effect of biochar on phosphorus sorption and clay soil aggregate stability

6
Effect of biochar on phosphorus sorption and clay soil aggregate stability Helena Soinne a,c, , Jarkko Hovi b , Priit Tammeorg b , Eila Turtola c a Helsinki University Centre for Environment (HENVI)/Department of Food and Environmental Sciences, P.O. Box 27, FIN-00014, University of Helsinki, Finland b Department of Agricultural Sciences, Plant Production Sciences, P.O. Box 27, FIN-00014, University of Helsinki, Finland c MTT Agrifood Research Finland, Plant Production Research, FI-31600, Jokioinen, Finland abstract article info Article history: Received 6 May 2013 Received in revised form 21 December 2013 Accepted 30 December 2013 Available online 25 January 2014 Keywords: Phosphorus Phosphate Soil structure Erosion Biochar Soil structure is one of the key properties affecting the productivity of soils and the environmental side effects of agricultural soils. Poor surface soil structure increases the risk of soil erosion by water and eroded clay-sized par- ticles can carry adsorbed phosphorus (P) to the surface waters, thus inducing eutrophication of receiving water- ways. Management practices, e.g. reduced tillage, used to reduce erosion can lead to enrichment of P in the uppermost soil layers, which leads to elevated risk for dissolved P loss in the runoff water. In this study, we aimed to identify whether biochar (BC) could be used to reduce clay soil erosion by improving aggregate stability. Moreover, we tested whether the BC addition would change the P sorption afnity of the soil and help to reduce the loss of dissolved P. One sandy and two clayey soils were amended with BC (0, 15 and 30 t ha -1 ) and after a 3- week incubation, a wet-sieving method was used to measure the release of colloidal particles and the stability of aggregates. The sorption of P onto soil surfaces was estimated with a Q/I (quantity/intensity) plot technique. The BC used here had a very low P sorption afnity and the BC addition did not increase the sorption of P in incubated soils. However, for the two clayey soils, the BC additions increased aggregate stability and reduced detachment of colloidal material. The BC thus induced changes in soil properties that could be benecial for erosion control and thereby aid in reducing particulate P losses from agricultural elds. © 2014 Elsevier B.V. All rights reserved. 1. Introduction In southern Finland, about 50% of the cultivated soils are clayey soils that have received phosphorus (P) fertilization for decades. Intensive fertilization leads to high concentrations of labile P in the surface soil, in- creasing the risk of particulate and dissolved P losses in surface runoff (Sharpley and Withers, 1994; Turtola and Yli-halla, 1999). Disturbance of cultivated soil by tillage weakens the soil aggregate structure (Oades, 1993) and promotes erosion, which is further enhanced by lower inltration rate and increased surface runoff. While clay particles are important for aggregate formation and a minimum of 15% clay con- tent of the soil is needed for the abiotic development of aggregate struc- ture (reviewed by Oades, 1993), presently in turn, the poor structure of clay soils leads to low strength under wet conditions and high risk of clay dispersion (Munkholm, 2011; Watts and Dexter, 1997). When dis- persed into water, clay-sized material can travel long distances carrying pollutants and nutrients, thus inducing eutrophication of receiving waterways. Reduced tillage has been effectively used to reduce the erosion and loss of particulate P from clayey elds (Puustinen et al., 2005; Turtola et al., 2007). However, the enrichment of P in the uppermost soil layer of the no-till soils may increase the loss of dissolved P in surface runoff (Puustinen et al., 2005; Tiessen et al., 2010). Tiessen et al. (2010) report- ed an increase of 12% in the annual total P export, due to the increase in dissolved P loss after converting to conservation tillage. Lehmann (2007) suggested that biochar (BC), a carbon-rich material produced by pyrolysis under anoxic conditions, can sorb phosphates, and recently results of reduced P leaching from BC-treated soils have been published. Laird et al. (2010) found that BC addition reduced P leaching after manure addition and Beck et al. (2011) found signicant reduction of total P in greenroof discharge water when the soil contained 7% BC. BC has been reported to have many positive effects on soil quality, e.g. increase in soil water retention, cation-exchange capacity (CEC) and microbial activity (Lehmann et al., 2006; Sohi et al., 2010). Similar characteristics have been attributed to the increase in natural soil or- ganic matter (OM) content. Liang et al. (2006) showed that application of BC may result in even larger CEC than does natural OM due to its larger surface area, higher negative surface-charge and surface charge density. Natural OM plays an important role in soil structure formation and a strong positive correlation between OM and soil aggregate stabil- ity has been reported (Chaney and Swift, 1984; Heinonen, 1955; Le Bissonnais and Arrouays, 1997; Tisdall and Oades, 1982). Despite the Geoderma 219220 (2014) 162167 Corresponding author at: Helsinki University Centre for Environment (HENVI)/ Department of Food and Environmental Sciences, P.O. Box 27, FIN-00014 University of Helsinki, Finland. Tel.: +358 9 19158324. E-mail address: Helena.soinne@helsinki.(H. Soinne). 0016-7061/$ see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geoderma.2013.12.022 Contents lists available at ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma

