降雨量对洱海流域稻季氮磷湿沉降通量及浓度的影响

为了探究洱海流域稻季氮磷湿沉降规律,于2016及2017年稻季在大理洱海流域收集湿沉降样品,分析样品中总磷(total phosphorus,TP)、总氮(total nitrogen,TN)、NO_3~--N、NH_4~+-N等指标及其变化规律。结果表明,TN、TP湿沉降通量主要受降雨量支配,2016与2017年稻季单次降水TN、TP湿沉降通量与降雨量均呈极显著的线性正相关关系。TN、TP湿沉降浓度总体上随降雨量的增大而减小,同时与是否发生连续降雨及是否大规模施肥有关。以2017稻季氮素湿沉降为例,2017年稻季NH_4~+-N和NO_3~--N湿沉降对TN的占比分别为53.1%和20.6%,湿沉降以NH_4~+-N为主。可溶性无机氮(dissolved inorganic nitrogen,DIN)对TN的占比随着降雨量的增大而减小,随着连续降雨的延续而增大。2016及2017年稻季湿沉降TN质量浓度分别为0.87~4.03和0.90~6.85 mg/L,均远大于湖泊富营养化阀值,对洱海水生生态系统产生不利影响。

Effect of precipitation on wet deposition flux and content of nitrogen and phosphorus in Erhai lake basin in rice season

In order to explore the regularity of nitrogen and phosphorous wet deposition during the rice-planting season in the Erhai lake basin, wet deposition samples were collected at the Dali Environment Comprehensive Monitoring Station of the Ministry of Agriculture, Xizhou, Dali, during the 2016 and 2017 rice-planting seasons. The contents of deposition samples, such as TP (total phosphorus), TN (total nitrogen), NO3--N, NH4+-N, and their changes were analyzed. The results showed that the wet deposition fluxes of TN and TP in the 2016 rice season were 2 250.21 and 133.87 kg/km2, respectively, and the wet deposition fluxes of TN and TP in the 2017 rice season were 1 034.26 and 73.37 kg/km2, respectively. In the 2016 and 2017 rice seasons, 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%. July and August in 2016 and 2017 had the largest wet deposition flux in the rice season, which had the highest rainfall. In 2016, the precipitation was 486.8 mm larger than 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 rainfall in 2016 and 2017 rice season showed a significant linear positive correlation with precipitation. The wet settlement concentration 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 of the 2017 rice-planting season 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 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 DIN (dissolved inorganic nitrogen) 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 concentrations in the rice season in 2016 and 2017 were 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, therefore, wet deposition in the rice season would have an adverse effect on the aquatic ecosystem in Erhai lake basin.