (Zech et al., 2014) (i.e. assignable to
Δ Nutnav in both cases). Leaching of P is insignificant, since P gets immobilized (Finck, 2007). We assume that some of the K provided
by the amendments may have been leached during the rainy season as
mentioned by Finck (2007) for light soils such as the present Andosol. There
were no signs of significant losses through soil erosion visible on the
experimental site.
From our findings we recommend the addition of urine and sanitized faeces to
the compost, since the matters provide a ready source of nutrients, accelerating, for example, compost's Nmin and total P content (compare
Table 2). Given that biochar can capture both nitrate and phosphate, as
shown by Gronwald et al. (2015) and Kammann et al. (2015), we assume that
combining urine and biochar as compost additives enriches compost with N and
P and reduces nutrient loss during and after composting. Especially, the
loss of N in the form of the greenhouse gas N2O can be reduced, as
shown by Larsen and Horneber (2015). In addition, urine can contribute to
the moisture required for successful composting.
Conclusions
To summarize: for beans and maize, crop biomass production and economic
yield were significantly improved by the application of CaSa compost. For
cabbage and onion, all three of the tested amendments were beneficial. The
amendments, and especially CaSa compost, improved the nutrient availability,
as revealed by vector nutrient analysis. This can be attributed to changes
in soil pH and the addition of nutrients.
Of particular significance was the observation that the P deficiency
affecting the local Andosol could be mitigated using CaSa compost. The
increase in available P achieved by the CaSa-compost treatment was more than
sufficient to supply the crops' requirement. Thus, we conclude that a gradual
increase in soil P could be achieved by a regular application of the CaSa compost.
Changes in the soil nutrient status (Δ Nut) along with
nutrients applied by the treatment (Nutapp) and the nutrients taken up
by the crop (Nutup).
Nutapp
Nutup
Δ Nut
Nutapp
Nutup
Δ Nut
P
P
P
K
K
K
g m-2
g m-2
g m-2
g m-2
g m-2
g m-2
Control Andosol
–
0.4
-0.4
–
3.3
-3.3
Biogas slurry
4.2
0.8
3.5
53.8
5.2
48.5
Compost
6.8
1.1
5.7
46.5
8.5
38.0
CaSa compost
13.8
1.7
12.3
63.5
10.7
52.5
Data based on three plots for each treatment.
The chosen rates of biogas slurry and standard compost supplementation were
sufficient to maintain the soil's pH, whereas the CaSa compost raised the
soil pH, improving its productivity immediately. Thus, we conclude that a
continuous program of composting and compost amendments over decades would
probably fully ameliorate the soil.
We further conclude, that the application of local available biogas slurry
needs to be tested for several crops before recommending the widespread
utilization of this matter as it may contain substances which could be
phytotoxic for plants if not applied in moderate quantities. In addition,
composting of biogas slurry prior to soil amendment, possibly with and
without biochar, is of certain practical relevance but needs preceding
scientific investigation to study the specific metabolisms taking place and
to identify the consequent N recovery efficiency.
Finally, we conclude that all the treatments, but especially CaSa compost,
are viable as substitutes for synthetic commercial fertilizers. We further
conclude that local smallholders with six people per household can produce
CaSa compost at an estimated rate of ∼ 5.1 m3 yr-1,
which would be sufficient to fertilize an area of
∼ 1850 m2 at the rate of 8.3 dm3 m-2 over the course of 3 years. By this means, it would be possible to fertilize about 30 %
of the average area cultivated by smallholders in Karagwe. Therefore, the CaSa approach needs to be integrated into farm-scale nutrient management by
conducting a detailed analysis of nutrient flows in the
farm household system and studying all potential additions and removals of
nutrients to and from the planted land.