Geoderma 206 (2013) 32–39

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Impacts of adding biochar on nitrogen retention and bioavailability inagricultural soil

Hao Zheng a,b, Zhenyu Wang a,⁎, Xia Deng a, Stephen Herbert b, Baoshan Xing b,⁎a College of Environmental Science and Engineering, Ocean University of China, Qingdao 266100, Chinab Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, USA

⁎ Corresponding authors. Tel.: +1 413 545 5212.E-mail addresses: wang0628@ouc.edu.cn (Z. Wang),

0016-7061/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.geoderma.2013.04.018

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 August 2012Received in revised form 11 April 2013Accepted 16 April 2013Available online 22 May 2013

Keywords:BiocharNitrogen (N)LeachingRetentionBioavailability

Interest in the use of biochar to improve soil productivity has rapidly increased. Nitrogen (N) loss, retention andbioavailability in biochar-amended soils fertilizedwith NH4

+–N and NO3−–Nwere studied using leaching and pot

experiments. NO3−–N leaching from the soils fertilized with NH4

+–N and NO3−–N was significantly reduced by

biochar addition. NH4+–N leaching from the NO3

−–N fertilized soil was also significantly reduced by biochar,while no significant effect was observed for the NH4

+–N fertilized soil. Also lower NH4+–N and NO3

−–N wereleached from NO3

−–N fertilized soil with maize (Zea mays L.) grown. Mitigation of N leaching loss followingbiochar addition is mainly attributed to the increase in soil water holding capacity (WHC), NH4

+ adsorptionand enhanced N immobilization. Biochar addition stimulated maize growth, both above and below ground.Biochar also increased N utilization efficiency (NUE) ofmaize but decreasedN accumulation efficiency (NAE), in-dicating that biochar addition may improve N bioavailability in agricultural soils. Therefore, reduction of Nleaching, and increase of N retention and bioavailability in agricultural soils can potentially decrease the N fertil-izer demand for crop growth.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Nitrogen (N) is an essential element for plant growth and a keyagricultural input into soils. To maximize grain yield, farmers espe-cially in developing countries often apply more N fertilizer than re-quired for maximum crop growth (Cui et al., 2010). China is theworld's largest N fertilizer producer and consumer (Wang et al.,2011a). In 2010, annual N fertilizer production and consumptionequated to 45.21 and 40.86 million tons (pure N), respectively, ac-counting for about one third of the world's total production and con-sumption (Center, 2011). In the past three decades, soil N losses havebecome more serious due to the overuse and low efficiency of N fer-tilizer, although N fertilizer has significantly contributed to the im-provement of crop yields in China (Wang et al., 2011a). The N useefficiency by crops is 30–35% in China's farmland (>50% in developedcountries), 45–50% of N was lost (Ju and Zhang, 2003). The overuseand low use efficiency of N fertilizer in China as well as in other coun-tries and regions (e.g., dead zone in the Gulf of Mexico) have alreadycaused serious environmental problems such as surface water eutro-phication (Jin et al., 2005), emissions of greenhouse gas nitrous oxide(N2O) (Kahrl et al., 2010), and soil acidification (Guo et al., 2010). Addi-tionally, excessive N application represents an unnecessary economicexpenditure and waste of resource for Chinese farmers. It is estimated

bx@umass.edu (B. Xing).

rights reserved.

that the amount of N fertilizer lost was 18.4 million tons in China in2010 alone, causing a direct economic loss of up to $ 4.38 billion(based on 45% loss, (Ju and Zhang, 2003)). Therefore, improving N fer-tilizer use efficiency and reducing N loss in soils are an urgent task.

N losses from the agricultural soils are resulted from gaseousemission (N2, N2O, NO, NH3), and ammonium (NH4

+) and nitrate(NO3

−) leaching (Robertson and Groffman, 2007). NO3− leaching has

been the main N loss pathway from Chinese agro-ecosystem. HighNO3

− concentration in water supplies can cause eutrophication(Huang et al., 2003) and health problems such as methemoglobine-mia and diabetes (Feleke and Sakakibara, 2002). Therefore, technicalsolutions have become obligatory to reduce N (NO3

− and NH4+)

leaching losses from agricultural soils and to improve N fertilizer uti-lization efficiency for sustainable crop production. Biochar, applied tosoils has currently generated considerable interest for scientists andpolicy makers (Atkinson et al., 2010), and may be a potential solution.