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Page 1: Effect of biochar on phosphorus sorption and clay soil aggregate stability

Geoderma 219–220 (2014) 162–167

Contents lists available at ScienceDirect

Geoderma

j ourna l homepage: www.e lsev ie r .com/ locate /geoderma

Effect of biochar on phosphorus sorption and clay soil aggregate stability

Helena Soinne a,c,⁎, Jarkko Hovi b, Priit Tammeorg b, Eila Turtola c

a Helsinki University Centre for Environment (HENVI)/Department of Food and Environmental Sciences, P.O. Box 27, FIN-00014, University of Helsinki, Finlandb Department of Agricultural Sciences, Plant Production Sciences, P.O. Box 27, FIN-00014, University of Helsinki, Finlandc MTT Agrifood Research Finland, Plant Production Research, FI-31600, Jokioinen, Finland

⁎ Corresponding author at: Helsinki University CentDepartment of Food and Environmental Sciences, P.O. BHelsinki, Finland. Tel.: +358 9 19158324.

E-mail address: [email protected] (H. Soinne)

0016-7061/$ – see front matter © 2014 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.geoderma.2013.12.022

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 May 2013Received in revised form 21 December 2013Accepted 30 December 2013Available online 25 January 2014

Keywords:PhosphorusPhosphateSoil structureErosionBiochar

Soil structure is one of the key properties affecting the productivity of soils and the environmental side effects ofagricultural soils. Poor surface soil structure increases the risk of soil erosion bywater and eroded clay-sized par-ticles can carry adsorbed phosphorus (P) to the surface waters, thus inducing eutrophication of receiving water-ways. Management practices, e.g. reduced tillage, used to reduce erosion can lead to enrichment of P in theuppermost soil layers, which leads to elevated risk for dissolved P loss in the runoff water. In this study, weaimed to identifywhether biochar (BC) could be used to reduce clay soil erosion by improving aggregate stability.Moreover, we tested whether the BC addition would change the P sorption affinity of the soil and help to reducethe loss of dissolved P. One sandy and two clayey soilswere amendedwith BC (0, 15 and 30 t ha−1) and after a 3-week incubation, a wet-sieving methodwas used to measure the release of colloidal particles and the stability ofaggregates. The sorption of P onto soil surfaces was estimated with a Q/I (quantity/intensity) plot technique. TheBC used here had a very low P sorption affinity and the BC addition did not increase the sorption of P in incubatedsoils. However, for the two clayey soils, the BC additions increased aggregate stability and reduced detachment ofcolloidal material. The BC thus induced changes in soil properties that could be beneficial for erosion control andthereby aid in reducing particulate P losses from agricultural fields.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

In southern Finland, about 50% of the cultivated soils are clayey soilsthat have received phosphorus (P) fertilization for decades. Intensivefertilization leads to high concentrations of labile P in the surface soil, in-creasing the risk of particulate and dissolved P losses in surface runoff(Sharpley and Withers, 1994; Turtola and Yli-halla, 1999). Disturbanceof cultivated soil by tillage weakens the soil aggregate structure(Oades, 1993) and promotes erosion, which is further enhanced bylower infiltration rate and increased surface runoff. While clay particlesare important for aggregate formation and a minimum of 15% clay con-tent of the soil is needed for the abiotic development of aggregate struc-ture (reviewed by Oades, 1993), presently in turn, the poor structure ofclay soils leads to low strength under wet conditions and high risk ofclay dispersion (Munkholm, 2011; Watts and Dexter, 1997). When dis-persed into water, clay-sizedmaterial can travel long distances carryingpollutants and nutrients, thus inducing eutrophication of receivingwaterways.

re for Environment (HENVI)/ox 27, FIN-00014 University of

.

ghts reserved.

