Effects Of Long-term Fertilization On Soil Nitrification And N₂O Emission In The Semiarid Loess Plateau

Nitrification and denitrification are both hot topics in the environmental researches.Recently, more attention was paid on improving nitrogen fertilizer use efficiency and reducingsoil N loss in upland agricultural soil in Loess Plateau. Most of the researches were focusedon the effects of different fertilization on soil nitrification, but rarely on denitrification process.Denitrification is one of the most important sources for N₂O emission in the nitrogen cycle.Therefore, it’s meaningful to investigate how the soil nitrification and denitrificationprocesses contribute to the whole nitrogen cycle and greenhouse gas emission. The study inthis thesis is based on a long-term fertilization experiment at Changwu Agri-ecologicalStation, in Changwu County, Shaanxi Province. We analyzed the soil water character, soilchemical, physical and other biological properties firstly. Then the soil samples wereincubated aerobically in the lab to investigate the differences in soil nitrification bycontrolling water content and incubation temperature. At the same time, an anaerobicincubation was done by monitoring NO, N₂O and N₂gases from denitrification. And also theabundance and community distribution of nitrifying microorganisms were compared underdifferent fertilization treatments. The results and conclusion of this thesis is important forimproving the fertilization managements and nitrogen use efficiency in field, and reducing thegreenhouse gas emission. The main results of the thesis are listed below:（1） Long-term fertilization changed the soil nutrient content. In manure treatments,higher values of organic carbon, total N and total P were found comparing to non-manuretreatments. Higher values of available P were found in P treatment and manure treatments.Higher values of available K were found in manure treatments. But there were no significantdifferences in soil pH and total K among fertilization treatments.（2） Significant differences were found between fertilization treatments on soil watercharacterize curve. Planting had great effect on soil water characters. Fallow soil had greaterwater holding ability than planted soils, but this can be improved by manure input.（3） The soil respiration rates were higher in manure treatments. A significant positivecorrelation was found between soil respiration rate and soil organic carbon content, showedthe relationship between fertilization and microbial activities. （4） Soil water content and incubation temperatures had big effects on soil nitrification.The nitrification process was stimulated under24%moisture condition. Lower nitrificationrates were found when incubations were done in15℃than in28℃. Lower temperature couldlower the nitrification potential and the maximum nitrification rate, and also lengthen the timewhen the maximum rate appeared.（5） Long-term fertilization had significant effects on soil nitrification. Either mineral Ninput or manure input could stimulate the nitrification process. No effect of P treatment wasfound in this study.（6） Long-term manure input induced the soil respiration rate and microbial activities,therefore induced the soil NO and N₂O production rates. Since the soil N₂production rate wasalso stimulated by manure input, no significant differences of denitrification product ratioN₂O/（N₂O+N₂） were found between manure and non-manure treatments. Comparing to theother non-manure treatments, mineral N input did not change denitrification rate so much, buthad a higher denitrification product ratio N₂O/（N₂O+N₂）. Mineral N input together withmanure reduced the effects of N fertilizer.（7） Significant differences of abundance were found with ammonia-oxidizer archaea（AOA） between fertilization treatments, but not in ammonia-oxidizer bacteria （AOB）. TheamoA gene copy numbers of AOB were between1.326×106¹.886×106copies·g1dry soil;the arch-amoA gene copy numbers of AOA were between1.329×10⁶⁴.510×10⁶copies·g^-1dry soil, and the differences between fertilization treatments were: NPM>NP>CK>P. Thearch-amoA gene copy numbers in NPM treatment is3.314times higher than in control soil.（8） Detrended canonical correspondence analysis （DCCA） was adopted to classtreatments by their environmental similarities, which indicated that P and NPM, CK and NPwere presented as two groups respectively; Canonical correspondence analysis （CCA） of4treatments and12environmental factors verified that the activity of ammonia oxidizer andammonia oxidation in different fertilizations was ordered as NPM>NP>CK>P.（9） Relationships were found between different environmental factors and theenvironmental similarity ordination of the treatments. The environmental factors reflecting theammonia oxidation process （such as the NO₃---N concentration, abundance of AOA and AOB）,and factors related to microbial growth conditions （such as the soil pH, moisture, totalnitrogen and organic carbon content） contributed mostly.