Synergy effects of biochar and polyacrylamide on plants growth

Environ Earth Sci (2015) 74:2463–2473 DOI 10.1007/s12665-015-4262-5

ORIGINAL ARTICLE

Synergy effects of biochar and polyacrylamide on plants growth and soil erosion control Sang Soo Lee1 • Haleem S. Shah1 • Yasser M. Awad1 • Sandeep Kumar2 Yong Sik Ok1

•

Received: 22 July 2014 / Accepted: 28 February 2015 / Published online: 14 March 2015 Ó Springer-Verlag Berlin Heidelberg 2015

Abstract Soil organic carbon (SOC) is one of the most critical factors determining soil quality or fertility. Recent survey has reported the severe degradation of SOC by soil erosion in agricultural fields throughout the world. To maintain soil quality or productivity, biochar (BC) or anionic polyacrylamide (PAM) has been recently suggested; however, the combination effects of BC and PAM have not been reported to date. This study evaluated the effect of BC, PAM or their mixture (BC?PAM) on soil quality, plant growth, and runoff and soil loss under simulated and natural rainfalls. Applications of BC promoted growth of soybean (C3 type) and maize (C4 type) plants and maintained soil physical properties such as water retention and stability. Our findings showed that BC?PAM was the best for plant growth, even other subject amendments were not worse. Addition of BC may lead to accelerate the metabolic-performance capacity of plants, especially C3 plant, due to sufficient C source. For runoff and soil loss tests, all amendments increased runoff compared to the control possibly due to clogging soil pore by viscous PAM solution application and decreased soil loss due to clay flocculation and aggregate stabilization by PAM, and water adsorbing capacity of BC. The use of BC?PAM can be a new, excellent strategy to promote plant growth and reduce soil loss; however, optimum application method should be considered carefully prior to its practical use.

& Yong Sik Ok soilok@kangwon.ac.kr 1

Department of Environmental Biology, Korea Biochar Research Center, Kangwon National University, Chuncheon 200-701, Korea

2

Department of Plant Science, South Dakota State University, Brookings, SD 57007, USA

Keywords Biochar Polyacrylamide Maize Soybean Soil quality Runoff Soil loss

Introduction Maintaining soil quality is a prerequisite for sustainable agriculture. Soil organic carbon (SOC) is one of the most critical factors determining soil fertility as an indicator of soil quality. The level of SOC determines the water and nutrients availabilities in the agricultural soils through a dynamics change. It also leads to alter physicochemical and biological soil properties (Arshad and Coen 1992; Reeves 1997).

However, the level of SOC in agricultural soils is degrading worldwide by intensive cultivation/grazing and soil erosion (Dalal et al. 1991). The erosion by water frequently results in the acceleration of topsoil loss along with the SOC and other macro/micro plant nutrients, thereby reducing crop yields and soil productivity consequently (Conforti et al. 2013; Jien and Wang 2013; Massey and Jackson 1952; Rogers 1941; Stallard 1998; Starr et al. 2000; Troeh et al. 1991).

Addition of plant residues into soils is be an excellent enhancer to ensure a sufficient level of SOC and helps preventing soil degradation (Chen et al. 2009; Mikha and Rice 2004; Novak et al. 2009; Sommerfeldt et al. 1988). It has been known as one of the best management practices (BMPs) to improve soil nutrients and water holding capability (WHC) (Reganold 1988), maintain soil aggregation (Grandy et al. 2002), and reduce the dosage of chemical fertilizers (Clark et al. 1998). However, due to exceed level of CO2 emission under global warming, fast decomposition/mineralization of plant residues is being in question nowadays (Chen et al. 2009).

Black carbon derived from biomass, also known as biochar (BC), is a pyrolysis byproduct at a relatively low

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temperature with no oxygen supply. Biochar is recently being highlighted as a soil amendment to improve soil quality, sequester C in a soil, and remediate organic/inorganic contaminants in a soil (Ahmad et al. 2012, 2014; Bruun et al. 2009; Glaser et al. 2001, 2002; Lehmann et al.

