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Birch (Betula spp.) wood biochar is a potential soil amendment to reduce glyphosate leaching in agricultural soils
Journal of Environmental Management 164 (2015) 46e52
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Research article
Birch (Betula spp.) wood biochar is a potential soil amendment to reduce glyphosate leaching in agricultural soils Marleena Hagner a, *, Sanna Hallman b, Lauri Jauhiainen c, Riitta Kemppainen c, €la €a €mo € c, Kari Tiilikkala c, Heikki Seta Sari Ra a b c
University of Helsinki, Department of Environmental Sciences, Niemenkatu 73, 15340 Lahti, Finland €skyla €, Department of Biological and Environmental Science, PO Box 35, 40014 Jyva €skyla €, Finland University of Jyva Natural Resources Institute (Luke), 31600 Jokioinen, Finland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 June 2015 Received in revised form 24 August 2015 Accepted 25 August 2015 Available online 3 September 2015
Glyphosate (N-(phosphonomethyl) glycine), a commonly used herbicide in agriculture can leach to deeper soil layers and settle in surface- and ground waters. To mitigate the leaching of pesticides and nutrients, biochar has been suggested as a potential soil amendment due to its ability to sorb both organic and inorganic substances. However, the efficiency of biochar in retaining agro-chemicals in the soil is likely to vary with feedstock material and pyrolysis conditions. A greenhouse pot experiment, mimicking a crop rotation cycle of three plant genera, was established to study the effects of pyrolysis temperature on the ability of birch (Betula sp.) wood originated biochar to reduce the leaching of (i) glyphosate, (ii) its primary degradation product AMPA and (iii) phosphorus from the soil. The biochar types used were produced at three different temperatures: 300 C (BC300), 375 C (BC375) and 475 C (BC475). Compared to the control treatment without biochar, the leaching of glyphosate was reduced by 81%, 74% and 58% in BC300, BC375 and BC475 treated soils, respectively. The respective values for AMPA were 46%, 39% and 23%. Biochar had no significant effect on the retention of water-soluble phosphorus in the soil. Our results corroborate earlier findings on pesticides, suggesting that biochar amendment to the soil is a promising way to reduce also the leaching of glyphosate. Importantly, the ability of biochar to adsorb agro-chemicals depends on the temperature at which feedstock is pyrolysed. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Biochar Glyphosate AMPA Phosphorus Leaching Pyrolysis temperature
1. Introduction Glyphosate (N-(phosphonomethyl) glycine) is a broadspectrum, nonselective and post-emergence organophosphate herbicide. It is an active ingredient in Roundup® and several other weed killing formulations commonly used in agricultural and nonagricultural systems (Baylis, 2000). For example, in Finland it accounted for almost 40% of all herbicide-active ingredients sold in 2010 (see Hagner, 2013). Glyphosate has unique sorption characteristics in the soil compared to other pesticides. Due to its rapid adsorption onto soil particles and relative fast degradation by microbes, glyphosate is assumed to be inactivated immediately after spraying and thus considered an environmentally safe herbicide (Giesy et al., 2000). However, the rate of degradation and sorption * Corresponding author. E-mail address: Marleena.Hagner@helsinki.fi (M. Hagner). http://dx.doi.org/10.1016/j.jenvman.2015.08.039 0301-4797/© 2015 Elsevier Ltd. All rights reserved.
of glyphosate depend on climatic conditions and soil properties (Gimsing et al., 2004a, 2004b). As a consequence, under certain environmental conditions glyphosate can leach to deeper soil layers and end up in surface- and ground waters (Battaglin et al., 2014; Borggaard and Gimsing, 2008; Laitinen et al., 2009; Kjaer et al., 2005). This may occur especially in northern ecosystems where glyphosate has a long persistence time due to cooler climates (Helander et al., 2012). As with pesticides, modern agriculture is dependent on phosphate fertilizers to replace the phosphorus removed via crop harvesting (Cordell et al., 2009). Simultaneously, excessive loads of phosphorus remain in the soil from where it can leach into adjacent aquatic ecosystems with well-known adverse effects on the quality of surface- and ground waters. The mobility of phosphorus in agricultural soils is controlled by several geo-chemical processes such as solubilization, complexation, adsorption and precipitation (Arai and Sparks, 2007). Judged by their chemical structure,
M. Hagner et al. / Journal of Environmental Management 164 (2015) 46e52
glyphosate and inorganic phosphates are strongly adsorbed to inorganic soil components, especially aluminium and iron oxides. This reaction leads to competition for sorption sites between glyphosate and phosphorus in the soil (Gimsing and Borggaard, 2002; Laitinen et al., 2008). Biochar, as a soil amendment, has been suggested to mitigate leaching losses of pesticides and nutrients. Biochar is a solid material produced by the thermochemical conversion (such as pyrolysis) of organic matter in an oxygen-limited environment. It has recently received much interest due to its ability to improve soil properties for enhanced plant production (IBI, 2014; Lehmann and Joseph, 2009; Verheijen et al., 2010). Moreover, biochar has received considerable interest as a soil amendment for the in situ stabilization of organic and inorganic contaminants. Evidence suggests that various biochar types have a high capacity to adsorb both organic (Beesley et al., 2010; Wang et al., 2010) and inorganic (Beesley et al., 2010; Cao et al., 2009) substances. For example, the sorption of phosphorus is reported to be strongly influenced by biochar in the soil (Chintala et al., 2014). The efficiency of biochar in pesticide and nutrient retention is likely to vary with regards to feedstock material and pyrolysis conditions (Mesa and Spokas, 2011). Slow and intermediate pyrolysis with residence times of a few minutes to several hours or days, and temperatures between 200 and 900 C are generally favoured in the production of biochar (Brown, 2009). A number of feedstock materials, including wood, animal litter, crop residues and solid waste, are in common use in the production of biochar (IBI, 2014; EBC, 2012). Pyrolysis temperature in particular plays a significant role in affecting biochar properties: the higher the pyrolysis temperature the higher the surface area, C content and aromaticity of biochar, and the lower the polarity and O and H content of biochar (Ahmad et al., 2014; Chen and Chen, 2009; Chen et al., 2012). A higher surface area, microporosity and hydrophobicity of biochar are usually associated with increased sorption potential of organic contaminants (Ahmad et al., 2014). Biochar produced at low pyrolysis temperatures on the other hand is characterized by a lower surface area and aromaticity but higher polarity and the amount of oxygen-containing functional groups on its surfaces, and may thus be more suitable in removing inorganic/ polar organic contaminants (Ahmad et al., 2014). Given the clear association between pyrolysis temperature and biochar properties, it is likely that the ability of biochar in retaining chemicals commonly applied in agriculture, such as glyphosate and phosphorus, is controlled by the temperature at which the feedstock material is pyrolysed. In a pilot study we showed that birch-derived biochar has the potential to reduce leaching of glyphosate in agricultural soils (Hagner et al., 2013). Here, our objective is to confirm this observation and extend the study design by including pyrolysis temperature as an explanatory variable. Consequently, we aimed to explore the effects of pyrolysis temperature on the ability of biochar to reduce the leaching of (i) glyphosate, (ii) its primary degradation product AMPA (aminomethylphosphonic acid) and (iii) phosphorus from the soil. We hypothesized that, due to the higher presence of polar functional groups in the surface layer, low-temperature biochar reduces the leaching of glyphosate/AMPA (polar organic chemicals) and phosphorus (inorganic chemical) from the soil more so than biochar pyrolysed at higher temperatures. This study forms part of a larger research program in which the impacts of pyrolysis temperature on the composition and chemical €s characteristics of birch-wood biochar is investigated (Fagerna €s and Kuoppala, unpublished). The effects of et al., 2014; Fagerna biochar characteristics on soil properties, plant growth and survival, and soil organisms were also investigated (Hagner et al., unpublished).
