Phosphorus adsorption and desorption in soil under different land use types in karst wetlands
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摘要:
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。
Abstract: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 (Qm), 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 (R2=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 Qm 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 (Qm, 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.
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Key words:
- karst wetland /
- phosphorus /
- adsorption /
- desorption /
- land use
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表 1 供试土壤的基本理化性质
Table 1. Basic characteristics of soils
土地利用类型 pH 总磷（TP）/mg·kg−1 有效磷（AP）/mg·kg−1 总有机碳（TOC）/g·kg−1 总氮（TN）/g·kg−1 黏粒/% 粉粒/% 砂粒/% 农田表层土 7.64±0.05 591.93±64.03 63.11±6.79 35.54±4.55 3.56±0.27 66.42 27.45 6.13 果园表层土 7.37±0.10 389.73±26.09 12.27±1.28 13.38±1.75 1.44±0.15 61.23 36.53 2.25 荒地表层土 6.15±0.04 223.31±11.05 25.19±5.80 20.83±8.15 2.20±0.64 76.50 20.18 3.32 河流底泥 7.20±0.08 345.55±32.31 30.37±3.79 46.57±4.26 4.84±0.55 / / / 农田深层土 7.75±0.04 407.23±93.01 71.23±5.39 16.32±14.35 1.49±1.05 64.96 31.27 3.77 果园深层土 6.96±0.22 354.54±82.16 26.52±4.66 10.96±2.29 1.17±0.20 54.62 30.27 15.11 荒地深层土 7.48±0.18 315.25±86.20 7.95±0.88 7.30±2.31 0.97±0.18 66.11 29.22 4.68 注：表中数值为平均值±标准差。 
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表 2 不同土地利用类型下土壤对磷的等温吸附参数
Table 2. Isothermal adsorption parameters of phosphorus in soils under different land use types
土地利用类型 R2 吸附能（K）/kJ·mol−1 最大吸附容量（Qm）/mg·kg−1 最大缓冲容量（MBC） 农田表层土 0.95** 0.146 1 388.89 202.84 果园表层土 0.97** 0.032 4 049.61 129.87 荒地表层土 0.91** 0.709 359.71 255.10 河流底泥 0.98** 0.034 4 961.61 171.82 农田深层土 0.93** 0.062 2 777.78 170.94 果园深层土 0.93** 0.119 1 923.08 228.83 荒地深层土 0.93** 0.086 2 325.58 200.40 注：** P < 0.01；MBC = K*Qm。
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表 3 不同土地利用类型下土壤对磷的吸附量与解吸量的关系
Table 3. Relationship between P desorption and adsorption by soil under different land use types
土地利用类型 y=ax+b R2 a b 农田表层土 0.079 5.908 0.93* 果园表层土 0.060 6.910 0.95* 荒地表层土 0.045 2.627 0.96* 河流底泥 0.119 9.987 0.96* 农田深层土 0.028 3.159 0.95* 果园深层土 0.017 15.310 0.92* 荒地深层土 0.038 10.522 0.93* 注：* P < 0.05。
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表 4 磷吸附和解吸参数与土壤理化性质的相关分析
Table 4. Correlation between the parameters of P adsorption/desorption and physical-chemical properties of soils
参数 pH 总磷 有效磷 总有机碳 总氮 黏粒 粉粒 砂粒 K −0.845* −0.525 −0.144 0.043 0.058 0.827* −0.889* −0.249 Qm 0.619 0.208 0.266 −0.285 −0.330 −0.577 0.821* −0.056 MBC −0.508 −0.388 −0.079 0.177 0.208 0.764 −0.917* −0.122 解吸率（a） 0.468 0.518 0.852* 0.124 0.043 0.102 0.189 −0.371 注：* P < 0.05。
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