2003, 2006; Rajapaksha et al. 2014; Vithanage et al. 2015; Zhang and Ok 2014). Mikan and Abrams (1995) found that the application of BC into a soil increases cation exchange capacity (CEC) by up to 40 % with a significant decrease of soil acidity (Mikan and Abrams 1995). Schmidt and Noack (2000) also found the increases of CEC and WHC in the soils treated with BC. Moreover, BC is advantageous to reuse/recycle waste materials, agricultural/industrial byproducts, useless biomass, and other natural/anthropogenic waste sources; therefore, BC is not only an effective soil amendment but also an environmentally friendly implement (Ahmad et al. 2013; Mohan et al. 2014; Warnock et al. 2007).

Since the 1990s, anionic polyacrylamide (PAM) is often considered as an effective way to reduce water turbidity and soil erosion (Lee et al. 2010). It has been known to improve soil aggregate (Agassi and Ben-Hur 1992), increase infiltration rate and soil water retention (Flanagan et al. 1997), and reduce runoff (Letey 1996) and soil erosion (Sojka et al. 2007). However, its effectiveness may be limited depending on the properties of soils such as clay content, electrolyte level, and organic matter (OM) (Lee et al. 2010). The objectives of this study were to determine the enhancement of soil quality/fertility and the change of C3 and C4 plants growth in soils amended with BC, PAM or their mixture. In addition, the effects of BC and PAM on runoff and soil loss were evaluated using simulated and natural rainfalls.

Materials and methods Soil and amendments The top layer (0–30 cm deep) of an agricultural soil was collected from Hongcheon in Gangwon Province, Korea (37°450 1200 N latitude, 127°510 1500 E longitude). Sampled soil was air-dried and passed through a 10 mm sieve for simulated rainfall experiment and a 2 mm sieve for soil characterization analysis. Soil was a loam having 31.1 % silt and 23.3 % clay, and the values of pH and EC were 6.42 and 0.03 dS m–1, respectively, as shown in Table 1.

Soil texture was determined by the Bouyoucos hydrometer method (Gee and Or 2002), OM by the wet digestion (Walkley and Black 1934), electrical conductivity (EC) and pH by 1:5 soil: water suspension, and exchangeable cations and CEC by an inductively coupled plasma (ICP) using 1 M NH4OAc solution (Sumner and Miller 1996).

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Commercially available granular anionic PAM (Soilfix G1, Ciba Chemical Co., Germany) having a high molecular weight (15 Mg mole–1) was employed in this study.

Granular PAM was dissolved with 340 mL tap water (EC 0.4 dS m–1 and pH 7.3) for 48 h at 21 °C, reaching to a concentration of 500 mg L–1. The pH value of aqueous PAM solution was 7.4. The BC derived from domestic oak tree was also used (Sootgage Company, Pocheon, Gyeonggi Province, Korea). The values of pH, EC, and organic carbon content of BC were 11.02, 0.75 dS m–1, and 88 %, respectively (Table 1).

Plant growth experiment Three kilograms of 10 mm sieved soils, having gravimetric water content of 5 %, were repacked into each pot. Each of three amendments, including BC at 10 Mg ha–1 (BC), PAM at 80 kg ha–1 (PAM), and its combination (BC?PAM), was applied to repacked pots along with the control (CK) which has no amendment. Characterization of soils amended with each amendment was done prior to experiment (Table 1). Each run was in triplicate.

The soybean (Glycine max) and maize (Zea mays L.) as representative C3 (Glycine max) and C4 (Zea mays L.) plants, respectively, were grown in amended pots.

Specifically, soybean and maize seeds soaked in water were sown in each pot. During 6 weeks cultivation, the water content in each pot was maintained at 70 % WHC. At 10 days after sowing, each pot was thinned to two seedlings. Selected parameters of plant growth including height, leaf area index (LAI), and chlorophyll readings (SPAD) were measured every week, and the soil water content was measured biweekly. Additionally, fresh and dry matter weights of soybean and maize were determined after harvest.

