Efficacy of Dicyandiamide as a Soil Nitrification Inhibitor in Citrus Production

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Abstract
Nitrification inhibitors such as dicyandiamide (DCD) help reduce leaching losses by retaining applied N in the ammoniacal form. Research objectives were to evaluate dicyandiamide added to ammonium sulphate nitrate (ASN) as a nitrification inhibitor in cultivated soils (Xeropsamments) and its effect on N uptake by citrus (Citrus sinensis (L.) Osbeck). In a greenhouse experiment, 10 g N as ASN either with or without 2% of DCD‐N were applied to plants grown in 50‐L pots filled with peat and sand (2:1). Addition of DCD to ASN resulted in significantly higher levels of NH+4‐N in the substrate than the ASN treatment, for at least 100 d. Drainage water from the ASN‐DCD treated pots was lower in NO−3‐N concentration during the first 60 d. Inclusion of DCD with ASN resulted in greater uptake of the fertilizer N by citrus plants. Under field conditions, fertilization of adult trees with ASN (600 g N tree−1) either with or without DCD (2% DCD‐N) was compared. In the ASN‐amended soil, nitrification was much more rapid than when DCD was added to the ASN. Fertilization with ASN + DCD maintained relatively high levels of NO−3‐N in soil longer than ASN alone. In another experiment, 60 g N as ASN either with or without DCD (2% DCD‐N) were applied to 3‐yr‐old citrus plants grown individually outdoors in containers and the distribution of NH+4‐N and NO−3‐N in the soil profile after consecutive flood irrigations was monitored. After the third irrigation, 68% of applied N was leached below 0.45 m in ASN amended soils, whereas only a very small amount of the applied N was retained above this depth. When DCD was added, only 20% of the applied N was lost by leaching at this time. The DCD nitrification inhibitor improved the N fertilizer efficiency and decreased NO−3 losses so as to minimize the economic and environmental risks that are inherent in irrigated production of citrus.

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