Agroforestry and Biochar for Climate Change Mitigation: Carbon Storage, Soil Carbon Cycling, and Greenhouse Gas Emissions
Creators: Gross, Cole D.
Publisher: name: University of Alberta Library
Year: 2022

<p>Agroforestry systems (AFS) and the application of organic amendments in croplands can contribute to carbon (C) sequestration and reduction in greenhouse gas (GHG) emissions from agricultural lands. However, previously understudied differences among AFS and organic amendments may underestimate their climate change mitigation potential. Additionally, land use and management practices that increase C inputs to soil may have dual effects on soil C dynamics, in some cases resulting in intensified loss (a.k.a. priming) of soil organic C (SOC) and initially reducing SOC storage. In a 3-year field study, I assessed various C stocks and GHG emissions across two common AFS (hedgerows and shelterbelts) and their component land uses: perennial vegetated areas with and without trees (woodland and grassland, respectively), newly planted saplings in grassland, and adjacent annual cropland in central Alberta, Canada. In the cropland, I also compared one-time additions of manure compost and its biochar derivative to a control to assess their effects on SOC and GHG emissions. Manure compost and biochar were applied at equivalent C rates (7 Mg C ha−1) and tilled into the surface 10 cm of soil. I further conducted a 150-d incubation in a controlled growth chamber to quantify SOC changes due to living roots and their C inputs in soils collected from the component land uses of the AFS, as well as those amended with manure compost and its biochar derivative, using the C stable isotope natural abundance technique. In the field study, nitrous oxide emissions were 89% lower under perennial vegetation relative to the cropland (0.02 and 0.18 g N m−2 y−1, respectively) between 2018 and 2020. Heterotrophic respiration in the woodland was 53% lower in shelterbelts relative to hedgerows (279 and 600 g C m−2 y−1, respectively) in 2020. Within the woodland, the deadwood C stock was more important in hedgerows (35 Mg C ha−1 or 7% of ecosystem C) than shelterbelts (2 Mg C ha−1 or &lt; 1% of ecosystem C), and likely affected C cycling in the woodland by enhancing soil labile C and microbial biomass in hedgerows. Total ecosystem C was 1.90–2.55 times greater within the woodland than in all other land uses. Shelterbelt and hedgerow woodlands contained 2.09 and 3.03 times more C, respectively, than adjacent cropland. In the cropland, biochar led to the sequestration of SOC at a rate of 2.5 Mg C ha−1 y−1 relative to the control. In 2018 and 2019, manure addition increased total GHG (sum of carbon dioxide, methane, and nitrous oxide as CO2-equivalents) emissions by 33%, on average, relative to both the control and biochar addition. In contrast, in 2020, biochar addition reduced total GHG emissions by 21% relative to both the control and manure addition. Results from the incubation experiment showed that priming of soil-derived C due to the influence of living roots was limited to the aggregated clay fraction (that is, clay within water-stable silt-size microaggregates), greatest in the surface soil (0.51–1.27 mg C g−1 soil), and similar across land uses. However, biochar minimized priming within the aggregated clay fraction. In soils with greater SOC content (that is, surface and woodland soils relative to subsurface and cropland soils, respectively), a larger proportion of clay-protected C was found in silt-size microaggregates. My findings are threefold: (1) AFS are important for fostering C sequestration and reducing GHG emissions and, in particular, retaining hedgerows (legacy woodland) and their associated deadwood across temperate agroecosystems is key to help mitigate climate change; (2) the application of biochar, rather than its manure compost feedstock, increased surface SOC sequestration and had either no effect on or reduced GHG emissions relative to the control; and (3) living roots can destabilize clay-protected C within silt-size microaggregates, leading to rapid and preferential decomposition of clay-protected C; however, biochar can stabilize clay-protected C within silt-size microaggregates under the influence of living roots. Moreover, root-driven stabilization or destabilization of clay-protected C within silt-size microaggregates may mediate SOC sequestration and SOC storage capacity, which has important implications for our understanding of SOC persistence and the underlying processes used in soil C models. To help meet climate change mitigation goals, I recommend incentivizing the retention and establishment of AFS on agricultural lands, as well as supporting and optimizing biochar application in agriculture.</p>