Nitrogen management in orchards should tend to improve the fertilizer N use efficiency for a sustainable agriculture where productivity, fruit quality and environment are reconciled. The use of specific nitrification inhibitors could increase the N fertilizer uptake and decrease the potential groundwater pollution by nitrate leaching. The aim of this experiment was to assess the effect of application frequency of the ammonium sulphate (AS) and the nitrification inhibitor (NI), 3,4-dimethylpirazole phosphate (DMPP) supply on: nitrate-N and ammonium-N seasonal changes in soil; N and Fe concentrations in the spring-flush leaves; and yield and fruit quality. The experiment was carried out with clementine cv. Nules (Citrus clementine Hort. Tanaka x Citrus reticulate Blanco) mandarins grafted on Troyer citrange (Citrus sinensis x Poncirus trifoliata) rootstock under field conditions during three consecutive years. The trees were fertilized with 324 Kg N ha -1 from which 192 Kg N ha -1 were applied as AS (21% NH 4 + -N) either with or without NI, and the remainder N came from irrigation water. The AS and AS+NI were split into 1, 2 or 4 applications per month by drip irrigation. The NH 4 + -N concentration in the 0-20 and 20-40 cm soil layers was significantly higher in the AS+NI treatment. By contrast, the NO 3 --N concentration was significantly higher in the soil treated only with AS. Moreover, the addition of NI to AS originated a significantly higher N and Fe concentrations in the spring-flush leaves. The yield was higher and some fruit quality parameters improved in trees fertilized with AS+NI compared to those fertilized only with AS.Additional key words: ammonium sulphate, fruit quality, fruit yield, N frequency, N soil. ResumenOptimización de la fertilización nitrogenada en cítricos en riego localizado mediante el inhibidor de la nitrificación DMPP Las prácticas agrícolas de fertilización nitrogenada deben mejorar la eficiencia de uso del nitrógeno (N) en una agricultura sostenible en que una mayor producción y calidad sea compatible con el medioambiente. El uso de inhibidores de la nitrificación (NI) podría incrementar la eficiencia de absorción de N disminuyendo la contaminación de las aguas subterráneas por lixiviación de nitratos. Este trabajo analiza el efecto de la frecuencia de aplicación del sulfato amónico (AS) y del inhibidor de la nitrificación 4-dimethylpirazol fosfato (DMPP) sobre los cambios estacionales del N-nítrico y N-amó-nico del suelo, la concentración de N y hierro en hojas de la brotación de primavera y la producción y calidad del fruto. El ensayo se llevó a cabo en una parcela comercial de clementina cv. Nules (Citrus clementine Hort. Tanaka x Citrus reticulata Blanco) injertadas sobre citrange Troyer (Citrus sinensis x Poncirus trifoliata) durante tres años consecutivos. Los árboles se fertilizaron con 324 Kg N ha -1 ; 192 Kg N ha -1 se aplicaron como AS (21% NH 4 + -N) sin y con NI, fraccionados en 1, 2 y 4 aplicaciones por mes, el resto procedió del agua de riego. La concentrac...

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