Reduced tillage has been effectively used to reduce the erosion andloss of particulate P from clayey fields (Puustinen et al., 2005; Turtolaet al., 2007). However, the enrichment of P in the uppermost soil layerof the no-till soils may increase the loss of dissolved P in surface runoff(Puustinen et al., 2005; Tiessen et al., 2010). Tiessen et al. (2010) report-ed an increase of 12% in the annual total P export, due to the increasein dissolved P loss after converting to conservation tillage. Lehmann(2007) suggested that biochar (BC), a carbon-rich material producedby pyrolysis under anoxic conditions, can sorb phosphates, and recentlyresults of reduced P leaching fromBC-treated soils have been published.Laird et al. (2010) found that BC addition reduced P leaching aftermanure addition and Beck et al. (2011) found significant reduction oftotal P in greenroof discharge water when the soil contained 7% BC.

BC has been reported to have many positive effects on soil quality,e.g. increase in soil water retention, cation-exchange capacity (CEC)and microbial activity (Lehmann et al., 2006; Sohi et al., 2010). Similarcharacteristics have been attributed to the increase in natural soil or-ganic matter (OM) content. Liang et al. (2006) showed that applicationof BC may result in even larger CEC than does natural OM due to itslarger surface area, higher negative surface-charge and surface chargedensity. Natural OM plays an important role in soil structure formationand a strong positive correlation between OM and soil aggregate stabil-ity has been reported (Chaney and Swift, 1984; Heinonen, 1955; LeBissonnais and Arrouays, 1997; Tisdall and Oades, 1982). Despite the

Page 2: Effect of biochar on phosphorus sorption and clay soil aggregate stability

163H. Soinne et al. / Geoderma 219–220 (2014) 162–167

many positive effects of BC on soil fertility, the knowledge of the effectsof BC addition on soil structure and erosion is limited.

Improving the surface soil aggregate stability would aid in reducingsoil erosion andmitigate the loss of particulate P fromagriculturalfields,while enhancing the sorption of P would reduce the risk of surface run-off loss of dissolved P. The aim of this study was to identify whether BChas the potential to a) increase soil aggregate stability and/or b) en-hance sorption of phosphate–phosphorus (PO4–P) and thus reducethe loss of particulate P and dissolved PO4–P from cultivated soils inFinland.

2. Material and methods

2.1. Biochar

BC was produced by pyrolyzing a mixture of Norway spruce (Piceaabies (L.) H. Karst.) and Scots pine (Pinus sylvestris L.) chips in a continu-ously pressurized carbonizer (Preseco Oy, Finland). The temperature ofthe carbonizer was constant at 550–600 °C during the 10–15 min pyrol-ysis process. After grinding, the particle size distribution (byweight)was38% b0.25 mm, 25.9%, 0.25–1 mm, 35.9% 1–5 mm and 0.1% 5–10 mm.For the incubation study, the BCwas sieved to obtain a b0.2-mm fractionthat was evenly mixed with the soil volume.

The pHH2O of BC was 8.9 (1:5, v/v). The total C content was903 g kg−1 (dry combustion with a VarioMax CN analyzer;Elementar Analysensysteme GmbH, Hanau, Germany). The ash content(23 g kg−1) was determined by dry combustion by ramping the tem-perature to 500 °C in 2 h, and maintaining it for 3 h (Jones and Steyn,1973) in a laboratory muffle furnace. The ash was dissolved with100 mL 0.2 M HCl and boiled for 30 min. The concentrations of P andmajor cations determined with an inductively coupled plasma opticalemission spectroscopy (Thermo-Fisher iCAP 6000; Thermo FisherScientific, Waltham, MA, USA) were: P: 0.2 g kg−1, Ca: 4.8 g kg−1, K:2.8 g kg−1, Fe: 0.4 g kg−1 and Mg: 0.8 g kg−1. The Brunauer EmmettTeller specific surface area (BET SSA) measured using nitrogen adsorp-tion technique with a Micromeritics Flowsorb 2300 gas adsorption an-alyzer was 11.8 m2 g−1.

2.2. Soil samples

The effect of BC on P sorption was tested using two fine-texturedsoils from Hyvinkää (silty clay loam, SiCL) and Jokioinen (Clay)(Endogleyic Stagnosol and Cutanic Vertic Luvisol, respectively, IUSSWorking GroupWRB, 2007), and one coarse-textured soil fromHelsinki(Sand, Endogleyic Umbrisol, IUSSWorking GroupWRB, 2007) sampledrandomly from the surface soil (Clay and Sand 0–10 cm; SiCL 0–5 cm)and mixed to form a bulked sample (Table 1). The aggregate stabilitytests were done only with the fine-textured soils (Clay and SiCL). Thefield moist samples were sieved (5 mm), homogenized and stored at+5 °C prior to the experiment.