Biochar is produced from biomass under partial or complete exclu-sion of oxygen, and is distinguished from charcoal by its use as asoil amendment for improving soil quality and carbon sequestration(Lehmann and Joseph, 2009). Biochar may improve soil physical prop-erties, increase soil pH and cation exchange capacity (CEC), enhancenu-trient retention and alter soil microbial populations, and augment cropyields. This has provided the incentive to study the effects of biochar ap-plication on soil N cycling (Lehmann et al., 2003; Ding et al., 2010;Steiner et al., 2010; Taghizadeh-Toosi et al., 2012). A number of studieshave reported that adding biochar to soils may increase net nitrificationrate (Deluca et al., 2006), stimulate N immobilization (Rondon et al.,

Table 2Description of experimental treatments for the leaching and pot experiments.

N fertilizer Replicate Biochar adding ratea

0% 1% 2% 5%

(NH4)2SO4 5 Lb L L LKNO3 4 L, L + M /c / L, L + MNo fertilizer 5 P P P P

a Biochar was added at the rate of 0%, 1%, 2% and 5% by weight.b “L” indicates the leaching experiment without maize growing, “L + M” indicates

the leaching experiment with maize grown, “P” indicates the pot experiment.c “/” means no treatment was set up at the biochar adding rate.

33H. Zheng et al. / Geoderma 206 (2013) 32–39

2007), reduce N2O emission (Spokas et al., 2009), decrease NH3 volatil-ization (Steiner et al., 2010), enhance ammonium-oxidizing bacterialabundance (Ball et al., 2010) and alter N availability for crops(Lehmann et al., 2003; Steiner et al., 2008). Biochar addition has alteredN transformation following the application of N containing substancessuch as bovine urine, swine manure, green-waste compost and bio-solids (Clough et al., 2010; Laird et al., 2010b; Knowles et al., 2011).However, the mechanisms underlying the N response to biochar addi-tions are not well understood, but according to these reports, it is rea-sonable to hypothesize that biochar may retain N and in theory retardN leaching losses (Steiner et al., 2008; Ding et al., 2010; Laird et al.,2010b; Knowles et al., 2011). Ding et al. (2010) reported that the appli-cation of 0.5% biochar to the surface soil layer reduced cumulativeleaching losses of NH4

+–N by 15.2% through NH4+ adsorption on the

biochar. Knowles et al. (2011) also reported that NO3−–N leaching

from biochar plus biosolids-amended soils was reduced to levels at orbelow the control treatments, the mechanisms, however, are unclear.

In this study, we focused on the N loss and retention in a biochar-amended soil. The specific objectives for this study were to 1) inves-tigate the impacts of the biochar amendment on N leached from anagricultural soil treated with different types of N fertilizer; 2) studythe N uptake by crops in the biochar-amended soil, and 3) evaluatethe N bioavailability in biochar-amended soil.

0

700

1400

2100

2800

3500

0 15 30 45 60 75

Cum

ulat

ive

leac

hate

(m

L)

Time (d)

0%char1%char2%char5%char

2500b

a

2. Materials and methods

2.1. Soil and biochar

A surface soil (0–20 cm) was collected from a cropland (120.38S,36.29E) after potatoes were harvested in the Chenyang district ofQingdao, Shandong province (China). This site was farmed in maize–vegetable rotation for the last 5 years. Urea and synthetic fertilizerwere two commonly used fertilizers in this area. The soil wasair-dried and ground to pass through a 2 mm sieve and thoroughly ho-mogenized. Soil pH was determined in a 1:2.5 soil to water suspension(Wang et al., 2013). Total organic carbon (TOC) wasmeasured using anoxidation method with potassium dichromate (Sciubba et al., 2012).Total N (TN) content of the soil samples was determined using an auto-matic azotometer (KDN-102F, Qianjian Ltd., Shanghai) (Bai et al., 2010).Total phosphate (TP) content was spectrophotometrically determinedusing phosphomolybdate blue (Parvage et al., 2012). NO3

−–N andNH4

+–N contents in the soil were analyzed using spectrophotometricmethods with phenol disulfonic acid and indophenol blue reagent, re-spectively (Bai et al., 2010). Bulk density was measured using a100 cm3 cylinder thatwas pressed into the soil (Zhang et al., 2010). Par-ticle size analysis was conducted by the hydrometer method (Spokas

Table 1Chemical and physical characteristics of the soil and biochar.

