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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Fertilizer alternatives for switchgrass
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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
46
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
Fertilizer alternatives for switchgrass
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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
63
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
77
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
Fertilizer alternatives for switchgrass
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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
105
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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Fertilizer alternatives for switchgrass
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Materials and Methods 116
117
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
Fertilizer alternatives for switchgrass
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water demand increased. Total irrigation was approximately 670mm yr-1 (Kimura et al., 147
2015). 148
149
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
Fertilizer alternatives for switchgrass
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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
Fertilizer alternatives for switchgrass
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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
226
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
Fertilizer alternatives for switchgrass
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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
243
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
253
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
Fertilizer alternatives for switchgrass
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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
Fertilizer alternatives for switchgrass
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302
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
314
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
331
Fertilizer alternatives for switchgrass
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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
344
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
Fertilizer alternatives for switchgrass
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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
Fertilizer alternatives for switchgrass
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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
Fertilizer alternatives for switchgrass
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
Fertilizer alternatives for switchgrass
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
Fertilizer alternatives for switchgrass
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
Fertilizer alternatives for switchgrass
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
Fertilizer alternatives for switchgrass
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
Fertilizer alternatives for switchgrass
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
Fertilizer alternatives for switchgrass
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
Fertilizer alternatives for switchgrass
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
Fertilizer alternatives for switchgrass
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
Fertilizer alternatives for switchgrass
30
2014
Harvest 1 2.8 39.2 a 53.5 368
Harvest 2 3.2 42.9 b 56.8 389 736
737
Fertilizer alternatives for switchgrass
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
Fertilizer alternatives for switchgrass
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
Fertilizer alternatives for switchgrass
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
Fertilizer alternatives for switchgrass
34
759
760 761