47
2. Materials and methods 2.1. Biochar properties Three types of biochar were produced by the VTT Technical Research Centre of Finland using a pilot-scale slow-pyrolysis unit. The pyrolysis process, as well as characteristics and analysis methods of the biochar types are reported in detail in Fagern€ as et al., 2014, Fagern€ as and Kuoppala, unpublished). The biochar types were produced using Betula sp. wood blocks as a feedstock, subject to three different temperatures: 300 C, 375 C and 475 C; referred to as BC300, BC375 and BC475. The biochar produced were crushed and passed through a 2 mm sieve before addition to the soil (see below). The specific surface area (BET-surface area analysed according to the Standard PANK 2401), water holding capacity (WHC) and other characteristics of the biochar types are presented in Table 1. The H/C and (O þ N)/C ratios were calculated to estimate the aromacity and polarity of the biochar types, respectively. 2.2. Establishment of the pot experiment The soil used in the experiment had no previous history of glyphosate application. Sandy silt soil was collected from an €rvi, SW-Finland in the autumn of organic agricultural field in Rehtija 2012. The soil had a pH of 6.02 (1:5 soil:water volume), water holding capacity (WHC) of 11%, electrical conductivity of 0.68 104 (S/cm) and pore volume of 47% (Sandbox-method, Eijkelkamp - Agrisearch equipment, 2013). The soil consisted of 22% of medium sand (grain size 200e600 mm), 56% fine sand (60e200 mm) and 21% of particles below 60 mm. The organic matter content of the soil was 5.6%. Concentrations of Ca, K, Mg, P and N were 760 mg/kg, 93 mg/kg, 34 mg/kg, 23 mg/kg and 0.45 mg/kg, respectively, analyzed in the laboratory of MTT Research Finland €kitie (1955). Earthaccording to the protocol by Vuorinen and Ma worms, larger stones and roots (diameter > 1 mm) were removed from the soil prior to use. Radish (Raphanus sativus), barley (Hordeum vulgare) and ryegrass (Lolium perenne) were used as test plants to mimic a crop rotation cycle of the three plant genera. The experiment was conducted in a greenhouse at MTT Agrifood Research Finland in Jokioinen in 2013, using 1500 ml flowerpots (Ø 11 cm, height 19 cm) with four holes (Ø 0.5 cm) at the bottom. The treatments consisted of soil (1 L ¼ 1320 g air dry soil, moisture Table 1 Characteristics of the biochar types produced at different pyrolysis temperatures: BC300 ¼ biochar produced at 300 C, BC375 ¼ at 375 C and BC475 ¼ at 475 C €s and Kuoppala, unpublished). (modified from Fagern€ as et al., 2014; Fagerna
Ash content (%) Volatile matter (%) C (%) H (%) N (%) O (%) S (%) Surface area (m2/g) WHC (%)a pH (1:5H2O) H/Cb (O þ N)/Cc Bulk density (g/cm3)d
BC300
BC375
BC475
0.5 48 72 4.9 0.2 23 0.01 2.2 23 5.1 0.07 0.32 0.41
0.7 30 80 3.9 0.3 15 0.01 6.4 24 5.2 0.05 0.19 0.41
1.0 17.2 89 3.1 0.3 7 0.01 44 288 7.5 0.04 0.09 0.45
a WHC: water holding capacity (24 h) i.e. the total amount of water a soil can hold (% of soil/biochar mass). b H/C: atomic ratio of H to C (mass). c (O þ N)/C: atomic ratio of sum of nitrogen and oxygen to carbon (mass), polarity index. d Bulk density (g/cm3) ¼ dry soil mass (g)/soil volume (cm3).
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M. Hagner et al. / Journal of Environmental Management 164 (2015) 46e52
content 1.0%) mixed with 1) 20 g of either BC300, BC375 or BC475, 2) 80 g of either BC300, BC375 or BC475 or 3) a control system without the addition of biochar (n ¼ 7 in each case). To estimate the risks and benefits that the biochar types may have as a soil amendment, moderately high to high concentrations of biochar were used in the experiment corresponding to an application rate of 20 or 80 t/ha at a tillage depth of 10 cm. The experimental design consisted of two simultaneously conducted identical pot “experiments” on 4 greenhouse tables each with 28 pots, altogether 112 pots (Fig. 1). The treatments were assigned to 28 pots on each table, according to a row-column experimental design: each column consisted of one control pot, one BC300 pot, one BC375 pot and one BC475 pot. The row-column design is efficient in greenhouse experiments because it takes into account two-dimensional gradients, while a randomized complete block design considers only one dimension (Williams et al., 2002). Biochar was properly mixed with the soil separately for each pot. After filling, the soils were fertilized with 25 ml of a 1.8% solution of Yara Combi 1 (NePeK: 14-5-21) and soil moisture content was set to 30% of fresh soil mass. The pots were randomly placed on a moist filter bed to ensure constant soil and air moisture during the experiments. An exclusion area - extra pots growing ryegrass e was established around the primary pots to reduce the edge effect (Fig. 1). The 7.5-month (9/2012-3/2013) experiment constituted of five phases: 1) stabilization period (5 weeks), 2) radish growth period (5 weeks), 3) barley growth period (6 weeks), 4) winter rest (4 weeks) and 5) ryegrass period (10 weeks). 2.3. Plant growth trials and soil sampling During the stabilization period the pots were kept in the greenhouse with a day/night temperature of 20/15 C. Drip irrigation was used to keep the water content (WC) above 30% in all pots. During the stabilization period, a germination inhibition assay with Lactuca sativa was carried out: 3 d after filling the pots, ten lettuce seeds were sown in each pot. The soil surface was sprayed with water and covered with a plastic film for 2 d to maintain stable soil moisture conditions. Additional irrigation was performed by spraying the soil surface daily. After seed germination, the plastic films were removed and germination of the lettuce seeds was checked 14 d after sowing and plants were uprooted. After the
Fig. 1. The greenhouse experimental design. Letters A-D represent the four tables on which the experimental pots resided. Numbers represents the pots on the tables: 1 ¼ Control, 2 ¼ BC300, 3 ¼ BC375 and 4 ¼ BC475, and in an exclusion area (5 & 6) around the primary exam pots. For Tables A and B, the biochar application level was 20 g/L and for C and D, 80 g/L of soil.
stabilization period, 10 radish (R. sativus) seeds were sown in the same pots from which the lettuce seedlings were removed. Temperature in the greenhouse was day/night 20/15 C. Irrigation was applied with an automated drip system; 35 ml twice a day. After 11 d, the crop was thinned to the three best seedlings per pot. These radish seedlings were left to grow for 25 d before harvesting. After removing the radish plants, the soil was removed from the pot, mixed properly, fertilized (50 ml, 1.8% Yara Combi 1) and put back into the same pot. On the next day, 10 barley (H. vulgare) seeds were sown in each pot. The pots were covered with a plastic film for 4 d to promote germination. Temperature in the greenhouse was adjusted to 16/10 C (day/night), with irrigation of 35 ml twice a day. Barley plants were left to grow for 6 weeks before harvest. Day/ night (light/dark) cycle during the growth period was always 16:8 h. An artificial winter (“rest period”) was introduced after the harvesting of barley. The pots were covered with brown craft paper and kept at 10 C without light at constant air humidity (45% RH) for four weeks. At the start of the last phase, the soils were again removed from the pots, mixed properly, fertilized (50 ml, 1.8% Yara Combi 1) and put into 1 L pots since the amount of soil decreased because of soil sampling (see below). Temperature of the greenhouse was adjusted to 18 C. Seeds of L. perenne were sown in the pots (ca. 150 seeds, 0.305 g/pot) and then covered with a plastic film for 4 d. Soils were sprayed daily (7 d) with water until the seeds germinated. Irrigation was applied with an automated drip system twice a day (10e23 ml). During the 10-week period, the yield was harvested six times (3, 4, 5, 7, 8 and 10 week after ryegrass was sown). Several soil parameters were assessed during the study. During each soil sampling event: at the end of the stabilization period (4 weeks), the radish growing season (10 weeks) and at the end of the experiment (30 week), three soil samples were taken from each pot using a corer (Ø 1.5 cm, 5 cm deep) and stored (1e5 d) at 5 C for the analyses of pH, microbial activity and biomass and nematodes. Results concerning responses of the three plant species and soil organisms to biochar amendments will be reported elsewhere (Hagner unpublished data). 