Soil Biochar

pH 6.02 Yield/% 33.4TOC/g kg−1 17.6 pH 9.45TN/g kg−1 1.33 C/% 73.4NH4

+–N/mg kg−1 63.2 H/% 3.01NO3

−–N/mg kg−1 NAa N/% 0.49TP/g kg−1 1.08 O/% 23.1Bulk density/g cm−3 1.13 NH4

+–N/mg kg−1 17.4Sand/% 29.2 NO3

−–N/mg kg−1 3.01Silt/% 57.2 Ash/% 10.0Clay/% 13.6 Zeta potential/mV −44.91CEC b/cmol(−) kg−1 15.9 CEC/cmol(−) kg−1 12.5WHC c/% 33.6 WHC/% 95.3Moisture content/% 2.19 Acid functional group/mmol g−1 1.16

Surface area/g m−2 2.84

a NA: Below the detection limit.b CEC: cation exchange capacity, cmol(−) kg−1.c WHC: water holding capacity, %.

and Reicosky, 2009). The soil was classified as a silt loam and its proper-ties are presented in Table 1.

Biochar was produced from giant reed (Arundo donax L.), a peren-nial C3 grass widely used in constructed wetlands for wastewatertreatment and ecological remediation in China, using a slow pyrolysismethod as described in Zheng et al. (2013). Giant reed was chosenbecause it can produce large quantities of biomass with low nutrient(in particular N) and pyrolysis at relatively high temperature pro-vides biochar with high stability and even lesser N. Following char-ring, the mass yield was recorded and the sample was milled topass a 2 mm sieve prior to further analysis. Total C, N, H, and O,pH, surface area, ash content, zeta potential, total acidic oxygen-containing groups, NH4

+–N and NO3−–N content were characterized

as described by Zheng et al. (2013). The properties of the biocharsample are presented in Table 1.

0 15 30 45 60 75Time (d)

0

500

1000

1500

2000

Cum

ulat

ive

leac

hate

(m

L)

0%char5%char0%char+plant5%char+plant

Fig. 1. The volume of cumulative leachate in the leaching experiments added with differ-ent types of N fertilizers: (a) NH4

+–N fertilizer; (b) NO3−–N fertilizer. The 0% char, 1% char,

2% and 5% char indicate that biochar was added at a rate of 0%, 1%, 2% and 5% (w/w) in thesoil columns, respectively. The 0% char + plant and 5% char + plant indicate that biocharwas added at a rate of 0% and 5% in the soil columns, respectively, where maize wasplanted.

34 H. Zheng et al. / Geoderma 206 (2013) 32–39

2.2. Soil column incubation and leaching

2.2.1. Preparation of soil columnsFree-draining soil columns were constructed with synthetic glass

tubes (length, 45 cm; tube internal diameter, 15 cm) and fitted withsynthetic glass end caps at the bottoms (Fig. S1). A synthetic glassplate with 0.5 mm holes was fixed at 5 cm from the bottom of thetube. A hole was drilled through the end caps and a drain tube (5 mmin diameter) was attached to the bottom of each column. The columnswere packed with 4 kg air-dried soil by tamping the columns as thesoil was added. Quartz sand (300 g) was placed on the top and bottomof the soil columns to prevent destruction of the soil surface duringwatering. To prevent soil loss, a fine nylon mesh (b0.5 mm) was at-tached to the base of the soil cores before packing. All columns werepacked to an initial bulk density of 1.13 g cm−3 as in the field.

2.2.2. N leaching from the NH4+–N and NO3

−–N fertilized soilsNH4

+–N and NO3−–N fertilizers are the main N fertilizers on the

market and many crops prefer to take up NH4+–N and/or NO3

−–Nfrom soils. Thus, the effect of biochar addition on N leaching fromthe NH4

+–N fertilized soil ((NH4)2SO4, equivalent to 600 mg N kg−1

soil) and NO3−–N fertilized soil (KNO3, equivalent to 600 mg N kg−1

soil) was studied, respectively (Table 2). Biochar was added at arate of 0%, 1%, 2% and 5% (w/w) in the NH4

+–N fertilized soil, and 0%and 5% (w/w) in the NO3

−–N fertilized soil. The biochar was mixedthoroughly with soil and N fertilizer before the columns were packed.After the column was packed, 1.36 L of double-distilled water wasadded using a perfusion tube over 3 days to reach the maximum

0

140

280

420

560

0 15 30 45 60 75

NH

4+-N

cum

ulat

ive

leac

hing

(m

g)

Time (d)

0 15 30 45 60 75Time (d)

0%char1%char2%char5%char

0

2

4

60%char5%char0%char+plant5%char+plant

a

c

NH

4+-N

cum

ulat

ive

leac

hing

(m

g)

