Effect of precipitation on wet deposition flux and content of nitrogen and phosphorus in Erhai lake basin in rice season
(2.Yunnan Institute of Water Resource and Hydropower Research, Kunming, China 650228)
【Abstract】In order to explore the regularity of nitrogen and phosphorous wet deposition in rice season in the Erhai lake basin, the wet deposition samples were collected at the Dali Environment Comprehensive Monitoring Station of the Ministry of Agriculture and Rural Affairs, Xizhou, Dali, in rice seasons in 2016 and 2017. The contents of deposition samples, such as total phosphorus (TP), total nitrogen (TN), NO3−-N, and NH4+-N, and their changes were analyzed. The results showed that the wet deposition fluxes of TN and TP in rice season in 2016 were 2 250.21 and 133.87 kg/km2, respectively, and the wet deposition fluxes of TN and TP in the rice season in 2017 were 1 034.26 and 73.37 kg/km2, respectively. In the rice seasons in 2016 and 2017, the amount of TN wet deposition which descended directly to the surface of Erhai Lake accounted for 16.7% and 7.7% of the annual TN lake load, and that of TP was 11.0% and 6.0%, respectively. July and August in 2016 and 2017 had the largest wet deposition fluxes in the rice season, which had the highest precipitation. The precipitation in 2016 was 486.8 mm larger than that in 2017. Correspondingly, the wet deposition fluxes of TN and TP in 2016 were 1 216 and 60.6 kg/km2 higher than those in 2017, respectively. The wet deposition fluxes of TN and TP were mainly controlled by precipitation. The TN and TP wet deposition fluxes of single precipitation in rice seasons in 2016 and 2017 showed a significant linear positive correlation with precipitation. The wet deposition concentrations of TN and TP decreased with the increase of rainfall, and it was related to whether there was continuous rainfall and large-scale fertilization. Taking the wet deposition of nitrogen in rice season in 2017 as an example, we could find that the correlation analysis of nutrient concentration and precipitation in the wet deposition showed that TP, TN, NO3−-N, NH4+-N, and precipitation were all in weakly negative correlation at the 0.05 level, and there was a strong correlation between TN, NO3−-N and NH4+-N at the 0.01 level. The ratios of wet deposition of NH4+-N and NO3−-N to TN were 53.1% and 20.6%, so NH4+-N was the main wet deposition. The ratio of dissolved inorganic nitrogen (DIN) to TN decreased with the increase of precipitation and increased with the continuity of precipitation. Ammonia volatilization was one of the main sources of atmospheric nitrogen wet deposition, but the change of nitrogen wet deposition flux in Erhai lake basin did not change with the change of ammonia volatilization rate in paddy fields. This was related to the complex planting structure and special three-dimensional climate in the basin. The randomness of precipitation and the uncertainty of the basin's wind direction determined that the changes of wet deposition fluxes were more complex. The wet deposition TN concentration in the rice seasons in 2016 and 2017 was 0.87–4.03 and 0.90–6.85 mg/L, respectively, much higher than the 0.20 mg/L threshold for eutrophication. The mass ratio of N and P in wet deposition was low N/P ratios (16.82 and 14.09, respectively), which was in favor of rapid growth of cyanobacteria, so the wet deposition in the rice season would have an adverse effect on the aquatic ecosystem in Erhai lake basin.
【Keywords】 nitrogen; phosphorus; precipitation; Erhai lake basin; wet deposition;
(Translated by LIU T)
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