2.4. Leaching of glyphosate and phosphorus Three weeks after the final experimental harvest (3/2013), ryegrass was grown again (height ca. 10 cm) and the pots on Table B (biochar application 20 g/L) and D (80 g/L) were treated with glyphosate (Roundup® Bio; Monsanto, Copenhagen, Denmark) mixed with tap water (15:1000). Glyphosate was sprayed according to the GEP protocol (Good Experimental Practice) used by Agrifood Research Finland (MTT), adopted from the EEC Directive 93/71/EEC. Before spraying, the pots were taken out of the greenhouse and glyphosate was spread using a 2 m wide sprayer at 50 cm height. The glyphosate dose per pot was calculated by placing 16 filterpapers with similar surface-area as the test pots between the pots during glyphosate spraying. The glyphosate dose per surface area was calculated from the mass difference of the filter-paper before and after spraying. The RoundUp dose was 0.11 g/pot comprising 1.5% glyphosate, i.e. 1650 mg/pot, corresponding glyphosate dose of 1300 g/ha. Grasses in the glyphosate treated pots withered and died between 3 and 21 d after spraying. After spraying, the pots were taken back to the greenhouse and returned to their original positions. The pots were kept moist and the temperature at 17:15 C day:night (16:8 h). After 4 weeks the pots were irrigated with 500e850 ml of 18e20 C tap water to mimic heavy rain. The water was applied gradually during a period of 1.5 h (average) so that 500 ± 25 ml of the leachate per pot was collected for the analysis of glyphosate, AMPA and solid material. The water
M. Hagner et al. / Journal of Environmental Management 164 (2015) 46e52
leachate was collected, quantified and stored in a freezer (18 C) until analyses. Solid material was separated from the melted leachates by centrifuging (15 min, 3500 rpm G). Glyphosate and AMPA concentrations in the liquid part of the melted leachates were measured using the UHPLC-MS-MS-method (2.23 Internal €nen et al., method of MTT Research Finland, LOQ 0.02 mg/l) (R€ asa 2013). The ability of the soil to retain phosphate-phosphorus (PO4eP) in the presence or absence of biochar was assessed from 200 g soil samples taken from the pots on Table C (biochar dose 80 g/L) in October 2013. After the last plant rotation cycle (see above: Plant growth trials and soil sampling) the pots on Table C were stored in the greenhouse at 10e20 C for six months without irrigation. Thereafter, the dry soils were homogenized by turning the pots upside-down and gently crushing the soil between fingers. Filter papers (Tervakoski, tesorb T42580 g/m2) were placed inside glass funnels and 200 g of the mixed soil was weighed and placed inside the filters. The funnels with the soil were irrigated with 18e20 C tap water to mimic heavy rain to obtain a 250 ml sample for the analysis of phosphorus (PO4eP) (Standard SFS 3025, Lachat QuikChem 800 Flow Injection Analysis) and total organic carbon (Apollo 9000 Total Organic Carbon (TOC) analyzer). 2.5. Statistical analyses Statistical analyses were performed using SPSS 20 (IBM). ANOVA was performed to examine the effects of biochar on the leaching of glyphosate, AMPA and TOC from the soil. Tukey's HSD test was used for multiple comparisons. Homogeneity of variances (Levene test) and normality of data were tested in accordance with the assumptions of ANOVA. In case of glyphosate concentrations, transformations (ln) were used to normalize the data. As concentrations of phosphate PO4eP in leachates were not normally distributed even after data transformations, the non-parametric KruskaleWallis and ManneWhitney tests were used. Correlations between the specific surface area of biochar and the concentrations of glyphosate, AMPA, PO4eP or TOC in the leachate were studied using Spearman correlation analysis. Spearman correlation was also used to detect correlations between PO4eP and TOC leaching. Pearson correlation analysis was used to test correlation between the amount of solid matter and concentration of glyphosate/AMPA in the leachates. 3. Results At the biochar application level of 20 g/L, none of the three biochar types affected either glyphosate (BC300: p ¼ 0.335, BC375: p ¼ 0.312 and BC475: p ¼ 0.866) or AMPA (BC300: p ¼ 0.780, BC375: p ¼ 0.571, BC475: p ¼ 0.551) concentrations in water leachates. However, the 80 g/L application of biochar reduced the leaching of glyphosate by an average 71% (BC300: p < 0.001, BC375: p ¼ 0.002, BC475: p ¼ 0.017) from the soil (Table 2, Fig. 2). The leaching of glyphosate was reduced by 81% in the BC300 treated soil, by 74% in BC375 and by 58% in BC475, while differences between the biochar types were statistically insignificant (p ¼ 0.315).
49
Altogether, 1.53% of the glyphosate added leached from the control pots while the respective percentages were 0.30, 0.42, 0.65% from BC300, BC375 and BC475, respectively. On average, biochar reduced AMPA leaching by 46% from BC300, 39% from BC375 and 23% from BC475 treated soils. However, due to large variation in AMPA concentrations in the water leachates, especially in the control soil, biochar had no statistically significant effect on AMPA leaching (p ¼ 0.106). The application of biochar did not affect the leaching of phosphorus (PO4eP) (p ¼ 0.328) or TOC (p ¼ 0.383) (Table 2). The amount of solid matter in the water leachates was significantly reduced by BC375 and BC475 (80 g/L) (BC375: p ¼ 0.045, BC475: p ¼ 0.003) (Table 2). There were no statistically significant correlations between the specific surface area of the biochar type and the leaching of glyphosate (r ¼ 0.308, p ¼ 0.118), AMPA (r ¼ 0.130, p ¼ 0.520), PO4eP (r ¼ 0.301, p ¼ 0.12) or TOC (r ¼ 0.134, p ¼ 0.495). No notable correlations were observed between solid matter and glyphosate (r ¼ 0.028, p ¼ 0.900) or AMPA (r ¼ 0.076, p ¼ 0.737) leaching. Neither was there a correlation between leaching of TOC and PO4eP (r ¼ 0.214, p ¼ 0.274). Biochar had no influence on soil pH at the application level of 20 g/L soil, while at the level of 80 g/L, biochar increased soil pH; being 5.5 in the control soil, 5.7 in BC300, 6.0 in BC375 and 6.3 in BC475 treated soil (BC300: p ¼ 0.592, BC375: p ¼ 0.012, BC474: p < 0.001). 4. Discussion 4.1. The effects of biochar pyrolysis temperature on glyphosate leaching We investigated the effectiveness of three types of biochar - all derived from Betula wood but pyrolysed at different temperatures in minimizing the leaching of glyphosate and its main degradation product AMPA in agricultural soil. As hypothesized, biochar, irrespective of the temperature at which it was pyrolysed, significantly reduced the concentrations of glyphosate in the water leachates. This suggests that biochar increases glyphosate adsorption in the soil. This is somewhat surprising as there is little evidence suggesting that glyphosate can be effectively adsorbed to organic matter. Instead, judged by its chemical structure, glyphosate is strongly adsorbed to inorganic soil components, especially aluminium and iron oxides (e.g. Piccolo et al., 1994). However, Piccolo et al. (1996) and Albers et al. (2009) reported that some specific organic components, such as purified humus substances, may bind with glyphosate. Like humus, biochar appears to have a high capacity to adsorb both organic (Beesley et al., 2010; Wang et al., 2010) and inorganic (Beesley et al., 2010; Cao et al., 2009) pollutants. For example, decreased leaching of herbicides, such as simazine (Jones et al., 2011), after the addition of biochar in the soil has been reported. In line with earlier findings on the ability of biochar to bind pesticides, we confirmed our earlier findings (Hagner et al., 2013) in a well replicated study design, that birch wood derived biochar can also influence the fate of the most commonly used herbicide e glyphosate e in the soil and thereby reduce its likelihood to leach out of the soil. It is noteworthy that in
Table 2 Leaching (mean (±SE), n ¼ 7) of glyphosate, AMPA, phosphorus (PO4eP), total organic matter (TOC) and solid matter from soils treated with the three biochar types at the application rate of 80 g of biochar per L of soil. BC300 ¼ biochar produced at 300 C, BC375 ¼ biochar produced at 375 C and BC475 ¼ biochar produced at 475 C.
