COS 1-1 - Age-Related Changes in Partitioning of Carbon in Perennial Grain and Annual Grain Cropping Systems

Abstract: The carbon balance of agroecosystems is of increasing importance for both their sustainability and toward the reduction of greenhouse gas emissions. Reduced disturbance and extensive root systems of perennial cropping systems are expected to build soil carbon. However, soil carbon accrual in perennial cropping systems can be limited, especially when perennial crops are established for short time periods. Although crop carbon uptake and partitioning to belowground pools (crop roots, soil and soil microbial communities) strongly controls soil carbon accrual rates in perennial cropping systems, little is known about how perennial crops partition carbon and whether carbon partitioning dynamics change over the production period of the perennial crop. To address this gap in research, we applied a ¹³CO₂ stable isotope tracer to one- and two-year old perennial grain (intermediate wheatgrass; IWG) and annual grain (spring wheat) cropping systems to evaluate carbon partitioning to belowground carbon pools over a 90-day study period. Results showed tracer carbon was recovered from both above- and belowground carbon pools immediately, indicating both perennial and annual grains rapidly assimilated carbon and translocated recently assimilated carbon into belowground pools. However, tracer carbon recovery from belowground pools (roots and soil) declined significantly between the first and second IWG production years. Further, the second-year IWG system allocated a greater proportion of recently assimilated carbon to aboveground tissues and lost more recently assimilated C to respiration when compared to the first-year IWG system. Together, these results suggest belowground inputs in IWG systems become limited as stands age, challenging the potential for these perennial cropping systems to build soil carbon over their production periods.