There has recently been concern in Brazil whether biological N₂ fixation (BNF) is capable of meeting the increased N needs of newly released more productive cultivars, as well as doubts about the advantages of annual reinoculation of seeds. Forty experiments were performed over 3 yr in oxisols containing at least 103 cells of Bradyrhizobium g^-1 in the State of Paraná, southern Brazil to estimate the contributions of BNF and of N fertilizer. The experiments were performed at two sites, Londrina and Ponta Grossa, under conventional (CT) or no-tillage (NT) systems, with two cultivars [Embrapa 48 (early-maturing) or BRS 134 (medium-maturity group)]. Treatments included non-inoculated controls without or with 200 kg of N ha^-1, and inoculation without or with N fertilizer applied at sowing (30 kg of N ha^-1), or at the R2 or R4 stage (50 kg of N ha^-1). Compared with the non-inoculated control, reinoculation significantly increased the contribution of BNF estimated by the N-ureide technique (on average from 79 to 84%), grain yield (on average 127 kg ha^-1, or 4.7%) and total N in grains (on average 6.6%). The application of 200 kg of N fertilizer ha^-1 drastically decreased nodulation and the contribution of BNF (to 44%), with no further gains in yield. Application of starter N at sowing decreased nodulation and the contribution of BNF slightly and did not increase yields, while N fertilizer at R2 and R4 stages decreased the contribution of BNF (to 77%) and also yields. Estimates of volatilization of ammonia ranged from 15 to 25% of the N fertilizer applied, and no residual benefits of the N fertilizer in the winter crop were observed. The results highlight the economical and environmental benefits resulting from replacing N fertilizer with inoculation in Brazil, and reinforce the benefits of reinoculation, even in soils with high populations of Bradyrhizobium. Key words: Biological nitrogen fixation, Bradyrhizobium, inoculation, N fertilizers, ureides, soybean

An interpretative framework for microbial biomass C (MBC), basal respiration, and the activity of soil enzymes cellulase, β-glucosidase, arylsulfatase, and acid phosphatase was developed for the clay...

The Cerrado—an edaphic type of savannah— comprises the second largest biome of the Brazilian territory and is the main area for grain production in the country, but information about the impact of land conversion to agriculture on microbial diversity is still scarce. We used a shotgun metagenomic approach to compare undisturbed (native) soil and soils cropped for 23 years with soybean/maize under conservation tillage—“no-till” (NT)—and conventional tillage (CT) systems in the Cerrado biome. Soil management and fertilizer inputs with the introduction of agriculture improved chemical properties, but decreased soil macroporosity and microbial biomass of carbon and nitrogen. Principal coordinates analyses confirmed different taxonomic and functional profiles for each treatment. There was predominance of the Bacteria domain, especially the phylum Proteobacteria, with higher numbers of sequences in the NT and CT treatments; Archaea and Viruses also had lower numbers of sequences in the undisturbed soil. Within the Alphaproteobacteria, there was dominance of Rhizobiales and of the genus Bradyrhizobium in the NT and CT systems, attributed to massive inoculation of soybean, and also of Burkholderiales. In contrast, Rhizobium, Azospirillum, Xanthomonas, Pseudomonas and Acidobacterium predominated in the native Cerrado. More Eukaryota, especially of the phylum Ascomycota were detected in the NT. The functional analysis revealed lower numbers of sequences in the five dominant categories for the CT system, whereas the undisturbed Cerrado presented higher abundance. High impact of agriculture in taxonomic and functional microbial diversity in the biome Cerrado was confirmed. Functional diversity was not necessarily associated with taxonomic diversity, as the less conservationist treatment (CT) presented increased taxonomic sequences and reduced functional profiles, indicating a strategy to try to maintain soil functioning by favoring taxa that are probably not the most efficient for some functions. Our results highlight that underneath the rustic appearance of the Cerrado vegetation there is a fragile soil microbial community.