Fig. 2. The effect of biochar on NH4+–N and NO3

−–N leaching from the biochar-amended sofertilized with NH4

+–N; (b) cumulative NO3−–N leaching from the soil fertilized with NH4

+–NNO3

−–N leached from the soil fertilized with NO3−–N. The 0% char, 1% char, 2% and 5% char in

respectively. The 0% char + plant and 5% char + plant indicate that biochar was added at

water holding capacity (WHC). After keeping for 7 days in a constanttemperature room (23 °C and 60–80% relative humidity), 250 mL ofdouble-distilled water was introduced on the top of each columnusing a slow dripping technique (∼4 h) with the aid of an infusionapparatus with a flow restricting needle mounted above the middleof each column in every leaching event (Fig. S1). The column wasleached every three days in the first four weeks, then every five andseven days in the following three and two weeks, respectively.

Moreover, in order to study the crop's roles in N leaching, maize(Zea mays L.), a chief crop in northern China, was planted in half ofthe columns packed with NO3

−–N fertilized soil, thereafter referredto as 0% char + plant and 5% char + plant. After the columns werepacked, a total of five maize seeds were sown per column at a depthof approximately 2 cm and then thinned to the best 3 following ger-mination. Leaching commenced one week after germination. Becauseof maize growth in this leaching experiment, the leaching was re-duced to 12 times during the two month incubation.

The leachate fromeach columnwas collected in 250 mLpolyethylenebottles for 12 h after the start of a leaching event and the leachate vol-ume was recorded. The leachate was filtered (0.45 μm glass fiber filter)and analyzed within 24 h. The NO3

−–N and NH4+–N contents in leachate

were determined using a spectrophotometric method, using phenoldisulfonic acid and Nessler's reagent, respectively (Major et al., 2010).The pH of collected leachate was determined using a glass electrode pHmeter (Accumet BASIC AB15, Fisher Scientific Co., USA). At the end ofthe leaching experiment, soil samples were collected at every 5 cmdepth from the top of the columns. A portion of each soil samplewas stored at 4 °C for NH4

+–N, NO3−–N and microorganism analysis,

NO

3- -N

cum

ulat

ive

leac

hing

(m

g)N

O3- -

N c

umul

ativ

e le

achi

ng (

mg)

0 15 30 45 60 75Time (d)

0

50

100

150

200

250

300

0 20 40 60Time (d)

0

500

1000

1500

2000

2500

0%char5%char0%char+plant5%char+plant

b

d

0%char1%char2%char5%char

ils fertilized with NH4+–N and NO3

−–N: (a) Cumulative NH4+–N leaching from the soil

; (c) cumulative NH4+–N leaching from the soil fertilized with NO3

−–N; (d) cumulativedicate that biochar was added at a rate of 0%, 1%, 2% and 5% (w/w) in the soil columns,a rate of 0% and 5% in the soil columns, respectively, where maize was planted.

35H. Zheng et al. / Geoderma 206 (2013) 32–39

and the remaining was air-dried for pH and TN analysis. The soil pH, TN,NO3

−–NandNH4+–Ncontentsweremeasured as described in Section 2.1.

Total microbial activity was determined using the fluorescein diacetatemethod (Wang et al., 2011b).

2.3. Pot experiments

Pot experiments were used to study N uptake by maize in thesame soil with biochar addition at the rates of 0%, 1%, 2% or 5%(w/w). No additional N fertilizer was added into the soil. All the treat-ments were replicated five times. Each nursery pot (100 mm diameterby 100 mm depth) was filled with 300 g of air-dried soil. The potswere gravimetrically adjusted to maintain 60% of maximum WHC andwere left to settle in the greenhouse for 1 week before seed sowing.Maize species was the same maize as used in the leaching experiment.The seed germination percentage was above 98%. Soil moisture contentwas kept at 60% of WHC. After 5 weeks, shoots and roots were sepa-rately harvested. Soils were divided into rhizosphere and non-rhizosphere soils by the hand shaking method (Wang et al., 2011b),and TN, NO3

−–N and NH4+–N contents were measured as described in

Section 2.1. The root parameters were obtained through WinRhizoimage analysis software (Epson Scanning and WinRhizo Pro. 2005).The shoots and roots were dried at 65 °C for dry biomass and TN analy-sis. TN content in shoots and roots was determined using the indophe-nol blue photometric method after digestion with sulfuric acid andhydrogen peroxide (Hoffland et al., 1989).

