Mahbubeh Gheitasi; Ali Reza Hosseinpur
Abstract
Introduction: Leafy vegetables such as spinach (Spinaciaoleracea L.) contain high levels of nitrate. Using nitrification inhibitors (NIs) such as 3,4-dimethylpyrazole phosphate (DMPP) is one of the strategies for reducing nitrate accumulation. Nitrification inhibitors are compounds that delay the biological ...
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Introduction: Leafy vegetables such as spinach (Spinaciaoleracea L.) contain high levels of nitrate. Using nitrification inhibitors (NIs) such as 3,4-dimethylpyrazole phosphate (DMPP) is one of the strategies for reducing nitrate accumulation. Nitrification inhibitors are compounds that delay the biological oxidation of ammonium to nitrite by depressing the activity of Nitrosomonas bacteria in soil. Soil properties such as texture, pH, organic matter, moisture, temperature and mineral nitrogen have important effects on the efficiency of NIs to delay nitrification. A pot experiment was conducted to investigate the effects of NI 3,4-dimethylpyrazole phosphate (DMPP) on soil mineral nitrogen (ammonium and nitrate) content, yield and nitrate concentration of spinach.
Materials and Methods: A completely randomized factorial design was carried out employing three factors consisted of nitrogen fertilizer type, soil type and spinach variety with three replications at Shahrekord University. Nitrogen fertilizers included urea, ammonium sulfate nitrate (ASN) and ASN plus DMPP (0.8 %). A no N fertilizer application was considered as control treatment. The soil factor contained 3 different soils with different physical and chemical characteristics. Two spinach varieties were smooth-leaf (Giant Santos) and wrinkled-leaf (Viking). The dose of applied nitrogen in all experimental treatments was 150 mg kg-1 soil that was applied in two split doses before sowing and after one month. The textures of three selected soils were loamy sand, loam and silty clay for the soils number 1, 2 and 3, 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. At 30 and 60 days after sowing, soil subsamples were taken to determine ammonium and nitrate content. The ammonium and nitrate concentrations (extracted with 0.5 M K2SO4) were determined calorimetrically using a spectrophotometer at a wavelength of 667 and 410 nm, respectively. At the end of the experiment, shoot fresh weight was determined and plants was mixed and dried to measure nitrate accumulation.
Results and Discussion: The results indicated that the application of ASN with DMPP led to significant increase of ammonium compared with ASN and urea fertilizers in three soils. At 30 days after sowing, the amount of this increase for ASN plus DMPP in comparison of ASN and urea were 182% and 78% for the soil number 1 (loamy sand), 105% and 65% for the soil number 2 (loam) and 89% and 74% for the soil number 3 (silty clay), respectively. By contrast, the application of ASN with DMPP led to significant decrease of soil nitrate in comparison of ASN and urea fertilizers in three soils. At 60 days after sowing, the amount of this decrease for ASN plus DMPP in comparison of ASN was 52%, 40% and 27% for the soils number of 1, 2 and 3, respectively. It means that the application of DMPP has slowed down the process of ammonium oxidation to nitrite. In fact, the addition of DMPP retained soil nitrogen as ammonium form for longer time. The application of NI DMPP also had positive effect on decrease of nitrate concentration in the soil. Unlike nitrate, ammonium is less susceptible to leaching and thus the application of DMPP can reduces nitrogen loss from the soil. However, the application of ASN with nitrification inhibitor DMPP in soils No. 2 (loamy sand) and No. 3 (loamy) significantly reduced shoot fresh weight of both spinach varieties compared with the similar treatment but without NI. This decrease was due to the toxic effects of high level of soil ammonium on the plant growth. While, in the soil No. 3 (silty clay) in Viking variety, the use of ASN plus DMPP resulted in significant increase of spinach shoot fresh weight to 29% in comparison with the same treatment but without NI. The highest and lowest values of shoot fresh weight (229 and 16.2 g pot-1, respectively) were obtained by Giant Santos variety in soil No. 3 (silty clay) with ASN plus DMPP and soil No. 1 (sandy loam) with no added N fertilizer. The application of ASN with nitrification inhibitor DMPP induced significant decrease of shoot nitrate concentration in spinach in comparison of ASN and urea. The amounts of this decrease for ASN plus DMPP in comparison with ASN and urea were 25.7% and 31.5% for the soil number 1 (loamy sand), 29.1% and 37.1% for the soil number 2 (loam) and 33.9% and 34.0% for the soil number 3 (silty clay), respectively. This decrease was due to ammonium nutrition of spinach plants.
