The Role of Soil Organic Matter and Manures in Sustainable Nutrient Cycling - Sustainable Management of Soil Organic Matter - page 267

γ -irradiation which effectively kills all active and dormant microorganisms

but may still allow for some enzyme activity (Powlson and Jenkinson,
1976).

Materials and Methods

The study was conducted with the sandy-loamy Ap horizon of a
Fluvisol from the Oderbruch area in Eastern Brandenburg, Germany.
After sample collection in autumn 1997, the soil was air-dried and
passed through a 2 mm mesh and then stored in the dark. It had the
following properties: 61% sand, 24% clay; pH 7.2; 6.4% C org ; CEC p
227 mmol c kg −1 .
For every incubation experiment, samples were moistened to 60%
water-holding capacity (15% w/w) and filled into open steel cylinders
(100 ml, 5 cm diameter) and pre-incubated at 5

° C for 24 h. A sub-set

of these samples was then sterilized by -irradiation (25-29 kGy), and a

further sub-set of these sterilized samples was re-inoculated by replacing
10% of the soil with non-sterile soil. For the temperature experiment, all
samples were then placed into sealed vessels of an incubation apparatus
with

γ

continuous CO 2 monitoring (Respicond, Nordgren SA) and

incubated for 2 weeks at 5, 20 or 35 C. Extracts from before and after

incubation were obtained from separate samples. For the long-term
experiment, non-sterile and re-inoculated samples were stored in sealed
containers with ample headspace to allow for aerobic conditions and
extracted repeatedly at 0, 2, 4, 8 and 12 weeks.
For the extraction, the cylinders were placed into a percolation
apparatus, where 250 ml of 1 mM CaCl 2 solution was drip-irrigated on
the samples at ~40 ml h −1 and the leachate collected at the base at

°

− 60 hPa,

thus m

membrane-filtered solutions, total DOC (Shimadzu 5050) and UV
absorbance at 280 nm were determined. The Cu complexation ability of
DOM was determined by measuring the solubility enhancement of CuO
in phosphate buffer solutions (pH 5.5) in the presence of DOM in
comparison with a DOM-free control. DOM degradability was assessed by
supplementing the extracts with N and P and inoculating with 1% fresh
soil solution. After a 5-day incubation period, the remaining DOC was
determined. Samples with glucose served as a control for the viability of the
inoculum.
All assays were conducted with six replicates, but extracts from two
samples were combined to composite samples in order to have sufficient
solution for the following fractionation and analyses.

allowing unsaturated flow in the soil sample. In the 0.45 µ