0

50

100

150

200

250

0-5cm 5-10cm 10-15cm 15-20cm

NH

4+-N

(m

g kg

-1)

Soil layer

0-5cm 5-10cm 10-15cm 15-20cmSoil layer

0%char

1%char

2%char

5%char

0

1

2

3

Tot

al N

(g

kg-1

)

0%char 1%char2%char 5%char

a a a a a

a a

a a

c

b

a

a

c

c

b

a ab b

c

a

c c

b a

c

b ab a a

c

b

ba

dc

Fig. 3. The effect of biochar addition on soil N content and microbial activity in the soil fertmicrobial activity. The different letters among the different amount of biochar addition indProduct and Service Solutions Software (SPSS 18.0). The 0% char, 1% char, 2% and 5% char inrespectively. The 0% char + plant and 5% char + plant indicate that biochar was added at

2.4. Statistical analysis

All results were expressed as the mean values. Analysis of vari-ance (ANOVA) was used to test all experimental data. The least sig-nificant difference (LSD at P b 0.05) test was applied to assess thedifferences using Statistical Product and Service Solutions Software(SPSS 18.0).

3. Results and discussion

3.1. Effect of biochar on leachate volume

Biochar additions significantly reduced (P b 0.05) the cumulativeleachate volume by 2.9–11.4% and 7.0–15.4% in the NH4

+–N and NO3−–N

fertilized soils, respectively (Fig. 1). As more biochar was added, cumu-lative leachate volume was decreased (r = −0.97, P b 0.01) (Fig. 1a).This implied that biochar increased soil WHC (Laird et al., 2010a),which is confirmed by the higher soil water content in the biochar-amended soils (Fig. S2). The mechanisms for biochar-provided waterretention are mainly attributed to the changes in soil surface area,bulk density, porosity, pore size distribution and aggregation of soil,and these could alter percolation patterns, residence time and flowpaths of the soil solution (Major et al., 2009; Verheijen et al., 2010).Atkinson et al. (2010) also suggested that if biochar contained sufficientamounts of humic substances, soil WHC could be largely increased. Thebiochar used in this study was fresh and had no humics, and pyrolytictemperature (500 °C) resulted in a low content of labile organicmatters

NO

3- -N

(m

g kg

-1)

0-5cm 5-10cm 10-15cm 15-20cmSoil layer

0-5cm 5-10cm 10-15cm 15-20cmSoil layer

0

5

10

15

20

25

30

350%char

1%char

2%char

5%char

0.0

0.1

0.2

0.3

0.4

0.5

Mic

robi

al a

ctiv

ity (

abs

g-1

soil)

0%char 1%char

2%char 5%char

b

a a

a

c

bc

a a

b

a a

b

a

a a

a a a a a

b b b

a

b

aab

a

a

a

a

ilized with NH4+–N. (a) NH4

+–N content; (b) NO3−–N content; (c) TN content; (d) total

icate significant difference, which was analyzed by LSD test (P b 0.05) using Statisticaldicate that biochar was added at a rate of 0%, 1%, 2% and 5% (w/w) in the soil columns,a rate of 0% and 5% in the soil columns, respectively, where maize was planted.

36 H. Zheng et al. / Geoderma 206 (2013) 32–39

in the biochar, thus, direct contribution of organic matter in the biocharmay be limited. In the plant-treatments, total leachate volume was sig-nificantly lower (P b 0.05) than that of non-plant grown treatments(Fig. 1b). This was caused by the combined effects of theWHC increaseinduced by biochar and the uptake of water by maize. Compared to the0% char + plant treatment, less leachate was collected from the 5%char + plant treatment during the first month, while more leachatewas collected in the following leaching period (Fig. 1b) due to the

0

3

6

9

12

Soi N

H4+

-N (

mg

kg-1

)

0%char 5%char

0.0

0.5

1.0

1.5

2.0

2.5

0%char 5%char

Tot

al N

(g

kg-1

)

0

0.4

0.8

1.2

1.6

Mic

robi

al a

ctiv

ity (

abs

g-1

soil)

0%char 5%char

* **

* *

*

* *

a

c

e

0-5cm 5-10cm 10-15cm 15-20cmSoil layer

0-5cm 5-10cm 10-15cm 15-20cmSoil layer

0-5cm 5-10cm 10-15cm 15-20cmSoil layer

Fig. 4. The effect of biochar addition on N content and microbial activity in soil fertilized witLSD test (P b 0.05) using Statistical Product and Service Solutions Software (SPSS 18.0). Theand 5% (w/w) in the soil columns, respectively. The 0% char + plant and 5% char + plant iwhere maize was planted.

inhibition of plant growth in the 5% char + plant treatment, possiblydue to the inhibition of maize germination by biochar (Rogovska etal., 2012), high water content induced by biochar application (Majoret al., 2009; Verheijen et al., 2010), or release of toxic components(e.g., PAHs) from the biochar along with its aging in soil (Hale et al.,2012).

