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Carbon dynamics in a biochar-amended loamy soil under switchgrass 2

Suzanne E. Allaire1*

, Benjamin Baril1, Anne Vanasse

2, Sébastien F. Lange

3, John MacKay

4, 3

Donald L. Smith5 4

*Corresponding author, email : suzanne.allaire@fsaa.ulaval.ca

5

1 Centre de Recherche sur les Matériaux Renouvelables, Pavillon 2480 Hochelaga, Université 6

Laval, Québec, Qc, Canada, G1V 7

2 Département de phytologie, Pavillon Comtois, 2425, rue de l'Agriculture, Université Laval, 8

Québec, Qc, Canada, G1V 0A6 9

3 Centre de Recherche en Horticulture, 2480 Hochelaga, Université Laval, Québec, Qc, Canada, 10

G1V 0A6 11

4 Centre d’étude de la forêt, Département des sciences du bois et de la forêt, 1030 rue de la 12

Médecine, Université Laval, Québec, Qc, Canada, G1V 0A6 13

5 Department of Plant Science, McGill University, Macdonald Campus, 21111 Lakeshore Road, 14

Ste-Anne-de-Bellevue, Qc, Canada, H9X 3V9 15

16

Abstract 17

Environmental impacts of switchgrass production for bioenergy could be reduced through 18

the use of biofertilizers rather than mineral fertilizers and through soil amendment with biochar. 19

The objectives of this study were to (1) assess the impact of biochar and biofertilizer on 20

switchgrass (Panicum virgatum L.) yield and parameters related to carbon dynamics; (2) correlate 21

carbon parameters with soil physico-chemical properties over the first two growing seasons, and 22

(3) develop a C budget. A complete randomized block design was installed in a sandy loam with 23

split plot treatment design, the main plots receiving 0 or 10 t ha-1

of biochar and the sub-plots 24

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receiving no fertilization, mineral N fertilization, or biofertilizers. Biofertilizers had no 1

significant impact on plant and soil. Biochar increased yield relative to the control treatment by 2

about 10% during the first year and root biomass by up to 50% after two years (P>0.1). Mineral 3

N fertilization also increased yield resulting in higher plant C sequestration after two years. 4

Biochar increased CO2 soil concentration (CO2-soil) by up to 50% but its impact on CO2 emission 5

flux (CO2-flux) changed over time. The impact of mineral fertilization on CO2-flux also varied with 6

time. Soil CO2 dynamics was mostly influenced by temperature, N and water content. Biochar 7

and fertilization treatments showed interactions on some plant and soil parameters. The highest C 8

sequestration budget was obtained with a combination of biochar and mineral N fertilization. The 9

equivalent of about one third of the increase in soil C content associated with biochar treatments 10

was respired away by soil microorganisms. Nearly one fourth of C sequestered by plants 11

remained in or at the soil surface (root and crop residues). 12

Keywords: Panicum virgatum L., carbon sequestration, CO2 emissions, soil carbon, soil gas 13

concentration 14

Résumé 15

Le biochar et les bioengrais pourraient aider à réduire les impacts environnementaux associés à la 16

production de panic érigé pour la bioénergie. Cette étude vise à (1) quantifier l’impact du biochar 17

et d’un bioengrais sur le rendement du panic érigé (Panicum virgatum L.), la séquestration de 18

carbone, le CO2 dans le sol (CO2-soil) et son émission (CO2-flux); (2) corréler les paramètres de 19

carbone avec les propriétés du sol et (3) évaluer le budget de carbone. Un plan expérimental en 20

tiroirs a été instauré sur un loam. Les parcelles principales ont reçu 0 ou 10 t ha-1

de biochar, les 21

parcelles secondaires ont reçu différents traitements d’engrais. Les bioengrais n’ont pas influencé 22

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les plantes et le sol. Le biochar a favorisé une augmentation de rendement de 10 % à la première 1

saison et la biomasse racinaire de 50 % après deux saisons. La fertilisation minérale a augmenté 2

les rendements après deux saisons. Le CO2-soil était jusqu’à 50 % plus élevé sous le biochar. Le 3

CO2-flux n’était pas toujours influencé par le biochar, mais a augmenté avec la fertilisation 4

minérale. La température, la teneur en azote et l’eau du sol ont influencé la dynamique du 5

carbone. Le meilleur budget de carbone a été obtenu avec l’utilisation combinée du biochar et de 6

la fertilisation azotée minérale. L’équivalent du tiers de l’augmentation de la teneur en carbone 7

du sol associée au biochar a été réémis par le sol. Environ le ¼ du carbone séquestré par les 8

plantes est resté dans le sol (plantes et sol). 9

Keywords: Panicum virgatum L., carbon sequestration, CO2 emissions, soil carbon, soil gas 10

concentration 11

Abbreviations: B, biochar treatment, NB, treatment without biochar, N-Bacteria, bacterial 12

fertilisation treatment, N-Full, mineral N fertilization treatment representing the full 13

recommended dosage, C-soil, carbon content in soil, CO2-soil, carbon dioxide concentration in soil, 14

CO2-flux, carbon dioxide surface emission fluxes, CV, coefficient of variation, θθθθv, volumetric 15

water content. 16

17

Introduction 18

The general goal of producing biomass for energy is to decrease overall greenhouse gas 19

(GHG) emissions from fossil fuel consumption without additional deleterious effects on the 20

environment. The major biofuel produced in the USA and Canada has been starch-based ethanol. 21

About 30% of the entire corn production is sold for ethanol production in the USA compared to 22

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about 10 % in Quebec (FPCCQ 2014). Switchgrass (Panicum virgatum L.) has been proposed as 1

a biofuel crop for mitigating climate change (Zan et al. 1997). It has a good potential for C 2

sequestration with its deep root system, efficient water-use (Ma et al. 2000), relatively low 3

nutrient requirement and good biomass production (Bransby et al. 1998). Davis et al. (2011) 4

showed that the amount of fuel per land area would be greater with switchgrass than with corn. 5

Switchgrass is a C4 perennial grass species native to North America. It offers the advantage of 6

being highly efficient in capturing sunlight energy as it gives up to 160 GJ ha-1

compared to 7

110 GJ ha-1

for corn and 45 GJ ha-1

for soybean (Samson et al. 2008). However, it has been 8

pointed out that energy production and C budgets are inefficient for bioenergy crop production 9

when considering the entire production cycle including: GHG emissions, the loss of soil carbon, 10

the energy for producing fertilizer, and the energy for transporting the crop to the power plant and 11

for discarding the residues. For switchgrass to become a valuable source of bioenergy, energy 12

consumption and GHG emissions related to its production must be reduced, and along with 13

overall environmental impacts. 14

Biochar application to soil has been suggested for plant production to improve overall 15

environmental impact of agriculture and to sequester C (Jeffrey et al. 2011; Lehmann et al. 2006; 16

