Is the enzymatic hydrolysis of soil organic phosphorus compounds limited by enzyme or substrate availability?

The mineralization of soil organic phosphorus (P) is catalysed by extracellular phosphatases that hydrolyse different non-phytate phosphomonoesters (e.g., sugar phosphates and nucleotides), inositol phosphates (e.g., phytic acids), and phosphodiesters (e.g., nucleic acids, phospholipids). The availabilities of both the organic P compound and the respective phosphatase enzyme jointly determine whether the hydrolysis reaction is substrate limited (enzyme availability > substrate availability) or enzyme limited (enzyme availability < substrate availability), with potential consequences for overall organic P composition and accumulation in soil. Our objective was to evaluate whether the hydrolysis of various organic P compounds in soil is limited by availability of substrate or enzyme. To this end, we combined the principles of enzyme activity assays and enzyme addition assays. Ten soils with contrasting properties and origins received either model organic P substrate, specific phosphatases or a combination of both, added either to a soil:water suspension or a soil:water filtrate. Soil indigenous activity of phosphatases was low in all filtrates, confirming that enzymes were mostly associated with the solid fraction of the soil. In soil suspensions, the rapid hydrolysis of added non-phytate phosphomonoester substrate by soil indigenous enzymes indicated high availability of non-phytase phosphomonoesterase enzymes. In combination with the low availability of non-phytate phosphomonoester substrate in soil suspensions, determined by adding enzymes to the suspensions, this indicated a substrate limitation for the hydrolysis of non-phytate phosphomonoesters in soil. In contrast, enzyme-limitation was found for phytate and phosphodiesters. A review of the available literature on the production, stabilisation and hydrolytic activity of each of the three analysed organic P substrates and the respective enzymes supported our findings. We therefore suggest that not only the production and stabilisation but also differences in available phosphatase enzymes strongly determine the occurrence of specific organic P compounds in soil.