The effects of straw or straw-derived gasification biochar applications on soil quality and crop productivity: A farm case study

Journal of Environmental Management 186 (2017) 88e95

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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman

Research article

The effects of straw or straw-derived gasification biochar applications on soil quality and crop productivity: A farm case study € ver a, Valentina Imparato b, Paul Henning Krogh c, Veronika Hansen a, Dorette Müller-Sto Lars Stoumann Jensen a, Anders Dolmer d, Henrik Hauggaard-Nielsen e, * a

University of Copenhagen, Department of Plant & Environmental Sciences, Thorvaldsensvej 40, 1821 Frederiksberg, Denmark Aarhus University, Department of Environmental Science e Environmental Microbiology & Biotechnology, Frederiksborgvej 399, 4000 Roskilde, Denmark Aarhus University, Department of Bioscience e Soil Fauna Ecology and Ecotoxicology, Vejlsoevej 25, Silkeborg, Denmark d Bregentved Estate, Koldinghus All e 1, 4690 Haslev, Denmark e Roskilde University, Department of People and Technology, Universitetsvej 1, 4000 Roskilde, Denmark b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 March 2016 Received in revised form 12 August 2016 Accepted 19 October 2016 Available online 1 November 2016

Thermal gasification of straw is a highly efficient technology that produces bioenergy and gasification biochar that can be used as a soil amendment, thereby returning non-renewable nutrients and stable carbon, and securing soil quality and crop productivity. A Danish on-farm field study investigated the impact of traditional straw incorporation vs. straw removal for thermal gasification bioenergy production and the application of straw gasification biochar (GB) on soil quality and crop production. Two rates of GB were applied over three successive years in which the field was cropped with winter wheat (Triticum aestivum L.), winter oilseed rape (Brassica napus L.) and winter wheat, respectively, to assess the potential effects on the soil carbon pool, soil microorganisms, earthworms, soil chemical properties and crop yields. The application of GB did not increase the soil organic carbon content significantly and had no effect on crop yields. The application of straw and GB had a positive effect on the populations of bacteria and protists, but no effect on earthworms. The high rate of GB increased soil exchangeable potassium content and soil pH indicating its potassium bioavailability and liming properties. These results suggest, that recycling GB into agricultural soils has the potential to be developed into a system combining bioenergy generation from agricultural residues and crop production, while maintaining soil quality. However, future studies should be undertaken to assess its long-term effects and to identify the optimum balance between straw removal and biochar application rate. © 2016 Published by Elsevier Ltd.

Keywords: Biochar Carbon sequestration Earthworms Soil chemical properties Soil quality

1. Introduction In the face of challenges such as the growing human population and the impact of climate change, a sustainable food and energy supply is becoming increasingly important (Smith et al., 2015). The agricultural sector can contribute to climate change mitigation by decreasing greenhouse gas (GHG) emissions and reducing CO2 in the atmosphere through soil carbon sequestration. Furthermore, new synergies between agriculture and energy production may have mitigation potentials by providing agricultural residues for bioenergy production to substitute fossil fuels (Smith and Olesen,

* Corresponding author. E-mail address: [email protected] (H. Hauggaard-Nielsen). http://dx.doi.org/10.1016/j.jenvman.2016.10.041 0301-4797/© 2016 Published by Elsevier Ltd.

2010). Thermal gasification efficiently produces bioenergy from a wide range of agricultural residues (Ahrenfeldt et al., 2013; Thomsen et al., 2015) and a valuable by-product, a carbon-rich material, biochar. Gasification biochar (GB) has been proven to be stable towards microbial degradation and therefore has a high carbon sequestration potential when it is incorporated into soil (Hansen et al., 2016). Furthermore, the positive effects of straw GB on soil cation exchange capacity, soil water retention and root development have been demonstrated (Bruun et al., 2014), in addition to a liming effect (Hansen et al., 2016) and fertiliser value (Müller€ ver et al., 2012). Thus, the use of straw GB as a soil amelioraSto tion and carbon sequestration agent is a promising approach for combining the production of bioenergy and maintenance of soil quality (Hansen et al., 2015), but the concept has not yet been fully

