| Taihu Basin is one of the most developing areas in China,and it is also one of the most serious acid deposition areas.In addition,the large-scale use of agricultural nitrogen fertilizer in the basin is also an important factor leading to soil acidification.Under dual effects of acid deposition and nitrogen application,acidification effect has a significant impact on the water environment in the basin.Taihu Lake is located in the downstream of the Yangtze River and is the third largest freshwater lake in China.Based on the study of natural water chemical changes in Taihu Lake,calcium and magnesium are main objects to analyze the acid deposition of watershed and the acidification of nitrogen fertilizer to the erosion of watershed carbonate rocks,as well as the influence of Ca and Mg(hardness)in water and sediments.In order to reveal the influence of acidification in the basin on the calcium and magnesium(hardness)in water historical data about acid deposition,soil in Taihu basin and water quality in Taihu Lake(1980-2018)and field sampling data(2017-2018)were analyzed.The results showed that acid deposition of the watershed accelerated dissolution of carbonate rock and caused the increasing of Ca2+and Mg2+in water.At the same time,the superposition of acid deposition and nitrogen fertilizer in the basin could further dissolve carbonate rock faster,but compared with acid deposition in the basin,the superposition of acidification only affected the agricultural soil.Through collecting literaturewe found that due to control of acid gas emissions in the basin and reduction of application of nitrogen fertilizer,pH in forest soil affected by acid deposition was not changed significantly in the past 20 years,however pH in agricultural soil affected by acid deposition and nitrogen fertilizer was a significant decreasing trend for nearly 40 years(p<0.01).Analysis of physical properties of the agricultural soils(double acidification by acid deposition and nitrogen fertilizer)and the forest soils(acidification by acid deposition),different forms of nitrogen and the occurrence of Ca and Mg showed that pH in surface agricultural soil(0-15 cm)was significantly lower than that in surface forest soil(0-15 cm)(p<0.05),while the deeper(>15cm)was opposite.Ammonium,nitrate nitrogen and total nitrogen in surface agricultural soil(0-15 cm)were significantly higher than that in surface forest(0-15 cm)soil(p<0.05),and nitrate nitrogen and total nitrogen in agricultural deeper(>15cm)soil were significantly higher than forest deeper(>15cm)soil(p<0.05),and ammonium nitrogen was opposite that was the result of varying degrees of nitrogen conversion.Due to nitrification and denitrification of soil in nitrogen conversion,pH in soil was negatively correlated with ammonium nitrogen,nitrate nitrogen and total nitrogen(p<0.05),organic matter in soil a positive correlated between ammonium nitrogen,nitrate nitrogen and total nitrogen(p<0.05).Exchangeable Ca,Mg,acid extracted Ca,Mg and total Ca and Mg in surface agricultural soil(0-15 cm)were significantly lower than that in surface forest(0-15 cm)soil(p<0.05),while there was no significant difference in the profile because acidification of agricultural soil was stronger than that of forest soil.Further analysis of effect of acidification on Ca and Mg in surface soil(0-15 cm)found that Ca,Mg in different forms in a soil layer(0-9 cm)in agricultural soil was significantly lower than that of b layer(9-15 cm),and there was no significant change in forest soil.In state exchangeable Ca(27.01%-30.15%)>acid extracted(2.31%-3.52%),exchangeable Mg(5.34%-6.64%)>acid extracted(0.28%-1.66%).The ratio of two forms of Ca of Mg to total amount was basically unchanged.Ca2+ and Mg2+ in Taihu Lake have responded to acidification of watershed.In the past 40 years,the concentration of Ca2+ and Mg2+ in water had shown significant increasing trends(p<0.01),and the average annual growth rate were 0.75 mg/(L a)and 0.16 mg/(L a),respectively.According to the analysis of Mann-Kendall mutation,the concentration of Ca2+ and Mg2+ in water were roughly divided into two stages:the first stage was before 1996,the concentration of Ca2+and Mg2+in water were increasing slowly,with the average values being 22.45 ± 3.36 mg/L and 6.47 10.47 mg/L,respectively;the second stage was from 1996 to the present,the concentration of Ca2+and Mg2+in water were increasing significantly,with the average values being 36.68 ± 6.97 mg/L and 9.70 ± 1.53 mg/L,respectively.By calculating,we found that Ca2+and Mg2+ in water were mainly derived from watershed carbonate rocks,followed by sewage discharge,while atmospheric precipitation has less impact.At the same time,sediment in Taihu Lake showed a corresponding correspondence,which Ca and Mg in surface sediment(0-10 cm)were significantly higher than that in deeper sediments(>10 cm).At the same time,physical properties in sediment also affected Ca and Mg in sediments.The results showed that Ca and Mg in sediments were negatively correlated with pH(p<0.05),and positively correlated with organic matter(p<0.05)in the whole section because organic matter was beneficial to adsorp and it was easy to release Ca and Mg in acidic conditions.Acidification caused by acid deposition and nitrogen fertilizer in the basin have important influences on Ca2+and Mg2+ in water.At present,concentration of Ca2+ and Mg2+(hardness)in water were increasing and conductivitycorrelated was positively with concentration of Ca2+and Mg2+ in water(p<0.01),and if not controlled,it will cause the problem of drinking water safety and risk of water salinization.Therefore,control the acidification of basin should be paid sufficient attentions. |
PDF Full Text Request