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Fertilizer alternatives for switchgrass

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Greener gas? Impact of biosolids on carbon intensity of switchgrass ethanol 1

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Sally Brown1* Manmeet Pannu1 and Steven C. Fransen2, 7

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1*School of Environmental and Forest Sciences, University of Washington Box 352100 9

Seattle, WA 98195, 10

2Washington State University, Irrigated Agriculture Research and Extension Center, 11

Prosser, WA 99350, 12

• Corresponding author phone: (206) 755 1396; email: [email protected]. 13

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Acknowlegements. This research was supported by the Genomic Science and 18

Technology for Energy and the Environment grant DE-SC0006869 from the Department 19

of Energy and by the Northwest Biosolids Association. 20

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mailto:[email protected]


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Keywords 24

Switchgrass, biosolids, total ethanol potential, fertilizer offsets, ethanol, nitrous oxide 25

Abstract 26

A field study compared biosolids and synthetic fertilizers on biomass yield, ethanol 27

production and N2O emissions of switchgrass (Panicum virgatum). Field measures of 28

N2O were limited and should be considered as qualitative rather than quantitative. 29

Minimal N2O emissions were observed the first year of the study (0.99 ±1.5 g N2O ha-1 d-30

1, for biosolids) with no difference between treatments. Biosolids were added in excess 31

of agronomic rates and gas samples collected immediately after irrigation for the 32

subsequent years to examine maximum N2O emissions. Mean year 2 emissions increased 33

for fertilizers to 1.8 g ± 8 g N2O ha-1 d-1 (n=131) and to 3.73 ± 10.2 g N2O ha-1 d-1 ( 34

n=130) for biosolids amended soils. Emissions in year 3 were similar to year 2. Yield 35

was similar and ranged from 3.7 5 to 11 1.1 and 5.0 ± 0.2 to 13.4 ± 1.7 Mg ha-1 for 36

biosolids and fertilizer, respectively. The potential ethanol yield was 365 ± 28 L Mg-1 37

and 374 ± 34 L Mg-1 for the biosolid and fertilizer grown grass. Greenhouse gas 38

emissions associated with fertilizer production were considered for N, P, and K and 39

totalled 1653 kg CO2e ha-1 . The equivalent credits for substitution of biosolids (18 Mg 40

ha-1) were -2492 kg CO2e ha-1. N2O emissions were calculated based on 1% of total N 41

applied for agronomic applications and were 8600 and 3500 g N2O ha-1 for the biosolids 42

and fertilizer treatments. Total carbon costs associated with fertilization totalled 2700 kg 43

CO2e ha-1 for fertilizer and 60 kg CO2e ha-1 for biosolids. Using measured N2O data 44

would have resulted in lower emissions for both treatments. 45

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Introduction 47

Recent studies have considered the impact of using synthetic fertilizers on the carbon 48

benefits associated with biofuels (Erisman et al., 2010; McGowan et al.; 2018; Roth et 49

al., 2015; Ruan et al., 2016). Use of synthetic fertilizers for biofuel production can 50

positively impact the carbon balance for biofuels by increasing biomass per ha (Erisman 51

et al, 2010; McGowan et al., 2018). However, this yield increase may not compensate for 52

potential carbon emissions associated with fertilizer manufacture and elevated N2O 53


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emissions associated with fertilizer use (Erisman et al., 2010; Griffing et al., 2014; 54

McGowan et al.; 2018; Roth et al., 2015; Ruan et al., 2016). Switchgrass, a perennial 55

warm-season grass, is viewed as a carbon friendly alternative to the annual warm-season 56

grass, corn (Zea mays). The range of fertilizer required for switchgrass production can 57

vary widely from no added fertility to 224 kg N ha-1(Kimura et al., 2015; McGowan et 58

al.; 2018; Ruan et al., 2016; Schmer et al., 2012) with application rates normally less than 59

corn (Wang et al., 2012). In addition, switchgrass can be grown on marginal lands, 60

reducing pressure on existing agricultural lands and the potential for emissions related to 61

converting native lands to agriculture (Schmer et al., 2012; Searchinger et al., 2008). 62

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Using recycled nutrients may be a means to further reduce the carbon intensity of 64

switchgrass derived biofuels. Alternative sources of fertility such as municipal biosolids 65

and manures can provide necessary nutrients without the embodied energy associated 66

with use of synthetic fertilizers (Brown et al., 2010; Liu et al., 2013; Smith et al., 2014). 67

This has been previously noted. Heller et al. (2003) suggested that using biosolids 68

instead of synthetic fertilizer for willow production would increase energy conversion 69

efficiency of cellulosic based biofuel by eliminating synthetic fertilizers. Esperschuetz et 70

al. (2016) observed improved yields and seed production of biomass crops with biosolid 71

applications to low- fertility soils, compared to urea. One study evaluated biosolids on 72

switchgrass biomass for theoretical ethanol potential (TEP) and found that switchgrass 73

fertilized with biosolids was comparable to grass grown using conventional fertilizers 74

(Liu et al., 2013). However, this study did not evaluate the impact of using biosolids on 75

the carbon intensity of the crop. 76

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Despite benefits associated with fertilizer avoidance, use of recycled nutrients may 78

enhance N2O emissions, thereby increasing the climate impact associated with using 79

these materials. Previous work has considered N2O emissions as part of the costs 80

associated with fertilizer use in biofuel production. In many cases default emissions 81

(typically 1-2% of total N applied) have been used to estimate N2O emissions from 82

switchgrass systems (Adler et al., 2007; Ranney and Mann, 1994; Schmer et al., 2014; 83

Spartari et al., 2005). Field studies have also measured N2O emissions of during 84


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switchgrass cultivation, typically finding emissions to be significantly lower than default 85

emission values (Oates et al., 2015; Schmer et al., 2012; Wile et al., 2014). For example, 86

