Agricultural bioenergy (‘bioenergy’) is a potential net-negative energy source if carbon (C) sequestered belowground during crop growth exceeds C emitted during growth and conversion. C enters soil predominantly through root production and turnover. Fine root biomass (FRB) is therefore a direct control over belowground C sink potential, and FRB distribution with increasing soil depth will control C sink magnitude below the plow layer (30 cm). FRB and its allocation by depth are inherently different among species, and fertilizer inputs will alter these root traits.
Root respiration is a major C source, contributing approximately 50% of total soil C flux. Root respiration is itself an independent root trait which varies among species and in fertilization response. Total root respiration is also directly impacted by total FRB.
We attempt to characterize how belowground autotrophic processes influence C sink vs. source in two bioenergy crops, Miscanthus x giganteus and Sorghum spp. We quantified FRB in both crops in successive depth increments spanning 1m (0-10, 10-20, 20-30, 30-50, and 50-100 cm) along a fertilization gradient (Miscanthus: 0, 112, 224, 336, 448 kg/ha/yr; Sorghum: 0, 112 kg/ha/yr), and also quantified root respiration rates along the same fertilization gradient.
Results/Conclusions
In both species, the top 10 cm contained the significantly greatest amount of FRB (p<0.0001) (177.24 + 14.62 g/m2 in Miscanthus; 65.65 + 6.43 g/m2 in Sorghum). At all depths below the plow layer, Miscanthus produced approximately 5x more FRB compared to Sorghum. With all depths summed, Miscanthus contained significantly greater (p<0.0001) FRB (471.06 + 27.17) than Sorghum (212.18 + 31.74). Root respiration rates were significantly lower in Miscanthus (p<0.0001, 6.02 + 0.25) than Sorghum (16.75 + 1.26).
Fertilization significantly reduced Miscanthus FRB (p<0.0001) and increased root respiration (p<0.0001). Post-hoc analyses revealed FRB in unfertilized plots (120.06 + 13.17 g/m2) was greater than all other fertilization rates (87.32 + 4.71, p<0.005). Unfertilized plots also had significantly lower root respiration (4.66 + 0.35) than all other fertilization rates (6.00 + 0.22 mg C-CO2/g FRB/day, p<0.005). Fertilization did not exert an effect on Sorghum FRB or root respiration rates.
Soil C storage will be greater in Miscanthus due to greater inputs. Despite lower root respiration rates in Miscanthus than Sorghum, total root respiration will be greater due to higher FRB. Overall, our results indicate greater C sink potential in Miscanthus than Sorghum.