岩溶湿地不同土地利用类型土壤磷的吸附解吸

Phosphorus adsorption and desorption in soil under different land use types in karst wetlands

文章选取桂林会仙岩溶湿地的3种不同土地利用类型(农田、果园、荒地)的表层土(0~20 cm)和深层土(20~40 cm)及河流底泥作为研究对象,利用Langmuir等温吸附方程拟合不同磷浓度的吸附曲线,计算出磷最大吸附容量(Qm)、磷吸附能(K)、最大缓冲容量(MBC),通过曲线拟合得到被吸附磷的解吸率(a)。结果表明:(1)荒地对外源磷吸附率高于农田、果园土壤及河流底泥;在中低浓度的磷添加下(≤2 500 mg·kg^(-1)),深层土对磷的吸附率均大于表层土;(2)Langmuir模拟揭示:河流底泥的Qm为4 961.61 mg·kg^(-1),但K很低(0.034 kJ·mol^(-1)),因此MBC较小(171.82);荒地表层土的磷吸附特征则与前者正好相反,MBC较大(255.10)。农田和果园土壤的磷吸附特征值介于两者之间;(3)土壤磷吸附主要受土壤质地及pH控制;河流底泥的a值(11.9%)高于其他土壤,表层土a值高于深层土,a值与土壤有效磷含量显著正相关。农田和果园对磷吸附量大,但固持能力弱,有较大的磷淋溶风险;荒地表层土则在湿地中起到固持磷、降低富营养化风险的作用;河流底泥的磷极易释放,是水体富营养化的长期磷源。

The adsorption and desorption of phosphorus(P)in soil are the main factors controlling the P availability and leaching risk.Soils in karst wetlands are characterized as being rich in calcium with pH close to neutral.However,it still lacks a systematic evaluation on the characteristics of P adsorption and desorption in the soil under different land use types in karst wetlands.Meanwhile,exploring the main influencing factors of P adsorption and desorption can provide a scientific basis for the prevention and control of surface source pollution in karst wetlands.This study investigated the characteristics of P adsorption and desorption in surface soils(0-20 cm)and deep soils(20-40 cm)under different land use types,namely,farmland,orchard and barren land,as well as river sediment in Huixian karst wetland,Guilin,China.Langmuir adsorption isotherm equation was applied to reveal the maximum P adsorption capacity(Q_(m)),energy of adsorption(K)and maximum buffering capacity(MBC).In addition,the desorption rate of adsorbed P was estimated through curve fitting.The relationship of the indices of P adsorption and desorption and soil physiochemical features were analyzed to reveal the impact of human activities.The results are shown as follows,(1)The barren soils had greater P adsorption rate than the soils from farmland,orchard and river sediment.The P adsorption rates of deep soils were higher than those of surface soils when the low P concentration(below 2,500 mg·kg^(−1))was added.(2)Langmuir equation showed good fits to the curves of soil adsorbed P contents and the corresponding P concentrations in the equilibrated solution of all soils(R^(2)=0.91-0.98,p<0.01).The characteristics of P adsorption in soil varied greatly among different land use types.The river sediment had the largest Qm(4,961.61 mg·kg^(−1))but very low K,resulting in a relatively low MBC.To the contrary,the surface soil of the barren land had the smallest Qm(359.71 mg·kg−1)but largest K,leading to the largest MBC among all soils.The indices of P desorption characteristics of the rice paddy land and orchard soil were in-between of the above two soils.(3)The P desorption rate of river sediment(11.9%)was higher than those of the other soils.Among all surface soils,the barren land had the lowest desorption rate(4.5%).The desorption rates of all deep soils were lower than those of the surface soils,indicating a greater P sequestration capability of the deep soils in karst wetlands.(4)In terms of the surface soil of farmlands and orchards,Qm increased by 286.11%and 1,025.80%;K decreased by 79.41%and 95.49%;desorption increased by 75.56% and 33.33%respectively,compared with the wasteland.This indicates that anthropogenic tillage raised the phosphorus adsorption sites in the soil.However,the binding energy between soils and the adsorbed P decreased.Therefore,the adsorbed P did not convert into a stable form.Under high external P load,it would enter into the phase of fast P desorption.In comparison,even though the river sediment had a great Qm,its K value was the lowest,leading to a small MBC and the greatest P desorption rate.This result indicates that the weakly bound iron/aluminum-P in the karst soil reduced and released P under anaerobic condition.Even though the barren soil had a low Q_(m) value,but its P adsorption and buffering capability was the greatest;therefore,the potential risk of P leaching was the lowest in this kind of soil.(5)The Pearson's correlation and principal component analysis(PCA)suggests that the indices of P desorption(Q_(m),K and MBC)were closely correlated with soil texture and pH,suggesting that land use change would affect the characteristics of P adsorption through changing soil physiochemical features.The P desorption rate was significantly correlated with soil available P,suggesting that the equilibrium of P desorption is the major control of available P content of soil.This study concludes that the characteristics of P adsorption and desorption are affected by different land uses in karst wetlands.The soils of farmland and orchards adsorb great amount of P,but with great potential risk for P leaching due to the weak retention strength.The barren soil retains P and plays an important role in reducing the risk for eutrophication owing to a high water connectivity in karst wetlands.The river sediment releases P easily,and therefore functions as a long-term P source for waterbody eutrophication.