Efficacy of phosphorus enriched post-methanation bio-sludge from molasses based distillery as P source to rice and wheat crops grown in a Mollisol: I. Laboratory and greenhouse evaluation with 32P-labeled sources

Laboratory and greenhouse experiments were carried out with 32P labeled sources to compare the kinetics of desorption, chemical transformation and efficacy of P enriched post-methanation bio-sludges from molasses based distillery (PEMB 1 and PEMB 2) against single superphosphate (SSP), a conventional P fertilizer. Phosphorus enrichment of bio-sludge was done with either SSP (PEMB 1), or with an equimolar mixture of SSP and ortho-phosphoric acid (PEMB 2). Desorption kinetics of added P (50 kg P2O5 ha− 1) showed a biphasic kinetics with an initial faster phase followed by a slower desorption phase. Desorption rate coefficient (K) for the initial faster desorption phase was the highest for PEMB 2 followed by PEMB 1 and the lowest for SSP. The K value for the slower desorption phase was also significantly higher for PEMB 2 than SSP. Sequential extraction of different chemical fractions of added P in the soil revealed that the P added as PEMB 1 or PEMB 2 was less transformed into occluded-P than SSP. In the greenhouse experiments, the dry matter yields (DMY) of rice with PEMB 2 at 37.5 kg P2O5 ha− 1 and with SSP at 50 kg P2O5 ha− 1 were statistically similar but the total P uptake with PEMB 2 at 37.5 kg P2O5 ha− 1 showed a significantly higher trend. The residual effect of PEMB 1 or PEMB 2 applied to rice at 37.5 kg P2O5 ha− 1 on DMY of subsequently grown wheat was significantly higher than that of SSP at 50 kg P2O5 ha− 1. The residual effect of PEMB 2 applied to rice at 37.5 kg P2O5 ha− 1 on the total P uptake by wheat grown after rice was significantly higher than that of SSP at 50 kg P2O5 ha− 1 or PEMB 1 at 37.5 kg P2O5 ha− 1. The residual effect of PEMB 2 applied to rice at 25.0 and 37.5 kg P2O5 ha− 1 on % P in plants derived from fertilizer (Pdff) was significantly higher than that of PEMB 1 at the corresponding application rates or SSP at 50 kg P2O5 ha− 1. The % utilization of residual P by the subsequently grown wheat after rice was the highest with PEMB 2 followed by PEMB 1 and the lowest with SSP. For the direct P fertilized wheat, the highest DMY was noted with PEMB 2 at 37.5 kg P ha− 1 which was significantly higher than the yields with PEMB 1 at 37.5 kg P ha− 1 or SSP at 50 kg P2O5 ha− 1. Total P uptake by wheat fertilized with PEMB 2 at 37.5 kg P2O5 ha− 1 was significantly higher than with PEMB 1 at 37.5 kg P2O5 ha− 1 but statistically similar with SSP at 50 kg P2O5 ha− 1. For both direct P fertilized rice and wheat, the values of Pdff were statistically similar with PEMB 2 at 37.5 kg P2O5 ha− 1 and with SSP at 50 kg P2O5 ha− 1. However, the % utilization of P by directly fertilized rice or wheat was significantly higher with PEMB 2 than with SSP or PEMB 1.