V. Hansen et al. / Journal of Environmental Management 186 (2017) 88e95

proven under field conditions. Concerns have been raised in relation to an increased removal of crop residues for bioenergy since straw incorporation, for example, is regarded as an important tool in maintaining soil organic carbon (SOC) content and soil quality (Powlson et al., 2011). It also promotes the abundance of earthworms (Kennedy et al., 2012) that play an important role in soil functioning, including organic matter decomposition, plant nutrient cycling and soil structure improvement (Kladivko, 2001). Unfortunately, there is very little understanding about the effect of biochar on soil fauna, including earthworms (Lehmann et al., 2011). Both negative (Li et al., 2011) and positive effects (Busch et al., 2012) of biochar on earthworms have been reported. However, these results are from laboratorybased studies using the compost worm Eisenia fetida. Marks et al. (2016) found negative effects on soil fauna in the field after three years of successive application of gasification biochar at 12 and 50 t ha
1, which was most likely due to the high content of polycyclic aromatic hydrocarbons (PAH). Thus, more research is needed into the effect of straw removal and biochar addition under field conditions, with particular attention paid to soil biota. A field trial on a Danish farm was therefore established as a case study for the concept of combining bioenergy and agricultural production by low-temperature gasification of straw residues and then recycling the residual biochar product back into the soil. The trial was located on the Bregentved Estate, which is an agricultural and forestry business comprising 3.465 ha of agricultural land and 3.054 ha of forest on Zealand in eastern Denmark. Bregentved's business concept is based on establishing management strategies for both agricultural and forestry land with a focus on sustainability and the climate-friendly production of food and energy. The main crops are wheat, barley and winter oilseed rape (ca. one third each). Approximately 6% of the produced straw is baled and used to generate the heat needed by the estate in a local, estate-owned incineration facility. The rest of the straw is incorporated in the field in order to sequester carbon and thereby contribute to soil quality improvements and climate change mitigation. The Bregentved Estate has been practising conservation agriculture (minimum tillage) in combination with straw incorporation since 2005 in order to increase soil organic matter and biota such as earthworms. This has brought about 30% reduction in the consumption of fuel for field operations compared to the traditional ploughing system (A. Dolmer, pers.com.). Since 2011, controlled traffic farming (CTF, which includes GPS-assisted permanent driving tracks for all machinery operations) has also been implemented to minimise soil compaction and improve water retention and infiltration. However, according to the estate manager, there may be a potential to remove and use a higher proportion of the cereal straw to produce bioenergy, provided that straw removal will not impair soil fertility. The present study was conducted in a farmer-controlled field following common practice and crop rotation. The aim of the study was to assess the agronomical and environmental consequences of a system combining bioenergy production and recycling of gasification biochar. Over a three-year trial period, the objectives were to investigate the effect of cereal straw and straw GB application on: (1) soil organic carbon, (2) microorganisms and earthworms, (3) soil chemical properties and (4) crop yield. 2. Materials and methods 2.1. Biochar production and characterisation A straw gasification biochar produced in a Low Temperature Circulating Fluidized Bed gasifier (LT-CFB) was used in this study. The LT-CFB gasifier applied (Ahrenfeldt et al., 2013) is designed to gasify biomass resources with a high content of low melting ash

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compounds (e.g. straw, manure or sewage sludge) that have proven difficult to convert in other thermal processes. The process is based on separate fast-pyrolysis and gasification fluid bed reactors with a circulating heating medium to transfer the heat from the gasification process to the pyrolysis. The process temperature is kept below the melting point of the ash components, i.e. maximum process temperatures of around 700e750  C. In this way, sintering of the ash and subsequent fouling (e.g. from potassium) or corrosion (e.g. from chlorine) of the plant unit operations are avoided, as these compounds leave the process in solid form as ash particles (Ahrenfeldt et al., 2013). The straw used for biochar production originated from winter wheat (Triticum aestivum L.) grown on Zealand, Denmark. Wheat straw was chopped prior to LT-CFB gasification for optimal gasifier operation. The biochar characteristics is given in Table 1. Nine different polycyclic aromatic hydrocarbons (PAHs, acenaphthene, fluorene, phenanthrene, fluoranthene, pyrene, benzo(bjk)fluoranthene, benzo(a)pyrene, indeno(1,2,3-cd)pyrene and benzo(ghi)perylene) were quantified after a Soxhlet extraction of 2 g samples with toluene for 48 h by Eurofins GfA (Hamburg, Germany). More details about the production process and further characteristics of the GB can be found in a study by Hansen et al. (2015). 2.2. Experimental design The field trial was established in a conventional agricultural field on the Bregentved Estate, Zealand, Denmark (55 220 N, 12 050 E) in August 2012 and was carried out for three successive growing seasons at the same site with permanent treatments. The climate is temperate and the long term annual average for 1961e1990 is a rainfall of 550 mm and a temperature of 9.1  C. In the experimental period the annual rainfall was 463, 501, 683 and 591 mm and the average annual temperature was 8.4, 8.6, 10.2 and 9.4  C in 2012, 2013, 2014 and 2015, respectively. The soil is a sandy loam and contains 14% clay (<0.002 mm), 14% silt (0.002e0.02 mm), 47%fine sand (0.02e0.2) and 24% coarse sand (0.2e2 mm). The total C content was 19.8 g kg
1 and total N 1.8 g kg
1. This field trial followed a three-year crop sequence, typical of many crop farms in Denmark, consisting of winter wheat (Triticum aestivum L. cv. Jensen) sown in 2012, winter oilseed rape (Brassica napus L. cv. Explicit) sown in 2013 and winter wheat (Triticum aestivum L. cv. Dacanto) sown in 2014. The preceding crop in 2012 was spring barley (Hordeum vulgare L.). The field trial was established in a randomized block design with four replicates. The experimental plots were 12  100 m. The trial involved six treatments: (1) control without cereal straw incorporation (Control), (2) control with cereal straw incorporation (Straw), (3) application of GB at a rate according to the amount of potassium (K) recommended for the crop (Low GB), (4) application of GB at a rate according to the amount of phosphorus (P) recommended for the crop (High GB), (5) without any application of

