Phosphorus application promoted the sequestration of orthophosphate within soil microorganisms and regulated the soil solution P supply in a temperate grassland in northern China: A 31P NMR study

Anthropogenic activities have made phosphorus (P) a new sustainability issue globally. To better understand the mechanisms of soil P dynamics responding to inorganic P fertilization (0-12.5 g P m(-2)yr(-1)), we conducted a 2-year field experiment in a temperate grassland in northern China. The P fertilizer was added once per year before the growth period as NaH2PO4. We randomly collected topsoil samples (0-10 cm) and analyzed the soil properties, microbial community, and extracellular enzyme activities, and quantified the soil P fractions using solution P-31 nuclear magnetic resonance (P-31 NMR) spectroscopy. P application promoted transformation from organic P to inorganic P driven by microbial carbon (C) demand and regulated by altered soil properties and microbial characteristics. P addition decreased orthophosphate diesters, especially DNA, due to increased hydrolysis under increased soil pH. This may increase hydrolysates such as alpha-glycerophosphate and choline phosphate. Orthophosphate monoesters, especially myo-inositol hexaphosphate, also decreased following increased P availability. This could be explained by increased alkaline phosphatase activity and reduced sorption due to a decreased metal (Ca, Al, and Fe) content. Furthermore, P fertilization increased soil NH4+-N, thereby increasing the growth of phosphonate-solubilizing microorganisms (Bacillus, Streptomyces, Bradyrhizobium, Mesorhizobium, Penicillium, Aspergillus, and Fusarium). Their increased growth also contributed to inorganic P solubilization and organic P mineralization due to their P-solubilizing specificity. Together, these factors increased inorganic P, especially orthophosphate, which was temporarily immobilized in microbial cells, to regulate the soil-solution P supply, and created a potentially available P sink in the grassland.

Automated summary

Anthropogenic activities have raised concerns about the sustainability of phosphorus (P) globally. In a 2-year field experiment in northern China, we examined the effects of inorganic P fertilization on soil P dynamics in a temperate grassland. Phosphorus fertilizer was applied annually before the growth period. We analyzed soil properties, microbial community, extracellular enzyme activities, and soil P fractions using P-31 NMR spectroscopy. The results showed that P application promoted the conversion of organic P to inorganic P, driven by microbial carbon (C) demand, and influenced by soil properties and microbial characteristics. The addition of P decreased orthophosphate diesters, especially DNA, due to increased hydrolysis under increased soil pH. Orthophosphate monoesters, particularly myo-inositol hexaphosphate, also decreased following increased P availability, attributed to increased alkaline phosphatase activity and reduced sorption due to decreased metal content. Additionally, P fertilization increased soil NH4+-N, promoting the growth of phosphonate-solubilizing microorganisms, which contributed to inorganic P solubilization and organic P mineralization. Overall, these findings highlight the complex mechanisms underlying soil P dynamics and the role of P fertilization in regulating P availability in grassland ecosystems.