2.3. Incubation experiment

The incubation experiment was carried out in the laboratory usingplastic 300-cm3 incubation containers. We used three BC applicationlevels corresponding to 0, 15 and 30 t ha−1 mixed in the 20-cm surface

Table 1Organic carbon (OC) content and texture of soils used in the incubation study.

Soils Texture OC (%)

Sand (%) Silt (%) Clay (%)

Clay 13 39 48 2.3SiCL 3 60 38 6.7Sand 92 6 2 2.8

layer and two moisture treatments; half of the incubation containerswere kept moist and half were left to dry freely during the incuba-tion and remoistened before the end of the incubation period. Thus,for each soil we used three BC rates with two moisture treatmentsreplicated three times. In the containers, 150 g soil (dry matter, DM)was weighed and in the treatments with added BC it was mixed in thesoil evenly. Next, the soil in the containers was compacted with200 g cm−2 pressure to obtain the following bulk densities: Clay,0.89; SiCL, 0.72; Sand, 0.96 g cm−3. The soil–water content during theincubation was set to field capacity (FC; volumetric water content inpF 2 determined prior to the experiment from compacted unamendedsoils with a pressure-plate apparatus, corresponding to 25%, 32% and18% for Clay, SiCL and Sand, respectively). Thewater contentwas adjust-ed by adding deionized water gently to the soil surface. The containerswere covered with plastic film and incubated at constant temperature(+20 °C). After 4 days at constant moisture, half of the incubation con-tainers were uncovered and left to dry freely for 16 (Clay and SiCL) or 19(Sand) days to reach moisture contents of 10% for Clay, 15% for SiCL and5% for Sand. During the last 3 days of the incubation, the dried soils wereslowly remoistened to the FC. The remaining containers were kept at FCthroughout the 20-day (Clay and SiCL) or 23-day (Sand) incubation. Themoisture content in the containers during the incubationwasmonitoredwith daily weighing, and deionized water was added accordingly. Final-ly, the incubation containers were inverted to loosen the structure, andthe pH and electrical conductivity (EC)weremeasured (soil:water 1:2.5,taking account of the initial moisture of the soil).

2.4. Aggregate stability test

Following incubation, the aggregate stability of the amended clayeysoils was testedwith thewet-sievingmethod as follows: 4-g aggregates(dry weight, DW) were placed on a 0.25-mm sieve and left to standin 100 ml of deionized water for 15 min. After a 15-min saturationtime, the wet-sieving apparatus (Eijkelkamp Agrisearch Equipment,Giesbeek, The Netherlands) was set in motion and the aggregateswere dipped into the water about 95 times for 3 min. The water anddetached soil material were then transferred into centrifuge tubes andthe suspension was left to settle for 21 h. A 25-ml sample was pipettedfrom the surface of the settled suspension into a turbidimeter cuvetteand the turbidity was measured by a HACH 2100N turbidimeter (HachCo. Loveland, CO, USA). The suspension was returned to the centrifugetube and centrifuged for 10 min (2600 g). The settled soil was furtherwet-sieved into b0.06-mm and 0.06–0.25-mm size fractions. The soilfractions were oven-dried (105 °C) and weighed to determine theirmasses. The mass of water-stable aggregates (WSA) was calculated asthe difference between the mass originally taken for the aggregate sta-bility analysis and the mass of the soil material detached during thewet-sieving.

2.5. Phosphorus sorption

One gram of incubated soil or soil + BC (DM) was shaken for21 h with 50 ml of deionized water, centrifuged and filteredthrough a 0.2-μm Nuclepore© polycarbonate filter to determinethe water-Extractable P (Pw) content. For the Q/I-plot, one gramof soil or soil + BC (DM) was mixed with 50 ml of a P solutioncontaining 0, 0.5, 1, 1.5 and 2 mg P l−1. After a 21-h equilibration,the suspensions were centrifuged and filtered through a 0.2 μmNuclepore© polycarbonate filter. For BC, the experiments wereperformed with a BC-to-solution ratio of 1:100, using P solutionsof 0, 1 and 2 mg P l−1. The filtrate P concentrations were mea-sured with a standard colorimetric flow injection analysis using aLachat QuickChem 8000 (Lachat Instruments, Hach Co., Loveland,CO.). The amount of adsorbed or desorbed P was calculated fromthe difference in the P concentrations in the solution before (I0)and after (I) the equilibration. For each incubation container, a

Page 3: Effect of biochar on phosphorus sorption and clay soil aggregate stability

Table 3Electric conductivity (dS m−1) measured from water suspension (1:2.5 v:v) afterincubation at 20 °C with biochar additions corresponding to 0, 15 and 30 t ha−1, andsummary of two-way variance analysis (biochar addition x drying treatment; SE =standard error, LSD = least significant difference) for EC. Significant F-values at p b 0.05(bold).

