The effect of sewage sludge biochar on peat-based growing media

A. Méndez, E. Cárdenas-Aguiar, J. Paz-Ferreiro, C. Plaza and G. Gascó

ABSTRACT Peat is the main component of growing media in horticulture. Increasing demand, environmental concerns and rising costs for peat make the search for alternative materials imperative. Much research has been performed aiming to fnd high quality and low cost substrates from diferent organic wastes such as compost and thus decrease peat consumption. Biochar is a carbon-rich material that has attracted important research as a soil amendment. However, its potential utilization as a peat substitute for growing media formulation remains less well explored. The aim of this study was to evaluate the efects of sewage sludge and sewage sludge biochar on peat properties as growing media and on lettuce (Lactuca sativa) growth. Sewage sludge transformation into biochar proved to be a sustainable waste management approach in order to promote their future use as growing media components. Addition of biochar from sewage sludge increased the N, P and K content of growing media. The biochar addition to peat at a 10%vol rate increased lettuce biomass production by 184–270% and the shoot length by 137–147% despite hydrophysical properties not being improved. Also, biochar addition had a positive efect on growing media microbial biomass which increased more than 966%. In spite of the higher metal concentration in biochar than in sewage sludge, their transfer to plants seems to be reduced when compared with direct sewage sludge use.

Introduction

Peat has a long history of use as the main component for growing media in horticulture. However, peat is a non-renewable resource obtained from peatlands, a major sink for carbon, and some countries limit peat mining due to environmental concerns. T i s reduced availability has been coupled to price increases (Strack 2008). Increasing demand, environmental concerns and rising costs for peat make it imperative to search for alternative growing media substrates in horticulture (Abad et al. 2001). An abundant body of literature has shown that organic residues such as urban solid wastes, sewage sludge, paper waste, pruning waste, pig litter, spent mushroom compost and even green waste, afer adequate composting, can be used as growth media instead of peat with very positive outcomes (Abad et al.

2001; Eklind et al. 2002; Pérez-Murcia et al. 2006; Ostos et al. 2008; Jayasinghe et al. 2010; Méndez et al. 2011; Luo et al. 2015).

Sewage sludge has the beneft of possessing a high concentration of organic matter and other nutrients essential to plants. Traditionally, this waste was used as an organic amendment to soils whereas its use as a growing medium component was limited as it is characterized by high salt lev­els, elevated metal concentrations and signifcant amounts of pathogenic organisms (Gascó et al. 2005; Gascó & Lobo 2007; Wang et al. 2008; Jamali et al. 2009). In recent years, much research has been performed in order to prepare biochar from diferent organic wastes, including sewage sludge (Hwang et al. 2007; Hossain et al. 2011; Méndez et al. 2012; Agrafoti et al. 2013; Lu et al. 2013). Biochar is the solid carbon-rich material obtained from biomass pyrolysis and has attracted wide­spread attention as a soil amendment as it could improve soil productivity and contribute to carbon storage (Lehmann & Joseph 2009; Lehmann et al. 2011; Enders et al. 2012). When using mineral-rich feedstocks such as sewage sludge or animal manure, the pyrolysed products tend to have a high ash content but lower organic carbon concentration. T e EBC (2012) recommended classifying the products from pyrolysis with carbon concentrations below 50% as bio-carbon- minerals. However, for the International Biochar Initiative, the limit for biochar is a 10wt% of organic carbon. Biochar has also been shown to infuence soil physical and chemical properties such as pH, porosity, bulk density and water holding capacity (Lehmann & Joseph 2009; Méndez et al. 2012; Lim et al. 2016). However, tailoring a specifc biochar for any particular application depends on its inherent prop­erties. Biochars with high recalcitrant carbon content may function as carbon fxation materials whereas those rich in available nutrients and minerals, such as the ones obtained from sewage sludge pyrolysis, could be used as amendments to improve soil fertility (Song & Guo 2012). Sewage sludge transformation into biochar has many advantages including the removal of pathogens and decreasing leaching and plant-available metal concentrations afer their addition to soil (Hossain et al. 2011; Méndez et al. 2012).

