Implications of Soil Biodiversity for Sustainable Organic Matter Management - Sustainable Management of Soil Organic Matter - page 379

26 gNg −1 soil), and one which had received inorganic N but no P and K

for 30 years ( nutrient-depleted ) (20% clay, 20% silt, 37% fine sand and 23%
coarse sand; organic C content, 1.25%; N content, 0.12%; inorganic P
(Olsen-P), 14

µ

gNg −1 soil).
14 C-labelled glucose (0.89 mg g −1 soil, specific activity: 560 kBq g −1 )
was added to the sieved soil. The soil was pre-incubated at 25

µ gg −1 soil; pH, 5.6; SMB nitrogen, 14 µ

C for 4
weeks. Soil samples were packed in PVC cylinders (3 cm long, 5.6 cm
diameter) closed at one end with a tightly attached 53

°

m nylon mesh.
Ryegrass ( Lolium perenne ) plants were grown in PVC cylinders (length
10 cm, inner diameter 4.4 cm). When root development at the bottom was
abundant, the cylinders were placed on top of the labelled soil in a model
rhizosphere system (Gahoonia and Nielsen, 1991). The plant cylinders
were screened from the labelled soil by the 53

µ

m nylon mesh. Root hairs
were able to penetrate the mesh, thus creating a one-dimensional model
root surface. The soil cylinders were placed on a sand bench, which
maintained soil moisture at 15% (w/w) corresponding to 50% of water-
holding capacity. Control tubes containing no plants were included to
check for non-rhizosphere effects on the soil.
Plant (four replicates) and control (two replicates) soil cylinders
were harvested after 15 days. At harvest, the labelled soil was frozen in
liquid nitrogen. The frozen soil cores were sliced parallel to the root mat at
0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10.0 and 15.0 mm distance. SMB
was measured using fumigation extraction (Brookes et al ., 1985; Vance
et al ., 1987), SMB N was measured as ninhydrin-reactive N (Amato and
Ladd, 1988; Carter, 1991) which was converted to biomass N using a k ninh
value of 0.20 (Joergensen and Brookes, 1990). SMB 14 C was determined by
liquid scintillation counting. Values were corrected to total

µ

14 C microbial

biomass using a k EC value of 0.45 (Wu et al ., 1990).
The effect of freezing on the SMB was tested by analysing frozen and
unfrozen soil samples for SMB N and SMB 14 C content.

Results

From the beginning of the experiment, the SMB N in the fertile soil was
twice as large as in the nutrient-depleted soil. At harvest, the SMB in the soil
fractions closest to the root mat had increased from 14 to > 40

gNg −1

µ

gNg −1 in the fertile soil (Fig.
5.5.1). The rhizosphere effect extended 1 mm into the nutrient-depleted soil
and 2.5 mm into the fertile soil.
After freezing, SMB N was reduced by 16.5% in the fertile soil and by
14.3% in the nutrient-depleted soil. SMB 14 C was reduced by 5.6 and 5.3%,
respectively.

in the nutrient depleted soil, and from 26 to 57 µ