Biochar(t ha−1)

Treatment EC (dS m−1)

Clay SiCL Sand

0 Moist 0.16 0.15 0.09Dried 0.15 0.14 0.09

15 Moist 0.11 0.11 0.05Dried 0.11 0.11 0.06

30 Moist 0.08 0.10 0.04Dried 0.08 0.10 0.04

SE (n = 3) 0.01 0.01 0.01LSD (p b 0.05) 0.01 0.02 0.01

F-values for Biochar (df 2) 98 17 35Drying (df 1) 0.7 0.8 0.7Interaction (df 2) 0.2 0.2 0.2

164 H. Soinne et al. / Geoderma 219–220 (2014) 162–167

total of 15 Q/I points (five P concentrations in triplicate) wereobtained and for the BC the number of points was six (threeconcentrations in duplicate). The Q/I points were fitted using amodification of the Freundlich adsorption Eq. (1) (Russel andPrescott, 1916)

Q ¼ Q0 þ k� In ð1Þ

where Q is the amount of P sorbed or, when negative values areobserved, desorbed, Q0 is a parameter for instantly labile P(Fitter and Sutton, 1975), I is the P concentration in the solutionafter the 21-h equilibration, and k and n are fitting parameters.

2.6. Statistical analyses

Statistical analysis was carried out as two-way analysis of variance(ANOVA) with BC level (3) and soil moisture (2) as fixed effects, sepa-rately for each soil. Each treatment was replicated three times and sothe total number of observations for each parameter measured was18. For pairwise comparisons, the least significant difference (LSD)was calculated for each parameter for each soil. All statistical analyseswere conducted using PASW Statistics 18 (SPSS Corp.).

3. Results

During the incubation, the BC amendments did not affect the dryingspeed of the soils, measured as gravimetric moisture, when left to dryfreely. During drying, the water content of the Clay decreased to 40%of the water content at pF 2, whereas for the SiCL, the respectivewater contentwas 47%. The Sand dried to 28% of the original water con-tent at pF2. The BC addition increased the soil pH over that of the controlsoils in treatments, in which the soil was maintained moist (Table 2).For soils dried during the incubation, the BC raised the pH over that ofthe control treatment in the Clay and SiCL soils. For the Clay and SiCL,drying alone raised the pH values over those of the continuously moistsoils.

The BC addition corresponding to 15 t ha−1 decreased electricalconductivity (EC) in all soils for both moisture treatments (Table 3).However, between the BC additions of 15 t ha−1 and 30 t ha−1, theconductivity was significantly different for both moisture treatmentsonly in the Clay soil. At any given level of BC addition, there was no dif-ference in EC between the continuously moist and dried soils (Table 3).

The concentration of Pw 1:100 in the BC was 5 mg kg−1. For soils, theSiCL had the lowest Pw 1:50 concentration and Clay the highest. Dryingdid not affect the Pw concentrations compared with those of soils keptcontinuously moist (Table 4). Addition of BC did not affect the amountsof Pw in Clay or SiCL thatwere kept continuouslymoist. However, for the

Table 2Soil pHmeasured fromwater suspension (1:2.5 v:v) after incubation at 20 °Cwith biochar(BC) additions corresponding to 0, 15 and 30 t ha−1, and summary of two-way varianceanalysis (BC addition x drying treatment; SE = standard error, LSD = least significantdifference) for pH. Significant F-values at p b 0.05 (bold).

Biochar(t ha−1)

Treatment pH

Clay SiCL Sand

0 Moist 5.4 6.2 5.9Dried 5.6 6.3 5.9

15 Moist 5.5 6.3 6.1Dried 5.6 6.3 6.1

30 Moist 5.7 6.4 6.3Dried 5.7 6.4 6.3

SE (n = 3) 0.033 0.006 0.009LSD (p b 0.05) 0.072 0.012 0.019

F-values for Biochar (df 2) 12.0 407 937Drying (df 1) 11.7 130 2.4Interaction (df 2) 5.3 6.6 1.6

dried Sand, BC additions of 15 and 30 t ha−1 increased the Pw over thatof the unamended control.

