Soil phosphorus accumulation changes with decreasing temperature along a 2300 m altitude gradient

Understanding the abundance of organic P in soil is a prerequisite for predicting the effects of climate change on P dynamics and availability in cold alpine regions. We sampled plant roots (up to 40 cm depth) and soils (100 cm depth) in grasslands along a 2300 m altitude gradient (1286-3589 m above sea level, mean annual temperature (MAT) from 9 to 0.3 degrees C) between alpine steppes on the Loess Plateau and alpine meadows on the Tibetan Plateau. A modified Hedley P fractionation in combination with root and microbial P, alongside phosphatase activity, was used to characterize P transformations depending on climate. Both the roots and microorganisms stored more P and produced higher phosphatase activity in cold meadow than in warm steppe soils. Total inorganic P (Pi) content decreased in cold meadow (MAT < 3 degrees C) compared with warm steppe soils because of the decrease of moderately labile Pi (extracted with diluted HCl) in cold meadow soils. Available P (NaHCO3) and labile Pi (NaOH) contents increased upslope, whereas the recalcitrant Pi (concentrated HCl) contents remained stable at all sites down to 100 cm soil depth. Labile (NaHCO3), moderately labile (NaOH) and recalcitrant (concentrated HCl) organic P (Po) contents sharply increased in cold meadow compared with warm steppe soils. The residue P in the 0-100 cm along the temperature gradient was similar to that of all Po fractions. The Po proportion in the total P was less than 10 % at sites with a MAT above 3 degrees C, but sharply increased up to 50 % at sites with a MAT below 0.7 degrees C. The greater root and microbial P uptake versus slower Po mineralization in cold meadows than in warm steppes increase the incorporation of Pi into organic pools in cold alpine areas. This accumulation of Po forms is a mechanism to prevent P losses by leaching as inorganic forms. The increased plant available P in cold meadows was associated with the increased phosphatase activity and Po contents compared with warm steppe soils. We conclude that the on-going climate warming could stimulate P cycling more in cold meadows than in warm steppe regions because Po dominated the total soil P in cold areas.