不同土地利用对洱海入湖河流溶解无机碳(DIC)浓度及其碳同位素组成(δ^(13)C_(DIC))的影响

Effects of Different Land Uses on the Concentrations of Dissolved Inorganic Carbon (DIC) and Its Carbon Isotopic Compositions (δ^(13)C_(DIC)) in the Rivers Flowing into the Erhai Lake

不同土地利用对河流中的溶解无机碳(DIC)浓度及其碳同位素组成(δ^(13)C_(DIC))的影响有很大差别,但其机制仍有待进一步揭示。本文以洱海入湖河流为研究对象,分别于2020年10月(雨季)和2021年1月(旱季)采集了洱海主要入湖河流的水样,测定了入湖河流的基本物理参数、主要阴阳离子及δ^(13)C_(DIC)组成,对其中岩性相似的流域进行了土地利用方式对河流DIC浓度及其δ^(13)C_(DIC)组成的影响分析。结果表明,洱海入湖河流的pH呈弱碱性,主要阳离子为Ca^(2+)和Mg^(2+),主要阴离子为HCO^(-)_(3)。入湖河流的DIC浓度在0.35~4.6 mmol/L之间,由于稀释作用雨季的DIC浓度小于旱季。河流δ^(13)C_(DIC)在-12.59‰~-2.99‰之间,北部流域、南部流域的河流以及洱海中部的白鹤溪、黑龙溪、中和溪和莫残溪的δ^(13)C_(DIC)值要比其它河流偏负。在流域内岩性相似情况下,洱海入湖河流中的DIC浓度随着流域内的建设用地面积占比的升高而增加,随着植被(林地+草地)覆盖面积占比的升高反而减小。流域内林地和草地面积占比越大δ^(13)C_(DIC)越偏正,耕地和建设用地面积占比越大入湖河流中的δ^(13)C_(DIC)越偏负。其中的原因是受到耕地施肥和污染物排放等人为活动的影响,耕地中的土壤CO_(2)升高及河流中生物活动增强,导致洱海入湖河流的DIC浓度升高及其δ^(13)C_(DIC)偏负,从而掩盖了林地和草地对河流中的DIC浓度及其δ^(13)C_(DIC)的影响规律。在土地利用的影响方面,洱海入湖河流中的DIC浓度受到河流建设用地的影响最大,耕地的影响相对于建设用地来说要弱;δ^(13)C_(DIC)也受到建设用地和耕地的控制,林地和草地对河流中的DIC浓度及其δ^(13)C_(DIC)的影响较小。

Different land uses have very different effects on the concentration of dissolved inorganic carbon(DIC)and its carbon isotope composition(δ^(13)C_(DIC))in rivers,but the mechanism still needs to be further revealed.This paper takes the main rivers flowing into the Erhai Lake as the research object.In October 2020(the rainy season)and January 2021(the dry season),samples of the main rivers entering the lake were collected,and the basic physical parameters,main anions and cations,and the composition ofδ^(13)C_(DIC) of the rivers were determined.The impact of land uses on the concentrations of river DIC andδ^(13)C_(DIC) was analyzed for the watersheds that are similarly affected by lithology.The results show that the rivers flowing into Erhai Lake is weakly alkaline,the main cations are Ca^(2+)and Mg^(2+),and the main anion is HCO^(-)_(3).The DIC concentrations in the rivers entering the lake are between 0.35 and 4.6 mmol/L.Due to the dilution effect,the DIC concentrations in the rainy season are lower than those in the dry season.Theδ^(13)C_(DIC) of rivers is between-12.59‰and-2.99‰.Theδ^(13)C_(DIC) values of rivers in the northern and southern basins,and Baihe,Heilong,Zhonghe and Mocan Creeks in the middle of Erhai Lake are more negative than other rivers.In the case of similar lithology in the basin,the DIC concentrations in the rivers flowing into Erhai Lake increase with the increase in the proportion of construction land in the basin,and decrease with the increase in the proportion of vegetation area.At the same time,the larger the proportion of vegetation in the basin,the more positive theδ^(13)C_(DIC),the larger the proportion of cultivated lands and construction lands,the more negative theδ^(13)C_(DIC) of the rivers flowing into the lake.The reason is that it is affected by human activities such as fertilization and pollutant discharge in the downstream cultivated lands.The increase of soil CO_(2) in cultivated lands and the increase of biological activities in the rivers lead to the increase of the DIC concentrations and the negativeδ^(13)C_(DIC) of the rivers flowing into the Erhai Lake,which mask the influence of the upstream vegetation on the DIC concentrations in the rivers and itsδ^(13)C_(DIC).In terms of the impact of land uses,the DIC concentrations in the rivers flowing into Erhai Lake are mostly affected by the construction lands downstream of the rivers,and the impact of cultivated lands is weaker than that of construction lands;δ^(13)C_(DIC) is also controlled by the construction lands and cultivated lands downstream,and upstream vegetation has little effect on the DIC concentrations and itsδ^(13)C_(DIC)in the rivers.