Plant roots and GHG mitigation in native perennial bioenergy cropping systems

Abstract

Native perennial bioenergy crops can mitigate greenhouse gases (GHG) by displacing fossil fuels with renewable energy and sequestering atmospheric carbon (C) in soil and roots. The relative contribution of root C to net GHG mitigation potential has not been compared in perennial bioenergy crops ranging in species diversity and N fertility. We measured root biomass, C, nitrogen (N), and soil organic carbon (SOC) in the upper 90 cm of soil for five native perennial bioenergy crops managed with and without N fertilizer. Bioenergy crops ranged in species composition and were annually harvested for 6 (one location) and 7 years (three locations) following the seeding year. Total root biomass was 84% greater in switchgrass (Panicum virgatum L.) and a four-species grass polyculture compared to high-diversity polycultures; the difference was driven by more biomass at shallow soil depth (0-30 cm). Total root C (0-90 cm) ranged from 3.7 Mg C ha À1 for a 12-species mixture to 7.6 Mg C ha À1for switchgrass. On average, standing root C accounted for 41% of net GHG mitigation potential. After accounting for farm and ethanol production emissions, net GHG mitigation potential from fossil fuel offsets and root C was greatest for switchgrass (À8.4 Mg CO2e ha À1 yr À1 ) and lowest for high-diversity mixtures (À4.5 Mg CO2e ha À1 yr À1). Nitrogen fertilizer did not affect net GHG mitigation potential or the contribution of roots to GHG mitigation for any bioenergy crop. SOC did not change and therefore did not contribute to GHG mitigation potential. However, associations among SOC, root biomass, and root C : N ratio suggest greater long-term C storage in diverse polycultures vs. switchgrass. Carbon pools in roots have a greater effect on net GHG mitigation than SOC in the short-term, yet variation in root characteristics may alter patterns in long-term C storage among bioenergy crops.

Grassland renovation has important consequences for C and N cycling and losses

“…According to Jungers et al. (), C‐pools in roots have a greater effect on net greenhouse gas (GHG) mitigation than soil organic C (SOC) in the short term. Changes over time in root characteristics may alter patterns in long‐term C storage (Figure ).…”

“…Compared to an old and unproductive sward, the newly sown grass and legume cultivars with their high growing potential can markedly contribute to the enhancement of the root C-pool (Marshall, Collins, Humphreys, & Scullion, 2016). According to Jungers et al (2017), C-pools in roots have a greater effect on net greenhouse gas (GHG) mitigation than soil organic C (SOC) in the short term. Changes over time in root characteristics may alter patterns in long-term C storage ( Figure 6).…”

Inconsistent effects of species diversity and N fertilization on soil microbes and carbon storage in perennial bioenergy cropping systems

“…Differences in soil C accrual patterns by depth could be related to differences in the root depth distribution, whereas a concentration of high C:N root biomass at the 0-15 cm depth may have resulted in higher soil C accrual at that specific depth interval. Although belowground biomass was not sampled in this study, root tissue quality, biomass and distribution of biomass by depth can explain a large portion of variability in soil C dynamics compared to planted species diversity (Fornara and Tilman, 2008;Jungers et al, 2017), thus explaining the lack of a species (Zemenchik and Albrecht, 2002) mixture treatment on total soil C in this study. Khan et al (2007) also observed reductions in soil C at depths >30 cm under fertilized, long-term crop rotations including corn, oats, soybean and alfalfa.…”

“…Total soil carbon C was determined using combustion analysis. Samples from 2017 were analyzed on an Elementar pyrocube (Elementar Americas, Inc., Ronkonkoma, NY, USA), with previous samples analyzed in a similar manner (Mangan et al, 2011;Jungers et al, 2017).…”

Plant Diversity and Nitrogen Addition on Belowground Biodiversity and Soil Organic Carbon Storage in Biofuel Cropping Systems

“…N fertilizer did not impact soil C accumulation, as other studies in cellulosic bioenergy cropping systems have found (Das et al, 2018;Kibet et al, 2015;Jungers et al, 2017;Valdez et al 2017). Fertilization can lead to increased decomposition of soil organic carbon (Khan et al, 2007), especially if N fertilizer application enhances the input of C that is easily accessible to the soil microbial community (Lin et al, 2019).…”