The plant height was measured at a bottom of stem to a tip of the highest leaf. Chlorophyll readings were taken by a SPAD-502 chlorophyll meter (Konica-Minolta Corporation, Osaka, Japan) and leaf area index was calculated from the ratio of leaf area to land surface. At the end of cultivation, the aerial and subterranean parts of plants were removed from the pots and were weighed. Then, these removed plant samples were dried for a week at 70 °C using a dry oven.

Simulated rainfall experiment Three kilograms of air-dried and 10 mm sieved soils, having gravimetric water content of 5 %, were packed in plastic test beds (200 mm wide 9 200 mm long 9 150 mm deep) for a simulated rainfall test in the laboratory (Fig. 1). This test bed was also designed for measurement of infiltration rate during rainfall event. Test bed was

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Table 1 Physicochemical characteristics of the soil used in this experiment (n = 3) OMa (g kg-1)

TCb (%)

pH (dS m-1)

ECc

–

–

88.0

11.02

232.91

11.21 d

–

6.42 b

22.99 a

–

6.60 a

Sand (g kg-1)

Silt (g kg-1)

Biochar (BC)

–

–

Loam soil

455.80

311.30

–

–

–

Texture

With BC

a

Clay (g kg-1)

NH4OAcd extractable cations Ca2? (cmol(?) kg-1)

Mg2? (cmol(?) kg-1)

K? (cmol(?) kg-1)

Na? (cmol(?) kg-1)

0.75

–

–

–

–

0.03 b

7.57 c

2.73 ab

0.27 b

0.18 a

0.04 a

8.33 a

2.67 b

0.30 a

0.15 c

With PAM

–

–

–

14.65 c

–

6.17 c

0.04 ab

7.83 bc

2.79 a

0.26 b

0.17 ab

With BC?PAM

–

–

–

18.10 b

–

6.62 a

0.04 a

7.97 b

2.54 c

0.28 ab

0.16 bc

Organic matter

b

Total carbon

c

Electrical conductivity

d

Ammonium acetate

Fig. 1 Soil test bed used for simulated rainfall experiment (200 mm wide 9 200 mm long 9 150 mm deep)

packed with a 50 mm layer of coarse sand on top of which a 100 mm layer of sampled loam soil was packed. The repacked soil in test beds had a bulk density of 1.10 ± 0.06 Mg m–3. To avoid interfacial water flow during rainfall event, bentonite slurry (1:8 bentonite: water) was injected around the test bed perimeter to a depth of 50 mm (Lee et al. 2010). Additionally, the packed soil test beds were under-saturated using tap water (EC 0.4 dS m–1 and pH 7.3) for 48 h to maintain intra-aggregate pore continuity and connectivity. Test beds were adjusted to a slope of 10 % and experimental run was in triplicate.

Rotating-boom rainfall simulator was employed (Swanson 1965). Simulated rainfall was applied at an intensity of 100 ± 2.7 mm h–1 with consideration of very high precipitation variation due to climate change in Asian monsoon region. The rainfall simulator was calibrated before and after every run. Runoff was collected through the V-trough and measured for 1 min every 5 min.

Collected runoff was weighed and then dried for 48 h at 105 °C. Natural rainfall experiment Pilot-scale plots were installed at an upland area in Chuncheon, Gangwon Province, Korea (37°940 7800 N lat., 127°750 3900 E long.). Total 12 experimental-plots (1 m wide 9 5.525 m long) including four amendments (CK, BC, PAM, BC?PAM) in triplicate were installed and their slope was adjusted to 2.58 ± 0.33 % (Fig. 2). The plots were established as randomized complete block design (RCBD). Prior to application of each amendment, a top layer of 200 mm was tilled after vegetation removal and left it for a month to stabilize soil aggregate. The same amounts of each amendment with a laboratory experiment were applied in the soil surface of stabilized plots. With intermittent rain events, runoff was collected during 24 h

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Fig. 2 Field plot at an upland area for natural rainfall experiment (1 m wide 9 5.525 m long)

and weighed. From collected runoff, 250 mL specimen was sampled and dried for a week at 105 °C for soil loss measurement. Average precipitation was 46.7 mm days–1 and maximum intensity of rainfall was 27.6 mm h–1 in three measurements of natural rainfall events on August 2013.

