Long-term continuous maize: Impacts on the soil microbiome and implications for residue management

Continuous maize (Zea mays L.) (CM) cropping has been proposed to increase soil organic carbon stocks through greater residue return to soils. These residues, however, can contribute to a yield decrease known as the continuous maize yield penalty (CMYP). The objectives of this research were to (1) evaluate the efficacy of residue management practices to mitigate the CMYP when compared to a maize–soybean [Glycine max (L.) Merr.] (MS) rotation and (2) determine effects of long-term rotation or residue sizing on soil microbial communities and carbon-cycling enzyme activities. Two long-term sites (17 and 19 years) with replicated blocks of CM and MS rotation were used. The yield response to management was similar at both sites, with an average CMYP of 2570 kg ha⁻¹ with no residue management, while chopping the residue along with broadcast ammonium sulfate (AMS) reduced the CMYP by 728 kg ha⁻¹ (28.3%). The CMYP was further reduced when a microbial blend was applied with AMS to the sized residues, reducing the CMYP by a total of 1303 kg ha⁻¹ (50.7%). Soil bacterial communities differed by site but were unchanged by crop rotation or residue sizing. Soil fungal assemblage, particularly arbuscular mycorrhizal fungi, was influenced by rotation and site. Sizing of residues resulted in higher soil cellobiohydrolase activity for CM, but not for MS. These findings indicate that the CMYP was not directly associated with the soil bacterial community, and that management to mitigate the CMYP may be aided by elevated levels of fungal diversity under CM.