Diversity of multifunctional phosphorus solubilizing bacteria in acid soils of diverse hill rice Ecosystems

In the anticipated scenario of global phosphorus (P) crisis, the exploitation of phosphate solubilizing bacteria (PSB) has been well recognized as an eco-friendly P management option so as to reduce the dependency on the dwindling inorganic-P (Pᵢ) fertilizer sources and increase agronomic P-use efficiency in acidic soils. The understanding on the incidence of PSB in ecological niches and their screening for multifaceted plant growth promoting (PGP) traits are essential from the view point of their habitat adaptability on application as biofertilizer and to obtain multiple benefits in acid soils. This study assessed the incidence of PSB in rice rhizospheric soils (pH 4.29 to 5.6) from five distinct hill rice ecosystems of North East India. Soil and rice root sampling were done in four states of NE India viz. Mizoram, Manipur, Arunachal Pradesh and Meghalaya. The diverse rice fields were categorised based on the slope degree, soil moisture content, management practices and soil contamination. Among 172 PSB isolates, the maximum Pᵢ dissolved from tricalcium phosphate (TCP) in Pikovskaya's broth (PB) was up to 1.2 μg mL⁻¹ h⁻¹. Only 15.1 and 40.7% isolates out of 172 showed aluminium phosphate (AlPO₄) dissolution and phytate mineralization abilities, respectively. Among the 60 PSB isolates (able to dissolve TCP and AlPO₄ and mineralize phytate), only 40, 83.3, 100 and 68.3% isolates were positive for the production of indole acetic acid (IAA) like substances (0.52 to 17.4 mg mL⁻¹ h⁻¹), N₂-fixation in nitrogen free bromothymol blue semi-solid agar (qualitative test), acid-phosphomonoesterase activity (0.16 to 3.24 mg mL⁻¹ h⁻¹) and carboxymethyl cellulose dissolution (1.0 to 16.0 mm zone diameter) activity, respectively. Acidic rice soils harbour efficient P solubilizers having multifaceted PGP traits. The significant correlation among the quantity of dissolved Pᵢ in TCP, AlPO₄ and phytate amended PB indicated that the screening of PSB isolate on TCP amended Pikovskaya plate could successfully trap the efficient PSB isolates from acid soils. The incidence of 52.3% of 21 efficient PSB isolates belong to genus Enterobacter (16S rRNA sequence-based identity) indicating that the genus Enterobacter is a potential candidate of PSB biofertilizer for application in strongly acid soils.