Statistics Data were analyzed using statistics software package SAS 9.1 (SAS 2004). Treatments means were compared using Tukey’s honestly significant differences (HSD) test at a 0.05 significance level.

Fig. 3 Change of water contents in the pots grown a no plant, b soybean (C3), and c maize (C4), amended with 10 Mg ha-1 biochar (BC), 80 kg ha-1 PAM (PAM), 10 Mg ha-1 ? 80 kg ha-1 PAM (BC?PAM), and the unamended control (CK). Dots and error bars indicate mean values and standard deviations, respectively (n = 3)

Results and discussion Change in water content by amendments Changes of water content in each pot were determined at 14, 28, and 42 days after sowing for each pot planted with soybean and maize, subjected to amendments of BC, PAM, and BC?PAM along with CK (Fig. 3). For no plant pot, no amendment effect was found in water content at 14 days; however, the applications of BC and BC?PAM amendments significantly increased the water content by 19.2 and

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5.7 %, respectively, at 28 days compared to the CK (all p values\0.05). At 42 days after sowing, the water content in the pots amended with BC, PAM, and BC?PAM were significantly increased by 10.3, 6.2, and 22.2 %, respectively, compared to the CK (all p values \0.05). Applications of BC and BC?PAM led to maintain soil moisture and the BC?PAM showed higher water retention than BC only.

For soybean and maize plants, no or little change of water content was observed at 14 days in response with the

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subjected amendments. For maize pots at 28 days, BC and PAM didn’t alter water content whereas BC?PAM increased water content significantly by 19.4 and 29.1 % in comparison with the CK (both p values \0.05) at 28 and 42 days, respectively. For soybean pots, no change of water content was detected with any amendment or cultivation period. No effect of water retention maintenance in soybean pots with amendments may be explained that soybean is known as a typical C3 plant requiring a higher amount of water than maize as a typical C4 plant because of a relatively high rate of photosynthesis or transpiration (Sage 2004).

Generally, BC?PAM was the best for maintaining water content. Our findings agree with previous studies that the application of BC or PAM can maintain water retention directly or indirectly because of a relatively large surface area of BC or a high WHC of PAM molecules (Kishimoto and Sugiura 1985; Sojka et al. 2007; van Zwieten et al.

2010). Kishimoto and Sugiura (1985) found that BC has relatively larger surface area (*200–400 m2 g–1) via pyrolysis at 100–200 °C and van Zwieten et al. (2010) also reported a surface area of 115 m2 g–1 of BC derived from paper-mill pyrolysis. Indirectly, the BC interacts with soil OM, minerals, and microorganisms and improves soil aggregate and structure, thereby maintaining water retention in soils (Verheijen et al. 2010). Regarding plant growth, BC increases plant available water, i.e., indicating -33 * -1500 kPa potential, by increasing soil macroaggregate by 20–130 % (Mbagwu and Piccolo 1997). PAM also contributes maintenance of water retention or conservation in soils by buffering the root zone against water loss, especially in situations where drought occurs (Letey et al. 1992). However, PAM does not reduce water demand by plants but enhance soil water storage in soils. Based on our findings, the mixture of BC and PAM is expected to be an excellent strategy for improving water retention or storage capacity of coarse-textured soils.

