Effect of bacterial biofertilizers, native arbuscular mycorrhizal fungi and soil amendments on soil and grain phosphorus availability of flooded rice in dry zone, Sri Lank

Rice is considered as the most important food crop in Sri Lanka and most of the Asian countries. Over the past three decades, rice farmers have become increasingly dependent on synthetic chemical phosphorous fertilizers, mainly triple super phosphate (TSP) as a source of phosphorous (P). However, dramatically increasing costs, serious environmental and health issues attached to chemical fertilizers, forced researchers to develop supplementary or alternate sources of P for rice. Hence, this study was done to evaluate the potential use of biofertilizers and natural soil amendments as alternatives to chemical P fertilizers in rice (Oryza sativa L.) farming in the dry zone of Sri Lanka. A field trial was carried out at Ranpathwela in Anuradhapura, North Central Province, Sri Lanka, during the Yala season in 2016. The experiment was designed as follows: CON: control; AMF: AMF inoculants (2 Mg ha-1); RAMF: ERP (153.3 kg ha-1) with the AMF inoculants (2 Mg ha-1); MC: mixed microbial culture (5 kg ha-1); RMC: ERP (153.3 kg ha-1) with a mixed microbial culture [consortium of Azospirillum sp., Pseudomonas sp. and Bacillus sp. (5 L ha-1)]; BC: biochar (6 Mg ha-1); CP: standard compost (10 Mg ha-1) and IF: inorganic synthetic fertilizer (125 N kg ha-1, 62.5 P2O5 kg ha-1 and 50 K2O kg ha-1). Three indigenous and two improved rice varieties were used. The experiment comprised of 24 plots and three replicates. Soil, roots and grains of rice were analyzed for phosphorus. Some other elements were shown to have an effect on plant available soil P such as aluminum and iron, were also estimated. It was revealed that significant differences (p<0.05) in variety, treatment and V x T interaction was observed in plant available soil phosphorus. The highest mean of available soil P was observed in TRSP2 x AMF (14.64 ± 0.12 mg kg-1) followed by TRSP3 x MC (8.87 ± 0.001 mg kg-1) interactions. The multifunctional microbial consortium in amended soil, including Azospirillum sp., Pseudomonas sp. Bacillus sp. and arbuscular mycorrhizal fungi, were capable of potentially increasing soil phosphorus.  Furthermore, the addition of rock phosphate did not always make a difference in soil P availability for rice farming systems.