Agriculture Reference
In-Depth Information
A. deNeergaardand J.Magid
RhizosphereEffects on Soil Microbial Biomass and Turnover
5.5
Rhizosphere Effects on
Soil Microbial Biomass Size
and Turnover in a Soil of
High and Low Fertility
A.
DE
N
EERGAARD
1
AND
J. M
AGID
2
1
Agroecology, Department of Agricultural Sciences, Royal
Veterinary and Agricultural University (KVL),DK-2630Taastrup;
and
2
Plant Nutrition and Soil Fertility Laboratory, Department of
Agricultural Sciences, Royal Veterinary and Agricultural University
(KVL),DK-1871Frederiksberg C, Denmark
Introduction
Various methods for studying the soil microbial biomass (SMB) in the
rhizosphere and its influence on soil organic matter have been developed.
Different measures, such as direct bacteria counting (e.g. Rovira
et al
.,
1974; Badalucco
et al
., 1996), ATP content (Youssef
et al
., 1989), enzyme
activity (Badalucco
et al
., 1996) or fumigation incubation (Helal and
Sauerbeck, 1986) have been used for determining SMB in the rhizosphere.
Likewise, several studies have investigated the decomposition of
14
C-
labelled soil organic matter (SOM) or plant material in the presence of
roots and have yielded conflicting results (e.g. Jenkinson, 1977; Sparling
et al
., 1982; Martin, 1987; Sallih and Bottner, 1988; Cheng and Coleman,
1990). The use of a model rhizosphere system (Gahoonia and Nielsen,
1991), combined with
14
C labelling of an SOM pool and the SMB, enabled
direct measurements of the SMB in distinct soil layers in close proximity to
roots and an investigation of its influence on the turnover of recently
formed microbial residues.
Materials and Methods
Two sandy loams were compared in the experiment, one with a normal
fertilization record (
fertile
) (22% clay, 21% silt, 37% fine sand and
20% coarse sand; organic C content, 1.31%; N content, 0.13%; inorganic
P (Olsen-P), 52 µgg
−1
soil; pH (0.01 M CaCl
2
), 5.7; SMB nitrogen,
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