Soybean and maize growth Plant growth in response to the applications of BC, PAM, and BC?PAM was investigated by pot experiment. Mean values of selected growth parameters for soybean and maize were determined at the end of 6 weeks cultivation, as shown in Table 2. Mean values of plant height ranged from 65.60 to 87.21 and from 47.07 to 51.70 for soybean and maize, respectively. The tallest height of soybean was measured in pot amended with BC?PAM and it was 24.8 % taller than the CK (p \ 0.05) while no difference among pots with different amendments was found for maize height. For soybean, addition of BC, i.e. BC and BC?PAM amendments, led to significant increases of its

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height compared to the CK. However, the addition of PAM didn’t bring any positive effect in this study except dry weight for soybean. These findings partially agree with a study of Lentz and Sojka (2009) that PAM application increased maize and bean yields up to 11.2 % by improving soil physical properties. We speculate that no effects of PAM on other growth parameters may result from a small pot size and short-term period of cultivation in this study compared to other studies conducted for a long-term field study.

For fresh/dry weight of soybean and maize, the addition of BC (i.e. BC and BC?PAM) was mostly effective. Biochar has been known as a soil amendment to increase nutrient availability and improve physicochemical conditions of soil, thereby promising a high agricultural productivity (Major et al. 2010). However, the stem diameter, leaf number, LAI, and SPAD of soybean and maize were not affected by BC addition. This could be delayed or inhibitory effects of BC on the nutrients uptake responsible for increasing chlorophyll content. The expectation of BC effects such as surface oxidation, CEC, and nutrient retention would be appeared gradually with time after its application into soils (Cheng et al. 2006, 2008; Major et al. 2010). It may also be attributed to irrigation schedule and immobilization of soil N associated with imbalance of C/N ratio, resulting from a high carbon content of BC (Jensen 1996).

Analysis of variance (ANOVA) was done to verify the effects of amendments and plants types on growth parameters as shown in Table 3. Applications of subjected amendments significantly affected stem diameter and dry weight (p \ 0.001) and the types of plants including soybean as C3 and maize as C4 were critical for all measured parameters of plant growth (p \ 0.001 for plant height, stem diameter, leaf number, LAI, SPAD, and dry weight; p \ 0.05 for fresh weight). The mixed effects of amendments by plant types on plant height, stem diameter, LAI, and dry weight were also significant. Variable responses of soybean and maize growth in response with the subjected amendments can be related to the dissimilar physiology of C3 and C4 plant types’ characteristics. Based on the physiological differences between C3 and C4 plants, the conditions of soil and atmosphere are very critical for their performances in growth (Almaroai et al. 2014; Taiz and Zeiger 2010; Ward et al. 2008). The BC or BC?PAM addition may accelerate the metabolic-performance capacity of C3 plants with sufficient C source from BC via a higher rate of photosynthesis than C4 plants (Sage 2004; van Zwieten et al. 2010; Verheijen et al. 2010; Wu et al.

2012). In consideration of plant types, the proper selection of BC and PAM, or their mixed applications would promise better efficacy on plant productivity and soil nutrient availability (Awad et al. 2012, 2013).

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Table 2 Mean values of growth parameters for soybean and maize in the pots subjected to 10 Mg ha-1 biochar (BC), 80 kg ha-1 PAM (PAM), and 10 Mg ha-1 ? 80 kg ha-1 PAM (BC?PAM) along with the unamended control (CK) (Mean ± SD; n = 3) Amendment

Plant height (cm)

Stem diameter (mm)

LAIa

Leaf number

SPADb

Fresh weight (g)

Dry weight (g)

