Using dual differential isotopic labeling of plant litter to track the contribution of labile and recalcitrant components to soil organic matter formation.
Michelle L. Haddix, Eldor A. Paul, and M. Francesca Cotrufo, Colorado State University
The classical view of soil organic matter (SOM) formation is that lignin-like compounds in plant material that resist decomposition are complexed or “humified” and eventually form stable SOM. The application of new techniques has revealed that SOM is mostly made of simple molecules, comprising microbial products and plant material in various stages of decomposition and lignin is not selectively preserved in soil. New plant stabilization frameworks have emerged proposing that mineral stable SOM is formed from the microbial processing of labile plant inputs and subsequent stabilization of those inputs onto mineral surfaces. We set out to test this new stabilization framework using a novel approach of dual 13C and 15N differentially labeled plant material where we are able to isotopically distinguish the metabolic and structural components within a single plant material. We were able to differentially label Big Bluestem (Andropogon gerardii) by growing seedlings in a dua l isotopic labeling chamber until maturity and, 21 days prior to harvest, removing the plants from the chamber to allow them to incorporate natural abundance 13C-CO2 and natural abundance 15N fertilizer into the metabolic plant components. Using this method we were able to achieve a greater than one atom % difference in 13C between the metabolic and structural components within the litter, but the 15N difference between these two plant components was not as strong. We then incubated this differentially labeled litter with soil at 35°C, for 386 days with destructive harvests at 14, 28, 147, and 386 days. CO2 was measured throughout the incubation and at each harvest the soil was fractionated into light fraction, sand, silt, and clay. We did not see a significant difference in the amount of litter biomass stabilized or the proportion of litter stabilized versus respired over time. We found isotopically labeled plant material already in the mineral fraction after 14 days of incubation. Only the metabolic litter component was found! in the sand, silt, or clay fraction with the structural component only being found in the light fraction. These results support the stabilization framework that mostly labile plant components are being stabilized onto mineral surfaces.