Long-term effects of biochar on soil physical properties

Geoderma 282 (2016) 96–102

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Long-term effects of biochar on soil physical properties Leigh D. Burrell a,⁎, Franz Zehetner a, Nicola Rampazzo a, Bernhard Wimmer b, Gerhard Soja b a b

Institute of Soil Research, University of Natural Resources and Life Sciences, Peter-Jordan-Str. 82, 1190 Vienna, Austria Austrian Institute of Technology, Standort Tulln, Konrad-Lorenz-Straße 24, 3430 Tulln, Austria

a r t i c l e

i n f o

Article history: Received 7 September 2015 Received in revised form 7 July 2016 Accepted 14 July 2016 Available online 21 July 2016 Keywords: Biochar Soil physical properties Soil aggregate stability Plant available water Bulk density Pore size distribution

a b s t r a c t A growing body of research into the effects of biochar on soil physical characteristics suggests that it is most effective in coarse-textured soils. In this study, we set out to test this theory by comparing the effects of a woodchip biochar on a Chernozem, Cambisol and a coarse-textured Planosol in a pot experiment. We also compared the effect of different biochars on the Planosol, including woodchip biochar, straw biochar, and two vineyard-pruning biochars produced at different pyrolysis temperatures. Three characteristics were measured as indicators of good soil structure: bulk density, soil aggregate stability and plant available water. The woodchip biochar induced greater decreases in bulk density in the coarse textured Planosol than in the other soils. It also had a greater effect on soil aggregate stability in the Planosol than in the Cambisol, but had no effect on the Chernozem. Woodchip biochar had no effect on plant available water in any of the three soils. Straw biochar was the most effective at improving soil aggregate stability in the coarse-textured Planosol, with a 98% increase. Straw biochar also improved plant available water in the Planosol by 38% relative to the control, compared with 24% and 21% increases in the vineyard-pruning biochars, produced at 525 °C and 400 °C, respectively. Our study supports the theory that coarse-textured soils have the most to gain structurally from biochar amendments. We also show that straw biochar was the most effective at improving soil aggregate stability and plant available water in a coarse-textured Planosol. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Meeting increasing global demand for food in the context of constrained resources and changing climate means that our agricultural systems must be both more productive and resilient (FAO, 2004). Innovative tools are required to help deal with these complex challenges, which has fuelled interest in biochar as a potential soil amendment to improve soil quality and crop productivity (Lehmann et al., 2006). Biochar is a porous, carbon-rich material produced by heating organic matter to temperatures of between 300 °C and 1000 °C in an environment with limited or no oxygen (Verheijen et al., 2010). Research into biochar as a soil amendment has been wide-ranging and results have been mixed due to the complexity of interactions between biochar, soils and crops (Lychuk et al., 2014). Meta-analysis of the effects of biochar suggests that it is most effective in acidic, degraded and coarse-textured soils (Jeffery et al., 2011; Crane-Droesch et al., 2013). The benefits are suspected to be derived from a liming effect, increases in cation

⁎ Corresponding author at: 1 Bennett Street, Newtown, NSW 2042, Australia. E-mail addresses: [email protected] (L.D. Burrell), [email protected] (F. Zehetner), [email protected] (N. Rampazzo), [email protected] (B. Wimmer), [email protected] (G. Soja).

http://dx.doi.org/10.1016/j.geoderma.2016.07.019 0016-7061/© 2016 Elsevier B.V. All rights reserved.

exchange capacity, sorption of organic matter, and changes in soil structure (Liu et al., 2012). Research into the effects of biochar on soil physical characteristics can be divided into two main concepts. The first is that by adding a porous substance to soil, it will inevitably have a direct effect on soil physical properties (de Melo Carvalho et al., 2014; Peake et al., 2014). Underpinning this theory are cases where total porosity, water-holding capacity or bulk density of biochar-amended soils have improved (Basso et al., 2013; Kammann et al., 2011). For example, Devereux et al. (2012) found that biochar added at a rate of 5% (w/w) decreased average pore size in the soil from 0.07 mm2 to 0.046 mm2. In their short run experiment, they also observed improvements in bulk density, as did Githinji (2014) and Mukherjee et al. (2014). Ulyett et al. (2014) attribute their observed reductions in bulk density to the lower density of biochar added to two coarse-textured soils. Quin et al. (2014) and Castellini et al. (2015) also suggest that this direct effect on soil bulk density explained observed increases in soil water retention close to saturation. Hardie et al. (2014) tested this direct effect theory in a 30-monthlong field experiment. They expected that biochar would increase plant available water in the soil through the addition of pores with a diameter of between 30 μm and 0.2 μm. However, they could not attribute

