Relative Phytotoxicity of Dicyandiamide and Availability of its Nitrogen to Cotton, Corn, and Grain Sorghum 1

The nitrification inhibitor, dicyandiamide (cyanoguanidine) (DCD), can improve fertilizer N efficiency; however, yield reductions and phytotoxicity from the use of DCD have been reported. A green­ house experiment was designed to determine the effect of DCD on growth, chlorophyll concentration and nutrient concentration of corn ( Zea mays L.), cotton ( Gossypium hirsutum L.), and grain sorghum [ Sorghum bicolor (L.) Moench.]. Dicyandiamide-N/urea-N combi­ nations of 0:60, 5:55, 10:50, 20:40, 30:30, 40:20, 50:10, and 60:0 mg kg soil were applied to pots containing a Norfolk sandy loam (fine­ loamy, siliceous, thermic, Typic Paleudults) cropped to each of the three species. Increasing the proportion of N as DCD decreased plant dry weight and leaf chlorophyll concentration and increased stem-leaf N concentrations. Nitrogen recovery decreased curvilin­ early from 103% to 4%. from 64% to -6%. and from 72% to 4% for corn, cotton, and sorghum, respectively. with increasing propor­ tion of N as DCD-N. The effects of DCD-N on stem-leaf tissue concentrations of P, K, Ca, Mg, Fe, Mn, Zn, and Cu varied with DCD-N concentration, plant species, and nutrient element. At lower DCD-N concentrations, most nutrient element concentrations were affected by uptake of NH; -N derived from urea; while higher con­ centrations of DCD-N resulted in increased nutrient element con­ centrations as a result of reduced plant growth. Additional Index Words: Zea mays, Gossypium hirsutum, Sorghum bicolor, nitrifiication inhibitor, DCD, nutrient composition, chloro­ phyll. Reeves, D.W., and J.T. Touchton. 1986. Relative phytotoxicity of dicyandiamide and availability of its nitrogen to cotton, corn. and gram sorghum. Soil Sci. Soc. Am. J. 50:1353-1357. D (cyanoguanidine) (DCD), C2H4N4, is a dimer of cyanamide, and is an effective ni­ trification inhibitor (Hauck, 1980; Nommik, 1958; Reddy, 1964a; Rodgers and Ashworth, 1982). In ad­ dition to its nitrification inhibiting properties, DCD contains 67% N. In soil, DCD undergoes decompo­ sition to ammonium and nitrate (Reider and Mi­ chaud, 1980; Amberger and Vilsmeier, 1979), and the nitrogen in DCD is thus eventually plant-available. The decomposition rate increases with organic carbon (Reddy, 1964a), Fe hydroxides (Amberger and Vils­ meier, 1979), and temperature (Vilsmeier, 1980). Data from Reddy (1964a) indicated that in the presence of an organic carbon source (sucrose), substantial min­ eralization of 67 mg kg DCD-N took place in a Cecil sandy loam after only 15 d. Reider and Michaud (1980) reported that rapid mineralization, in three soils (DCD­ N, to NH; -N and NO3-N) began after 28 d and was complete after approximately 70 d. Contribution of USDA-ARS, Soil-Plant Interaction Research Unit, in cooperation with Dep. of Agronomy and Soils, Alabama Agric. Exp. Stn., Auburn Univ., AL 36849. Received 30 Jan. 1986. Research Agronomist, USDA-ARS, and Associate Professor, re­ spectively, Auburn Univ., AL 36849. Although DCD is an effective nitrification inhibitor and can improve fertilizer N efficiency, yield reduc­ tions and phytotoxicity from the use of DCD have been reported. Cowie (1918) reported toxicity symp­ toms in pot trials with barley ( Hordeum vulgare L.) from concentrations of nitrogen as DCD (DCD-N) ex­ ceeding 18 mg kg soil. In field trials, however, no injurious effects were noted from 30 kg ha of DCD­ N. Nommik (I 958) noted leaf injury symptoms in oats ( Avena sativa L.) when rates of DCD-N exceeded 28 kg ha. Reddy (1964b) reported that 16.7 mg kg DCD-N decreased dry matter yields of wheat ( Triti­ cum aestivum L.), oats ( Avena sativa L.), maize ( Zea mays L.), cotton ( Gossypium hirsutum L.), and to­ mato ( Lycopersicon esculentum Mill.), but Increased yields of Coastal bermuda grass [ Cynodon dactylon (L.) Pers.]. The decreases in dry weights were, in part, de­ pendent on the interaction between plant species and N source. In a field experiment with wheat (Sommer and Rossig, 1978), DCD in combination with urea increased N uptake, but reduced grain yields by 10%. Maftoun and Sheibany (1979) reported that DCD de­ creased green and dry tissue weights of pot-grown soy­ beans [ Glycine max (L.) Merr.]. The soil concentra­ tion of DCD-N needed to reduce fresh and dry weights by 50% was 72 and 58 mg kg, respectively. Visual symptoms of DCD phytotoxicity, other than reduced growth, on wheat (Allison et al., 1925; Reddy, 1964b), barley (Cowie, 1918), oats (Nommik, 1958; Reddy, 1964b), and corn, tomato, and cotton (Reddy, 1964b) include leaf tip and margin chlorosis and necrosis. These symptoms suggest that one effect of DCD phy­ totoxicity may be reduced synthesis or increased deg­ radation of chlorophyll. The only report of DCD phytotoxicity to corn and cotton limited DCD-N rates to 3.3, 6.7, and 16.7 mg kg (Reddy, 1964b). No efforts have been made to quantify DCD’s phytotoxicity at rates that could result from banded application of N formulated with DCD. In addition, the relative phytotoxicity of DCD to grain sorghum [ Sorghum bicolor (L.) Moench.] has not been