Assessment of soil carbon storage and greenhouse gas emissions in perennial bioenergy crops on low productive agricultural land in southwestern Ontario, Canada

Abstract

Low productive agricultural lands with less suitability for annual row crop production have been recommended globally for growing perennial bioenergy crops. This is due to the low input requirement of perennial bioenergy crops and their ability to produce large biomass on low productive agricultural lands as well as rejuvenating these lands for possible future annual row crop production. In addition to eliminating competition with food crops for land occupation, growing perennial bioenergy cops on low productive agricultural lands provides climate change mitigation benefits through soil carbon (C) sequestration and reduced greenhouse gas (GHG) emissions. In this dissertation, four studies were conducted to evaluate soil C pools and GHG (N2O and CO2) emissions in unfertilized perennial bioenergy crops of miscanthus (Miscanthus giganteus L.), switchgrass (Panicum virgatum L.), and willow (Salix miyabeana L.) on low productive agricultural land in southern Canada to understand the climate benefit of growing perennial bioenergy crops on low productive agricultural land relative to a natural regrowth vegetation (successional) site. The first study in Chapter 2 quantified the nitrous oxide (N2O) and carbon dioxide (CO2) emissions in miscanthus, switchgrass, willow, and a successional site over two growing seasons to understand the GHG mitigation benefits of the bioenergy crops compared to the successional site. The static chamber method which consisted of installing two polyvinyl chloride (PVC) chambers (25 cm height, 10 cm radius) to a 10 cm depth in each bioenergy crop replicate and three chambers in the successional site was used to quantify N2O and CO2 emissions in the perennial bioenergy crops and successional site. Gas from each chamber headspace was sampled at time t = 0, 15 and 30 minutes once every two weeks for the 2020 and 2021 growing seasons (May to October). The sampled gas was brought to the laboratory and analyzed on a gas chromatograph for N2O and CO2 in each bioenergy crop and successional site. A repeated measure and univariate ANOVA was used to quantify differences between N2O and CO2 fluxes and cumulative emissions from the bioenergy crops and successional site over the two growing seasons. Mean N2O and CO2 fluxes during the two growing seasons ranged from –0.022 to 0.087 µg N2O-N m–2 hr–1 and 0.010 to 0.266 mg CO2-C m–2 hr–1, respectively. Whereas mean N2O fluxes did not differ (p>0.05) among land use type and between years, mean CO2 fluxes significantly differed (p<0.05) among land use types and between years. The 2-year total cumulative N2O emissions ranged from 93.75 µg N2O-N m–2 to 174.50 µg N2O-N m–2 and was significantly higher (p<0.05) in switchgrass than the successional site. Of the 2-year total cumulative N2O emissions, 51% in the successional site and switchgrass occurred in 2020, while 63% and 69% in willow and miscanthus, respectively, occurred in the same year. The 2-year cumulative CO2 emissions on the other hand ranged from 760.68 mg CO2-C m–2 to 1009.04 mg CO2-C m–2 and was not significantly different (p>0.05) among land use type. Although soil parameters such as temperature and available nitrogen (N), which can provide favorable conditions for CO2 emissions from soil microbial respiration, were significantly higher (p<0.05) in 2020 compared to 2021, the absence of any statistically significant (p>0.05) differences in CO2 emissions between both years suggests that the majority of the emitted CO2 may have been contributed by root respiration rather than soil microbial respiration. This is because root respiration can constitute the majority of CO2 emissions from the soil. Moreover, the overall combined average emissions which ranged from 4.96 Mg CO2-eq ha–1 6-month–1 to 8.18 Mg CO2-eq ha–1 6-month–1 were significantly higher (p<0.05) in switchgrass compared to the successional site. The implication of this is that, unlike miscanthus and willow, converting low productive agricultural land to switchgrass may result in an overall higher emission. This means that the intended benefits of reduced GHG may actually worsen emissions – if what occurs at this experimental scale is true at a global scale with switchgrass cultivation. The objective of the second study (Chapter 3) was to evaluate the impact of repeated soil freeze-thaw on soil C and N cycling and resultant N2O and CO2 emissions in low productive agricultural land under different perennial bioenergy crops and a successional site to understand their GHG mitigation benefits under a changing climate. In this study, a 49-day laboratory incubation experiment was conducted to compare the impact of freeze-thaw cycles on N2O and CO2 emissions in miscanthus, switchgrass, and willow to a successional site and to understand the processes controlling the N2O and CO2 emissions in these crops. The results showed that freeze-thaw cycles caused a decline in dissolved organic C (DOC) and dissolved inorganic N (DIN) concentrations but enhanced the dissolved organic N (DON) and nitrate (NO3–). Although freeze-thaw decreased water stable soil aggregates in all the bioenergy crops and successional site, this did not have any statistically significant (p>0.05) impact on N2O and CO2 emissions, suggesting that the N2O and CO2 emitted during the freeze-thaw cycles may have originated mostly from cellular materials released from lysis and death of microbial biomass rather than from soil aggregate disruption. Cumulative