L.D. Burrell et al. / Geoderma 282 (2016) 96–102

any observed changes in the soil to the internal porous structure of the biochar. In their case, improvements in near-saturated hydraulic conductivity and soil water content at − 0.1 kPa were attributed to increased earthworm activity. This leads us to the second concept, which is that the addition of biochar can support soil structure building processes via indirect means. These processes may include: providing improved habitat for soil microorganisms (Pietikainen et al., 2000; van Zwieten et al., 2009), through favourable association with soil organic matter and improved aggregation (Lehmann et al., 2011; Fletcher et al., 2014), or by improving plant growth thereby enhancing rhizosphere effects (Joseph et al., 2010). An increase in soil aggregate stability was reported by Herath et al. (2013) and Lu et al. (2014) when analysing different biochars added to different soils. The authors attributed these effects to biochar-carbon combining with clay mineral phases to form macro-aggregates. This is to say that although the catalyst for these processes may come from a direct effect such as a change in bulk density, the processes that follow may be of more importance in the long-term. The inherent complexity of biochar, which can change dramatically depending on feedstock and pyrolysis parameters (Verheijen et al., 2010; Demirbas, 2004), makes it difficult to isolate interactions with dynamic soil processes. However, this complexity also lends support to both concepts being relevant, perhaps at different points throughout an experiment or field trial. For example, a short-run pot experiment may be useful for determining the initial direct effects, such as changes in water holding capacity (Novak et al., 2009). However, changes induced by cropping, consolidation, biochar hydrophobicity, weathering of biochar particles, and washout of ash and soluble elements are unlikely to be captured in this type of study. Conversely, long-term field trials are much more likely to yield results that represent these indirect effects over time. A general disadvantage is that they are usually adversely impacted by environmental variability (Liu et al., 2013). In order to limit some of these methodological issues for this study, we chose to take samples from an old biochar pot experiment that had involved several cropping cycles (Kloss et al., 2014) and then been left to fallow outside over two years. As the soil/biochar mixtures had a chance to equilibrate, it is hoped that the measurements are more representative of the long-term effects of biochar, and that the issues of disturbance should be minimised. The exposure to the outside environment and vegetative effects means that some of the factors included in field trials are also captured without the variability. Using this resource meant we were able to take advantage of existing data to deepen our analysis and include temporal factors. Our study had two main objectives. The first was to test the idea that biochar has the most positive effect on the physical properties of sandy acidic soils. We did this by comparing the effect of a typical woodchip biochar on three very different agricultural soils. Our second objective was to test whether there are characteristics of different types of biochars that make them more effective amendments in coarse-textured soils. We chose three indicators of good soil structure for our two comparisons: bulk density, soil aggregate stability and plant available water.

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2. Materials and methods 2.1. Soils and biochars Three agricultural soils were used for the study from a prior project (see Kloss et al., 2014); a Planosol (N48°46′32.9″, E15°14′28.6″), a Chernozem (N48°19′52.6″, E15°44′20.5″) and a Cambisol (N47° 13′46.0″, E15°50′40.6″) (see Table 1). The soils were originally taken from the top 30 cm of each profile, air-dried and homogenised. Large aggregates were broken down and rocks N3 cm were excluded. Four biochars were selected from three feedstocks including mixed woodchips (pyrolysis temp. 525 °C), wheat straw (Triticum aestivum L., pyrolysis temp. 525 °C) and vineyard-prunings (Vitis vinifera L., pyrolysis temp. 525 °C and 400 °C) (see Table 2). Basic measurements such as pH (in CaCl 2 ), cation exchange capacity, electrical conductivity (ratio 1:10) and water-soluble cations (ratio 1:20) were taken as per standard methodologies (see Kloss et al., 2012). Biochar was mixed with soils at a rate of 3% by weight, and soil-biochar mixtures were filled into 17 litre pots (diameter: 23.5 cm, height: 40 cm) at a defined bulk density. The pots were planted with three consecutive crops — mustard (Sinapis alba L.), barley (Hordeum vulgare L. cv. Xanadu) and red clover (Trifolium pratense L.) — between November 2010 and December 2011, and fertilised according to common agricultural practice (see Kloss et al., 2014). The experiment was run in a glasshouse for this period and pots were then left to fallow outside for two years. 2.2. Soil physical characteristics In November 2013, the clover was removed from the pots and both disturbed soil samples and undisturbed 200 cm3 cores were taken at a depth of 15 cm. Three main parameters were chosen as indicators for changes in soil physical characteristics: bulk density, soil aggregate stability and plant available water. Plant available water was measured using the pressure chamber method (according to Richards, 1948). Four undisturbed cores from each treatment type were saturated, weighed, and pressure applied at 6 kPa, 30 kPa and 1.5 MPa. Samples were weighed between each pressure step and then oven dried at 105 °C for 24 h. Gravimetric water content was determined as the difference between the dried and wet weights at each pressure step and converted to volumetric water content. Plant available water was taken as the water held between 6 kPa and 1.5 MPa. Soil aggregate stability was determined using a wet sieving device (according to Murer et al., 1993). Fine earth (b2 mm) was air-dried for seven days and sieved to collect aggregates of between 1 mm and 2 mm in diameter. Four grams of aggregates were weighed and placed on a 0.25 mm sieve which was mechanically raised and lowered (42 cycles/min) for 5 min in distilled water. Weakly aggregated material fell through the sieve, leaving the stable aggregates, sand, organic particles and biochar. These materials were then oven-dried at 105 °C for 24 h and weighed. Samples were then immersed in 0,1 mol Na4P2O7·H2O for 5 min to breakdown the stable aggregates and sieved again, leaving only the sand, organic particles and biochar. Samples were then dried at

