The effect of solid waste of olive oil mill and Pseudomonas bacteria on phosphorus availability and some biological characteristics of a calcareous soil

Agro-industrial wastes of olive mill processing are an important environmental problem in olive-oil producing countries. The incorporation of organic wastes such as solid olive waste into soil can be useful for improving soil condition. Phosphorus is an essential nutrient for plant growth. Concurrently, it is also known that phosphate-solubilizing bacteria (PSB) are important for P-solubilization from insoluble phosphorus compounds due to large volume of solid waste generated in olive oil mill processing, especially in Guilan province, and increasing demand for phosphorus fertilizers in agriculture, this study was conducted with the aim of increasing the available phosphorus from solid olive waste in the presence of Pseudomonas solubilizing phosphorus bacteria in calcareous soil. Solid waste obtained from an olive oil mill (Rudbar, Guilan province), and loamy soil employed in this study was collected of Lowshan from the topsoil (0‒30 cm). Native pseudomonas with the ability of phosphorus solubilizing was isolated in Sperber medium plate containing insoluble phosphorus and then phosphorus solubilizing index (PSI) was calculated. The treatments for soil incubation consisted of three levels of solid waste include 0 (W0), 2 (W2) and 4 (W4) percent and three levels of bacteria, without bacteria (Pn), isolated pseudomonas (Pi) and pseudomonads CHA0 (Pch) as an index bacteria in 11 sampling times. Soil samples were thoroughly mixed with solid wastes and after inoculating with bacteria (106 cell/g) were incubated at 28°C in 70% of the WHC. Sampling was carried out at 0, 2, 7, 14, 28, 42, 56, 86, 116, 146 and 176 days of incubation. pH, organic carbon, microbial basal respiration, microbial biomass carbon, available phosphorus and phosphatase activity were measured. A multi-level factorial experiment with complete randomized design was employed in three replications. Mean comparisons were done by Duncan method at p≤ 0.05. Data analysis was carried out on SAS. The effect of solid waste (W), bacteria (B) and time (T) and their interaction effects on soil studied properties were significantly difference (p≤0.01). Results of the mean comparison of the effect of the different levels of waste and bacteria interaction showed that pH decreased with increasing the amount of solid waste and bacteria activity. The most basal respiration was observed in W4Pcha treatment. With increasing the levels of solid waste, the amount of organic carbon also increased and in W4Pcha treatment reached to 3.1%. The maximum amount of microbial biomass carbon was observed in treatments that received solid waste. The maximum amount of available phosphorus was observed in W4Pi (99.1 mg kg-1) and the minimum amount was observed in W0Pn. The activity of phosphatase enzyme also increased with increasing the levels of waste in Pi and Pch bacteria. The results of the effect of waste and time interaction showed a rapid but temporary increase in basal respiration in response to waste. Significant changes were observed in basal respiration, biomass carbon and organic carbon in soils that received solid waste compared to control soil. Adding of solid waste with phosphorous solubilizing bacteria increased phosphatase activity (as an indicator of microbial activity and phosphorous mineralization) in soil. The application of bacterial and solid waste treatment is a solution that, in addition improving the biological properties of the soil, can lead to increase availability of phosphorus, reduce the using of phosphate fertilizers use and the sustainable management of this type of solid waste.