Although there are several studies concerning the efects of biochar from sewage sludge applied directly to agricultural soils, concerning heavy metal mobility and the response of soil biological properties (Méndez et al. 2012; Paz-Ferreiro et al. 2012), there is an absence of studies contemplating the use of sewage sludge biochar as a growing media component. Vaughn et al. (2013) studied the use of biochar from hard wood pellets and pelletized wheat straw as replacements for soilless substrates containing vermiculite and peat moss. Tese authors established that biochar can act as a substitute for peat but only at doses lower than 15% (v/v). However, Zhang et al. (2014) concluded that the highest quality growth medium and the best Calathea growth were achieved mixing composted green waste with 30% biochar made from coconut husk fbre pyrolysed at approximately 450 °C and 0.7% of humic acids. Dumroese et al. (2011) found that pelletized biochar from agricultural and forestry residues worked well when substituted for peat at a rate of 25% (v/v) but at higher levels led to unsat­isfactory results possibly due to high C/N ratios. Tian et al. (2012) examined the feasibility of using biochar substrate made from green waste as a replacement for some or all of the original peat substrate commonly used for the cultivation of the ornamental plant Calathea rotundifola cv. Fasciata. Tese authors found that addition of biochar at 50%vol decreased the air space of peat growing media and did not improve nutrient concentration or pH, but plant biomass was substantially greater with peat plus biochar than with peat alone.

Diferences observed in the biochar efect on peat-based growing media properties could be due to the diferent species cultivated, diferences in peat and other growing media components, and difer-ences in biochar properties that are greatly infuenced by feedstock, the choice of pyrolysis conditions and biochar particle size distribution. T e aim of this study was to evaluate the efects of sewage sludge and sewage sludge biochar on peat-based growing media and on lettuce (Lactuca sativa) growth. As pyrolysis of sewage sludge could reduce metal lixiviation and content of pathogenic organisms, it was hypothesized that biochar from sewage sludge could be used as an additive to peat-based growing media.

Table 1. composition and formulation of diferent growing media.

Growing medium

Pt Pt/Sl1 Pt/Sl2 Pt/B1 Pt/B2

Components

Peat Peat/Sewage sludge 1 Peat/Sewage sludge 2 Peat/Biochar 1 Peat/Biochar 2

Formulation (vol/vol)

100 90/10 90/10 90/10 90/10

Formulation (weight/weight)

100 71.6/28.4 64.3/35.7 75.8/24.2 67.9/32.1

Materials and methods

Selection and preparation of raw materials

T e raw materials used in this study were a brown commercial peat (PT), two sewage sludges (SL1 and SL2) selected from two municipal plants from Madrid Province (Spain) and two biochars prepared from pyrolysis of the sewage sludges at 450 °C (B1 and B2, respectively). Pyrolysis was performed using a Heron 12/PR-300 electrical furnace (Heron, Madrid, Spain) with temperature control. In both cases, sewage sludges were placed in a capped ceramic cup which was introduced inside a covered nickel recipient. T e cavity between the two recipients was flled with fuel coke particles (<1 mm). As the temperature increases the O2 present in air is consumed by the fuel coke particles and samples are pyrolysed in the inert atmosphere generated. In all cases, the temperature was increased at a rate of 3 °C min-1 and the fnal temperature was maintained for 2 h. Sewage sludges were air-dried and sieved to 2 mm before starting the experiment. T e moisture content was 2.4 and 2.2 wt% for SL1 and SL2, respectively.

Growing media formulation

Growing media were prepared by adding sewage sludges (SL1 and SL2) or sewage sludge biochars (B1 and B2) to peat at a 10%vol rate. Table 1 summarizes their volume/volume and weight/weight ratios. Commercial peat (PT) was used as control material.