Table 1 Basic soil characteristics, measurements and analyses undertaken by Kloss et al. (2014). Soil type

pH (CaCl2)

CEC (mmolc kg−1)

Carbonate (w.-%)

OC (w.-%)

clay (w.-%)

silt (w.-%)

sand (w.-%)

Texture class

Planosol Chernozem Cambisol

5.4 ± 0.0a 7.4 ± 0.1c 6.6 ± 0.1b

75.1 ± 0.0a 208.6 ± 2.3b 209.4 ± 1.2b

0.0 ± 0.0a 15.8 ± 0.1b 0.0 ± 0.0a

1.64 ± 0.02b 1.50 ± 0.01a 2.36 ± 0.02c

10.7 16.9 32.7

19.6 61.4 37.6

69.8 21.6 29.7

Sandy loam Silt loam Clay loam

Different letters indicate significant difference within one column (P b 5%; Tukey's test). ± corresponds to standard error. CEC: cation-exchange capacity; OC: organic carbon.

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Table 2 Biochar characteristics, measurements and analyses undertaken by Kloss et al. (2014). Biochar feedstock

Pyrol.

Straw Woodchip Vineyard Vineyard

525 525 525 400

ash cont. (w.-%)

pH (CaCl2)

EC (mS cm−1)

CEC (mmolc kg−1)

BET -N2 SSA (m2 g−1)

28.10 15.20 7.70 4.30

9.7 ± 0.0c 8.9 ± 0.0b 8.8 ± 0.0b 8.3 ± 0.0a

5.2 ± 0.1d 1.6 ± 0.0c 1.1 ± 0.0a 1.5 ± 0.0b

148.5 ± 0.4d 93.0 ± 1.1b 78.8 ± 0.8a 123.5 ± 0.8c

12.26 ± 0.6c 26.41 ± 0.6d 4.85 ± 0.0b 1.69 ± 0.0a

(°C)

Water-sol. Cations (mg kg−1) Na

K

Mg

Ca

41 119 29 27

16,410 4890 3070 2960

5.5 4.8 72 223

1.1 11 189 864

Different letters indicate significant difference within one column (P b 5%; Tukey's test). ± corresponds to standard error. EC: electrical conductivity; CEC: cation-exchange capacity; BETN2 SSA: Brunauer-Emmett-Teller specific surface area (N2 adsorption).

105C° for 24 h and weighed again. The Soil Aggregate Stability (SAS) was then calculated as: SAS ¼

A−B 100 in weight% W−B

where A is stable aggregates, sand, organic particles and biochar, B is sand, organic particles and biochar, and W is the air dried weight of the sample. Statistical analysis was conducted using one way ANOVA, Tukey's post hoc test (P b 0.05), and t-test (P b 0.05) (SPSS V22). 2.3. Biochar measurements Biochar bulk density and total porosity were measured via mercury porosimetry (POROTEC Pascal 140/440, Hofheim, Germany) according to Brewer et al. (2014) (see Table 3). The biochar bulk densities were used to calculate the theoretical bulk density of each treatment as: Theoretical ρb ¼

1 As ρb s

1 1 Abc: þ ρb bc

where ρb is bulk density, S is soil, bc is biochar, and A is application rate by weight; 97% for soil and 3% for biochar. Note that the measured bulk densities after three years, rather than the packed bulk densities, were used in this calculation. Biochar was extracted from soil samples after three years and particles were broken to expose the internal structure. Scanning electron microscopy images were taken of all biochars at 150, 500, 2000 and 5000× magnification with a high-resolution scanning electron microscope (Quanta 250 FEG) using an Everhart-Thornley detector. 3. Results 3.1. Electrical conductivity, soil pH and bulk density Soil pH and electrical conductivity were measured at the beginning of the pot experiment (0 days) (data from Kloss et al., 2014). Electrical conductivity was measured again three years later upon commencement of this project (see Table 4). An initial increase in electrical conductivity was seen in all biochar-amended Planosol treatments compared with the control. Straw biochar, which contained high Table 3 Biochar physical characteristics determined by mercury porosimetry. Biochar feedstock

Straw Woodchip Vineyard Vineyard

Pyrol.