Soil WHC increase may enhance nutritional benefits, especially forsoil solution mobile elements (e.g. N, P and K). Additionally, WHC

Soi N

O3- -

N (

mg

kg-1

)0

4

8

12

16

0%char 5%char

0.0

0.4

0.8

1.2

1.6

2.0

2.4

non-rhizosphere rhizosphere

Tot

al N

(g

kg-1

)

Soil

0%char 5%char

*

* *

*

b

d

0-5cm 5-10cm 10-15cm 15-20cmSoil layer

h NO3−–N fertilizer. The asterisk indicates significant difference, which was analyzed by

0% char, 1% char, 2% and 5% char indicate that biochar was added at a rate of 0%, 1%, 2%ndicate that biochar was added at a rate of 0% and 5% in the soil columns, respectively,

0.0

0.5

1.0

1.5

0% 1% 2% 5%

Mai

ze b

iom

ass

(g p

ot-1

)

Biochar amount

0% 1% 2% 5%Biochar amount

leaf stem root

0

0.4

0.8

1.2

Soil

TN

(g

kg-1

)

a

a

a

a

a

b

b

a

b

c

a

b

bc

A

B B B

aab

bab

b

Fig. 5. The effect of the biochar addition on maize biomass and soil TN in pot experi-ments: (a) biomass; (b) soil TN. The different small and capital letters indicated signif-icant difference analyzed by LSD test (P b 0.05) using Statistical Product and ServiceSolutions Software (SPSS 18.0).

37H. Zheng et al. / Geoderma 206 (2013) 32–39

increase may reduce irrigation frequency or irrigation volume, whichin turn may reduce the risk of nutrient leaching loss, especially N.

3.2. Effect of biochar on N retention in soils fertilized with differentN fertilizers

Application of biochar to soils not only increased WHC, but alsoaffected the amount of N leached. The NH4

+–N leaching from thebiochar-amended soils varied with the application of N fertilizer type(Fig. 2). Biochar addition had no significant influence on the cumulativeamount of NH4

+–N leached from the NH4+–N fertilized soil (Fig. 2a),

while it markedly decreased (P b 0.05) the NH4+–N leaching from the

NO3−–N fertilized soil (Fig. 2c). Cumulative amount of NH4

+–N leachedfrom the NO3

−–N fertilized soil was much smaller compared to theNH4

+–N fertilized soil, mainly due to the different content of NH4+–N

in the soils. Biochar could retain NH4+–N in soils through acid functional

groups (e.g. carboxyl and hydroxyl) on its surface via cation exchange(Clough and Condron, 2010; Zheng et al., 2013), but fresh biochar's cat-ion exchange capacity can be limited and depends on the feedstocksand conditions of manufacturing biochar (Taghizadeh-Toosi et al.,2012). The NH4

+–N content in the NH4+–N fertilized soil was in excess

of the maximum adsorption capacity of the soil and biochar, hencethe large amount of NH4

+–N was leached from the NH4+–N fertilized

soil. But NH4+–N adsorption on the biochar may be the reason for the

reduction of NH4+–N leaching from the biochar-amended soil with

NO3−–N addition.Biochar additions significantly decreased (P b 0.05) the cumulative

amounts of NO3−–N leaching following either NH4

+–N or NO3−–N fertil-

izer application (Fig. 2b, d). For the NH4+–N fertilized soil, the presence

of NO3−–N (Fig. 2b) indicated that nitrification occurred in the soils, and

that nitrification was slower during the first 18 days (Fig. S3). With in-creasing biochar rates, less NO3

−–Nwas leached (r = −0.91, P = 0.09)(Fig. 2b). For the NO3

−–N fertilized soil, the NO3−–N leaching reached a

cumulative maximum relatively quickly (Fig. 2d). Biochar may adsorbNO3

− from aqueous solution (Zheng et al., 2013), which may be one ofthe reasons for the reduction of NO3

−–N leaching.Apart from the reduction of leachate volume and adsorption of

NH4+–N and NO3

−–N on the biochar, biochar may affect N transforma-tion in the soils (Ball et al., 2010; Clough and Condron, 2010), whichmay be responsible for the reduction of N leaching from the biochar-amended soils. The soil with biochar had higher TN (Figs. 3c, 4c, andS4). Thismay have resulted from the enhancement of N immobilizationin the biochar-amended soils. It was confirmed by the increase ofmicrobial activity in both NH4