The BC used in this experiment had much lower P sorption affinitythan that of any of the soils (Fig. 1), and its addition did not changethe P sorption affinity of the Clay or SiCL (Fig. 1a and b). However, forSand, the Q/I plots showed lower sorption affinity after BC addition(Fig. 1c). The effect of BC addition on the Q/I plots was similar formoist and dried soils (data not shown).

A BC addition of 30 t ha−1 to the clayey soils (Clay and SiCL) in-creased the amount of WSA in soils kept moist (7–11%) and in soilsdried during incubation (8–9%) comparedwith those of the unamendedcontrol treatment (Fig. 2). Formoist and dried SiCL soils, a BC addition of15 t ha−1 differed from that in the unamended soils. Only in the Clay,drying increased the WSA in the control and in the BC-amended soils.

In the BC-amended soil, the detachment of colloids during wet-sieving expressed as turbidity (nephelometric turbidity units NTU g−1)decreased (Table 5). For both soils, the 15 t ha−1 application was suffi-cient to lower the turbidity values (for Clay and SiCL 41% and 19%,respectively) and the application of 30 t ha−1 did not further affect thecolloid detachment. However, for Clay soil, the drying treatment wasas effective as the BC applications in reducing the detachment ofcolloid-sized particles and thus in improving the aggregate stability.While BC reduced the mass of small-sized soil particles that were de-tached during wet-sieving, there were no significant changes in de-tached mass of the N0.06-mm soil particles. The soils with BC additionwere also clearly darker and the aggregates did not adhere to eachother, making the textures appear lighter.

Table 4Water-extractable (1:50 w:v) phosphate–phosphorus (PO4–P) in soils after incubation at20 °C with biochar additions corresponding to 0, 15 and 30 t ha−1, and summary of two-way variance analysis (biochar addition x drying treatment; SE = standard error,LSD = least significant difference) for PO4–P. Significant F-values at p b 0.05 (bold).

Biochar(t ha−1)

Treatment PO4–P (mg kg−1)

Clay SiCL Sand

0 Moist 13.9 6.2 8.4Dried 13.1 6.1 7.5

15 Moist 12.8 6.9 7.8Dried 12.9 6.5 8.9

30 Moist 13.9 6.6 9.1Dried 15.3 6.5 9.5

SE (n = 3) 0.6 0.3 0.4LSD (p b 0.05) 1.2 0.9

F-values for Biochar (df 2) 5.1 0.3 5.9Drying (df 1) 0.3 0.5 0.3Interaction (df 2) 1.9 0.9 2.9

Page 4: Effect of biochar on phosphorus sorption and clay soil aggregate stability

-60

-40

-20

0

20

40

60

80

0 1 2mg

P k

g-1

a)

-60

-40

-20

0

20

40

60

80

0 1 2

c)

-60

-40

-20

0

20

40

60

80

0 1 2

mg P l-1

0 t/ha15 t/ha30 t/haBC 0 t/ha15 t/ha30 t/haBC

b)

Fig. 1. Phosphorus quantity/intensity (Q/I) plots for a) Clay b) SiCL and BC c) Sand. All soils were incubated with biochar (BC) corresponding to additions of 0, 15 and 30 t ha−1. The Q/Ipoints determined are marked separately (three for each concentration and each BC treatment) and the line is the fitted Freundlich equation.

165H. Soinne et al. / Geoderma 219–220 (2014) 162–167

4. Discussion

The relatively small but consistent increase in soil pH (0.1–0.4 units)due to the BC addition was similar to that reported in the literature forwoody BCs with amendment rates of 10–40 t ha−1 (Streubel et al.,2011). The more pronounced increase in pH for coarse-textured sandcompared with that of the clayey soils was probably due to the lowerbuffering capacity of coarser soils against pH changes. Increase in soilpH reportedly enhances the solubility of PO4–P (Goldberg and Sposito,1984; Hartikainen and Yli-Halla, 1996) and in here; the dried sandysoil demonstrated an increase in Pw together with an increase in pH.On the other hand, Parvage et al. (2013) reported decreased Pw concen-trations in BC-amended soils that coincided with a BC-induced increasein pH.