Bulk density

(°C)

(g cm

525 525 525 400

0.22 0.47 0.44 0.36

−3

)

Total porosity (%) 84.3 66.7 72.0 68.8

amounts of water-soluble K (cf. Table 2), caused the strongest increase in electrical conductivity. This initial increase was also found in the woodchip biochar treatments for all three soil types, however the increase was far less pronounced in the Chernozem. After three years, the electrical conductivity in the straw and 525 °C vineyard-pruning treatments returned to a comparable level to the control. The reduction was far less pronounced in the 400 °C vineyard-pruning and woodchipbiochar treatments. Electrical conductivity increased over time in both control and biochar-amended treatments in the Chernozem and Cambisol. Bulk density was determined via packing using a predetermined bulk density and measured three years later for the comparison (see Table 5). The initially lower bulk density of the biochar treatments persisted over three years, with the exception of the woodchip biochar-amended Cambisol. Bulk density increased in all control treatments over time. 3.2. Soil aggregate stability Soil aggregate stability was higher relative to the control in all biochar-amended Planosol treatments after three years, with 92%, 37%, 28% and 50% relative increases in the straw, 525 °C vineyard-pruning, 400 °C vineyard-pruning and woodchip biochar treatments, respectively (see Fig. 1). This effect was also observed in the woodchip biocharamended Chernozem with a 26% relative increase, but not in the Cambisol. 3.3. Plant available water Straw biochar had the greatest effect on plant available water in the Planosol, followed by vineyard-pruning produced at 525 °C and 400 °C with respective increases of 38%, 24% and 21% (see Fig. 2). This increase was achieved through a greater volume of water being held at 6 kPa. Although the woodchip biochar-amended Planosol also showed similar increases, the gains were offset by an increase in water held at 1.5 MPa (see Table 6). The woodchip biochar-amended Chernozem and Cambisol did not show an increase in plant available water. Table 4 Electrical conductivity (EC) and pH (taken in 2010 and 2013) of soils amended with 3% biochar. Soil type

Treatment

pH at day 0 (CaCl2)

EC at day 0 (μS/cm)

EC after 3 years (μS/cm)

Planosol Planosol Planosol Planosol Planosol Chernozem Chernozem Cambisol Cambisol

Control Straw Woodchip Vineyard 525 °C Vineyard 400 °C Control Woodchip Control Woodchip

5.4 ± 0.00a 6.7 ± 0.00e 6.8 ± 0.00e 6.5 ± 0.00c 6.5 ± 0.00c 7.2 ± 0.00f 7.4 ± 0.00g 6.6 ± 0.00d 6.3 ± 0.00b

43 ± 0.02a 189 ± 1.62g 121 ± 4.39d 95 ± 1.27c 91 ± 0.12c 137 ± 0.23e 154 ± 2.54f 82 ± 0.17b 136 ± 2.08e

59 ± 0.00b 55 ± 0.58a 113 ± 0.33e 55 ± 0.00a 72 ± 0.33c 178 ± 0.00h 171 ± 0.33g 101 ± 0.00d 167 ± 0.33f

Different letters indicate significant difference within one column (P b 5%; Tukey's test). ± corresponds to standard error.

L.D. Burrell et al. / Geoderma 282 (2016) 96–102 Table 5 Bulk density (BD) after a three year pot experiment. Soil

Biochar treatment

Original packed BD

BD measured after 3 years

Calculation BD of soil fraction



1.35 1.28 1.32 1.31 1.25 1.28 1.23 1.17 1.11

1.43 ± 0.02f 1.17 ± 0.01a 1.24 ± 0.02bc 1.27 ± 0.01cd 1.23 ± 0.01abc 1.36 ± 0.01ef 1.22 ± 0.02abc 1.31 ± 0.01de 1.18 ± 0.01ab

– 1.23 1.35 1.34 1.31 – 1.29 – 1.24

Different letters indicate significant difference within one column (P b 5%; Tukey's test). ± corresponds to standard error.