Chemical and physical properties of raw materials and growing media

Individual raw materials and growing media, prepared according to volume ratios showed in Table 1, were characterized as follows. pH and electrical conductivity (EC) were determined using a ratio sample:water 1:10 (weight:volume) with a Crison micro-pH 2000 and a Crison 222 conductivity meter (Alella, Spain), respectively (Rhoades 1996; Tomas 1996).

Cation exchange capacity (CEC) was determined with NH4OAc/HOAc pH 7.0 (Sumner & Miller 1996). Later, Na, K, Ca and Mg in the extract were determined in a Perkin Elmer AAnalyst 400 Atomic Absorption Spectrophotometer. (Shelton, CT, USA). Na, K, Ca and Mg were determined with NH4OAc/HOAc at pH 7.0 (Rhoades 1982) and then analysed with a Perkin Elmer AAnalyst 400 Atomic Absorption Spectrophotometer. (Shelton, CT, USA). NKjeldahl content was analysed by the Kjeldahl method (Bremner 1996). Extractable phosphorus was determined by the Olsen method (Watanabe & Olsen 1965). Blue colour in the fltered extract, developed with molybdate-ascorbic acid reagent, was measured with a Shimadzu UV-1203 spectrophotometer (Kyoto, Japan) at 430 nm. T e organic carbon (CK2Cr2O7) was analysed by the Walkley-Black method (Nelson & Sommers 1996). For sewage sludges and biochars the soluble organic carbon (SOC) was determined using the analytical method of Nelson and Sommers (1996) afer sample extraction in a ratio sample:water 1:10 (m/v) and agitation for 1 h. Total metal concentration in biochar and sewage sludges was determined using a Methohm 746 Voltamperimetry Trace Analyzer (Gallen, Switzerland) afer sample digestion with 3:1 (v/v) concentrated HCl/HNO3 for sewage sludges and biochars USEPA-3051a method (USEPA 1997) and with HNO3 for plants. Mobile forms of metals in biochars and sewage sludges were extracted using 0.1 M CaCl2 (Ure et al. 1995).

Hydrophysical characteristics of the substrates tested were determined by the following methods. A container of known volume was flled with the growing medium and slowly completely saturated by gradually pouring water onto the surface. T e n , the container was placed over a watertight pan and several drainage holes were performed on the bottom to allow all the free water to drain out of the container. By recording the volume of water that drains, the container volume flled with air, and hence air space, was determined. Aferwards, the entire content of saturated growing medium was weighted and emptied into an aluminium pan and dried at 105 °C for 24 h. T e water holding capacity was calculated from the weight of the media before and afer drying and the total porosity was calculated as the addition of the air space and the water holding capacity. All experiments were performed in triplicate.

Plant growth experiments

Crops of lettuce were established in pots with an approximate volume of 380 cm³, which were 75% flled with the growing media. Ten lettuce seeds were sown in each pot and the pots incubated with 12:12 light: dark cycles at a controlled temperature of 20–25 °C for 25 days. During this time, sam­ples were regularly watered with distilled water. Each treatment was replicated three times. Neither pesticides nor fertilizers were applied before or during the study. Afer 25 days, stem and root lengths were determined. Subsequently, plants were dried at 85 °C for 24 h to remove moisture and the total dry weight was determined for plants, stems and roots. At the end of the experiment, a combined sample of stems and roots of 10 plants, grown in each pot, was washed using deionized water, dried at 80 °C for 48 h, fnely ground and stored at 5 °C until used for metals analysis. Zn, Cd, Pb and Cu concentrations were determined using a Methohm 746 Voltamperimetry Trace Analyzer (Gallen, Switzerland) afer dried tissues were digested in nitric and sulphuric acid following the procedure described by Gérard et al. (2000).

Microbial biomass

T e microbial biomass carbon of growing media afer lettuce growth experiments was determined by the chloroform fumigation–extraction method (Vance et al. 1987). T e diference in the carbon content of the fumigated and unfumigated extracts was converted to microbial biomass carbon by applying a factor (Kc) of 0.45 and was expressed in milligrams per kilogram of dry soil.

