Effects of Nitrification Inhibitor 3,4-Dimethylpyrazole Phosphate (DMPP) on the Nitrogen Use Efficiency of Spinach

Large amounts of nitrogen (N) fertilizers are being applied to optimize yield in vegetable production. Nitrogen use efficiency in vegetable fields is low due to high application of N fertilizers in frequent cultivation, short growth cycles and their shallow rooting system. Nitrification inhibitors (NI) are compounds that retard the biological oxidation of ammonium to nitrite by depressing the activity of Nitrosomonas bacteria in soil. In different studies, the positive effects of these compounds on the reduction of N losses from soil and increase of N use efficiency and crop yield have been demonstrated. The 3,4-dimethylpyrazole phosphate (DMPP)  is a very popular nitrification inhibitor around the world. The efficacy of this molecule depends on climatic conditions and soil properties including of texture, pH, organic matter, moisture, temperature and mineral nitrogen. In this experiment, the effects of NI 3, 4-dimethylpyrazole phosphate on the N use efficiency of two spinach varieties were investigated in different soils.
A pot experiment was conducted in a completely randomized design with a factorial arrangement with three replications at Shahrekord University. Experimental factors were different N fertilizer sources, soil types and spinach varieties. Three N fertilizer sources consisted of urea, ammonium sulfate nitrate (ASN) and ASN plus DMPP (0.8 %). A no added N fertilizer treatment was considered as the control. The soil factor contained three different soils with different physical and chemical characteristics. The textures of the soils No. 1, 2 and 3 were loamy sand, loam and silty clay, respectively. Three selected soils were non-saline (EC1:2=0.14-0.31 dS m-1) and alkaline (pH1:2=7.9-8.0). Organic carbon and calcium carbonate equivalent (CCE) ranged from 0.26 to 0.35%, and 28.5 to 36.2%, respectively. Two spinach varieties were smooth-leaf (Giant Santos) and wrinkled-leaf (Viking). The used soils were mixed homogenously with 100 mg P kg−1 soil as triple super phosphate, 5 mg Fe kg−1 soil as Fe-EDDHA, 15 mg Zn kg−1 soil as ZnSO4.7H2O, 5 mg Mn kg−1 soil as MnSO4.H2O and 2.5 mg Cu kg−1 soil as CuSO4.5H2O. Nitrogen was applied at the rate of 150 mg kg-1 soil in two split doses before sowing and after one month. Twelve seeds were sown in 7 kg soil in plastic pots, and then placed in a greenhouse. The pots were thinned to 7 seedlings per pot after plant establishment. One week before harvesting, 10 measurements were done using a chlorophyll content meter to determine chlorophyll content index of leaves. At the end of the experiment, shoot dry weight was determined and plants were mixed and dried to measure N concentration. Finally, shoot N uptake and N use efficiency were calculated in different treatments.
In the present study, spinach plants fertilized with ASN+DMPP had a better appearance (dark green color) than those grown without DMPP. The results indicated that application of ASN with DMPP led to significant increase of leaf chlorophyll content index in comparison of ASN and urea fertilizers in all studied soils. Application of DMPP slowed down the process of ammonium oxidation to nitrite. Thus, this increase may be due to the role of ammonium in N nutrition of spinach plants treated with DMPP. This may be explained by the fact that ammonium has a positive effect on the synthesis of polyamines, cytokinins and gibberellins. The presence of these two phytohormones retarded senescence and chlorophyll degradation in plants. However, adding ASN to DMPP resulted in a significant decrease of shoot dry weight as compared with the ASN and urea fertilizers in soils No. 1 (loamy sand) and 2 (loam). In soil No. 3, shoot dry weight was not affected in plants fertilized with ASN+DMPP. Also, agronomic and physiological efficiencies of N significantly decreased by applying ASN+DMPP in comparison with ASN. It seems that application of DMPP strongly delayed the ammonium nitrification to nitrate, and consequently the soil nitrate availability appears not to be synchronized with spinach N needs. Due to short growth cycle of spinach, low availability of nitrate resulted in decreased shoot dry weight of spinach. The highest N use efficiency was observed is soil No. 2 (loam) and Giant Santos had more N use efficiency than Viking.
The results demonstrated that using ASN+DMPP led to yield loss, and we cannot recommend its application as a nitrogen fertilizer for spinach. However, application of ASN+DMPP is an effective strategy for improving qualitative appearance (dark green color) of spinach. Also, all studied indices were not affected in plants fertilized with ASN and urea. Therefore, application of both fertilizers is recommended for spinach production under similar conditions of the present study.