Amending sugarcane monoculture through rotation breaks and fungicides: effects on soil chemical and microbial properties, and sucrose yields

Sugarcane (Saccharum spp. hybrids) monoculture was amended with leguminous rotation breaks (cowpea, Vigna unguiculata; and soybean, Glycine max) and compared with growers’ practice (sweet corn, Zea mays var. saccharata rotation; and bare fallow). After incorporation of rotation-break residues, fungicidal treatments (mancozeb, mefenoxam and azoxystrobin) were applied on seed-cane pieces laid in the furrows before row closure, with the objective of determining effects of both crop rotation and fungicides on soil properties and sugarcane agronomic performance. Aboveground biomass yields of sweet corn, soybean and cowpea were 5.54, 5.17 and 4.48 t ha–1, and carbon:nitrogen ratios of sweet corn, soybean and cowpea crop residues were 25.47, 11.92 and 11.61, respectively. Following residue incorporation, phospholipid fatty acid analyses of soil microbial communities at pre-plant and early-growth stages of sugarcane indicated significant differences in abundance of Gram-positive bacteria, actinomycetes and fungi biomarkers, whereas no differences were found in Gram-negative bacteria and arbuscular mycorrhizal fungi. At pre-planting, fungi:bacteria ratios in sweet corn and bare fallow plots were significantly higher than in cowpea or soybean rotation plots and similar to ratios at early-growth sampling. Soybean rotation produced higher cane yield than sweet corn, and both soybean and bare fallow produced higher sucrose yield than sweet corn rotation in 2016 plant cane, but no significant yield differences occurred in 2017 plant cane. The results of 2016 plant cane persisted in the 2017 first ratoon, where the sweet corn rotation had lower yields than cowpea and soybean rotations. All three fungicides significantly improved cane yield and sugar yield compared with the untreated check in plant cane, with both mancozeb and mefenoxam performing similarly in 2016, but with mefenoxam performing better in 2017 plant cane. Overall, introduction of cowpea and soybean rotations, coupled with seed-piece fungicidal application, seems a promising practice for improving sugarcane yields on Histosols.