Small soil C cycle responses to three years of cover crops in maize cropping systems

Cover crops are touted for their ability to improve many ecosystem functions in annual cropping systems. In addition to water and nutrient retention, cover crops may influence C cycling by increasing total C inputs to the agroecosystem, stimulating microbial populations, altering main crop residue decomposition rate, or changing litter chemistry over time. We assessed whether annual (rye) or perennial (bluegrass) cover crops in maize cropping systems influenced maize residue decomposition (litterbags) or microbial communities (shotgun metagenomics) in soil and litter, and whether these cover crops had an effect on microbially active pools of C: particulate organic matter (POM) C and N, or potentially mineralizable C (PMC) after three years of cover crops. Neither cover crop affected litterbag decay rates or microbial composition relative to no cover crop controls. However, both cover crop types increased PMC indicating that microbially-available C was boosted by cover crops. Total POM and POM-N were higher with bluegrass cover crops. These modest effects of cover crops on dynamic soil C pools suggest the generally positive long-term effects of cover crops on soil protection and nutrient retention are related to incremental shifts in quantity, timing and quality of C inputs.