Soybean (Glycine max) CK

65.60 ± 28.04 b

4.94 ± 0.51 bc

6.00 ± 2.30 a

2.41 ± 0.35 ab

31.75 ± 3.60 a

14.51 ± 3.66 a

2.28 ± 0.26 b

BC PAM

87.09 ± 24.88 a 79.31 ± 32.65 ab

5.05 ± 0.42 ab 4.70 ± 0.51 c

6.74 ± 2.31 a 6.13 ± 2.23 a

2.41 ± 0.17 ab 2.35 ± 0.15 b

29.81 ± 3.79 a 31.46 ± 2.61 a

14.33 ± 3.57 a 14.36 ± 3.89 a

3.09 ± 0.02 a 3.04 ± 0.23 a

BC?PAM

87.21 ± 26.52 a

5.33 ± 0.49 a

6.89 ± 2.37 a

2.52 ± 0.20 a

31.10 ± 2.49 a

15.04 ± 2.84 a

3.48 ± 0.15 a

10.41 ± 0.80 b

Maize (Zea mays L.) CK

51.70 ± 9.62 a

4.12 ± 0.42 b

4.59 ± 0.65 a

8.85 ± 0.38 ab

21.93 ± 5.26 a

BC

49.59 ± 9.46 a

4.47 ± 0.37 a

4.94 ± 0.71 a

8.46 ± 0.48 b

22.06 ± 4.55 a

11.76 ± 0.09 ab

2.16 ± 0.02 a

PAM

47.07 ± 10.70 a

4.17 ± 0.46 b

5.00 ± 0.73 a

8.95 ± 0.28 a

22.79 ± 3.10 a

12.69 ± 0.73 ab

1.93 ± 0.06 ab

BC?PAM

48.90 ± 10.16 a

4.21 ± 0.42 ab

4.96 ± 0.66 a

8.66 ± 0.76 ab

22.59 ± 3.45 a

13.59 ± 1.65 a

1.96 ± 0.12 ab

a

Leaf area index

b

Chlorophyll meter

1.70 ± 0.15 b

Table 3 Analysis of variance (ANOVA) in growth parameters of soybean and maize subjected to 10 Mg ha-1 biochar (BC), 80 kg ha-1 PAM (PAM), and 10 Mg ha-1 ? 80 kg ha-1 PAM (BC?PAM) along with the unamended control (CK) Source

df

Plant height

Stem diameter

Leaf number

LAIa

SPADb

Fresh weight

Dry weight

0.651

\0.001***

0.034*

\0.001***

0.546

0.806

0.001**

P[F Amendment

3

\0.001***

0.2084

0.032*

Plant

1

\0.001***

\0.001***

\0.001***

\0.001***

Amendment 9 plant

3

0.009**

0.003**

0.6029

0.002***

446.750

0.207

2.891

0.149

13.760

6.658

0.023

0.388

0.476

0.201

0.986

0.592

0.335

0.960

Error MS R2

0.057

0.372 \0.001***

* p \ 0.05, ** p \ 0.01 and *** p \ 0.001 a

Leaf area index

b

Chlorophyll meter

Dry weights of aerial and subterranean parts Dried aerial and subterranean parts of soybean and maize plants were weighed to compare the effects of subjected amendments on aerial and subterranean parts of each plant (Fig. 4). For aerial part of soybean, the BC?PAM application recorded the highest value among all amendments (p \ 0.05) and the application of BC or PAM was higher than the CK (both p values \0.05); however, no difference was found in pots amended with BC and PAM. The BC, PAM, and BC?PAM applications significantly increased dried weights of soybean aerial parts by 28.1, 25.4, and 34.9 % compared to the CK (all p values \0.05). On the other hand, for subterranean part of soybean, no difference of each dried weight was found when amendments were applied. For the dry weight of maize aerial part, the BC showed the highest among all subjected amendments (p \ 0.05). Interestingly, although the dried weights of maize aerial parts in the pots amended with the PAM and

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BC?PAM were slightly increased, these increases were not significant compared to the CK. On the other hand, all amendments increased the dried weights of maize subterranean part when compared to the CK (all p values \ 0.05).

