Diurnal variations in the content and oxygen isotope composition of phosphate pools in a subtropical agriculture soil

Phosphorus (P) is an essential nutrient that affects crop yields, soil ecosystems and environmental pollution. The oxygen isotope composition of phosphate (δ¹⁸OP) in soil is mainly controlled by P source, temperature, the δ¹⁸OW of soil water, and biological processes and has been used as a unique tracer for the sources and interconversion of P pools within soil. Understanding if and how the diurnal variation of the environmental factors affects the δ¹⁸OP of soil P pools is thus indispensable to elucidating the biogeochemical cycling of different P pools and to a better use of soil δ¹⁸OP as a tracer. Soil samples from the same location with different moisture contents were collected every 3 or 12 h for 7 days for the measurement of the content and oxygen isotope composition of soil water and P pools. The results show that soil temperature, moisture and the δ¹⁸OW of soil water had clear diurnal variations, with δ¹⁸OW decreasing with increasing soil moisture. However, both the content and δ¹⁸OP of different soil P pools did not show a significant diurnal variation. These characteristics indicate that short-term diurnal changes in environmental conditions and biological activities have little effect on the content and δ¹⁸OP of soil P pools. Therefore, the diurnal sampling time has no effect on the content and δ¹⁸OP of soil P pools. However, the equilibrium δ¹⁸OP value represents the complete exchange of oxygen isotopes between phosphate and water. Comparisons of the measured values with the equilibrium value could decipher the degree of P recycling by organisms. This study shows that the mean, not the measured (at the sampling time), temperature and δ¹⁸OW of soil water should be used to calculate the representative equilibrium δ¹⁸OP value of soil to avoid bias.