Statistical analysis

T e signifcance of the diferences among means was assessed by analysis of variance. When signifcant diferences (p < 0.05) were observed, Duncan’s multiple range test was implemented. T e Statgraphics Centurion XVI.I. sofware (Statpoint technologies, Warrenton, USA) was used for the calculations.

Results and discussion

Sewage sludge and biochar characterization

Table 2 shows the main properties for sewage sludges and corresponding biochars. T e pH value of sewage sludge increased afer pyrolysis and transformation to biochar. T e literature indicates that typically the pH of biochar is alkaline and that the pH of most biochars increases with pyrolysis tem­perature (Song & Guo 2012). T e CK2Cr2O7 and hydrosoluble carbon concentration (SOC) decreased afer pyrolysis of sewage sludge being indicative of higher stability for biochars. According to Calvelo Pereira et al. (2011), carbon oxidation with potassium dichromate could refect the degree of biochar carbonization and could therefore be used to estimate the labile fraction of C in biochar. T e Nkjeldhal

concentration of SL2 was 2.92 times higher than NKjeldhal of SL1. In both cases, their value decreased

Table 3. Main properties of peat and peat-based growing media containing sewage sludges (Sl1, Sl2) and biochars (B1 and B2).

raw pH EC (dS m 1 ) CEC (mmol C N ( w t % ) C/N P (mg kg1) K (mg kg 1 ) (+) K2Cr2o7 kjeldhal

material 100 g-1) (wt%) Pt 6.8 ± 0.2 b 0.04 ± 0.01 c 102.9 ± 5.7 a 14.3 ± 0.2 a 0.51 ± 0.06 d 27.9 38 ± 5 c 500 ± 15 d Pt/Sl1 6.5 ± 0.1 c 0.96 ± 0.10 b 87.8 ± 4.5 b 13.5 ± 0.2 b 0.76 ± 0.04 c 17.8 191 ± 10 a 800 ± 34 a Pt/Sl2 6.4 ± 0.1 c 1.82 ± 0.25 a 89.4 ± 4.0 b 14.4 ± 0.3 a 1.79 ± 0.10 a 8.0 184 ± 9 a 750 ± 25 ab Pt/B1 7.4 ± 0.2 a 0.83 ± 0.08 b 86.1 ± 2.0 b 12.7 ± 0.1 c 0.64 ± 0.05 c 19.8 109 ± 8 b 670 ± 15 c Pt/B2 6.9 ± 0.1 b 1.79 ± 0.15 a 81.7 ± 1.5 c 12.4 ± 0.1 c 1.33 ± 00.8 b 9.2 96 ± 5 b 700 ± 20 bc

Notes: ec: electrical conductivity; cec: cation exchange capacity; cK2cr2o7: organic carbon concentration; Pt: peat; Sl1: sewage sludge 1; Sl2: sewage sludge 2; B1: sewage sludge 1 biochar; B2: sewage sludge 2 biochar. Values in column followed by the same letter are not signifcantly diferent (p = 0.05) using the Duncan test. table shows the mean value ± standard deviation.

afer pyrolysis, indicating N transformation to inorganic forms and formation of some N volatile compounds.

T e total and mobile concentration of Zn, Cd, Pb and Cu in sewage sludges and corresponding biochars was determined. Except for Cu, similar total values were obtained for SL1 and SL2. However, the concentration of mobile forms of Cu and Cd was higher for SL2 than for SL1. In both sewage sludges, the total metal concentration increased with sewage sludge transformation into biochar due to ash enrichment during pyrolysis. However, the mobility of Cd, Pb and Cu was lower in the biochar than in the sewage sludges. Similar results were previously observed (Hwang et al. 2007; Méndez et al. 2012) and demonstrate that pyrolysis of sewage sludges is an interesting management tool as it decreases the mobility of heavy metals.

