2.1 Study site description and experimental design
A long-term field nutrient addition experiment was carried out in Hezuo, Gansu, eastern Qinghai-Tibetan Plateau of China (N34°55’, E102°53’, 3,000 m above sea level). Hezuo has a mean annual temperature of 2.4°C and a 30-yr mean annual precipitation of 550 mm (1986 to 2015; Institute of Hezuo Meteorology). The site belongs to a typical alpine meadow with soil and vegetation characteristics as described previously (Li et al., 2014).
The experiment consists of four treatments (N alone, P alone, NP together, and a control without any nutrient addition (Control)) with five, 5 m × 5 m, replicates. Nitrogen and/or P in the form of urea and sodium dihydrogen phosphate anhydrous were applied annually (10 g m-2 yr.-1) since 2009. For a complete description of the experimental design and establishment see Li et al. (2014).
Soil samples of 500 g were sampled from a 0-20 cm depth randomly in each treatment plot in August 2018, sieved thru a 2-mm to remove roots, homogenized and divided into two portions. One portion was stored at 4 °C for determination of soil available N (AN), microbial biomass C (MBC), microbial biomass N (MBN), microbial biomass P (MBP) and for the priming experiment. The other portion was air dried and subsequently used for pH, SOC, TN and total P (TP) analysis.