4. Discussion 4.1. Bulk density The initial lower packed bulk densities in all biochar treatments reflect the direct dilution of soil with the lower-density biochars. Straw biochar, with its particularly low density, had the greatest effect on the Planosol, and woodchip biochar showed comparable reductions across the three soils of between 3.9 and 5.1%. The bulk density measurements after three years show consolidation in all three control treatments, as is to be expected (Streck and Cogo, 2003). However, there was a clear resistance to consolidation in all biochar treatments. This meant that the long-term improvements to bulk density were 13.3%, 10.3% and 9.9% in the woodchip biocharamended Planosol, Chernozem and Cambisol, respectively. Quin et al. (2014) also found that biochar made from woody residues had a greater

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Table 6 Volumetric water contents held at different pressures. Different letters indicate significant differences within columns (P b 5%, Tukey's test). ± indicates standard error. Soil


Biochar treatment


Measured water content (vol.%) at: 6 kPa

30 kPa

1.5 MPa

24.6 ± 0.2a 29.1 ± 0.4c 27.2 ± 0.7b 27.8 ± 0.2bc 27.5 ± 0.4bc 31.4 ± 0.5d 32.8 ± 0.4d 36.7 ± 0.2e 37.4 ± 0.6e

19.6 ± 0.3a 21.6 ± 0.4b 21.1 ± 0.3ab 21.6 ± 0.3b 20.9 ± 0.1ab 25.1 ± 0.4c 25.6 ± 0.5c 31.56 ± 0.3d 32.03 ± 0.6d

11.1 ± 0.3a 10.5 ± 0.4a 13.1 ± 0.3b 10.8 ± 0.4a 11.1 ± 0.3a 16.2 ± 0.3c 16.4 ± 0.3c 20.1 ± 0.4d 20.7 ± 0.4d

effect on bulk density in a course-textured soil than in soils with higher clay content. The scale of the improvement also supports a study from Laird et al. (2010). They found large decreases in bulk density in a biochar-amended soil after 500 days at even lower application rates. The straw biochar appeared to have the greatest effect of the four biochars on the Planosol. However, the difference between the straw and vineyard-pruning biochar treatments was not statistically significant. We wanted to see whether measured differences could be attributed to the different densities of the biochars. To do this, we calculated what the theoretic bulk density of the biochar treatments should be based on the dilution effect. In every case, measured bulk densities were lower than calculated bulk densities (see Table 5), indicating that the effects are not solely attributed to the dilution of the soils with a porous material. Laird et al. (2010) discuss similar findings and suggest that biochar is acting as a soil conditioner. Biochar may be supporting the microbial communities

Fig. 1. Soil aggregate stability (SAS) of a sandy, acidic Planosol amended with four different biochars (3% application), and a Cambisol and Chernozem amended with a woodchip biochar (3% application). Different letters indicate significant differences (P b 5%, Tukey's test). Error bars indicate standard error.

Fig. 2. Plant available water (PAW), between 6 kPa and 1.5 MPa, of Planosol amended with four different biochars, and Cambisol and Chernozem amended with woodchip biochar. PAW expressed as percentage of total soil volume. Different letters indicate significant differences (P b 5%, Tukey's test). Error bars indicate standard error.

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Fig. 3. Correlation between initial electrical conductivity (EC) and soil aggregate stability (SAS) three years after biochar (BC) amendment. Figures reported in percentage difference relative to control treatment. EC measurements taken by Kloss et al. (2012). Error bars indicate propagated standard error.

associated with the building and maintenance of soil structure through improved aeration, availability of nutrients or better moisture retention (Lei and Zhang, 2013). 4.2. Soil aggregate stability Due to a range of methodological, temporal and material factors, the literature on the effects of biochar on soil aggregate stability has yielded mixed results (Sun and Lu, 2014; Liu et al., 2013; Soinne et al., 2014). In our case, the biggest increases were observed in the Planosol and straw biochar induced the greatest improvement. Liu et al. (2014) also found wheat straw biochar to be very effective and Sun and Lu (2014) found that straw biochar out-performed woodchip biochar in improving soil aggregate stability.

Potential causes behind the observed increase in soil aggregate stability were investigated. Interestingly, the initial electrical conductivity taken when the biochars were first added to the soils three years prior correlated very strongly with the current soil aggregate stability measurements (see Fig. 3). Here the relative difference in soil aggregate stability between biochar-amended treatments and their respective controls were plotted against the relative difference in electrical conductivity. The initial biochar addition brought with it a considerable input of soluble salts (cf. EC and water-soluble cations in Table 2). Increasing electrolyte concentration is known to shrink the diffuse double layers of soil colloids and promote flocculation (Bohn and McNeal, 1983; McBride, 1997). In the flocculated state, other aggregation mechanisms could have acted more efficiently, thus sustaining the soil aggregate stability despite the leaching of soluble salts over time. For

Fig. 4. Scanning electron microscopy (SEM) images of biochar (BC). Scale bars for 50 μm are shown at the base of each picture. a = straw BC, b = vineyard-pruning BC (pyrolysis 400 °C), c = vineyard-pruning BC (pyrolysis 525 °C), d-f = woodchip BC. Arrow showing hyphae location. Scale bar of 50 μm provided in bottom right of each image.


