COS 83-3 - Rhizodeposition and microbial communities underlying mineral-associated organic matter destabilization

Background/Question/Methods

Quantifying how plants and microbes can destabilize the large, long-lived mineral-associated organic matter (MAOM) pool in soils is critical to understand carbon storage and nutrient cycling. Our recent lab experiments suggest that compounds commonly released by roots can destabilize and mobilize organic matter off minerals via chemical and microbial mechanisms, opening the organic matter to microbial attack. We have developed plant viral infection as a tool to manipulate rhizodeposition and probe its effect on MAOM destabilization. Using a common grass-infecting virus (barley yellow dwarf virus, BYDV), we previously found that oat plants (Avena sativa) grown either hydroponically or in soil exhibit strongly reduced overall biomass, photosynthesis and transpiration (per unit leaf), and root:shoot ratio when infected. Solutes gathered from hydroponic liquid around roots of infected plants differed in chemistry from solutes around uninfected roots, and drove stronger MAOM mobilization and mineralization when applied to soil in lab assays. Based on these findings, we hypothesize that in soil-grown oats, BYDV infection will intensify MAOM mobilization and mineralization per unit root, but that infection-induced reduction in root biomass may reduce whole soil column mineralization of MAOM.

Results/Conclusions

We are testing these hypotheses in a greenhouse experiment with uninfected and BYDV-infected oats (infection status confirmed with RT-PCR). To spur strong nutrient foraging, plants were established in sandy loam soil with low nutrient availability, mixed 1:1 with sand for drainage. Prior to planting, we added labeled MAOM (¹³C-glucose bound to ferrihydrite) to soil. Over time, ¹³C-MAOM mobilization and mineralization are being detected as ¹³C-CO₂ evolution from pots. In the low-nutrient substrate, growth and photosynthesis rates of infected and uninfected plants are more similar than in the higher nutrient environments considered earlier. We speculate that nutrient stress reduced the growth potential advantage of uninfected plants, masking virus treatment effects. Despite similar growth patterns, however, virus-infected plants used less water at the whole-plant level, consistent with other reports of infection-induced changes in plant water use. To probe these interactions further, we continue to measure soil respiration, leaf-level gas exchange, and soil solutions, and will evaluate rhizosphere and bulk soil metatranscriptomes along with plant biomass C:N and root:shoot ratios at harvest.