Increasing dry weight of plants commonly promises a high agricultural productivity. Addition of BC led to increase dry weights of soybean and maize in this study. It might be a consequence of soil quality improvement. By adding BC, the soil fertility can be maintained by increasing soil carbon, infiltration, water retention, soil aggregate, and soil nutrient availability. Ishii and Kadoya (1994) and Yamato et al. (2006) reported the similar increase of dry weight of plant root by improving soil physicochemical properties after the BC application into soils. This change may also result from the positive effects of BC on plant nutrition availability (Glaser et al. 2002; Lehmann et al. 2003; van Zwieten et al. 2010), SOC (Novak et al. 2009), soil aggregate (Glaser et al. 2002),

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Runoff and soil loss: simulated versus natural rainfall

Fig. 4 Dry weights of aerial and subterranean parts for soybean (C3) and maize (C4) grown in the pots amended with 10 Mg ha-1 biochar (BC), 80 kg ha-1 PAM (PAM), and 10 Mg ha-1 ? 80 kg ha-1 PAM (BC?PAM), along with the unamended control (CK). Same letters above the bars are not significantly different according to Tukey’s studentized range test (p \ 0.05; n = 3)

and cation status (Chan et al. 2008). Glaser et al. (2002) has insisted that the BC application can increase WHC, SOC, pH, and CEC with low soil strength. A similar increase of CEC by the BC application was also reported by Liang et al. (2006) and Novak et al. (2009) showing increases of soil pH, OM, Mn, and Ca, and decreases of S and Zn, by adding BC into agricultural soils. van Zwieten et al. (2010) attributed sufficient amounts of P and K in the soil to BC addition.

Application of PAM has been known as a soil amendment to improving soil physical properties. The higher yields of soybean and maize with PAM in this study can be partially explained with enhancement of soil physical properties, such as tensile strength, aggregate stability, root penetration, pore continuity, water retention etc. (Agassi and Ben-Hur 1992; Barry et al. 1991; Busscher et al. 2007; Terry and Nelson 1986). Lee et al. (2008) also observed that the application of PAM increases soil porosity and aeration, possibly building more favorable soil condition for plant growth.

Runoff and soil loss from soils amended with BC, PAM, and BC?PAM were determined using a rainfall simulator and natural rainfall, along with the CK. For a simulated rainfall experiment, all amendments increased runoff significantly, whereas soil loss was decreased compared to the CK (all p values of \0.05; Fig. 5). Runoff in the pots amended with BC, PAM, and BC?PAM was increased by 14.8, 22.9, and 23.2 %, respectively, compared to the CK. Although no difference of runoff in pots amended with PAM and BC?PAM was found, these amendments increased runoff the most. These increases of runoff may be mainly resulted from PAM addition. We speculate that PAM solution at a concentration of 500 mg L–1 used in this study may plug or clog soil pores in soil aggregate. Sirjacobs et al. (2000) and Lentz (2003) pointed out that viscous PAM solution can clog soil pores, mainly through macropores, and reduce infiltration, thereby increasing runoff.

This finding was also agreed with Lee et al. (2010) showing that the aqueous PAM solution at 40 kg ha–1 significantly increased runoff by an average of 4 % compared to that at 20 kg ha–1 or no treated silt loam soil with relatively high OM content. However, the soil loss was significantly decreased with all amendments compared to the CK (all p values of \0.05), especially in a pot amended with BC?PAM which showed the most effective in reducing soil loss. Soil loss was reduced by 19.9, 23.8, and 42.1 % in the pots amended with BC, PAM, and BC?PAM, respectively.

The fact that a proper amount of PAM application effectively reduces soil loss in the most types of soil was well documented. Different ion charges and molecular weights of PAMs were tested on different types of soils having various clay content, CEC, OM etc., for verifying its practical use for soil erosion control (Lee et al. 2010, 2013; Lentz and Sojka 2009; Mamedov et al. 2010; Sirjacobs et al. 2000; Sojka et al. 2007). PAM stabilizes silt and clay soils via clay flocculation by bonding long polymer chains with soil particles (Sojka et al. 2007); however, I should be noted that it only stabilizes soil aggregation, not improve poor structure of a puddled soil (Agassi and Ben-Hur 1992; Cook and Nelson, 1986). Our findings agree with Zhang and Miller (1996) showing 48–66 % reduction of soil loss on Cecil sandy loam (clayey, kaolinitic, thermic Typic Kanhapludults) in a ridge furrow.

