Enhanced available phosphorus in paddy fields applying biochar and water-saving irrigation together: The role of alkaline phosphomonoesterase-harboring microorganisms.

Organic phosphorus mineralization is one of the important pathways for phosphorus mobilization. However, the level of organic phosphorus mineralization in rice paddies under the coupling of water-saving irrigation and biochar has yet to be determined. To address this knowledge gap, we investigated the interplay between available phosphorus fractions, alkaline phosphomonoesterase activity, and bacterial communities encoding alkaline phosphomonoesterase in water-saving irrigated paddy fields subjected to different biochar applicated rates (0 t ha, 20 t ha, and 40 t ha), compared with a control (flooding irrigation with 0 t ha biochar application). The findings revealed that changes in alkaline phosphomonoesterase activity were linked to variations in phoD gene numbers and the composition of phoD microbial communities. Water-saving irrigation, as opposed to flooding irrigation, notably enhanced the capacity of organic phosphorus mineralization in paddy fields. This enhancement was supported by a 27.94%-45.84% increase in alkaline phosphomonoesterase activity and a 23.00%-36.06% decrease in enzyme-extractable phosphorus in water-saving irrigated paddy fields vs. flooding irrigated paddy fields. Simultaneously, the important role of Firmicutes among phoD-harboring microorganism in water-saving irrigated rice paddies was detected. However, the increased capacity of microbes-mediated organic phosphorus mineralization under the condition of water-saving irrigation alone was insufficient to offset the resulting phosphorus depletion. Biochar could improve the available phosphorus content, which promoted competition among phoD-carrying microorganisms, raising the risk of phosphorus loss. The combined approach of water-saving irrigation and 40 t ha biochar application enhanced the availability of phosphorus and simultaneously mitigated the risk of phosphorus loss, which is proved to be a more suitable field management strategy for paddy fields in southern China.