Understory vegetation interacts with nitrogen addition to affect soil phosphorus transformations in a nutrient-poor Pinus sylvestris var. mongolica plantation

Globally increasing atmospheric nitrogen (N) deposition has aggravated phosphorus (P) limitation in many forests around the world. However, how N deposition affects soil P transformations and what is the role of understory vegetation in regulating these N deposition impacts are largely unknown. Here, we examined soil P transformation characteristics (P fractions, adsorption capacity, and related soil physicochemical properties) in a nutrient-poor Mongolian pine (Pinus sylvestris var. mongolica) plantation that has been subjected to six years of N addition and understory removal treatments in northern China. Nitrogen addition alone significantly elevated labile and moderately labile organic P concentrations by 1.2 times and 24.1%, but decreased labile inorganic P and occluded P concentrations by 42.8% and 41.5%. Nitrogen addition elevated maximum P adsorption capacity, but did not affect maximum buffer capacity. Understory removal alone significantly reduced occluded P concentration by 38.6% but did not affect other P fractions. There were significantly negative interactions between understory removal and N addition on labile inorganic P, all organic P fractions and occluded P concentrations, as N addition effects on these variables were non-significant when the understory vegetation was removed. These results indicate that N addition depressed mineralization of organic P, but accelerated solubilization of occluded P, and these N addition effects were alleviated by understory removal. Significantly positive correlations between soil pH, microbial biomass and P fractions suggest that reduction in soil pH and microbial biomass are largely responsible for the variation in soil P fractions. These findings greatly improve our understanding of the consequences of N addition on soil organic and inorganic P transformations and the role of understory vegetation in affecting the responses of soil P transformation to N deposition.