BC300 BC375 BC475 Control
Glyphosate (mg/L)
AMPA (mg/L)
PO4eP (mg/L)
TOC (mg/L)
Solid matter (mg/L)
10.3 14.2 20.9 50.9
0.36 0.40 0.48 0.60
938.7 969.1 946.9 1016.6
19.1 22.6 19.8 18.3
41.9 33.4 20.4 52.7
(1.73) (3.42) (6.82) (5.64)
(0.02) (0.05) (0.07) (0.08)
(16.6) (18.2) (41.8) (28.2)
(1.2) (2.6) (1.2) (1.8)
(2.0) (1.5) (0.9) (1.2)
50
M. Hagner et al. / Journal of Environmental Management 164 (2015) 46e52
Fig. 2. Effects (mean ± SE, n ¼ 7) of the three biochar types (BC300, BC375 and BC475) pyrolysed at differed temperatures (300 C, 375 C and 475 C) on glyphosate (left panel) and AMPA (right panel) leaching from the soil at application levels 20 and 80 g biochar/L soil. Statistically significant differences within each biochar application level when compared to the control (no biochar addition) are marked with asterisks * (p < 0.01).
the current study, glyphosate was analysed from filtered water samples while a considerable portion of glyphosate can also be bound and leached with colloidal soil particles (Kjaer et al., 2005; Yang et al., 2015). However, as biochar also reduced the wash-out of solid matter of solid matter in the current study, it is likely that biochar decreased the overall leaching of glyphosate in our experimental system. Even though the quality of biochar types varied highly due to divergent pyrolysis temperatures (Fagern€ as et al., 2014; Angin and €z, 2014; Keiluweit et al., 2010), no statistically significant Senso differences in the concentrations of glyphosate and AMPA in the leachates between the biochar types were observed. However, biochar produced at the lowest temperature tended to be more effective in reducing glyphosate and AMPA leaching than biochar produced at the highest temperature. In contrast, several studies have reported increased adsorption of pesticides e e.g. terbuthylazine, pyrimethanil and diuron e to biochar produced at increased pyrolysis temperatures (Wang et al., 2010; Yu et al., 2006, 2010). It is commonly concluded that an increased BET surface area and abundance of micropores in biochar are responsible for the enhanced adsorption capacity (Mesa and Spokas, 2011; Wang et al., 2010; Yu et al., 2010). In our study, BC475 had the highest BET surface area but this product also had the lowest influence on glyphosate and AMPA leaching. Thus, the BET surface area of biochar seems not to be the most important property controlling the adsorption of glyphosate on the surface of biochar. The surface layers of biochar contain polar oxygen-containing functional groups (e.g. carboxyl, hydroxyl, and phenolic groups), which have a high potential to sorb polar organic and inorganic contaminants (Ahmad et al., 2014). As glyphosate is a highly polar organic compound, the detected differences in sorption properties of biochar pyrolysed at different temperatures in the current study may well stem from differences in their surface characteristics. Further, a decrease of the polarity index [(O þ N)/C] of biochar indicates a reduction of the surface polar groups (carboxyl, amino, and phenolic) when pyrolysis temperature increases (Chen and Chen, 2009; Wan et al., 2014). Indeed, Sun et al. (2011) indicated that the proportions of polar functional groups on biochar surfaces increase the sorption of polar fluorinated herbicides on biochar. In line with Chen et al. (2008), we suggest that biochar types produced at low temperatures have greater sorption capacity (e.g. due to higher polarity) e for at least the tested e polar organic contaminants than biochar produced at higher temperatures.
A complementary factor contributing to the ability of biochar to retain glyphosate is hydrogen bonding, which has been suggested to be an interaction mechanism between glyphosate and purified humic substances (Piccolo et al., 1996; Mazzei and Piccolo, 2012). Hydrogen bonding may also, at least partly, explain why all three biochar types had a positive influence in retaining glyphosate in the soil in our study. As for biochar with its large content of oxygencontaining functional groups, the glyphosate molecule contains several electronegative atoms (Wauchope, 1976; Piccolo and Celano, 1994), which enables hydrogen bonding between biochar and glyphosate. However, Piccolo et al. (1996) found that other structural properties of humic substances can also affect glyphosate sorption capacity. In their study, the most adsorbing humic materials had a lower content of aromatic C than the least adsorbing and most aromatic materials (Piccolo et al., 1996). This was true also in our study in which BC300 with the lowest content of aromatic structures (indicated by the highest atomic ratio of H and C) had the greatest effect in reducing glyphosate leaching. In addition, as glyphosate is an ionic herbicide, biochar-induced changes in soil pH may affect its sorption to soil particles. According Gimsing et al. (2004a) a decrease in soil pH resulted in increasing glyphosate adsorption in the soil. Consequently, increase in soil pH may increase the leaching of glyphosate. However, in our study each biochar type increased soil pH but decreased the leaching of glyphosate. Whether the observed differences in the leaching of glyphosate between the various biochar types were/were not affected by the biochar-inducing changes in the soil pH remains open. In summary, birch wood originated biochar appears to have a substantial capacity in reducing glyphosate leaching from agricultural soils when the application dose is high (80 t/ha). The role of various functional groups, but also other structural and electrochemical properties affecting biochar sorption capacity, should be addressed to better understand biochar-induced effects on glyphosate and other chemicals in the soil. 4.2. The effects of biochar on phosphorus leaching Contrary to our hypothesis, biochar did not reduce PO4eP leaching from the study soils. Biochar addition has been reported to increase (Guo et al., 2014) or decrease (Laird et al., 2010; Novak et al., 2009) P levels in leachate solutions. For example, Laird et al. (2010) reported that biochar reduced leaching of manure-
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originated P. Similarly to the current study, Soinne et al. (2014) reported that sorption of dissolved P was not affected by biochar addition. Instead, they concluded that biochar could be beneficial for erosion control and in reducing particulate P losses. In our study, we analyzed only the dissolved fraction of phosphorus while it is probable that a large proportion of phosphorus was adsorbed in soil clay particles (Hingston et al., 1967). In addition, the drying of soil during 6 months of storage may also have had an effect on P behaviour. Thus, we can only conclude that according our study, biochar application had no significant impact on the leaching of dissolved phosphorus. Furthermore, even though glyphosate and phosphorus share similar binding sites in the soil, our results imply that birch wood biochar has specific properties that reduces glyphosate but not phosphorus leaching. To increase P sorption capacity of biochar, pre-treatment of feedstock or post-treatment of biochar will be needed. 4.3. Conclusions Our results corroborate earlier findings suggesting that biochar as a soil amendment is a promising way to reduce the leaching of harmful substances. It is noteworthy, however, that the amount of biochar applied in this study, 80 t/ha may be an unrealistic application dose for general agricultural purposes. However, biochar additions in large quantities can be an effective way to reduce leaching of various contaminants, including pesticides and nutrients, at specific target sites, such as outlets of drainage systems and agricultural systems on steep slopes. The interactions between various types of biochar, glyphosate and phosphorus in the soil are complex and evidently influenced by a variety of factors, such as the quality of the soil, vegetation, and biochar properties. Importantly, the ability of biochar to sorb agro-chemicals depends on the temperature at which feedstock is pyrolysed. The development of technologies to utilize biochar to promote plant yield and to control the leaching of agrochemicals is in its infancy. More data from field conditions are urgently needed. Acknowledgements The authors thank the Finnish Funding Agency for Technology and Innovation (40226/11) (Tekes) and several enterprises (e.g. Raussi Ltd., Charcoal Finland Oy and Biolan Oy) for funding this project. Thank you for the Maa-ja Vesitekniikan Tuki ry and University of Helsinki for funding of glyphosate analyses. We thank the University of Helsinki, VTT Technical Research Centre of Finland and Natural Resources Institute Finland (Luke) personnel who participated in the research. We also thank Johan Kotze for checking the English and Anna-Lea Rantalainen for her valuable comments. References Ahmad, M., Rajapaksha, A.U., Lim, J.E., Zhang, M., Bolan, N., Mohan, D., Vithanage, M., Lee, S.S., Ok, Y.S., 2014. Biochar as a sorbent for contaminant management in soil and water: a review. Chemosphere 99, 19e33. Albers, C.N., Banta, G.T., Hansen, P.E., Jacobsen, O.S., 2009. The influence of organic matter on sorption and fate of glyphosate in soil e comparing different soils and humic substances. Environ. Pollut. 157, 2865e2870. €z, S., 2014. Effect of pyrolysis temperature on chemical and surface Angin, D., Senso properties of biochar of rapeseed (Brassica napus L.). Int. J. Phytorem. 16, 684e693. Arai, Y., Sparks, D.L., 2007. Phosphate reaction dynamics in soils and soil components: a multiscale approach. Adv. Agron. 94, 135e179. Battaglin, W.A., Meyer, M.T., Kuivila, K.M., Dietze, J.E., 2014. Glyphosate and its degradation product AMPA occur frequently and widely in U.S. Soils, surface water, groundwater, and precipitation. J. Am. Water Resour. Assoc. 50, 275e290. Baylis, A.D., 2000. Why glyphosate is a global herbicide: strengths, weaknesses and prospects. Pest Manage. Sci. 56, 299e308. nez, E., Gomez-Eyles, J.L., 2010. Effects of biochar and Beesley, L., Moreno-Jime
