Ammonium Oxidation Kinetics in the Presence of Nitrification Inhibitor Dicyandiamide (DCD) in some Calcareous Soils of Chaharmahal va Bakhtiari Province

Nitrification inhibitors (NIs) are compounds that retard the biological oxidation of ammonium to nitrite by depressing the activity of Nitrosomonas bacteria in the soil. Many popular NIs such as nitrapyrine (NP), dicyandiamide (DCD) and 3,4-dimethylpyrazole phosphate (DMPP) are produced and used in agricultural soils. Dicyandiamide is a very popular NI in some of the world countries. It delays nitrification process in the soil through its bacterial static property. It is easy to blend with commercial fertilizers such as urea, due to its low volatile nature. Application of urea in combination with nitrification inhibitor DCD lengthens nitrogen presence in soil as ammonium form. It has several beneficial effects for agriculture and enhances environmental protection. Studying the ammonium oxidation kinetics in the presence of nitrification inhibitor DCD can provide the experts in agriculture with very useful information regarding the ammoniumdurability in different soils. This research has been done to study the effect of using NI dicyandiamide on the kinetics of ammoniumloss in some calcareous soils of Chaharmahal Va Bakhtiari province, Iran.
This research was conducted as factorial using completely randomized design with two factors of nitrogen fertilizer type and soil type with three replications at laboratory conditions. In this experiment, nitrogen fertilizer type included 2 levels of: 1- urea 2- urea plus nitrification inhibitor DCD (3.2%). A no added nitrogen fertilizer was considered as control treatment.The soil factor also consisted of 5different soils with a wide variation in soil physical and chemical characteristics. Five selected soils were non-saline (EC1:2=0.14-0.76 dS m-1) and alkaline (pH1:2=7.5-8.2). Organic carbon and cation exchange capacity (CEC) ranged from 0.48 to 2.34% and 10 to 30 cmolc kg-1, respectively. The dose of applied nitrogen in all experimental treatments was 50 mg kg-1 N as urea. Forty-five containers containing different soils were incubated at 20°C for 105 days. At 1, 7, 14, 21, 28, 35, 49, 63, 77, 91 and 105 days after adding urea and ureaఠ, soil subsamples were extracted to determine ammonium content. The ammonium concentration (extracted with 0.5 M K2SO4) was determined colourimetrically using a spectrophotometer at a wavelength of 667 nm. Then zero, first and second order equations were calibrated on the residual ammonium in the soil using SAS 8.02 and the best equation was seleted on the basis of coefficients of determination (R2) and standard error of the estimate (SE). In addition, ammonium half-time and nitrification inhibitor index were calculated.
The results indicated that the first order equation was able to describe ammonium oxidation kinetics of the soil in all of the experimental treatments (control, urea and ureaఠ). The average values of R2 and SE of first order equationwere 0.915 and 1.51 in control, 0.903 and 4.98 in urea treatment and 0.863 and 4.92 in ureaఠ treatment, respectively. It means that ammonium oxidation kinetics is dependent on the ammonium concentration in soil. In all study soils, the slope of the first order equation in the urea treatment with DCD has been less in comparison to similar treatment but without NI. This may be explained by the fact that application of DCD has slowed down the process of ammonium oxidation to nitrite. The application of urea with DCD resulted in increase of ammonium half-life (calculated with first order equation) in the soil comparing to urea fertilizer without NI in all of the studied soils. The amount of this increase for DCD was 34.8, 31.6, 31.1, 25.1 and 40.4 days for the soils number of 1 2, 3, 4, and 5, respectively. Increasing the presence of ammonium in soil can be considerable for agricultural and environmental purposes. The maximum nitrification inhibitor indexes were 11.3% and 28.1% after 28 days of incubation in soils number 1 and 3, respectively. These nitrification inhibitor index values are in agreement with observations by other researchers.
The results showed that nitrification inhibitor DCD is compound with a high capacity for extending ammonium presence in studying soils under conditions of this experiment. However, its efficiency was dependent to physical and chemical properties of soil.According to the results, first order equation was the best equation for describing ammonium oxidation kinetics in tested soils fertilized with urea and ureaఠ.