Fate of nitrogen applied to corn second crop and its effect on soybean cultivated after

Maize (Zea mays L.) second crop has potential for sowing in intercropping with forages of the genus Urochloa spp. and Megathyrsus spp., which can modify the microbial population of the rhizosphere and affect the dynamics of nitrogen (N) in the production system. This management improves the nutritional conditions of the system, and the availability of N through the cycling of the nutrient by the residues, and there may be a reduction in the losses of NO3- by leaching and nitrous oxide (N2O). The N cycling favors the nutrition of maize, however the amount of N fertilizer absorbed by this crop in intercropping with forage still need to be clarified. Studies that better explore the agronomic potential of forages with maize and soybeans sown in succession are extremely important. To confirm the hypothesis that forages have the ability to improve N cycling, acting to reduce losses, studies were conducted between 2018 and 2021 in Botucatu, with the objectives of (1) evaluate the influence of forages on the number of copies of the main genes related to the N cycle and N2O emission in soybean (Chapter 1), (2) to verify if the maize - forage intercrop interferes in the recovery of 15N fertilizer applied in the crop and the fate of 15N in soybean in succession (Chapter 2), and (3) assess whether the consortium mitigates N2O emissions and what influence it has on NH3 losses (Chapter 3). To achieve the proposed objectives, the following field experiments were carried out. Study 1; in January 2018, the sowing of the forages Megathyrsus maximus (Guinea grass) and Urochloa ruziziensis (Ruzigrass - common) was carried out, which were conducted for seven months, the experiment also had a management without forage. In sequence, the sowing of soybeans for the 18-19 harvest took place. To evaluate gene abundance, soil collection was carried out in September, October 2018 and January 2019 and during the soybean cycle 18-19, sampling of N2O emission was carried out. Studies 2 and 3; After the removal of soybean 18-19, in February 2019, second crop maize was sowed intercropped with forages and single. From this season, the experiment started to have subplots referring to doses of 120 kg ha-1 of N and the control without N. The management with second crop maize and summer soybean was carried out for two crop years, 19-20 and 20-21. In study 2, the fate of 15N in the soil–plant system in maize and soybean crops was evaluated, with plant and soil collections. For study 3, greenhouse gas and NH3 emissions were collected during the crop cycle and in the off-season, with greater intensity after sowing and topdressing with N in maize. The abundance of AOA genes prevailed over AOB and soybeans cultivated on these forages emitted more N2O. When absorbed by maize, the 15N fertilizer presented grains as its main destination, however the soil is still the largest reservoir of 15N fertilizer in soybean crops. Higher N2O losses occurred at the beginning of the experiment, and throughout the experiment there was a predominance of CH4 absorption. The system with forages did not reduce the number of copies of AOB or AOA, however these have a good capacity for N absorption, storing it in their shoots, allowing a greater absorption by the soybean cultivated afterwards. CH4 is no less absorbed with fertilized maize and soybeans in rotation, while the main route of gaseous N loss is by volatilization of NH3 in species managed with and without forage. (AU)