Wile et al. (2014) applied up to 120 kg N ha-1 on a sandy loam to fine sandy loam soil, 87

observing < 1 kg N2O-N ha-1 emissions the first growing season and < 0.2 kg N2O-N ha-1 88

the second on established switchgrass. Higher N2O (17.0 kg N2O-N ha−1) emissions were 89

observed on fine textured soil where urea was applied at 112- 134 kg N ha-1 to 90

switchgrass (2016). Ruan et al. (2016) found high daily emissions during the growing 91

season at 270 ± 25 g N ha−1 d−1from fertilizer treatments receiving 196 kg N ha-1yr-1 on 92

an Alfisol. They reported increases in annual emissions in the 2nd and 3rd year of the trial 93

that were attributed to residual N from fertilization. This increase may have been related 94

to higher rainfall and associated anaerobic conditions for each successive year (74, 102, 95

and 112 cm, respectively). McGowen et al. (2018) reported increasing N2O emissions 96

with increasing N application rates for switchgrass grown on a fine textured soil in 97

Kansas. Emissions were measured year round with frequency of measurements increased 98

during the growing season. Emissions were below default values for lower rates of N 99

application and above default for higher rates (50-150 kg N ha-1). To our knowledge, 100

there have not been studies done where the impact of organic fertilizers, such as biosolids 101

or animal manures, on N2O emissions from switchgrass. In addition, no studies have 102

been conducted with field data to evaluate the impact of organic sources of fertilizer on 103

the energy intensity of switchgrass ethanol. 104

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This study evaluated the feasibility of producing switchgrass based ethanol using 106

biosolids as an alternative to synthetic fertilizers in irrigated fields in the Pacific 107

Northwest. In addition to measuring yield and TEP, we also measured N2O emissions 108

from the field plots across three growing seasons. Results were used to estimate the 109

relative GHG emissions from switchgrass grown with synthetic fertilizers and grass 110

grown using municipal biosolids. In addition to limited field measures of N2O, default 111

N2O emissions were also calculated. The results were used to evaluate whether an 112

alternative source of fertilizer would reduce greenhouse gas emissions associated with 113

switchgrass cultivation for biofuel production. 114

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Materials and Methods 116

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Study sites. 118

A replicated field trial established at the USDA-ARS Integrated Cropping Systems 119

Research Field Station near Paterson, in Benton County, WA (4556N, 11929 W; 114 120

m) on a Quincy Sand (mixed, mesic Xeric Torripsamment) was used for this trial. 121

Average annual rainfall in the area is 20 cm. The average high temperature in the area is 122

18.6 C with a mean low temperature of 6.5 C. The soil in the study had a surface (0-15 123

cm) bulk density of 1.33 Mg m-3, organic C concentration of 3.7 g kg-1, and total N 124

concentration of 0.73 g kg-1. The pH of the top 15 cm measured 6.6. The mineral 125

fraction of the soil in the surface 15 cm consisted primarily of sand (917 g kg-1) with 56 g 126

kg-1 silt and 27 g kg-1 clay (Kimura et al., 2015). The study began in the spring of 2012 127

using existing plots that had been established in 2004 (Collins et al., 2010; Kimura et al., 128

2015). The original study tested high and low fertilizer application rates and three 129

varieties of switchgrass. Original plots measured 7.6 x 7.6 m. in a completely 130

randomized design with three replicates of each treatment. The high rate fertilizer 131

application consisted of 224 kg N, 114 kg P2O5, 440 kg K2O, and 8 kg S ha-1 as a split 132

application with half applied as the grass broke dormancy and the remaining half applied 133

after the first harvest. The low rate consisted of half of the high rate, also applied as a 134

split application. During the three years between the study of Kimura et al. (2015). and 135

this study a reduced blanket balanced application of macronutrients were applied 136

annually. For this study the high rate fertilizer plots with the Kanlow cultivar were 137

randomly split, with one half of the plot receiving conventional fertilizers at the high rate 138

detailed above and the other half receiving municipal biosolids. For the first year of this 139

study we also included the low fertilizer rate but this was discontinued to add a control. 140

The original plot design did not include a control (no fertilizer) treatment. Fertilizer 141

addition to the low rate Kanlow plots was stopped for the second and third year of the 142

study and these plots were used as a control treatment. The plots are irrigated during the 143

growing season with above ground sprinkler irrigation starting in March or April and 144

ending after the 2nd cutting of the switchgrass. Plots were irrigated 2x per week in the 145

early portion of the growing season and increased to 3x per week as temperature and 146


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water demand increased. Total irrigation was approximately 670mm yr-1 (Kimura et al., 147

2015). 148

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The biosolids used for the trial were obtained from local municipalities (2012-3 from 150

Wapato, WA; 2014 from Mabton, WA) and had been dried to reach pathogen reduction 151

requirements for unrestricted use. The biosolids from Wapato contained 30.3 g kg-1 total 152

N, 16.0 g kg-1 total P, and 3.9 g kg-1 total K. The biosolids from Mabton contained 48.5 g 153

kg-1 total N, 22.3 g kg-1 total P, and 4.6 g kg-1 total K. For the first season, Wapato 154

biosolids were applied at 18 dry Mg ha-1 (545 kg total N) in a single application at the 155

start of the growing season. This rate was expected to match the N supplied by the 156

fertilizer based on the expectation of 40% mineralization for the first season (Gilmour et 157

al., 2003; Henry et al., 1999; Rigby et al., 2016). This application rate is below the WA 158

