Effects of different nitrogen application levels on nitrous oxide emission from the soil of a ‘Kuerlexiangli' pear orchard

Abstract: 【Objective】‘Kuerlexiangli'pear is a characteristic fruit industry in Xinjiang, where there is a large scale production. However, nitrogen content in the soils is low in‘Kuerlexiangli'pear orchards. Through the application of nitrogen fertilizer, soil nitrogen contents can be increased to meet the nitrogen demands of pear trees. In order to increase the yield of‘Kuerlexiangli'pear, farmers often use large amounts of fertilizer, which are excessive for the demands of pear trees. Excessive fertilization leads nitrogen enrichment in the soil, and reduces nitrogen use efficiency. The surplus nitrogen in the soil will be converted into ammonia and nitrogen oxides, which will reenter the atmosphere, causing air pollution, intensifying greenhouse effect, and leading to a series of ecological and environmental problems. Therefore, we studied the effects of different nitrogen treatments on soil N2O emissions with in-situ field monitoring and indoor analysis, and determined the appropriate amount of nitrogen for pear orchard soil.【Methods】We conducted fertilization experiments with different amount of nitrogen fertilizer in a 6-year-old‘Kuerlexiangli'pear orchard. Five nitrogen treatments were applied: nofertilizer (N0 P0 K0) , no nitrogen fertilizer (N0 PK) , low nitrogen amount 150 kg· hm-2 (N1 PK) , medium nitrogen amount 300 kg· hm-2 (N2 PK) and high nitrogen amount 450 kg· hm-2 (N3 PK) . During the growth period of pear trees (from April to September) , the N2O emissions from the soil in the pear orchard were collected and monitored by applying the static box-gas chromatography method.【Results】In different nitrogen treatments, N2O emission flux was lowest at night (20:00—8:00) , followed in an increasing order by that in morning (8:00—12:00) , at noon (12:00—16:00) and in the afternoon (16:00—20:00) , and the range of change was from 0.060 to 0.310 g· hm-2· h-1. In April and June, the N2O fluxes of soils treated with nitrogen (N1 PK, N2 PK and N3 PK) were significantly higher than those in other months (May, July, August and September) . On the 15 th of May, July, August and September, the N2O fluxes under different fertilization treatments increased slightly compared to the soil N2O flux on the5 th and 25 th of the corresponding months. At nighttime (except April 5 and June 5) , soil N2O fluxes was not significantly different between soils treated with nitrogen fertilizer and that with no nitrogen fertilizer. It varied from 0.060 to 0.150 g· hm-2· h-1. At the daytime (from April to July) , N2O flux of soil treated with nitrogen was significantly greater than that with no nitrogen fertilization, and the difference in the afternoon on April 5 was especially prominent. It ranged from 0.0120 to 0.310 g· hm-2· h-1.The N2O flux among different treatments was in the order of N1 PK < N2 PK < N3 PK. The daily accumulation of soil N2O emissions was lower in the nighttime than in the daytime. The N2O emission from soils treated with nitrogen showed a fluctuating upward and downward trend at nighttime, but it decreased gradually in the control group. There was no significant difference in the daily soil N2O emission among different fertilization treatments, and the range was from 0.720 to 1.800 g · hm-2· d-1. The N2O emission of soil treated with nitrogen showed a fluctuating upward and downward trend at daytime, but the N2O emission in the control soil decreased gradually. The daily N2O emission in the soil treated with nitrogen was significantly greater than in the control. It ranged from 0.900 to 3.160 g· hm-2· d-1 in nitrogen treatments but varied from 0.840 to 1.352 g· hm-2· d-1 in the control. The daily N2O emission in the soils treated with nitrogen was in the order of N1 PK < N2 PK < N3 PK. The N2O emission during the annual growth period among different treatments was in the order of N0 P0 K0< N0 PK < N1 PK-2· a-1, respectively. It increased with the increase in nitrogen application rate. The N2O emission in N1 PK, N2 PK, and N3 PK treatments increased by 26.10%, 32.67%, and 53.50% compared with N0 PK treatment, respectively.When nitrogen application rate was 450 kg · hm-2, the N2O flux in the afternoon, diurnal cumulative amount and the cumulative amount during annual growth period were significantly greater than the other treatments. Temperature, irrigation and application of nitrogen fertilizer could all affect N2O emission from the soil. The application of nitrogen fertilizer had the greatest impact on N2O emission, which increased with the nitrogen application rate. When nitrogen application rate was 150, 300, and450 kg· hm-2, the N2O emission coefficients was 0.060%, 0.038% and 0.041%, respectively.【Conclusion】based on the comprehensive effects of nitrogen application on the production and environmental benefits, it was recommended to apply nitrogen at a dose of 300 kg· hm-2 in pear orchard. However, the nitrous oxide emission of soil in pear orchard is affected by many factors such as temperature, nitrogen application amount, irrigation, etc., and there are interactive effects between these factors. Therefore, the isotopic tracer technology should be used to study the action mechanism of nitrogen fertilizer on soil N2O emission in future.