Equations for Soil Microbial Biomass C and N Calculation

VARIATE:

DEFINITION AND EQUATION:

CO2_C:

CO2 mineralized in 10 day incubation of unfumigated soil at 25 C (µg C g-1 soil):

CO2_C = (B-T) * M*6/W

where
B = Titre for blank base trap (mL),
T = Titre for sample base trap (mL),
M = Molarity of HCl,
W = dry weight of soil (g)

CO2_F:

CO2 mineralized in 10 day incubation of chloroform fumigated soil at 25 C (µg C g-1 soil):

CO2_F = (B-T) * M*6/W

where
B = Titre for blank base trap (mL),
T = Titre for sample base trap (mL),
M = Molarity of HCl,
W = dry weight of soil (g)

CFIBio_C:

Microbial biomass C (µg C g-1 soil):

CFIBIO_C = 1.73 * CO2_F – 0.56 * CO2_C

The constants 1.73 and 0.56 are derived from regression of CFI data against microscopically determined biomass C (Horwath et al., 1996. Defining a realistic control for the chlororform fumigation – incubation method using microscopic counting and 14C substrates. Can. J. Soil Sci. 76: 459-467).

NF:

N mineralized in 10 day incubation of fumigated soil (µg N g-1 soil):

NF = (NH4F+NO3F) – (NH4I+NO3I)

where:

NH4F
(µg/g) = ammonium concentration in soil extract (ppm-N) from fumigated soil after 10 day incubation * soil extract volume (100 mL) / dry weight of extracted soil (g)

NO3F
(µg/g) = nitrate concentration in soil extract (ppm-N) from fumigated soil after 10 day incubation * soil extract volume (100 mL) / dry weight of extracted soil (g)

NH4I
(µg/g) = ammonium concentration in soil extract (ppm-N) from soil before fumigation * soil extract volume (100 mL) / dry weight of extracted soil (g)

NO3I
(µg/g) = nitrate concentration in soil extract (ppm-N) from soil before fumigation * soil extract volume (100 mL) / dry weight of extracted soil (g)

NC:

N mineralized in 10 day incubation of unfumigated soil (µg N g-1 soil):

NC = (NH4C+NO3C) – (NH4I+NO3I)

where:

NH4C
(µg/g) = ammonium concentration in soil extract (ppm-N) from unfumigated soil after 10 day incubation * soil extract volume (100 mL) / dry weight of extracted soil (g)

NO3C
(µg/g) = nitrate concentration in soil extract (ppm-N) from unfumigated soil after 10 day incubation * soil extract volume (100 mL) / dry weight of extracted soil (g)

NH4I
(µg/g) = ammonium concentration in soil extract (ppm-N) from soil before fumigation * soil extract volume (100 mL) / dry weight of extracted soil (g)

NO3I
(µg/g) = nitrate concentration in soil extract (ppm-N) from soil before fumigation * soil extract volume (100 mL) / dry weight of extracted soil (g)

CFIBIO_N:

Microbial biomass N (µg N g-1 soil):

CFIBIO_N = CFIBIO_C

(0.56
*
(NF-pNC)

/

(CO2_F-p*CO2_C)

+

0.095

where
p = 0.29 * (CO2_F/CO2_C) + 0.23

The constants 0.56 and 0.095 are derived from regression of the amount of CO2-C and mineral-N released on fumigation of soil to which organisms of known C and N contents have been added