Conclusion: In all studied soils, application of ASN with nitrification inhibitor DMPP is recommended for diminishing nitrate content in both spinach varieties (Giant Santos and Viking).
Fatemeh Rakhsh; Ahmad Golcchin
Abstract
Introduction: Mobilization and stabilization of organic matter in soils represent a set of complex processes involving the processing and decomposition of organic matter by diverse communities of soil fauna and microorganisms, as well as chemical-physical interactions with mineral particles of soil. ...
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Introduction: Mobilization and stabilization of organic matter in soils represent a set of complex processes involving the processing and decomposition of organic matter by diverse communities of soil fauna and microorganisms, as well as chemical-physical interactions with mineral particles of soil. Clay minerals have high effects on the soil organic matter dynamics. Clay minerals with the physical protection of organic matter play an important role in reducing the rate of decomposition of organic matter. The effects of soil texture on the soil organic matter dynamics have been investigated in many studies, but the effects of exchangeable cations and clay types on mineralization of organic nitrogen and microbial biomass nitrogen have not been given much attention. For this reason, the aim of this study was to evaluate the effects of types and clay contents and exchangeable cations on the mineralization of organic nitrogen and microbial biomass nitrogen.
Material and Methods: Appropriate amounts of homoionic Na-, Ca- and Al- clays from Georgia kaolinite, Illinois illite and Wyoming montmorillonite were mixed with pure sand to prepare artificial soils with different clay contents, exchangeable cations, and clay types. The artificial soils have zero, 5 and 10% clay from Georgia kaolinite, Illinois illite and Wyoming montmorillonite that their clay minerals saturated with Ca, Na and Al. Alfalfa plant residues were incorporated into the artificial soils and the soils were inoculated with microbes from a natural soil and incubated for 60 days and concentration of NH4-N and NO3-N were measured every 15 days. In the artificial soil samples, microbial biomass nitrogen was measured by the fumigation-extraction method in the end time of incubation period.
Results and Discussion: The results of this study showed that the percentage of mineralized nitrogen in the two-month incubation period, was higher in the pure sand than in soils containing 5% and 10% clay, indicating that clay contents influence the capacity of soils to protect and store organic nitrogen. Microbial biomass nitrogen increased as the amount of clay in the soil increased. The highest and lowest amounts of microbial biomass nitrogen measured in soils with 10% clay (9.26 mg per 50 g dry soil) and pure sand (4.31 mg per 50 g dry soil), respectively. There was a significant influence of exchangeable cations on the percentage of mineralized nitrogen and microbial biomass nitrogen. The microbial biomass nitrogen and the percentage of mineralized nitrogen were highest in Ca-soils and lowest in Al-soils. The percentage of mineralized organic nitrogen in two months of incubation period was highest in soils with Georgia kaolinite clay and lowest in soil with Wyoming montmorillonite clay. The amounts of microbial biomass nitrogen in soils with Wyoming montmorillonite clay were lower than soils with Georgia kaolinite and Illinois illite clays. The percentage of mineralized organic nitrogen increased as the incubation period increased. The results of this study indicated that organic nitrogen mineralization rates and microbial biomass nitrogen were affected by types and clay contents and exchangeable cations and interaction of organic matter with clays and is an important process as it slows soil organic matter decomposition.
Conclusions: Mixing the alfalfa residues with artificial soils and incubation samples allowed to study the effects of types and clay contents and exchangeable cations on the percentage of NH4+-N, NO3--N, mineralized nitrogen, and microbial biomass nitrogen. Soils with different clay contents have different surface areas and cation exchange capacities; therefore, it is concluded that organic nitrogen storage of soils is, partly, controlled by the surface areas, cation exchange capacity and physical protection provided by the soils. Nitrogen mineralization and the amounts of microbial biomass nitrogen were different in soils with different exchangeable cations. It is concluded that exchangeable cations exert their influence on microbial biomass and hence nitrogen dynamics by controlling the size and activity of the microbial population through modifying the physicochemical characteristics of microbial habitats. Since various clay minerals have different specific surface areas and cation exchange capacity and the physicochemical changes induced in the soil environment as a result of variations of exchangeable cations is much greater in soils with higher cation exchange capacity and specific surface area. It seems the effects of clay mineralogy on the dynamics of organic materials and microbial biomass, in part, arise from the type of exchangeable cations present on the exchange sites of the clay minerals.
