摘要: The first step of nitrification (i.e., the oxidation of ammonia to nitrate) is catalyzed by nitrifiers, such as ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA). However, the impact of soil warming on the activity and abundance of nitrifiers under different nitrogen (N) fertilization conditions remains poorly understood. A long-term field warming experiment has been conducted since October 2008 at the Luancheng Agro-Ecosystem Experimental Station of Chinese Academy of Sciences in the North China Plain, during which soil temperature was increased by 1.5℃ using infrared heaters (power, 1 000 W) placed 2 m above the soil surface. In 2018, we investigated soils from the control (no warming) and warming treatment plots for potential nitrification rate (PNR), abundance of AOB and AOA at 10 cm and 20 cm soil depth under two N fertilization conditions:without N fertilization (N0) and with 240 kg(N)·hm-2·a-1 fertilization (N1). Soil PNR, nitrate (NO3--N), and ammonium (NH4+-N) contents were spectrophotometrically assessed, and the abundance of functional genes was investigated via real-time quantitative PCR. Warming increased PNR and NO3--N content under N1 treatment and decreased them under N0 treatment (P < 0.05). Moreover, warming significantly increased AOB abundance under N1 treatment (P < 0.05), whereas it decreased the abundance of both AOA and AOB under N0 treatment, at both soil depths. Compared with N0, N1 exhibited substantial decrease in AOA/AOB ratio, suggesting that compared with warming without N fertilization, warming with N fertilization exhibited higher stimulation of AOB growth than of AOA growth. Conclusively, this study suggests that AOB significantly and positively responded to warming with N fertilization, whereas both AOA and AOB significantly and negatively responded to warming without N fertilization. This study provides an understanding of nitrifier activity and the response of ammonia-oxidizing microorganisms to warming conditions and N availability.
摘要: The present study aimed to presenting the source and environmental behavior of nitrate in various environmental media by sampling and analyzing the soil in the unsaturated zone, surface water, and groundwater of a typical area irrigated with reclaimed water in the North China Plain. This study also identified the sources of groundwater nitrate pollution in the area, and determined the effects of different irrigation conditions on nitrate migration in the soil in the unsaturated zone. In the Xiao River basin, which is seriously affected by reclaimed urban water, nitrate concentrations in groundwater ranged from 4.0 mg·L-1 to 156.6 mg·L-1, nitrate in the groundwater had formed a high-value area of 2 km from the river channel and 70 m in depth. And nitrate moved down 1-2 m every year. The correlation between nitrate and chloride ion indicated that reclaimed urban water was the main source of nitrate in the unsaturated zone, surface water, and groundwater of the area irrigated with reclaimed water. Geoprobe system was used to obtain continuous soil core samples from the typical unsaturated zones to study the influence of reclaimed water on the vertical distribution of NO3--N in thick unsaturated zone. The average contents of NO3--N in the reclaimed water and groundwater irrigation areas were 137.0 mg·kg-1 and 107.7 mg·kg-1, with the peak values of 523.2 mg·kg-1 at a soil depth of 1.20 m and 725.9 mg·kg-1 at a depth of 0.85 m, respectively. The correlation analysis between soil nitrate and chloride ion indicated that the NO3--N in the reclaimed water irrigation area was mainly affected by reclaimed urban water, whereas in the groundwater irrigation area it may because the use of agricultural nitrogen fertilizers. By comprehensively analyzing the relationship between groundwater age and nitric acid, the historical process of the nitrogen source causing the groundwater nitrate pollution could be reproduced on a time scale. Under the unique hydrogeological background of the North China Plain, the nonpoint source pollution of the farmland has a limited effect on groundwater pollution, while the risk of groundwater nitrate nitrogen pollution is relatively high around the reclaimed water river.