Beaton et al. (1960) found that charcoal has the capacity to sorbP and suggested a mechanism of hydrogen bonding between the

0

10

20

30

40

50

60

70

80

90

100

Moist Dried

Clay

%

SE (n=3) 1.7LSD (p<0.05) 3.6F values for Biochar 6

Drying 15Interaction 0.3

0 t ha-1

Fig. 2. Water-stable aggregates (WSA, %) in two different clayey soils (Clay and SiCL) after a 3-summary of two-way variance analysis (BC addition x drying treatment) for WSA. Significantp b 0.05) for each soil.

phosphate and charcoal surface. However, the ability of charcoal tosorb P was limited compared with that of soil, which is in line withour results. The BC used in the present study had a relatively low surfacearea and its amendment did not increase the ability of soil to sorb P inany of the incubated soils. In fact, for the coarse-textured Sand, BC addi-tions reduced the sorption of P leading to the tendency for BC-amendedsoil to maintain higher P concentration in the surrounding solutioncompared with that of soil without BC addition. Considering the lowsorption capacity of BC, the mixture of soil and BC should indeed showa lower sorption capacity than the original soil. However, since the BCproportion was only 3% of the mass at maximum, not all of the differ-ences detected in the Q/I plots could be explained solely by mixing ofthe materials. Nelson et al. (2011) reported a temporary increase in Pavailability in soil after BC addition and suggested inhibition of P sorp-tion byBC. Literature results suggest that natural OMmay increase P sol-ubility through competition with P for sorption sites (Bolan et al., 1994;

Moist Dried

SiCL

SE (n=3) 1.0LSD (p<0.05) 2.2F values for Biochar 28

Drying 2Interaction 1

30 t ha-115 t ha-1

week incubation with biochar (BC) (additions corresponding to 0, 15 and 30 t ha−1) andF-values at p b 0.05 (bold). The error bars represent the least significant difference (LSD,

Page 5: Effect of biochar on phosphorus sorption and clay soil aggregate stability

Table 5Turbidity (NTU g−1, Nephelometric Turbidity Unit g−1), andmass of soilmaterial (g g−1) detached from1 g of soil or soil + biochar (BC)mixture duringwet sieving after incubatingwithBC additions corresponding to 0, 15 and 30 t ha−1, and summary of two-way variance analysis (BC addition × drying treatment; SE = standard error, LSD = least significant difference)for the parameters measured. Significant F-values at p b 0.05 (bold).

Biochar(t ha−1)

Treatment Clay SiCL

Turbidity b0.06 mm N0.06 mm Turbidity b0.06 mm N0.06 mm

(NTU g−1) (g g−1) (NTU g−1) (g g−1)

0 Moist 158 0.22 0.03 62 0.25 0.04Dried 80 0.17 0.03 67 0.25 0.04

15 Moist 94 0.19 0.03 50 0.20 0.03Dried 66 0.15 0.03 48 0.20 0.04

30 Moist 87 0.18 0.03 49 0.17 0.03Dried 40 0.12 0.02 48 0.20 0.04

SE (n = 3) 12 0.014 0.002 3 0.010 0.003LSD (p b 0.05) 27 0.031 7 0.021

F-values for Biochar (df 2) 10.7 6.6 2.5 16.4 25.4 2.1Drying (df 1) 25.3 17.9 0.9 0.1 1.0 2.3Interaction (df 2) 2.1 0.4 0.0 0.7 1.0 0.4

166 H. Soinne et al. / Geoderma 219–220 (2014) 162–167

Sibanda and Young, 1986) but the influence of organic compounds on Psolubility may also increase with input of P in the material applied(Guppy et al., 2005). Here, the BC contained less Pw than the soil andtherefore the BC addition could not have increased the total amount ofPw in the soil + BC mixture. Thus, for the coarse-textured soil, the re-sults showing decreased P sorptionmay have resulted from the increasein soil pH or from competition for P sorption sites in the BC-amendedsoil. Together, the results show that the BC used here did not act as aneffective sorption surface for P.