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Contents lists available at ScienceDirect
Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Research article
Birch (Betula spp.) wood biochar is a potential soil amendment to reduce glyphosate leaching in agricultural soils Marleena Hagner a, *, Sanna Hallman b, Lauri Jauhiainen c, Riitta Kemppainen c, €la €a €mo € c, Kari Tiilikkala c, Heikki Seta Sari Ra a b c
University of Helsinki, Department of Environmental Sciences, Niemenkatu 73, 15340 Lahti, Finland €skyla €, Department of Biological and Environmental Science, PO Box 35, 40014 Jyva €skyla €, Finland University of Jyva Natural Resources Institute (Luke), 31600 Jokioinen, Finland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 June 2015 Received in revised form 24 August 2015 Accepted 25 August 2015 Available online 3 September 2015
Glyphosate (N-(phosphonomethyl) glycine), a commonly used herbicide in agriculture can leach to deeper soil layers and settle in surface- and ground waters. To mitigate the leaching of pesticides and nutrients, biochar has been suggested as a potential soil amendment due to its ability to sorb both organic and inorganic substances. However, the efficiency of biochar in retaining agro-chemicals in the soil is likely to vary with feedstock material and pyrolysis conditions. A greenhouse pot experiment, mimicking a crop rotation cycle of three plant genera, was established to study the effects of pyrolysis temperature on the ability of birch (Betula sp.) wood originated biochar to reduce the leaching of (i) glyphosate, (ii) its primary degradation product AMPA and (iii) phosphorus from the soil. The biochar types used were produced at three different temperatures: 300 C (BC300), 375 C (BC375) and 475 C (BC475). Compared to the control treatment without biochar, the leaching of glyphosate was reduced by 81%, 74% and 58% in BC300, BC375 and BC475 treated soils, respectively. The respective values for AMPA were 46%, 39% and 23%. Biochar had no significant effect on the retention of water-soluble phosphorus in the soil. Our results corroborate earlier findings on pesticides, suggesting that biochar amendment to the soil is a promising way to reduce also the leaching of glyphosate. Importantly, the ability of biochar to adsorb agro-chemicals depends on the temperature at which feedstock is pyrolysed. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Biochar Glyphosate AMPA Phosphorus Leaching Pyrolysis temperature
1. Introduction Glyphosate (N-(phosphonomethyl) glycine) is a broadspectrum, nonselective and post-emergence organophosphate herbicide. It is an active ingredient in Roundup® and several other weed killing formulations commonly used in agricultural and nonagricultural systems (Baylis, 2000). For example, in Finland it accounted for almost 40% of all herbicide-active ingredients sold in 2010 (see Hagner, 2013). Glyphosate has unique sorption characteristics in the soil compared to other pesticides. Due to its rapid adsorption onto soil particles and relative fast degradation by microbes, glyphosate is assumed to be inactivated immediately after spraying and thus considered an environmentally safe herbicide (Giesy et al., 2000). However, the rate of degradation and sorption * Corresponding author. E-mail address: Marleena.Hagner@helsinki.fi (M. Hagner). http://dx.doi.org/10.1016/j.jenvman.2015.08.039 0301-4797/© 2015 Elsevier Ltd. All rights reserved.
of glyphosate depend on climatic conditions and soil properties (Gimsing et al., 2004a, 2004b). As a consequence, under certain environmental conditions glyphosate can leach to deeper soil layers and end up in surface- and ground waters (Battaglin et al., 2014; Borggaard and Gimsing, 2008; Laitinen et al., 2009; Kjaer et al., 2005). This may occur especially in northern ecosystems where glyphosate has a long persistence time due to cooler climates (Helander et al., 2012). As with pesticides, modern agriculture is dependent on phosphate fertilizers to replace the phosphorus removed via crop harvesting (Cordell et al., 2009). Simultaneously, excessive loads of phosphorus remain in the soil from where it can leach into adjacent aquatic ecosystems with well-known adverse effects on the quality of surface- and ground waters. The mobility of phosphorus in agricultural soils is controlled by several geo-chemical processes such as solubilization, complexation, adsorption and precipitation (Arai and Sparks, 2007). Judged by their chemical structure,
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glyphosate and inorganic phosphates are strongly adsorbed to inorganic soil components, especially aluminium and iron oxides. This reaction leads to competition for sorption sites between glyphosate and phosphorus in the soil (Gimsing and Borggaard, 2002; Laitinen et al., 2008). Biochar, as a soil amendment, has been suggested to mitigate leaching losses of pesticides and nutrients. Biochar is a solid material produced by the thermochemical conversion (such as pyrolysis) of organic matter in an oxygen-limited environment. It has recently received much interest due to its ability to improve soil properties for enhanced plant production (IBI, 2014; Lehmann and Joseph, 2009; Verheijen et al., 2010). Moreover, biochar has received considerable interest as a soil amendment for the in situ stabilization of organic and inorganic contaminants. Evidence suggests that various biochar types have a high capacity to adsorb both organic (Beesley et al., 2010; Wang et al., 2010) and inorganic (Beesley et al., 2010; Cao et al., 2009) substances. For example, the sorption of phosphorus is reported to be strongly influenced by biochar in the soil (Chintala et al., 2014). The efficiency of biochar in pesticide and nutrient retention is likely to vary with regards to feedstock material and pyrolysis conditions (Mesa and Spokas, 2011). Slow and intermediate pyrolysis with residence times of a few minutes to several hours or days, and temperatures between 200 and 900 C are generally favoured in the production of biochar (Brown, 2009). A number of feedstock materials, including wood, animal litter, crop residues and solid waste, are in common use in the production of biochar (IBI, 2014; EBC, 2012). Pyrolysis temperature in particular plays a significant role in affecting biochar properties: the higher the pyrolysis temperature the higher the surface area, C content and aromaticity of biochar, and the lower the polarity and O and H content of biochar (Ahmad et al., 2014; Chen and Chen, 2009; Chen et al., 2012). A higher surface area, microporosity and hydrophobicity of biochar are usually associated with increased sorption potential of organic contaminants (Ahmad et al., 2014). Biochar produced at low pyrolysis temperatures on the other hand is characterized by a lower surface area and aromaticity but higher polarity and the amount of oxygen-containing functional groups on its surfaces, and may thus be more suitable in removing inorganic/ polar organic contaminants (Ahmad et al., 2014). Given the clear association between pyrolysis temperature and biochar properties, it is likely that the ability of biochar in retaining chemicals commonly applied in agriculture, such as glyphosate and phosphorus, is controlled by the temperature at which the feedstock material is pyrolysed. In a pilot study we showed that birch-derived biochar has the potential to reduce leaching of glyphosate in agricultural soils (Hagner et al., 2013). Here, our objective is to confirm this observation and extend the study design by including pyrolysis temperature as an explanatory variable. Consequently, we aimed to explore the effects of pyrolysis temperature on the ability of biochar to reduce the leaching of (i) glyphosate, (ii) its primary degradation product AMPA (aminomethylphosphonic acid) and (iii) phosphorus from the soil. We hypothesized that, due to the higher presence of polar functional groups in the surface layer, low-temperature biochar reduces the leaching of glyphosate/AMPA (polar organic chemicals) and phosphorus (inorganic chemical) from the soil more so than biochar pyrolysed at higher temperatures. This study forms part of a larger research program in which the impacts of pyrolysis temperature on the composition and chemical €s characteristics of birch-wood biochar is investigated (Fagerna €s and Kuoppala, unpublished). The effects of et al., 2014; Fagerna biochar characteristics on soil properties, plant growth and survival, and soil organisms were also investigated (Hagner et al., unpublished).
