COS 166-4 - Identifying optimal plant traits and agricultural practices to maximize soil carbon benefits under varying atmospheric carbon dioxide levels

Background/Question/Methods

New efforts seek to design sustainable agricultural systems that restore soil carbon (C), thus contributing to endeavors to slow global climate change. To assess the potential of these efforts, there is a critical need to enhance our predictive understanding of the impact of plant traits and agricultural practices on soil C pools both under current conditions and under elevated CO₂. While field studies that have experimentally elevated CO₂ show enhanced aboveground C sequestration by plants, the belowground response remains inconclusive. Given the paucity of empirical data, our objective was to model the optimal combination of plant traits and agricultural management practices that maximize soil C stocks under ambient and elevated CO₂. To meet this objective, we used the Fixation and Uptake of Nitrogen-Bioenergy Carbon, Rhizosphere, Organisms, and Protection model (FUN-BioCROP). FUN-BioCROP dynamically predicts the impact of C spent by plants to acquire nitrogen on microbial decomposition. We used the model to identify the optimal combination of plant traits (e.g., litter chemistry, rooting depth, root C exudation) that led to the greatest soil C gains. We then examined the ability of bioenergy feedstocks that vary widely in plant traits to increase soil C under elevated CO₂.

Results/Conclusions

Our model experiments showed that plants with lower quality litter resulted in the production of more unprotected soil C and less protected soil C compared to plants with high quality litter. Deeper rooting zones and higher root exudation rates both increased protected soil C. Agricultural practices that increased soil C stocks in the model included reduced soil tillage frequency and depth, use of organic matter amendments as opposed to nitrogen fertilizers, and increased residue following harvest. Under elevated CO₂, the model projected higher SOC stocks in all simulated bioenergy feedstocks with the perennial feedstocks (miscanthus, switchgrass) gaining relatively more soil C than the annual feedstocks (corn-corn-soybean rotation). This projection can be attributed to the persistent root systems of perennials allowing for greater C contributions to the soil profile than in the annual systems whose roots and associated soils are disturbed each year at harvest. However, the perennial soil C was composed of a larger proportion of unprotected C, increasing its vulnerability to loss under warmer conditions. Collectively, our results suggest that plant traits are an important driver of the ability of sustainable agricultural practices to enhance soil C under current and projected increases in atmospheric CO₂.