The effect of BC on aggregate stability was evident from the increaseof WSA and the decrease in turbidity and small-sized soil particlesreleased during the wet-sieving. Similarly, Liu et al. (2012) reported in-creasedWSA after an11-month incubation for BC amended soils havingclay content higher than 13%. Annabi et al. (2007) found organic mate-rial resistant tomineralization to improve aggregate stability by increas-ing aggregate interparticular cohesion. Here, increased interparticularcohesion would explain reduced detachment of b0.06 particles duringwet-sieving in BC amended soils. Furthermore, the decrease in turbiditysuggests enhanced contact formation between soil particles in BCamended soils during the incubation. Lin et al. (2012) showed thatcontact between soil mineral surfaces and BC can be formed throughcarboxylic and phenolic functional groups on the aged BC surfaces.Having high CEC, BC may contribute to structure stabilization throughcation bridge formation. With BC, ash that contains cations is alsoadded to the soil and according to Etiégni and Campbell (1991) thecations in wood ash are readily soluble. The addition of readily solublecations should increase soil EC, but in this study, the EC was lower inBC amended soils compared to that of the unamended soils. This sug-gests that BC acted as a sorptive surface rather than as a source of cat-ions and furthermore, the increased sorption of ions supports theformation of cationic bridges.

Since drying is an important factor in soil structure formation, the ef-fect of BC on aggregate stability was compared with that of drying-induced changes during the incubation. Indeed, the drying treatmentdecreased the turbidity values for Clay having greater clay contentthan SiCL. This was in agreement with the study of Kemper andRosenau (1984), who reported greater cohesion in 49% clay than in15% clay after air-drying. Drying causes colloidal particles to flocculateas the electric double-layer becomes thinner and particles come closeenough for short-range attractive forces to become significant (Dexteret al., 1988). Thus, the lower clay content of SiCL soil may explain thelack of drying-induced changes in aggregate stability.

Similarly, an increase in ionic strength leads to flocculation of colloi-dal particles, while lower ionic strengthmay induce colloid detachment(Lægdsmand et al., 1999). Since the BC-induced decrease in EC did notlead to increased release of clay particles, the stabilizing effect of BC

on the soil aggregate structure was able to overcome the effect oflower salt concentration. As stated above, the lower EC in BC-amendedsoilsmay, in fact, have indicated structural stabilization through cationicbridge formation.

Soils with BC amendment were incubated at the same gravimetricmoisture content, based on the FC measurement for soils without BCaddition. However, BC pyrolyzed at relatively high temperatures(400–700 °C) increases soil water retention capacity (Karhu et al.,2011; Kinney et al., 2012) suggesting that the BC-amended soils maynot have been, in fact, at FC during the incubation. Karhu et al. (2011)reported an 11% increase in water retention capacity after BC additionof 9 t ha−1. To attain the FC, the BC-amended soils need higher gravi-metric moisture, and when incubated at the same gravimetric moistureas unamended soils the actual matric potential was probably lower.Lower matrix potential increases the surface contacts of colloids(Kjærgaard et al., 2004) while the amount of clay dispersed decreaseswith decreasing water content (Watts and Dexter, 1997). Thus, thelower matric potential of BC soils may have reduced the colloid detach-ment and, together with cationic bridge formation, these may have ledto increased aggregate stability.

Many agricultural soils subjected to intensive cultivation suffer frompoor aggregate stability (Tisdall et al., 1978), which is enhanced by soiltillage and associated mineralization of OM (Amézketa, 1999). Wattsand Dexter (1997) showed that clay dispersal with no mechanical pre-treatment correlates positively with water content, but that the phe-nomenon slows down with increasing OM content. In this study, thereduced dispersal of colloidal material after BC addition resembles theeffect of natural organicmatter on colloid detachment. Therefore, BC ad-ditions may have potential to maintain or improve soil aggregate struc-ture and help to maintain soil productivity as well.

5. Conclusions

The BC used in this experiment did not have the affinity to sorbphosphate that could be expected to lead to sorption-based reductionin dissolved P leaching from agricultural soil. Instead, increase in pH orBC-induced inhibition of P sorption may improve the availability of re-cently added P and even reduce the need for P fertilization in BC-amended soils. This incubation study showed however, that BC hasthe potential to improve aggregate stability in clay soils, which may bebeneficial for reducing particulate P load from agricultural fields. Re-peated BC additions could be used to reduce the deteriorating effect oftillage on aggregates and even to improve the structural stability of cul-tivated clay soils. However, these promising findings need to be con-firmed by long-term field experiments.

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Acknowledgment

We wish to thank Tommi Peltovuori for the assistance in fittingthe sorption equations and Asko Simojoki for the advice and assistanceprovided during the experiments. We are grateful to Sampo Tukiainen(Preseco Oy) for providing the BC. We also thank Miia Collander,Johanna Muurinen and Maria Vähätalo for their assistance in BC analy-sis. This researchwas funded by theMinistry of Agriculture and Forestryof Finland and the Helsinki University Centre for Environment (HENVI).

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