Nitrogen Stock in a Clayey Rhodic Hapludox Under No-till.

Nitrogen (N) is an essential nutrient in most agricultural cropping systems. However, N cycling is complex and due to the large number of reactions which is subject to and its high instability in the soil, N is the most difficult nutrient to be handled in tropical regions, resulting in low nutrient use efficiency. So, a proper N management is essential for a sustainable, economically and environmentally sound agriculture. We evaluated the N stock in soil down to 0.8 m deep as affected by cropping systems under no-till. The experiment has been conducted since 2006 on a clayey Rhodic Hapludox, in Botucatu, state of 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. Soil samples were taken in December 2012 (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), before soybean sowing. Undisturbed samples (sampling rings) were taken for determination of the bulk density. The total–N was determined in deformed samples by CHNS Elemental Determinator. The N stock was calculated as follows: N stock = N (%) × bulk density (g cm^-3) × depth of soil layer (cm). The interaction between fall/winter and spring crops was only significant for 0.2-0.4 m layer. The differences in this layer probably contributed to the result of total stock (up to 0.80 m), which differ between spring crops. The largest stock of N was found in the systems with cober crob (12.31 Mg ha^-1) or indian hemp (11.56 Mg ha^-1).