Sohi et al. 2010). Biochar is a product of pyrolysis of any matter containing C (e.g. organic 17

matter, tires, plastics) under temperature ranging from 350°C to 800°C without or with very low 18

oxygen concentration (Laird et al. 2009; Allaire and Lange 2013). It can be used as a soil 19

amendment and has the advantage of recycling crop residues. Biochar can increase soil C 20

content, including a large portion that remains in the soil for hundreds of years (Woolf 2008) as 21

well as soluble and slowly degradable pools (Lehmann et al. 2006). It also has the potential to 22

reduce soil density and protect against compaction (Sohi et al. 2010, Verheijen et al. 2010), both 23

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of which decrease energy needs for crop production. It has been reported to improve water 1

budget, crop yield, N and C cycles, and reduce overall GHG emissions (Woolf 2008), all of 2

which reduce its environmental impact. Biochar is viewed as a good soil amendment not only 3

because of its ability to improve soil physico-chemical properties, but also because it seems to 4

favor root and microbial activities (Lehmann et al. 2006). Researchers reported that biochar had 5

larger impacts on soil and plants under poor soil conditions and in dry climates, than in well-6

structured soils under sufficient rainfall (Verheijen et al. 2010). The importance of biochar varies 7

with soils, climate, and crops (Sohi et al. 2010, Jeffrey et al. 2011), but its impact on soil under 8

switchgrass production in Quebec, a cold humid climate, is not well known. 9

Since biochars typically contain a low level of N, they cannot be used as a source of N for 10

plants. Besides mineral fertilization, biofertilizers such as atmospheric N fixing rhizobacteria 11

(Fuentes-Ramirez et al. 2006) may offer an interesting environmental alternative. They seem to 12

improve plant productivity (Lal and Tabacchioni 2009), favor C sequestration (Sohi et al. 2010), 13

and require little energy for their production or for their application since they are usually applied 14

by seed inoculation. Their use does not release as much GHG as N fertilizers and they do not 15

pollute surface waters. However, their efficiency for switchgrass production is unknown. 16

Because the aim of producing switchgrass for energy is to reduce GHG emissions and 17

improve the C budget, a better understanding and quantification is needed for CO2 emissions and 18

C dynamics under standard and alternative switchgrass production systems. In addition to 19

microbial and plant activities, soil surface CO2 emission fluxes (CO2-flux) are influenced by 20

parameters related to: (1) soil physical properties such as temperature, moisture, texture, density, 21

and gas diffusion (Allaire et al. 2012, Smith et al. 2003), (2) soil biochemical properties such as 22

organic matter content, N and C cycles (Sainju et al. 2008), (3) environmental conditions such as 23

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rain, temperature, and field morphology (Allaire et al. 2012), and (4) crop management such as 1

fertilization, tillage, and amendment (Kiss et al. 2009). However, these factors vary in the field 2

and interact in a complex manner so that it is very difficult to predict their relative and combined 3

effects on CO2-flux and C dynamic in soil. As a result, gas and C dynamics in soils under 4

switchgrass production with or without biochar or biofertilizer is poorly understood. 5

The objectives of this study were (1) to assess and compare the impacts of biochar, N 6

fertilizer and biofertilizers switchgrass biomass above and belowground and plant C 7

sequestration and on CO2 soil concentration (CO2-soil) and CO2-flux, (2) correlate C components 8

with physico-chemical soil properties, and (3) evaluate a C budget over two growing seasons in a 9

sandy loam soil. It was expected that biochar will increase plant biomass, soil microbial activity, 10

water content and favour overall more C sequestration budget. 11

Material and methods 12

Experimental site and design 13

This study was carried out at the experimental farm of Laval University at St-Augustin-14

de-Desmaures (Lat. 46° 45’ 00’’ N- Long 71° 27’ 00” O) near Quebec City on the St-Lawrence 15

plain with 2300 UTM (22). The site was used for soybean production in 2005, for corn from 16

2006 to 2008 and oat in 2009. The climate is humid temperate; data from an on-site weather 17

station gave a 30-year (1970-2000) average air temperature of 16, 19, 18, and 13 °C with 18

monthly precipitation of 110, 119, 120, 124 mm for June, July, August and September, 19

respectively. The sandy loam soil (Gasser and Collin 2004) at this site is classified as a Gleyic 20

Podzol (Soil Science Society 1999) formed from alluvial deposits. Plowing was completed 21

during fall 2009. 22

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A complete randomized block design was used for this experiment. An experiment with a 1

split plot treatment layout was set up in a sandy loam with biochar (0 to 10 t ha-1) as the main 2

plots and fertilizer types as the sub-plots. Fertilizer treatments included: (1) without fertilization 3

(Control), (2) with calcium ammonium nitrate at a rate of 50 kg N ha-1

corresponding to 100% of 4

the recommended dose as the mineral fertilizer (N-full); and (3) with N fixing rhizobacteria (N-5

bacteria) as the biofertilizer. The plots were 6 m long x 1.62 m wide and included 9 rows spaced 6

at 0.18 m interval. The main plots include an amendment with (B) or without biochar (NB). This 7

biochar application rate was selected based on previous researches in Quebec climate (Husk and 8

Major 2010) and economic considerations. The biochar was produced by Pyrovac Inc. (Québec, 9

Canada) through slow pyrolysis of coniferous wood at about 500°C. Biochar was manually 10

applied and incorporated into the first 0.05 m of the soil. Biochar characteristics were analyzed 11

following standard methods as listed in Table 1. Based on the biochar C concentration of 0.636 12

kg kg-1

, 10 Mg dry matter ha-1

biochar rate added 6.36 Mg C ha-1

was added to the soil. 13

Mineral fertilizers were applied by hand right after biochar incorporation and during the 14

following spring after biomass harvest. For this paper, only the main plots with the following 15

subplots were considered: Control, N-Full, and N-Bacteria from only 3 blocks, resulting in 18 16

plots used in this paper (3 blocks x 2 main plots (B vs NB) x 3 treatments). 17

N-fixing bacteria were a mixture of Paenibacillus polymyxa, Rahnella sp., Serratia sp. 18

and Pseudomonas sp. with peat moss and seeds. Peat was mixed with the seeds in order to 19

improve adherence. About 24 h prior to application, bacteria inoculum was mixed to 10 kg of 20

mixture (seed-peat) at a rate of about 107 - 10

8 cfu mL

-1 in an aseptic environment. These 21

mixtures were then kept at 20±1°C in plastic tubes. These plastic tubes containing the mixture 22

were open to let the seeds dry about three hours prior to seeding. Bacteria were not re-applied 23