+–N and NO3−–N fertilized soils by biochar

addition (Figs. 3d, 4e). This enhanced N immobilization process couldcreate a temporary reservoir of organic N, which would reduce thepotential for inorganic N leaching in highly leached soils (Lehmann etal., 2003). However, nitrification affected by biochar in both soils wasnot consistent. For the NH4

+–N fertilized soil, more NH4+–N and less

NO3−–N were retained in the biochar-amended soil (Fig. 3a–b), imply-

ing the inhibition of nitrification. For the NO3−–N fertilized soil, the situ-

ation was exactly opposite (Fig. 4a–b), and the enhancement ofnitrification may have occurred. In the plant-treatments, biochar addi-tion did not alter NH4

+–N and NO3−–N contents in both rhizosphere

and non-rhizosphere soils, but significantly (P b 0.05) increased TNcontent in non-rhizosphere soil (Fig. 4c). It may be concluded that thebiochar addition retained more N in soil with plants, because theamount of rhizosphere soil is relatively small compared to thenon-rhizosphere. The total microbial activity was also increased bybiochar in the maize-treatment, suggesting that the enhancement of Nimmobilization occurred. Ammonia volatilization in agricultural soilsis common and favored at alkaline pH and at high concentrations ofNH4

+–N (Robertson and Groffman, 2007). The soil pH was significantlyincreased with biochar addition (Fig. S5) due to the alkaline minerals inthe biochars (Zheng et al., 2013), but not to the level (pH > 8) thatwould result in significant ammonia volatilization from the soils

(DeLuca et al., 2009). Therefore, ammonia volatilization in both leachingexperiments should be negligible. Moreover, in the plant-treatments,the pH was not changed in the rhizosphere soil amended with or with-out biochar compared to that of non-rhizosphere (Fig. S5c). This couldhave resulted from the short time incubation with maize.

3.3. Effect of biochar on crop growth

Research has suggested that biochar applications to soils may in-crease seed germination, plant growth, and crop yields (Lehmann etal., 2003; Rondon et al., 2007; Graber et al., 2010). In pot experiment,biochar addition stimulated the maize seedlings' growth (Fig. S6). Thetotal seedling biomass was significantly higher in the biochar-amendedsoil compared to that of the control (Fig. 5a). However, the biomassof each plant components and total biomass did not increase with in-creasing biochar rate (Fig. S7), suggesting that excessive biochar additionmay not lead to higher biomass and the biochar rate should be controlledat an optimal level. Themechanisms responsible for the beneficial effectof biochar on plant growth have been proposed as the direct contribu-tion of biochar through its intrinsic elemental and compositional nutri-ents (e.g. N, P and K), and improvement of physical properties of thesoils resulting in benefits for root growth and/or nutrient and water

Table 3Rhizosphere parameters of plants in the pot experiment.

Biochar amount Lengthm

Volumecm3

SAa

cm2Tips RSb NAEc

mg m−1NUEd

g mg−1

0% 4.27 ± 0.89ae 1.00 ± 0.07a 83 ± 13a 2116 ± 544a 0.22 ± 0.04a 0.96 ± 0.23b 0.57 ± 0.09a1% 7.61 ± 0.93b 1.37 ± 0.08b 108 ± 16ab 2282 ± 598a 0.20 ± 0.03a 0.42 ± 0.02a 0.94 ± 0.04b2% 8.32 ± 1.16b 1.52 ± 0.19b 126 ± 16b 2462 ± 121a 0.25 ± 0.01a 0.41 ± 0.16a 1.02 ± 0.05b5% 9.74 ± 1.86b 1.59 ± 0.24b 139 ± 21b 3023 ± 648ab 0.25 ± 0.05a 0.29 ± 0.06a 1.18 ± 0.06b

a Surface area of root.b Ratio of root to shoot.c Accumulation efficiency of nitrogen.d Utilization efficiency of nitrogen.e Different small letter behind the values indicated significant difference, which was analyzed by LSD test (P b 0.05) using Statistical Product and Service Solutions Software (SPSS

18.0).