47
2. Materials and methods 2.1. Biochar properties Three types of biochar were produced by the VTT Technical Research Centre of Finland using a pilot-scale slow-pyrolysis unit. The pyrolysis process, as well as characteristics and analysis methods of the biochar types are reported in detail in Fagern€ as et al., 2014, Fagern€ as and Kuoppala, unpublished). The biochar types were produced using Betula sp. wood blocks as a feedstock, subject to three different temperatures: 300 C, 375 C and 475 C; referred to as BC300, BC375 and BC475. The biochar produced were crushed and passed through a 2 mm sieve before addition to the soil (see below). The specific surface area (BET-surface area analysed according to the Standard PANK 2401), water holding capacity (WHC) and other characteristics of the biochar types are presented in Table 1. The H/C and (O þ N)/C ratios were calculated to estimate the aromacity and polarity of the biochar types, respectively. 2.2. Establishment of the pot experiment The soil used in the experiment had no previous history of glyphosate application. Sandy silt soil was collected from an €rvi, SW-Finland in the autumn of organic agricultural field in Rehtija 2012. The soil had a pH of 6.02 (1:5 soil:water volume), water holding capacity (WHC) of 11%, electrical conductivity of 0.68 104 (S/cm) and pore volume of 47% (Sandbox-method, Eijkelkamp - Agrisearch equipment, 2013). The soil consisted of 22% of medium sand (grain size 200e600 mm), 56% fine sand (60e200 mm) and 21% of particles below 60 mm. The organic matter content of the soil was 5.6%. Concentrations of Ca, K, Mg, P and N were 760 mg/kg, 93 mg/kg, 34 mg/kg, 23 mg/kg and 0.45 mg/kg, respectively, analyzed in the laboratory of MTT Research Finland €kitie (1955). Earthaccording to the protocol by Vuorinen and Ma worms, larger stones and roots (diameter > 1 mm) were removed from the soil prior to use. Radish (Raphanus sativus), barley (Hordeum vulgare) and ryegrass (Lolium perenne) were used as test plants to mimic a crop rotation cycle of the three plant genera. The experiment was conducted in a greenhouse at MTT Agrifood Research Finland in Jokioinen in 2013, using 1500 ml flowerpots (Ø 11 cm, height 19 cm) with four holes (Ø 0.5 cm) at the bottom. The treatments consisted of soil (1 L ¼ 1320 g air dry soil, moisture Table 1 Characteristics of the biochar types produced at different pyrolysis temperatures: BC300 ¼ biochar produced at 300 C, BC375 ¼ at 375 C and BC475 ¼ at 475 C €s and Kuoppala, unpublished). (modified from Fagern€ as et al., 2014; Fagerna
Ash content (%) Volatile matter (%) C (%) H (%) N (%) O (%) S (%) Surface area (m2/g) WHC (%)a pH (1:5H2O) H/Cb (O þ N)/Cc Bulk density (g/cm3)d
BC300
BC375
BC475
0.5 48 72 4.9 0.2 23 0.01 2.2 23 5.1 0.07 0.32 0.41
0.7 30 80 3.9 0.3 15 0.01 6.4 24 5.2 0.05 0.19 0.41
1.0 17.2 89 3.1 0.3 7 0.01 44 288 7.5 0.04 0.09 0.45
a WHC: water holding capacity (24 h) i.e. the total amount of water a soil can hold (% of soil/biochar mass). b H/C: atomic ratio of H to C (mass). c (O þ N)/C: atomic ratio of sum of nitrogen and oxygen to carbon (mass), polarity index. d Bulk density (g/cm3) ¼ dry soil mass (g)/soil volume (cm3).
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content 1.0%) mixed with 1) 20 g of either BC300, BC375 or BC475, 2) 80 g of either BC300, BC375 or BC475 or 3) a control system without the addition of biochar (n ¼ 7 in each case). To estimate the risks and benefits that the biochar types may have as a soil amendment, moderately high to high concentrations of biochar were used in the experiment corresponding to an application rate of 20 or 80 t/ha at a tillage depth of 10 cm. The experimental design consisted of two simultaneously conducted identical pot “experiments” on 4 greenhouse tables each with 28 pots, altogether 112 pots (Fig. 1). The treatments were assigned to 28 pots on each table, according to a row-column experimental design: each column consisted of one control pot, one BC300 pot, one BC375 pot and one BC475 pot. The row-column design is efficient in greenhouse experiments because it takes into account two-dimensional gradients, while a randomized complete block design considers only one dimension (Williams et al., 2002). Biochar was properly mixed with the soil separately for each pot. After filling, the soils were fertilized with 25 ml of a 1.8% solution of Yara Combi 1 (NePeK: 14-5-21) and soil moisture content was set to 30% of fresh soil mass. The pots were randomly placed on a moist filter bed to ensure constant soil and air moisture during the experiments. An exclusion area - extra pots growing ryegrass e was established around the primary pots to reduce the edge effect (Fig. 1). The 7.5-month (9/2012-3/2013) experiment constituted of five phases: 1) stabilization period (5 weeks), 2) radish growth period (5 weeks), 3) barley growth period (6 weeks), 4) winter rest (4 weeks) and 5) ryegrass period (10 weeks). 2.3. Plant growth trials and soil sampling During the stabilization period the pots were kept in the greenhouse with a day/night temperature of 20/15 C. Drip irrigation was used to keep the water content (WC) above 30% in all pots. During the stabilization period, a germination inhibition assay with Lactuca sativa was carried out: 3 d after filling the pots, ten lettuce seeds were sown in each pot. The soil surface was sprayed with water and covered with a plastic film for 2 d to maintain stable soil moisture conditions. Additional irrigation was performed by spraying the soil surface daily. After seed germination, the plastic films were removed and germination of the lettuce seeds was checked 14 d after sowing and plants were uprooted. After the
Fig. 1. The greenhouse experimental design. Letters A-D represent the four tables on which the experimental pots resided. Numbers represents the pots on the tables: 1 ¼ Control, 2 ¼ BC300, 3 ¼ BC375 and 4 ¼ BC475, and in an exclusion area (5 & 6) around the primary exam pots. For Tables A and B, the biochar application level was 20 g/L and for C and D, 80 g/L of soil.
stabilization period, 10 radish (R. sativus) seeds were sown in the same pots from which the lettuce seedlings were removed. Temperature in the greenhouse was day/night 20/15 C. Irrigation was applied with an automated drip system; 35 ml twice a day. After 11 d, the crop was thinned to the three best seedlings per pot. These radish seedlings were left to grow for 25 d before harvesting. After removing the radish plants, the soil was removed from the pot, mixed properly, fertilized (50 ml, 1.8% Yara Combi 1) and put back into the same pot. On the next day, 10 barley (H. vulgare) seeds were sown in each pot. The pots were covered with a plastic film for 4 d to promote germination. Temperature in the greenhouse was adjusted to 16/10 C (day/night), with irrigation of 35 ml twice a day. Barley plants were left to grow for 6 weeks before harvest. Day/ night (light/dark) cycle during the growth period was always 16:8 h. An artificial winter (“rest period”) was introduced after the harvesting of barley. The pots were covered with brown craft paper and kept at 10 C without light at constant air humidity (45% RH) for four weeks. At the start of the last phase, the soils were again removed from the pots, mixed properly, fertilized (50 ml, 1.8% Yara Combi 1) and put into 1 L pots since the amount of soil decreased because of soil sampling (see below). Temperature of the greenhouse was adjusted to 18 C. Seeds of L. perenne were sown in the pots (ca. 150 seeds, 0.305 g/pot) and then covered with a plastic film for 4 d. Soils were sprayed daily (7 d) with water until the seeds germinated. Irrigation was applied with an automated drip system twice a day (10e23 ml). During the 10-week period, the yield was harvested six times (3, 4, 5, 7, 8 and 10 week after ryegrass was sown). Several soil parameters were assessed during the study. During each soil sampling event: at the end of the stabilization period (4 weeks), the radish growing season (10 weeks) and at the end of the experiment (30 week), three soil samples were taken from each pot using a corer (Ø 1.5 cm, 5 cm deep) and stored (1e5 d) at 5 C for the analyses of pH, microbial activity and biomass and nematodes. Results concerning responses of the three plant species and soil organisms to biochar amendments will be reported elsewhere (Hagner unpublished data). 