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during the experiment and we did not attempt to measure their presence in soil, their survival or 1

their activities during the experiment. 2

The switchgrass (Panicum virgatum sp.) variety ‘upland Cave in Rock’ was selected for 3

its good performance in Quebec (Samson 2007). It was seeded at a rate of 10 kg ha-1

on June 11, 4

2010 (Wintersteiger seeder) after secondary tillage with spacing between rows of 0.18 m. 5

Atrazine was applied before emergence in 2010 for control of broad-leaf weeds. 6

Yield and biomass carbon content 7

Above ground biomass was manually sampled during fall 2010 and fall 2011 on rows 2 8

and 8 over a row segment of 1.5 m in length with the biomass cut 7 cm above ground while it 9

was harvested in spring 2011 with a harvester (model PH554 OHV, Hobbs) over the entire plots. 10

Biomass leftover in each plot after harvesting (the lowest 7 first cm) was evaluated following 11

hand clipping using two quadrats of 0.50 m in length x 0.50 m wide. 12

Root biomass was sampled with a 0.08 m diameter auger 0.3 m long. Only the center of 13

the cores at the selected depths was kept to avoid contamination. Composite samples of 8 cores 14

from each plot were taken from both row and interrow samples at 0-0.15 m and 0.15-0.3 m 15

depths in November 2011. Weeds were removed prior to sampling. The samples were soaked 16

during 16 h in 500 mL of sodium hexametaphosphate (100 g L-1

). They were then washed with a 17

hydropneumatic elutriation instrument using 760 and 250 µm sieves (Boehm 1981). Root 18

cleaning was completed by hand. 19

A representative subsample of 300 g of the above ground and root biomass was weighed, 20

oven dried at 70°C for 3 days to determine dry matter content in each plot. Root biomass density 21

was estimated using a method slightly adapted from Ma et al. (2000). For C content, above 22

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ground biomass subsamples were ground and passed through a 1-mm sieve and then dried prior 1

to combustion for C content. Roots were ground with a laboratory mill and analysed with a CNS 2

analyser (Carlo-Erba, model NA 1500) for total C. Concentration values were multiplied by dry 3

yield to calculate C sequestration. 4

CO2 emission fluxes and soil C concentration 5

Gas emissions were measured near the edge of each plot where the soil received the same 6

treatment, but where switchgrass aboveground biomass was removed. Switchgrass roots were 7

allowed to grow into this space, but not the weeds. 8

Carbon dioxide emissions from the soil to the atmosphere (CO2-flux) were measured 9

several times during 2010 and 2011using acrylic closed chambers (Rochette and McGinn 2005) 10

that were 1.22 m long, 0.76 m wide and 0.115 m high. All chambers had a reflective surface on 11

their top to decrease greenhouse effect within the chamber. Soil disturbance in the chamber was 12

reduced to a minimum, and atmospheric pressure was maintained with a small hole without 13

significant loss of gases. CO2-flux measurements were performed as rapidly as possible to 14

minimize the impact of microclimate changes caused by the chamber. Gas samples were taken 15

during 18 min at 3 min interval. There was no significant change in soil temperature during 16

measurement. Six chambers were simultaneously used on six plots and the chambers were rotated 17

to complete the 18 plots within two hours. This precaution minimizes the effect of varying sun 18

intensity and temperature during measurement events on CO2-flux. 19

Gas samples (10 mL) were withdrawn through septa with hypodermic gastight syringes 20

(10 mL, Becton-Dickinson 309643, Franklin Lakes, NJ, USA) and were immediately inserted 21

into gastight vials (10 mL model 5182-0838, Agilent, Wilmington, DE, USA) that were 22

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previously vacuumed and capped with aluminium seal (20 mm, 224178-01, Wheaton, Milleville, 1

NJ, USA) and rubber butyl septa (Wheaton cat. 224 100–202, Millville, NJ, USA) specifically 2

chosen for CO2 (Lange et al. 2008). 3

Access tubes of 5 mm diameter were installed at 0.15 and 0.3 m depth about 5 cm apart 4

from the flux chambers. The access tubes were purged two hours prior to gas sampling. A 10 mL 5

gas sample was withdrawn in each plot to obtain CO2-soil at each depth using the same syringes as 6

for CO2-flux. Access tubes and thermocouples remained in the field for two growing seasons. 7

CO2 concentration in the vials (emissions and soil concentration) was measured the within 8

48 h using a gas chromatograph (6890 N Agilent, Wilmington, DE, USA) with a 30 m HP-9

PLOT-Q column (19095P-QO4PT) and a TCD detector. Helium (UH-T 5.0, Praxair, Darbury, 10

CT, USA) was the carrier gas. Soil CO2-flux were calculated with the linear portion of the curve 11

representing temporal change in concentration that occurred within the chamber (Rochette and 12

McGinn 2005). 13

A 0.02 m diameter auger 0.3 m long was used for extracting composite soil samples at 0.1 m 14

depth in order to evaluate soil C content during both growing seasons. The auger was used to 15

directly reach depth greater than the desired one to prevent surface soil from falling into the hole 16

and contaminate the soil at the depth of interest. The soil was dried at 70°C for 3 days and sieved 17

through a 250 µm sieve. Soil C content was obtained with the instant combustion method (CN 18

Eager 1112) and reported on a per hectare basis using the equations of Ellert and Bettany (1995). 19

Other soil properties 20

Prior to planting and incorporating biochar at the beginning of the experiment, composite 21

soil samples were randomly extracted from each block at 0.05-0.15 and 0.15-0.3 m depth. These 22

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samples were used for chemical analyses. The soil contained 216, 293, 5472, and 291 kg ha-1

of 1

total P, K, Ca, and Mg. Texture analysis, using the hydrometer method (Gee and Bauder 1986) 2

indicated a sandy loam soil containing 55% of sand, 25% of silt and 20% of clay. Intact soil cores 3

were also extracted from the same depths for measuring bulk density (Grossman and Reinsch 4

2002) varying between 1.4 g cm-3

at 0.15 m and 1.6 g cm-3

at 0.30 m depth. Surface bulk density 5

was not measured. This soil is considered well structured (stable sub-angular aggregates) 6

relatively rich soil (Gasser and Collin 2004). 7

In addition, 0.15 m long TDR probes were vertically inserted into the soil at a depth of 0-8

0.15 m for measuring the volumetric water content (θv, m3 m

-3) through a connection to a 9

TDR100 (Campbell Scientific) and using the Topp et al.’s equation (Topp et al. 1980). Type T 10

thermocouples (copper-constantan) were installed at 0.15-m and 0.3 m depths immediately after 11

seedling. Soil water content and temperature were measured with a datalogger (CR23X 12

Micrologger, Campbell Scientific, Logan, UT) at the same time as CO2-soil and CO2-flux 13

measurements. They were measured 9 times during 2010 and 5 times during 2011. 14

Statistical analyses 15

Descriptive statistics for yield, biomass C content, CO2-flux, and soil physico-chemical properties 16

were obtained with SAS 9.2 (SAS 2008). Data were first submitted to a Box-Cox transformation 17