State Department of Ecology recommended rate that assumes 30% mineralization (Henry 159

et al., 1999). This rate was expected to supply a similar amount of plant available N as 160

the fertilizer treatment. A recent review noted the percentage of organic N that 161

mineralizes in biosolids ranges from 7% (compost) to 47% (aerobic digestion) (Rigby et 162

al., 2016). For the second season and third seasons, biosolids were applied in a split 163

application to mimic fertilizer application at a season loading rate of 36 dry Mg ha- , 164

deliberately in excess of fertilizer requirements (1090 and 1746 kg total N, respectively). 165

The high biosolids application rate for the last two years of the study was used to evaluate 166

a worst case impact of excess N loading on N2O emissions. These rates are well in excess 167

of what is required for plant growth and were included for research purposes only. These 168

rates would not be permitted for large-scale application. Previous studies on biosolids 169

have used excessive loading rates as a means to estimate worst case scenarios (Tian et al., 170

2006). 171

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Harvest and tissue analysis 174

Biomass was harvested in mid-July and at the end of September/ early October using a 175

John Deere F935 tractor and a 0.9 m flail harvester with biomass cut to a stubble height 176

of 15 cm (Kimura et al., 2015). An interior section was cut from each plot and weighed. 177


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A subsample (800 g) was oven dried at 50 C to adjust yield to a dry weight. Biomass 178

samples were analysed for digestibility, acid detergent fiber and lignin, sugars, and crude 179

protein using a FOSS 6500 near-infrared reflectance spectroscopy (NIRS). Ethanol yield 180

was estimated using NIRS Consortium grass hay equation (Liu et al., 2013). These 181

parameters have been related to the potential bioenergy value of different feedstocks 182

(Anderson et al., 2008; Liu et al., 2013). The TEP was calculated by estimating the 183

cellulose and hemicellulose fractions using the following equation as described in Liu et 184

al.(2013) 185

H = (% Cellulose+ (% Hemicellulose x 0.07)) x 172.82 186

P= (% Hemicellulose x 0.93) x 176.87 187

TEP (L Mg-1) = (H+P) x 4.17 188

In the calculation H and P represent hexose and pentose carbohydrates. Total ethanol 189

production per ha was estimated by multiplying dry biomass yield (Mg) by TEP. 190

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Sampling. 192

Gas samples were collected two – four times per month during the growing season for 193

three years. Gas samples were collected from three sites per plot using static chambers 194

(10 cm dia X 15 cm length) at three 10 minute intervals for a total time of 30 minutes 195

and stored by injection into pre-evacuated 12 mL Exetainer vials (Labco Ltd., UK) 196

(Parkin and Venterea, 2011). N2O was measured within one week on a gas 197

chromatograph (GC) (GC-2014, Shimadzu with an autoinjector) equipped with an 198

electron capture detector (ECD) and Hayesep-D capillary column (length: 21.3 m, inner 199

diameter: 0.32 mm), with N2 as the carrier gas. Samples were run using a 4 point 200

calibration curve with known standards run every 10 samples. We attempted to collect 201

gas samples when soils were saturated in order to capture the highest fluxes of N2O. 202

Previous work found fluxes to be correlated with wet soil measured as water filled pore 203

space (Johnson and Barbour, 2016; Oates et al., 2015; Ruan et al., 2016). The soil in this 204

study, Quincy sand, was coarse textured so that drainage after irrigation was expected to 205

be rapid. Gas samples from 2012 were collected both prior to and/or several hours after 206

irrigation to increase the potential of capturing peak emissions characteristic of saturated 207

soils (Johnson and Barbour, 2016; Oates et al., 2015; Ruan et al., 2016). For 2013-4, 208


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samples were collected prior to and immediately (<15 minutes) after irrigation for most 209

months. Samples in April and July of all years were collected immediately after fertilizer 210

application and irrigation, maximizing the potential to capture high fluxes (Oates et al., 211

2015). Soil samples were collected for each gas sampling event in 2013 and 2014 and 212

analysed for total nitrate (Bremner and Keeney, 1966). Distance to the field site 213

prevented more frequent collection of samples. 214

Partial life cycle assessment 215

Greenhouse gases 216

To estimate the full impact of fertilizer substitution, we calculated a greenhouse gas 217

balance for ethanol production with a focus on fertilization. Boundaries for this analysis 218

are shown in Table 1. The primary benefit considered was the estimated total ethanol 219

produced per hectare. Total ethanol production was calculated from biomass yield and 220

biomass TEP. Net emissions associated with producing and burning both conventional 221

gasoline and ethanol and the relative energy density of ethanol compared to gasoline 222

were considered. Due to the lower energy density of ethanol, 1.45 l of ethanol would be 223

required to match the energy equivalence of gasoline with associated emissions of 224

approximately 1 kg of CO2e (Hofstrand). 225

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Greenhouse gas costs/benefits for growing the switchgrass were limited to a 227

consideration of the use of fertilizers. These included energy requirements for 228

conventional fertilizer manufacture for N, P, and K. For municipal biosolids, this cost is 229

taken as a credit for avoidance of emissions (Brown et al., 2010; Lal, 2004; Ney and 230

Schnoor, 2002). A CO2e of 4 kg for each kg of fixed N was used for calculations (Brown 231

et al., 2010, 2014; Lal, 2004) and was based on total N applied. No fertilizer offset were 232

considered for the control treatment. For the biosolids the offset for N fertilizer avoidance 233

was based on total N applied in the biosolids in 2012 when the material was applied at 234

agronomic rates (total N of 545 kg) (Cogger et al., 2013ab; Liu et al., 2013). Offsets 235

based on total fertilizer applied has been used previously (Brown et al., 2010). This 236

approach eliminates the need to consider what fraction of total N, P, and K for any type 237

of fertilizer is actually plant available. It is also the same approach that is used for N2O 238

emissions. Values of 2 kg CO2e per kg P and 1.8 kg CO2e per kg potassium (K) 239


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accounted for the energy used to produce these nutrients (Brown et al., 2010; Shresta, 240

