Phosphorus limitation in microorganisms leads to high phosphomonoesterase activity in acid forest soils

We addressed the mechanistic basis for a negative correlation between soil pH and phosphomonoesterase activity, often found in various soils. Also in the present study, a significant negative correlation was observed between soil pH and phosphomonoesterase activity measured at pH 6.5 in Japanese acidic forest soils (3 Inceptisols, 3 non-allophanic Andisols, and 2 allophanic Andisols). A hypothesis that higher activity of phosphomonoesterase in acid forest soils results from increased synthesis of phosphomonoesterase by microorganisms in response to P limitation was tested. Soils with lower pH showed a lower optimum pH for phosphomonoesterase activity and greater activity at the optimum pH than other soils. To assess nutrient limitations of the soil microbial community, the effects of addition of C, N, or P on phosphomonoesterase and dehydrogenase activities, which is an intracellular enzyme and the activity of which reflects overall microbial activity, were examined in the soil samples. Addition of P increased dehydrogenase activity in some forest soils. Also, microorganisms in some soils were co-limited by C, N, and P. Response ratios (RR) of phosphomonoesterase and dehydrogenase activities in P-amended soil to their activity in non-amended soil were used to evaluate the response of soil microorganisms to P limitation. The ratio of RR-dehydrogenase to RR-phosphomonoesterase was strongly correlated with phosphomonoesterase activity at the optimum pH (P < 0.01). The results indicate that P limitation accounts for higher phosphomonoesterase activity in the more acid forest soils.