(Box and Cox 1964) when required, to improve the normality of their distribution. The 18

generalized MIXED linear procedure (SAS 2008) provided the main analyses. Since some soil 19

properties such as CO2-soil and CO2-flux were repeated in time, the procedure MIXED included a 20

repeated procedure with multivariate unstructured covariance model. Given the large natural 21

variability in soil parameters such as CO2-soil, CO2-flux and the variability in soil C content due to 22

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manual application, the effects were considered significant at P=0.1 level and we considered a 1

strong effect at P=0.05. The means were separated with LSD test. Since the statistical distribution 2

and transformation were different between years, they were separately analyzed. In addition, root 3

biomass from 0-0.15 m and 0.15-0.3 m depth were pooled to improve statistical analyses. 4

5

Results and discussion 6

Plant C sequestration 7

The average switchgrass yield was close to 6 Mg ha-1

in fall 2010 and ranged from 11 to 8

14 Mg ha-1

in fall 2011 (Table 2). The yields represent an efficient establishment during the first 9

year (Kiss et al. 2009) relative to an expected yield of 8-12 Mg ha-1

as measured in other 10

experimental sites with a mature crop in Quebec (CRAAQ 2008). The yield of the second year 11

was higher than those obtained from established crops under similar climatic conditions (5.5 Mg 12

ha-1

and 6.9 Mg ha

-1 in Bolinder et al. 2002). Other studies have reported 3.2 to 35 Mg ha

-1 in 13

different sites across the USA with lower yields in North Dakota and the highest obtained in 14

southwestern states (Liebig et al. 2008). Follett et al. (2012) also measured yields ranging from 15

3.5 to 11 Mg ha-1

for the same cultivar as in this study. This high yield was explained in part with 16

its well-developed root system (Ma et al. 2000). The soil at this site was a loam from a fluvial 17

deposit, with a good texture, which represents suitable soil properties for switchgrass production 18

(Best and Campbell 1971). 19

Root biomass was about half that of the above ground biomass ranging from 4.3 to 7.8 20

Mg ha-1

(Table 2). The root biomass was similar to those of 5.3 Mg ha-1

found by Bolinder et al. 21

(2002) and those of 4.6 to 8.4 Mg ha-1

for several cultivars including those of 4.7 Mg ha-1

for 22

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‘Cave in Rock’ measured by several authors (Bransby et al. 1998; Frank et al. 2004; Follet et al. 1

2012). The high variability observed in root biomass was due to collars near soil surface. 2

Biochar improved yield (above ground biomass) by about 10% and root biomass by 40% 3

during the establishment year (Table 2). The high water retention of the biochar (Table 1) may 4

have helped plant growth during the driest part of the summer in a dry year (Table 3) since water 5

content (θv) was slightly higher with the biochar treatment that in the control treatment (Fig. 1). 6

Indeed, it is usually assumed that biochar increases soil water content (Sohi et al. 2010). 7

Comparatively, the second growing season received frequent rainfalls during the entire summer, 8

resulting in a moist soil throughout 2011 (Fig. 1, Table 3). The biochar had no effect on plant 9

growth during the second year probably because water content was not limiting during this year. 10

In addition, seed germination may have been more influenced by biochar during the first year 11

because it was applied in the first 0.05 m where seed germination occurred. The reason for the 12

greater effect of biochar on root biomass than above ground biomass is not clear. It may be in 13

part due to the very high variability of root biomass estimate (which includes the collars 14

sometimes present and sometimes not present in the samples). 15

Switchgrass typically responds to N fertilization after at least one year of growth (Martel 16

and Perron 2008) and above a certain level of fertilization. Martel and Perron (2008) suggested 17

that at least 50-60 kg N ha-1

should be applied for switchgrass production in Quebec. Vogel et al. 18

(2002) observed increased yield when more than 75 kg N ha-1

was applied in Nebraska and Iowa. 19

The same trends were observed in this study with no yield response during the establishment and 20

an increase during the second year (Table 2). 21

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N-fixing bacteria did not influence plant growth, yield and C content (Table 2). Since 1

bacterial survival and activities were not determined, this treatment will not be further discussed. 2

The C content of the above ground biomass was within the expected range with a 3

minimum of 36% and a maximum of 51% of C content on a dry biomass basis in 2011. The 4

sequestration resulted in 2.7 to 3.1Mg C ha-1

in 2010 and to 5.0 to 6.6 Mg C ha-1

in 2011. 5

Comparatively, Liebig et al. (2008) obtained between 3 to 5 Mg C ha-1

in Iowa for several 6

cultivars, Bolinder et al. (2002) about 3 Mg C ha-1

in Quebec, while Zan et al. (1997) reported 5.5 7

Mg C ha-1

for the same cultivar in a warmer region of Quebec. 8

Biochar and mineral N fertilization increased the aboveground C sequestration (Table 2) 9

mainly because of a higher yield since plant C contents were not affected by treatments (data not 10

shown). Variation in harvested C (Table 2) was thus largely due to variation in biomass yield. 11

Liebig et al. (2008) also reported no difference in plant C content for a large range of fields 12

during 5 years of production. 13

Soil C content and soil CO2 concentration 14

Soil C content and CO2 emissions are important for establishing C budget. CO2 soil 15

concentration may help in understanding C dynamics in soil. In the present study, the initial soil 16

C content was low, with an average of 1.5 to 2.8 %. Differences in soil C content ranged from 17

nearly 0 to 9.7 Mg C ha-1

(Table 2) over the two-year period. The large variability observed in 18

soil C content (CV in Table 3) was due to the abundance of very fine roots (<250 µm) that could 19

not be removed from the soil samples, biochar movement by runoff at the soil surface observed 20

after intense rainfalls, and potentially to movement of soluble C in the soil profile since both 21

biochar (Table 1) and fresh organic matter contained soluble C. 22

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Differences in C content (∆C) between the beginning and the end of the study was about 9 1

times higher with biochar than without biochar (Table 2). Of course, this change was largely 2

associated with the biochar itself, but N fertilization also had an impact. Full-N fertilization 3

favored higher increases in soil C content as it probably activated microbial activity and root 4

development, both increasing biomass turnover (Halvorson and Wienbold 2002). A significant 5

interaction between biochar and fertilization indicated a synergy between both treatments (Table 6

2). The treatment without biochar and with full-N resulted in no or a very small change in soil C 7

content. The higher increase in C content with full-N treatment and biochar (Table 3) may 8

indicate that biochar may improve nutrient use by roots and bacteria resulting in increased C 9

addition to soil. 10

The capacity of perennial grasses to affect soil properties over time and the importance of 11

soil properties for plant growth are well documented (Follett 2001). It is generally accepted that 12

changes in soil C content only slowly respond to changes in management. However, other studies 13

also reported differences in soil C content after the first year of switchgrass production (Woolf, 14