1998). No additional fertilizers were added to the biosolids amended plots or the control 241

plots. 242

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Nitrous oxide emission were considered to be a cost for all treatments. Prior research has 244

indicated that use of biosolids can result in significant increases in soil carbon (Brown et 245

al., 2011, Powlson et al., 2012). Due to the relatively low cumulative applications of 246

municipal biosolids and the short duration of this study, we did not measure changes in 247

soil carbon for this balance. We modelled the balance using default values of 1% total N- 248

N2O for fertilizer and biosolids treatments with a 298 x correction factor (DeKlein et al., 249

2006; Forster et al., 2007). A balance estimated from measured values is shown in the 250

supporting information. As our N2O measures were limited, this was considered as a 251

hypothetical case. 252

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The GHG balance was done using data for three years for the fertilizer treatment. The 254

balance was carried out based on the first year application rate and associated default 255

N2O emissions for biosolids, as this was the year where agronomic rates were used. N2O 256

emissions and biomass yield for the two subsequent seasons are reported. Factors 257

consistent across all treatments including irrigation, field preparation and harvesting, 258

were excluded from this analysis as they did not vary across treatments. Fertilizer offset 259

values were taken from prior work (Brown et al., 2010). Emissions associated with 260

biosolids stabilization and transport was included. Dewatering, transport and application 261

emissions ranged from 0.02-0.03 Mg CO2e per dry Mg biosolids. For the agronomic 262

application rate used in 2012, this totalled 0.54 Mg CO2e per ha-1, reflecting a round trip 263

haul distance of 216 km. As a basis of comparison, emissions for biosolids (25% solids) 264

at a greater haul distance (710 km round trip) would have totalled 0.124 Mg CO2 per dry 265

Mg biosolids or 2.0 Mg CO2e for a 16 Mg ha-1 application (Brown et al., 2010). 266

Statistical analysis 267

Data were analysed using SPSS version 19 (SPSS, 2005). Sources and rates of fertilizer 268

treatments are fixed effects while years and replications were considered as random 269

effects. The significance of main effect means and interactions was examined using an 270


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ANOVA for yield, plant tissue characteristics, and daily N2O flux measures. Mean 271

separations were conducted using the Waller Duncan procedure. Significant differences 272

were designated with p < 0.05. 273

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Results and Discussion 275

N2O fluxes. 276

Nitrous oxide emissions by moisture status and month of sampling are shown in Table 1. 277

Very low N2O emissions were observed for the first year of the study. Emissions from 278

the biosolids amended plots (0.99 ±1.5 g N2O ha-1 d-1, n=56) were statistically similar to 279

emissions from the fertilizer treatment (0.99 ±1.3 g N2O ha-1 d-1, n=56). It is important to 280

note that this year the biosolids were applied at agronomic rates (total N 545 kg ha-1) with 281

fertilizer added at 224 kg ha-1. There were no differences in emissions based on moisture 282

status of the soil (p <0.772). For this year we measured gas emissions several hours after 283

irrigation when the soil was expected to be wet. For the two subsequent years we 284

measured gas fluxes immediately after irrigation in an attempt to capture spikes 285

associated with high soil moisture as well as prior to irrigation when soils were expected 286

to have relatively low water content. Biosolids application rates were also increased in 287

an attempt to capture N2O spikes. Potentially as a result of the change in measurement 288

time and/or increased loading rates, both moisture status and soil treatment were 289

significant factors for the following two growing seasons. In 2013 emissions were higher 290

on post irrigation soil (3.64 g N2O ha-1 d-1) in comparison to pre irrigation (below 291

detection) or day of irrigation (0.61 g N2O ha-1 d-1 ) (several hours after irrigation) soil 292

(p< 0.0001). Treatment was also significant (p<0.018) with emissions lowest in the 293

control treatment (below detection, n= 102). Mean emissions in the fertilizer treatment 294

(1.8 g ± 8 g N2O ha-1 d-1, n=131) were statistically similar to both the control and the 295

biosolids amended soils (3.73 ± 10.2 g N2O ha-1 d-1, n=130). There was also a significant 296

treatment by irrigation interaction. In the pre irrigation soil, emissions were statistically 297

similar across all treatments (<0.72). In the post irrigation soil, the biosolids amended 298

soil had the highest mean N2O emissions (6.9 g N2O ha-1 d-1), with fertilizer (3.57 g N2O 299

ha-1 d-1) also elevated in comparison to the control (below detection). Total N applied in 300

the biosolids treatment this year was1090 kg ha-1. 301


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A similar pattern was observed in the final year of the study with moisture being a highly 303

significant factor (p<0.003), fertilizer treatment close to statistically significant (p<0.08) 304

and a significant irrigation x treatment interaction (p<0.04). Emissions from post 305

irrigation soil were higher than from pre irrigation soil. The control soils had the lowest 306

overall emissions (0.7 ± 0.13 g N2O ha-1 d-1, n=117) with fertilizer (1.89 ± 14 g N2O ha-1 307

d-1, n=117) statistically similar to both the control and biosolids (2.6 ± 5.2 g N2O ha-1 d-1, 308

n=117) amended soils. Again, emissions were statistically similar across all treatment in 309

the pre irrigation soil. In the post irrigation soil emissions from the fertilizer treatment 310

(3.71 g N2O ha-1 d-1) were statistically similar to the biosolids amended soil (4.51 g N2O 311

ha-1 d-1) and both treatments were higher than the control soil (0.08 g N2O ha-1 d-1). Total 312

N added in the biosolids treatment this year was 1746 kg ha-1. 313

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Nitrous oxide can be formed during both nitrification and denitrification reactions. 315

