Nitrogen IN SOIL AS Affected by Cropping SYSTEMS UNDER NO-till.

Nitrogen (N) dynamics in soil-plant system is complex and a proper management of the N is essential for the sustainability of agricultural activity. Cropping systems appropriate to each region, which include growing cover crops, may be able to improve N utilization by crops and to increase N content in soil. We evaluated the contents of soil total–N, organic–N and inorganic–N and soybean yield in cropping systems after five years under no-till. The experiment has been conducted since 2006 on a clayey Rhodic Ferralsol, in Botucatu, state o São Paulo, Brazil. Congo grass (Brachiaria ruziziensis), grain sorghum (Sorghum bicolor) and a mix of both were cropped during fall/winter. In the spring, pear millet (Pennisetum glaucum), cober crop [Sorghum bicolor (L.) Moench x Sorghum sudanense Piper Stapf] and indian hemp (Crotalaria juncea) were cropped in sub-plots. Soybean (Glycine max (L.) Merril) was cropped as a summer crop. In the harvest made in 2010/2011 the interaction between fall/winter and spring crops was not significant for soybean yield. Soybean yield was affected only by fall/winter crops, which was higher (3223 kg ha^-1) in systems where congo grass was included. After five years there were no differences in soil total and organic–N among the cropping systems. Highest NH₄⁺–N (18.7 mg dm^-3) and NO₃^-–N (10.5 mg dm^-3) contents at 0-10 cm depth were found in rotations including sorghum and congo grass in fall/winter and indian hemp in the spring. At 10-20 cm depth the highest NH₄⁺–N and NO₃^-–N content was found in the system with single grain sorghum and indian hemp. Overall, the results show the efficiency of indian hemp to increase the availability of inorganic–N in the soil.