Table 1 Chemical characteristics of the straw gasification biochar for each year of the field trial, n.d. ¼ not determined.

C (%) K (%) P (%) Mg (%) pH (water) SPAHa (mg kg
1)

Year 1

Year 2

Year 3

34.8 5.8 0.36 n.d. n.d. n.d.

32.08 5.69 0.54 0.43 10.5 2.5

32 5 0.41 0.48 n.d. 2.2

a Sum af acenaphthene, fluorene, phenanthrene, fluoranthene, pyrene, benzo(bjk)fluoranthene, benzo(a)pyrene, indeno(1,2,3-cd)pyrene and benzo(ghi) perylene.

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fertiliser P (No P) and (6) without any application of fertiliser K (No K). No P and No K treatments were included in the second and third year of the trial in extra plots treated equally to the Straw treatment during the first year. The moist GB (ca. 50% water content) was applied by a standard lime and fertiliser spreader (Bredal K105) and incorporated to 15 cm depth by a disc harrow (Horsch Joker) in the first year and by an airseeder combined with a harrow (Horsch) in the second and third year. All field operations were performed and managed by the farm workers following their common practice (Table 2). All treatments received 192, 233 and 189 kg mineral N ha
1 in years 1, 2 and 3 respectively. The control treatment was also fully fertilised with P and K according to the recommended fertiliser application rates for the different crops (Table 3). The straw and GB treatments received P and K with the added material and any deficits according to the recommended fertiliser application rates were subsequently replenished with mineral P and K (Table 3). For yield measurement, a total area of 320 m
2 (2  80 m) was harvested at maturity in each experimental plot using an experimental plot combine harvester (Haldrup C-85) with 2 m wide header. The grain was dried, cleaned and weighed. In the control and GB treatments, the cereal straw was left in the field, baled and removed, whereas in the straw treatment, the cereal straw was chopped and spread evenly on the soil surface during harvest and subsequently incorporated a few days later using disc a harrow (Horsch Joker).

2.3. Soil analyses and surface CO2 flux measurement Soil sampling was carried out at 0e25 cm depth to determine the P, K and Mg content and at 0e15 cm depth to determine total organic carbon (TOC) and pH using a soil auger (2 cm in diameter). From each plot, 10 subsamples were taken and bulked into one composite sample. For TOC analyses, 50 mg of dry and ball-milled soil were weighed into tin capsules to be measured on an elemental analyser (Elementar Analysensysteme GmbH, Hanau, Germany). The soil pH was determined in Milli-Q water at a soil:water ratio of 1:5 (w/v). The measurements of plant available P, K and Mg were carried out by a commercial laboratory according to the Danish standard procedures for soil nutrient analysis (Sørensen and Bülow-Olsen, 1994). The soil surface CO2 flux was measured with an infrared gas analyser (LI-COR 8100, Lincoln, Nebraska, USA) in four treatments: Control, Straw, Low GB and High GB. Three plastic pipes (12 cm high and 10 cm in diameter) per plot were pressed into the soil to a depth of 9 cm and remained in the field for the rest of the measuring period. All plants inside the pipes were continuously removed. The survey chamber with a volume of 1570 cm3 was placed on top of the pipe and the CO2 flux was measured over a period of 2 min. The CO2 flux was measured from one day after the sowing of oil seed rape in August 2013 until May 2014, and the