N2O emissions measured over the 49-day incubation period ranged from 148 mg N2O-N m–2 to 17 mg N2O-N m–2 and were highest (p<0.05) in miscanthus followed by willow, switchgrass, and successional site. Cumulative CO2 on the other hand was highest (p<0.05) in the successional site than any of the bioenergy crops and ranged from 25,262 mg CO2-C m–2 to 15,403 mg CO2-C m–2 after the 49 days. Higher N2O emissions in the miscanthus and willow than switchgrass and successional site were attributed to accelerated N losses as N2O due to possible changes in soil microbial biomass and structure as influenced by the different crop species. Results from this study point to a potential influence of soil microbial biomass and microbial diversity in regulating C and N cycling in the different bioenergy crop species resulting in the varied responses of these crops to N2O and CO2 emissions during soil freeze-thaw. Hence, reducing soil freezing intensities in perennial bioenergy crops on low-productive agricultural lands may be important for decreasing freeze-thaw-related N2O and CO2 emissions in miscanthus, switchgrass, and willow. This reduction can lead to climate change mitigation benefits under a changing climate scenario, characterized by more frequent soil freeze-thaw events in cold temperate regions where these crops are cultivated. To understand how miscanthus, switchgrass, and willow influence long-term soil organic carbon (SOC) storage and stabilization compared to a successional site, the third study (Chapter 4) evaluated and compared C storage in whole soil and different soil-size fractions in the perennial bioenergy crops to a successional site. The proportion of C contributions from the different perennial bioenergy crops to different soil-sized fractions were also determined. Soil sampled to 30 cm depth from the different bioenergy crops were physically fractionated into macro-sized (250-2000 µm), micro-sized (53-250 µm), and silt + clay (<53 µm) fractions using the dry-sieving method. SOC concentrations and stocks were determined in whole and fractionated soil using an elemental analyzer while the proportion of C contributions from the different perennial bioenergy crops to different soil-sized fractions were determined using the 13C natural abundance technique. Due to the absence of fertilization and potential decreased organic residue input to the soil from lower biomass yield, after 12 years of cultivation, the miscanthus and willow only posted marginal gains in SOC concentrations that were 2.5% higher compared to the baseline SOC concentrations in 2009, with the switchgrass recording 3.7% lower SOC concentrations. Soil organic C stocks ranged from 5686 to 7002 g C m–2 and were significantly higher (p<0.05) in the successional site than switchgrass and willow but not in miscanthus. Of the three bioenergy crops, only the miscanthus maintained SOC stocks comparable to the successional site owing to the ability of unfertilized miscanthus to produce higher biomass on low productive agricultural lands compared to switchgrass and willow. The distribution of soil-sized fractions and SOC in the soil-sized fractions under the perennial bioenergy crops were consistent with the aggregate hierarchy model proposed by Tisdall and Oades (1982) and followed an increasing order of silt + clay < micro- < and macro-sized fractions. Significantly higher (p<0.05) SOC in miscanthus in micro-sized and silt + clay fractions were contained in 20–30 cm depth compared to surface soil layers. These higher micro-sized and silt + clay-associated SOC in miscanthus in deeper soil depths compared to the switchgrass and willow indicates the vital contribution miscanthus could play in long-term C stabilization in deeper soil layers. The ẟ13C signatures of the different soil-sized fractions under the perennial bioenergy crops to 30 cm revealed a higher proportion of C contribution by the crops were contained in the micro-sized fractions compared to macro-sized fractions, indicating their significant contribution to more stabilized C pools with long-term positive impact on climate change mitigation. The objective of the fourth study (Chapter 5) was to use the Century model to predict and compare long-term SOC dynamics in miscanthus, switchgrass, and willow to a successional site and a row crop system to understand the soil C storage potential of the different bioenergy crops over the long-term. The Century model was calibrated using the site-specific soil biophysical characteristics, climate, and land management practices of the study site to predict SOC stocks for 162 years. Average SOC stocks over the 162-year simulation period were highest in miscanthus (8521 g C m–2), followed by the successional site (6877 g C m–2), switchgrass (6480 g C m–2), willow (5448 g C m–2) and lowest in the row crop system (3995 g C m–2). Higher SOC stocks in the miscanthus than the successional site indicates that, despite frequent biomass harvest, perennial bioenergy crops can accumulate higher C in soil over the long-term than when a low productive agricultural land is left to undergo secondary regrowth. This may, however, depend on the crop species, since the miscanthus was the only bioenergy crop that reached pre-cultivation (1911) SOC levels of 8288 g C m–2. Moreover, the perennial bioenergy crops enhanced SOC in the slow fraction, whereas row crops depleted organic C in this fraction. This indicates the vital contribution of perennial bioenergy crops in long-term SOC sequestration and their role in climate change mitigation, especially when grown on low productive agricultural lands.