2008) without biochar application. Davis et al. (2011) indicated that switchgrass almost always 15

increased soil C content by, on average, 27 Mg C ha-1

, representing about a 1.9% yearly change. 16

Zan et al. (1997) observed about a 45% increase in soil C content near the soil surface (the 0- to 17

0.15-m depth) after replacing annual crops by switchgrass. Also, Garten and Wullschleger (1999) 18

reported that 19-31% of the soil C content changed after several years of root growth and 19

senescence of switchgrass. Follet et al. (2012) observed up to 50% increase in C content in 0-1.5 20

m depth corresponding to about 2 Mg C ha-1

yr-1

. Schmer et al. (2011) reported a change of 0.1 to 21

1.2 Mg C ha-1

yr-1

at a 0-0.3 m depth over several years. However, Follet et al. (2012) observed 22

that half of the total increase in soil C content was below 0.3 m depth. Schmer et al. (2011) 23

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reported a change of up to 3.3 Mg C ha-1

yr-1

at 1.2 m depth. Deeper soil measurements in this 1

study may have increased the estimated changes in soil C content under switchgrass. 2

Average CO2-soil and its coefficient of variation (Table 3) were similar to ranges observed 3

for other crops in Canadian studies (Allaire et al. 2012). The maximum values in 2010 were 4

lower than in 2011 (Table 3). They indicate either good exchange, lateral diffusion, or a slower 5

respiration than in 2011, in part because plant roots were much less developed and the soil was 6

dryer in 2010. As usual for agricultural soils, the CO2-soil concentration at 0.3 m depth was 20 to 7

50% higher than at 0.15 m (Fig. 2, Table 3) because the exchange with the atmosphere decreases 8

its concentration near the soil surface (Allaire et al. 2012). The concentration in 2011 nearly 9

reached 141 µg mL-1

during the hottest days of the growing season, in what was a particularly 10

wet summer (Table 3). The high concentration indicates either low exchange with the atmosphere 11

and high soil respiration or both. Low gas exchange occurs in a wet soil because of its low air-12

filled porosity, which considerably reduced gas movement in soil. Soil water content was one of 13

the sources of CO2-soil variation (Fig. 3). In addition, the rain water of 2011 infiltrated into the soil 14

during the hottest days of the summer, which likely activated microbial and root activities 15

(Rochette and Angers, 1999) resulting in higher concentrations of CO2-soil. 16

Biochar significantly influenced CO2-soil (Table 4). It increased CO2-soil concentration 17

during both years and at both depths (Fig. 2). This was probably due to the soluble C in biochar 18

(Table 1) and in microelements (Verheijen et al. 2010), as well as its impact on soil water content 19

and specific surface area (Table 1). The influence of biochar at 0.3 m remained significant during 20

the two-year period (Fig. 2). This is probably due to the downward movement of soluble C 21

released from the biochar (Table 1) in the soil profile that fed microorganisms, and roots 22

extending deeper in the soil profile over time. Therefore, biochar has a very short term effect on 23

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soil properties as observed in this study in addition to long term effect as observed in other 1

studies (Lehmann et al. 2012; Sohi et al. 2010). 2

Comparatively, N fertilization influenced CO2-soil, but differently between depths (Table 3

4). Forward multiple regressions indicated a linear correlation between soil N content and 4

fertilization (data not shown, P=0.02) and between N content with CO2-soil (P=0.04). It is known 5

that N and C cycles are intricate. In this case, mineral fertilization, by influencing N content may 6

have indirectly influenced CO2-soil by stimulating microbial and root activity. 7

CO2 fluxes 8

The CO2-flux ranged from -8 µg m-2

s-1

to 160 µg m-2

s-1

(Table 3). Sequestration rather 9

than emissions occurred during coldest conditions measured in this study such as during early 10

spring and late fall 2011 (Fig. 4). Similar observations were reported for switchgrass by Skinner 11

et al. (2010). Schmer et al. (2012) also showed a similar trend during switchgrass production with 12

increasing rate from 15 µg m-2

s-1

(ex: late spring, day 160) to 69 µg m-2

s-1

in the warmest 13

months (ex: day 180) and going down to 15 µg m-2

s-1

toward the end of the growing season. As 14

observed for most agricultural soils (Allaire et al. 2012), the coefficient of variation nearly 15

reached 100% (Table 3). 16

The average flux for 2011 was about 20% higher than in 2010 (Tables 3) probably 17

because July and August emitted significantly more in 2011 than in 2010 (Fig. 4). Although the 18

seasonal average CO2-soil was almost 4 times higher in 2011 than in 2010 (Table 3 Fig. 2), the 19

emissions were in the same order of magnitude between years (Table 3, Fig. 4). Either, more 20

accumulation in the soil, sporadic high emissions, more reabsorption, or more lateral movement 21

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may have occurred during 2011 because of a wetter soil, in addition to change and movement of 1

soluble C (not measured), change in root growth and depth compared to 2010. 2

The cumulative flux was calculated with a linear interpolation between measuring events. 3

It ranged between 1.2 to 5 Mg C ha-1

for both growing seasons combined with an average of 3.3 4

Mg C ha-1

for all treatments combined (data not shown but used in the calculation of soil CO2 5

emissions in Table 5). Roughly, the C lost by CO2-flux represents more than one third of that 6

added to the soil through biochar amendment, or about 25% of total plant C captured over the 7

establishment period (Table 5). Comparatively, Frank et al. (2004) estimated that C loss by CO2-8

flux was about 44% of the total plant sequestration, a higher loss, although similar methods and 9

instrumentation were used in both studies and over about the same period of the year but in a 10

different region. 11

A strong interaction was observed between biochar treatment and time (Table 4, Fig. 4). 12

The addition of biochar did not influence CO2-flux right after its application in 2010 but had a 13

positive effect later on toward the end of the growing season (Fig. 4). In 2011, CO2-flux under 14

biochar treatment was lower than under no biochar throughout most of the season (Fig. 4). 15

Mineral N fertilization not only increased CO2-soil but also CO2-flux (Table 4) by almost doubling 16

the emission rate, but only when the soil was warmer (Fig. 4). It was probably limiting only when 17

microbes and roots were the most active. Mineral fertilization tended to promote higher 18

respiration throughout most of the year in 2010, but it was not significant during 2011 (Fig. 4). 19

The impact of fertilization was also probably indirect since CO2-soil in 2011 and CO2-flux of both 20

years were correlated with soil N contents (data not shown, P=0.001). Lee et al. (2006) observed 21

similar results with switchgrass under mineral fertilization, and an increase in CO2-flux with 22

manure application. 23

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Stepwise multiple regressions linear relationship between soil properties, mainly CO2-soil 1

and CO2-flux as dependant variables. As expected, stepwise multiple regressions indicated that CO2-2

soil (P=0.02) was highly related to soil water content (Fig. 3) but weakly correlated to CO2-flux 3