measuring frequency ranged from daily at the beginning to once a month at the end of the measuring period. Soil moisture was measured in the field at the same time as soil CO2 flux using TDR equipment (HydroSense II CS 659, Campbell scientific, Australia) at 12 cm soil depth. 2.4. Earthworms Earthworms were sampled in the field in year 2 of the field trial, one month after the GB application, using a combined hand-sorting and chemical extraction method. For the chemical extraction, allyl isothiocyanate (AITC) was diluted in 96% alcohol to give a 5 g L
1 solution. This solution was further diluted by water to reach a concentration of 0.1 g L
1 (Zaborski, 2003). Two soil blocks of 33 cm2 were excavated to a depth of 20 cm per plot for handsorting in the field. After excavation, three portions of 1 L AITC solution were applied to the pit at 15-min intervals to expel deepburrowing earthworms. Earthworms were collected after the application of each portion, counted and kept in plastic buckets with a little soil until species identification in the laboratory using the key of Sims and Gerard (1985). 2.5. Microorganisms For quantification of culturable bacteria and protists, aqueous suspensions were prepared by shaking 25 g field-moist soil in 125 ml sterile tap water using a Multi-Wrist® Shaker (LAB-Line Instruments, Illinois, USA) for 5 min at maximum speed. The suspensions were sedimented for 1 min and the supernatants used for measurement. For the quantification of culturable bacteria, 100 ml of ten-fold serial dilutions of the soil suspensions were plated in triplicate on 1/10 tryptic soy agar plates containing 50 mg L
1 of the fungal inhibitor Delvocid® Instant (DSM Food Specialties, Delft, The Netherlands). Plates were incubated at 15  C in the dark and the number of colonies was counted at 10
3 dilution after 1, 5 and 7 days to observe new emerging colonies. The total number of culturable bacteria was reported as number of CFU per gram of soil dry weight. For the quantification of culturable protists, the most probable number (MPN) method was used, following the modification of Winding and Oberender (2012). Briefly, 100 ml of 0.3 g L
1 Tryptic Soy Broth were placed in each of the wells in 96-well microtiter plates (Thermo Scientific, Denmark). Fifty ml soil suspensions were added to the first column of wells and a three-fold dilution series of this suspension was obtained by sequentially inoculating the next columns on the plate. The plates were incubated at 15  C in darkness and the wells inspected for Protista growth after three weeks using an inverted microscope (Olympus, microscope, 40, Olympus Europe, Hamburg, Germany). Protist densities were calculated according to Jarvis et al. (2010) and presented as numbers per gram of soil dry weight.

Table 2 Field operations and sampling dates during the three-year trial. Field operation/Year

1 Winter wheat

2 Oil seed rape

3 Winter wheat

Cereal straw incorporation Biochar application Harrowing Sowing Soil samples Soil CO2 flux Earthworm sampling Soil microorganism sampling Harvest

05 Aug 12 20 Sep 12 02 Oct 12 03 Oct 12 Nov 12 e e e 08 Aug 13

e 11 Aug 13 e 13 Aug 13 Jun 13 14 Aug 13e14 May 14 e Nov 13 22 Jul 14

07 Aug 14 08 Sep 14 e 11 Sep 14 Sep 14 e 07 Oct 14 e 21 Aug 15

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Table 3 Application rates of straw and straw gasification biochar (GB). Phosphorus (P) and potassium (K) added with GB and straw and additional mineral P and K to meet the requirements for winter wheat (year 1), winter oil seed rape (year 2) and winter wheat (year 3). Total carbon (C) applied in the different treatments and total input of cereal straw, GB and C during the three years. Year

Treatment

Cereal straw

GB

GB/straw-P

GB/straw-K

fertiliser-P

fertiliser-K

C added

0 0 862 8333 0 0 1441 5926 0 0 0 0 470 1900 0 0 0 0 2773 16,159 0 0

0 3 3 30 0 4 8 32 0 0 0 3 2 10 0 0

0 50 50 483 0 35 82 337 0 0 0 27 42 170 0 0

30 27 27 0 32 28 24 0 0 32 10 7 8 0 0 10

50 0 0 0 82 47 0 0 82 0 42 15 0 0 42 0

0 1115 300 2900 0 1593 462 1901 0 0 0 0 154 462 0 0 0 5416 916 5263 0 0


1

Kg ha 1

2

3

Total input

a

Control Straw Low GB High GB Control Straw Low GB High GB No Pa No K Control Straw Low GB High GB No P No K Control Straw Low GB High GB No P No K

0 2478 0 0 0 3540 0 0 0 0 0 0 0 0 0 0 0 6018 0 0 0 0

Treatments No P and No K were applied in years 2 and 3 only.

