2.4. Biochemical properties of soil
The soil biological activity measurements included enzymatic activity
and soil respiration. Dehydrogenase activity (DhA) was determined by the
method in Thalmann (1968) using triphenyltetrazolium chloride (TTC) as
the electron acceptor. Samples were incubated at 37±2 °C for 24 hours.
The enzymatic activity of the dehydrogenases was determined by
colourimetry using a Backman DU 640 spectrophotometer (Beckman Coulter,
Woonsocket, USA) at a wavelength of 546 nm.
Urease activity (Ure) was determined by the Zantua and Bremner (1975)
method, with urea as a substrate, after a 18 hour incubation at 37 °C.
The enzymatic activity of ureases was determined by colourimetry using a
Backman DU 640 spectrophotometer at a wavelength of 470 nm.
Basal respiration (BR) was determined according to the ISO 16072
(International Organization for Standardization, 2002) method. Moist
soil subsamples (20 g) were incubated at 20±2 °C for 24 hours. The
released CO2 was absorbed in a solution of 0.05 M NaOH
and precipitated as barium carbonate with the addition of a 0.5 M
BaCl2 solution. The unconsumed sodium hydroxide was
titrated with 0.1 M HCl in the presence of phenolphthalein as an
indicator, and the amount of CO2 was calculated (ISO
16072: 2002). Substrate-inducted respiration (SIR) was determined by the
quantification of carbon dioxide evolution shortly (6 hours) after
glucose addition (aqueous solution equivalent to 10 g glucose per kg of
soil) (ISO 14240-1: 1997). The respiratory-activation quotient (QR) was
calculated by dividing the BR rate by the SIR rate according to ISO
17155 (2012).