(Fig. 5). The fact that CO2-soil and CO2-flux did not follow the same trends all the time, although 4

they were correlated, may be due to several factors. Some processes occurred near the soil 5

surface in part because of biochar application, which cannot be captured by measuring soil 6

properties at 0.15 m and 0.30 m depth as was done for CO2-soil. CO2 may have formed at the 7

surface where biochar was applied, or transformed (Sohi et al. 2010), captured or laterally 8

deviated near the soil surface. In addition, small pressure gradients caused by wind and water 9

infiltration (e.g. during rainfall) create high temporarily convective fluxes than cannot be detected 10

by the flux chamber method nor by soil gas concentration in depth. 11

Soil temperature at different depths (P=0.01) and N content (P=0.01) measured at 0.15 m 12

depth also explained some of CO2-flux variability. The warmer soil of July and August (between 13

days 175 and 225,) corresponded to higher emission fluxes (Fig. 1 and 5). Frank et al. (2004) and 14

Lee et al. (2006) also observed values of CO2-flux that coincided with changes in soil temperature 15

under switchgrass production with higher emissions during the warmest months. Further studies 16

are needed to assess current outcome related to the relationship with N content. 17

Carbon budget 18

The C budget (Table 5) was obtained by combining plant, soil and management budgets. 19

The plant and soil budgets were calculated from data obtained in this study while the 20

management budget was calculated using data from the literature. The use of plants after 21

harvesting (e.g. biochar for sequestration vs for energy) was not considered. The plant C budget 22

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was most likely underestimated since switchgrass roots grow to 3 m deep (Liebig et al. 2008) 1

whereas we only measured to 0.3 m deep. It was partly compensated by an underestimation of 2

CO2-flux because winter and spring were not included in the budget. In addition, industrial biochar 3

production, such as used for this biochar, tends toward zero emissions because pyrolysers reuse 4

and recirculate energy and gases within the process (Allaire and Lange 2013). We therefore 5

consider zero emissions for manufacturing biochar in this budget. 6

Based on a C budget, the greatest C sequestration by plant was obtained with biochar 7

irrespective of fertilization (Table 5). This result is in agreement with several authors who also 8

considered biochar as a good way of improving the C budget (Woolf 2008, Lehmann et al. 2006). 9

The relative importance of soil C sequestration on the net C budget was about the same as that of 10

the plants. In practice, the impact of biochar on the C budget relative to that of plants will tend to 11

be lower over time because long intervals are likely to occur between biochar applications. The 12

equivalent of about one third of the increase in soil C content in the biochar treatments was 13

respired away by the soil. Most of the biochar remained in the soil. Nearly one fourth of C 14

sequestered by plants remained in or at the soil surface (root and crop residues) to be degraded 15

into soil organic matter or lost through CO2-flux at later times. The C lost by management 16

represented a small portion of the C budget. This is true for the first years of establishment, but 17

loss of C from management is expected to gain in relative importance over time as biochar 18

addition will likely be rare. The best overall C budget occurred with the biochar treatment 19

combined with the mineral N fertilization followed by biochar alone, then by fertilization alone 20

(Table 5). Based on these results, improved biomass production with mineral N fertilization does 21

not have a negative impact on the C budget as long as it is combined with biochar amendment. 22

Implications for energy biomass production 23

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Several implications could be listed from these findings: (1) Switchgrass can grow fast and lead 1

to high yields, even as early as the second year of growth. (2) Switchgrass slightly responded to 2

biochar amendment at a rate of 10 Mg ha-1

in a loamy sand. Considering switchgrass as not 3

demanding in terms of environmental conditions and fertilization, the soil of this study to be 4

relatively favorable to plant growth, and the low level of biochar addition to soil, we expect 5

stronger influence of biochar with more sensitive plants or under poorer soil conditions in short 6

and long terms periods, thus more valuable financial return of biochar application. (3) Because 7

biochar influenced soil properties at a rate of 10 Mg ha-1

in a cold humid climate in a loamy soil 8

(e.g. humidity and temperature), we expect that it will help germination during spring time and 9

plant growth during dry summers not only in loamy soils, but also in sandy soils. (4) Because 10

biochar influenced CO2-soil, and biochar and fertilization treatments showed interactions on some 11

plant and soil parameters, potential benefits associated with changes in soil properties should be 12

considered when evaluating its application irrespective of yield effects. (5) The best C budgets 13

were obtained with biochar application with or without mineral N fertilization, but the long term 14

impact of biochar on the C budget will highly decrease depending upon the frequency of biochar 15

application. (6) Averaged over the first two years of establishment, switchgrass, produced for 16

bioenergy, can immediately start sequestering C as long it is accompanied with an amendment 17

high in C such as biochar. 18

19

Conclusions 20

Switchgrass may be produced for bioenergy, but for being efficient in terms of energy and 21

environment, one should consider at decreasing input energy and matter for its production. In 22

addition, its global environmental impact should be as low as possible. In this study, switchgrass 23

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22

was produced with biochar and/or with N-fixing bacteria hoping to sequester C and reducing 1

input materials. Swithgrass produced up to 14 Mg dry matter ha-1

over the first two years of 2

establishment, that is efficient compared to other perennial plants. Biochar slightly increased 3

yield by about 10% during the first year and root biomass by up to 50% after two years, 4

increasing C sequestration. Biochar had the highest influence on CO2-soil at 0.15 and 0.3 m depths 5

with up to 50% increase while its influence on CO2-flux varied over time. Mineral N fertilization 6

tended to increase CO2-soil and CO2-flux. Variations in CO2-flux were mostly explained by CO2-soil 7

followed by temperature, and soil N content. The interaction between CO2-soil and CO2-flux with 8

soil properties changed during the growing seasons. Biochar and fertilization treatments showed 9

interactions on some aspects of plant and soil. Using plant C, soil C content, C losses through 10

emissions, we found that the best C budget was obtained with a combination of biochar and 11

mineral N fertilization. The equivalent of about one third of the increase in soil C content in 12

biochar treatments was respired away by the soil. Nearly one fourth of C sequestered by plants 13

remained in or at the soil surface (root and crop residues) to be degraded into soil organic matter 14

or lost through CO2-flux at later times. Since switchgrass, that is a non exigent plant, responded to 15

biochar in a good soil with a low application level of biochar, we expect that more sensitive 16

plants, poorer soils and higher application of biochar should result in much stronger impact of 17

biochar on C budget. Also, biomass production for energy is often pointed out as not efficient in 18

terms of C budget, the results indicate that short term biomass production with biochar 19

application have the potential for having an efficient C budget. 20

21

Acknowledgment 22

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The authors wish to thank the FQRNT and NSERC for their financial support. They are 1

grateful for the technical support of Valérie Bélanger and François Marquis who provided aid in 2

plant and biochar analyses, respectively. The team also thanks Benjamin Dufils, Marie-Pier 3