2.6. Statistics Statistical analyses of the data were performed in R, version 3.1.1 (R Core Team, 2013). The experimental set up was a randomized block design and the data were analysed in a linear mixed-effects model from the R-package lme4 with treatments as a fixed effect and block as a random effect. The differences between treatments within each year were analysed using least-square means from the R-package lsmeans. P values for differences between the treatments were adjusted according to the Tukey method. All differences at P  0.05 were reported as significant. Prior to analysis, data were tested for homogeneity of variance and normality of residuals using the Wally plot (R MESS package) and log transformed if necessary. 3. Results 3.1. Soil carbon turnover and sequestration Soil surface CO2 flux was generally highest during the autumn, decreased in all treatments during the winter months and increased again during spring (Fig. 1A). Results indicated an effect of soil moisture on overall soil respiration rates, as the CO2 flux rate decreased with decreasing soil moisture on day 17 and increased again after rainfall (Fig. 1B and C). The CO2 flux rate was significantly higher in the Straw treatment compared to the Control during the first 40 days, except on day 17. The CO2 flux rate tended to be lower in the High GB treatment compared to the Straw treatment, although only significantly on day 22. The soil moisture was highest in the Control treatment and lowest in the Straw treatment, and the difference was statistically significant during the first 40 days. Neither the application of GB nor of straw had any significant effect on the content of soil TOC content measured in years 2 and 3 (Fig. 2). 3.2. Effect on soil organisms The earthworm abundances were dominated by the endogeic

Fig. 1. Field surface CO2 flux rate as a measure of soil respiration in the second year of application: (A) during the entire campaign and (B) during the first 40 days. (C) Volumetric water content measured in the field. Both measurements started one day after sowing of oil seed rape in August and continued for 250 days until May. Values presented are means with standard error bars (n ¼ 4). Control ¼ straw removed, Straw ¼ straw incorporated, Low GB ¼ low rate of gasification biochar, High GB ¼ high rate of gasification biochar.

earthworms Aporrectodea chlorotica, Aporrectodea rosea and Aporrectodea caliginosa, whereas less than 10% consisted of the anecics Aporrectodea longa and Lumbricus terrestris and the epigeic

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Fig. 2. Total organic carbon (TOC) measured at 15 cm soil depth in years 2 and 3 of the field experiment. For treatment abbreviations, see Fig. 1. Values presented are means with standard error bars (n ¼ 3).

earthworms Lumbricus festivus and Lumbricus castaneus (Fig. 3). The treatments had no significant effect on earthworm abundances, although a slightly lower number of endogeic earthworms could be observed when the low rate of GB had been applied compared to the control. The application of straw and GB significantly increased the abundance of soil culturable bacteria compared to the control (Fig. 4A). Almost the same effect was observed in the case of protist abundance, where the application of straw and the high rate of GB significantly increased the protist abundance (Fig. 4B).

3.3. Soil chemical properties The soil chemical characterisation over the three agronomic years shows that the addition of the high rate of GB significantly increased the soil exchangeable potassium (K) content compared to the control gradually in years 2 and 3, resulting in a 62% increase in year 3 (Table 4). The exchangeable magnesium (Mg) content was also higher in this treatment, however, the difference was not statistically different. Omitting fertiliser application in the No P and No K plots in the second and third year did not result in consistently decreased levels of available P and K compared to the fertilised plots. The application of straw and GB significantly increased pH in year 2 and 3 compared to the control (Fig. 5). The application of the high rate of GB increased the soil pH from 6.95 to 7.42 (7% increase) in year 2 and from 7.3 to 7.8 (7% increase) year 3.

Fig. 4. A) Soil cultivatable bacteria (CFU) and B) most probable numbers (MPN) of protozoa measured after two years of the treatment application. For treatment abbreviations, see Fig. 1. Values presented are the means with standard error bars (n ¼ 3). Different letters indicate significant differences between treatments (P < 0.05).

Table 4 Soil chemical characteristics for the three years measured under winter wheat (year 1), oil seed rape (year 2) and winter wheat (year 3) at 25 cm depth. For treatment abbreviations, see Fig. 1. Presented values are means ± SEM (n ¼ 4). Values followed by star indicate significant differences between treatments within each year (p < 0.05). Year

Treatment

Olsen P Mg kg

1

2

3

a

Control Straw Low GB High GB Control Straw Low GB High GB No Pa No K Control Straw Low GB High GB No P No K

32.8 30.4 30.4 29.2 26.1 24.8 23.1 24.4 22.5 26.3 23.5 22.7 21.3 21.9 21.9 23.5


1

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

Exch. K

Exch. Mg

soil (DW)

1.48 2.15 0.98 1.56 1.84 0.73 1.84 2.37 1.21 1.33 2.61 1.06 1.05 1.39 1.96 1.98

127 137 125 150 116 120 131 189 135 129 190 182 194 309 165 195

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

2.79 1.17 4.31 2.63 7.22 8.52 7.34 15.09* 8.97 8.69 10.41 11.86 13.23 19.90* 20.26 6.59

44.5 45.2 39.8 42.4 42.7 51.9 44.9 53.7 47.8 47.0 38.7 43.5 37.5 48.0 40.5 44.6

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

3.80 3.83 1.35 2.53 3.10 5.04 3.80 5.77 5.01 4.46 2.94 4.50 3.07 6.24 4.29 4.22

Treatments No P and No K were applied in years 2 and 3 only.

