Crop diversification and seed inoculation strategies effects on soil microbial community in soybean cropping systems

Crop diversification has gained importance in Brazilian soybean (Glycine max L.) cropping systems, usually cultivated in soybean/2^nd season maize (Zea mays L.) successions. Brachiaria grass (Urochloa spp.), a forage highly grown in Brazilian livestock systems, can be a suitable option for the soybean systems diversification. Brachiarias are well adapted to tropical conditions, produce high amounts of above and belowground biomass, have high nutrient cycling capacity, and release exudates known as biological nitrification inhibitors (BNI). All these traits might increase soybean yield and nutrient use efficiency in the agroecosystem.

Brazilian cropping systems rely on plant growth-promoting bacteria (PGPB), like seed inoculation of soybean with the nitrogen-fixing bacteria Bradyrhizobium, alone or in combination with Azospirillum, to replace mineral N fertilizers.

In this study, we aimed to investigate the soil bacterial community (activity and diversity) response to the diversification of soybean/maize cropping systems with Urochloa ruziziensis and inoculation with different combinations of PGPB. We hypothesize that inoculation with PGPB and diversification of the system with maize intercropped with Brachiaria will enhance microbial community activity and diversity.

A 5-year experiment has been conducted in Londrina (Paraná State, Southern Brazil) in a randomized complete block design with a split-plot arrangement and six replicates. Main plots consisted of soybean during the cash crop season (S: soybean without inoculation; Si: soybean inoculated with Bradyrhizobium; Sc: soybean co-inoculated with Bradyrhizobium + Azospirillum). Sub-plots consisted of different diversification systems after the cash crop season (M: succession with maize; M+U: maize intercropped with U. ruziziensis; Mi+Ui: maize intercropped with U. ruziziensis, both inoculated with Azospirillum). After the soybean harvest in the 2021/2022 cropping season, soil samples were taken at the 0-10 cm soil layer. We analyzed soil enzymes (arylsulfatase, β-glucosidase, and acid phosphatase), environmental factors (soil pH and nutrients), and the 16S gene sequence.

Preliminary results suggest an increase in the relative abundance of some bacterial phyla with Brachiaria. The phylum Proteobacteria, which harbors numerous PGPB, showed higher relative abundance in the cropping systems with Brachiaria, independently of the inoculation strategy in the summer soybeans. On the other hand, for the Nitrospirota phylum, which contains nitrite-oxidizing bacteria, higher relative abundance was observed in S/MiBi and Si/MiBi, compared with Sc/MiBi. Additional results on bacterial community diversity and composition and their relationship with microbial activity and environmental indicators will be discussed.

This study provides novel insights into how crop diversification combined with PGPB affects the soil microbial community and nitrogen dynamics, supporting agricultural and soil management practices to achieve more sustainable production systems.