Rhizosphere Priming of Cover Crop Decomposition.

Rhizosphere priming is a process in which plant belowground carbon (C) allocation stimulates soil organic matter (SOM) decomposition. Rhizosphere priming has largely been investigated in non-agricultural ecosystems or in pot studies, yet it may be an important, overlooked mechanism in agricultural soil organic matter dynamics. Furthermore, knowledge gaps remain in our understanding of how soil-plant-microbe interactions regulate the fate of cover crop litter inputs in agroecosystems. We hypothesized that corn (Zea mays) would increase (relative to soils without plants) the decomposition rate of cover crop litter and that the corn effect would differ by preceding cover crop type. We tracked the C and N from corn, clover (Trifolium pratense) and rye (Secale cereale) cover crop litter, and background SOM in plots following clover, rye, or no cover crop (fallow) in a 2-year study within an organic cropping systems experiment in Pennsylvania. The field had a C₃ cropping history, allowing the use of C stable isotopes to differentiate between corn-derived and C₃ cover crop and SOM sources. In the first year of the study, corn stimulated C and N loss from litterbags of clover residue by 8 and 17%, respectively, compared to plots without corn. Corn had no effect on rye decomposition. Relative to a no-corn plot, corn increased soil respiration in plots following clover and fallow by an average of 40%, but corn did not increase respiration following rye. This increased respiration was from corn-derived C and not from increased decomposition of SOM or recent cover crop inputs. We did not find an increased contribution of corn-derived C in dissolved organic C or microbial biomass in any of the cover crop plots, likely because these measures are temporally variable and are small in comparison to the large pool of background C₃-derived C. Decomposition rates are a more integrated measure that shows cumulative effects over time, and we found clear evidence of corn priming the decomposition of N-rich cover crop litter. To supplement the existing results, we will be including a second year of data.