However, it is considered to occur most commonly during denitrification in fine textured, 316

wet soil. Sufficient soil nitrate is a precursor for denitrification. Soil nitrate measures 317

from 2013 show similar and low NO3- concentrations for the fertilizer and biosolids 318

amended soils with the highest measured NO3- (3.68 ± 1 mg kg-1) in the fertilizer 319

treatment for the June sampling and a similar value for the biosolids treatment (3.68 ± 320

2.15 mg kg-1) in the April sampling( Figure 1). This was mirrored by low N2O 321

emissions. With the higher biosolids loading rate in 2014 there was also a general 322

increase in soil NO3-. Nitrate values for both the biosolids and the fertilizer treatments 323

were highest in April, (43.6 ± 20 and 10.5± 7.5 mg kg-1, respectively). Nitrous oxide 324

emissions for post irrigation during this month were statistically similar for fertilizer and 325

biosolids (10.75 ± 17.5 and 5.4±3.5 g ha-1 day-1, respectively). Soil nitrate values for the 326

both treatments decreased for all subsequent measures, with N2O emissions for the 327

biosolids also decreasing. The low NO3- concentrations, particularly for the 2013 328

growing season suggest a low potential for denitrification and associated N2O emissions 329

as was observed in our limited sampling. 330

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The relatively low observed N2O emissions may be related to the surface application of 332

the amendment, coarse soil texture, and restricted moisture, all of which reduce the 333

potential for saturated conditions that are typically associated with N2O release (limited 334

nutrient diffusion and less stimulation of nitrifying microbial communities) (Gaillard et 335

al., 2016; Johnson and Barbour, 2016; Rochette et al., 2008; Ruan et al., 2016). A study 336

showed that finer soil texture (silt loam soils) had 80-158% greater N2O emissions than 337

coarser soil textures (loamy sand and sandy loam) such as that at this study site (Rochette 338

et al., 2008). The authors also observed that 74-98% of N2O emissions were related to 339

denitrification. The relatively low observed emissions in this study are likely related to 340

rapid drainage as a result of coarse soil texture. Surface application of amendments also 341

likely improved aeration. However, it is also possible that with limited sampling we did 342

not capture spikes in emissions that would have been closer to default values. 343

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Biomass Yield. 345

Treatment (p<0.001), year (p< 0.0001) and year * harvest* treatment interaction (p< 346

0.001) were significant. Across all harvests, per harvest yield from the biosolids 347

treatment (8.78 t ha-1) was similar to yield from the fertilizer (8.61 t ha-1) and higher than 348

the control (4.3 t ha-1). With the exception of the first harvest in 2012 where switchgrass 349

treated with biosolids had lower biomass yields than fertilizer-amended plots, yields in 350

the biosolids-amended plots were statistically similar to the plots that received synthetic 351

fertilizer (Table 2). Despite high rates of application for the last two years of the study, 352

yields in biosolids amended soils remained similar to controls, indicating that maximum 353

yield potential had been reached. There was no control treatment for the first year of the 354

study. For the final two years, both the biosolids-amended and fertilized plots yielded 355

more biomass than the control plots for the last three of four harvests of the study. 356

Biomass yield from the control for the first harvest of 2013 (9.93 Mg ha-1) was similar to 357

other treatments, likely as a result of fertilizer carry over from the previous season, but 358

decreased significantly by the second harvest (2.8 Mg ha-1). Yield summed across both 359

harvests ranged from 10.7 (year 1) to 22.8 (year 2) Mg ha-1 in the fertilizer treatment with 360

yield in year three of 16.1 Mg ha-1. Corresponding yields in the biosolid treatments were 361

9.5 (year 1), 20.1 (year 2) and 20.4 (year 3) Mg ha-1. Yield in the control treatment 362


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measured 12.7 Mg ha-1 for the second year of the study then declined to 4.5 Mg ha-1 for 363

the final year. This biomass reduction was likely due to the depletion of residual fertility 364

in this treatment from the first year of the study when low rates of fertilizers were applied 365

to plots. First year biomass results from this study are within the range reported for 366

switchgrass (Wang et al., 2012) but below what had previously been observed on these 367

plots (Kimura et al., 2015). The yields for the second and third years for both the 368

fertilizer and biosolids treatments were similar to those previously reported (Kimura et 369

al., 2015). We saw a significant biomass yield response with added fertility from the 370

fertilizer and biosolids treatments in comparison to the control. Previous work has shown 371

a mixed response to fertilizer addition with some studies showing significant yield 372

increases and other studies showing a less pronounced impact of fertilizer addition on 373

switchgrass yields (McGowan et al., 2018; Ruan et al., 2016; Schmer et al., 2012). This 374

may be related to differences in switchgrass varietals. Previous work on this site showed 375

a more pronounced response to fertilizer addition for Kanlow, a lowland variety in 376

comparison to the two upland varietals used in the previous trial (Kimura et al., 2015). 377

378

Fuel quality and quantity. 379

Forage analysis was carried out for both harvests for the 2013 and 2014 growing seasons 380

(Table 2). Switchgrass from the control treatment had lower lignin than grass from the 381

fertilizer or biosolids treatments. No statistical differences were found for acid detergent 382

fiber (ADF), digestibility of neutral detergent fibers at 48h (NDFD), or TEP between the 383

control, biosolids-amended and fertilized switchgrass for both harvests and both years. 384

Our results support previous work that biosolids application had no significant impact on 385

the chemical composition of switchgrass compared to unfertilized grass (Liu et al., 2013). 386

Total ethanol potential ranged from 323 ± 14 L Mg-1 to 388 ± 11 L Mg-1 for the fertilizer 387

treatment across all harvests. TEP from the biosolids treatment varied from 359 ± 28 388

LMg-1 to 398 ± 35 L Mg-1. Across both the fertilizer and biosolids treatments, harvest 389

and year impacted TEP, with the lowest yield observed for the first harvest in 2013 390

(Table 2). Both similar and higher TEP from switchgrass have been reported (Liu et al., 391