3.4. Crop yield Fig. 3. Abundance of endogeic, epigeic, anecic earthworms and total amount of earthworms measured in the field after three years of treatment application. For treatment abbreviations, see Fig. 1. Values presented are the means with standard error bars (n ¼ 4). Different letters indicate significant differences between treatments for the total number of earthworm individuals (P < 0.05).

A comparison of crop yields over the three agronomic years showed no significant effect of treatments (Table 5). Wheat yields in the third year were significantly higher than in the first year.

V. Hansen et al. / Journal of Environmental Management 186 (2017) 88e95

Fig. 5. Soil pH measured at 15 cm soil depth in years 2 and 3 of the field experiment. For treatment abbreviations, see Fig. 1. Presented values are means with standard error bars (n ¼ 4). Different letters indicate significant differences between treatments (P < 0.05), small letters indicate year 2 and capital letters year 3 of the field trial.

Table 5 Effect of treatments on crop yields (dry matter) over three years: winter wheat (year 1), winter oil seed rape (year 2) and winter wheat (year 3). For treatment abbreviations, see Fig. 1. Presented values are means ± SEM (n ¼ 4). Treatments

Winter wheat

Oil seed rape

Winter wheat

± ± ± ± ± ±

9461 ± 293 9370 ± 232 9434 ± 321 9837 ± 161 9853 ± 304 10,304 ± 467


1

kg grain ha Control Straw Low GB High GB No P No K

8526 8510 8768 8608 e e

± ± ± ±

461 193 394 121

4387 4227 4459 4126 4238 4242

281 83 250 129 156 234

4. Discussion 4.1. Carbon sequestration In accordance with Hansen et al. (2016) who used the same GB material, the higher soil CO2 fluxes after straw addition compared to GB addition suggest that GB is more recalcitrant towards microbial degradation (Fig. 1). However, the application of GB over three years did not increase the SOC content markedly (Fig. 2). This is most likely due to the relatively high initial content of SOC in the field (around 2% corresponding to approximately 63 t C ha
1 in the top 25 cm), the relatively low application rate of GB during those three years (5.4 t ha
1 yr
1 on average for the high rate of GB) and the relatively low C content of the GB (approximately 34%), which corresponded to an average application of ca. 1.8 t C ha
1 yr
1 (Table 3). Assuming that at least 90% of GB-carbon is stable towards microbial degradation (Hansen et al., 2016), the expected SOC increase after three years of application would only be 7%, while after ten years it would reach 26%, increasing the SOC content to 2.1 and 2.5% respectively. These findings are in accordance with a study by Tammeorg et al. (2014b), in which the application of 5 t biochar ha
1 in a three-year field experiment on a sandy loam soil did not increase the SOC content, while 10 t did. Therefore, increasing the content of SOC with GB application is suggested as a long-term strategy. 4.2. Effect on soil organisms The application of GB had no negative impact on soil organisms (Figs. 3 and 4). The abundance of earthworms in the Control and Straw treatment was at a similar level as that in a study by Tammeorg et al. (2014a). However, it was at the lower end of the