Growing media characterization

Physical and chemical properties of the peat growing media were greatly afected by the addition of sewage sludges and biochars (Table 3). T e pH values of peat-based growing media containing sewage sludges were within the established limits for an optimal substrate (5.3-6.5) (Abad et al. 2001) whereas those of peat mixed with biochars showed higher pH than recommended for optimal substrates due to the higher pH of both biochars. However, similar results were obtained previously by diferent authors who used biochars as growing media components with good results for growth of diferent species (Tian et al. 2012). For example, the acid loving ornamental calathea developed greater total biomass when grown in a 1:1 mixture of peat moss and biochar than when grown in either 100% peat or 100% biochar, even though the pH value of the peat-biochar substrate was higher (7.11) than 100% peat (6.2) (Tian et al. 2012). Other authors, however, recommended the treatment of biochars with organic acids such as citric acid in order to optimize their pH value for use as growing media (Vaughn et al. 2015).

T e EC of the growing media was strongly afected by the addition of sewage sludges and biochars, especially afer addition of SL2 and B2 due to the elevated EC of those two raw materials. CEC of peat slightly decreased afer sewage sludge or biochar addition. However, all growing media show values higher than 80 mmol(+) 100 g-1. Variations in CK2Cr2O7 and Nkjeldhal values reduced the C/N ratio, espe­cially for peat mixtures with SL2 and B2. Finally, addition of biochars and sewage sludges increased the P and K concentration with respect to peat alone.

Table 4 shows hydrophysical properties for the growing media. Although there are no universal standards for substrate physical properties, there are ranges within which most potting substrates utilized in the commercial production of horticultural crops fall. As an example, total porosity values range from 50 to 85%, container capacity values vary from 45 to 65% and air space ranges from 10 to 30% (Yeager et al. 1997). T e bulk density of an optimal substrate should be lower than 0.40 g cm-3

(Abad et al. 2005). As shown by data in Table 4, the bulk densities of peat and corresponding mixtures with sewage sludges and biochars were within the optimal range. Air space, water holding capacity and total porosity were not greatly afected by biochar addition, whereas sewage sludge addition slightly decreased water holding capacity and total porosity of growing media. Tian et al. (2012) found that

Table 4. Hydrophysical properties of peat and peat-based growing media containing sewage sludges (Sl1, Sl2) and biochars (B1 and B2).

Growing medium Bulk density Air space (vol%) Water holding capacity (vol%) Total porosity (vol%)

Pt 0.16 ± 0.005 b 7.9 ± 0.6 a 63.2 ± 1.8 a 71.2 ± 2.0 a Pt/Sl1 0.17 ± 0.008 b 7.2 ± 0.2 a 58.1 ± 1.0 b 65.3 ± 2.4 b Pt/Sl2 0.17 ± 0.007 b 6.4 ± 0.2 b 57.0 ± 1.3 b 63.4 ± 2.2 b Pt/B1 0.18 ± 0.005 ab 7.6 ± 0.1 a 65.4 ± 1.5 a 73.0 ± 2.1 a Pt/B2 0.19 ± 0.003 a 7.6 ± 0.1 a 66.5 ± 2.3 a 74.0 ± 3.0 a

Notes: Pt: peat; Sl1: sewage sludge 1; Sl2: sewage sludge 2; B1: sewage sludge 1 biochar; B2: sewage sludge 2 biochar. Values in column followed by the same letter are not signifcantly diferent (p = 0.05) using the Duncan test. table shows the mean value ± standard deviation.

Figure 1. Dry biomass produced by peat-based growing media. Note: (Bars followed by the same letter are not signifcantly diferent (p = 0.05) using the Duncan test).

addition of a green waste biochar at a 50% rate to a peat moss medium with air space of 25.95% had no efect on the total porosity or water holding capacity but signifcantly decreased air space. Vaughn et al. (2015) prepared biochar digestate-based substrates with physical values within the suggested ranges for total porosity but higher values for air space and lower values for water holding capacity. Diferences between the present results and previous studies (Tian et al. 2012; Vaughn et al. 2015) could be due to the smaller particle size for the biochars and sewage sludges used in the present experiment, the characteristics of the peat used and diferences in biochar properties. It is known that biochar characteristics are widely infuenced by feedstock properties, pyrolysis conditions (Zhao et al. 2013) and particle size (Liang et al. 2016).

