Resumo

The nitrogen cycling is crucial for the soil functioning and for the maintenance of the plant growth. Within this cycle, the biological nitrogen fixation is the major natural source for nitrogen (N) input in soils, what is typically carried out by free-living diazotrophs, as well as symbiotic bacteria, active only in association with plants. Additionally, λ- and β-Proteobacteria (AOB) and Thaumarchaeota (AOA) play a role in nitrification, being also considered as biomarkers for soil quality. Among other factors, agronomic practices can influence the microbial communities of soil, possibly interfering in the role of such organisms in the promotion of plant growth. Here we examine, in three production units of sugarcane allocated in the State of Sao Paulo (Brazil), the abundance of such communities and the possible occurrence of shifts caused by managements of plantations or geographical distance among samples. In each of the three areas, 10 sugarcane fields were sampled, where plants present contrasting characteristics of managements. Community sizes were studied by quantifying the nifH, bacterial and archaeal amoA genes. Quantities of diazotrophs communities were higher than AOA and AOB communities, ranging from 10⁶ to 10⁷ per gram of dry soil, while nitrification-related genes showed similar values, around 10⁶ (AOA) and 10³ (AOB). The quantification of ammonium and nitrate in these soils indicated the prevalence of nitrate (average of 16,4 mg N/kg^-1 dry soil) over ammonium (average of 12,0 mg N/kg^-1 dry soil) in our analyzed areas, possibly indicating the occurrence of nitrification in the analyzed samples. The further deep understanding of the drivers of key functional genes can help us to better understand changes in microbial activity in the environment. From these present databases, we know that the abundance of these functional communities involved in N cycle vary in our samples and further analysis will generate new contributions to our understanding of the global distribution patterns of this functional genes and key driving factors for the assembling of such communities.