example, biochar surfaces may attract labile organic matter, thus offering substrates for microbes which, in turn, may build further aggregates through the excretion of mucilage (Liang et al., 2010). The improved soil conditions may also be favourable to mycorrhizal fungi (Fletcher et al., 2014). Indeed, hyphae were observed deep within the biochar particles, particularly in the straw biochar (see Fig. 4). These secondary processes may be maintaining soil aggregate stability after the effect of the increased salt input has subsided (Bearden and Petersen, 2000). 4.3. Plant available water The fact that woodchip biochar had no effect on plant available water in any treatment was a little unexpected, but supports the findings of Hardie et al. (2014). Similar results have also been reported by Abel et al. (2013) and Ulyett et al. (2014) where water held at permanent wilting point was also increased in soils amended with woodchip biochar. In these cases, it was suggested that the higher specific surface area in the woodchip biochar increased its ability to hold more water at high matric potentials. In our study, specific surface area was greater in the woodchip biochar (see Table 2), however no correlation was observed in relative or absolute terms. The straw biochar had the most positive effect on plant available water in the Planosol. When Koide et al. (2014) compared the effect of a switch grass biochar on four different soils, they found that it had the most positive effect on plant available water in a sandy loam. However, they also found improvements in loam and clay loam soils. When Sun and Lu (2014) made a comparison of different biochars applied to a Vertisol of comparable texture to the Cambisol in our study, they also found that the straw biochar improved plant available water and that woodchip biochar had no effect. Given the scale of the improvement induced by the straw biochar in our study, it is conceivable that it could also have a positive effect on less sandy soils. 5. Conclusions By applying a woodchip biochar to three different soils, we were able to show that its effects on bulk density and soil aggregate stability were greatest in a coarse textured Planosol. This supports the suggestions from Jeffery et al. (2011) and Crane-Droesch et al. (2013) that sandy, acidic soils have the most to gain from biochar amendments. The lack of improvement in plant available water for all woodchip biochar treatments shows that the improvements to soil structure do not necessarily apply across all indicators. This is why the comparison of different types of biochar applied to the Planosol was important. It suggests that some of the negative results we see in other studies may be derived from the biochar and the soil types being poorly suited. Where the woodchip biochar failed to improve plant available water, the other three biochars succeeded. Straw biochar was particularly effective, which could suggest that using a grass-based feedstock rather than the more widely used woodchips may yield more positive results. Biochar's recalcitrant nature and ability to interact with a range of soil functions potentially makes it an attractive long-term investment in soils. If biochar is to be applied successfully in the future, it needs to be proven as an effective soil amendment in its own right, rather than just a sink for carbon (Sohi et al., 2009). Matching biochars to specific soils will be a critical step in applying this soil amendment more widely. Acknowledgements The authors gratefully acknowledge the financial support from the Austrian Research Promotion Agency (FFG, project no. 825438). The authors would also like to thank the University of Natural Resources and Life Sciences and the Austrian Institute of Technology (AIT) for supplying the facilities and equipment required to undertake the study. Special

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thanks also goes to Christian Mayer for his technical support throughout the project.

