Soil Storage and Annual Increment of Nitrogen in Brazilian Cropping Systems without Nitrogen Fertilization.

We aimed to evaluate the nitrogen (N) storage and the annual increment of N (AIN) in a Rhodic Hapludox soil under a long-term cropping systems experiment. The experiment has been ongoing under no-till since 2006 in Botucatu, São Paulo, Brazil. Ruzigrass (Brachiaria ruziziensis) and grain sorghum (Sorghum bicolor) were grown in the Fall/Winter, either alone or intercropped (plots); and pearl millet (Pennisetum glaucum), forage sorghum (Sorghum bicolor) and sunn hemp (Crotalaria juncea) were grown in the spring, in sub-plots. In the summer, soybean (Glycine max (L.) Merrill) was planted in all sub-plots. There was no N fertilization. Soil samples were taken in April 2012 and April 2014 (at the depths of 00-0.1, 0.1-0.2, 0.2-0.4, 0.4-0.6 e 0.6-0.8 m), always after soybean harvest. The total–N was determined by Elemental Determinator. Undisturbed samples were also taken for determination of bulk density. The AIN was calculated by difference between the final soil N storage and the soil N storage in 2012 (considering the sum of all layers). Considering the Fall/Winter, the largest N storage was found in the Ruzigrass / Grain sorghum intercropping plots (8.99 Mg ha^-1). In the Spring, the largest soil N storages were found in the Sunn hemp and forage sorghum subplots. The combination (Fall/Winter – Spring) Ruzigrass / Grain sorghum intercropping – Sunn hemp also provided the highest values of IAN (61 kg ha^-1 year^-1). Looking the experiment historical, we might see a correlation between these results and the characteristics of the plants used in the systems. Based on that, we conclude that the adoption of cropping systems that provide high intake of plant residues, high production of roots and inclusion of plants able to fix atmospheric N is an important strategy for improve soil quality and assure the sustainability of the system.