Available online at www.inia.es/sjar ISSN: 1695-971-X Improvement of N fertilization by using the nitrification inhibitor DMPP in drip-irrigated citrus trees

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 concentra-tions 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 Pon-cirus 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 % NH4+-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 NH4+-N concentration in the 0-20 and 20-40 cm soil layers was significantly higher in the AS+NI treatment. By contrast, the NO3--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