An approach to nitrogen balance in olive orchards

The nitrogen balance was estimated in two olive orchards at two different localities in order to provide data for rational fertilization practices for olive orchards, to account for the lack of response of olive trees to nitrogen fertilization, and to study the environmental impact of the current practices of nitrogen fertilization. The experiments were established in 1994, and the nitrogen balance was determined for 2001-2007 on unfertilized plots and on fertilized plots that received annual applications of 1 or 1.15 kg N tree(-1) applied to the soil and split in two times in early spring. The nitrogen balance was estimated as the difference between nitrogen inputs and nitrogen outputs. Nitrogen applied with fertilizers, provided by rainwater and supplied by irrigation water were determined as nitrogen inputs. Nitrogen lost by leaching and by ammonia volatilization, and nitrogen removed by crop and pruning, represent nitrogen outputs determined in this work. Also, nitrogen from the net mineralization or immobilization was determined as an input or output, respectively. The nitrogen balance was close determining the difference between inorganic nitrogen (NO3--N and NH4+-N) contents in the soil at the end and at the beginning of each year. Results indicate that, under natural conditions represented by unfertilized plots, nitrogen removed by crop and pruning was equivalent to nitrogen mineralization of soil organic matter. Since nitrogen lost by leaching was very low under these conditions, ammonia volatilization was almost insignificant, and nitrogen supplied by rainwater was higher than these outputs, the results could explain why leaf nitrogen concentrations remained above deficiency for many years in these orchards without nitrogen applications. These results suggest that in fertile soils annual applications of nitrogen are not necessary to produce a good crop. When nitrogen was applied annually to these orchards, nitrogen net mineralization was significantly reduced and nitrogen net immobilization into soil organic matter increased. Simultaneously, nitrogen losses by leaching and ammonia volatilization also significantly increased. These results reveal the disruption of the nitrogen balance when unnecessary nitrogen fertilizers are applied to the orchards, which may cause environmental damage. (C) 2011 Elsevier B.V. All rights reserved.