Changes in soil phosphorus fractions after 9 years of continuous nitrogen addition in a Larix gmelinii plantation

• The key message N addition decreased soil inorganic P availability, microbial biomass P, and acid phosphatase activity in the larch plantation. Soil inorganic P availability decreased after N addition due to the changes in both microbial properties and plant uptake. • Context Soil phosphorus (P) availability is considered an important factor in influencing the biomass production of plants. Sustained inputs of nitrogen (N) through atmospheric deposition or N fertilizers, particularly in temperate forests, may change the composition and availability of P and thus affect long-term forest productivity. • Aims The objective of this study was to assess soil P availability, P fractions, and microbial properties including microbial biomass P and acid phosphatase activity after 9 consecutive years of N addition in a larch ( Larix gmelinii ) plantation, northeastern China. • Methods From 2003 to 2011, NH 4 NO 3 was added to replicate plots (three 20 m × 30 m plots) in the larch plantation each year at a rate of 100 kg N ha −1 year −1 . Soil samples from 0–10-cm and 10–20-cm depths were collected in N addition plots and control (no N addition) plots. • Results N addition significantly decreased soil NaHCO 3 -Pi (Pi is inorganic P), microbial biomass P, and acid phosphatase activity but increased the NaOH-Pi concentration. N addition appeared to induce a decrease in soil inorganic P availability by changing pH and P uptake by trees. In addition, N addition significantly decreased the NaOH-Po (Po is organic P) concentration, possibly because of increased P mineralization. However, the total P and other P fractions were unaffected by N fertilization. • Conclusion Our results suggested that N addition enhanced P uptake by trees, whereas it reduced soil inorganic P availability as well as microbial biomass and activity related to soil P cycling in the larch plantation.