93

range in comparison with results from a study by Kennedy et al. (2012), where the number of earthworms ranged from 100 to 700 per m
2 in a minimum tillage system with straw incorporation. In the present study, the application of GB had no significant effect on the total abundance of earthworms (Fig. 3). The endogeic earthworms that are horizontal burrowers and topsoil feeders were not affected by the high rate of GB, indicating no tendency of inhibition. There is currently no explanation for the tendency of lower earthworm abundance in the treatment with the low rate of GB. In contrast, Tammeorg et al. (2014a) reported a positive, though not significant, effect of wood biochar application of 30 t ha
1 on earthworm density and biomass in a boreal region field study. This difference might be due to the lower GB rate applied in the present study or to the characteristics of the GB such as lower C content. The removal of the straw did not decrease earthworm abundance within the time scale of this experiment, which is in agreement with Kennedy et al. (2012), who reported that the earthworm abundance decreased significantly only after three years of straw removal. Thus even though not shown in the present study, longterm annual straw removal may have a negative impact on earthworm populations. The straw removal in the Control treatment resulted in a significantly lower abundance of cultivable bacteria and protists in the soil compared to the Straw treatment (Fig. 4), which was also reflected in a lower soil surface CO2 flux (Fig. 1A). This indicates that the microbial abundance may give a faster indication of slow changes in organic matter content than measurements of total soil organic matter (Powlson et al., 1987). Furthermore, the straw removal led to a higher soil moisture content in comparison to straw incorporation (Fig. 1C), which may be due to increased bulk density and decreased water infiltration capacity of the soil (Sarkar et al., 2003). The application of the high rate of GB increased the bacterial population compared to the Control, indicating that the impact of straw removal may be counteracted by the addition of GB. However, it should be kept in mind that the bacterial numbers presented are only representative for a given instant and for the culturable species. Therefore, we cannot draw any overall conclusion on the impacts of GB addition on the microbial community in the field over time. The straw incorporation resulted in higher CO2 fluxes and accordingly a higher bacteria population compared to the Control and partly also to the GB treatments, which may be regarded as being contradictory to the goal of increasing carbon sequestration and soil quality. However as Janzen (2006) suggests, a major part of the benefits of soil organic matter is derived from its decay, not from its accumulation. Indeed, biological degradation of incorporated straw leads to an improvement in soil structure in the form of increased aggregate stability, which is crucial for the soil's ability to resist erosion and compaction (Le Bissonnais, 1996). In an incubation study, Hansen et al. (2016) observed that application of straw increased aggregate stability compared to a treatment without straw, while application of the recalcitrant straw GB had no effect. However, stubble and roots will still remain in the field after straw removal and contribute a substantial input of easily degradable C. Results from different studies on the amount of C originating from stubble, roots and rhizodeposition of winter wheat vary between 0.6 and 2 t C ha1 yr1, depending on climate, soil type and measurement techniques among other factors (Powlson et al., 2011; Hulugalle et al., 2012; Liu et al., 2014). In fact, €tterer et al. (2011) suggested that root-derived carbon contribKa uted relatively more to stable soil C pools compared to the same amount of above-crop residue-derived carbon. 4.3. Agronomic value The application of the high rate of GB increased plant-available

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K (Table 4), which was in accordance with other studies of gasifi€ ver et al., 2012; cation biochars from straw and wood (Müller-Sto Borchard et al., 2014) and indicates the K-fertilising value of cereal straw-based biochars (Table 5). However, applying cereal straw derived biochars at too high rates or too frequently may lead to over fertilization of K and potentially to K loss by leaching. The available content of P did not change during this field trial, which was in agreement with other field studies on fertile temperate soils (Jones et al., 2012; Tammeorg et al., 2014b). Since the soil content of plant available P in the treatment without any P addition did not decrease compared to the other treatments, no conclusion could be drawn about substitution of mineral P by GB addition. However, a P fertilization effect of straw gasification biochar has been shown in €ver et al., 2012). The high rate of GB, pot experiments (Müller-Sto which was only 5 t ha
1 yr
1 on average in the present study, increased the soil pH (Fig. 5). This was due to the high alkalinity of the GB, probably resulting from the comparatively high temperature of the gasification process and the straw content of base cations (Enders et al., 2012; Smider and Singh, 2014), and indicated an efficient liming value. Many other biochar materials from pyrolysis processes have to be applied at very high rates (>50 t ha
1) in the field (Jones et al., 2012; Rogovska et al., 2014) or to low pH tropical soils (Abiven et al., 2015; Martinsen et al., 2015) before a significant increase in soil pH is observed. The lack of effect of biochar application on already relatively high crop yields (Table 5) was consistent with other studies from the temperate region on sandy loam soils (Tammeorg et al., 2014b; Nelissen et al., 2015). The high background fertility of the soil probably masked some potentially positive effects of biochar application, as for example the treatments with no P or K added did not decrease crop yields, indicating that the soil content of plantavailable P and K was sufficient to supply the crop demand in the respective growing season. Thus, field experiments over longer periods and in soils with a low nutrient (especially P) content in temperate regions are needed in order to investigate the potential of GB to substitute mineral fertilisers. Taking into account the present and future costs of mineral P and K fertilisers and the fact that they are obtained from non-renewable resources, access to nutrients through GB may be a more sustainable solution for farmers in the long-term perspective. The fertilising value of GB could even be improved by using P-rich feedstocks, such as manure fibres or sewage sludge (Kuligowski et al., 2010; Thomsen et al., 2015). Gasification of sewage sludge and returning the biochar to the field would allow the recycling of more nutrients from society back to agricultural soils with concurrent production of bioenergy. Furthermore, a biochar optimised with regard to its ratio of C, P and K could be produced by combining different C, P and K-rich feedstocks, such as wood and straw residues and sewage sludge.