2.4. Leaching of glyphosate and phosphorus Three weeks after the final experimental harvest (3/2013), ryegrass was grown again (height ca. 10 cm) and the pots on Table B (biochar application 20 g/L) and D (80 g/L) were treated with glyphosate (Roundup® Bio; Monsanto, Copenhagen, Denmark) mixed with tap water (15:1000). Glyphosate was sprayed according to the GEP protocol (Good Experimental Practice) used by Agrifood Research Finland (MTT), adopted from the EEC Directive 93/71/EEC. Before spraying, the pots were taken out of the greenhouse and glyphosate was spread using a 2 m wide sprayer at 50 cm height. The glyphosate dose per pot was calculated by placing 16 filterpapers with similar surface-area as the test pots between the pots during glyphosate spraying. The glyphosate dose per surface area was calculated from the mass difference of the filter-paper before and after spraying. The RoundUp dose was 0.11 g/pot comprising 1.5% glyphosate, i.e. 1650 mg/pot, corresponding glyphosate dose of 1300 g/ha. Grasses in the glyphosate treated pots withered and died between 3 and 21 d after spraying. After spraying, the pots were taken back to the greenhouse and returned to their original positions. The pots were kept moist and the temperature at 17:15 C day:night (16:8 h). After 4 weeks the pots were irrigated with 500e850 ml of 18e20 C tap water to mimic heavy rain. The water was applied gradually during a period of 1.5 h (average) so that 500 ± 25 ml of the leachate per pot was collected for the analysis of glyphosate, AMPA and solid material. The water
M. Hagner et al. / Journal of Environmental Management 164 (2015) 46e52
leachate was collected, quantified and stored in a freezer (18 C) until analyses. Solid material was separated from the melted leachates by centrifuging (15 min, 3500 rpm G). Glyphosate and AMPA concentrations in the liquid part of the melted leachates were measured using the UHPLC-MS-MS-method (2.23 Internal €nen et al., method of MTT Research Finland, LOQ 0.02 mg/l) (R€ asa 2013). The ability of the soil to retain phosphate-phosphorus (PO4eP) in the presence or absence of biochar was assessed from 200 g soil samples taken from the pots on Table C (biochar dose 80 g/L) in October 2013. After the last plant rotation cycle (see above: Plant growth trials and soil sampling) the pots on Table C were stored in the greenhouse at 10e20 C for six months without irrigation. Thereafter, the dry soils were homogenized by turning the pots upside-down and gently crushing the soil between fingers. Filter papers (Tervakoski, tesorb T42580 g/m2) were placed inside glass funnels and 200 g of the mixed soil was weighed and placed inside the filters. The funnels with the soil were irrigated with 18e20 C tap water to mimic heavy rain to obtain a 250 ml sample for the analysis of phosphorus (PO4eP) (Standard SFS 3025, Lachat QuikChem 800 Flow Injection Analysis) and total organic carbon (Apollo 9000 Total Organic Carbon (TOC) analyzer). 2.5. Statistical analyses Statistical analyses were performed using SPSS 20 (IBM). ANOVA was performed to examine the effects of biochar on the leaching of glyphosate, AMPA and TOC from the soil. Tukey's HSD test was used for multiple comparisons. Homogeneity of variances (Levene test) and normality of data were tested in accordance with the assumptions of ANOVA. In case of glyphosate concentrations, transformations (ln) were used to normalize the data. As concentrations of phosphate PO4eP in leachates were not normally distributed even after data transformations, the non-parametric KruskaleWallis and ManneWhitney tests were used. Correlations between the specific surface area of biochar and the concentrations of glyphosate, AMPA, PO4eP or TOC in the leachate were studied using Spearman correlation analysis. Spearman correlation was also used to detect correlations between PO4eP and TOC leaching. Pearson correlation analysis was used to test correlation between the amount of solid matter and concentration of glyphosate/AMPA in the leachates. 3. Results At the biochar application level of 20 g/L, none of the three biochar types affected either glyphosate (BC300: p ¼ 0.335, BC375: p ¼ 0.312 and BC475: p ¼ 0.866) or AMPA (BC300: p ¼ 0.780, BC375: p ¼ 0.571, BC475: p ¼ 0.551) concentrations in water leachates. However, the 80 g/L application of biochar reduced the leaching of glyphosate by an average 71% (BC300: p < 0.001, BC375: p ¼ 0.002, BC475: p ¼ 0.017) from the soil (Table 2, Fig. 2). The leaching of glyphosate was reduced by 81% in the BC300 treated soil, by 74% in BC375 and by 58% in BC475, while differences between the biochar types were statistically insignificant (p ¼ 0.315).
49
Altogether, 1.53% of the glyphosate added leached from the control pots while the respective percentages were 0.30, 0.42, 0.65% from BC300, BC375 and BC475, respectively. On average, biochar reduced AMPA leaching by 46% from BC300, 39% from BC375 and 23% from BC475 treated soils. However, due to large variation in AMPA concentrations in the water leachates, especially in the control soil, biochar had no statistically significant effect on AMPA leaching (p ¼ 0.106). The application of biochar did not affect the leaching of phosphorus (PO4eP) (p ¼ 0.328) or TOC (p ¼ 0.383) (Table 2). The amount of solid matter in the water leachates was significantly reduced by BC375 and BC475 (80 g/L) (BC375: p ¼ 0.045, BC475: p ¼ 0.003) (Table 2). There were no statistically significant correlations between the specific surface area of the biochar type and the leaching of glyphosate (r ¼ 0.308, p ¼ 0.118), AMPA (r ¼ 0.130, p ¼ 0.520), PO4eP (r ¼ 0.301, p ¼ 0.12) or TOC (r ¼ 0.134, p ¼ 0.495). No notable correlations were observed between solid matter and glyphosate (r ¼ 0.028, p ¼ 0.900) or AMPA (r ¼ 0.076, p ¼ 0.737) leaching. Neither was there a correlation between leaching of TOC and PO4eP (r ¼ 0.214, p ¼ 0.274). Biochar had no influence on soil pH at the application level of 20 g/L soil, while at the level of 80 g/L, biochar increased soil pH; being 5.5 in the control soil, 5.7 in BC300, 6.0 in BC375 and 6.3 in BC475 treated soil (BC300: p ¼ 0.592, BC375: p ¼ 0.012, BC474: p < 0.001). 4. Discussion 4.1. The effects of biochar pyrolysis temperature on glyphosate leaching We investigated the effectiveness of three types of biochar - all derived from Betula wood but pyrolysed at different temperatures in minimizing the leaching of glyphosate and its main degradation product AMPA in agricultural soil. As hypothesized, biochar, irrespective of the temperature at which it was pyrolysed, significantly reduced the concentrations of glyphosate in the water leachates. This suggests that biochar increases glyphosate adsorption in the soil. This is somewhat surprising as there is little evidence suggesting that glyphosate can be effectively adsorbed to organic matter. Instead, judged by its chemical structure, glyphosate is strongly adsorbed to inorganic soil components, especially aluminium and iron oxides (e.g. Piccolo et al., 1994). However, Piccolo et al. (1996) and Albers et al. (2009) reported that some specific organic components, such as purified humus substances, may bind with glyphosate. Like humus, biochar appears to have a high capacity to adsorb both organic (Beesley et al., 2010; Wang et al., 2010) and inorganic (Beesley et al., 2010; Cao et al., 2009) pollutants. For example, decreased leaching of herbicides, such as simazine (Jones et al., 2011), after the addition of biochar in the soil has been reported. In line with earlier findings on the ability of biochar to bind pesticides, we confirmed our earlier findings (Hagner et al., 2013) in a well replicated study design, that birch wood derived biochar can also influence the fate of the most commonly used herbicide e glyphosate e in the soil and thereby reduce its likelihood to leach out of the soil. It is noteworthy that in
Table 2 Leaching (mean (±SE), n ¼ 7) of glyphosate, AMPA, phosphorus (PO4eP), total organic matter (TOC) and solid matter from soils treated with the three biochar types at the application rate of 80 g of biochar per L of soil. BC300 ¼ biochar produced at 300 C, BC375 ¼ biochar produced at 375 C and BC475 ¼ biochar produced at 475 C.