38 H. Zheng et al. / Geoderma 206 (2013) 32–39

retention and acquisition (Atkinson et al., 2010). Lehmann et al. (2003)attributed the increase of rice and cowpea biomass to K, P and, possibly,Cu nutrition provided by their biochar. The biochar used in our studycontained little available N (20.4 mg kg−1, Table 1) and the direct Nsupply (equivalent to 4.1 mg) can be neglected. However, Graber et al.(2010) reported that biochar application (1–5%) significantly enhancedthe growth of pepper (Capsicum annuum L.) and tomato (Lycopersicumesculentum Mill.) and their productivity, and they assigned the positiveeffects to the shifts in microbial populations towards beneficial plantgrowth promoting rhizobacteria or fungi, due to either chemical or phys-ical attributes of the biochar. This was consistent with our microbial ac-tivity data, being increased following biochar addition (Figs. 3d, 4e), andmay be used to interpret the improvement of maize growth in thebiochar-amended soils.

The improvement of plant growth in soil should come with the re-sponse of plant root and rhizosphere, which are of importance towater and nutrient uptake, carbohydrate storage and growth regulation(Wang et al., 2001). Rhizosphere is the interactive interface betweenplant and soil, and thus any change in root morphology is a key mecha-nism for the plants to adapt to soil changes such as pHalteration and nu-trient improvement (Hinsinger et al., 2003). In our study, the maizeroots grown in biochar-amended soil were obviously bigger than thoseof the control (Fig. S6d–g). The length, volume, surface area and tips ofmaize roots were significantly increased by 78.2–128.2%, 36.6–58.9%,30.2–67.3% and 7.8–42.9%, respectively (Table 3). Root developmentis remarkably sensitive to variations in supply and distribution of in-organic nutrients in the soil, and may quickly respond to the soil nu-trient variation (Forde and Lorenzo, 2001). The improvement of rootmorphology would certainly strengthen the nutrient bioavailabilityof plants in soils (Wang et al., 2001).

3.4. Effect of biochar on N bioavailability

The addition of biochar can influence N bioavailability and alterbioavailability of other nutrients (Major et al., 2009; Steiner et al.,2010; Taghizadeh-Toosi et al., 2012). Two parameters were usedhere to evaluate the N bioavailability, namely N accumulation efficiency(NAE, the amount of N intake per unit root length/volume/SA/tips)and N utilization efficiency (NUE, the amount of biomass producedby per unit N) (Wang et al., 2007). The NAE significantly decreasedfrom 0.96 mg m−1 in the control soil to 0.29–0.42 mg m−1 in thebiochar-amended soils, while NUE showed a reverse trend (Table 3).This suggested that less N was absorbed by the maize seedlings butmore biomass was produced using the equivalent amount of N in thebiochar-amended soil compared to that of the control, thus, biochar ad-dition decreased N fertilizer demand for maize growth. Therefore, theapplication of biochar in the soil enhanced the N bioavailability formaize growth in our study.

The enhancement of N bioavailability and N retention in the biochar-amended soilsmay result in potential agronomic and environmental ben-efits, such as reduction in fertilizer application, decline of the cost of food

production and mitigation of N2O emission. However, it is noted thatthe effects of biochar on N leaching and bioavailability were based onthe short-term (b3 month) experiments in this study. The biocharadded in soils can be aged with the formation of oxygen-containingfunctional groups (e.g. carboxyl and hydroxyl) (Cheng and Lehmann,2009), carbonmineralization (Zimmerman, 2010) and adsorption of or-ganicmatters from the soils (Zhang et al., 2011) over time. The effects ofaged-biochar on N cycles may be different from the fresh biochar. Forexample, the aged-biochar with newly-formed oxygen-containingfunctional groups could adsorb more NH4

+ than the fresh ones. There-fore, works based on the long-term effects of biochar amendment onthe N cycle in soils are warranted.

4. Conclusion

Biochar addition reduced NH4+–N and NO3

−–N leaching followingNO3

−–N addition. Less NH4+–N and NO3–N were leached from the

NO3−–N fertilized soil with maize grown. The mitigation of N leaching

losses following biochar addition was attributed to the soil WHC in-crease, NH4

+ adsorption and enhanced N immobilization. Biochar addi-tion stimulated maize growth, both above and below ground. The NAEand NUE were used to estimate the N bioavailability. Biochar additionincreased NUE of maize but decreased NAE. These data indicated thatbiochar addition could reduce N leaching, increase N retention andbioavailability in agricultural soils, and thus may decrease N fertilizerdemand for crop growth.

Acknowledgments

This study was supported by the Ocean PublicWelfare Scientific Re-search Project (2013418021), the Scientific and technological project ofQingdao (10-3-3-21-nsh), and USDA Hatch Program (MAS 00982).

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.geoderma.2013.04.018.

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