Soil carbon stocks, microbial stoichiometry, and enzyme activities across a 54‐year chronosequence of continuous maize versus maize–soybean rotations

Crop rotational diversity is hypothesized to enhance soil organic carbon (SOC) surface stocks, yet the long‐term SOC response to maize ( Zea mays L.)–soybean [ Glycine max (L.) Merr.] (MS) relative to continuous maize (MM) is inconsistent despite the greater residue C inputs in MM. We quantified SOC concentration and stock, microbial biomass carbon (MBC) and nitrogen (MBN), mineralizable C, and β‐glucosidase, cellobiohydrolase, phenol oxidase, and peroxidase activities at 0‐ to 15‐cm and 15‐ to 30‐cm depths in 19‐, 26‐, 41‐, and 54‐year rotation durations on Argiudolls across central‐north Illinois. Surface (0–15 cm) SOC stocks were similar between MS and MM across 19–54 years. The logarithmic attenuation of absolute relative annual (RAD abs , |% year −1 |) and absolute annual (AD abs , |Mg C ha −1 year −1 |) SOC stock differences suggested a trajectory toward equilibrium over half a century. Soil β‐glucosidase activity and MBC:MBN converged between rotations by 19 years, equilibrating earlier than SOC stock differences. Microbial biomass (MBC and MBN) was largely similar between rotations. Higher residue C inputs in MM increased mineralizable C and cellobiohydrolase and peroxidase activities, which, along with similar SOC stocks as MS, suggests that the greater residue C inputs were offset by increased decomposition. Rotation effects on microbial biomass and enzyme activities were largely limited to the 0‐ to 15‐cm depth and attenuated at 15‐ to 30‐cm depth. Collectively, our findings indicate that microbial physiological adaptations, specifically stoichiometric constraints and enzymatic shifts toward accelerated decomposition, decouple residue input quantity from long‐term SOC stocks and lead to equilibrium.