The agroforestry systems have been widely promoted as one sustainable agricultural system, mainly for developing areas, where the external use of resources are not feasible. The aim of this study was to evaluate the impact of one conventional and four agroforestry systems on the contents of total soil nitrogen, mineral nitrogen and different organic nitrogen fractions after five years of experimental cultivation on a Luvisol (Alfisol). The study area was in the semi-arid zone at the National Caprine Research Center (CNPC) of Embrapa, Ceará, Brazil. The following treatments were tested: agrosilvopasture (AGP), silvopasture (SILV), traditional agroforestry in 1998 and 1999 (TR98), traditional agroforestry in 2002 (TR02), conventional cropping (CC), and seasonally dry native vegetation ("Caatinga") (CA). Total-N, N-NH4+, N-NO3-, microbial-N, light organic matter N (free and occluded) and N in the humic substances were evaluated. Results indicated high nitrate-N (NO3-) content in all treatments, accounting for 10.3 to 23.5 % of the total N. The CC treatment reduced the total N and N in humic substances by 38 and 44 %, respectively. The AGP and TR98 agroforestry treatments significantly reduced N in the light organic matter and in humic substances. On the other hand, the SILV system preserved, and in some cases, improved the concentration of the soil. This system can therefore be recommended as a sustainable alternative for soil management in the semi-arid region of Ceará state.
mineral nitrogen; microbial nitrogen; humic substances; semi-arid; silvopasture
