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

Abstract

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.