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).