2.3. Soil sampling and laboratory analyses
The experiment was conducted on August 12, 2018 for in situ simulation of the litter decomposition of exogenous N and P addition, and the nylon net bags were retrieved on the 90th, 270th and 360th days of decomposition, respectively. After retrieval, the surface of the net bags was gently brushed, and plant fine roots and gravel were picked out from the bags, followed by washing the soil in the bags with ultrapure water. Each treatment (3 bags) was mixed well and dried in an oven at 60 ℃ for 72 h to a constant weight, then weighed and recorded after passing through a 1 mm steel sieve, and crushed through a 0.149 mm steel sieve for determination of total carbon, total nitrogen, total phosphorus, lignin and cellulose concentration of litter. At the same time, soil was collected from the 0-10 cm layer under the nylon net bags. Three bags of soil samples from each sample were mixed into one bag, totaling 30 bags.
Plant total carbon was determined by concentrated sulfuric acid-potassium dichromate oxidation method, total nitrogen by concentrated sulfuric acid-hydrogen peroxide-boiling-Kaeschner method, and total phosphorus by molybdenum antimony anti-colorimetric method (Lu, 1999); lignin and cellulose were determined by acid detergent fiber method (ADF method); soil organic carbon (SOC) was determined using potassium dichromate external heating method, total nitrogen (TN) by Kaeschner method, total phosphorus (TP) by molybdenum antimony anti-colorimetric method. 3,5-dinitrosalicylic acid colorimetric method for the determination of soil invertase (INV) and cellulase (CBH), sodium phenol-sodium hypochlorite colorimetric method for the determination of urease (UG), sodium hydrogen phosphate colorimetric method for the determination of alkaline phosphatase (AP), and o-phenylene triol colorimetric method for the determination of polyphenol oxidase (POX) (Blair et al., 1994).