Lettuce growth

Figure 1 shows dry biomass produced on peat-based growing media. T e substitution of peat by 10%vol of SL1, B1 and B2 signifcantly increased lettuce growth compared with peat alone. Traditionally, the

£ 10

o 8

b b

Root

Shoot

peat peat+SLl peat + SL2 peat+Bl peat+B2

Figure 2. root and aerial part length for lettuce grown in diferent growing media. Note: (Bars followed by the same letter are not signifcantly diferent (p = 0.05) using the Duncan test).

enhancement of plant growth afer substitution of container media with conventional composts has been attributed to changes in water availability, additional or increased microbial activity, increased availability of macro and micronutrients, augmentation of activities of critical enzymes, or production of plant growth-promoting materials by microorganisms (Atiyeh et al. 2001). Pérez-Murcia et al. (2006) observed a signifcant increase in the dry and fresh weights of broccoli aerial parts with increasing sewage sludge content in the growing medium. Similar efects have been reported with respect to the efects of composts (Garcia-Gomez et al. 2002). However, the addition of SL2 to peat led to let­tuce mortality afer germination. A combination of several factors could be responsible for this. T i s growing medium (SL2) shows the lowest aeration space, lowest water holding capacity, high pH and elevated conductivity. Terefore, SL2 showed the highest concentration of SOC which could lead to phytotoxicity when mixed with peat.

Figure 2 shows root and shoot part length for lettuce plants in diferent growing media. T e addi­tion of SL1 signifcantly increased shoot length but there was not efect on root length. T e potting mixture containing 10% of SL1 showed lower porosity and aeration than peat and this could in turn afect lettuce root growth. Addition of biochars B1 and B2 did not afect root growth but signifcantly increased shoot length. According to Frankenberger and Arshad (1995) microorganisms not only mineralize complex substances into plant available nutrients but can also synthesize a whole series of biologically active substances including plant growth regulators.

Microbial biomass carbon

Figure 3 shows C microbial biomass for peat-based growing media afer lettuce growth. Addition of sewage sludges and biochars resulted in statistically signifcant increases in microbial biomass carbon. For sewage sludges, the highest increment was produced with SL1 whereas for biochars, the highest microbial biomass was with the mixture of peat and B1. Tese results were in accord with those of

680

580

480

280

180

80

-20 T T+SL1 T+SL2 T+B1

Figure 3. Microbial biomass carbon in peat-based growing media after lettuce growth. Note: (Bars followed by the same letter are not signifcantly diferent (p = 0.05) using the Duncan test).

T+B2

Zhang et al. (2014) who found addition of biochar or humic acids, and especially a combination of both materials, increased the microbial biomass of growing media. Previously, T ies and Rilling (2009) suggested that, because of its porous structure, its high internal surface area and its ability to adsorb soluble organic matter, gases and inorganic nutrients, biochar was a highly suitable habitat for microbial growth. Partially, the benefcial properties of biochar on microbial populations observed in the present experiment can be attributed to an increase in pH (Lu et al. 2015). T e chemical stability of the biochar material means that soils microorganisms will not readily be able to utilize the carbon as an energy source or the nitrogen and possibly other nutrients contained in the carbon structure. However, depending on the type of biochar, a fraction may be readily leached and therefore mineral-izable and in some cases it has been shown to stimulate microbial activity and increase the abundance of microorganisms (Lehmann et al. 2011; Lu et al. 2015). Tese results indicate that more studies will be necessary in order to understand the infuence of biochar on peat microorganisms.

