At the beginning of the
Over the second year, the influence of the application of organic amendments on the soil nitrogen dynamic was less evident. One year after application (spring), organic-amended soils showed low availability of mineral nitrogen, despite better conditions for an increase in N mineralization processes. These results concur with Ochoa-Hueso et al. (2013), who found a minimum of mineral N availability in spring in a semi-arid Mediterranean soil. Three months later (summer, 15 months after application) we found a slight increase in NH4+-N and NO3−-N content in amended soils, probably due to increased temperatures and an early rainfall 2-APB (see Fig. 2) which promotes soil microbial activity. However, a response to a drought period – typical of the Mediterranean climate – was observed in autumn. In this sampling (18 months after application) we found a sharp increase in NO3−-N content at both monitored depths (Fig. 5). The increase in NH4+-N content in summer and its decrease in subsequent samplings may be due to the nitrification process, as observed from the increase in NO3−-N content. Although the drought effect could decrease soil microbial activity, many authors have demonstrated that nitrifying communities may be metabolically active in a semi-arid soil during the dry season (Gleeson et al., 2008, Parker and Schimel, 2011 and Sullivan et al., 2012). However, we observed high potential nitrification only in MSWC soils. Cruz et al. (2008) found a high NO3−-N content in a Mediterranean soil during the dry season. As previously discussed, this leads to a greater likelihood of the nitrate moving into the deeper layers of the soil after autumn and early winter. The rainfall accumulated until the sampling (100 mm, 21 months after application) may cause an increase in NO3−-N (20–40 cm) content, causing it to be higher than the topsoil content.