In case of BC application, the impact on soil structure and stability which are critical factors determining soil erosion rate was poorly reported although positive effects of BC on plant nutrient availability or loss, WHC, CEC, drought tolerance, and contamination mitigation were widely understood by many studies (Ogawa et al. 2006;

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Fig. 5 Runoff and soil loss from the soil test beds amended with 10 Mg ha-1 biochar (BC), 80 kg ha-1 PAM (PAM), and 10 Mg ha-1 ? 80 kg ha-1 PAM (BC?PAM), along with the unamended control (CK), subjected to simulated rainfall having a 100 mm h-1 rainfall intensity. Same letters above the bars are not significantly different according to Tukey’s studentized range test (p \ 0.05; n = 3)

Fig. 6 Runoff and soil loss from the pilot-scale plots amended with 10 Mg ha-1 biochar (BC), 80 kg ha-1 PAM (PAM), and 10 Mg ha-1 ? 80 kg ha-1 PAM (BC?PAM), along with the unamended control (CK), subjected to 46.7 mm days-1 natural rainfall.

Same letters above the bars are not significantly different according to Tukey’s studentized range test (p \ 0.05; n = 3)

Vithanage et al. 2014; Yu et al. 2006). Interestingly, our results found the reduction of soil loss by up to 19.9 % in BC amended soils subjected to a 100-mm h–1 simulated rainfall, compared to the CK. It may be explained with physicochemical properties of BC and its action in wetting soils. We speculate that BC may lead to maintain water retention and possibly stabilize the soil macro-aggregate to trap water molecules by bridging cations and soil particles under coulombic and van der Waals forces. Applied BC in soil or soil surface may also absorb or buffer raindrop energy to eliminate soil particle detachment, soil crusting or sealing (Lee et al. 2008).

For a 33-mm days–1 natural rainfall, no difference of runoff was found in field soils amended with BC and BC?PAM along with the CK (Fig. 6). Unexpectedly, runoff was only increased in a field soil amended with PAM, showing 17.9 % higher than other amendments or the CK (p \ 0.05). As mentioned, a relatively high density

of PAM solution might clog soil pore on surface, thereby increasing runoff in a field soil. The reason for no increasing runoff in a field soil amended with BC or BC?PAM compared to the CK is that the offset effects of BC addition, such adsorbing or retaining water in BC surface or particle, may be involved (Verheijen et al.

2010). However, the soil loss was significantly decreased by 49.6, 71.1, and 70.4 % in field soils amended with BC, PAM, and BC?PAM, respectively, compared to the CK (all p values of\0.05). This trend was very similar with the findings of simulated rainfall experiment in this study and can be explained with the same reasons suggested. It should be noted that the natural rainfall experiment was conducted in a concept of gross volume of rainfall a day without intensity or variation adjustment; therefore, data are limited to understand the mechanism of reducing soil loss with subject amendments, but may face reality in nature.

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Conclusions This study evaluated the effect of BC, PAM or their mixture (BC?PAM) on soil quality, plant growth, and runoff and soil loss under simulated and natural rainfalls. Applications of BC led to maintain water retention or storage capacity and the BC along with PAM was the best. The BC?PAM also showed the highest growth rate on both C3 and C4 plants in general. Application of PAM alone indicated no negative effects on plant growth, but it was not effective as well. In comparison with C3 and C4 plants against subject soil amendments, BC addition (i.e. BC, BC?PAM) may lead to accelerate the metabolic-performance capacity of C3 plants with BC’s sufficient C compared to C4 plants. For runoff and soil loss, the subject amendments generally increased runoff possibly due to clogging soil pore by viscous PAM solution application and decreased soil loss due to clay flocculation and aggregate stabilization by PAM, and water adsorbing capacity of BC. Acknowledgments This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2013R1A1A2057582).

Conflict of interest of interest.

The authors declare that they have no conflict

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