Nitrogen Utilization And GHG Emissions Under Reduced Nitrogen Fertilization In The Semiarid Loess Plateau

The concentrations of green house gas（GHG） in the atmosphere has an increasing trend year by year. Global climate change has attracted great attention of government all over the world. The agricultural soil was the main source of GHG such as N₂O and CH₄, of which the total emissions respectively account for 60% and 50% of the total global emission. Dry land areas account for 70% of China’s total territory, and areas of the semiarid arable land account for 43% of total arable land. The Loess Plateau in the northwest belongs to classical rain-fed agricultural area, has an area of 600 million km², with farmland area of 145.8 million km², of which 70% belongs to rainfed agricutural area. Chemical fertilization was the main measure of this area to improve soil fertility and ensure grain yield. However, effects of chemical fertilization on GHG emissions from agricultural source remian unclear. Therefore, the mechanism and influencing factors of grain yield and GHG emissions under different nitrogen fertilization regimes were explored to provide theoretical foundation for the area to establish scientific and reasonable farmland fertilization treatments.Taking the spring maize（Xianyu 335） as a case study, the experiment were established in the Changwu State Key Agro-ecological Experimental Station, Changwu, Shannxi, China. Five nitrogen fertilization treatments were constructed in the rain-fed spring maize cropland, including control（CK）, conventional N fertilization rate（Con）, optimal N fertilization rate（Opt）, optimal N fertilization rate plus nitrification inhibitor（Opt+DCD）, and optimal N fertilization rate with slow release urea（Opt+SR）, to determine the effect of reduced nitrogen（N） fertilization practices on greenhouse gas emissions, grain yields and soil nitrate and ammonium nitrogen contents in the Loess Plateau, China. In the three-year study（April 2013 to September 2015）, the GHG was sampled and determined with a static chamber and gas chromatography-based system, and the soil nitrate and ammonium nitrogen contents were measured with continuous flow analyzer, with a total of 9000 gas samples, 2700 topsoil samples, 2700 profile soil samples and 15 sets of plant samples were sampled. In the study, variance analysis and correlation analysis were used to analyze data acquired from laboratory. The results were summarized as follows:（1） Compared with Con, nitrogen fertilization amount of Opt、Opt+DCD �'� Opt+SR were generally consistent reduced by 25% with the spring maize yield maintained, which significantly increased the nitrogen fertilizer agronomic efficiency, partial nitrogen fertilizer productivity, and physiological efficiency（P<0.05）. The yield of each treatment ranged from 10.1 to 10.46 t·ha^-1 for 2013, 11.41 to 12.23 t·ha^-1 for 2014 and 17.59 to 17.72 t·ha^-1 for 2015. Nitrogen fertilization significantly increased soil water consumption; however, no significant difference among treatments was observed for the soil water consumption of spring maize.（2） Nitrogen fertilization significantly increased N₂O emission rates during spring maize growth（P<0.05）. All the emission rates under different nitrogen fertilization regimes increased rapidly after fertilization application, which reached maximum within two to three days and remained high up to 10 days and fell rapidly; and the emission rates maintained low all over the growth period since then. The emission amounts of N₂O within 10 days after nitrogen fertilization account for 20% of the total emission all year round. Furthermore, the rainfall events that more than 40 mm were the main influencing factor of N₂O emission. The cumulative emissions of rainfall-induced N₂O accounted for 6.4%, 12.5% and 9.6% for 2013, 2014 and 2015, respectively. The nitrate nitrogen content showed a positive correlation with N₂O contents within 10 days after fertilization, with a multi-linear equation of Y=a X+b（R2=0.80⁰.84,P<0.05）. It was concluded that the agricultural soil in the Loess Plateau was the absorption sink of CH₄, with no significant effect of nitrogen fertilization on CH₄ absorption observed.（3） Reduced nitrogen fertilization significantly reduced the cumulative emissions of agriculture-source GHG（P<0.05）. Compared with the Con(1.87 kg N₂O-N · ha^-1), the greatest decrease was in the Opt+DCD（46.0%）, followed by Opt+SR（34.7%）, and then the Opt（24.6%）. The GWP was 838、619.7、358、457 kg CO₂-eq · Mg-1 respectively for the Con, Opt, Opt+DCD and Opt+SR. Compared with the Con, the total GHG emission intensity of the Opt, Opt+DCD and Opt+SR was significantly reduced by 24.3%, 55.0% and 43.4%, respectively（P<0.05）.（4） Nitrogen fertilization substantially elevated the cumulative soil profile nitrate nitrogen（P<0.05）. The content of soil profile nitrate nitrogen ranged from 33.5 to 148.9 kg·ha^-1 for 0-100 cm and 8.9 to 92.8 kg· ha^-1 for 100 200 cm, and averaged at 72.7 kg·ha^-1 and 49.7 kg·ha^-1, respectively. Compared with the Con, the contents of soil profile nitrate nitrogen were significantly reduced by 24.3%, 55.0% and 43.4%（P<0.05）, respectively for the Opt, Opt+DCD and Opt+SR.