Previous Cropping Systems and Nitrogen Application in Soybean Yield.

We aimed to evaluate soybean yield after fall/winter cropping systems associated to contrasting conditions of nitrogen availability. The experiment was performed under field conditions in a distroferric Red Nitisol of Itambe, Parana, Brazil, in 2014/2015 and 2015/2016 seasons. The experimental design was a 7x2 split-plot in randomized blocks with 4 replications. The cropping systems were maize (Zea mays); maize intercropped with Urochloa ruziziensis; maize intercropped with Urochloa brizantha; Urochloa ruziziensis; Urochloa brizantha; millet (Pennisetum glaucum); and association of radish (Raphanus sativus L.) with black oat (Avena strigosa). These cropping systems received 0 or 100 kg ha^-1 of N (as ammonium nitrate). In 2014/2015 season, the best systems were U. ruziziensis, U. brizantha and P. glaucum, differing and exceeding R. sativus L. associated to A. strigosa. In 2015/2016 season, however, soybean after the association of these last two species had the best results. The nitrogen application did not influenced soybean yield at both seasons and systems. It is necessary to state that there was a 20-day drought after the soybean was sown in 2014/2015 season, while in 2015/2016 there were high rates of rainfall. Consequently, in the first season the plots which had higher residues input and better soil surface cover may have retained moisture from the previous rainfall, favoring the establishment and yield of soybean. The opposite occurred in the second season, when the presence of denser layers of soil cover may have reduced the water evaporation, creating an unfavorable environment for plant growth, due to low gas exchange and favoring root pathogens. Thus, the association of radish and black oat in the last season reduced the soil compaction with the aggressive root system, allowing a better water drainage; also, the lower accumulation of biomass of these plants on soil surface enabled the water evaporation, reflecting in a better soybean yield.