Effect Of Biochar On Nitrogen Migration And Ammonia Oxidation In Farmland Soil

The nitrogen fertilizer was widely used in agriculture in order to obtain higher grain yield, which also brought a lot of negative effects. The low fertilizer-nitrogen use efficiency lead to nutrients leaching, pollute the surface water and ground-water resources which was the main form of expression. Such problems posed a significant threat to the agricultural environment. Therefore, an economic and efficient method to absorb the soil nitrogen was an urgent problem to solve, which could reduce nitrogen loss from the soil, improve nitrogen utilization rate and control the inorganic nitrogen pollution.In recent years, the biochar as a soil conditioner had attracted more and more attention. The main raw materials were plants or other agricultural wastes. The biochar was produced in limited or absence of oxygen under high temperature conditions, which was rich in carbon content, had stable structure and large specific surface area. The biochar has stronger ability to adsorb NO3-and NH4+. In order to explore the adsorption characters of inorganic nitrogen in aqueous solution by maize straw- and corn cob-derived biochars, the adsorption kinetics of NH4+-N, NO3--N and NO2--N were studied. The adsorption processes of NH4+-N and NO3--N were fitted by Langmuir and Freundlich isothermal adsorption models, and the adsorption mechanisms were also elucidated. The results showed that the maize straw- and corn cob-derived biochars produced at 400℃ and 600℃ were both alkaline(400℃<600℃). As for the same raw material, the biochar produced at 600℃ showed relatively higher alkaline oxygen-containing functional group content and lower acidic oxygen-containing functional group content compared with the biochar produced at 400℃. The biochars produced at 400℃ had a stronger adsorption capacity to NH4+-N(the equilibrium adsorption amounts of maize straw- and corn cob-derived biochars were 4.22 and 4.09mg/g, respectively). However, the biochars produced at 600℃ had a stronger adsorption capacity to NO3--N and NO2--N(for NO3--N: the equilibrium adsorption amounts of maize straw- and corn cob-derived biochars were 0.73 and 0.63mg/g, respectively; for NO2--N: 0.55 and 0.35mg/g, respectively). Compared to NO3--N and NO2--N, all the four kinds of biochar showed stronger adsorption capacity to NH4+-N, and the equilibrium adsorption amounts of NH4+-N were 4.29-20.2 times more than NO3--N/NO2--N. The adsorption of NH4+-N and NO3--N were in accord with pseudo second-order kinetic model, which was mainly based on chemical adsorption. The isothermal adsorption model study showed that the adsorption of NH4+-N and NO3--N in aqueous solution by maize straw- and corn cob-derived biochars could be described by Freundlich model, and the multi-layer adsorption was the major adsorption mechanism.In order to study the fresh and aged maize straw-derived bochars’ effects on soil nitrogen migration, the column leaching experiment was conducted. The biochar could reduce the leaching of soil inorganic nitrogen significantly during the leaching events. Inorganic nitrogen concentration and p H showed a single peak, which increased at first and then decreased.Compared with the control, the peak time of NH4+-N leaching was delayed, which achieved a sustained release effect. And the role of the aged biochar was more significant than the fresh biochar. For NO3--N, the peak time of adding aged biochar was same with the control, but the peak time of adding fresh biochar advanced with the control. Therefore the aged biochar had important significance to reduce nitrogen loss and improve nitrogen utilization rate. In the leaching experiment of 40 days, the potential ammonia oxidation rate and ammonia-oxidizing bacteria amount and urease activity of the soil surface were larger compared to the fresh biochar’ column, while was opposite in the deep column(>5cm). The reason may be the aged biochar had a stronger adsorption capacity to NH4+-N and more inhibition of migration. The distribution of potential ammonia oxidation rate and ammonia-oxidizing bacteria amount and urease activity of the different soil column depths may be related to the vertical migration and distribution of NH4+-N content.In order to explore the effect of wheat straw-derived biochar on farmland soil ammonia oxidation, wheat straw-derived biochar was mixed with soil by the proportion of 0%, 2%, 5%, 10%. The soil potential ammonia oxidation rate, ammonia-oxidizing bacteria amount, urease activity and p H were determinated at regular intervals, which was aimed at proclaiming the mechanism of.the change of soil ammonia oxidation under the addition of biochars. The alkaline soil potential ammonia oxidation rate was promoted significantly after adding biochar. More biochar was added, the effect was more obvious. The soil potential ammonia oxidation rate increased with the culture time. Except soil NH4+-N content, ammonia-oxidizing bacteria amount, urease activity, p H and NO3--N content all showed increasing trends during the incubation. The correlation analysis indicated that soil potential ammonia oxidation rate and ammonia-oxidizing bacteria amount, urease activity, p H, NO3--N content was in significantly positive correlation(r>0.709, P<0.01), and with NH4+-N content was in significantly negative correlation(r=-0.413, P<0.01); The enhanced ammonia oxidation induced by biochar incorporation in arable loess soil would aggravate NO3- leaching to groundwater and impair ammonium bioavailability in agriculture.