Soil Microbial Attributes and Soybean Yield Response to Off-Season Crop Diversification in an Oxisol in Southern Brazil

Soybean (Glycine max) cropping in Brazil often relies on soybean/maize or soybean/fallow systems. Diversification with off-season crops can improve soil biological health and soybean yield. The aim of this study was to evaluate the effects of off-season crops on soil microbial attributes and soybean yield over a seven-year experiment carried out in Londrina, Parana, Brazil. The treatments included five off-season cropping systems: (i) maize (Zea mays) and (ii) wheat (Triticum aestivum) as cash crops, (iii) ruzigrass (Urochloa ruziziensis) and (iv) showy rattlebox (Crotalaria spectabilis) as cover crops, and (v) fallow. Soil microbial attributes and soybean yield were assessed over 2020/2021 and 2022/2023 cropping seasons. Ruzigrass had the highest straw yield (8515 kg ha(-1) year(-1)), while showy rattlebox produced the lowest amounts (509 kg ha(-1) year(-1)) among the off-season crops. Ruzigrass and maize improved the soil organic carbon levels compared with fallow, whereas maize promoted the lowest amount of soil labile-C compared with the other treatments. Maize, ruzigrass, and wheat also increased the soil microbial biomass carbon. Cropping ruzigrass as off-season cover crop also improved N-cycling traits (microbial biomass N, and total inorganic N), microbial respiration, and the activity of beta-glucosidase, arylsulfatase, acid phosphatase, and glutaminase. Principal component analysis of soil microbiological and chemical attributes revealed a separation among the off-season treatments, specially distinguishing ruzigrass from showy rattlebox and fallow. Ruzigrass provided the highest soybean yield in succession (4119 kg ha(-1) year(-1)) compared with fallow (3525 kg ha(-1) year(-1)). These results highlight ruzigrass as option to diversify the soybean production system, improving soil microbial attributes and soybean yield. Our findings also add on the understanding of crop diversification as sustainable agricultural strategy for promoting soil health.