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

Abstract

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. 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. 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. From 2003 to 2011, NH4NO3 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. N addition significantly decreased soil NaHCO3-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. 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.