Heavy metal transfer

T e evaluation of heavy metal transfer from sewage sludge or biochars to the plant is important in order to avoid food chain hazards and for optimisation of their use as growing media and seedlings. Table 5 shows the Zn, Cd, Pb and Cu concentrations of lettuce grown in peat and peat-based growing media. T e permitted values for consumption of Cd and Pb, in industrial crops are 0.2 and 0.3 mg kg-1, respectively, on a fresh weight basis (EEC 2001). T e metal concentration of Zn, Cd and Cu was lower in lettuce grown in media containing 10%vol of sewage sludge SL1 or biochars B1 and B2 than in plants grown in peat alone. T i s efect could be due to metal complexation or dilution efect, as the highest biomass production was obtained afer sewage sludge and biochar addition to plants. Pérez-Murcia et al. (2006) have shown that, in general, the presence of sewage sludge in the medium increases the contents of the heavy metals studied in the aerial parts for broccoli. However, Jayasinghe et al. (2010) studied the efect of partial substitution of peat in growth media by sewage sludge sugarcane trash based compost and synthetic aggregates on the physical and chemical characteristics of the growth media and on the growth and nutrition of lettuce and concluded that increased concentrations of total

d

d

c

b

a

Table 5. Metal concentration of lettuce plants grown in peat and peat-based growing media containing sewage sludges (Sl1, Sl2) and biochars (B1 and B2).

Medium

Pt Pt/Sl1 Pt/B1 Pt/B2

z n

0.43 ± 0.04 a 034 ± 0.03 b 0.29 ± 0.01 c 0.10 ± 0.01 d

Metal concentration (mg k g - 1 fresh weight)

Cd Pb

0.10 ± 0.01 a 0.13 ± 0.02 b 0.06 ± 0.01 b 0.09 ± 0.01 c 0.05 ± 0.01 b 0.07 ± 0.01 c 0.01 ± 0.01 c 0.21 ± 0.04 a

Cu

0.38 ± 0.05 a 0.16 ± 0.04 b 0.02 ± 0.01 c 0.12 ± 0.04 b

Notes: Pt: peat; Sl1: sewage sludge 1; Sl2: sewage sludge 2; B1: sewage sludge 1 biochar; B2: sewage sludge 2 biochar. Values in column followed by the same letter are not signifcantly diferent (p = 0.05) using the Duncan test. table shows the mean value ± standard deviation.

and extractable trace elements in the growth medium did not increase the trace element concentra­tions in plant tissues compared with the peat control. Ostos et al. (2008) studied the efect of a partial substitution of peat for compost on the growth and nutrition of a native shrub (Pistacia lentiscus). Composts were prepared from pruning and municipal solid wastes or pruning waste and sewage sludge. Tey found that the substitution of peat by the municipal solid waste and sewage sludge-based compost in the medium afected plant height and biomass positively. T i s fnding was in spite of the micronutrients Cu, Mn and Zn at concentrations below the ranges considered normal. Tese authors attributed that to a possible dilution efect derived from the greater biomass reached by the plants in the compost-based substrates that could cause comparatively low concentrations in relation to peat alone.

Comparing the efect of SL1 and B1 addition, the Zn and Cu concentrations in plants (Table 5) were lower afer biochar addition in spite of the higher metal concentration in B1 than in SL1. T i s could be due to the higher pH in growing media containing biochar which limits heavy metal mobil­ity complexation reactions between metals and biochar functional groups and the higher biomass reached by plants in PT/B1.

Conclusions

Addition of biochar from sewage sludge increased the N, P and K concentrations of growing media. Addition of biochar from sewage sludge to peat increased microbial biomass and lettuce biomass production despite hydrophysical properties not being improved. In spite of the higher metal concen­trations of biochar compared with raw sewage sludge, their transfer to plant seems to be reduced when compared with direct sewage sludge use. In conclusion, sewage sludge transformation into biochar could reduce the metal transfer of sewage sludge to plants increasing nutrient content of growing media. Pyrolysis of sewage sludge to prepare biochar is an interesting waste management approach in order to promote their future use as growing media component.

Disclosure statement

No potential confict of interest was reported by the authors.

ORCID

E. Cárdenas-Aguiar http://orcid.org/0000-0002-9686-0333

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