Phosphorus solubilizing bacteria growth and effects on soil phosphorus adsorption-desorption characteristics in reclaimed soils

Large coal mining has greatly increased the subsidence area of land. Therefore, soil remediation is necessary for relieving stress on cultivated lands and protecting ecological environment in subsidence areas. Phosphorus (P) solubilizing bacteria in soil, an important role in P cycling, can convert insoluble phosphate into available P. Application of P solubilizing bacteria is an effective bio-measure in improvement of reclaimed soil. However, the effects of P solubilizing bacteria on P adsorption-desorption in reclaimed soil have been less concerned. In this study, reclamation soil samples were collected in mining-driven subsidence areas and laboratory incubation experiments conducted via application of P solubilizing bacteria and other different fertilizers (glucose, G; urine, U; organic fertilizer, M) on reclaimed soils. The soil available P, organic P contents and P adsorption-desorption characteristics were investigated after different times of incubation. The objectives of the study were to determine the P solubilizing bacteria effects on soil P nutrient and provide reference for rapid fertility of reclaimed soils. Seven treatments were conducted — control (no bacteriaP and fertilizers, CK); P solubilizing bacteria only (B); organic fertilizer only (M); P solubilizing bacteria and glucose (BG); P solubilizing bacteria and urine (BU); P solubilizing bacteria, glucose and urine (BGU); and P solubilizing bacteria, glucose, urine and organic fertilizer (BGUM). The amount of P solubilizing bacteria in one pot with 500 g soil was 5 mL P solubilizing bacteria fermented liquid (with P·solubilizing bacteria 1.2×10⁸ CFU·mL^-1). The results showed that the amount of P solubilizing bacteria in soil initially increased and then decreased during the incubation period. Under BGUM treatment, the amount of P solubilizing bacteria decreased from 1.0×10⁶ CFU·g^-1 at the start of the experiment to 3.3×10⁴ CFU·g^-1 after 60 days of cultivation, which was 300, 367, 1 650 and 3 300 times of those of BGU, BG, BU and B treatments, respectively. Soil available P contents of M, B and BGUM treatments were 172.27 mg·kg^-1, 3.00 mg·kg^-1 and 188.9 mg·kg^-1 higher than that of CK. Therefore, the application of P solubilizing bacteria or organic fertilizer obviously improved soil available P content and the effect was more significant under combined application of glucose, urine, organic fertilizer and P solubilizing bacteria. The P isothermal adsorption and desorption curves showed that with increasing exogenous P concentration, P adsorption and desorption decreased in each treatment. The Langmuir equation was the model that best described P adsorption isotherm under different treatments. The maximum soil P adsorption capacity was smallest under BGUM treatment, which was 119.05 mg·kg^-1 lower than that of CK. In addition, P adsorption coefficient under BGUM treatment also significantly decreased. The average desorption rate under BGUM treatment was 33.20%, which was significantly higher than those under other treatments. Overall, BGUM treatment had the highest amount of P solubilizing bacteria and average desorption rate. It also significantly improved soil available P content and influenced the maximum buffering capacity of soil P. Therefore, BGUM treatment was considered to be the best among all the treatments for the application of P solubilizing bacteria in soil reclamation. The results of this study suggested that suitable carbon source, nitrogen source and organic fertilizer were necessary during P solubilizing bacteria application in reclaimed soil.