BC300 BC375 BC475 Control
Glyphosate (mg/L)
AMPA (mg/L)
PO4eP (mg/L)
TOC (mg/L)
Solid matter (mg/L)
10.3 14.2 20.9 50.9
0.36 0.40 0.48 0.60
938.7 969.1 946.9 1016.6
19.1 22.6 19.8 18.3
41.9 33.4 20.4 52.7
(1.73) (3.42) (6.82) (5.64)
(0.02) (0.05) (0.07) (0.08)
(16.6) (18.2) (41.8) (28.2)
(1.2) (2.6) (1.2) (1.8)
(2.0) (1.5) (0.9) (1.2)
50
M. Hagner et al. / Journal of Environmental Management 164 (2015) 46e52
Fig. 2. Effects (mean ± SE, n ¼ 7) of the three biochar types (BC300, BC375 and BC475) pyrolysed at differed temperatures (300 C, 375 C and 475 C) on glyphosate (left panel) and AMPA (right panel) leaching from the soil at application levels 20 and 80 g biochar/L soil. Statistically significant differences within each biochar application level when compared to the control (no biochar addition) are marked with asterisks * (p < 0.01).
the current study, glyphosate was analysed from filtered water samples while a considerable portion of glyphosate can also be bound and leached with colloidal soil particles (Kjaer et al., 2005; Yang et al., 2015). However, as biochar also reduced the wash-out of solid matter of solid matter in the current study, it is likely that biochar decreased the overall leaching of glyphosate in our experimental system. Even though the quality of biochar types varied highly due to divergent pyrolysis temperatures (Fagern€ as et al., 2014; Angin and €z, 2014; Keiluweit et al., 2010), no statistically significant Senso differences in the concentrations of glyphosate and AMPA in the leachates between the biochar types were observed. However, biochar produced at the lowest temperature tended to be more effective in reducing glyphosate and AMPA leaching than biochar produced at the highest temperature. In contrast, several studies have reported increased adsorption of pesticides e e.g. terbuthylazine, pyrimethanil and diuron e to biochar produced at increased pyrolysis temperatures (Wang et al., 2010; Yu et al., 2006, 2010). It is commonly concluded that an increased BET surface area and abundance of micropores in biochar are responsible for the enhanced adsorption capacity (Mesa and Spokas, 2011; Wang et al., 2010; Yu et al., 2010). In our study, BC475 had the highest BET surface area but this product also had the lowest influence on glyphosate and AMPA leaching. Thus, the BET surface area of biochar seems not to be the most important property controlling the adsorption of glyphosate on the surface of biochar. The surface layers of biochar contain polar oxygen-containing functional groups (e.g. carboxyl, hydroxyl, and phenolic groups), which have a high potential to sorb polar organic and inorganic contaminants (Ahmad et al., 2014). As glyphosate is a highly polar organic compound, the detected differences in sorption properties of biochar pyrolysed at different temperatures in the current study may well stem from differences in their surface characteristics. Further, a decrease of the polarity index [(O þ N)/C] of biochar indicates a reduction of the surface polar groups (carboxyl, amino, and phenolic) when pyrolysis temperature increases (Chen and Chen, 2009; Wan et al., 2014). Indeed, Sun et al. (2011) indicated that the proportions of polar functional groups on biochar surfaces increase the sorption of polar fluorinated herbicides on biochar. In line with Chen et al. (2008), we suggest that biochar types produced at low temperatures have greater sorption capacity (e.g. due to higher polarity) e for at least the tested e polar organic contaminants than biochar produced at higher temperatures.
A complementary factor contributing to the ability of biochar to retain glyphosate is hydrogen bonding, which has been suggested to be an interaction mechanism between glyphosate and purified humic substances (Piccolo et al., 1996; Mazzei and Piccolo, 2012). Hydrogen bonding may also, at least partly, explain why all three biochar types had a positive influence in retaining glyphosate in the soil in our study. As for biochar with its large content of oxygencontaining functional groups, the glyphosate molecule contains several electronegative atoms (Wauchope, 1976; Piccolo and Celano, 1994), which enables hydrogen bonding between biochar and glyphosate. However, Piccolo et al. (1996) found that other structural properties of humic substances can also affect glyphosate sorption capacity. In their study, the most adsorbing humic materials had a lower content of aromatic C than the least adsorbing and most aromatic materials (Piccolo et al., 1996). This was true also in our study in which BC300 with the lowest content of aromatic structures (indicated by the highest atomic ratio of H and C) had the greatest effect in reducing glyphosate leaching. In addition, as glyphosate is an ionic herbicide, biochar-induced changes in soil pH may affect its sorption to soil particles. According Gimsing et al. (2004a) a decrease in soil pH resulted in increasing glyphosate adsorption in the soil. Consequently, increase in soil pH may increase the leaching of glyphosate. However, in our study each biochar type increased soil pH but decreased the leaching of glyphosate. Whether the observed differences in the leaching of glyphosate between the various biochar types were/were not affected by the biochar-inducing changes in the soil pH remains open. In summary, birch wood originated biochar appears to have a substantial capacity in reducing glyphosate leaching from agricultural soils when the application dose is high (80 t/ha). The role of various functional groups, but also other structural and electrochemical properties affecting biochar sorption capacity, should be addressed to better understand biochar-induced effects on glyphosate and other chemicals in the soil. 4.2. The effects of biochar on phosphorus leaching Contrary to our hypothesis, biochar did not reduce PO4eP leaching from the study soils. Biochar addition has been reported to increase (Guo et al., 2014) or decrease (Laird et al., 2010; Novak et al., 2009) P levels in leachate solutions. For example, Laird et al. (2010) reported that biochar reduced leaching of manure-
M. Hagner et al. / Journal of Environmental Management 164 (2015) 46e52
originated P. Similarly to the current study, Soinne et al. (2014) reported that sorption of dissolved P was not affected by biochar addition. Instead, they concluded that biochar could be beneficial for erosion control and in reducing particulate P losses. In our study, we analyzed only the dissolved fraction of phosphorus while it is probable that a large proportion of phosphorus was adsorbed in soil clay particles (Hingston et al., 1967). In addition, the drying of soil during 6 months of storage may also have had an effect on P behaviour. Thus, we can only conclude that according our study, biochar application had no significant impact on the leaching of dissolved phosphorus. Furthermore, even though glyphosate and phosphorus share similar binding sites in the soil, our results imply that birch wood biochar has specific properties that reduces glyphosate but not phosphorus leaching. To increase P sorption capacity of biochar, pre-treatment of feedstock or post-treatment of biochar will be needed. 4.3. Conclusions Our results corroborate earlier findings suggesting that biochar as a soil amendment is a promising way to reduce the leaching of harmful substances. It is noteworthy, however, that the amount of biochar applied in this study, 80 t/ha may be an unrealistic application dose for general agricultural purposes. However, biochar additions in large quantities can be an effective way to reduce leaching of various contaminants, including pesticides and nutrients, at specific target sites, such as outlets of drainage systems and agricultural systems on steep slopes. The interactions between various types of biochar, glyphosate and phosphorus in the soil are complex and evidently influenced by a variety of factors, such as the quality of the soil, vegetation, and biochar properties. Importantly, the ability of biochar to sorb agro-chemicals depends on the temperature at which feedstock is pyrolysed. The development of technologies to utilize biochar to promote plant yield and to control the leaching of agrochemicals is in its infancy. More data from field conditions are urgently needed. Acknowledgements The authors thank the Finnish Funding Agency for Technology and Innovation (40226/11) (Tekes) and several enterprises (e.g. Raussi Ltd., Charcoal Finland Oy and Biolan Oy) for funding this project. Thank you for the Maa-ja Vesitekniikan Tuki ry and University of Helsinki for funding of glyphosate analyses. We thank the University of Helsinki, VTT Technical Research Centre of Finland and Natural Resources Institute Finland (Luke) personnel who participated in the research. We also thank Johan Kotze for checking the English and Anna-Lea Rantalainen for her valuable comments. References Ahmad, M., Rajapaksha, A.U., Lim, J.E., Zhang, M., Bolan, N., Mohan, D., Vithanage, M., Lee, S.S., Ok, Y.S., 2014. Biochar as a sorbent for contaminant management in soil and water: a review. Chemosphere 99, 19e33. Albers, C.N., Banta, G.T., Hansen, P.E., Jacobsen, O.S., 2009. The influence of organic matter on sorption and fate of glyphosate in soil e comparing different soils and humic substances. Environ. Pollut. 157, 2865e2870. €z, S., 2014. Effect of pyrolysis temperature on chemical and surface Angin, D., Senso properties of biochar of rapeseed (Brassica napus L.). Int. J. Phytorem. 16, 684e693. Arai, Y., Sparks, D.L., 2007. Phosphate reaction dynamics in soils and soil components: a multiscale approach. Adv. Agron. 94, 135e179. Battaglin, W.A., Meyer, M.T., Kuivila, K.M., Dietze, J.E., 2014. Glyphosate and its degradation product AMPA occur frequently and widely in U.S. Soils, surface water, groundwater, and precipitation. J. Am. Water Resour. Assoc. 50, 275e290. Baylis, A.D., 2000. Why glyphosate is a global herbicide: strengths, weaknesses and prospects. Pest Manage. Sci. 56, 299e308. nez, E., Gomez-Eyles, J.L., 2010. Effects of biochar and Beesley, L., Moreno-Jime
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