2013; Morrow et al., 2006). Total ethanol potential may vary based on the harvest 392

schedule with higher lignin concentrations in switchgrass harvested once per season. 393


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Overall TEP yields were lower for the first harvest in 2013 compared to the second 394

harvest. Yields were similar for both harvests in 2014. 395

396

Total Fuel production 397

Total fuel production was estimated by multiplying harvested biomass (Mg ha-1) times 398

TEP (Table 3). There were no significant differences in TEP for the biosolids amended 399

versus the fertilized switchgrass for the 4 harvests where samples were analysed. Mean 400

TEP from 2013-2014 was used to estimate the total fuel production for 2012. Data from 401

the 2014 harvest was used for the control as there was no control treatment in 2012. The 402

fertilized grass yielded about 10% more ethanol per hectare than the biosolids for 2012-403

2013 and about 25% less for the 2014 growing season. Across all harvests the total 404

ethanol ha-1 in the fertilizer treatment (3480 L ha-1) was statistically similar to the 405

biosolids treatment (3794 L ha-1). Both of these treatments showed higher ethanol 406

production than the control (1522 L ha-1). These results confirm that biosolids is an 407

effective substitute for fertilizer for switchgrass intended for ethanol production (Lui et 408

al., 2013). 409

410

Balance calculations 411

The total GHG savings associated with switchgrass ethanol in comparison to gasoline 412

were calculated by comparing the estimated CO2e emissions associated with production 413

and combustion of conventional gasoline and switchgrass ethanol and the energy 414

densities of each fuel (Hofstrand; Wang et al., 2012) (Figure 2). The CO2e per MJ of 415

switchgrass ethanol was significantly lower than of gasoline, however the energy density 416

of the ethanol is also lower, meaning that more ethanol would be required to generate the 417

same energy as a liter of gasoline. A factor of 3.2:1 was used for kg of CO2eL-1 for 418

carbon emissions of gasoline in comparison to switchgrass based ethanol (Hofstrand; 419

Wang et al., 2012). 420

421

The emissions/ credits associated with using fertilizers (N, P, and K) and municipal 422

biosolids were calculated to quantify the benefits/ offsets associated with using biosolids 423

as an alternative fertilizer source for switchgrass (Table 3). The biosolids used in 2012 424


15

contained 16.0 g kg-1 total P, and 3.9 g kg-1 total K (Table 3). For the biosolids the offset 425

for N fertilizer avoidance from the agronomic loading rate (total N of 545 kg) amounted 426

to a credit of 2.18 Mg CO2e (Cogger et al., 2013ab; Liu et al., 2013). The CO2e for the 427

total P in the biosolids totalled 0.26 Mg in 2012. The credit for K was much smaller (31- 428

kg CO2e), as the biosolids contained low concentrations of K. Here, K provided via 429

biosolids was much less than was supplied by synthetic fertilizer. These results suggest 430

full-scale switchgrass production with biosolids amendments would require additional K 431

fertilization (Kimura et al., 2015). The fertilizer offsets from biosolids-amended (2012) 432

represented significant CO2e savings. 433

434

While switchgrass is often touted as an ethanol feedstock with lower fertilizer 435

requirements than corn, adding sufficient fertility to maximize yield can be beneficial. 436

Previous work on these plots showed that a high percentage of the added fertilizer was 437

taken up by the switchgrass (Kimura et al., 2015). These results also show a strong 438

response to fertilizer addition. 439

440

N2O emissions 441

The default emissions based on an N application rate of 224 kg ha-1 (using an equivalence 442

of N2O as N2O-N *44/28) were 3.52 kg N2O ha-1 yr-1(Farrell et al., 2006). 443

Based on 1% of total N, default emissions for the biosolids were 8.6 kg N2O ha-1 yr-1 444

(Farrell et al., 2006). For subsequent years when above agronomic rates of biosolids were 445

applied, the default emissions based on 1% of total N applied would have totalled 17 and 446

27 kg N2O ha-1 yr-1 for 2013 and 2014, respectively (DeKlein et al., 2006). The CO2e for 447

fertilizer N2O was 1.05 and 2.55 Mg CO2e for the fertilizer and biosolids, respectively. 448

For the higher rates of biosolids addition, the default N2O emissions totalled 5.1 and 8.2 449

Mg CO2 for 2013 and 2014. The higher N2O defaults were considered in the net balance 450

to be conservative. The measured N2O emissions in this study were much lower than 451

those that would have been expected based on default factors. An estimate of total 452

emissions based on field measures is shown in the supporting information. For the first 453

year of the study when agronomic rates of biosolids were applied, our estimate of total 454

emissions were 0.13 and 0.14 kg N2O ha-1 yr-1 for the biosolids and fertilizer, 455


16

respectively. Recent studies have found lower than default emissions of N2O from soils 456

where organic amendments have been used in lieu of synthetic fertilizers (Charles et al., 457

2017; Rochette et al., 2018). The dry amendment in combination with coarse soil texture, 458

low natural precipitation and perennial crop in this study are all factors that have been 459

associated with reduced N2O emissions. For example, across all types of organic 460

amendments Charles et al (2017) found an emissions factor of 0.57 ±0.30% of total N. 461

Within this, they observed that emissions on coarser textured soils were 2.8x less than on 462

finer textured soils. 463

464

Net Balance. 465

Data, as detailed above was compiled to evaluate the impact of alternative fertilizers on 466

carbon for ethanol production (Figure 2). This was done using data from years 1 for 467

biosolids, 1-3 and fertilizer years and years 2 and 3 for the control. The balance was split 468

into costs and benefits (SI). The benefits are defined as net ethanol production based on 469

biomass yield and TEP. Costs were considered in reference to fertilizer use and transport 470

(biosolids only). These included emissions associated with fertilizer manufacture and 471

