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 (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 Q_m (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 Q_m (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, Q_m 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 Q_m, 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.