Amyot, Samuel Richard, Claudia Sylvain and Jennifer Granja for field measurement, sampling 4

and data collection. They also thank the managers of the experimental farm of Laval University 5

for their collaboration. 6

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____________________________________________________________________________________

Table 1. Initial physico-chemical properties of biochar

Property Units Average CV (%) Reference

Physical properties

Ash g g-1

0.10 10 CEAEQ 2003

Mean weight diameter mm 1.38 0.5 IBI 2011

Uniformity index g g-1

*100 2.65 0.5 Allaire and Parent 2004

Bulk density Kg m-3

270 3.6 Grossman and Reinsch 2002

Solid density Kg m-3

1547 5.1 Flint and Flint 2002

Total porosity m3 m

-3 0.83 5.1 Flint and Flint 2002

Water retention at saturation g g-1

*100 250 20 Allaire and Parent 2004

Specific surface area m2 g-1 5.3 14 ASTM D6556-10

Chemical properties

pH-H2O -- 7.3 1.9 CPVQ 1997

Electrical conductivity µHoms 123 6.2 CPVQ 1997

Ctotal g g-1

*100 63.6 1.4 CPVQ 1997

Csoluble mg g-1

0.04 62 Amacher et al. 1990

Ntotal g g-1

* 100 0.42 2.4 ASTM E1941

Psoluble cmol

+ kg

-1 1.42 1.8 Amacher et al. 1990

Nasoluble cmol

+ kg

-1 5.46 3.7 Amacher et al. 1990

CEC cmol+ kg

-1 60.1 2.7 CPVQ 1997

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____________________________________________________________________________________________________________

Table 2. Effect of treatments on yield (above ground biomass as exportable for bioenergy) and root biomass, carbon content in

biomass, and change in soil carbon content (ΔC) during the experiment

zCombined samples from 0-0.15 m and 0.15-0.3 for improving statistical analysis.

Treatments Biomass C content ΔC

Above ground Roots Above ground Roots Soil

Fall-10 Spring-11 Fall-11 Fall-11z Fall-10 Spring-11 Fall-11 Fall-11 Fall-11 – Spring-10

(Mg dry matter ha-1

) (Mg C ha-1

) (ΔMg C ha-1

)

Main plots, biochar treatments

B 6.53 a 2.48 12.55 7.44 a 3.00 a 1.21 5.87 3.49 a 8.65 a

NB 5.90 b 2.51 11.66 4.90 b 2.70 b 1.22 5.50 2.25 b 0.19 b

Subplots, fertilization

Control 6.45 2.45 11.04 b 6.63 2.92 1.20 5.16 b 3.02 3.59 b

N-Bacteria 5.95 2.47 11.45 b NA 2.78 1.20 5.41 b NA 4.87 a

N-Full 6.23 2.55 13.82 a 5.71 2.85 1.24 6.49 a 2.72 4.80 a

Interactions between treatments

B + Control 6.76 2.47 11.27 b 7.78 3.05 1.22 5.26 3.61 6.63 b

B + N-Bacteria 6.59 2.42 12.27 b NA 3.11 1.18 5.80 NA 9.72 a

B + N-Full 6.24 2.55 14.13 a 7.10 2.83 1.25 6.56 3.38 9.52 a

NB + Control 6.15 2.43 10.81 c 5.48 2.79 1.19 5.06 2.43 0.54 c

NB + N-

Bacteria

5.32 2.52 10.64 c NA 2.46 1.23 5.02 NA 0.009 d

NB + N-Full 6.22 2.55 13.52 a 4.32 2.86 1.24 6.43 2.06 0.007 d

Page 32 of 43C

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B: biochar, NB: no biochar, N-Control: control treatment without fertilization, N-bacteria: biofertilizer treatment, N-Full: full dosage

of mineral fertilization

(Bold numbers indicate significant effect at P=0.1, bold and underlined numbers indicate significant effect at P=0.05 resulting in

groups identified by letters). The mean separation letters apply to means in columns.

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_________________________________________________________________________________________________

Table 3. Descriptive statistics of soil properties during both years

C-soil CO2-soil CO2-flux N θv

Temperature

Depth (m) 0-0.15 0-0.15 0.15 0.3 Surface 0-0.15 0-0.15 Surface 0.15 0.3

(g g

-1 *

100)

(Mg C ha-

1)

(µg mL-1

) (µg m-2

s-1

) (g g-1

*

100)

(m3 m

-3) (°C)

2010

n events 2 2 9 9 9 2 9 9 9 9

n obs. 168 168 155 154 161 168 162 161 155 162

Mean 1.84 28.20 11.55 16.86 44.91 0.27 0.18 26.40 21.16 20.98

Min 1.47 23.61 0.60 2.11 -7.48 0.20 0.07 16.07 9.17 11.84

Max 2.84 42.49 27.59 34.83 156.40 0.36 0.34 36.08 25.93 25.89

CV 29.1 28.8 59.7 53.8 70.0 27.4 46.6 35.8 37.5 33.7

2011

n events 2 2 5 5 8 2 5 5 5 5

Mean 1.99 30.60 53.28 73.21 34.54 0.27 0.27 22.65 17.44 17.13

Min 1.29 20.29 14.68 14.42 -8.68 0.15 0.18 11.14 7.28 9.31

Max 2.76 42.55 123.67 141.47 160.89 0.38 0.33 31.35 23.64 23.08

CV 37.9 38.2 57.6 52.7 96.5 37.5 18.9 23.4 37.4 22.7

CV: coefficient of variation (%), n: number of events or number of observations

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Page 35 of 43C

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______________________________________________________________________________________________________________

Table 4. Significance of treatment (P > F value), depth and time on soil properties measured several times during each growing

seasons (Bold and underlined numbers indicate significant effect at P=0.05, bold numbers indicate significant effect at P=0.1)

Treatment C-soil CO2-soil CO2-flux θv Temperature

2010 2011 2010 2011 2010 2011 2010 2011 2010 2011

Biochar 0.0009 0.02 0.03 0.037 0.41 0.56 0.09 0.02 0.85 0.42

Fertilization 0.0001 0.71 0.69 0.35 0.11 0.12 0.59 0.31 0.24 0.015

Depth - - 0.0001 0.001 - - - - 0.14 0.11

Time z 0.0001

z 0.0001 0.0001 0.013 0.0015 0.0001 0.0003 0.0001 0.0001

Biochar + fertilization 0.02 0.02 0.33 0.3 0.40 0.82 0.5 0.31 0.68 0.02

Biochar + Depth - - 0.0027 0.6 - - - - 0.22 0.84

Fertilization + depth - - 0.0074 0.89 - - - - 0.72 0.29

Biochar + time - - 0.0092 0.033 0.09 0.10 0.0012 0.31 0.0003 0.30

Fertilization + time 0.0001 0.093 0.68 0.53 0.016 0.02 0.99 0.16 0.78 0.14

Depth + time - - 0.0001 0.20 - - - - 0.0001 0.005

Biochar + fertilization + depth - - 0.26 0.45 - - - - 0.93 0.71

Biochar + fertilization + time 0.52 0.27 0.07 0.87 0.13 0.35 0.32 0.73 0.19 0.84

Biochar + depth + time - - 0.63 0.48 - - - - 0.46 0.90

Fertilization + depth + time - - 0.17 0.74 - - - - 0.64 0.61

Biochar + fertiliz + depth +

time

- -

0.93 0.80 - - - - 0.98 0.92

z Csoil was measured only twice. In this case, the time effect corresponds to the difference between Spring 2010 and Fall 2011.