effects would be most beneficial. Besides straw gasification biochar application, there are also other options to counteract carbon export from fields, such as growing catch crops or importing biochar produced from wood residues (Blanco-Canqui, 2012; Peltre et al., 2016). In a field study, Mutegi et al. (2011) concluded that a fodder radish catch crop had the potential to offset the carbon exported with barley straw removed for energy production. Thus catch crops, typically grown during autumn before a spring crop, could mitigate soil carbon depletion from cereal straw export on around one third of the fields at Bregentved, which are sown with spring barley. Furthermore, around 20.000 t of wood material is produced annually in the forests at Bregentved, which would result in around 2.000 t biochar with an average C content of 60% (Hansen et al., 2015) if all of this was used for bioenergy production by gasification. To counteract the 2 t C removed by straw, a further 600 ha could be amended with this wood GB every year. As wood biochar often reveals higher stability, specific surface area and porosity compared to biochars derived from crop residues, further carbon sequestration and improvement of soil quality may be reached with this approach (Kloss et al., 2012; Hansen et al., 2015). According to the Bregentved Estate, annual straw incorporation also presents some challenges, such as N immobilisation during autumn and consequently less N for cover crops or subsequent autumn sown, N demanding cash crops such as oilseed rape or winter wheat. Thus, straw removal may lead to more available N and thereby better growth of catch crops and more C input to the soil. Furthermore, the straw incorporation into the soil requires high fuel use and, according to the field staff, in unsatisfactory seedbed preparation at times. Since biochar is a very light material, the storage, handling and spreading of biochar on the field is also a challenge. In the present study, biochar was sprinkled with water when it left the gasifier to avoid re-ignition. The moisture content of 50% also minimised dust losses during handling and application to the field. However, immediate incorporation of biochar is still needed to avoid drying and loss of the material by wind. In summary, straw removal for energy production in some years only combined with the return of biochar and other land use practices, such as growing catch crops or the input of extra carbon with wood biochar derived from forestry residues, may lead to a system in which agriculture can be sustainably combined with energy production (Powlson et al., 2011; Blanco-Canqui, 2012). The Bregentved Estate is looking into this as a possible business option in future.

4.4. The Bregentved case

5. Conclusions

At farm level, not all fields are likely to receive GB every year. In the present study, 0.4 t GB was produced from 1 ha of cereal straw, assuming an average straw production of 4 t ha
1. For example, 1.2 t of biochar could be produced from approx. 3 ha of cereal straw to fertilize 1 ha of land according to the K demand of the crop. The total annual cereal straw production on the Bregentved Estate is around 8.000 t yr
1, which would yield around 800 t GB yr
1 if all straw was used for bioenergy production. This means that of the estate's 3.465 ha, around 670 ha could be amended with 1.2 t GB ha
1 yr
1. An application rate of 1.2 t GB ha
1 with 33% C would return 0.4 t C ha
1 to the field, which is less than the 2 t C removed with straw, but more stable and long-lasting in the soil. The GB could be concentrated on low-fertility soils, such as sandy soil (Bruun et al., 2014), or soils with low C and P content, where the

The results presented in this case study suggest that a three-year application of straw gasification biochar (GB) to a temperate highly productive sandy loam soil improved specific soil chemical properties. The straw GB may thereby be used as a K fertiliser and liming agent to substitute mineral fertiliser and traditional lime. No negative effects on soil microorganisms and earthworms were detected after the addition of GB. Similarly important, it was possible to handle moistened GB without any major constraints using standard farm machinery. However, soil amendment with GB did not increase SOC content or affect the crop yields. Nevertheless, it is important to investigate how much straw can be removed in order to combine agricultural and energy production without having any negative long-term effects on soil organisms and soil structure.

V. Hansen et al. / Journal of Environmental Management 186 (2017) 88e95

Acknowledgments This research was supported by a grant from the VILLUM Foundation VKR022521. The study was also partially supported by EU-FP7-PEOPLE-2011-ITN (Trainbiodiverse, project number 289949). We are grateful to DONG Energy for providing us with the straw gasification biochar and for supporting the field trial. We thank the Bregentved Estate for providing access to the field site and especially to Lars Erik Nielsen for agronomic assistance in the field. The Strategic Projects programme 2014-2016 of DCE - Danish Center for Environment and Energy, Aarhus University, provided support to the earthworm study. References Abiven, S., Hund, A., Martinsen, V., Cornelissen, G., 2015. Biochar amendment increases maize root surface areas and branching: a shovelomics study in Zambia. Plant Soil 45e55. http://dx.doi.org/10.1007/s11104-015-2533-2. Ahrenfeldt, J., Thomsen, T.P., Henriksen, U., Clausen, L.R., 2013. 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