The effect of vermicompost enriched with phosphate solubilizing bacteria on phosphorus availability, pH and biological indices in a calcareous soil

The shortage of phosphorus is one of the major problems of calcareous soils. One of the most effective and economical ways to increase phosphorus availability is through the addition of organic fertilizers. Vermicompost is the most desirable organic material and bio fertilizers, but the high usage of vermicompost to achieve optimal performance and the presence of organic insoluble phosphates are limited the use of vermicompost. One of the best way is that enriched vermicompost with plant growth-promoting bacteria, especially phosphate solubilizing bacteria.

Vermicompost sample was produced in Vermicompost Research Center of Tehran University. 18 isolates with ability of organic and inorganic phosphate solubilizing were isolated. Finally, two isolates 53 and 22 which had high solubilizing capacity of inorganic and organic insoluble phosphate were purified and identified. An incubation experiment was conducted in the Split plot design based on time to investigate the enrichment of vermicompost with phosphate solubilizing bacteria on available phosphorus, pH, microbial respiration, alkaline phosphatase and dehydrogenase enzyme activity in pots with 4 kg of soil consist of 5 treatments, including T1: vermicompost (6%) + B53; T2: vermicompost (6%) + B22; T3: positive control (Triple superphosphate fertilizer at 50 mg/kg); T4: vermicompost (6%) and T5: negative control (soil without vermicompost and bacteria), in 3 replicates. The treatments were incubated for 30 days and after incubation, the effect of treatments on phosphorus availability, pH and biological indices was measured.

The results of the 16S rRNA gene sequence identified isolate 22 as Serratia marcescens with 99% similarity and isolate 53 as Pseudomonas aeruqinosa with 98% similarity. the use of vermicompost enriched with phosphate solubilizing bacteria (T1 and T2) increased available phosphorus (59.2% and 100%), microbial respiration (58% and 61%), Alkaline phosphatase activity (34 and 41%) and dehydrogenase activity (102.6% and 13.7%) significantly compared to control treatment (T4), but reduced pH (4.7% and 4.4%). According to the results, bacterial enriched treatment Serratia marcescens (T2) was more able to increase phosphorus and biological indices.

The application of vermicompost enriched with phosphate solubilizing bacteria (T1 and T2) cusses increase of the available phosphorus, microbial respiration, alkaline phosphatase enzyme activity and dehydrogenase activity and soil pH reduction. The treatment enriched with Serratia marcescens (T2) had more ability to increasing phosphorus availability and biological index. The results showed that vermicompost enrichment with phosphate solubilizing bacteria can be a suitable alternative for reducing the use of phosphate fertilizers and as a suitable strategy for better vermicompost management in calcareous soils in the future.