References Abel, S., Peters, A., Trinks, S., Schonsky, H., Facklam, M., Wessolek, G., 2013. Impact of biochar and hydrochar addition on water retention and water repellency of sandy soil. Geoderma 202–203, 183–191. Basso, A.S., Miguez, F.E., Laird, D.A., Horton, R., Westgate, M., 2013. Assessing potential of biochar for increasing water-holding capacity of sandy soils. Glob. Chang. Biol. Bioenergy 5, 132–143. Bearden, B.N., Petersen, L., 2000. Influence of arbuscular mycorrhizal fungi on soil structure and aggregate stability of a vertisol. Plant Soil 218, 173–183. Bohn, H.L., McNeal, B.L., 1983. A qualitative derivation of the diffuse double layer. J. Agric. Educ. 12, 26–28. Brewer, C.E., Chuang, V.J., Masiello, C.A., Gonnermann, H., Gao, X., Dugan, B., Driver, L.E., Panzacchi, P., Zygourakis, K., Davies, C.A., 2014. New approaches to measuring biochar density and porosity. Biomass Bioenergy 66, 176–185. Castellini, M., Giglio, L., Niedda, M., Palumbo, A.D., Ventrella, D., 2015. Impact of biochar addition on the physical and hydraulic properties of a clay soil. Soil Tillage Res. 154, 1–13. Crane-Droesch, A., Abiven, S., Jeffery, S., Torn, M.S., 2013. Heterogeneous global crop yield response to biochar: a meta-regression analysis. Environ. Res. Lett. 8 (4). de Melo Carvalho, M.T., de Holanda Nunes Maia, A., Madari, B.e., Bastiaans, L., van Oort, P.A.J., Heinemann, A.B., Soler da Silva, M.A., Petter, F.A., Marimon Jr., B.H., Meinke, H., 2014. Biochar increases plant-available water in a sandy loam soil under an aerobic rice crop system. Solid Earth 5, 939–952. Demirbas, A., 2004. Effects of temperature and particle size on bio-char yield from pyrolysis of agricultural residues. J. Anal. Appl. Pyrolysis 72, 243–248. Devereux, R.C., Sturrock, C.J., Mooney, S.J., 2012. The effects of biochar on soil physical properties and winter wheat growth. Earth and Environmental Science Transactions of the Royal Society of Edinburgh. vol. 103, pp. 13–18. Fletcher, A.J., Smith, M.A., Heinemeyer, A., Lord, R., Ennis, C.J., Hodgson, E.M., Farrar, K., 2014. Production factors controlling the physical characteristics of biochar derived from phytoremediation willow for agricultural applications. Bioenergy Res. 7, 371–380. Food and Agriculture Organization of the United Nations (FAO), 2004A. Drought-resistant Soils: Optimization of Soil Moisture for Sustainable Plant Production. FAO Information Division, Rome. Githinji, L., 2014. Effect of biochar application rate on soil physical and hydraulic properties of a sandy loam. Arch. Agron. Soil Sci. 60, 457–470. Hardie, M., Clothier, M., Bound, S., Oliver, G., Close, D., 2014. Does biochar influence soil physical properties and soil water availability? Plant Soil 376, 347–361. Herath, H.M.S.K., Camps-Arbestain, M., Hedley, M., 2013. Effect of biochar on soil physical properties in two contrasting soils: an Alfisol and an Andisol. Geoderma 209, 188–197. Jeffery, S., Verheijen, F.G.A., van der Velde, M., Bastos, A.C., 2011. A quantitative review of the effects of biochar application to soils on crop productivity using meta-analysis. Ecosyst. Environ. 144, 175–187. Joseph, S.D., Camps-Arbestain, K.M., Lin, Y., Munroe, P., Chia, C.H., Hook, J., van Zwieten, L., Kimber, S., Cowie, A., Singh, B.P., Lehmann, J., Foidl, N., SmernikI, R.J., Amonette, J.E., 2010. An investigation into the reactions of biochar in soil. Aust. J. Soil Res. 48, 501–515. Kammann, C.I., Linsel, S., Gößling, J.W., Koyro, H.W., 2011. Influence of biochar on drought tolerance of Chenopodium quinoa: wild and on soil–plant relations. Plant Soil 345, 195–210. Kloss, S., Zehetner, F., Dellantonio, A., Hamid, R., Ottner, F., Liedtke, V., Schwanninger, M., Gerzabek, M.H., Soja, G., 2012. Characterization of slow pyrolysis biochars: effect of feedstocks and pyrolysis temperature on biochar properties. J. Environ. Qual. 41, 990–1000. Kloss, S., Zehetner, F., Wimmer, B., Buecker, J., Rempt, F., Soja, G., 2014. Biochar application to temperate soils: effects on soil fertility and crop growth under greenhouse conditions. J. Plant Nutr. Soil Sci. 177, 3–15. Koide, R.T., Nguyen, B.T., Skinner, R.H., Dell, C.J., Peoples, M.S., Adler, P.R., Drohan, P.J., 2014. Biochar amendment of soil improves resilience to climate change. Glob. Chang. Biol. 7, 1757-1707. Laird, D.A., Fleming, P., Davis, D.D., Horton, R., Wang, B., Karlen, D.L., 2010. Impact of biochar amendments on the quality of a typical Midwestern agricultural soil. Geoderma 158, 443–449. Lehmann, J., Gaunt, J., Rondon, M., 2006. Bio-char sequestration in terrestrial ecosystems a review. Mitig. Adapt. Strateg. Glob. Chang. 11, 403–427. Lehmann, L., Rillig, M.C., Thies, J., Masiello, C.A., Hockaday, W.C., Crowley, D., 2011. Biochar effects on soil biota - a review. Soil Biol. Biochem. 43, 1812–1836. Lei, O., Zhang, R., 2013. Effects of biochars derived from different feedstocks and pyrolysis temperatures on soil physical and hydraulic properties. J. Soils Sediments 13, 1561–1572. Liang, B., Lehmann, J., Sohi, S.P., Thies, J.E., O'Neill, B., Trujillo, L., Gaunt, J., Solomon, D., Grossman, J., Neves, E.G., Luizão, F.J., 2010. Black carbon affects the cycling of nonblack carbon in soil. Org. Geochem. 41, 206–213. Liu, X., Feng, P., Zhang, X., 2012. Effect of biochar on soil aggregates in the Loess Plateau: results from incubation. Int. J. Agric. Biol. 14, 975–979. Liu, X., Zhang, A., Ji, C., Joseph, S., Bian, R., Li, L., Pan, G., Paz-Ferreiro, J., 2013. Biochar's effect on crop productivity and the dependence on experimental conditions - a metaanalysis of literature data. Plant Soil 373, 583–594.

