2.4 Soil property analysis
Soil MBC and MBN were determined using the chloroform fumigation
extraction method (Wu et al., 1990; Jenkinson et al., 2004). One
subsample (20 g) was directly extracted with
K2SO4 solution (60 mL; 0.5 M); another
(also 20 g) was first fumigated with ethanol-free chloroform in the dark
for 24 h and then extracted with K2SO4solution (60 mL; 0.5 M). The extracted C in the fumigated and
unfumigated samples was measured after acidification using the
Shimadzu TOC-VWP analyzer
(Shimadzu), and inorganic N (NO3- and
NH4+) was extracted with 2 M KCl and
analyzed using a continuous-flow analyzer (Fiastar 5000; Foss Tecator
AB, Höganäs, Sweden). The TN of the soil samples was determined using an
elemental analyzer (MACRO cube, Elementar, Germany).
MBC and MBN were calculated as the difference in C and N contents
between fumigated and non-fumigated sample extracts, adjusted by a
proportionality coefficient (kEC = 0.45, kEN = 0.54) to account for the
extraction efficiency. The 0.5 M
K2SO4-extractable C from the unfumigated
sample was considered as DOC (Vance et al., 1987).
The leaching water samples collected for
NO3-(NO3--L) and
NH4+(NH4+-L) were analyzed using a
continuous-flow analyzer (Fiastar 5000; Foss Tecator AB, Höganäs,
Sweden). Leaching water DOC (DOC-L) was measured using the Shimadzu
TOC-VWP analyzer (Shimadzu).
The metabolic quotient (qCO2) and microbial carbon-use
efficiency (CUE) were calculated using previously published methods
(Anderson and Domsch, 1993; Sinsabaugh et al., 2013).
qCO2 was defined as the ratio of C in respired
CO2 and soil microbial biomass (qCO2 =
CO2-C/MBC) and the CUE was defined as the amount of C in
the microbial biomass relative to the sum of the CO2-C
and MBC [CUE = MBC/(MBC + CO2-C)].