N2O emissions. The control treatment received credits for TEP only. The alternative 472

source of fertility for the study; municipal biosolids received credits for avoidance of 473

fertilizer use. All treatments received debits for N2O emissions based on default values. 474

Total amount of N added to the soil was used for the calculation. No debits for N2O 475

emissions were taken for the control treatment. The same exercise, carried out with N2O 476

emissions extrapolated from our measured values is shown in the Supporting 477

Information. The analysis was done on a per hectare basis to determine the impacts of 478

the alternative sources of fertility on relative biomass yield. 479

480

The CO2e associated with ethanol production from conventionally fertilized switchgrass 481

was -8.6 Mg ha-1 in 2012, -18.3 Mg ha-1 in 2013, and -12.9 Mg ha-1 in 2014. Energy 482

costs for N fertilization were 1.65 Mg CO2e annually (Figure 2). Estimates of annual 483

N2O emissions based on our measures ranged from 0.043 to 0.126 Mg CO2e ha-1 (SI). 484

Using default factors for N2O emissions the cost of using fertilizer was 1.05 Mg CO2e ha-485

1 yr-1. This represented 15% (2013) to 31% (2012) of the total cost of production. 486


17

Growing switchgrass without fertilizer significantly reduced yield resulting in a credit of 487

-10.7 (2013) and -4.1 (2014) Mg CO2e ha-1. It is likely that further cultivation without 488

additional fertility would further reduce yields, suggesting that for this soil, growing 489

switchgrass biomass for bioenergy without fertilizer is not a viable long-term option 490

(Kimura et al., 2015). Substituting municipal biosolids for the fertilizer resulted in a 491

slight but not statistically significant reduction in fuel yield and associated CO2e credits 492

for both 2012 and 2013 seasons (-7.8 and -16.5 Mg CO2e ha-1, respectively) and an 493

increase in 2014 (-16.8 Mg CO2e ha-1). Credits associated with fertilizer avoidance 494

(considering the agronomic loading rates used in 2012) more than compensated for this, 495

resulting in an additional credit of -2.49 Mg CO2e ha-1 each year. Using default factors for 496

N2O and considering transport related emissions, the overall credit was still greater than 497

seen with fertilizer use (-7.7, -13.8, and -11.1 Mg CO2e ha-1 for 2012, 2013, and 2014, 498

respectively). In 2012, fertilizer costs for biosolids were about 7% of the fuel benefits. In 499

subsequent years, the higher yields helped to compensate for higher default N2O 500

emissions. If the observed N2O emissions were indicative of actual emissions across the 501

full growing season, the benefits associated with substitution of municipal biosolids for 502

fertilizer would have been even more pronounced. Studies have estimated that synthetic 503

fertilizer production constitutes 37-67% of the energy associated with biofuel production 504

(Heller et al., 2003; Wang et al., 2012). Using default emission factors for N2O, 505

fertilizer costs were equivalent to 30% of the fuel benefits for the fertilizer treatment for 506

the first year of this study. This decreased to 14% in 2013 with higher grass yields and 507

was 21% in 2014. In comparison, fertilizer costs for biosolids were close to zero in 2012. 508

509

Conclusion 510

The results from this study suggest that growing switchgrass without supplemental 511

fertilizer is not viable for marginal low fertility soils, like the Quincy sand used in this 512

study. Municipal biosolids can provide additional benefits for switchgrass based ethanol 513

when they are used at agronomic rates and they do not require long-distance transport. 514

These benefits are contingent on using agronomic rates of amendments, availability of 515

amendments in proximity to farms to limit transport related emissions, and N2O 516

emissions at or below default levels. Benefits associated with their use will be greater if 517


18

crops are grown on coarse textured soils where N2O emissions are likely to be lower than 518

default values (Rochette et al., 2008). These findings are also likely applicable to cases 519

where other organic residuals such as animal manures are used. 520

521

522

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703


25

List of Figures 704

705

Figure 1. Soil NO3- measures from the 0-15 cm depth from samples collected 706

immediately after N2O sampling during the 2013 and 2014 growing seasons. Means ± 707

standard error are shown. 708

709

Figure 2. Net carbon balance for ethanol production from switchgrass including total 710

ethanol (TEP * yield), CO2e for fossil fuel replacement (CO2e credit l ethanol-1 * number 711

of liters ha-1), and fertilizer offsets for all harvests. Fertilizer debit includes energy 712

associated with N, P, and K production where used, as well as for default N2O emissions. 713

Credit includes the CO2e for fertilizer avoidance (2012 agronomic rates for biosolids). 714

Transport costs were also considered for biosolids. 715

716

717

718


26

Table 1. Factors considered and excluded from the carbon accounting. Those shown in green represent carbon credits while those 719

shown in red represent sources of carbon emissions 720

721

722

Fertilizer Biosolids

Included

Fuel offsets (yield x TEP)

Fertilizer manufacture

Fertilizer avoidance offsets

N2O emissions

Transport

Excluded

Soil carbon sequestration

Fugitive gas avoidance from landfill

diversion

Landfill energy recovery

Fugitive gas avoidance from

combustion diversion

Energy recovery from combustion

723


27

Table 1. Nitrous oxide emissions (g ha-1 day-1) for each sampling time. Post samples were collected immediately following sprinkler 724

irrigation in 2013-4 and within hours of irrigation in 2012. Pre samples were collected immediately prior to irrigation. Means SD 725

are shown. Means shown in bold within the same month are significantly different (p <0.05), while means shown in bold and italic 726

indicate a significant (p <0.05) interaction (treatment * moisture). Shading signifies two sampling events within a month. 727