102


Liu, Z., Chen, X., Jing, Y., Li, Q., Zhang, J., Huang, Q., 2014. Effects of biochar amendment on rapeseed and sweet potato yields and water stable aggregate in upland red soil. Catena 123, 45–51. Lu, S.G., Sun, F.F., Zong, Y.T., 2014. Effect of rice husk biochar and coal fly ash on some physical properties of expansive clayey soil (Vertisol). Catena 114, 37–44. Lychuk, T.E., Izaurralde, R.C., Hill, R.L., McGill, W.B., Williams, J.R., 2014. Biochar as a global change adaptation: predicting biochar impacts on crop productivity and soil quality for a tropical soil with the Environmental Policy Integrated Climate (EPIC) model. Mitig. Adapt. Strateg. Glob. Chang. http://dx.doi.org/10.1007/s11027-014-9554-7. McBride, M.B., 1997. A critique of diffuse double layer models applied to colloid and surface chemistry. Clay Clay Miner. 45, 598–608. Mukherjee, A., Lal, R., Zimmerman, A.R., 2014. Effects of biochar and other amendments on the physical properties and greenhouse gas emissions of an artificially degraded soil. Sci. Total Environ. 487, 26–36. Murer, E.J., Baumgarten, A., Eder, G., Gerzabek, M.H., Kandeler, E., Rampazzo, N., 1993. An improved sieving machine for estimation of soil aggregate stability (SAS) Soil Structure/Soil Biota Interrelationships. Geoderma 56, 539–547. Novak, J.M., Lima, I., Xing, B., Gaskin, J.W., Steiner, C., Das, K.C., Ahmedna, M., Rehrah, D., Watts, D.W., Busscher, W.J., Schomberg, H., 2009. Characterization of designer biochar produced at different temperatures and their effects on a loamy sand. Ann. Environ. Sci. 3, 195–206. Peake, L.R., Reid, B.J., Tang, X., 2014. Quantifying the influence of biochar on the physical and hydrological properties of dissimilar soils. Geoderma Vol. 235– 236, 182–190. Pietikainen, J., Kiikkila, O., Fritze, H., 2000. Charcoal as a habitat for microbes and its effect on the microbial community of the underlying humus. Oikos 89, 231–242.

Quin, P.R., Cowie, A.L., Flavel, R.J., Keen, B.P., Macdonald, L.M., Morris, S.G., Singh, B.P., Young, I.M., Van Zwieten, L., 2014. Oil mallee biochar improves soil structural properties - a study with X-ray micro-CT. Agric. Ecosyst. Environ. 191, 142–149. Richards, L.A., 1948. Porous plate apparatus for measuring moisture retention and transmission by soils. Soil Sci. 66, 105–110. Sohi, S., Lopez-Capel, E., Krull, E., Bol, R., 2009. Biochar, climate change and soil: a review to guide future research. CSIRO Land and Water Science Report. 5, pp. 17–31. Soinne, H., Hovi, J., Tammeorg, P., Turtola, E., 2014. Effect of biochar on phosphorus sorption and clay soil aggregate stability. Geoderma 162–167. Streck, E.V., Cogo, N.P., 2003. Reconsolidation of the soil surface after tillage discontinuity, with and without cultivation, related to erosion and its prediction with rusle. Rev. Bras. Ciênc. Solo 27, 141–151. Sun, F., Lu, S., 2014. Biochars improve aggregate stability, water retention, and pore-space properties of clayey soil. J. Plant Nutr. Soil Sci. 177, 26–33. Ulyett, J., Sakrabani, R., Kibblewhite, M., Hann, M., 2014. Impact of biochar addition on water retention, nitrification and carbon dioxide evolution from two sandy loam soils. Eur. J. Soil Sci. 65, 96–104. van Zwieten, L., Kimber, S., Morris, S., Chan, K., Downie, A., Rust, J., Joseph, S., Cowie, A., 2009. Effects of biochar from slow pyrolysis of papermill waste on agronomic performance and soil fertility. Plant Soil 327, 235–246. Verheijen, F.G.A., Jeffery, S., Bastos, A.C., van der Velde, M., Diafas, I., 2010. Biochar application to soils - a critical scientific review of effects on soil properties, processes and functions. EUR 24099 EN. Office for the Official Publications of the European Communities, Luxembourg.