COS 228-3 - CANCELLED - Unlocking Deep Soils: Linking depth gradients in roots, microbial activity, and soil carbon

Abstract: Sustainable production of bioenergy that offsets downstream CO₂ emissions requires building soil carbon (C) at both shallow and deep depths. While research suggests that deep-rooted perennial bioenergy crops may offer the greatest potential for increased soil C, much of this research has focused on shallow soils. This focus on shallow soils neglects deep soils which account for nearly a third of all soil C. Our objective was to link depth gradients in root biomass with microbial activity and soil C stocks down to 1 meter under the deep-rooted perennial bioenergy crop, Miscanthus. To do this, we dug five, quantitative soil pits under 20-year-old Miscanthus plots in Champaign-Urbana, IL. We extracted all the soil and root biomass in a 1m x 30 cm x 1m volume and separated the soils into five depths. To meet our objective, we measured microbial respiration, the ratio of mineral associated organic C (MAOC) to particulate organic C (POC), nitrogen mineralization and nitrification, and enzyme activity.

Our results show that depth declines in root biomass were the primary driver of microbial activity and soil C stabilization gradients. For microbial activity, we found that in the top 30 cm where root biomass was the greatest, there was nearly an order of magnitude more net nitrogen mineralization and nitrification, and nearly 2-fold greater microbial respiration and enzyme activity than in the soil depths below 30 cm. Soil C was more evenly distributed with depth, but the top 30cm still contained 56% of the total C. In addition, there was a strong shift in the ratio of MAOC-to-POC with depth. The top 15cm had more POC (MAOC-to-POC=5.54); whereas the soil in the 85-100cm depth was dominated by MAOC (MAOC-to-POC=30.77). These results suggest that fundamentally different processes drive soil C stabilization with depth in Miscanthus soils. In the top 30cm of soils, the low MAOC-to-POC ratio is likely due to greater inputs that exceed the capacity of microbial decomposition and the saturation of available mineral surfaces. By contrast, in deeper soils, the opposite is likely occurring. Given that these depths have 75% of the total soil mass but only 46% of the total soil C, there may be the potential to build more MAOC.