2.2 Soil analyses
A total of 11 soil physicochemical properties were measured to assess the overall multifunctionality of the rhizosphere soil based on the soil multinutrient index proposed by Jiao et al. (25). The soil physicochemical properties measured were pH, soil organic matter (SOM), soil organic carbon (SOC), total nitrogen (N; TN), AP, TP, nitrate N (NO3--N), ammoniacal N (NH4+-N), microbial biomass carbon (C; MBC), microbial biomass N (MBN), dissolvable organic C (DOC), and dissolvable organic N (DON). Rhizosphere soil multinutrient index was used to quantify soil multinutrient cycling, and the index values were calculated based on rhizosphere soil nutrient properties that had been normalized using log-transformation (11, 24).
Soil pH was measured using a glass electrode with a soil-water ratio of 1:2.5 (v/w). Soil SOM was determined by potassium dichromate oxidation with external heating(4). Soil TN was determined using the Kjeldahl method(4), and soil TP was determined using the molybdenum antimony anticolorimetric method after perchloric acid–sulfuric acid digestion (42). Soil NH4+-N and NO3--N were extracted using 0.5 mol L-1 KCI and then analyzed using a continuous flow analyzer (AutoAnalyzer-AA3; Seal Analytical, Norderstedt, Germany). Soil MBC and MBN were determined using the chloroform fumigation–extraction method with conversion factors of 0.45 and 0.54, respectively (29). Soil DOC and DON were measured using a total organic C analyzer (vario TOC select; Elementar Analysensysteme, Frankfurt, Germany) after extraction with 0.5 mol L-1 K2SO4(33).