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_________________________________________________________________________________

Table 5. Estimated carbon budget in switchgrass production during the first two years of establishment

Control B N-Full B+ N-Full

(Mg C ha-1

)

Plant sequestrationz 11.47 13.14 12.59 14.02

Plant exportsy -7.85 -8.31 -9.29 -9.39

Soil sequestrationw 0.54 6.63 0.007 9.52

Soil CO2 emissionsv -2.58 -2.56 -3.99 -3.18

Emissions for fertilizer and

biochar manufacturingu 0 0 -0.02 -0.02

Emissions from equipmentt -0.060 0.062 -0.062 -0.063

Plant budgetx 3.22 4.83 3.30 4.63

Soil budget 2.04 4.07 -3.98 6.34

Management budget -0.06 -0.06 -0.08 -0.08

Net C budget 1.52 8.84 -0.76 10.87 z Summation of all plants parts over two seasons (above ground for fall 2010, Spring 2011, and fall 2011, roots for fall 2011)

y Summation of harvested biomass for energy production (above ground for fall 2010 and 2011)

x Equivalent to leftover from spring harvest and root biomass of 2011

w Difference between soil C concentration at the beginning of the experiment and in fall 2011.

v Total emissions of both growing seasons assuming (1) no emissions before Julian day 145 (May 25

th) and (2) no emissions after

Julian day 264 (September 22nd

), (3) linear interpolations between measurement events

u From Davis et al. (2011)

t Emissions related to machinery used in this type of production such as plowing, disk, seedbed preparation, application of fertilizer

and biochar, mowing and baling (different from seedling year and established year), values from Adler et al. (2007)

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38

Fig. 1. Soil water content and temperature in 2010 and 2011 as influenced by biochar treatment (B: biochar, NB: without biochar).

Vertical error bars are standard errors.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

150 175 200 225 250 275 300

Water content (m

3m-3)

Date (Julian)

B in 2010

B in 2011

NB in 2010

NB in 2011

10

12

14

16

18

20

22

24

150 175 200 225 250 275 300

Temperature (°C)

Date (Julian)

B 2010

B 2011

NB 2010

NB 2011

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39

Fig. 2. CO2 soil concentration (CO2-soil) (average of all fertilisation treatments and replicates) with (B) and without biochar (NB) for

both depths (0.15 and 0.30 m) during 2010 and 2011. Vertical error bars are standard errors.

0

20

40

60

80

100

120

150 175 200 225 250 275 300

CO2concentration (µµ µµg ml-1)

Date (Julian)

2011

B at 0.15 m

B at 0.30 m

NB at 0.15 m

NB at 0.30 m0

5

10

15

20

25

30

35

150 175 200 225 250 275 300

CO2concentration (µµ µµg ml-1)

Date (Julian)

2010

B at 0.15 m

B at 0.30 m

NB at 0.15 m

NB at 0.30 m

Page 39 of 43C

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Fig. 3. Relationship between CO2 soil concentration (CO2-soil) and volumetric water content at 0.15 m depth in 2010 and 2011.

CO2 (2010) = 51.2 (water content) + 2.2 R² = 0.34 P=0.006

CO2 (2011) = 383 (water content) - 51.7 R² = 0.22 P=0.04

1

10

100

0.0 0.1 0.2 0.3 0.4

CO

2co

nce

ntr

ati

on

(µµ µµ

g m

l-1)

Water content (m3 m-3)

2010 2011

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Fig. 4. CO2 emission fluxes (CO2-flux) influenced by biochar treatments (average of all fertilizer treatments and replicates) and by

-20

0

20

40

60

80

100

140 160 180 200 220 240 260 280 300

CO

2fl

ux

(µµ µµ

g m

-2s-1

)

Date (Julian)

Control

N-Bact

N-full

2010

-20

0

20

40

60

80

100

140 160 180 200 220 240 260 280 300

CO

2fl

ux

(µµ µµ

g m

-2s-1

)

Date (Julian)

Control

N-Bact

N-Full

2011

-20

0

20

40

60

80

100

140 160 180 200 220 240 260 280 300

CO

2fl

ux

(µµ µµ

g m

-2s-1

)

Date (julian)

B 2010

NB 20102010

-20

0

20

40

60

80

100

140 160 180 200 220 240 260 280 300

CO

2fl

ux

(µµ µµ

g m

-2s-1

)

Date (julian)

B 2011

NB 20112011

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fertilization treatments (average of all biochar treatments and replicates) during both growing seasons 2010 and 2011. Vertical error

bars are standard errors.

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Fig. 5. Correlation between CO2 emission fluxes (CO2-flux) at 0.15 m and 0.30 m depths with CO2 soil concentration (CO2-soil) and

temperature at the same depths

CO2-flux = 0.35 CO2-soil + 10.0

R² = 0.09 P=0.05

-25

0

25

50

75

100

125

150

175

0 50 100 150

CO

2fl

ux

(µµ µµ

g m

-2s-1

)

CO2 soil concentration (µµµµg ml-1)

0.15 m depth CO2-flux = 0.35 CO2-soil + 1.90

R² = 0.13 P=0.03

-25

0

25

50

75

100

125

150

175

0 50 100 150

CO

2fl

ux

(µµ µµ

g m

-2s-1

)

CO2 soil concentration (µµµµg ml-1)

0.30 m depth

CO2-flux = 4.22 Temperature - 37.9

R² = 0.25 P=0.01

-25

0

25

50

75

100

125

150

175

5 10 15 20 25

CO

2fl

ux

(µµ µµ

g m

-2s-1

)

Temperature (°C)

0.15 m depth CO2-flux = 5.73 Temperature - 63.3

R² = 0.30 P=0.008

-25

0

25

50

75

100

125

150

175

5 10 15 20 25

CO

2fl

ux

(µµ µµ

g m

-2s-1

)

Temperature (°C)

0.30 m depth

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