COS 65-1 - The climate mitigation potential and soil microbial response of cyanobacteria-fertilized bioenergy crops in a cool semi-arid cropland

Background/Question/Methods

Bioenergy carbon capture and storage (BECCS) systems can serve as decarbonization pathways for climate mitigation. Perennial grasses are a promising second-generation lignocellulosic bioenergy feedstock applicable to BECCS, but optimizing their sustainability, productivity, and climate mitigation potential requires an evaluation of their greenhouse gas (GHG) dynamics and nitrogen (N) fertilizer strategies. Land-use transitions to bioenergy cropping systems and crop-soil feedbacks from N-fertilization can lead to negative consequences including increased emissions of GHGs and loss of soil organic carbon (SOC). Soil microbiome dynamics play a role in possible environmental tradeoffs with productivity, but these are poorly understood due to the paucity of data from agroecosystems. Here, we examine the climate mitigation potential and soil microbiome response to establishing two perennial grasses, tall wheatgrass (Thinopyrum ponticum; C3), and switchgrass (Panicum virgatum; C4), in a semi-arid agroecosystem under two fertilizer applications, a novel cyanobacterial biofertilizer (CBF) and urea. Finally, we examine shifts in soil microbial composition resulting from crop establishment and fertilizer regime.

Results/Conclusions

In contrast to the C4 crop, the C3 crop achieved 98% higher productivity (2.3 versus 6.7 Mg ha^-1) despite low background soil fertility, modest N-fertilization application (50 kg N ha^-1), and no irrigation. Importantly, CBF fertilization displayed high efficacy as it produced the same biomass enhancement as urea. Furthermore, we observed an annual soil carbon loss of 0.08 Mg SOC ha^-1 yr^-1 under the C4 crop and an increase of 0.25 Mg SOC ha^-1 yr^-1 under the C3 crop. This pattern was driven by partitioning of SOC accumulation by depth, with increases occurring at the deeper soil depth (15-30 cm) and no change at the shallower depth (0-15 cm). Non-CO₂ greenhouse gas fluxes across all treatments were low. We detected crop and depth specific changes in the soil fungi and bacteria, that included shifts in functionally important taxa to the cycling of carbon and nitrogen. These included increases in the relative abundance of arbuscular mycorrhizal fungi under the C3 and increases of pathogenic fungi and ammonia-oxidizing bacteria under the C4 grass. Taken together, these findings highlight the potential of CBF-fertilized C3 crops as a bioenergy feedstock in semi-arid regions as a part of a climate mitigation strategy.