728

March April May June July August September

Post Pre Post Pre Post Pre Post Pre Post Pre Post Pre Post Pre

Paterson

2012 Control 0.23 ±

0.35 0.2 ±

0.1

0.3 ±

0.2

Biosolids 1.5 ±

1.5

1.36

± 1.7

0.26 ±

0.34

0.39 ±

0.2

0.16±

0.08 1.9 ±

2.1

0.1 ±

0.1

Fertilizer 1.16

± 0.8

1.5 ±

1.6

0.19 ±

0.07

0.21 ±

0.07

0.32 ±

0.39 2.1 ±

1.8

0.3 ±

0.6

2013 Control 0.1 ±

0.3 0 ± 1 0 ± 1

0.15 ±

0.2

0 ±

0.14

1 ±

0.5 0 ± 0

0.1 ±

0.2

Biosolids 11.8 ±

25.3 6.8 ±

8.6

1.2 ±

1.2

0.55 ±

0.5

0.5 ±

0.5

16.2 ±

19 1 ± 1.5

1.7 ±

1.5

0.5 ±

0.6

3.2 ±

6 0 ± 3

Fertilizer 22.1 ±

14.5 0.2 ±

0.2

1.7 ±

1

0.2 ±

0.1

0.03 ±

0.2 7 ± 16 2.5 ± 5

1.1 ±

1.5

0 ±

0.3

0.3 ±

0.7

0 ±

6.6

2014 Control 0.1 ±

0.1

0 ±

0.1

0.1 ±

0.1 0.1 ±

0.05

0 ±

0.1

0.1 ±

0.2

0.1 ±

0.1

0 ±

0.1 0 ± 0.1

0.2 ±

0.1

0.2 ±

0.2

Biosolids 0.7 ±

0.8

0.8 ±

0.9

5.4 ±

3.5 3 ± 5

2.9 ±

0.8

4.1 ±

8.5

1.1 ±

1.1

3.4 ±

4.9

1.3 ±

1.5

6.6 ±

13

1.5 ±

2.1


28

Fertilizer 0 ± 0.2 0.5 ±

0.6

10.7 ±

17.5

1.6 ±

0.3

1.3 ±

0.9

4.3 ±

11 0 ± 27

12.9 ±

29

6.7 ±

12

1.5 ±

2.7

0.7 ±

1

729


29

Table 2. Kanlow switchgrass biomass yield (dry Mg ha-1) and composition for each harvest. The first harvests were in July and the 730

second harvests in late September to mid-October each year. Means 1 SD are shown. Composition was determined using NIRS 731

analysis and is shown as a function of nitrogen source shown by harvest and year (2013-4). There were no significant differences as a 732

result of fertilizer types. Means followed by different letters within the same harvest are significantly different (p < 0.05). 733

734

735

2012 2013 2014

1 2 1 2 1 2

Yield dry Mg ha-1

Control 9.9 2.1 2.8 0.3 a

2.7 0.6

a

1.8 0.3

a

Fertilizer 5.0 0.2 5.7 1.3 13.4

1.7 9.4 1.2 b

7.7 1.8

b

8.4 1.8

b

Biosolids 3.7 0.5 5.8 1.6 11 1.1 9.1 0.7 b 10.1 1.3

b

10.3 1.6

b

Lignin

Acid

detergent

fiber Digestibility NDF >48 hr Total Ethanol Production

Nitrogen Source

liters Mg-1

Control 2.78 ± 0.8

a 40 ± 1.5 54.2 ± 3.7 373 ± 33

Fertilizer 3.4 ± 0.7

b 41 ± 1.8 53.8 ± 3.2 365 ± 28

Biosolids 3.4 ± 0.6

b 41 ± 2 55 ± 4.5 374 ± 34

Harvest and Year

2013

Harvest 1 4.1 b 41.6 b 49.7 a 330 a

Harvest 2 3.6 a 40 a 56.5 b 384 b


30

2014

Harvest 1 2.8 39.2 a 53.5 368

Harvest 2 3.2 42.9 b 56.8 389 736

737


31

Table 3. Kanlow switchgrass biomass yield (sum of two harvests) per year, estimated total ethanol production (yield * TEP) per ha, 738

and the CO2e per ha for the different treatments. As TEP was not determined for the 2012 harvest year due to sample loss, values were 739

calculated based on the mean TEP of each treatment for the 2013-2014 growing seasons. Total applied N, P, and K and CO2e for N, P 740

and K fertilizer offset or debit 741

742

743 Yield Total Ethanol CO2e

2012 2013 2014 2012 2013 2014 2012 2013 2014

Mg ha-1 Liters Ethanol

ha-1

Mg ha-

1

Control 12.7 4.5 4534 1755 10.7 4.1

Fertilizer 10.7 22.8 16.1 3906 8322 5877 8.6 18.3 12.9

Biosolids 9.5 20.1 20.4 3553 7517 7630 7.8 16.5 16.8

Total

Nitrogen

applied

CO2e-

N

Total

Phosphorus

applied

CO2e- P

Total

Potassium

applied

CO2e-

K

Total

Fertilizer

Cost/ offset

kg

Control

Fertilizer 224 896 50 100 365 657 1653

Biosolids 545 -2180 128 -256 31.2 -56.16 -2492

744


32

Figure 1. Soil NO3- measures from the 0-15 cm depth from samples collected 745

immediately after N2O sampling during the 2013 and 2014 growing seasons. Means ± 746

Standard error are shown. 747

748

749

750


33

Figure 2. Net carbon balance for ethanol production from switchgrass including total 751

ethanol (TEP * yield), CO2e for fossil fuel replacement (CO2e credit l ethanol-1 * number 752

of liters ha-1), and fertilizer offsets for all harvests. Fertilizer debit includes energy 753

associated with N, P, and K production where used, as well as for default N2O emissions. 754

Credit includes the CO2e for fertilizer avoidance (2012 agronomic rates for biosolids). 755

Transport costs were also considered for biosolids. 756

757

758


34

759

760 761

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