Effects Of Saline-alkali Soil On Nitrogen Transformations

Salinization is a process of soil degradation that is increasing in importance and attracts worldwide attention. Soils containing high levels of soluble salts may seriously affect agricultural production. The accumulations of soluble salts not only promote nutritional imbalances, but also lower the osmotic potential of the soil solution, possibly to the extent of decreasing crop yields. These types of saline-alkali soil are either derived from natural or anthropogenic causes. Due to shallow buried depth of groundwater table and high evaporation under arid and semi-arid conditions, water loses and salts are retained, which leads to salt accumulation and further to soil salinization. Intensive agricultural practices, such as long-term heavy fertilization and irrigation with saline waters, commonly speed up the development of these unfavorable conditions under arid conditions. Microbial activities, nitrogen transformations and decomposition of organic materials are affected by high concentrations of salts. Particularly nitrogenous fertilizers play an important role in the agricultural production. Nitrogen loses through ammonia volatilization, nitrate leaching, denitrification and other channels due to many reasons, which affects nitrogen utilization ratio. Soil salinization has become a worldwide resource issue and ecological problem. Rational utilization of saline land is an important issue faced by human and a necessary requirement for sustainable development of human society. Therefore, the study of effects of salinity on nitrogen transformations is very important to provide a scientific basis for quantitative evaluation and prediction of nitrogen pollution of groundwater and scientific and rational fertilizer application and improve utilization ratio of nitrogenous fertilizers.The researches on effects of salinity on N transformations started very early abroad, and people began to study in the early 20th century. Foreign scholars have reported the effects of salinization on nitrogen mineralization, nitrification, denitrification, ammonia volatilization, and the research contents were extensive and fruitful. Common influence factors are salinity, exchangeable sodium percentage (ESP), electrical conductivity (EC), etc, among them salt factor includes NaCl2, Na2SO4, NaHCO3, CaCO3, CaSO4, etc. The researches on effects of salinity on N transformations started later interiorly, and had not been reported before 1990s. Despite the late start, few studies, and large research gap between domestic and foreign, the researches involved the effects of salinity on N mineralization, nitrification, denitrification, and ammonia volatilization, etc, and filled the research blanks, and made valuable research results. Currently, domestic influence factors include salt concentration, EC, ESP, etc, and the research level is lower and further to study.The effects of soda salinization on N transformations were examined from two aspects. One was to simulate the effects of NaHCO3 on N transformations as the main component of saline-alkali soil; the other was to simulate the effects of different levels of manual simulated saline-alkali soils on N transformations by manual ratio. The emphasis of the two approaches was different, but the methods were similar, and particularly the second approach was considered as a trial. The two approaches could be compared with each other and authenticated mutually. The contents of the experimental studies included four parts:(1) adsorption experiment, to study the effect of salinization on NH4+-N adsorption; (2) N mineralization experiment, to study the effect of salinization on organic nitrogen mineralization; (3) nitrification experiment, to study the effect of salinization on NH4+-N nitrification; (4) denitrification experiment, to study the effect of salinization on NO3--N denitrification.Experimental results are as follows:(1) There was no significant effect of NaHCO3 on NH4+-N adsorption at low NaHCO3 concentrations (100 mg/L and 300 mg/L); NH4+-N was not conductive to adsorption with NaHCO3 concentrations increasing (500mg/L and 800 mg/L). Under the experimental conditions NH4+-N adsorptions affected by various NaHCO3 concentrations obeyed Langmuir linear adsorption isotherm.(2) N mineralization was inhibited severely by NaHCO3 concentration increasing, and net N mineralization rates decreased, that net N mineralization rate was negatively correlated with NaHCO3 concentration. (3) Nitrification was less affected by low NaHCO3 concentrations; the inhibition of nitrification was severe with NaHCO3 concentrations increasing. Net nitrification rate was negatively correlated with NaHCO3 concentration.(4) There was no significant effect of mixed concentrations changes of NaHCO3 and Na2CO3 on denitrification, and no significant effect on disappearance rate of nitrate was observed. Denitrifying bacteria were with strong saline-alkali tolerance.(5) The effects of manual simulated saline-alkali soils by leaching on N transformations were not satisfactory. Leaching strongly changed the original physical and chemical properties of the soil, and destroyed the indigenous microorganisms and their living environment, to the extent that the microorganisms were difficult to reproduce.