Influence of nitrite accumulation in association with seasonal patterns and mineralization of soil nitrogen in a semi-arid pine forest

To identify factors that influence the relatively high productivity of a semi-arid pine afforestation system in southern Israel, we investigated inorganic nitrogen deposition and mineralization for more than 2 years. To this end, we measured bulk and dry deposition, in situ N-mineralization over the seasonal cycle, and the potential activity of nitrifying microorganisms by soil slurry incubations. There was a small increase in bulk N deposition in the forest, compared with shrubland, but no change in dry deposition. An unexpected rapid increase in nitrite concentration in the forest soil was observed after soil rewetting by the first winter rains, which could not be explained by deposition. This was accompanied by a decrease in ammonium and only a slight increase in nitrate concentrations. Only a small increase in nitrite and a rapid increase in nitrate concentration in the mineral soil were observed in the surrounding shrubland. Soil slurry incubations from the forest sites exhibited significant delay in nitrite, compared with nitrate accumulation (up to 50 h under lab conditions) in samples taken in the dry season, but not in the wet season. This indicated different rates of ammonium and nitrite oxidation that are most likely linked to differential activation of different microbial populations after the summer stress. The initial oxidation process of ammonia to nitrate, upon soil rewetting in semi-arid environments, appears to occur as a partially uncoupled two-step process, as opposed to a rapid continuous one in wetter environments. This may have implications for the synchronization of nitrate availability to plants and therefore for high forest productivity and nitrogen use efficiency. Forest productivity in the semi-arid regions, in turn, is becoming increasingly more important with persistent predictions of warming and drying trends over the entire Mediterranean basin and other regions.